## Vol. 65, No. 12 (2016)

##### 2016-06-20

###### REVIEW

## EDITOR'S SUGGESTION

2016, 65 (12): 128504.
doi: 10.7498/aps.65.128504

Abstract +

Oxide semiconductor is regarded as one of most suitable active materials of thin-film transistors (TFTs) for driving organic light-emitting diodes because of its advantages of high mobility, low-temperature processing, good electrical uniformity, visible-light transparency, and low cost. Currently oxide TFTs have been successfully applied to the backplanes of the flat-panel displays. This review gives a comprehensive understanding of the development process of oxide TFTs. In the present article, we review the major trend in the field of oxide TFTs. First, the questions of how to achieve high-mobility and high-stability oxide semiconductors are introduced, and the carrier transport mechanism is also addressed. Next, the device structures and the fabrication processes of the oxide TFTs are introduced. The electrical instability of the oxide TFTs is also discussed, which is critical for their applications in backplanes of the flat-panel displays. Especially, the mechanism of the threshold voltage instability of the oxide TFTs under negative bias illuminant stress is discussed in detail. Finally, the applications of oxide TFTs in flat-panel displays, such as active matrix organic light-emitting diodes and flexible displays, are addressed.

###### GENERAL

2016, 65 (12): 120201.
doi: 10.7498/aps.65.120201

Abstract +

Singular systems are also called descriptor systems. Compared with normal systems, singular systems have become one of effective tools which can describe and characterize varieties of real systems, because they can better describe physical properties of the systems. Up to now, the analyses and syntheses of singular systems have been widespread applied to linear matrix inequality (LMI) method. The method requires known systems to have accurate mathematical model information, however, real systems have difficulties in obtaining their accurate mathematical models, owing to the fact that real systems are frequently subjected to all kinds of interferences, uncertainties, and nonlinear factors. Specially, in the case of singular systems, obtained LMI conditions often have a constraint equation or LMI is semi-definite, which makes it more difficult to solve LMI. Therefore, in order to avoid the above two problems occurring in settling state tracking issue for singular systems, in the meantime, for convenience of computating control algorithm and information storage, in this paper we propose a discrete-time iterative learning control algorithm for a class of discrete-time singular system with repetitive running characteristics in finite time interval. The specific process is divided into two steps. First, the class of discrete-time singular system is decomposed into normal discrete-time state equation and algebraic equation form by nonsingular transformation. Accordingly, the singular system state is also decomposed into two parts. Among them, the dimension of the first part state is equal to singular matrix rank and another is equal to system dimension minus singular matrix rank. In addition, the control law of last iterative learning is modified by using two tracking errors at two different times: one error is real-time tracking error generated from the comparison between the first part state and its desired state and another is tracking error at a previous time generated from comparison between the second part state and its desired state. And thus a new control law of next iterative learning is obtained, such that, as for any given real singular system, its state may completely track the desired state as long as selected learning gain can satisfy the convergence condition of the algorithm. Further, the convergence of the control algorithm is theoretically proved by compression mapping method, and thus its sufficient convergence condition is given in the sense of -norm. The results indicate that the proposed iterative learning control algorithm can make system state realize the perfect tracking of desired state as iteration number gradually increases in finite time interval, and the convergence of the algorithm only depends on system parameters and learning gain rather than initial value of control variable. The simulation example finally verifies the effectiveness of the proposed algorithm.

2016, 65 (12): 120301.
doi: 10.7498/aps.65.120301

Abstract +

Quantum entanglement is one of the most fundamental properties of quantum mechanics. Because of the nonlocality, quantum entanglement is widely used in quantum computation and quantum information. Considering the fact that thermal fluctuation suppresses quantum effects, the concept of thermal entanglement is introduced to refer to the idea that the effect of temperature should be viewed as external control in the preparation of entangled state. It has been found that nanoscale single molecular magnet has a novel quantum effect at low temperature. Furthermore, single-molecular magnet is viewed as a promising candidate for realizing encoding and manipulation of quantum information. Na9[Cu3Na3(H2O)9(-AsW9O33)2]26H2O (denoted as {Cu3} for convenience) is one of the typical representatives of nanoscale single molecular magnets. In this paper, we will theoretically analyze the properties of tripartite entanglement in {Cu3} with an external magnetic field in thermal equilibrium. The tripartite negativity is used to characterize the tripartite entanglement. The tripartite negativity of {Cu3} single molecular magnet is calculated numerically by using the equivalent spin model and experimental fitting parameters. We consider the magnetic fields along the vertical and the parallel directions of triangular spin ring, respectively, and the case with a tilted magnetic field is also discussed in this paper. It is shown that the magnitude and direction of magnetic field, and temperature have importance effects on the tripartite negativity of the system. It is found that the larger extra strong magnetic field will inhibit the generation of the quantum state of tripartite entanglement at higher temperature. In addition, compared with the magnetic field along the parallel direction of triangular spin ring and the tilted magnetic field, the magnetic field along the vertical direction of triangular spin ring obtains larger values of tripartite negativity under the same temperature and magnetic field. We also plot the variations of the critical temperature with the magnetic field along different directions, and from the critical temperature-magnetic field phase diagrams one can obtain the range of parameters in which the tripartite entanglement of the system exists. We also find that entanglement revival behaviors may occur in the specific range of parameters. Therefore, the properties of the tripartite entanglement in the {Cu3} triangular spin ring can be controlled and enhanced by choosing appropriate magnitude and direction of the magnetic field and temperature.

2016, 65 (12): 120302.
doi: 10.7498/aps.65.120302

Abstract +

Quantum communication is the interdisciplinary science of quantum mechanics and telecommunication theory. It has advantages of perfect information security and high efficiency in transmission. In recent years, the theoretical and experimental results show that quantum communication systems have the superiority over the traditional communication systems. Quantum communication systems are hopeful for solving the information security problems that everyone is facing today, therefore, they possess broad application prospects and are forming a research hotspot of the telecommunications field currently. On the other hand, Voice over Internet Protocol (VoIP) is the method to transmit the digitized packet voice in Internet around the world. The advantages of VoIP are that it can carry voice, data, video, telephone conference, electronic commerce, and electronic mail economically. VoIP can realize the information storage and retransmission easily and flexibly. However, VoIP also encounters the problem of information security. We are trying to combine the quantum communications network and the VoIP system together and build a brand new network named quantum VoIP network which combines the advantages of both quantum communications and VoIP. The data packets may be delayed and lost in a queue up with a router due to the congestion and link failure during the transmission of quantum information. In order to ensure the performance of quantum VoIP system, the routing optimization strategies are proposed in the paper. The relay technology based on entanglement swapping is adopted. The multiuse quantum communications are realized by giving priority to the quantum channels with the least relay nodes. Theoretical analysis and simulation results show that when the data transmission links are fail to work properly and routers are in congestion, adopting the routing optimization strategies in M/M/m queuing system with the bit error rate (BER) of quantum bit setting to be 0.2 and the number of common channels increasing from 4 to 8,, the percentage of call failure in quantum communication network decreases from 0.25 to 0.024, and the maximum throughput of quantum networks increases from 64 kbps to 132 kbps. In comparison, when the number of common channels is set to be 4 andthe BER of the quantum bit is from 0.3 to 0.1, the maximum throughput of quantum networks increases from 41 kbps to 140 kbps. Thus it can be concluded that the routing optimization strategies proposed in this paper can improve the performance of quantum VoIP system significantly.

2016, 65 (12): 120501.
doi: 10.7498/aps.65.120501

Abstract +

Recently, the dynamics problems of nonlinear systems driven by noises have attracted considerable attention. The researches indicate that the noise plays a determinative role in system evolution. This irregular random interference does not always play a negative role in the macro order. Sometimes it can play a positive role. The various effects of noise are found in physics, biology, chemistry and other fields, such as noise-induced non-equilibrium phase transition, noise-enhanced system stability, stochastic resonance, etc. Especially, in the field of biology, the effects of noise on life process are significant. At present, a large number of researchers have studied the kinetic properties of the neuron system subjected to noises. However, these studies focus on the Gaussian noise, while the researches about non-Gaussian noise are less. In fact, it is found that all the noise sources among neuronal systems, physical systems and biological systems tend to non-Gaussian distribution. So it is reasonable to consider the effects of the non-Gaussian noise on systems, and it is closer to the actual process. Therefore, it has some practical significance to study the FHN system driven by the non-Gaussian noise and analyze the kinetic properties of this system. In this work, we study the stationary probability distribution (SPD) in FitzHugh-Nagumo (FHN) neural system driven by correlated multiplicative non-Gaussian noise and additive Gaussian white noise. Using the path integral approach and the unified colored approximation, the analytical expression of the stationary probability distribution is first derived, and then the change regulations of the SPD with the strength and relevance between multiplicative noise and additive noise are analyzed. After that, the simulation results show that the intensity of multiplicative noise, the intensity of additive noise, the correlation time of the non-Gaussian noise and the cross-correlation strength between noises can induce non-equilibrium phase transition. This means that the effect of the non-Gaussian noise intensity on SPD is the same as that of the Gaussian noise intensity. However, the non-Gaussian noise deviation parameter cannot induce non-equilibrium phase transition. Moreover, we also find that the increases of the multiplicative noise intensity and the cross-correlation strength between noises are conducive to the conversion of the exciting state into the resting state. And with the additive noise intensity and the correlation time increasing, the conversion of the resting state into the exciting state becomes obvious. Meanwhile, the increase of non-Gaussian noise deviation parameter increases the probability of staying in the resting state.

2016, 65 (12): 120502.
doi: 10.7498/aps.65.120502

Abstract +

In a neuronal system, propagation speed of neuronal information is mainly determined by the length, the diameter, and the kind of the axons between the neurons. Thus, some communications between neurons are not instantaneous, and others are instantaneous or with some negligible delay. In the past years, effects of time delay on neuronal dynamics, such as synchronization, stochastic resonance, firing regularity, etc., have been investigated. For stochastic resonance, it has been reported recently that stochastic multi-resonance in a neuronal system can be induced by time delay. However, in these studies, time delay has been introduced to every connection of the neuronal system. As mentioned in the beginning, in a real neuronal system, communication between some neurons can be instantaneous or with some negligible delays. Thus, considering the effect of partial time delay (time delay is called as partial time delay if only part of connections are delayed) on neuronal dynamics could be more meaningful.In this paper, we focus on discussing effect of partial time delay on response amplitude of a Watts-Strogatz neuronal network which is locally modeled by Rulkov map. With the numerically obtained results, we can see that partial time delay can induce a stochastic multi-resonance which is indicated by the multi-peak characteristics in the variation of response amplitude with partial time delay. Namely, partial time delay could also induce stochastic multi-resonance in a neuronal system. Moreover, we also find that optimal response amplitude can be reached in much wider range of the partial time delay value when delayed connections are less (i.e., the partial time delay probability is small). This is different from the case in which all connections are delayed, where response amplitude become optimal only when time delay is nearly the multiples of external signal's period. But the range of the partial time delay value becomes narrower and narrower with the increasing of the partial time delay probability and when the partial time delay probability is large enough, response amplitude becomes optimal only when time delay is nearly the multiples of external signal period. It is similar to the case where all connections are delayed. Furthermore, effects of random rewiring probability and total link number in the neuronal network on partial time delay induced stochastic multi-resonance are also studied. It is found that partial time delay induced stochastic multi-resonance is robust to random rewiring probability but not robust to total link number. Stochastic resonance is a very important nonlinear phenomenon in neuroscience, thus, our obtained results could have some implications in this field.

2016, 65 (12): 120503.
doi: 10.7498/aps.65.120503

Abstract +

A nanoscale memristor can replace the nonlinear part of a chaotic system, which can greatly reduce the physical size of the chaotic system. More importantly, it can enhance the complexity of the chaotic system and the randomness of signals. In this paper, a new memristor-based chaotic system is designed based on a new three-dimensional autonomous chaotic system. In order to study the complex dynamic characteristics of the memristive system, the chaotic system is investigated by the theoretical derivation, numerical simulation, stabilization of equilibrium points, and Lyapunov exponent spectrum. The influences of different parameters on the phase diagram and the stability of equilibrium point of this system are also discussed in detail. It is interesting that when system parameters a and c take different values, the location and stability of the equilibrium point of the system will be changed, then two scrolls of the system will be overturned at a different angle, and it will produce a different degree of aliasing between the two scrolls. Parameter b has a large variable range, when it is changed, and the system will transform into three kinds of classical chaotic systems defined by Vaněček and Celikovsk. These indicate that the memristor-based chaotic system has a lot of valuable dynamic behaviors, so it has applications in the field of secure communication, information processing etc. Field programmable gate array (FPGA) technology has a large capacity and high reliability, which is widely used in modern digital signal processing. And with the development of FPGA technology, applying FPGA technology to realizing the chaotic systems has gradually become a hot topic. Moreover, the improved Newton iteration method is used to design a square root operator of memristor in this paper by using verilog hardware description language (verilog HDL) which only needs three times iteration to reach the required accuracy. The results of FPGA hardware are consistent with the numerical simulation results. It breaks through the previous bottleneck that the chaotic system based on titanium dioxide memristor can only be simulated in computer, which is of great significance for further studing of memristor, and provides a reference for further research on the memristor-based chaotic system and applications in secure communication and information processing.

###### ATOMIC AND MOLECULAR PHYSICS

2016, 65 (12): 123201.
doi: 10.7498/aps.65.123201

Abstract +

High-order harmonic generation (HHG) from the interaction among intense laserfields and atoms and molecules has attracted much attention. It is of the paramount importance and is still a rapidly growing field due to its potential to produce coherent and bright light within the uv and soft X-ray region and to generate attosecond pulses. Generally speaking, a typical spectrum of HHG shows that for the first few harmonics decrease rapidly, then present by a broad plateau of almost constant conversion efficiency, and end up with a sharp cutoff. In a recent experiment, it is verified that the field enhancement induced around the bow-tie elements with a 20-nm gap allows the generation of extremeultraviolet light directly from the output of a single femtosecond oscillator of 100-kW peak power. With the development of the HHG in the vicinity of metallic nanostructure from atomic responses, the harmonic generation in the vicinity of metallic nanostructure from molecules has also been investigated. In this paper, HHG from H2+ in bowtie-shaped nanostructure is investigated by solving the one-dimensional time-dependent Schrdinger equation within the non-Born- Oppenheimer approximation by the splitting-operator fast-Fourier transform technique. We find that the spatial position of the inhomogeneous field inside the nanostructure has a great influence on the harmonic spectrum. When the spatial position of the inhomogeneous field is translated from 30 a.u. to -30 a.u., the cutoff of the HHG can be extended and the smoother supercontinuum harmonic spectrum is formed. The underlying physical mechanism can be well demonstrated by the time-frequency distribution, the three-step model, the ionization probability and electric field of the driving laser. The harmonic order as a function of the ionization time and emission time can be given by the semi-classial three-step model. The trajectory with an earlier ionization time but a later emission time as a long electronic trajectory, and the trajectory with a later ionization time but an earlier emission time as a short electronic trajectory. The interference between the long and the short trajectories will lead to a modulated structure of the supercontinuum. When the spatial position of the inhomogeneous field is translated from 0 a.u. to 30 a.u., the cutoff of the HHG can be shortened and there are short and long electronic trajectories contributing to each harmonics and bringing about more modulations. When the spatial position of the inhomogeneous field is translated from 0 a.u. to -30 a.u., the cutoff of the HHG can be extended and there is only a short electronic trajectory contributing to each harmonics and the smoother supercontinuum harmonic spectrum is formed. The effects of the carrier-envelope phase on HHG is also demonstrated. When the carrierenvelope phase is -0.2, the cutoff of the HHG is extended. When the carrier-envelope phase is -0.2, the cutoff of the HHG is shortened. But we find that with the change of the carrier-envelope phase, their overall trends are the same, that is, the cutoff of the HHG is extended when the spatial position of the inhomogeneous field is translated from 30 a.u. to -30 a.u..

2016, 65 (12): 123301.
doi: 10.7498/aps.65.123301

Abstract +

The plasma activated water has great application prospects in the fields of environmental protection, biomedicine, food safety, et al., due to its unique chemical activity. In this work, the plasma activated physiological saline is successfully generated by using hollow fiber-based cold microplasma jet array running in physiological saline solution. This design can lead to an obvious increase in the contact area between microplasmas and treated physiological saline solution, thus improving the chemical reaction efficiency of short-lived species. The influences of working gases such as He, N2, O2 and air on the sterilization efficiency of E. Coli by using this plasma activated physiological saline are investigated as a function of discharge time. As the discharge time increases from 10 to 180 s, the sterilization efficiency of the plasma activated physiological saline significantly increases. It is found that the bactericidal efficiency of the air discharge activated physiological saline is highest. For a discharge time of 120 s, the sterilization efficiency of E. Coli in this plasma activated physiological saline can reach as high as 99.999%. The pH value of this air discharge activated physiological saline is achieved by using acidity meter and as the discharge time increases from 10 to 60 s, the pH value decreases from 7.3 to 3.1 and the physiological solution becomes acidic. This may be due to the NOX produced in the plasma reacting with water and producing nitric and nitrate acids. The reactive oxygen species generated in the plasma activated physiological saline are supposed to be O3 and H2O2. The concentrations of O3 and H2O2 are identified by using UV-visible absorption spectra and chemical deposition methods. The strong absorption peak of O3 in UV-visible absorption spectrum is at a wavelength of 253.7 nm. The concentration of O3 is calculated by using Beer-Lambert Law. As the discharge time increases, the concentration of O3 in the plasma activated physiological saline obviously increases. For a discharge time of 60 s, the concentration of O3 is 43.1210-3 mol/L and nearly saturated. The concentration of H2O2 is obtained by the total amount of reactive oxygen species, which is calculated by using the chemical deposition method, deducting the O3 content. As the discharge time increases from 10 to 180 s, the concentration of H2O2 increases from 1.510-3 to 4.710-3 mol/L. The analyses of experimental data from various methods indicate that air discharge activated physiological saline containing a variety of long-lived reactive oxygen species, such as H2O2 and O3, is very effective in killing E. Coli cells in the acidic saline solution. The air discharge activated physiological saline can provide a means to store the advanced oxidation species induced by the discharge for sterilization applications.

2016, 65 (12): 123601.
doi: 10.7498/aps.65.123601

Abstract +

The structure and thermodynamic properties of Cu-Co alloy cluster with Cu atoms distributed in inner layer and outer surface of Co cluster are investigated by the molecular dynamics simulation combining with an embedded atom potential method. The results demonstrate that there are huge differences in structure, energy and melting point between the inner layer and outer surface of Co clusters due to various doping layers comprised of the same number of Cu atoms. The different doping positions of Cu atoms in Co cluster make atoms shift towards lower energy state. However, after relative movement, the supplementary deposition of subsequent atoms leads to the relatively non-diffusive phase transformation of cluster structure. The segregations of Cu atoms from inner layer to outer surface of Co cluster are the main reason for the enormous difference in melting point between the inner layer and outer surface of Co clusters with the same percentage of Cu atoms.

###### ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS

## EDITOR'S SUGGESTION

2016, 65 (12): 124201.
doi: 10.7498/aps.65.124201

Abstract +

Single wavelength illumination is used in the traditional ptychography. Even though using multi-wavelength to improve image quality, it takes the scheme of illuminating in turn due to the requirement of coherence. So far, the addition of incoherent modes has been regarded as a nuisance in diffractive imaging. Here we propose a scheme of incoherent ptychography and an algorithm of information multiplexing that uses the multi-wavelength illumination simultaneously, which are demonstrated in experiment and simulation. Compared with the scheme of traditional ptychography, it can recover not only the object well, but also the spectral response of the object, probes of complex value and spectral weight of each wavelength respectively. This method obtains much information about the object and owns the multichannel and multispectral merits. Meanwhile, by means of color image coding, this method can retrieve true color images and enhance the image quality. The proposed algorithm has strong robustness. Besides, we also investigate how many modes can be recovered by this method. The work may open up possibilities for information multiplexing in ptychography and multispectral microscopy imaging over various applications.

2016, 65 (12): 124202.
doi: 10.7498/aps.65.124202

Abstract +

Cavity optomechanics becomes a promising field in quantum and nano technologies. Motivated by the optomechancial experiment with the membrane located in a high-finesse optical cavity and theoretical treatment on two membranes cavity optomechanics, we here study the optomechanical interaction of the system consisting of triple membranes within an optical cavity. The increase of membranes will increase the normal modes of the cavity and mechanical fields, and thus enrich the forms of optomechanical interaction. Firstly, we use the transfer matrix and resonance transmission methods to obtain the dispersion relation between the eigen-frequencies of the optical modes and the mechanical motions. Owing to the existence of triple mechanical membranes, the system possesses different forms of collective mechanical motion, and here we focus on the center-of-mass (COM) motion and relative motion of the equally placed membranes. The numerical solutions of the dispersion relation show that the optical eigenmodes are comprised of a group of closely spaced avoided-crossing quaternion of wave numbers, which arise from the transmission and reflection of the optical field at the membranes and the tunneling couplings between subcavity modes. Moreover, the change of each eigen wave number along each form of the mechanical motion is different, which implies the different forms of optomechanical coupling between eigenmodes and mechanical motions. Then, to achieve the explicit expressions of the optomechanical coupling, it is sufficient to use the perturbation method under the equilibrium condition of the system, where the amplitude of mechanical motion is much smaller than the optical wavelength. With using the implicit function differentiation theorem, the optomechanical coupling strengths between the four optical modes and the COM and relative mechanical motions are obtained respectively. We find that the strong quadratic optomechanical coupling between the optical modes and COM motion can be achieved, and the linear and quadratic couplings between the optical modes and relative motion can both be realized. By tuning the laser to pump different optical modes, we can choose either the linear or the quadratic coupling to the relative motion. Our method is universal to multi-membrane system, and the results may provide some references to theoretical and experimental investigations on the multi-membrane cavity optomechanics.

2016, 65 (12): 124203.
doi: 10.7498/aps.65.124203

Abstract +

Spin-flip model (SFM) is a mostly used approach to analyzing the nonlinear dynamics of vertical-cavity surface-emitting laser (VCSEL), and therefore the value selections of some key parameters in this model are crucial. In this work, based on experimentally measured dynamical characteristics of a 1550 nm vertical-cavity surface-emitting laser (1550 nm-VCSEL) under free running and parallel optical injection, some key parameters (field decay rate k, total carrier decay rate N, linewidth enhancement factor , active medium birefringence rate p, spin relaxation rate s, and active medium linear dispersion rate a) are estimated. Through experimentally measuring the noise spectrum of the laser, the relaxation oscillation frequency and the damping rate of the relaxation oscillations are calculated, and the photon lifetime can be preliminary estimated. After further amending the photon lifetime by considering the effect of the gain saturation on the damping rate of the relaxation oscillations, the value of k is determined. Based on the function relation between the laser relaxation oscillation frequency and the electrical pumping, the value of N is obtained. By experimentally acquiring the dynamical distribution mapping of the laser under parallel optical injection, the minimum Hopf bifurcation point of the Hopf bifurcation curve can be found, and then the value of is roughly estimated. According to the frequency difference between the two polarization components of the laser in the measured optical spectrum, the value of p can be calculated. The value of s is obtained by using the relationship between s and p. On the basis of the above determined parameter values, the value of a can be specified by numerically simulating the optical spectrum of the laser and comparing with experimentally obtained results. Moreover, by comparing the experimentally measured dynamical mapping of optical injection VCSEL with corresponding dynamical mapping simulated on the basis of the above mentioned parameters, the value of is rectified. Finally, further simulated results agree with relevant experimental observations.

2016, 65 (12): 124204.
doi: 10.7498/aps.65.124204

Abstract +

Four-petal Gaussian beam is a special type of Gaussian beam, and its propagation properties are widely used in micro optics, optical communication and splitting technology. Recently, the generations and the properties of different types of hollow beams have become a hot research topic, such as research on hollow optical vortex beams. The Gyrator transform can be used to fulfill the mode conversion of laser beam. In this paper, based on the Gyrator transform, the analytical expression of four-petal Gaussian beam passing through such a transform system is derived, and the intensity distribution and the corresponding phase distribution associated with the transforming four-petal Gaussian beam are analyzed by numerical simulations. It is found that the four-petal Gaussian beam can be transformed into rectangular hollow beam by Gyrator transform, under the appropriate conditions of the beam order, the beam parameter, the transform angle of Gyrator transform, and the waist width. For the beam order n=m=3, the transform angle of Gyrator transform = 0.4133, the beam parameter K=30, and the waist width = 0.9, the rectangular hollow optical vortex beams can be obtained. Under such conditions, the maximum intensities appear in the four corners, and they are almost uniform on the four sides. The effects of the beam parameters, the transform angle, and the beam order on the distributions of intensity and phase of the rectangular hollow beam are analyzed in detail. The numerical results show that for the beam parameter K10, the rectangular hollow beam always is obtained, and for a lager beam parameter, the intensity distribution of the rectangular hollow beam is more uniform. Different beam order generates different type of hollow beam. For example, for n=m = 2, = 1.2, K = 30, and = 0.5409, a new strange circular hollow beam with solid circular nucleus can be obtained. The transform angle of Gyrator transform has a significant effect on the energy distribution of the hollow beam. When the transform angle changes in a small range, the uniformity of the intensity distribution of the rectangular hollow beam is lost. The bigger the transform angle change, the more serious the loss of uniformity of the hollow beam intensity is. The size of the hollow beam bright ring is determined by the waist width of the four-petal Gaussian beam: the larger the waist width, the smaller the bright ring is. The results further enriches the applications of Gyrator transform system and the four-petal Gaussian beam in the beam shaping.

2016, 65 (12): 124205.
doi: 10.7498/aps.65.124205

Abstract +

With the development of infrared optics, low-loss waveguide materials are required. Especially, low-loss optical fiber development for far-infrared application has become a focus. Chalcogenide Ge-As-Se-Te(GAST) glasses and fibers for far-infrared light are prepared and investigated in this paper. The thermal properties and the infrared transmissions are reported. The influences of oxygen and hydrogen on the glass transmission and fiber attenuation are discussed. Low-loss GAST fiber with a structure of fine core/cladding is reported by a novel extrusion method (0.46 dB/m at 8.7 m, 1.31 dB/m at 10.6 m, base loss being under 1 dB/m from 7.2 to 10.3 m). Here, the glasses are prepared by traditional vacuum melt-quenching and vapor distillation method. Structure and physical properties of GAST glass system are studied with X ray diffractions and thermal expansion instrument. Optical spectra of GAST glass system are obtained by spectrophotometer and infrared spectrometer. Main purification processes with different oxygen-getters (magnesium and aluminum) are disclosed. The fiber attenuation is measured by the cut-back method with an Fourier transform infrared spectroscopy spectrometer. The lowest loss of this fiber can be reduced to 1.32 dB/m at 10.6 m, as it has a structure of Ge20As20Se15Te45 core and Ge20As20Se17Te43 cladding. The results show that these glasses are well transparent in a wide infrared window from 1.1 to 22 m, and these glass fibers can transmit far-infrared light up to 12 m, thus the GAST glass system is one of good candidates for far-infrared transparent materials. The fiber attenuation can be reduced effectively by the reasonable purification and novel extruded-processing. These fibers are suited for the power delivery of CO2 laser.

2016, 65 (12): 124206.
doi: 10.7498/aps.65.124206

Abstract +

Photonic crystal has drawn much attention because of its application in molding the flow of light, which can be used in optical communication, optical storage and computing. In theory, plane wave expansion method, finite difference time domain (FDTD) method and transfer matrix method are widely used methods to study photonic crystal, and each of them has its own advantages and disadvantages.Here, a new method i.e. mixed variational method is introduced to study the photonic crystals, which is from the work of anti-plane shear waves in periodic layered elastic composites. The calculations of this method are direct and require no iteration, which accurately and efficiently produce the entire band structure of the composite and other field characteristics. Moreover, the composite cell in this method may consist of any number of units of any variable permittivity and permeability.Firstly, based on the variational principle, the Lagrangian density of electro-magnetic field is obtained. Then through the surface integral of the Lagrangian density in the unit cell, the Lagrangian is acquired. The first variation of Lagrangian with respect to electric field and magnetic field yields a set of Euler-Lagrange equations. Approximate solutions in explicit series expressions subject to the Bloch periodicity are substituted into the above equations. Minimization of Lagrangian with respect to the electric field and magnetic field results in an eigenvalue problem, and to solve it, the band structure of the composite is yielded. Electrical field, magnetic field, group velocity and energy flux density are also calculated. Secondly, we use the above method to study a two dimensional air-rod unit cell system. Bandgaps with respect to different structural parameters are plotted, which are the same as the results from the plane wave expansion method and FDTD method. In theory, the entire band structure can be calculated with our method. There are more gaps for TE wave than for TM case. By constant frequency contours, it is shown that there is a gap between the first and the second pass band for TE wave, however, there is no a gap for the corresponding TM wave. The directions of group velocity for the first and the second bands are shown in the contours. Electrical field, magnetic field and energy flux in cells illustrate the energy distribution, and the energy-flux directions and the group-velocity directions are also essentially the same. Lastly, we apply this mixed variational method to one-dimensional media-air slab and three dimensional sphere-air structure. The obtained band results accord with those reported previously former, which demonstrates that our method is universal and correct.In the present work, a mixed variational approach is proposed to produce the entire band structure of the composite for unit cells with any arbitrary properties. Explicit expressions are developed for the band, electrical field, magnetic field, group velocity and energy flux.

2016, 65 (12): 124207.
doi: 10.7498/aps.65.124207

Abstract +

The Gabor zone plate is an ideal zone plate with single focus spot, which has the potential applications in spectroscopy, X-ray imaging, etc. However, the Gabor zone plate is very difficult to prepare because of its sinusoidal transmission characteristic, thereby restricting its applications. Traditionally, the zone plate is prepared on the transparent substrate such as quartz glass, polyimide, etc. This restricts the applications of Gabor zone plates in the extreme ultraviolet and soft X-ray frequency band due to the strong absorption of quartz and polyimide in such bands.In this work, we report a method of preparing the self-standing binary Gabor zone plate by using the focused ion beam direct writing. By combining the techniques of focused ion beam and chemical wet etching, the binary Gabor zone plate with self-standing and curved structure is fabricated. The main characteristic parameters of the Gabor zone plate are as follows: the diameter of 1400 m, the radius of the first zone 90 m, the outset zone number of 60, and a gold absorber thickness of 500 nm. The focusing properties of the self-standing binary Gabor zone plate are measured at different transfer distances with a 355 nm laser. The experimental results show that the high-order focus is removed with only the first-order focus spot reserved, and the focal distance is 2.28 cm, which is in agreement with the theoretical value of 2.41 cm. The self-standing Gabor zone plate is free from the influence of the substrate. Therefore, this kind of binary Gabor zone plate has potential applications in ultraviolet and soft X-ray regions.

2016, 65 (12): 124301.
doi: 10.7498/aps.65.124301

Abstract +

Ship underwater radiated noise is one of the most important ocean ambient noise sources, and building a reasonable model for the ship underwater radiated noise is helpful for understanding the physical mechanism and reducing research cost of ship underwater radiated noise. The quasi-periodic random sound pulse sequence signals act well in explaining the rhythm and the power spectrum variation of the ship underwater radiated noise, and reveal that there are not any real sinusoidal components in ship radiated noise signals, which come from the non-linear transformation of the signals, and the analysis of some representative experimental data of ship radiated noise also supports this idea. Based on this, the explosion-type cosine pulses are used as the units of quasi-periodic random sound pulse sequences. This model can generate the power spectrum with a peak, and the peak location can change with ship velocity or ship type. The power spectrum variation characteristics of quasi-periodic random sound pulse sequences consisting of the explosion-type cosine pulses are in good agreement with the measured ship underwater radiated noise data, which shows that this model is of important practical value.

2016, 65 (12): 124401.
doi: 10.7498/aps.65.124401

Abstract +

In this paper, we present an inversion estimation method of thin film parameters based on thermal effects induced by laser irradiation. Firstly, the theoretical model of classical Fourier heat conduction of thin film irradiated by laser is established, and the analytical solutions of temperature fields are obtained by using Laplace transform. Then, the inversion model and the iteration algorithm are established based on the nonlinear conjugate gradient method on condition that the thermal conductivities of the film and the substrate are selected as inversion parameters and the temperature fields of the thin film surface in different irradiation times are selected as measured data. In view of the fact that the sensitivity coefficient plays a decisive role in determining the accuracy and efficiency of the nonlinear conjugate gradient iteration inversion algorithm, we derive the closed form expressions of the sensitivity coefficients for the thermal conductivities of the film and the substrate based on the above analytical solutions of the temperature fields, and this closed form expressions can improve the accuracy and efficiency of the thin film parameter inversion significantly. Taking four kinds of metal films (aluminum, silver, copper and gold) with glass substrate for example, the accuracies of the analytical solutions of temperature fields are verified by comparing with the numerical results from the finite element method in the existing literature, and it can ensure the accuracies of the sensitivity coefficients in the process of iteration inversion. Finally, the thermal conductivities of the above four kinds of thin films are estimated by using the presented iteration inversion method. The accuracy and efficiency of the parameter inversion are verified by investigating and analyzing the inversion results of the parameters for different random noises and different iterative initial values. The inversion results show that this method has a high accuracy and efficiency, and it only needs less than 20 iteration times to convergence when the iteration stop error is 10-7. The smaller random noise is added in the measured data, and the less iteration times to convergence are needed. It can achieve higher convergence efficiency even in the iterative initial values from the inversion results that differ greatly for the case of 5% random noise. This inversion method of thin film parameters is not only applicable to the inversion of the thermal conductivity, but can also be used to inverse the parameters such as the reflection coefficient or the absorption coefficient. The presented method has a certain guiding significance for the parameters inversion and the parameters optimization in the process of the laser processing or the laser damage.

2016, 65 (12): 124501.
doi: 10.7498/aps.65.124501

Abstract +

Using a direct shear-box capable of very low shearing rate, we measure the force-displacement curve of cyclic, large-amplitude shear, and also the total plastic displacement residual after each cycle, for samples of glass beads. As the shear rate decreases, we observe a transition from normal, elastoplastic behavior to pure elastic behavior, with reducing residual, or total plastic, displacement after each cycle. Remarkably, this transition is also observed for large amplitude of the cyclic shear, up to 90% of the failure value. The force-displacement relation is necessarily rate-dependent during this transition. These experimental results demonstrate that granular solids may respond in a purely elastic manner, both for low amplitude force oscillations of high frequencies (such as sound) and for large amplitude ones of low frequencies, implying that the granular matter has a purely elastic regime, in which the theory of elasticity holds fully true. This regime has been overlooked in the literature, probably because its deformation rate is nearly two orders of magnitude lower than those typically used. Theoretically, the present measurements support granular solid hydrodynamics, or the fact that strong deviation from elastoplastic dynamics and rate independence take place in the low frequency limit, with a rate-dependent transition to the classic theory of elasticity.

2016, 65 (12): 124701.
doi: 10.7498/aps.65.124701

Abstract +

A flag flapping in the wind is a classical fluid-structure interaction problem that concerns the interaction of elastic bodies with ambient fluid. The fluid-flag interaction can give rise to three self-sustained oscillation states under certain conditions, i.e., stretched-straight state, periodic state, and chaotic state. This paper reports an experimental study of a cantilevered polydimethylsiloxane (PDMS) flag flapping in a uniform flow at a periodic state. A heavy flag is well designed, with metal strips imbedded in the PDMS sheet. Immersing the elastic but self-sustaining flag into the water flow, we use the time-resolved particle image velocimetry (PIV) and visualization techniques to obtain the whole flow field around the midspan of the flapping flag. A unique PIV image processing method is used to measure the near-wall flow velocities around the flap-ping elastic flag at the periodic state. The image processing technique adopts a radon transform technology to determine the moving interface in the particle images. The interface velocity distribution is subsequently calculated. Artificial particles of uniform size with the interface velocity are added into the flag region. Therefore, the whole velocity field over a flapping period is accurately obtained, giving the basic data to analyze the flag flapping. It is found that there exists an inflection point in the envelope curve of the flag flapping. Based on the analyses of the flapping states and velocity fields, a unified flapping Strouhal number (St = 2Af/U) is proposed by choosing the amplitude of the inflection point as the characteristic length, which is similar to the Strouhal number of the flow around a circular cylinder over the same range of Reynolds number.

2016, 65 (12): 124702.
doi: 10.7498/aps.65.124702

Abstract +

A proper orthogonal decomposition (POD) based hybrid surrogate model and the applications to transonic flow reconstructions are presented in the paper. In the implementations, the radial basis function (RBF) model response instead of the least-square linear regression is employed in order to improve the coefficients of POD basis modes; moreover, an adaptive sampling strategy with both the model response error and sample independence taken into account is studied to reduce the sample number, while maintaining sufficient response accuracy. Firstly, the POD-RBF surrogate model is studied and compared with the least-square-based POD through pressure reconstruction studies on the twodimensional blade surface. The results demonstrate that the non-linear model response method significantly improves the coefficients of the basis modes and thus the averaged description error. Meanwhile, the beneficial gains on the convergence performance of the response error versus the number of basis modes are obtained. Then by comparing with the uniform sampling and the resampling strategy with taking only the response error into account, the adaptive sampling method proposed in the paper obtains the best performance on reducing the averaged description error. Finally, the flow characteristics of the flow fields on the suction surface, at the blade tip, in the blade passage of the sampled three-dimensional transonic compressor rotor blades with different spanwise sweeps based on the baseline blade, NASA Rotor 67 are illustrated through the flow basis modes. Compared with the suction flow, the flow at the blade tip contains more intensive flow characteristics including shock, tip-leakage flow and shock-leakage interaction, resulting in a higher averaged description error. Besides, the missed flow fields in the passages of the test blades are reconstructed from the flow basis modes by using the adaptive POD-RBF hybrid model and the corresponding aerodynamic parameters are then predicted. The spanwise distributions of the circumferentially averaged aerodynamic parameters at the blade outlet reconstructed from POD-RBF model are consistent well with the numerical solutions. The results demonstrate that the adaptive POD-RBF hybrid surrogate model is effective and accurate enough for reconstructing the transonic flow. In order to further evaluate the response performance of the adaptive POD-RBF model, statistic analysis is carried out for a group of hybrid models with different sampling strategies and different numbers of samples. Generally, although the number of adaptive samples is much less, the mean value and standard deviation of the adaptive model are close enough to those of the static model with sufficient uniform samples. Besides, the standard deviations of a lot of aerodynamic parameters of interest exhibit significant peaks near the blade tip, further demonstrating that the flow at the blade tip is more intensive in the three-dimensional transonic rotor blade passage.

###### PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES

2016, 65 (12): 125201.
doi: 10.7498/aps.65.125201

Abstract +

Black-body irradiation method can be utilized for measuring the instantaneous temperatures of electrons and lattice in dielectric machined by the ultrashort laser. One ultrashort laser pulse, of which the pulse energy and pulse duration are 240 J and 599 fs respectively, is focused into the fused silica by objective lenses with a magnification of 10 times. The focal point is at the position of 874 m. The microstructure induced by laser near the focal point is 16 m wide and 104 m long. The central region of the microstructure is heavily damaged, and the marginal region is slightly modified. The black-body irradiation spectra are recorded by the system that is composed of objective lenses, a fiber with two lenses, a spectrometer and an intensified charge coupled device (ICCD). Furthermore, other imaging elements can also be used as alternative to objective lenses, for measuring black-body spectra. The image point, which is conjunctive with the machined region due to the imaging effect of the objective lenses, is coupled into the fiber by one lens. Another lens collimates the diverging light beam from the fiber. The collimated light is incident into the spectrometer and dispersed on the ICCD. Because the minimum gate width of ICCD is much larger than the coupled time of electron and lattice, the temperature of electron equals that of lattice when they are characterized by the black-body irradiation method. The temperatures of the electrons and the lattice are regarded as the temperature of dielectric. When the system acquires the reflection peak of incident ultrashort laser, the delay is set to be 0 ns, and the central wavelength of the peak is 784 nm. Therefore, to eliminate the reflection peak, the second harmonic and supercontinuum spectra, the delay for black-body irradiation acquirement is set to be above 6 ns and the machined region should be confined inside the dielectric. The system collects the black-body spectra emitted by the heat-affected zone in fused silica 981 ns after the fused silica has been irradiated by single ultrashort laser pulse. And then the spectra are fitted by the Planck formula to obtain the temperature of dielectric. After the dielectric is processed by the ultrashort laser pulse, the valence electrons of the dielectric transit to the conduction band via strong filed ionization and avalanche ionization. The plasma with high temperature and pressure moves outward in the form of shockwave. The shockwave transfers energy by convection after fused silica has been machined by laser pulse. Due to inverse Bremsstrahlung effect during the avalanche ionization, nearly all the incident laser energy is absorbed by the fused silica. The irradiated energy is only 1.3% of the absorbed energy, so the ways of heat transfer are mainly convection and heat diffusion. 21 ns later the shock wave turns into acoustic wave, so central gaseous fused silica affects the surrounding region through heat diffusion and the temperature of fused silica decreases slowly. The temperature of fused silica is 5333 exp(-t/1289) K at time t (unit: ns). The temperature drops down to room temperature 3.72s after the fused silica has been irradiated by one ultrashort laser pulse. If another laser pulse arrives at fused silica before 3.72s, the temperature rises on the basis of the previous laser pulse. In other words, the heat accumulation effect cannot be ignored if the repetition rate of ultrashort laser is more than 269 kHz.

###### CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

2016, 65 (12): 126101.
doi: 10.7498/aps.65.126101

Abstract +

Understanding the complicated kinetic process involved in nanoparticle self-assembly is of considerable importance for designing and fabricating functional nanostructures with desired properties. In this work, using the stopped-flow absorption technique, we investigate kinetic behaviors involved in gold nanorod assembly mediated by cysteine molecules. Further combining with SEM microstructural analyses of the assembly structure of gold nanorods, we establish the correlations between the kinetic parameter and the assembled structure. The dynamical surface plasmonic absorptions of gold nanorods are monitored during the formation of GNRs chains with different assembly rates. And the acquired kinetic data are analyzed in the frame of the second-order theoretical model, which has been widely used in the literature for linear assembly of gold nanorods. We find that the second-order theoretical model for describing the kinetic behaviors is merely limited to the case of slow assembly process of gold nanorods, but shows large deviation when the assembly process is relatively fast. We, therefore, propose in this work a new kinetic model on the basis of the logistic function, to make kinetic analyses for the different assembly rates of gold nanorods. Compared with the second-order theoretical model, this new logistic function model possesses an extended validity in describing the kinetic behaviors of both the slow and relatively fast nanorods assembly. Particularly, due to introduction of a new parameter, i.e., the exponential parameter p, the logistic function model enables a more accurate description of the kinetic behavior at a very earlier assembly stage (e.g., on a millisecond scale), in addition to quantifying the assembly rate T0. More importantly, the value of p derived from the new logistic function model allows us to establish the kinetics-structure relationship. The slow assembly process that produces mainly the one-dimensional linear chains of nanorods, is featured by the value of kinetic parameter p close to 1. By contrast, for the relatively fast assembly process that results in the formations of irregular zigzag chains even two-dimensional assembled structures of nanorods, the value of kinetic parameter approaches to 2. Furthermore, in the present study, the kinetic parameter p based on the logistic model might be related to the fractal dimension (Df) of the aggregated structures of the gold nanorods self-assembly processes. These results suggest that the logistic function model could provide the kinetic features for directly quantifying the fractal structures of the nanorods assembly. We believe that the new kinetic analysis method presented in this work could be helpful for an in-depth understanding of the kinetics-structure-property relationship in self-assembled plasmonic nanostructures.

2016, 65 (12): 126102.
doi: 10.7498/aps.65.126102

Abstract +

Ammonium perchlorate (NH4ClO4) is a highly energetic oxidizer widely used in solid propellants and explosives. Under extreme pressure conditions, significant changes are observed in the structures and properties of NH4ClO4. However, many studies of structural transformations of NH4ClO4 under high pressures have not formed a more consistent conclusion. In this study, the structural, electronic, and elastic properties of NH4ClO4 are investigated by first-principles calculations based on the density functional theory with dispersion correction (DFT-D) method in a range of 0-15 GPa. The unit cell volume and lattice parameters are optimized by GGA/PBE-TS, which leads to good agreement with the experimental structure parameters at 0 GPa, suggesting the reliability of the present calculation method. The calculated P-V data are fitted to the third-order Birch-Murnaghan equation of state, and the result provides better agreement with experimental result than other calculations for the unit cell with a volume V0 and bulk moduli B0 and B'. The comprehensive analyses of the lattice parameters, bond lengths, and hydrogen bonds under high pressure indicate that three structural transformations occur in NH4ClO4 at 1 GPa, 4 GPa, and 9 GPa. With increasing pressure, hydrogen bonding interaction gradually increases, and intra- and intermolecular hydrogen bonds are present in crystals. Results obtained from the band structures and state densities under high pressure indicate that NH4ClO4 exhibits good insulating properties. Valence band shifts towards low energy, conduction band shifts towards high energy, and electronic localization is enhanced. The charge density differences and Mulliken charge populations at different pressures reveal that the covalent interaction between the N-H and Cl-O bonds increases, and the ionicity of crystal decreases. The band gaps in different structural transition regions exhibit different linear increase trends with increasing pressure. The calculated elastic constants of NH4ClO4 satisfy elastic stability criteria of orthorhombic systems at pressures ranging from 0 GPa to 15 GPa, indicating that NH4ClO4 is mechanically stable. The bulk modulus, shear modulus, and Young's modulus are estimated by the Voigt-Reuss-Hill approach. The Cauchy pressures and B/G values indicate that NH4ClO4 exhibits ductility, attributed to the fact that NH4ClO4 is an ionic crystal, and ionic bonds are non-directional bonds; hence, NH4ClO4 is ductile and can be easily bended or reshaped. The results indicate that the ductility properties of NH4ClO4 increase with increasing pressure. All calculated properties are in excellent agreement with the available experimental results. These results will not only help to understand the structural transformations of NH4ClO4 under high pressures but also provide an important theoretical reference for the safe application of NH4ClO4 in solid propellants and explosives.

2016, 65 (12): 126301.
doi: 10.7498/aps.65.126301

Abstract +

The widely used energetic material 1, 3, 5-triamino-2, 4, 6-trinitrobenzene (TATB) is an extremely powerful explosive and known for its extraordinary insensitivity to external stimuli (i.e., shock, friction, impact). TATB crystal exhibits graphitic-like sheets with significant inter- and intra-molecular hydrogen bondings within each layer and weak van der Waals (vdW) interactions between layers. Although TATB has been extensively studied both theoretically and experimentally, a fully understanding of its unique detonation phenomenon at a microscopic level is still lacking. Before establishing the exact pathway through which the initial energy is transferred, a fundamental knowledge of both the lattice vibrations (phonons) and molecule internal vibrations must be gained at the first step. Recently, it has been demonstrated that density functional theory (DFT) is inadequate in treating conventional energetic materials, within which dispersion interactions appear to be major contributors to the binding forces. In the present work, phonon spectrum and specific heat of TATB crystal are calculated in the framework of DFT with vdW-DF2 correction, which has been validated in our previous studies of the equation of state, structure and vibration property of TATB crystal under pressures in a range of 0-8.5 GPa. Structure optimization is preformed at zero-pressure, followed by calculating the equation of state, crystal density and lattice energy. The computed results are found to fit well with the experimental and other theoretical values. Frozen phonon method is used to calculate the phonon spectrum and phonon density of states. We find that the phonon density of states reaches its maximum at a vibration frequency of 2.3 THz, which is in good agreement with the strong absorption peak at 2.22 THz observed by THz spectroscopy. The assignment of several Raman active vibrations of TATB above 7.5 THz is given, and a comparison with other published results is also made in this study. Furthermore, the contributions of different phonon vibration modes to the specific heat are derived from the phonon density of states. The number of doorway modes (i.e., the low frequency molecular vibrations that is critical to detonation initiation) of TATB in a range of 6.0-21.0 THz is estimated based on the phonon density of states. It is shown that the phonon modes in a range of 0-27.5 THz would contribute 93.7% of the total specific heat at room temperature. By combining a Mulliken population analysis of TATB with the relative contribution of phonon vibration modes to the specific heat at 300-600 K, we conclude that C-NO2 bond might be the trigger bond of TATB during thermolysis.

###### CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

2016, 65 (12): 127101.
doi: 10.7498/aps.65.127101

Abstract +

Using first principles calculations within density functional theory, we investigate multiphase property and phase transition of monolayer MoS2. All the quantities are calculated using the Vienna ab initio simulation package. Calculations are performed within the generalized gradient approximation with van der Waals corrections (optimized Perdew- Burke-Ernzerhof-vdW). The cutoff energy of plane-wave is set to be 400 eV. The atomic plane and its neighboring image are separated by a 15 vacuum layer. The k-meshes for the structure relaxation and post analysis are 11111 and 19191 respectively.Firstly, we obtain the geometry configurations of 2H-MoS2, 1T-MoS2 and ZT-MoS2 phases through structure relaxing. The lattice constants of 2H-MoS2 are a = 3.190 and b= 5.524 , and total energy is -39.83 eV which means that it is the most stable phase. The lattice constants of 1T-MoS2 are a = 3.191 and b = 5.528 , and total energy is -38.21 eV, which means that it is the most unstable phase. Both 2H-MoS2 and 1T-MoS2 have a three-layer structure with two S layers sandwiching one Mo layer. The difference of 1T-MoS2 from the 2H-MoS2 is the upper S layer shifting. The ZT-MoS2 derives from 1T-MoS2 through lattice distortion. The lattice constants of ZT-MoS2 are a = 3.185 and b = 5.725 , and total energy is -38.80 eV. The total energy determines the following stability order of three phases: 2H-MoS2 ZT-MoS2 1T-MoS2. Our computed results agree well with the other computed and experimental results. Band structure and density of states confirm that 1T-MoS2 is metallic and ZT-MoS2 is semiconducting. But the bandgap of ZT-MoS2 phase is only 0.01 eV. Then we compute the intrinsic carrier mobility values of 2H-MoS2 and ZT-MoS2 at 300 K with the deformation potential theory. The carrier mobility of 2H-MoS2 is between 100 cm2 V-1 s-1 and 400 cm2V-1s-1. Owing to ZT-MoS2 carrier effective mass decreasing obviously, the carrier mobility of ZT phase rises to 104 cm2V-1s-1. The great carrier mobility of ZT-MoS2 covers the shortage of 2H-MoS2 and expands the applications of monolayer MoS2.After obtaining the intrinsic properties of three phases, we investigate the phase transition of monolayer MoS2. Adsorption energy becomes more accurate with van der Waals corrections. Through comparing the adsorption energy, we conclude that the stabilities of Li absorbed on the surfaces of three phases are in the following order: 1T-MoS2 ZTMoS 2 2H-MoS2, which is opposite to the stability order of the three phases. It means that 1T-MoS2 absorbs Li more easily than 2H-MoS2. Finally we compute the energy pathways of the phase transition from 2H-MoS2 to 1T-MoS2. Introducing an electron makes the energy barrier of 2H-1T transition change from 1.85 eV to 1.49 eV. Increasing electron concentration reduces the difficulty in producing phase transition. Li intercalation plays the same role as an electron and the energy barrier drops to 1.24 eV. In conclusion, the MoS2 electron concentration change is the key reason for phase transition. The study results may provide guidance for the preparation and characterization of monolayer MoS2.

2016, 65 (12): 127102.
doi: 10.7498/aps.65.127102

Abstract +

In recent years, GaN doped with Gd (GaN:Gd) has attracted much attention due to its potential applications in spintronic devices since the high temperature ferromagnetism and the colossal magnetic moment were observed in GaN:Gd. However, the microscopic nature of ferromagnetism in GaN:Gd still is controversial. We investigate the crystal parameters, magnetic moment, formation energies, and electronic structures of the defect complexes formed by Gd and native Ga (or N) vacancies in GaN by using the first-principles method based on the density functional theory. The calculated results show that the energy band gap of GaN:Gd becomes indirect and its width becomes small compared with that of GaN. The lattice constants of GaN:Gd expand due to the larger ionic radius of Gd than that of Ga atom, while they shrink when the Gd atom and Ga vacancies coexist. In the case of the isolated Gd dopant, the Gd-4f electrons lead to a magnetic moment of about 7 B in GaN:Gd. For the defect complex, one Ga vacancy can introduce a magnetic moment of about 2 B, while N vacancy has little effect on the total magnetic moment. In addition, when we focus on the defect complex composed of Gd and five neighboring Ga vacancies, we find that the magnetic moment of per Gd atom and the total magnetic moment depend strongly on the concentration and position of Ga vacancies. When the Ga vacancies are distributed loosely near the Gd atom, the magnetic moment of Gd atom increases slightly, while for the closely-distributed Ga vacancies the Gd magnetic moment can be increased by 2 B. We infer that the interactions among Ga vacancies result in the large magnetic moment of Gd atom. It is also found that the formation energy is very small when the Ga vacancies are distributed thickly around the Gd atom in GaN:Gd. Our results are in qualitative agreement with the results from other studies (Thiess A et al. 2012 Phys. Rev. B 86 180401; Thiess A et al. 2015 Phys. Rev. B 92 104418), where Ga vacancies were proposed to tend to cluster in GaN:Gd and induce the large magnetic moment of Gd. Moreover, the effect of distance between the Gd atom and Ga vacancies on the Gd magnetic moment is also discussed. It is found that the Gd magnetic moment is relatively large when Ga vacancies are close to the Gd atoms.

2016, 65 (12): 127201.
doi: 10.7498/aps.65.127201

Abstract +

The monoclinic phase (M phase) VO2 film is prepared on quartz glass substrate by a model MSP-3200 three-target co-sputter coater with RF magnetron reactive sputtering. The optical properties in incident energy ranges of 0.5-3.5 eV (350-2500 nm) and 0.083-0.87 eV (1400-15000 nm) of VO2 film are investigated by spectroscopic ellipsometry with variable temperature attachment. The good results are determined point by point with the three Lorentz harmonic oscillator dispersion models in the range of 0.5-3.5 eV and four Gaussion harmonic oscillator dispersion models in the range of 0.083-0.87 eV in the state of semiconductor below the transition temperature, while adding seven Lorentz harmonic oscillator dispersion models in the high temperature metallic state film results in the characteristic absorption peaks. The results show that the refractive index of the semiconductor state of VO2 film is maintained at maximum 3.27 and extinction coefficient k is close to zero in the near infrared-mid infrared, which is due to the fact that the absorption of semiconductor thin film in the VIS-NIR range is derived from the free carrier absorption and d// orbital of the semiconductor film has less electron density. The refractive index n of high temperature metallic state VO2 film has an obviously increasing trend in the near infrared-the mid infrared which is larger than the refractive index of the semiconductor state when the incident light energy is 0.45 eV. Extinction coefficient k increases rapidly in the near infrared, which is because the density of free carrier increases in the range of 0.5-1.62 eV and electron transition absorption augments within the V3d band. When the incident energy less than 0.5 eV, k value changes gently in the film because free carrier concentration and flow rates are stable.

## EDITOR'S SUGGESTION

2016, 65 (12): 127401.
doi: 10.7498/aps.65.127401

Abstract +

Single-layer FeSe film grown on SrTiO3(001) surface (STO surface) by molecular beam epitaxy has aroused a great research boom ever since the discovery of its huge superconductive energy gap which indicates a possible critical temperature (Tc) higher than the liquid nitrogen temperature. The interface enhanced superconductivity with a Tc above 100 K is revealed in an in situ electrical transport measurement by using a four-point probe installed in a scanning tunneling microscope (STM). Consequent research interest in multi-layer FeSe films grown on STO surface is also increasing. The quality of thick FeSe film, however, has not been well studied yet in previous studies, although it is related to the sample properties including superconductivity. Here, reflection high-energy electron diffraction (RHEED) is used to monitor the growths of multi-layer FeSe thin films on STO surface under different growth conditions. Combing the RHEED results with STM observations taken at various FeSe coverages, we find that the intensity evolution of the RHEED pattern in the early growth stage can be well explained by the step density model but not by the widely known facet model. The intensity evolution of the FeSe(02) diffraction streak exhibits a single-peak oscillation in the growing of the first layer of FeSe. As the oscillation does not depend on the grazing angle of the high-energy electron beam, the FeSe(02) diffraction streak is very suitable for calibrating the FeSe growth rate. In contrast, the intensity of the specular spot exhibits different evolution pattern when the grazing angle of electron beam is changed. It is found in STM observations that only at an appropriate substrate temperature and a growth rate can the high-quality multi-layer FeSe films be grown on STO substrates. If the growth temperature is too high, the FeSe molecules nucleate into islands so that FeSe films with various thickness values eventually come into being on the STO surface. If the growth temperature is too low, a different phase of FeSe film is formed. The optimal growth temperature is in a range from 400 ℃ to 430 ℃, within which a two-layer FeSe film grown at a low rate (0.15 layer/min) coveres the whole STO surface with a negligible number of small FeSe islands. In contrast, a larger growth rate is necessary for growing thicker FeSe film. This is because FeSe islands tend to come into form at steps when the growth rate is too low, which is more distinct in a thicker FeSe film. An STM image of 80-layer FeSe film grown under an optimal condition, i.e., the substrate temperature of 420 ℃ and the growth rate of 2.3 layer/min, shows that it is in a perfect layer-by-layer growth mode. These experimental results are useful for growing high-quality multi-layer FeSe films on STO substrates, which could be critical for studying their physical properties and relevant physical phenomena.

2016, 65 (12): 127501.
doi: 10.7498/aps.65.127501

Abstract +

Half-metallic ferromagnet, in which the electrons with one spin band are metallic and the electrons with another spin band are semiconducting, is believed to be the most promising spin-injector material for spintronic devices, such as spin valves, spin filters, spin diodes, and magnetic tunnel junctions. The main advantages of half-metallic Heusler alloy over other half-metallic systems are their relatively high Curie temperatures and structural similarity to important binary semiconductors that are widely utilized in the industry. Thus far, half-metallicity has been predicted theoretically or confirmed experimentally in a limited number of Heusler alloys. Exploring new half-metallic Heusler alloys is necessary. In this study, the full-potential linearized augmented plane wave (FP_LAPW) method under density functional theory is utilized to investigate the electronic structures and magnetisms of semi-Heusler alloys CoCrTe and CoCrSb. In the calculations, the generalized gradient approximation (GGA) in the scheme of Perdew-Bueke-Ernzerhof is adopted to treat the exchange-correlation potential. The cutoff parameter is set to be Rmt Kmax =9, where Rmt is the smallest atomic sphere radius and Kmax is the maximum value of the reciprocal lattice vector. Meshes (131313 k-points) are used in the first Brillouin zone integration. Self-consistent calculations are considered to be convergent only when the integrated charge difference between the last two iterations is less than 110-4 e/cell. Spin-polarized calculations of the electronic structure for the semi-Heusler alloys CoCrTe and CoCrSb are performed. The calculations reveal that CoCrTe and CoCrSb at their equilibrium lattice constants are half-metallic ferromagnets with half-metallic gaps of 0.28 and 0.22 eV and total magnetic moments of 3.00 and 2.00 B per formula unit, respectively. The calculated integer total magnetic moments (in B) are consistent with the Slater-Pauling rule, Mt = Zt-18, where Zt denotes the total number of valence electrons and Mt means the total magnetic moment (in B) per formula unit. Moreover, the spin moment of the Cr atom is obviously larger than those of the Co, Te, and Sb atoms. Co, Te and Sb are all antiferromagnetically coupled to Cr for CoCrTe and CoCrSb. The electronic structures of CoCrTe and CoCrSb are also calculated as their lattice constants change from -13% to +13% relative to the equilibrium lattice constant. The calculated results indicate that CoCrTe and CoCrSb can maintain their half-metallicities and retain their total magnetic moments of 3.00 and 2.00 B per formula unit even as their lattice constants change from -11.4% to 9.0% and from -11.2% to 2.0%, respectively. The semi-Heusler alloys CoCrTe and CoCrSb should be useful in spintronics and other applications.

2016, 65 (12): 127502.
doi: 10.7498/aps.65.127502

Abstract +

A soft/hard bilayer system with mutually orthogonal anisotropies is considered in this paper. The easy axis of the hard layer is perpendicular to the film plane, and the easy axis of the soft layer is parallel to the film plane. Pt84Co16 is chosen as the soft layer material, and TbFeCo is chosen as the hard layer material. The one-dimensional continuum micromagnetic model is used. The characteristics of nucleation fields, angular distribution and hysteresis loops are studied. The calculation results show that the nucleation field decreases rapidly and even turns negative with increasing soft layer thickness. This negative nucleation field is caused by the demagnetizing field and the easy axis orientation of the soft layer which is parallel to the film plane. Both of these two factors can induce an effective in-plane uniaxial anisotropy, which will tend to align the magnetization of the soft layer parallel to the film plane. As the magnetocrystalline anisotropy constant K of the soft layer is very small, the negative nucleation field mainly comes from the demagnetizing field of the soft layer. The angular distribution calculation shows that the change rate of magnetization deviation angle (degree per nanometer) along z axis in the soft layer is faster than that in the hard layer. The angular change rate could be adjusted by varying the anisotropy constant ratio, exchange energy constant ratio, or external field. When the anisotropy constant ratio Ks/Kh (soft/hard) or exchange energy constant ratio As/Ah (soft/hard) increases, the angular change rate ratio (soft/hard) decreases. Especially when both Ks/Kh and As/Ah increase at the same time, the angular change rate in the hard layer could become faster than that in the soft layer. If the anisotropy constant Ks becomes larger, it is more difficult for the magnetization in the soft layer to deviate from its easy axis than before. This will also enhance the pinning effect of the magnetization in the soft layer, and reduce the difference in deviation angle between the two boundaries of the soft layer. When the exchange energy constant As increases, the magnetization tends to become parallel to the neighboring magnetization, which also reduces the angular change of magnetization in the soft layer. As the anisotropy constant is roughly proportional to the square of spontaneous magnetization, the effect of spontaneous magnetization on the angular change rate comes from the anisotropy constant change. The simulation for the hysteresis loops shows that the saturation field strength increases while the remanence decreases with increasing both the values of Ks and As.

2016, 65 (12): 127801.
doi: 10.7498/aps.65.127801

Abstract +

Chalcogenide glass has been considered to be a promising optical material for infrared (IR) transmission and nonlinear optics because of its favorable physical properties such as wide IR transparent windows, high linear and nonlinear refractive indices, and tunable photosensitivity. In many optical designs and practical applications, the refractive index (n) and optical bandgap (Eg) are two important parameters. Aiming to evaluate the composition dependence of the n and Eg in Ge-As-S chalcogenide glasses, a series of glasses with different stoichiometric characteristics are synthesized in quartz tubes under vacuum by the melt quenching technique. The structure, n and Eg of the glass are investigated by Raman spectroscopy, ellipsometry, and diffused reflectance spectroscopy, respectively.To eliminate thermal effects on the measured Raman spectra, the data are corrected by the Bose-Einstein thermal factor. Raman spectrum analyses indicate that Ge-As-S glass has a continuous network structure with interconnected [GeS4] tetrahedra and [AsS3] pyramids forming the backbone. When S amount is excess, S chains or S8 rings emerge. When S amount is deficient, As4S4/As4S3 molecules are formed, and even a large number of As-As/Ge-Ge homopolar bonds appear in the structure. The n values at different wavelengths are obtained by fitting the ellipsometry data with the Sellmeier dispersion model. The values of molar refractivity (Ri) of Ge, As and S elements are evaluated by using the measured n and density (d) of the investigated glass. The optimal values of Ri at 2-10 m for each element are RGe=9.83-10.42 cm3/mol, RAs=11.72-11.87 cm3/mol, and RS=7.78-7.86 cm3/mol, respectively; and the values decrease with increasing wavelength. The n of Ge-As-S glass is well quantitatively correlated to the d and the Ri of constituent elements, so that its value can be predicted or tailored within 1% deviation. A method to determine reliable Eg of a glass is proposed based on diffuse reflectance spectrum (DRS) of glass powders. To determine Eg of a glass, the absorption coefficient () is required to be as low as ～104 cm-1. For a 1-mm-thick bulk glass, the detection limit of a spectrophotometer is typically 100 cm-1. To obtain a reasonable Eg, the sample thickness used for the measurement must be less than 10 m. Such a thin glass sample is difficult to prepare. In comparison, DRS of glass powers measured using a spectrophotometer is able to provide valid absorption data in a 104 cm-1 range required for Eg determination. In this proposed method, the Kubelka-Munk function F(R), which is proportional to of the glass, is calculated from the measured DRS on the glass powders. The F(R) is calibrated by using the DRS of a glass (e.g. As2S3) with a known Eg. Using the same F(R) absorbance value, Eg of the Ge-As-S glass is determined based on DRS of powders measured under the same condition. The Eg of Ge-As-S glass is broadly correlated to the average bond energy of the glass. The glass containing more S atoms tends to show a higher average bond energy, and therefore exhibits a larger Eg.

2016, 65 (12): 127802.
doi: 10.7498/aps.65.127802

Abstract +

Magneto-optical information storage has been a hot research subject for several years. FePt exhibits abundant physical properties and has received much attention as a candidate material. Its alloy film with perpendicular anisotropy and small grain size has important applications in magnetic recordings due to the large intrinsic magnetic anisotropy which ensures long-time thermal stability of nanometer sized bits. However, the large coercive field of FePt is a significant factor that hinders its application. As is well known, the magnetic anisotropy in FePt alloy can be precisely modulated by carbon-doping, and as a result, the coercive field of FePt film can be modified effectively with the carbon dopant. On the other hand, the microscopic mechanism of magnetic storage relies on the motion of spin system. Ultrashort femtosecond laser has been demonstrated to be a very effective tool to investigate the dynamical coupling among different degrees of freedom, such as electron, spin and lattice in a ferromagnetic film. The research on spin dynamics has become a new frontier of condensed matter physics, which is crucial for ultrafast magnetic recording materials. In this work, by using the time-resolved magneto-optical Kerr effect spectroscopy, we study the ultrafast spin dynamics of two FePt alloy films with different carbon dopants under the applied magnetic field along the film surface. The FePt alloy films with different carbon dopants are fabricated on silicon substrates by the sputtering method. The main experimental findings in this work are as follows. (i) The transient Kerr signal is linearly proportional to the magnetization with the magnetic field up to 0.8 T, while the transient reflectivity of the film is independent of the applied magnetic field. (ii) For FePt alloy films with different coercive fields, it is found that the demagnetization time of the film with smaller coercive field is significantly faster than that of the larger counterpart: the former shows 0.8 ps demagnetization time, and the latter has a magnitude of 1.2 ps. The demagnetization times for both soft and hard magnetic films are independent of the applied magnetic field. (iii) With ultrafast laser pulse radiation, we observe the propagation of acoustic phonon with a resonance frequency of ～ 49 GHz, and the frequency of the acoustic phonon is independent of the applied magnetic field. From the above, the spin dynamics of the samples shows strong correlation with carbon-doping. Our experimental findings are desired for basic research as well as for the design and development of novel magneto-optical devices.

###### INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

2016, 65 (12): 128101.
doi: 10.7498/aps.65.128101

Abstract +

Monolayer molybdenum disulfide (MoS2) has recently aroused the great interest of researchers due to its direct-gap property and potential applications in electronics, catalysis, photovoltaics, and optoelectronics. Chemical vapor deposition (CVD) has been one of the most practical methods of synthesizing large-area and high-quality monolayer MoS2. However, The process of preparation is complex and cumbersome. Here we report that high-quality monolayer MoS2 can be obtained through using sulfurization of MoO3 by a simple and convenient CVD on sapphire substrates.The substrate cleaning is simplified. Substrates are cleaned in detergent solution, deionized water and acetone without sopropanol or piranha solution (H2SO4/H2O2=3:1) in sequence, avoiding their potential dangers. The MoO3 powder (Alfa Aesar, 99.995%, 0.02 g) is placed in an alumina boat, and a sapphire substrate is faced down and is placed 6 cm away from MoO3 powder in the same boat. The sapphire substrate is placed in the center of the heating zone of the furnace. Another alumina boat containing sulfur powder (Alfa Aesar, 99.999%, 0.2 g) is placed upstream with respect to the gas flow direction in the low temperature area. We adopt an atmospheric pressure chemical vapor deposition method, so it does not require a vacuum process. After 30 min of Ar purging, the furnace temperature is directly increased from room temperature to 800 ℃ in 30 min, reducing the heating steps. After 60 min, the furnace is cooled down naturally to room temperature. Optical microscopy (OM) images, Raman spectra and photoluminescence (PL) are all obtained by confocal Raman microscopic system (LabRAM HR Evolution). From the OM images, we can see that isolated islands (triangles) have edge lengths up to 50 m, which is far larger than that grown by micromechanical exfoliation. The color of the triangles is uniform, which has a strong contrast with the substrate. We can obtain a preliminary result that the sample is a uniform monolayer MoS2. Raman spectra are collected for MoS2 samples on sapphire substrates. Two typical Raman active modes can be found: E2g1 at 386.4 cm-1 and A1g at 406 cm-1 ( =19.6 cm-1), which correspond to single-layered MoS2 sample. Raman mapping shows that the sample is a uniform monolayer MoS2. The PL spectrum of MoS2 shows a pronounced emission peak at 669 nm, which is consistent with other reported results for MoS2 thin sheets obtained from exfoliation methods. When the layer number of MoS2 decreases, with its bandgap transforming from indirect to direct one, the fluorescence efficiency will be significantly enhanced. So the results further prove that the sample is high-quality monolayer MoS2.

2016, 65 (12): 128401.
doi: 10.7498/aps.65.128401

Abstract +

The low-signal-gain versus frequency slope is often a highly desirable property of traveling wave tube (TWT) used in a communication system. The gain ripple is usually caused by internal reflexions of forward and backward waves in the TWT. Random fabrication error may have a detrimental effect on the performance of TWT. The quartic equation including backward wave models the effect of reflection to analyze the effect of Gain ripple from many small circuit errors in a TWT operating under small-signal condition. We present a transfer matrix method (TMM) to correctly calculate the transmission and reflection of the wave incident respectively from left and right at a single isolated joint. The TMM, which links the input signal to output signal that includes the feedback signal from the reflections at multiple joints to the output end, can calculate the gain ripple of multiple internal reflections. Appling this method to several numerical examples, we look at how small signal gain is affected by a single isolated discontinuity and many small randomly distributed discontinuities. In particular, we investigate the effects of random perturbations of Pierce velocity detuning parameter b and Pierce gain parameter C on the small signal gain at different values of space charge 4QC. The computed result agrees with that from Chernin's model. We find that reflections may significantly increase the statistical effects on the gain. A further conclusion is that the standard deviation of gain, dgain, increases with b gradually, but the ratio of the backward wave power to the forward wave power at x=0 decreases with b when standard deviation of pierce velocity detuning parameter, b, is more than 1.5. In another example, the effects of two discontinuities of pitch distribution and many small random pitch errors on gain ripple are reported for a G-band TWT. We find that larger pitch error and longer distance for the discontinuities may produce a larger ripple in the small-signal-gain versus frequency. Many small discontinuities may produce a large gain ripple, and the gain ripple grows as the level of pitch error increases. These effects of random fabrication errors become increasingly important for very high frequencies, such as 1 THz, at which TWTs are currently being designed and built.

2016, 65 (12): 128501.
doi: 10.7498/aps.65.128501

Abstract +

Density of localized states (DOS) over the band-gap determines the electrical and instability characteristics in the indium zinc oxide thin film transistor (IZO TFT). In order to propose an accurate extraction method for DOS in the bulk region, low frequency noise and multi-frequency capacitance voltage characteristics are measured and analyzed in this paper. Firstly, the relationship between surface potential and gate voltage is extracted based on subthreshold I-V characteristics. The extraction results show that the surface potential increases with the increase of gate voltage in the sub-threshold region. When the Fermi level is close to the bottom of conduction band, the increase of surface potential should be saturated. Secondly, drain current noise power spectral densities in the IZO TFTs under different operation modes are measured. Based on carrier number fluctuation mechanism, the flat-band voltage noise power spectral density is extracted and localized state near IZO/SiO2 interface is then calculated. By considering the emission and trapping processes of carriers between localized states, the distribution of bulk trap density in the band-gap is extracted based on low frequency noise measurement results. The experimental results show an exponential tail state distribution in the band-gap while NTA is about 3.421020 cm-3eV-1 and TTA is about 135 K. Subsequently, contact resistances are then extracted by combining capacitance-voltage characteristics with I-V characteristics in the linear region. The extrinsic parasitic resistances at gate, source, drain are separated. By considering charges trapped in the localized states and free carriers, the distributions of deep states and tail states in the active layer of IZO TFT are extracted through multi-frequency capacitance-voltage characteristics. The experimental results also show an exponential deep state and tail state distribution in the band-gap while NDA is about 5.41015 cm-3eV-1, TDA is about 711 K, NTA is about 1.991020 cm-3eV-1, and TTA is about 183 K. The above two proposed extraction methods are compared and analyzed. The deviation between two extraction results may relate to the existence of neutral trap in the gate dielectric which is also an important source of low frequency noise in the IZO TFT.

2016, 65 (12): 128502.
doi: 10.7498/aps.65.128502

Abstract +

Oxide thin film transistor with an oxide channel layer is investigated to cater to the requirements of transparent electronics for the high mobility, good uniformity, and large band gap. Owing to its special conduction mechanism, high carrier mobility can be realized even in the amorphous phase. Oxide-based thin films have been prepared by using a number of methods, such as pulsed laser deposition, chemical vapor deposition, radio-frequency sputtering and solution-derived process. Solution processing is commonly used in TFT applications because of its simplicity and potential application in printed device fabrication. In the solution process, the conductivity of multicomponent oxide films can be controlled by incorporating charge-controlling cations. In this paper, bottom-gat topcontact thin film transistors are fabricated by using solution processed InGaZnO channel layers. The effects of annealing temperature and Ga content on the properties of thin film transistor are examined. Optical transmittance of InGaZnO thin film is greater than 80% in the visible region. Electrical characteristics of InGaZnO thin film transistor are improved by increasing annealing temperature. The threshold voltage of solution-processed InGaZnO transistor decreases from 6.74 to -0.62 V with annealing temperature increasing from 250 to 400 ℃, owing to the increase in electron concentration in the active layer. A lower annealing temperature suppresses the generation of carriers outside of the control of Ga cations. X-ray photoelectron spectrum measurement shows that the electron concentration increases because oxygen vacancies generate electrons. The incorporation of Ga into a InZnO compound system results in reducing the carrier concentration of the film and an off-current of thin film transistor. As the Ga ratio is increased at an identical In and Zn content, the carrier concentration of the film decreases and the threshold voltage of thin film transistor shifts towards the positive direction. As the content of Ga is increased in the oxide active layer of transistor, the subthreshold amplitude decreases, and the on/off ratio is improved. This is a consequence of the Ga ions forming strong chemical bonds with oxygen as compared with the Zn and In ions, acting as a carrier suppressor. The performances of thin film transistor with an atomic ratio of In: Ga: Zn=5:1.3:2 are optimized as follows: saturation mobility of 0.43 cm2/(Vs), threshold voltage of -1.22 V, on/off current ratio of 4.7104, subthreshold amplitude of 0.78 V/decade.

2016, 65 (12): 128503.
doi: 10.7498/aps.65.128503

Abstract +

Many memristors fabricated by different materials share the characteristics which are similar to the memory and learning functions of synapse in biological systems. These characteristics include memorizing and forgetting function and learning-experience behavior. A memristor model was proposed in the published paper [Chen L, Li C D, Huang T W, Chen Y R, Wen S P, Qi J T 2013 Phys. Lett. A 377 3260] to describe the memorizing and forgetting function of this kind of memristor. This model includes three state variables , and . The change of w describes the variation of the conductance of the memristor, a function fE () is used to the input voltage's influence on the change of , and are used to describe the its forgetting effect. The simulation analyses of this model in the published papers [Chen L, Li C D, Huang T W, Hu X F, Chen Y R 2016 Neurocomputing 171 1637] and [Meng F Y, Duan S K, Wang L D, Hu X F, Dong Z K 2015 Acta Phys. Sin. 64 148501] showed that this model can also describe the learning-experience behavior. This model is further studied in this paper to show its detailed characteristics. The analyses of the state equations of the original model show that these state equations cannot restrict the state variables in their permissible interval because the window function is not appropriately used in all the state equations, and the original window function cannot force the state equation to be identical to zero either when corresponding state variable reaches its bound. An improved window function is introduced and the appropriate utilization of this window function is discussed to deal with this problem. The upper bound of is defined in the modified model to describe the saturation of that has been observed in the experimental studies of this kind of memristor. The behaviors of the modified state equations are different from those of the original ones only when the state variables reach their bounds, and this modified model has the same ability to describe the memristor's memorizing and forgetting function and learning-experience behavior as original one. The behaviors of the model when the input voltage is not negative are discussed based on the state equations and their analytical solution when the input is the repeated voltage pulses, and the results of the discussion are used to explain how a model designed according to the memorizing and forgetting function can also describe the learning-experience behavior. The analysis shows that the increased rising speed of the state variable w in the stimulating process is caused by increasing the values of and , and the learning-experience behavior described by this model would also be influenced by the value of :a smaller initial value of state variable in the learning-experience experiment would lead to a more obvious learning-experience behavior. The analytical results are also used to design an estimation method based on the learning-experience experiment to estimate the parameters and function in the state equation. The further discussion shows that this proposed estimation method can also be used to verify the reasonability of the assumption used in the state equations that the derivatives of and are proportional to fE (V).

2016, 65 (12): 128701.
doi: 10.7498/aps.65.128701

Abstract +

A continuous wave cavity ringdown spectroscopy based on a double-locking loop is proposed to improve the shortcoming of low acquisition rate of concentration in traditional scheme. A small portion of laser is separated to pass through a C2H2 reference cell, used to lock the laser frequency to the 1+3 band P(9)e absorption line of C2H2 at 6534.3634 cm-1 by the 1st harmonic demodulation of the frequency modulation spectroscopy. The remaining portion is incident on a high finesses cavity to observe the ringdown events. Meanwhile, the reflected light of cavity is used to extract the error signal to lock the laser based on the PDH frequency locking technique. As a consequence, the frequency drift of the laser and the jitter of the cavity length are improved, therefore a more relatively accuracy result is expected. The laser light is dual frequency modulated by a fiber coupled electro optic modulator (FEOM)in the above system. In order to optimize, to some extent, the asymmetry of the error signal caused by the residual amplitude modulation due to the inconsistency of the laser polarization direction with the extraordinary axis of the FEOM, the demodulation phase is adjusted carefully until the error signal is smoothed up and close to symmetry. Then, the effect of locking loop is examined. The frequency of laser, based on the measurement by a wavelength meter, is more stable and the relative frequency discrimination between the laser and the longitudinal mode of cavity is about 9.8 kHz. In addition, the PDH locking, ensuring the efficient coupling of the laser with the cavity, can gain a high acquisition rate of the concentration information. In order to obtain a complete ringdown event, the frequency of square wave to the fiber coupled acoustic optical modulator (FAOM) is limited to 30 kHz with the duty cycle of 85%, which is determined by the ringdown time and re-lock time. However, there exists a relatively large random noise in a series of ringdown time measurements of empty cavity, which is mainly caused by the errors of fitting and measurement. For the further improvement of the accuracy of experiment, an efficient digital filter, Kalman filter which can suppress the noise considerably at no expense of real-time capability, is used. The standard deviation of the ringdown time is reduced from 0.00333 to 0.00153. According to Allan variance analysis, the detection limit can reach 410-9 cm-1 for a 2 s integration time. Finally, the C2H2 gases with different concentrations from 100 ppb to 5 ppm are measured to demonstrate the linear response of this system.