Vol. 61, No. 18 (2012)
2012, 61 (18): 180101. doi: 10.7498/aps.61.180101
A new form of power matrix is proposed to correct the unreasonable points in traditional statistical multi-input multi-output (MIMO) channel model which mishandles different power variations in different polarization direction. Based on the analysis of modeling theory of different diversities, a composite calculation method of power matrix and covariance matrix is proposed to build a general-purpose statistical channel model which exactly represents statistical characteristics under the effects of space-diversity and polarization-diversity, and thus can be applied to arbitrary diversity MIMO systems.
Study on derivation and optimization algorithm about thin plate bending large deformation higher-order nonlinear partial differential equations
2012, 61 (18): 180201. doi: 10.7498/aps.61.180201
For a thin plate bending large deformation problem, variational principle is applied, and higher-order nonlinear partial differential equations about thin plate bending large deformation is established. Based on difference method and dynamic design variable optimization method, making unknown deflection of discrete coordinate points as design variables, differential equations sets of the discrete coordinates points as building objective function, a dynamic design variable optimization algorithm for computing thin plate bending deflection is proposed. Universal computing program is designed. Practical example about rectangular thin plate with fixed boundary under uniform load is analyzed. Comparing the program computing result with finite element solution. Effectiveness and feasibility of the method are verified. This method can be used to solve engineering problem.
The mode competition is the problem to be solved urgently in high-order mode gyrotron. Based on the generalized theory of transmission line, a group of first order transmission line equations that can accurately describe the coupling modes in a gradually-varying cavity is studied. Through analysing the expressions of the coupling coefficients between the various modes, a gradual-varying cavity for a gyrotron is designed. It is proved that the power in spurious modes in this novel cavity may be kept far below that in the line-joint cavity. The normalized amplitudes of non-operating modes can reach -100 dB in cavity and -88.6 dB in travelling wave guide.
2012, 61 (18): 180301. doi: 10.7498/aps.61.180301
In this paper we consider the situation that four identical two-level atoms are separately trapped in coupled single-mode optical cavities and each atom resonantly interacts with cavity via a one-photon hopping. The three-body entanglement dynamics in each cavity is studied. The influences of atom-cavity coupling constant on the three-body entanglement are discussed. The results obtained using the numerical method show that the three-body entanglement displays the collapse-revival phenomena when atom-cavity coupling constant is larger than a fixed value. On the other hand, the three-body entanglement is the result of the coherent superposition of the two-body entanglements.
2012, 61 (18): 180302. doi: 10.7498/aps.61.180302
The solid-state superconducting circuit-QED (quantum electrodynamics) system is a promising candidate for quantum computing and quantum information processing, which serves also as an ideal platform for quantum measurement and quantum control studies. In this context, a large number of cavity photons may be involved in the quantum dynamics and will degrade the simulation efficiency. To avoid this difficulty, it is helpful to eliminate the degrees of freedom of the cavity photons, and obtain an effective master-equation description which contains only the qubit states. In this work, we examine two such schemes, the adiabatic elimination (AE) and the more recently proposed polaron transformation (PT) approaches, by comparing their results with exact numerical simulations. We find that in the absence of qubit-flip, which is a specific quantum nondemolition (QND) measurement, the PT scheme is superior to the AE method. Actually, in this case the PT scheme catches the measurement dynamics exactly. However, in the presence of qubit-flip such as for qubit oscillation measurement, the PT scheme is no longer better than the AE approach. We conclude that both schemes, in the weak measurement regime, can work almost equally well. This corresponds to strong cavity damping or weak coupling between the qubit and cavity photons. Out of this regime, unfortunately, one has to include the cavity photons into numerical simulations and more advanced methods/techniques are waiting for their exploration in this field.
The tunneling phenomena of the Fermi superfluid gases in unitarity by manipulating the Fermi-Fermi scattering length
2012, 61 (18): 180303. doi: 10.7498/aps.61.180303
In the present work, we investigate the tunneling dynamics of superfluid Fermi gas in a double-well potential in a unitarity regime by adjusting the Fermi-Fermi scattering length. The scattering length can affect the quantum transition dramatically. In a certain regime, the complete population transfer between two modes can be obtained. However, in some other regimes, the quantum transition can be completely blocked.
2012, 61 (18): 180501. doi: 10.7498/aps.61.180501
The frequency invariance and phase shift of the first-order steady state moment of system response are deduced for fractional order linear oscillator. Moreover, through the theoretical analysis, the amplification effect of fractional order linear system on the amplitude of first-order steady state moment is verified in stochastic resonance mechanism. Duffing detector is used to realize weak signal detection based on parameter perturbation sensitivity and noise immunity of chaos. Numerical simulations show that this scheme can effectively extract the weak sinusoidal signal from the strong colored noise background and significantly reduce the signal-to-noise ratio threshold in comparison with traditional chaotic detection method.
2012, 61 (18): 180502. doi: 10.7498/aps.61.180502
The detection of weak harmonic signals has important practical value. In this paper, the detection of weak harmonic signals in strong noise is realized with the controlled Chen's system. Dynamics can be divided into slowly varying dynamics and fast varying dynamics, so a slowly varying dynamics is obtained by an averaging method. The critical values of detection parameters are determined, which lead to a sudden change of system dynamical behavior from periodic orbit to stable equilibrium point. Weak harmonic signals can be detected by adjusting the detection parameters and observing the change of system variables. Simulation results show that weak harmonic signals in strong noise can be detected accurately with this system. Compared with existing detection methods with chaotic oscillator, this method is of stronger immunity to noise, and the accurate range of parameter threshold can be obtained through theoretical analysis, which enables its popularization and application in relevant fields.
Modified function projective lag synchronization for multi-scroll chaotic system with unknown disturbances
2012, 61 (18): 180503. doi: 10.7498/aps.61.180503
In this paper, we treat the modified function projective lag synchronization between two non-identical multi-scroll chaotic systems with unknown disturbances. By utilizing the lyapunov stability theory, the fuzzy adaptive controller and parameter update law are derived to make the state of chaotic system achieve modified function projective lag synchronization. The controller can keep the systems stable even when the disturbance changes. Finally numerical simulations are performed to show the effectiveness of the proposed method.
Field programmable gate array-based chaotic encryption system design and hardware realization of cell phone short message
2012, 61 (18): 180504. doi: 10.7498/aps.61.180504
More and more people having cell phone, they must pay more attention to message security of their personal privacy, in which cell phone short message security is the most important. In this paper, we focus on designing an encryption hardware system of cell phone short message with chaos and A5/1 hybrid algorithm and realizes it on field programmable gate array. In the encrypted core, chaotic sequences of logistic map are used as initial keys of A5/1 algorithm for encrypting short messages, and in this paper we prove the relationship between linear correlation and circle correlation in the pseudo-randomness research of chaotic sequences serving as the initial keys. The SIM300 modules are used to realize cell phone functions, simulate short messages of common cell phone and the encrypte short messages after processing by our system by wireless transmitting through the base station. In addition, in this paper we design a software test platform, and it can show the results of this hardware system on computer. The test results indicate that the proposed system and its hardware implementation can encrypt/decrypt short messages and ensure their normal transmission, which can improve the security of short message and ensure its integrity and reliability during transmitting.
Peaked soliton solutions and interaction between solitons for the extended (2+1)-dimensional shallow water wave equation
2012, 61 (18): 180505. doi: 10.7498/aps.61.180505
By an improved Riccati mapping approach and a variable separation approach, a new family of variable separation solutions (including solitory wave solutions, periodic wave solutions, and rational function solutions) of the extended (2+1)-dimensional shallow water wave (SWW) equation is derived. According to the derived solitary wave excitation, we obtain some special peaked soliton structures and study the interaction between solitons.
Fractal eroded safe basins in a forced Holmes-Duffing system and its control by delayed velocity feedback
2012, 61 (18): 180506. doi: 10.7498/aps.61.180506
A forced Holmes-Duffing system is considered in this paper. A delayed velocity feedback is opplied to the system. The erosion of safe basin, caused by the periodic excitation, and the effects of delayed velocity feedback on the controlling of the fractal eroded safe basin are investigated. The conditions of fractal erosion of the basin boundary are obtained by the Melnikov method. Then considering the time delay as a variable parameter, the evolutions of safe basin with time delay are presented numerically by the 4th Runge-Kutta and the point-to-point mapping method. It is found that the increase of time delay can enhance the threshold of the fractal erosion of the basin boundary under a weak and positive feedback gain so as to reduce the basin erosion. These imply that the delayed velocity feedback can control the basin erosion of the system effectively.
2012, 61 (18): 180601. doi: 10.7498/aps.61.180601
A frequency measurement system for dual frequency He-Ne laser is set up based on a fiber femtosecond optical frequency comb and an external cavity diode laser. Using a Rb clock as a frequency standard, the diode laser that is locked to the optical frequency comb is traced to the Rb clock, and then the frequencies of the orthogonal polarized lasers are measured by beating with the locked diode laser at the same time. Locking the diode laser to the 1894449 th comb mode, the absolute frequency of the diode laser is 473612190000.0 ± 2.7 kHz, with a relative frequency uncertainty of 5.7×10-12. A commercial dual frequency He-Ne laser is measured to test the system, and the results show that the mean absolute frequencies of the horizontal polarized laser and the vertical polarized laser are 473612229934 kHz and 473612232111 kHz, respectively, with a relative Allan deviation of 5.2×10-11 at 1024 s, and the mean split frequency is 2.177 MHz with a standard deviation of 2 kHz.
Method on double-pass acousto-optic frequency shifter in absolute distance measurement using Fabry-Pérot interferometry
2012, 61 (18): 180701. doi: 10.7498/aps.61.180701
In order to realize nanometer-scale absolute distance measurements based on Fabry-Pérot interferometry for long-range displacement measurement of the moving coil in Joule balance, the acousto-optic frequency shifter (AOFS) in double-pass configuration is presented, and a tunable frequency difference in a range of 200 MHz is achieved. The focus length of the lens is determined by analyzing the relationship of the tradeoff between the AOFS modulation bandwidth and its diffraction efficiencies; the beam spot of the first-order diffraction beam is guaranteed by accurately positioning the focused beam according to the distribution of the zero-order diffraction beam spot. The experimental single-pass and double-pass peak diffraction efficiency of the AOFS are 79.54% and 61.41%, respectively; the tunable frequency difference of 440-640 MHz, which is twice the single-pass modulation bandwidth output of 220-320 MHz, is obtained by the beat note between the incident beam and the first-order diffraction beam of the double-pass AOFS, and has a good signal-to-noise ratio. Theoretical analysis shows that a folded Fabry-Pérot cavity length displacement of about 53 mm can be measured through the tunable frequency difference achieved by means of double-pass AOFS.
The influence of ionization efficiency on the multiply charged ions produced by laser-clusters interaction
2012, 61 (18): 182101. doi: 10.7498/aps.61.182101
The productions of multiply charged ions in the interactions of intense nanosecond laser pulse with clusters have aroused broad interests in molecular physics. Benzene, cyclohexene and cyclohexane clusters are chosen to study the effect of multiphoton ionization (MPI) efficiency on the relative intensity of multiply charged ions, as they possess similar molecular structures and the same element constitutions. They are ionized with a 5 ns Nd-YAG nanosecond laser. The carbon charge state produced by cyclohexene and cyclohexane is about 4; while by benzene is only about 3. The ratios of C3+/C2+ for cyclohexane, cyclohexene and benzene are 1.1, 0.6 and 0.4, respectively. The relative MPI efficiencies of three molecules are measured to be in the magnitude sequence of benzene > cyclohexene > cyclohenane by diffusion beam. Higher MPI efficiency of molecules can cause more than one molecules to be ionized at the edge of laser pulse, the Coulomb repelling force between adjacent ions leads clusters to early split into small size ones, which will prevent the production of the highly charged ions.
ATOMIC AND MOLECULAR PHYSICS
Strain effect on the intercalation potential of the layered Mn-contained lithium ion batteries cathode materials: a first principles method
2012, 61 (18): 183101. doi: 10.7498/aps.61.183101
The strain effects on the intercalation potentials of LiMnO2 and Li2MnO3 are investigated by the first principles method, and the relationship between the intercalation potential and the strain is given in the form of elastic response. All the modes of strain reduce the intercalation potential and the effect is anisotropic. Most of the single modes reduce the potential by less than 0.1 V when the strains are 5%. The bonding between the host layers is rather sensitive to the strain perpendicular to the host layer when the lithium vacancy left by lithium extraction is in the lithium layer, thus that strain brings more reduction to the intercalation potential; whereas for the Li2MnO3 system when lithium is extracted form the transition metal layer, the strain along the host layer brings more reduction to the potential. For the Li2MnO3-stabilized LiMO2 (M=Mn, Ni, Co) solid solution system, the strain can keep the voltage of the high potential charging stage lower than the cut-off voltage, and open up the migrating pathway of lithium in the transition metal layer, therefore the charging can last a long time and larger charging capacity is achieved.
2012, 61 (18): 183201. doi: 10.7498/aps.61.183201
In order to study the effect of substrate on the focusing laser beam in atom lithograph, the simulations of optical potential of Gaussian laser standing wave in the presence and the absence of straight edge diffraction are performed based on the scalar optical theory. The similarities and differences in optical potential between the diffractive Gaussian and non-diffractive Gaussian laser beam are fully indicated by the three-dimensional and cross-sectional figure. The effects of the distance between laser axis and substrate, the distance between laser waist and reflective mirror are fully studied.
Experimental investigation on ionized ultracold molecules formed in a magneto-optical trap by time-of-flight mass spectroscopy
2012, 61 (18): 183301. doi: 10.7498/aps.61.183301
We use time-of-flight mass spectroscopy to detect the ultracold ground state Cs2 molecules formed directly by a photoassociation due to the trapping laser beams and repumping laser beam in a magneto-optical trap. We investigate the dependences of the Cs2+ intensity on operating voltage of micro-channel-plate, accelerating field intensity and duration time of the accelerating field. The experimental results are consistent with theoretical fitting. We obtain the best optimal parameters suited to our experimental condition, which is important for the future experiment on ultracold molecule photoassociation and photoionization spectroscopy.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2012, 61 (18): 184101. doi: 10.7498/aps.61.184101
A metamaterial absorber is designed and applied to the waveguide slot antenna. The absorber is composed of two metallic layers separated by a lossy dielectric spacer. The top layer consists of etched oblique cross-gap patch set in a periodic pattern and the bottom one is of a solid metal. By fine adjusting the geometric parameters of the structure, we can obtain a polarization-insensitive, wide-incident-angle ultra-thin absorber that has an absoption of 99.1% and a thickness of only 0.01λ. The absorber is applied to the waveguide slot antenna. The radar cross secction (RCS) of antenna is reduced by 3 dB or more in an the operation band from 5.48 GHz to 5.7 GHz, and by 5 dB or more in the boresight direction from -25° to +25°, and the most reduction value exceeds 12 dB, but the forward gain of antenna reduces only 0.53 dB. The experimental results are in good agreement with the simulation results, which verifies that the absorer has a good characteristic of RCS reduction and can be applied to the stealth of antenna.
2012, 61 (18): 184201. doi: 10.7498/aps.61.184201
High resolution pulse shaping based on the frequency comb has been widely used in microwave photonics, spectroscopy and communication optics and so on. To describe and evaluate the resolution capability of such a pulse shaping system, the ray tracing method is adopted to analyze the spatial dispersions of four schemes like single grating, parallel gratings, single grating with focus and anti-parallel gratings with focus. The spot spacings and sizes of different wavelengths can be determined from the modeling. As indicated by the calculation results, the latter two structures are advantageous to achieve high resolution pulse shaping; frequency combs with long wavelength, large mode spacing and big spot size are favorable for space dispersion; high grating groove density, small incident angle and multi passes in the dispersion system are conducible to the achievement of high resolution. The criterion for resolution would bring on some spot overlap noise.
2012, 61 (18): 184202. doi: 10.7498/aps.61.184202
The sensitivity of the second-order parameter of the Henyey-Greenstein phase function based on the spatially-resolved diffuse reflectance within the two-point-source approximation to the Delta-P1 approximation model is studied, and the analytical expression of the sensitivity is derived. The results show that the analytic solution of the Delta-P1 approximation model for reflectance contains the second-order parameter of the scattering phase function compared with the diffusion approximation model, and the second-order parameter has significant influence (the extreme value greater than 30%) on the analytic solution for spatially-resolved diffuse reflectance with small source-detector separations ( 1.5 mm) no matter whether the absorption is weak or strong. The research has theoretical significance for obtaining the optical information abour tissue with the second-order parameter .
Investigation on the characteristics of electromagnetic scattering from large-scale rough surface of layered medium
2012, 61 (18): 184203. doi: 10.7498/aps.61.184203
A physical model for the electromagnetic scattering from large-scale rough surface of layered medium is established. Stratton-Chu integral equation and Kirchhoff approximation are adopted to calculate the electromagnetic scattering field. The Gaussian random rough surface is selected to simulate the actual rough surface of layered medium. The scattering echo is detected with a single-station nadir-looking radar sounder. The effects of surface and subsurface's roughness on the scattering filed are examined both theoretically and numerically. The effect of upper layer's electric properties (permittivity and conductivity) on the scattering field is also investigated and some analysis for the numerical results is presented.
2012, 61 (18): 184204. doi: 10.7498/aps.61.184204
The lock-in threshold can be obtained by reducing the rotation rate. The lock-in threshold obtained using this method is inconsistent with the angle random walk derived from Allan variance. A dynamic burn-in grating model is proposed. The burn-in effect happens in the mirror not only in the standing wave state, but also in the traveling wave state, and it responds dynamically to beat frequency, which is different from previous research results. It can be derived that the lock-in threshold increases with the decrease of rotation rate for this model, and many puzzling phenomena of the ring laser gyro can be explained.
2012, 61 (18): 184205. doi: 10.7498/aps.61.184205
The steady-state nonvolatile two-step, two-color holographic recording performance is studied theoretically for LiNbO3:Cu:Ce based on the two-center model, with taking into account the direct electron transfer between the deep-trap center Cu+/Cu2+ and the shallow-trap center Ce3+/Ce4+ due to the tunneling effect. The results show that the total space-charge field is determined by the space-charge field on the deep-trap center, and the direct electron exchange between the Cu+/Cu2+ and the Ce3+/Ce4+ levels through the tunneling effect dominates the charge-transfer process in the two-step, two-color holographic recording. Therefore, the material parameters related to this direct tunneling process play a key role in the two-step, two-color holography performance.
2012, 61 (18): 184206. doi: 10.7498/aps.61.184206
Detections and manipulations of quantum optical state at single-photon level have received much attention in the current experiments. Here, by numerically calculating the time-evolved Wigner functions, we investigate the dynamics of the typical non-classical state, i.e., few-photon superposition states in a dissipating optical microcavity. It is shown that the negativity of their Wigner function vanishes with dissipation. But this does not imply that all the non-classical features of the dissipative quantum state disappear. In fact, it is shown that the value of the second-order correlation function g(2)(0) (which serves usually as the standard criterion of a typical non-classical effect, i.e., the anti-bunching of photons, if g(2)(0)g(2A)(0) varies with the cavity dissipation and thus could be used to describe the physical effects of the dissipative cavity. Finally, we discuss the experimental feasibility of our proposal with a practically-existing cavity QED system.
Effects of modulation of atomic injection and exit rate on spatial evolution of phase-dependent lasing without inversion gain and intensity in an open V-type system
2012, 61 (18): 184207. doi: 10.7498/aps.61.184207
The propagation effect in an open V-type three-level atomic system with spontaneously generated coherence is studied by numerical calculation. The results show that changing the relative phase between the probe and driving field has an important effect on variations of gain and intensity of lasing without inversion (LWI) with propagation distance, i.e. spatial evolution; variation of ratio (S) between the atomic injection rate and atomic exit rate (r0) has a remarkable modulation role in phase-dependent spatial evolution. In a certain range of S (r0), with value of S (r0) increasing, LWI gain and intensity increase and the propagation distance needed for reaching an intensity maximum value becomes longer; In a corresponding closed system, the maxima of LWI gain, intensity and the propagation distance along which the gain can be produced are smaller than in an open system. The Doppler effect also has an obvious influence on spatial evolutions of LWI gain and intensity, values of LWI gain and intensity are smaller in the presence of Doppler effect than in the absence of Doppler effect.
2012, 61 (18): 184208. doi: 10.7498/aps.61.184208
By introducing sub-wavelength grating into an oxide-confined 980-nm vertical-cavity surface-emitting laser (VCSEL), the VCSEL with a stable polarization is realized. A sub-wavelength zero-order grating period is smaller than the wavelength in the semiconductor medium. This choice avoids a reduction in efficiency due to higher diffracting light. After the grating is etched, the threshold gains of E-fields parallel and perpendicular to the grating groves direction are different. The polarization therefore would be controlled. Good laser characteristics are obtained: the orthogonal polarization suppression ratio is over 12 dB and the threshold current increases just 7.14%.
2012, 61 (18): 184209. doi: 10.7498/aps.61.184209
We experimentally investigate the plasma grating created by two intersecting ultrashort laser pulses, and its effects on the supercontinuum generated from femtosecond laser filamentation. The enhanced supercontinuum generation is observed, with a plasma grating formed. And an energy transfer of the supercontinuum is demonstrated, from one pulse to the other, due to the diffraction property of plasma grating. The theoretical calculation accords well with the experimental result.
2012, 61 (18): 184210. doi: 10.7498/aps.61.184210
Nonlinear phase noise structure is studied in the background of long-haul interferometric fiber sensing system. The influence of each factor is analyzed and evaluated in detail, and suppression methods are discussed briefly. The results show that phase noise of the system mainly includes the phase noise converted from intensity noise, the phase noise caused by nonlinear effects induced laser linewidth variations as well as the phase noise introduced by self phase modulation and cross phase modulation. Stimulated Brillouin scattering and four-wave mixing can cause intensity noise and then it turns into phase noise. For the fiber sensing system which has a narrow detecting bandwidth, the corresponding noise induced by four-wave mixing can always be neglected. Stimulated Brillouin scattering, four-wave mixing and modulation instability can cause laser linewidth broadening, leading to the increase of phase noise. When the number of channels is large, the phase noise introduced by cross phase modulation cannot be ignored. The obtained results provide a good guidance for the practical applications of long-haul interferometric fiber sensing systems.
2012, 61 (18): 184211. doi: 10.7498/aps.61.184211
We consider a one-dimensional thermal nonlinear medium with a step in the linear refractive index at the sample center. It is found that there exist multipole interface solitons around the interface of the sample. In this paper, we address the existences of three types of solutons for fifth-order and sixth-order interface solitons. The three types of interface solitons have diferent pofiles, beam widths, mass centers, existence and stability regions. The three types of fifth-order solitons each have a stability region, and two of them have the same regions. However for sixth-order interface solions, only one type solution has a stability region.
2012, 61 (18): 184212. doi: 10.7498/aps.61.184212
In satellite remote sensing, the relationship between bidirectional reflectance and optical depth is the foundation of the retrieval of the aerosols' optical depth. According to asymptotic integral theory, a new approach to dealing with bidirectional reflectance of atmospheric without artificial stratification in the single-scattering approximation. The results show that the accuracy of analytical solution obtained by the new method is high. We formulate the analytical bidirectional reflectance with the vertical continuity of the inherent optical properties. Moreover, analytical solution is relatively easy to explain some of most import features for radiative transfer process.
Nonlinear absorption properties of Cu(II)meso-tetra(4-sulfonatopheny1) porphine in TiO2/SiO2 organic-inorganic gel using Z-scan technique
2012, 61 (18): 184213. doi: 10.7498/aps.61.184213
The Cu(II)meso-tetra(4-sulfonatopheny1) porphines (Cu(II)-TPPS) with various concentrations are incorporated into TiO2/SiO2 to form organic and inorganic composite gels via sol-gel process. With a Nd:YAG laser of 532 nm wavelength and 7 ns pulse width, the curves of Z-scan are measured under the condition of open aperture. Nonlinear absorption parameters of these materials, which are attributed to the reverse saturation absorptions of monomer and dimer of Cu(II)-TPPS, are obtained by fitting the above curves with Z-scan theroy. The result indicates that the nonlinear parameter of gel increases with doping concentration increasing. The nonlinear absorption coefficients of gels in 1.11×10-4 mol/L A2, 1.48×10-4 mol/L A3 and 3.01×10-4 mol/L A4 doping concentration are 1.705×10-11, 1.892×10-11, and 4.854×10-11 m/W, respectively. The effects of monomer and dimer on the nonlinear parameter are discussed. The reduction of the nonlinear parameter results from the increases of dimer and multimer as the doping concentration increases. The damage threshold of the gel is also measured and can reach up to ～5 J/cm2.
Sensitivity of spatially-resolved diffuse reflectance to optical parameters in the hybrid diffusion approximation
2012, 61 (18): 184214. doi: 10.7498/aps.61.184214
Based on a theoretical model for spatial diffusion reflection with two-point-source hybrid diffusion approximate to a semi-infinite homogeneous turbid medium, expressions of the sensitivities for the absorption coefficient μa and the reduced scattering coefficient μ's are derived, and the effect of the sensitivities of the two optical parameters μa and μ's on diffuse reflectance is studied systematically in small volumes of highly absorbing medium. The results show that the sensitivity of the absorption coefficient increases approximately linearly with the increase of source-detector separation ρ, with a slope proportional to 1.4 (μ's/μa)1/4, and also indicate an optimal probe spacing ρopt at which a variable reduced scattering coefficient has a smallest effect on the measurment of optical absorption in the turbid medium. The value of ρopt is approximately equal to 3.4 times transport mean free path. The study is of great significance for the measurment of the optical properties of biological tissues and the application of diffuse reflecance spectroscopy technology.
2012, 61 (18): 184215. doi: 10.7498/aps.61.184215
Low-efficiency is an important issue in "plug and play" quantum key distribution system for practical application. The inefficient reasons of system and the current solutions are analyzed. And then a new quantum key distribution scheme with multi-wavelength pulses is proposed, in which a broadband optical source and the WDMs are combined to generate the optical pulses with four wavelengths. Moreover, the WDMs can also be used as a filter to extract the signal in the proposed scheme. Compared with other schemes, this scheme is highly stable, more securate and independent of the response speed of the control systems. The scheme can be used in high efficiency "plug and play" quantum key distribution system in practice.
Design and analysis of two-stage transmitted-reflected concentration PV/thermal system with spectral beam splitter
2012, 61 (18): 184216. doi: 10.7498/aps.61.184216
For the traditional photovoltaic (PV) system which is under a full spectral concentrating condition, only a part of solar energy can be used by solar cells for PV power generation, the other part is not effective for the PV power generation and can result in temperature of solar cells increasing. High temperature can reduce the PV transformation efficiency of solar cells. In this study, a concentration PV/thermal system with beam splitter is proposed, which is composed of a linear Fresnel lens, a concave lens, a beam splitter, solar cells and a thermal receiver. The selective transmission coating on the splitter is designed to improve the incident condition of the sunlight on the surface of solar cells and reflect the noneffective part of sunlight to the thermal receiver for recycling. Theoretical analysis of the PV/thermal system is made and the results show that compared with the traditional concentration PV system, the two-stage transmitted- reflected concentration spectral beam splitting PV/thermal system has a high efficiency.
2012, 61 (18): 184217. doi: 10.7498/aps.61.184217
A new collimator array based on single collimating lens and fiber array is proposed in the paper. An in-depth study is conducted on the two realizing methods, one is with glue in the optical path and the other is without glue in the optical path. Based on Gauss optics transmission matrix and q parameter theory, the relationship among the variables is deduced and computed theoretically, simulated virtually and confirmed experimentally. Good agreement between theoretical results and simulation, experimental results is obtained. The mechanical and packaging designs of the two realizing methods are first studied, then the proposed collimator arrays, with good performance are made. Both the theoretical and experimental results show that the scheme has the merits of easy-to-make, low cost, easy-to-package, good performance, good scalability, etc., which can strongly support the development of reconfigurable optical add-drop multiplexer system and optical cross-connect system.
The generation and transmission research of the fan-shaped multi-beam intense relativistic electron beams
2012, 61 (18): 184218. doi: 10.7498/aps.61.184218
Compared with the beam of conventional relativistic klystron, each beam of the multi-beam relativistic klystron has a low perveance and low space charge force, but it has a high conversion efficiency of beam-wave. According to these requirements, in this paper we investigate the generation and transmission of fan-shaped multi-beam intense relativistic electron beams by the experiment and the simulation with using the three-dimensional software, and analyse the electrostatic field distribution of the cathode end and the influence on the generation of the electron beams by establishing a three-dimensional model of electron gun. The emission currents by the particle-in-cell simulation, then the beam spot pictures of electron beam transmission in a hollow drift tube and multiple fan-shaped hole drift tube by the particle tracking solver are obtained. The theoretical analysis and explanation with the aid of the sheet beam theory are presented. The simulation and experimental results show that the beams rotate not only around their own center, but also around the center of the system in the transmission process of the electron beams in the hollow drift tube. Thus we can increase the transmission efficiency by rotating multiple fan-shaped hole drift tube to align the beams.
2012, 61 (18): 184301. doi: 10.7498/aps.61.184301
A layered co-focal elliptical-cylindrical acoustic cloak is designed based on the effective medium theory. The distribution of acoustic field is simulated using the finite element method. The cloak possesses the properties of low-reflection outside and wavefront-bending in the cloak shell. The relationship between the cloaking effect and the number of discrete layers, the direction of incident wave are also investigated. The effective working frequency band can be broadened by using more layers. As the cloak is of a line-transformed type, its properties are associated with the direction of incident wave. Only when the incident wave is parallel to the major axis of the ellipse, can the cloak have the best performance. In addition, the cloak is of a cylindrical structure as the focus of the ellipse-cylinder is small enough. This work is expected to be helpful for realizing acoustic cloaks with complex shapes.
Generation of negative pressure of underwater intensive acoustic pulse and cavitation bubble dynamics
2012, 61 (18): 184302. doi: 10.7498/aps.61.184302
In this paper, the high speed photography and the pressure measurement are used to study the propagation and focusing process of the underwater intensive acoustic pulse which is reflected from an ellipsoidal reflector. The experimental results indicate that the reflector contribute to both the sound energy focusing and the generation of a negative pressure portion of the intensive acoustic pulse, and the cavitation bubble cluster will be induced to grow accordingly. Based on the experimental results, a sound propagation model derived from Kirchhoff diffraction integral and the equation of bubble dynamics proposed by Qian and Xiao [Qian Z W and Xiao L 2003 Chin. Phys. Lett. 20 80; Qian Z W and Xiao L 2008 Chinese Physics B 17 3785] are used to numerically study the propagation of the intensive acoustic pulse and the bubble motion. The numerical results indicate that the "wake wave" and "edge wave" will evolve into the negative pressure portion of the intensive acoustic pulse in front of the far focus of the reflector and the "center wave" will evolve into the negative pressure part beyond the far focus of the reflector. When the bubble is subjected to the reflected wave, it will be compressed and oscillate periodically during the positive pressure phase, and it will expand during the negative pressure phase of the incident pulse. And after the reflected wave passes, the typical processes of expanding, collapsing and rebounding can be seen clearly from the photographs. The research results have practical significance for understanding the propagation characteristic of the underwater intensive acoustic pulse with an ellipsoidal reflector.
2012, 61 (18): 184401. doi: 10.7498/aps.61.184401
Coupled simulation of hypersonic flow and heat exchange is investigated. Moreover, structural stress based on pseudo static assumption is also studied. The unsteady Navier-Stokes equations are solved for the flow field, and SST k-ω model is used as the turbulent model. The unsteady heat conduction equation is solved for the structure. At the fluid-structure interface the flow field acquires temperature boundary condition from the structure and the structure temperature field obtains heat flux boundary condition from flow field. The method proposed here is proved by the slipstream experiment of the infinite circular pipe. The transformation of the temperature and stress of the two-dimensional circular pipe is analyzed in detail during aerodynamic heating. The results show that the high temperature field is gradually enlarged with the increase of aerodynamic heating time. At the same time the structural thermal strain is also gradually enhanced. The minimal deformed area will appear when θ is equal to 60°. The thermal deformation of circular pipe has very little effect on the flow field.
2012, 61 (18): 184701. doi: 10.7498/aps.61.184701
Via molecular dynamics simulations employing an embedded-atom-method potential, we investigate the microscopic process and dynamical properties of shock-induced micro-jet from a grooved aluminum surface. For a large range of shock pressure, we obtain the micro-jet morphology variation, its mass spatial distribution and mass-velocity distribution. The amorphous state and release melting during the jetting are both analyzed using the central symmetry parameter, where the effect law of release melting on the micro-jet is obtained. It is found that the micro-jet mass keeps a linear increase with the piston velocity prior to release melting; the micro-jet mass is enhanced evidently after release melting; while the velocity of release melting is above a threshold, the jetting mass shows a linear increase with the piston velocity again, where the strength of material can be neglected.
2012, 61 (18): 184702. doi: 10.7498/aps.61.184702
The processes of different droplets impacting the solid surface are recorded using a high-speed digital came and the effects of the impact parameters on the droplet impact are studied. The results show that the viscosity of droplet plays a decisive role in spreading process, and surface tension has a leading influence on recoiling process. These two factors jointly determine the oscillation characteristics. The maximum spread factor increases with impact velocity increasing, however, the time from the beginning to achieve the maximum spreading factor shows different variation rules due to surface tension.
2012, 61 (18): 184703. doi: 10.7498/aps.61.184703
Granular matter is a complex energy dissipation system. The friction and the viscous contacts among particles can dissipate effectively the system energy caused by external impact load. The force chain structure in granular system can extend the local impact in spatial dimension and expand the instantaneous impact in temporal dimension, thus to obtain the effective shock-absorbing effect. To investigate the absorbing capacity of granular matter under an impact load, in the present study, we develop an experimental system, in which a rock ball impacts granular matter in a cylinder under gravity, and the impact force on the cylinder bottom is measured with three load cells. The influences of particle size, material propery, thickness of granular matter on shock-absorbing capacity are discussed. The results show that irregular particles have more shock absorbing capacity, while the large-size particles have a slightly higher shock absorbing capability than the small-size particles. The thickness of granular matter, H, is a key parameter to affect the shock-absorbing. Critical thickness, Hc, is obtained in the experiments. The shock-absorbing capacity of granular matter is enhanced with H increasing when HHc, while H has little influence on shock-absorbing when H>Hc. The resutls above are obtained with constant impact energy. Critical thickness Hc should be a function of impact energy and will be determined in the next study. With the experiments on shock-absorbing capacity of granular matter, it can reveal basic mechanical behaviors of granular materials and be applied in mechanical vibration absorptions.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
A new kind of particle in tokamak, which is different from passing partilce and trapped particle, is found. This kind of particle can fly directly out of the boundary due to the drift motion. The loss cone in tokamak, which is composed of these particles, is studied. At the same time, the loss rates due to the banana orbit in different tokamaks are calculated.
2012, 61 (18): 185202. doi: 10.7498/aps.61.185202
Previous studies have shown that the technology of beam smoothing may effectively control parameter instabilities within the laser-plasma interaction, and greatly reduce stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS). However, the recent experimental results on NIF revealed a much higher SBS and SRS than expected, one possible reason is due to the coherence between incident laser beams. In our research, two laser beams from Shenguang II facility are employed to irradiate an Au plate target, the energy and spectra of the backscattered light are measured in different coherent conditions. The results show that the driven-laser beams strongly interfere with each other, and usually the backscatter becomes strong gradually with the increase of coherent degree between the incident beams.
2012, 61 (18): 185203. doi: 10.7498/aps.61.185203
An inactivation mechanism of A549 cancer cells is studied by using a dielectric barrier discharge (DBD) plasma needle. The influence of oxygen concentration, which is injected into helium plasma afterglow region through a stainless steel tube, is investigated. The neutral red uptake assay provides a qualitative observation of morphological differences between the dead cells and the viable cells after plasma treatment and a quantitative estimation of cell viability under different conditions. In the treatment process at a fixed power of 24 W, the inactivation efficiency of helium-oxygen plasma depends mainly on the exposure time and percentage of added oxygen in helium plasma. Experimental results show that the best parameters of the process are 150 s treatment time, 800 mL/min He with 3% O2 addition and separation of needle-to-sample 3 mm. According to the helium-oxygen emission spectra of the plasma jet, it is concluded that the reactive species (for example, OH and O) in the helium-oxygen plasma play a major role in the cell deactivation.
2012, 61 (18): 185204. doi: 10.7498/aps.61.185204
In this paper, the particle-in-cell/Monte Carlo method simulation algorithm with independent intellectual property is described in-depth. The discharge properties of the J-PARC multicusp ion source are investigated using this algorithm. The discharge mechanism of the J-PARC ion source is discussed, and the ion source volume production efficiency is analyzed under single correction magnet and double correction magnets. The results show that under the single correction magnet, a maximum volume production efficiency can be obtained when the distance between the center of the correction magnet and the top of the ion source is 50 mm; fixed the top one of double correction magnet, the longer the distance between magnets, the lower the volume production efficiency is.
2012, 61 (18): 185205. doi: 10.7498/aps.61.185205
In this paper, the particle-in-cell/Monte Carlo method simulation algorithm with independent intellectual property is described in-depth, and discharge properties of the JAERI 10 A multicusp ion source are investigated by using the algorithm. The effects of discharge parameters (such as air pressure, discharge electrode position, magnitute of the filter magnetic field, discharge voltage, etc) on volume production efficiency are analyzed. The results show that in order to obtain high volume production of negative hydrogen ions, filter magnetic field should not be too large, and the discharge voltage should not be too small, but the filter magnetic field and discharge voltage should be appropriately selected, and primarily regulating air pressure and discharge location simultaneously can optimize the ion source.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
Electron-beam induced abnormal expansion in a silica-shelled gallium microball-nanotube structure (Retracted Article)
2012, 61 (18): 186102. doi: 10.7498/aps.61.186102
Under electron-beam irradiation of heteroshape-heteroscale structure of silica-shelled Ga microball-nanotube, an abnormally large and fast volume expansion of liquid Ga is observed. First, we analyze the processes and phenomena about experiment, and the abnormal expansion process can be regarded as a quasi-static process. Then in the framework of quasi-static thermodynamics, according to the Fourier heat conduction law the relative volume variation with temperature is further quantitatively discussed, and the relative expansion rate and expansion coefficient in system are obtained. At the same time it is found that abnormal expansion coefficient of system under electron-beam irradiation is 5-9 times the general thermal expansion coefficient. Finally, it is pointed out that the abnormal expansion is due to the gallium atom ionization effect and the retention effect resulting from a few electrons retaining in the liquid gallium system under electron-beam irradiation. In essence, both ionization effect and the retention effect make the particle densities of liquid systems increased dramatically, resulting in volume expansion abnormally large and fast of liquid Ga.
2012, 61 (18): 186103. doi: 10.7498/aps.61.186103
Quality properties and internal defects of unintentionally doped GaN films grown on 0.3 vicinal sapphire (0001) substrates by MOCVD are investigated by TEM. The results show that plenty of dislocations in the GaN films prepared on vicinal sapphire substrates are annihilated in the areas with a distance of 0.8 m away from substrates, and that dislocations gather in the GaN films. Based on these phenomena, a mechanism for dislocation annihilation in the GaN film prepared on vicinal substrate is proposed, which is capable of explaining the fact that vicinal substrates are able to improve the qualities of GaN films.
The evolution of microstructure and mechanical properties of Ti/Al composite synthesized by accumulative roll-bonding
2012, 61 (18): 186104. doi: 10.7498/aps.61.186104
Ti/Al multilayered composite, mainly composed of ultrafine equiaxed grains with a mean size of 200-300 nm is successfully synthesized by accumulative roll-bonding. The microstructure evolution and mechanical properties of the multilayered Ti/Al compound are investigated.
Improvement on performance of Si-based Ge PIN photodetector with Al/TaN electrode for n-type Ge contact
2012, 61 (18): 186105. doi: 10.7498/aps.61.186105
Large contact resistance due to Fermi level pinning effect at the interface between metal and Ge strongly restricts the 3 dB bandwidth of Ge photodetectors. In this paper, the Ge PIN photodetectors fabricated on silicon-on-insulator substrates, respectively, with Al and Al/TaN electrodes are comparatively studied. It is found that 3 dB bandwidth of photodetector with 24 μm mesa diameter using an Al/TaN stack electrode is improved by four times more than that of the same structure Ge PIN photodetector using an Al electrode under -1 V bias at 1.55 μ. In addition, the dark current is reduced by one order of magnitude, and optical responsivity is enhanced by two times. These results suggest that a thin metallic TaN layer as an electrode can effectively passivate the Ge surface and alleviate the Fermi-level pinning effect, thus reducing the contact resistance and the recombination current at the interface. TaN can be considered as a promising electrode material for Ge device applications.
2012, 61 (18): 186106. doi: 10.7498/aps.61.186106
Based on the concept of residual bond, a physical model quantitatively describing the relationship between the melt structure and viscosity of metal is established by mathematical deduction, and the microscopic nature of viscosity is considered to be the evolution of the size d of the residual-bond structure of melt. Using this model, the kinematic viscosityies of the magnesium and aluminum melt in a certain region above the melting point are calculated, and the functional relations vMg=3.1710-7+3.0410-7d and vAl=1.6510-7+1.0510-7d are obtained, which accord with the experimental data measured by the method of crucible rotating oscillation damping. This model reveals the microscopic naturs of the micro-inhomogeneity and viscosity of the melt structure from the view of chemical bond, which provides a new way to calculate the viscosity of melt and is of significance for understanding the relationship between microstructure and macroscopic properties of liquid metal.
Problem and modification to the equation of state for liquid and solid-liquid mixed phase in Grover model
2012, 61 (18): 186201. doi: 10.7498/aps.61.186201
According to the definition of disordered entropy, we obtain the equation of state (EOS) of liquid metals. Then due to the fact that the fraction of liquid present in the mixed phase region is assumed to be not linear with temperature but linear with volume, the incorrect hypothesis of the fraction of liquid present in Grover model is clarified. The more accurate formula to express the EOS of solid-liquid mixed phase is obtained. The calculations from Grover model show that the solidus, liquidus and melting line do not coincide in the P-T graph as they should be. The modified results are shown to be in good coincidence with the three lines in the P-T graph.
2012, 61 (18): 186202. doi: 10.7498/aps.61.186202
The Nd0.7Sr0.3MnO3 compounds are treated at a thermal pressure (HTP) of temperature 1273 K and pressure 9 GPa. The results show that the crystal structure and space group of samples keep unchanged while the lattice and structural parameters, especially the microscopic structure change remarkably, which produces significant influence on magnetoelectric transport of the Nd0.7Sr0.3MnO3 ceramic. For electrical transport of the HTP sample, no electroresistance (ER) effect occurs when loaded current is less than 1.5 mA, however, an ER effect around 200% takes place when the loaded current goes up to more than 1.5 mA. Interestingly, the peak at -T curve disappears by replacing a platform, which can return to a peak if a magnetic field is applied again. The formation of intergranular phase and insulating behaviour of sample under thermal pressure condition are suggested to be responsible for the unique transport properties.
2012, 61 (18): 186301. doi: 10.7498/aps.61.186301
The influence of phonon on the properties of polaron and qubit in spherical shell quantum dot is studied by solving accurately the time-independent Schrödinger equation, Lee-Low-Pines unitary transformation and variation methods. The numerical results indicate that phonon effect leads to a lower energy of ground (or excited) state of polaron than electronic energy of ground (or excited) state and the increased oscillating period of a qubit, and the phonon effect becomes more obvious with outer radius increasing when inner radius is const. The numerical results also show that the phonon effect cannot influence the amplitude of probability density distribution of electrons in quantum bit, and that the probability density distribution of electrons is dependent on co-ordinate and time and its amplitude is maximal in centre spherical surface and but zero in boundary surface. The probability density of electrons at each position oscillates periodically with time.
2012, 61 (18): 186401. doi: 10.7498/aps.61.186401
The splitting of particles precipitating from solid solutions, e.g. Ni-based alloy, is studied with the phase field method. The simulation results show that in the single particle system, the nucleuses of crystal with the sizes of 80l to 90l (l= 12.18 Å) split during ageing. The splitting is the result of the interaction between elastic energy and interface energy. During the earlier stage of ageing, the sharpening along of the interface of the initial spheric shape particle will lead to the solute beneficiation at the corner but impoverishment in the center of the particle, it is the splitting incubation stage (SIS). The total interface energy (TIE) appears as being of horizontal step during SIS. The particles split at 300τ (τ=4.65 s) after the SIS and at the end of splitting the TIEs reach their maxima and the total elasic energy (TEE) reaches minimum at 1000τ. The horizontal step during SIS and the extreme points of TIE and TEE are the representative features of splitting. The TIE has SIS but no extreme point lying on TIE and TEE when the particle sizes are bigger than 90l. For the particles with sizes smaller than 80l, the TIE increases up monotonically.
First-principle calculations of elastic, electronic and thermodynamic properties of TiC under high pressure
2012, 61 (18): 186501. doi: 10.7498/aps.61.186501
First-principle investigations of the elastic, electronic and thermodynamic properties of TiC in NaCl structure under high pressure are conducted by using the plane-wave pseudopotential method and quasi-harmonic Debye model. The obtained lattice parameters, elastic constants and moduli at p=0 GPa and T=0 K are in very good agreement with the available experimental data and other theoretical results. According to the analysis of the density of states, the Ti-C bond becomes stronger with pressure increasing. The values of bulk modulus, thermal expansion coefficient, Debye temperature, entropy, Grüneisen parameter and heat capacity (CV) at different pressures and temperatures are obtained successfully by using the quasi-harmonic Debye model. The influence of pressure on bulk modulus, thermal expansion parameter and Debye temperature is greater than that of temperature. The CV decreases with the increase of pressure under the same temperature and tends to the Dulong-Petit limit at high temperature.
2012, 61 (18): 186801. doi: 10.7498/aps.61.186801
The electronic structures at the interfaces of iron phthalocyanine (FePc)/TiO2(110), FePc/C60 and FePc:C60 blends are studied in situ by synchrotron radiation-based ultraviolet photoelectron spectroscopy (SRUPS). It is found that the interaction between organic molecules and the surface of reduced rutile TiO2(110) is stronger than that of the stoichiometric TiO2(110) interface. The energy level alignments at the FePc/C60 interface and FePc:C60 blends are drawn based on the evolutions of the interfacial electronic structures. From the SRUPS spectra, the band bending energies are found to be 0.45 eV in the C60 layer and 0.1 eV in the FePc layer at the FePc/C60 interface. The interface dipole energy is 0.2 eV at the FePc/C60 interface. The offsets between the HOMO of FePc and LUMO of C60 are 0.85 eV at FePc/C60 and 1.04 eV at FePc:C60 blends, which indicates that the blend films can improve the efficiency of the relevant optical-electric devices.
2012, 61 (18): 186802. doi: 10.7498/aps.61.186802
In this paper, we use molecular dynamics simulation to analyze the diffusion and dissociation barriers of homogeneous (Cu2) and heterogeneous dimer (Ag2, Pd2) on the surface of Cu(100), (111) surface. We explore the diffusion and dissociation process and characteristics of those dimers on Cu surface and compare their diffusion and dissociation barriers of dimer, the binding energies of the dimer and substrate, substrate surface textures, temperatures, etc. The semiempirical EAM potential is used in the simulation.
2012, 61 (18): 186803. doi: 10.7498/aps.61.186803
B-doped Ti film is fabricated by direct current magnetron sputtering. The doping concentration, surface morphology, crystal structure, crystal grain diameter and stress are characterized by X-ray photoelectron spectroscopy, scanning electron microscopy and X-ray diffraction, respectively. It is found that, with the increase of B doping, the crystal grain diameter decreases monotonically and reaches a minimum of 1.3 nm at a B doping concentration of 5.50 at.%. The B-doped Ti film presents a compact columnar structure at that concentration. The stress of Ti film changes from compressive stress to tensile stress when B is doped.
Confinement effect and interface effects on the thermoelectric properties of nano-ceramics: theoretical study
2012, 61 (18): 187201. doi: 10.7498/aps.61.187201
The influences of electron and phonon confinement and interface effects on the thermoelectric performance of SrTiO3 nano-ceramic are studied by using the Boltzmann transport equations and density functional calculations. Theoretical calculations show that the figure of merit of nano-ceramic is greatly improved up to 0.8 at room temperature. The improvement is due mainly to the phonon confinement effect and the electronic energy filtering effect at the grain boundary. The electron confinement effect and the interface phonon scattering effect play a supporting role. These results may be conducive to the design of high-performance thermoelectric nano-ceramic.
Numerical investigation of the metal-insulator-metal waveguide filter based on the arc-shaped resonator
2012, 61 (18): 187301. doi: 10.7498/aps.61.187301
A metal-insulator-metal waveguide with an arc-shaped resonator is designed, and the transmission properties are numerically investigated by the finite-difference time-domain method. The results show that the transmission peaks in the transmission spectra result from the resonance of the surface plasmon polaritons in the arc-shaped resonator. The effects of the structural parameters of the arc-shape resonator on the transmission properties are also studied. In addition, this structure can also act as a splitter and achieve a dual function of filter and splitter for subwavelength waveguide.
In this paper, the dielectric spectra of ZnO varistor ceramics are measured by Novocontrol wide band dielectric spectrometer in a temperature range of -160℃-200℃ and frequency range of 0.1 Hz-0.1 MHz. It is found that electron transportation can be characterized by the flat region on a low frequency side of σ'-f curve. The Schottky barrier height is 0.77 eV obtained from σ'-f curve, which is consistent very well with the data from I-V curves given in other literature. On the basis of back-to-back double Schottky barrier model, Schottky barrier height corresponding to electron transportation across grainboundary is explained to be the energy difference between interface state and barrier top. According to this explanation, Schottky barrier height will increase linearly with the increase of DC voltage applied. The linear variation of barrier height with the increase of DC voltage applied is confirmed experimentally. Finally, the theoretical value of averaged grain size is obtained to be 6.8 μm, which is almost identical to 6.5 μm measured from SEM images. Therefore, the macroscopic electrical properties and the microstructure can be expressed at the same time by dielectric spectra.
2012, 61 (18): 187303. doi: 10.7498/aps.61.187303
The material characteristics and one of the preparation methods, atomic layer deposition of Al2O3 are introduced. The passivation mechanisms (chemical passivation and field-effect passivation) of Al2O3 films are demonstrated comprehensively, and optimization methods from the angles of film thickness, thermal stability and stack passivation are illuminated. The application of Al2O3 passivation in the crystalline silicon solar cell is provided, including passivated emitter rear locally diffused cell and passivated emitter and rear cell. Finally, the future study of the Al2O3 passivation process and the application to industry production are proposed.
2012, 61 (18): 187304. doi: 10.7498/aps.61.187304
The semiconducting Cu2ZnSnS4 (CZTS) thin film with high absorption coefficient has long been recognized as a novel solar cell material. In this work, the performances of n-ZnO:Al/i-ZnO/n-CdS/p-CZTS solar cells are analyzed by using semiconductor theory. The influences of doping concentration, thickness, and defect states of CZTS layer and the doping concentration and thickness of CdS layer on the performances of the solar cells and the temperature characteristics are investigated. The calculated results show that the CZTS layer is a main absorption layer in the solar cell. The changes in doping concentration and thickness of CZTS layer have significant influence on the conversion efficiency of the solar cell. The density of defect states in CZTS can sharply degrade the photovoltaic performances. The influences of the doping concentration and thickness of CdS layer can be neglected. The calculated results show that the optimal n-ZnO:Al/i-ZnO/n-CdS/p-CZTS structure has open-circuit voltage of 1.127 V, short circuit current density of 27.39 mA/cm2, fill factor of 87.5%, and conversion efficiency of 27.02%. In addition, the temperature gradient of conversion efficiency is -0.14%/K. These results reveal the promising photovoltaic characteristics of CZTS thin film serving as a solar cell absorber.
2012, 61 (18): 187305. doi: 10.7498/aps.61.187305
Organic light-emitting diode with a structure of ITO/CuPc/NPB/Alq3/LiF/Al is fabricated. The excitons of the device are produced by laser irradiation using two kinds of laser beams which are at 442 nm and 325 nm, and the evolutions of the excitons are controlled by a small bias (which is either positive or negative, and ensures that the device does not turn on). The photo-induced magneto-conductance (PIMC), which is the dark current of the device showing no magnetic response at a small bias, is also measured at the same time. It is found that unlike the magneto-conductance in the electrical injection case, the PIMC presents significantly different results at the positive and negative small bias. The PIMC of the device increases rapidly in a range of 0-40 mT at a small forward bias, then increases slowly with the further increase of magnetic field, and finally becomes saturated gradually. But in the case of small reverse bias, although the PIMC of the device also first increases rapidly with the increase of magnetic field (0-40 mT), but it decreases after its maximum value has been reached. By using a composite model of electron-hole pairs and the theory of hyperfine interaction, the PIMC effect at the forward bias can be explained by analyzing the effects of the applied magnetic field on the micro-processes of the light-generated carrier of the device. When the device is in the case of reverse bias, due to the fact that the relationship of the energy-band of each organic layer provides the necessary conditions for the interactions between exciton and charge, the decrease of PIMC in high magnetic-fields can be attributed to the mechanism of reaction between triplet exciton and charge.
2012, 61 (18): 187306. doi: 10.7498/aps.61.187306
The structural and electronic properties of spinel LiMn2O4 and its Al doping system LiAl0.125Mn1.875O4 are investigated within the density functional theory in both the generalized gradient approximation (GGA) and the GGA with Hubbard U correction (GGA+U). The results from the GGA method suggest that LiMn2O4 has a cubic structure and the valences of Mn ions are all +3.5, which is unable to explain the Jahn-Teller distortions in the material. The band structure of LiMn2O4 predicted by the GGA method is also inconsistent with experimental result. With the GGA+U method, the low temperature structures of LiMn2O4 and its Al doping system LiAl0.125 Mn1.875O4 are shown to be orthogonal, the two different valence states of Mn, i.e., Mn3+/Mn4+ ions, are then determined, which is then able to explain the Jahn-Teller distortion in octahedron Mn3+O6 and the non-existence of distortion in octahedron Mn4+O6. These results are in good accordance with experimental data. Their band structures by GGA+U calculations are also consistent with experimental results. The GGA+U calculations on the LiAl0.125Mn1.875O4 indicate that with the replacement of an Mn by Al, the crystal structure and electronic properties are not significantly changed, but the Jahn-Teller distortion in octahedron Al3+O6 can be effectively eliminated, which could improve the performance of the anode materials based on LiMn2O4. The phenomenon is in consistent with the electrochemical experiments.
The nonreciprocal of second harmonic generation with dielectric/antiferromagnetic/dielectric structure in Voigt geometry
2012, 61 (18): 187501. doi: 10.7498/aps.61.187501
We calculate the second harmonic generation (SHG) from structures with dielectric/antiferromagnetic(AF)/dielectric in Voigt geometry. The calculations are performed with a nonlinear transfer matrix method and examples of nonreciprocal SHG are given. In particular, we study the nonreciprocal SHG in four different configurations when the direction of an applied magnetic field and the order of dielectric films are reversed. The nonreciprocal SHG can be realized in a resonance zone of AF, including THz frequency range. The increasing of the angle of incidence wave can enhance the effect of nonreciprocal SHG. In terms of applications, AF may be of interest in signal processing in the THz.
2012, 61 (18): 187502. doi: 10.7498/aps.61.187502
The elementary excitation spectra of the one-dimensional spin-1/2 XY model in the ferrimagnetic diamond chain at low temperature are calculated by using invariant eigenvector in this paper. And the elementary excitation energies in different cases are discussed. Therefore, analytic solutions of the three critical magnetic field intensities HC1, HC2 and Hpeak in the system are given. It is found that the analytic solutions of three critical magnetic field intensities are correct from the law of the magnetization changing with temperature under different external magnetic fields, and it is explained by the contributions of the three elementary excitations to the magnetization. The external magnetic field dependent magnetization presents a 1/3 magnetized plateau at low temperature. The variation of magnetic susceptibility either with temperature or with external magnetic field shows a double peak structure, this phenomenon indicates that the double peak structure originates from the competition among the ferromagnetic exchange interaction energy of intramolecular electronic spin parallel arrangement in dimer, the antiferromagnetic exchange interaction energy of intermolecular electronic spin antiparallel arrangement in dimer-monomer, the thermal disorder energy and the spin magnetic moment potential energy related to external magnetic field.
2012, 61 (18): 187503. doi: 10.7498/aps.61.187503
The hysteresis behaviors domain structures and temperature coefficients of coercivity are investigated in Sm(CobalFe0.1 Cu0.1Zr0.033)6.9, which is aged at 810℃ and slowly cooled with a rate of 0.5℃/min, and then quenched at different temperatures. It is found that the demagnetization cures show two steps clearly as the alloys are quenched at 600℃, which means that there should have two pinnings on the domain wall, and its domain structure appears more as a zigzag shape domain, which means that there should be a small gradient of Cu distribution in the 1:5 cell boundary phase and a small domain wall pinning in the cell boundary phase. The maximum domain wall pinning should be at the interface between the 1:5 cell boundary phase and 2:17 cell phase. As the alloys are quenched at a lower temperature, the steps in the demagnetization cures disappear. At the same time, their domain structures become narrower, and show more attached domains, which means that a lower domain wall energy is in the 1:5 cell boundary phase and that the maximum domain wall pinning should be in the center of the 1:5 cell boundary phase. As the maximum domain wall pinning is at the interface between the 1:5 cell boundary phase and 2:17 cell phase, the coercivity will show an abnormal temperature dependence. While as the maximum domain wall pinning is in the center of the 1:5 cell boundary phase, the coercivity will decrease with temperature increasing. As the testing temperature rises to 500℃, the coercivities for all samples nearly come to the same values, and the maximum domain wall pinnings all should come to the interface between the 1:5 cell boundary phase and 2:17 cell phase.
2012, 61 (18): 187504. doi: 10.7498/aps.61.187504
The magnetic domain evolution behaviors of ferromagnetic (FM) monolayer and ferromagnetic (FM)/antiferromagnetic (AFM) bilayer are compared and analyzed. The results indicate that the equivalent width, mass and velocity of magnetic domain wall are changed, then the coercivity is enhanced and the exchange bias is present due to the exchange coupling between anti- ferromagnetic and ferromagnetic layer. The results also show that the equivalent width, mass and velocity of magnetic domain wall for FM /AFM bilayers system could be varied with the change of the net magnetization of antiferromagnetic layer, the magnetic anisotropy constants of FM and AFM layer, the exchange coupling constant of antiferromagnetic layer, interface exchange coupling constant and the temperature, and the relevant influences on the coercivity and exchange bias are discussed. So, the physical mechanisms of the emergence of exchange bias and enhancement of coercivity are discovered by the formation and evolution of the domain wall.
Study on the variation of static dielectric constant with temperature and the corresponding orientational correlation in polar liquids by using Weiss's molecular field theory
2012, 61 (18): 187701. doi: 10.7498/aps.61.187701
There is no widely accepted microscopic theoretical model of the static dielectric constant of liquids so far. This is mainly because the orientational correlation between molecules in liquids, belonging to the strong correlation systems, is still not clear. In this paper, the variations of the static dielectric constants of 4 kinds of polar liquids, i.e., water, methanol, ethanol and 1-propanol, specifically the Curie-Weiss constant, Curie temperature and Weiss molecular field factor with temperature are studied according to the Weiss's molecular field theory (WMFT), and it is concluded that 1) ferroelectric correlations (FCs) and anti-ferroelectric correlations (AFCs) between molecules coexist in the liquids, and FC is stronger than AFC, as well as FC becomes weak and/or AFC stronger with temperature decreasing; 2) WMFT of homogeneous structure cannot quantitatively describe the abnormally large values of εs of the liquids at low enough T. It can be expected that the coexistence of FC and weak AFC must lead to the spatial distribution of the correlation orders, and consequently, we propose an WMFT of coarse grain approximation of the spatial distribution correlation orders, and use the WMFT to explain why the static dielectric constant varies rapidly with temperature. The above results can benefit the understanding of the liquid physics, including the glass transition mechanism.
Theoretical analysis of response characteristics for the large exponential-doping transmission-mode GaAs photocathodes
2012, 61 (18): 187901. doi: 10.7498/aps.61.187901
A new-type GaAs photocathode with ultrafast time response, that is, the large exponential-doping transmission-mode GaAs photocathode, is discussed in detail. The response characteristics, including quantum yield, time and spatial resolution, are numerically simulated. The analysis results show that the transit response time of the photo-excited electrons for the GaAs photocathode is extremely shortened, because the built-in electric field in GaAs layer formed by the large exponential-doping mode is benefitcial to the photoelectron transport process of GaAs photocathodes. The response time can reach about 10 ps when the thickness of GaAs dgorption layer is around, which shows that the novel NEA cathode has a better feature of temporal response than that of traditional GaAs photocathode. In addition, the quantum yield will reach ～10%-20% in the whole special response range, and the spatial resolution is improved obviously. The analysis results indicate that with high quantum efficiency guaranteed, the large exponential-doping NEA cathode overcomes the limitation of time response of traditional GaAs NEA cathode and improves the spatial resolution, which indicates that the new NEA cathode is expected to meet the demands of high-speed device and photoelectron device, and promote the further development and applications of NEA cathodes.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
Preparation and characteristic of phase transition vanadium oxide thin films by rapid thermal process
2012, 61 (18): 188101. doi: 10.7498/aps.61.188101
Metal-insulator phase transition VOX thin film is fabricated on a Si 100 substrate after the metal V thin film, prepared by direct current facing targets magnetron sputtering has been rapidly thermally treated first in pure oxygen environment and then in pure nitrogen environment. The thermal treatment conditions are 430℃/40 s, 450℃/40 s, 470℃/40 s, 450℃/30 s, 450℃/50 s in pure oxygen environment and 500℃/15 s in pure nitrogen environment. XRD, XPS, AFM and SEM are imployed to analyze the crystalline structure, valentstate and the components, morphology of the thin film. The electrical and optical characteristic of the thin film are analyzed by the Four-point probe method and THz time domain spectrum technology. Results reveal that after 450℃/40 s rapid thermal treatment in pure oxygen environment the metal V thin film turns into VOX thin film which has low properties of phase transition. Before and after heating, the change of resistivity reaches 2 orders of magnitude and the range of the THz transmission intensity shows smooth change. In order to improve the properties of phase transition, the VOX thin film is treated by 500℃/15 s rapid thermal process in pure nitrogen environment. After that, we find that the thin film shows a good phase transition performance, accompanied by a sheet square resistance drop of above 3 orders of magnitude and a 56.33% reduction in THz transmission intensity.
2012, 61 (18): 188701. doi: 10.7498/aps.61.188701
Aberrations will degrade trapping performance of optical tweezers. In the holographic optical tweezers, aberrations originate not only from optical elements but also from holographic phase hologram of optical traps designed by a certain algorithm. We utilize a spatial light modulator to imprint Zernike polynomials phase hologram for correcting some certain aberrations in holographic array optical tweezers which are caused by grating and lens algorithm. The results show that third-order Zernike term can effectively correct coma due to the algorithm in the optical train, and the trap stiffness for 2 μm microns diameter polystyrene beads can reach 40%. Further comparison between different Zernike term aberration correction effects demonstrates that coma caused by grating and lens algorithm in the holographic array optical tweezer has the same serious influence on tweezer trapping performance as the aberrations originating from optical elements. Meanwhile, based on first-order Zernike term aberration correction results it can be obtained that grating and lens algorithm are robust with first-order Zernike aberrations. The correcting of aberrations for algorithm in holographic optical tweezers has great significance for improving the tweezer trapping performance and deepening the understanding of specific algorithm.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
A new method of obtaining perturbation vertical profiles in estimating the atmosphere gravity wave parameters
2012, 61 (18): 189201. doi: 10.7498/aps.61.189201
The gravity wave is the fundamental dynamic process in the atmosphere, and one of the important theoretical studies of atmosphere dynamics is to estimate the gravity wave parameters. Due to the inherent deficiency of the curve fitting method for the vertical perturbation profile (VPP), a new method is proposed in obtaining the profile for estimating the reasonable gravity wave parameters. Based on the features of cone of influence in wavelet transformation and its maximum value, which is considered as the critical value for background profile and perturbation profile, the VPP is obtained with a 10th-order Butter Worth highpass filter for the signal with the magnitude less than the critical value. With the Morlet wavelet and Fourier power spectrum analyses for the numerically simulated zonal wind and meridional wind, it is found that there are neither distinct background signals nor false signal including in the VPPs, which overcomes the deficiency of curve fitting method. Therefore, the proposed method is excellent compared with the widely used curve fitting method in obtaining the VPPs, and the reasonable atmosphere gravity wave parameters could be estimated with the new method.
2012, 61 (18): 189202. doi: 10.7498/aps.61.189202
In this paper, we analyze the stability of solution of the nonlinear function of physics, the Logistic function. It is found that the solution has a special character that it can change abruptly from one stable state to another when the initial value and parameters of function are selected. Abrupt change level and abrupt change rate are related to parameters of function, which can be described by defining the abrupt change intensity index. By using the character of solution, we build an ideal time series to imitate climate abrupt change in mean of climate system, investigate what behaviors the recovery rate and recovery force can have when the system approaches to a critical threshold, and to ascertain how it warns the abrupt change of the system early. Besides, we also find that even the system is disturbed by some noise signals, the recovery rate and recovery force also make an early response to the arrival of the abrupt change of system. Finally, the result of testing the Pacific Decadal Oscillation (PDO) index showes that the early warning of the abrupt change appeared in 1973, much more early than the abrupt change of PDO index happening in 1976/1977, which means that the recovery rate and recovery force can be used as the early warning signals of the abrupt change in mean.
A modified method to configure the parameters of the bilateral filtering for synthetic aperture radar image speckle reduction
2012, 61 (18): 189501. doi: 10.7498/aps.61.189501
Bilateral filtering can effectively smooth synthetic aperture radar (SAR) images with preserving the edges, but it is difficult to configure the parameters of bilateral filtering to the optimum. The iterative method to configure the parameters with high precision and efficiency has been presented, but the iterative process may be aborted incorrectly. In this paper, we present a modified configuration method to get the optimal trade-off parameters, and give the proof of the convergence of the method. The experimental results on real SAR images show that the modified method can not only obtain equivalent visibility as the iterative method, but also higher accuracy of the gray value similarity variance than that under identical iterative precision. With the iterative precision improved, the convergence rate of the modified method is faster than that of the iterative method.