Vol. 61, No. 4 (2012)
2012, 61 (4): 040501. doi: 10.7498/aps.61.040501
Parameter-induced stochastic resonance is an important method of detecting weak signal from noise, but under stable noise background, this method has not been reported. In this paper, we study the parameter-induced stochastic resonance in an overdamped system with stable noise. Our investigation discloses that the stochastic resonance can be realized by tuning the system parameter under stable noise background; when the nonlinear term parameter is turned, the resonant effect becomes weakened as the stability index decreases. But when the linear term parameter is turned, the resonant effect becomes strengthened as the stability index decreases. Our observation is significant for understanding the positive role of stable noise in weak signal detection, which is helpful for understanding the effects of different stable noises on stochastic resonance systems.
2012, 61 (4): 040502. doi: 10.7498/aps.61.040502
In this paper, we present an adaptive method to improve the synchronization of complex networks. We summarize the effects of the average degree of network and the size of network on the synchronization. By this adaptive method, We obtain a power low relationship between the synchronous capacity and the size of network. At the same network size, this method can enhance the network synchronous capacity several orders of magnitude higher than that of unweighted network. When the network size is larger, it is more efficient to improve synchronous capacity.
2012, 61 (4): 040503. doi: 10.7498/aps.61.040503
Passive dynamic walking becomes an important development for walking robots due to its simple structure and high energy efficiency, but it often falls. The key to this problem is to ascertain its stable gaits and basins of attraction. In order to handle the discontinuity, massive numerical computation is unavoidable. In this paper, we first propose an algorithm to compute Poincar maps in heterogeneous platforms with CPU and GPU, which can take the best performance of the newest heterogeneous platforms and improve the computing speed by more than a hundred times. With this algorithm, we study the simplest walking model by sampling massive points from the state space. We obtain high resolution images of the basin of attraction, and reveal its fractal structure. By computing the relation between the stable gaits and their basins and by varying the slop k, we find a new three-period stable gait and a period-doubling route to chaos, and we also study the new gait and its basin.
2012, 61 (4): 040504. doi: 10.7498/aps.61.040504
In this paper, a four-dimensional hyperchaotic system is constructed by adding a state feedback controller into a chaotic system. Numerical simulations and theoretical analysis show that this four-dimensional system will take on periodic,complex periodic, quasi-periodic chaotic hyperchaotic dynamical behaviours as parameters vary. Moreover, an electronic circuit diagram is designed for demonstrating the existence of the hyperchaos. Finally, an adaptive tracking controller is proposed to acquire adaptive tracking control for this hyperchaotic system.
2012, 61 (4): 040505. doi: 10.7498/aps.61.040505
A joint of image encryption and compression is investigated, and a novel joint encryption and compression scheme is proposed. In our scheme, the encryption process is performed before compression. Making use of the properties of discrete wavelet transform (DWT) and Set Partitioning in Hierarchical Trees (SPIHT) coding, confusion is restricted to the interior of single subband image itself and so image details are retained. Furthermore, diffusion preserves the two most significant bits (MSBs) and the sign bit, which contain important information about the image. The experiments show that the proposed algorithm possesses a good visual quality of the reconstructed image and a high encryption speed.
2012, 61 (4): 040506. doi: 10.7498/aps.61.040506
A prediction algorithm of multivariable chaotic time series is proposed based on optimized extreme learning machine (ELM). In this algorithm, a presented composite chaos system and mutative scale chaos method are utilized first to search and optimize the parameters of ELM for improving the generalization performance. Then the optimized ELM is used to predict the multivariable chaotic time series of Rossler coupled system for single step and muti-step, and the scheme is compared with the congeneric method, which shows the validity and stronger ability against noise of the developed algorithm. Finally, the relation between prediction result and number of hidden neurons is discussed.
2012, 61 (4): 040507. doi: 10.7498/aps.61.040507
A class of the characteristic collective dynamic behaviors, i.e., the frozen random patterns, in a globally coupled both-discontinuous-and-non-invertible-map lattices are studied. The coupling-strength dependences of the mean order parameters and the largest Lyapunov exponents are calculated and analyzed. The result shows that, given the initial values for the dynamical variables, the system will reach its complete or partial synchronization state when the coupling strength is beyond some critical value, where the frozen random pattern appears. These phenomena reveal that there are coexisting attractors in the system, and thus the structure and the distribution of the frozen random patterns sensitively depend on the choice of the initial dynamics variables. The interesting event is that the system can be modulated to some regular states of motion by the coupling among lattices even when the single maps are in the chaotic states, which may have some important applications in controlling chaos. The rich dynamical behaviors mentioned above are due to the interplay between the discontinuity and the non-invertibility in the map.
Reseach on accommodation parameter of linear bridging relation for blunt cone in transitional regime
2012, 61 (4): 040508. doi: 10.7498/aps.61.040508
Due to the complex flow in the transitional regime and the nonlinearity of the flow control functions, when a launch verhicle is designed, the aerodynamic heating is predicted mainly by engineering method. The accommodation parameter of linear bridging relation which is used to calculate the heat flux in transitional regime is fixed to fit the blunt cone which is widely used. The results of the bridging relation are validated by using the DSMC method. The validation shows that the accommodation parameter suited for the blunt cone and ensuring the linear bridging relation may be used to calculate the heat flux of blunt cone in a transitional regime.
2012, 61 (4): 040701. doi: 10.7498/aps.61.040701
A three-dimensional (3D) optical model is developed for small color pixels in color filter liquid-crystal-on-silicon (CF-LCOS) microdisplays. The 3D optical model includes an LC electromechanical analysis of color LC cells, a calculation of optical reflectance using the extended Jones matrix, and a standard RGB (sRGB) representation of the optical reflectance in the pixel array. The simulated optical reflectance accords well with the experimental result.
2012, 61 (4): 040702. doi: 10.7498/aps.61.040702
The high-pressure tungsten carbide (WC) radius-anvil is analyzed and studied based on the finite element method (FEM). The results indicat that under the same transfer efficiency of pressure, the lifetime of high-pressure WC radius-anvil is longer than that of the traditional anvil, which can be enhanced about 3.05%16.75%. The highest sample cell pressure generation by the new design of high-pressure WC radius-anvil increases about 5% (from 5.80 GPa to 6.09 GPa) compared with that by the traditional anvil, which can be attributed to the technology of radius-bevel. The high-pressure WC radius-anvil will be indeed very useful to broaden the synthetic region of functional materials. Further more, in this work, the operational costs of cubic high-pressure apparatus is reduced and the cubic anvil type high pressure techniques is improved in many important aspects.
2012, 61 (4): 040703. doi: 10.7498/aps.61.040703
We present the generation of terahertz pulses from a bulk GaP emitter fabricated with a micro-pyramid anti-reflection output coupling layer. The anti-reflection layer is designed by the graded effective refractive index principle. The micro-pyramid structure is manufactured by micro fabrication technique. The experimental results demonstrate that the micro structure layer can not only increase the output coupling efficiency but also scatter the transmitted pump power.
THE PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
2012, 61 (4): 041201. doi: 10.7498/aps.61.041201
In this paper, we investigate the joint production of charged top-pions pair and Z gauge boson in the topcolor-assisted technicolor (TC2) at the International Linear Collider (ILC) via the process e+e- → Zπt+πt-. We calculate the production cross section of the process e+e- → Zπt+πt- and find that the cross section of the process e+e- → Zπt+πt- can reach 1 fb in a reasonable parameter space of the TC2 model. Considering the main decay mode of charged top-pion πt+ → tb, we find that such a production may provide copious events at the ILC.
2012, 61 (4): 042501. doi: 10.7498/aps.61.042501
In order to further study the radiation of the relativistic electron beam-ion channel experimentally and theoretically, the propagation of a relativistic electron beam in neutral gas and its self-focusing process are investigated. Particle in cell (PIC) simulation shows that the electron beam can self-focus and transmit the dynamically loaded plasma through impact ionization. The transverse and the longitude inhomogeneities of the ion background have significant effects on the transport properties of the electron beam. Base on these researches, a model of transmission of electron beam in a transverse non-uniform ion background is supposed. And the condition of self-focus is given. The numerical results show that the transverse inhomogeneity will lead to the mixed phase transmission of the electron beam, and the inner electrons can defocus near the focus point, which is consistent with the PIC simulation. The PIC simulation also shows that due to the self-focusing of the electron beam, there are much more ions to be ionized at the focus point, which will capture the lower-energy electrons after collision, the capture electron effect will significantly reduce the efficiency of the transmission of the electron beam. But the distribution of the captured electrons in the longitude direction is quasi-periodic, which acts as the electrostatic Wiggler field. These may achieve the dynamical loading of the electrostatic Wiggler field. These results give new clues to the further study of electron beam-plasma system in experiment and the establishment of theoretical models.
ATOMIC AND MOLECULAR PHYSICS
2012, 61 (4): 043101. doi: 10.7498/aps.61.043101
Using the time-dependent multilevel approach (TDMA) and combining the B-spline expansion technique and the model potenial method of atom, the properties of Rydberg sodium atom are investigated. The energy level structure of high excited states and population transfer of sodium atom in a microwave field are studied by numerical calculation, and quantum states are controlled and manipulated by microwave field. The results show that the method we used can be applied to the investigation of the properties of alkali-metal atoms in external field, that the population is very sensitive to the parameters of chirped rate and field amplitude, and that the population can be completely transferred into the target state and trapped there by changing the chirped rate and field amplitude.
2012, 61 (4): 043102. doi: 10.7498/aps.61.043102
The geometric configurations, electronic structures, vibrational frequencies, and average binding energies of NaBn(n=19) clusters are studied using the B3LYP DFT method at 6-311+G(d) level. The stabilities of the ground states of NaBn(n=19) clusters are analysized by means of energy gaps, and secondary energy differences between the ground state structures. And the polarizations are studied.
2012, 61 (4): 043103. doi: 10.7498/aps.61.043103
For developing novel high strength materials, we study the crystal structures, electronic properties and mechanical properties of hcp-C3 carbon bulk rings based on the first principles method. Electronic structure analysis shows that the viable sp3 hybridization appears in this carbon allotrope. It caused the mechanical properties of the carbon bulk ring to exhibit strong orientation dependence. Along the  orientation, the carbon bulk ring demonstrates an elastic modulus of 1033 GPa, a Tensile strength of 124.17 GPa, and a compressive strength of 381.83 GPa. Furthermore, along the  orientation, the compressive strength reaches 458.34 GPa, which shows the excellent mechanical properties of hcp-C3 carbon bulk rings.
2012, 61 (4): 043104. doi: 10.7498/aps.61.043104
In order to find how the external electric field affects the SiN molecule completely, in the present work the density functional B3P86/6-31(g) method is adopted to optimize the ground state structure and the time dependent density functional theory TDDFT/6-31(g) is used to study the absorption spectra, emission spectra, excited energies, oscillator strengths and dipole moments of SiN molecule under different external electric fields. We find that the absorption spectrum, excited energy, oscillator strength and dipole moment of SiN molecule are affected strongly by external electric field. One of the valuable results is that the absorption spectra in violet light and blue wavelength ranges of SiN molecule each have a red shift. The luminescence mechanism of visible light for SiN molecule is also investigated and compared with the experimental data.
2012, 61 (4): 043105. doi: 10.7498/aps.61.043105
Using the density functional theory(B3LYP) method, the 6-311++G(3df,2pd), AUG-cc-PVTZ, AUG-cc-PVQZ basis sets for H and effective core potentials for Y, the energies, equilibrium structure and harmonic frequency of the ground states of YH(D,T) molecules are calculated. Based on the theory of atomic and molecular reaction statics, the reasonable dissociation limits of the ground states of YH(D,T) molecules are derived. By comparing the calculation results with the existing experimental and theoretical values, we find that the mixed basis sets LANL2TZ/AUG-cc-PVQZ are most suited for the calculation of the molecules. Consequently, the potential energy surfaces of the ground states of YH(D,T) molecules are scanned at the B3LYP/LANL2TZ/AUG-cc-PVQZ level of theory. The potential energy curves of the ground states of YH(D,T) molecules are obtained by the least square fitting to the Murrell-Sorbie potential energy function. The spectroscopic constants (Be, e, e, ee, De) and force constants ((f2, f3, f4)are calculated and compared with experimental results, indicating that the calculation results are in good agreement with the experimental data.
2012, 61 (4): 043201. doi: 10.7498/aps.61.043201
The ratio of radiation energy density to matter energy density is an important parameter to distinguish the characteristic of atomic processes in plasma. Actoring to this parameter, atomic processes in plasma can be divided into two typical categories: collision-dominated and radiation-dominated. According to numerical simulation, atomic processes of these two categories have different characteristics. The LTE state can be quickly reached in the collision-dominated plasma. However in the radiation-dominated plasma, the temperature of bound electrons, the ionization degree, and the temperature of free electrons have different relaxation time scales. There is some kind of quasi-LTE state.
Experimentally studying the scheme on exciting coherent population trapping resonances with lin//lin configuration
2012, 61 (4): 043202. doi: 10.7498/aps.61.043202
Coherent population trapping (CPT) atomic clocks are prevailingly realized by exciting CPT resonance with circularly polarized bichromatic coherent light (σ+-σ+ scheme), while atom utilization efficiency is relatively low in the scheme. For improving efficiency, we experimentally study the scheme of realizing CPT resonance through the interaction between atoms and parallel linearly polarized bichromatic coherent light field (lin//lin scheme). In the same experimental conditions the experimental results show that the signal-noise ratio of the CPT resonance signal is twice, and the differential slope is 1.65 time higher than those of σ+-σ+ scheme. The experimental results reveal that the scheme is an ideal candidate for low power consuming atomic clock and magnetometer.
2012, 61 (4): 043203. doi: 10.7498/aps.61.043203
In this paper, the dynamic properties and the spontaneous emission spectrum of V-type three-level atom near left-handed material slab are investigated. The two orthogonal dipoles of the decay from the two upper levels to the lower level are considered. Due to the focusing and the phase compensation effects of the left-handed materials, the indirect quantum interference between two orthogonal spontaneous emission transitions leads to the fact that the populations in the two upper levels influence each other, the decay rate is enhanced or decreased and the spontaneous emission spectrum is narrowed or widened, depending on the initial atomic state.
2012, 61 (4): 043204. doi: 10.7498/aps.61.043204
With the revised UCL wave code, the reactance matrix can be calculated when an electron impacts on an ion, so that the scattering matrix and the collision strength are calculated. With the collision strength, the electron broadening of the resonance line is studied. Specifically, the widths and the shifts of resonance lines emitted from Ar+17 and Ar+16 are calculated under the different values of electron temperature and electron density.
Ultracold 70S Cs Rydberg atoms are obtained by two-photon excitation in a magneto-optical trap, and detected by using the state-selective pulse field ionization technique. The evolution of 70S Rydberg atoms is investigated by changing the delay time between excitation laser and ionization electric field and the excitation duration. Blackbody radiation is taken into account to explain experimental result. The experimental result is found to be consistent with theoretical value.
2012, 61 (4): 043206. doi: 10.7498/aps.61.043206
In this paper, the Raman scattering spectra of cold atomic gas in different environments are closely calculated. Comparing the spectra from the atoms in free and trapped environments, the calculation result gives a right spectrum structure for the free atoms and reveals a distinct high-gain spectrum exhibiting a comb-like structure for the harmonic trapped atoms. The spectral peaks of the trapped spectrum are uniformly distributed and the distance between adjacent spectral peaks exactly equals the frequency of the trap. The nonlinear gain scheme and the gain conditions of the Raman scattering process are clearly given in this paper.
Theoretical study of the P-branch spectral lines in d1+b1+ electronic state transition of NbN molecule
2012, 61 (4): 043301. doi: 10.7498/aps.61.043301
The accurate P-branch spectral lines of high-lying rotational quantum states of (1,1) band in d1+b1+ electronic state transition of NbN molecule are obtained in this work using the analytical formula proposed in Sun's previous work. The formula not only reproduces all known experimental spectral lines excellently, but also generates the correct values of the unknown spectral lines up to J=80 that are unavailable experimentally for this band.
A density functional method DFT/B3LYP with SDD basis for Pu and 6-311+G* basis for O is used to study HOMO energy level, LUMO energy level, energy gap, and harmonic frequency of PuO ground state molecule under different inner radiation fields ranging from -0.005 to 0.005 a.u.. The results show that the magnitude and the direction of the electric field have important effects on these characteristics of PuO molecule. The HOMO energy level is found to decrease, but the LUMO energy level, energy gap and Fermi energy level are found to increase with the increase of electric field. The electron which occupies orbital is difficult to stimulate to empty orbital and transform into excited state. The PuO molecule is more stable in inner radiation field, and it can prevent O2, H2 and so on from proliferating to superficial inner layer and corroding the plutonium surface, which contributes to the plutonium corrosion prevention in inner radiation field.
2012, 61 (4): 043303. doi: 10.7498/aps.61.043303
By adjusting the polarized angle of a two-color laser field that is synthesized by a 12 fs/2000 nm and 12 fs/800 nm, we find that the spectrum of the high-order harmonic generation has a “funnel structure” in the second plateau when θ is π/2. And when θ is π/6, a bandwidth of supercontinuum about 290 eV is observed. We illustrate the structure of the high-order harmonic generated spectrum by the time-frequency analysis. An isolated attosecond pulse is obtained by intercepting an arbitrary 70 eV width in the supercontinuum plateau, and whose full width at half maximum (FWHM) is about 60 as. When we intercept a narrow width of the bandwidth of supercontinuum, a linear polarized attosecond pulse with FWHM 94 as is obtained. All those are convenient to generate and use attosecond pulse in experiment.
The theoretical calculation of (e,2e) triple differential cross sections of Ag+ (4p,4d) in coplanar asymmetric geometry
2012, 61 (4): 043401. doi: 10.7498/aps.61.043401
The three-body distorted-wave Born approximation is used to calculate the (e,2e) triple differential cross sections (TDCSs) of Ag+(4p10) and Ag+(4d10) in different kinematical variables in coplanar asymmetric geometry. The angles 4, 10 and 20 are selected as the scattering electron angles. We find that the position of binary peak or the dip between split peaks are not in the direction of momentum transfer, which is probably ascribed to one kind of double-binary collision. We also find that the binary peaks show abnormal splits for Ag+(4p10). Such abnormal splits indicate that an (e,2e) process for inner valence orbital of ionic target becomes more complicated than for outer valence orbital. Furthermore, beside the binary peak and the recoil peak, some pronounced peaks appear at certain ejected angles in the (e,2e) TDCSs of Ag+(4p10) and Ag+(4d10). We consider that these pronounced peaks are probably related to one kind of double-binary collision.
Momentum transfer dependence of the triple differential cross section for electron impact ionization of helium at incident energy of 150 eV
2012, 61 (4): 043402. doi: 10.7498/aps.61.043402
The momentum transfer dependence of the cross section for electron impact ionization of helium at an incident energy of 150 eV is investigated by using the BBK model and the DS3C model. The results of the present work are compared with experimental data. The structure of the cross section is analyzed, and the exchange effect is discussed.
2012, 61 (4): 043403. doi: 10.7498/aps.61.043403
The momentum transfer dependence of the cross section for electron impact ionization of helium for (e,2e) process is investigated by use of BBK model and modified BBK model. The results of the present work are compared with experimental data. The structure of the cross section is analysed, and the effective shield is discussed in detail in the final state.
2012, 61 (4): 043404. doi: 10.7498/aps.61.043404
In the edge plasma, hydrogen and its isotopic molecule will be ionized via collisions with the energetic electrons in the plasmas. The cross section data for the electron impact non-dissociative ion of the hydrogen and its isotopic molecule, which are unavailable from the literature are calculated on the basis of Morse potential function, Franck-Condon theory and semi-classical Gryzinski method. The change of cross section with electron energy and the effect of vibrational level of ground state on the cross section are presented in the paper. The results show that the vibrational excitation plays an important role in the process of ion.
Calculation study of the geometry structures and electronic properties of heterofullerenes C19M(M=Cr,Mo,W)
2012, 61 (4): 043601. doi: 10.7498/aps.61.043601
The possible geometrical structures of C19M(M=Cr,Mo,W) molecules are optimized by using the density functional theory (B3LYP) at the LANL2DZ level. For the ground state structures of C19M(M=Cr, Mo, W) clusters, the physical and the chemical properties are studied. The results show that the kinetic stabilities of the C19M clusters with different M atoms are almost the same. Theis thermodynamic stabilities are obviously increased with the increase of atomic number. It can be found from the frontier orbital that the M atoms have the effects on the orbits more or less. M atom contribution to the orbits roughly increases with M atomic number increasing. A great many of positive charges accumulate on the M atoms in C19M clusters. Their aromaticity decreases with the increase of atomic number.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2012, 61 (4): 044201. doi: 10.7498/aps.61.044201
According to the relationship between the first-order Doppler of HF radar and the wind direction over the sea surface, a new method of predicting wind direction is proposed with considering the combination of neural network and the method of the beam sampling. Using the simulated data under different input and output parameters of neural network, the wind direction is predicted when the expansion factor is odd number, the combination of the neural network and the beam sampling is utilized to predict the wind direction when the expansion factor is even number, which can eliminate the fuzziness of the wind direction. The predicted result shows a good agreement with the simulated data. From the predicted results by the neural network and the beam sampling, it is find that the error of the wind direction is about 4-6 and the mean error of the expansion factor of the wind direction is 0.26, which provides a new idea and method of predicting the wind field over the sea surface.
Electromagnetically induced negative refraction induced by microwave field driving hyperfine levels transition
2012, 61 (4): 044202. doi: 10.7498/aps.61.044202
Electromagnetically induced negative refraction induced by microwave field driving hyperfine level transition is studied by proposing a quasi-lambda type four-level system. Negative refraction characteristics are observed when hyperfine levels within ground state are driven by a microwave field which interacts with electric dipole moment or magnetic dipole moment at different hyperfine levels. In addition, two optical transitions between ground state and excited state are driven by two coupling fields respectively, and the frequency bandwidth of negative refraction is controlled by the detuning of two coupling fields. It is shown that frequency bandwidth of negative refraction is much narrower under off-resonant coupling field than under the resonant coupling field and there is a significant difference in behavior between positively detuned coupling field and negatively detuned coupling field.
2012, 61 (4): 044203. doi: 10.7498/aps.61.044203
The anti-Stokes frequency conversion based on four-wave mixing (FWM) has been widely used to generate short-wavelength radiation for high resolution imaging, direct excitation of electronic molecular transitions, and so on. For achieving more effective anti-Stokes conversion, we use the Ti: sapphire laser with a central wavelength of 810 nm and a pulse width of 120 fs as a pump source, and the degenerated FWMs of the higher mode and the fundamental mode are achieved respectively in 0.5 m long and 3 m long photonic crystal fibers (PCFs) with a zero dispersion wavelength of fundamental mode around 820nm in our experiment. The anti-Stokes signals around 560nm are generated efficiently at the fundamental phase matching. The maximum power ratios of anti-Stokes signal at 562 nm to the residual pump component and the Stokes signal are above 33:1 and 25:1, respectively. The maximum conversion efficiencies are achieved to be up to 48% and 34% in theory and experiment, respectively. And then the variation laws of the phase matching and the output spectrum with pump power, wavelength and the fiber length are obtained and the discrepancy between theoretical and experimental results is analyzed. Moreover, the effects of more factors on experimental results are discussed.
Time-resolved spectrum characteristics of instantaneous plasma generation and evolution processes in nanosecond laser-Cu-target
2012, 61 (4): 044204. doi: 10.7498/aps.61.044204
The physical processes in the generation and the evolution of copper plasma induced by a high power nanosecond laser pulse are investigated with time-resolved spectrum. In the solid-gas-liquid phase transition, the intensity of reflected beam evolving with time markedly reveals the absorption of laser energy in the laser-copper interacting zone. So the phase transitions on the surface of copper target result in the fact that intensity of reflected beam evolving with time obviously presents a double-peak structure. Meanwhile, the appearance times of the plasma-gas-liquid phase transition on the target surface is advanced with the increase of peak-power of laser pulse. Therefore, these transient properties can be an efficient approach to diagnosing the phase transitions of copper target when the laser irradiates the target surface.
Numerical simulation and experimental study of thermal-induced-depolarization in 2 m Cr,Tm,Ho:YAG laser
2012, 61 (4): 044205. doi: 10.7498/aps.61.044205
The theory of thermal-induced-depolarization is analyzed, the distribution of depolarization is numerically simulated, and the result is verified by the polarized light interferometry. The experimental result is coincident with the numerical simulation, which shows that the distribution of depolarization in the cross section of Cr,Tm,Ho:YAG is crisscross, and increases with the pump energy increasing. The direction of the worst depolarization is 45 with respect to the polarization direction of the polarizer. According to the numerical simulation, we employ a /4-plate to compensate the depolarization of a high-energy Cr,Tm,Ho:YAG laser. The pulse energy increases by 24% after compensation.
A high power photonic crystal fiber laser oscillator based on nonlinear polarization rotation mode-locking
2012, 61 (4): 044206. doi: 10.7498/aps.61.044206
A high power femtosecond fiber laser based on a Yb-doped large mode area photonic crystal fiber operating in an all-normal dispersion regime is experimentally demonstrated. A ring laser cavity is used without any elements for dispersion compensation. Stable mode-locking is achieved through nonlinear polarization rotation and the dissipative function of a filter. The laser directly outputs 1.03 ps chirped pulses at a 49.09 MHz repetition rate for an average power of 10 W, corresponding to a pulse energy of 202 nJ. The pulses are compressed to 95.5 fs with a grating pair outside the cavity.
Multi-beam laser heterodyne measurement with ultra-precision for the glass thickness based on oscillating mirror sinusoidal modulation
2012, 61 (4): 044207. doi: 10.7498/aps.61.044207
In this paper, We propose a novel method of measuring glass thickness by means of multi-beam laser heterodyne. Based on the Doppler effect and the heterodyne technology, loading the information about the glass thickness into the frequency difference of the multi-beam laser heterodyne signals by sinusoidal frequency modulation of the oscillating mirror that is in a simple harmonic motion, we can simultaneously obtain many values of the glass thickness through the multi-beam laser heterodyne signal demodulation. Processing these values by weighted-average, the glass thickness can be obtained accurately. This novel method is used to simulate the measurements for different glass thicknesses by MATLAB, the obtained result shows that the relative measurement error obtained by using this method is just 0.008%.
2012, 61 (4): 044208. doi: 10.7498/aps.61.044208
Based on the Strehl criterion of coherent lens imaging system, the focal depth (FD) of lensless Fourier digital micro-holographic system is deduced. The FD expressions of the holographic systems with symmetrical and unsymmetrical reference point sources are obtained. The results show that the FD of lensless Fourier digital micro-holographic system is dependent not only on the optical wavelength and the numerical aperture of the recording system but also on the offset of the reference optical source, which is different from the FD of lensless Fourier digital micro-holographic system. The validities of the theoretical analysis are demonstrated by the computer simulations and experiments.
2012, 61 (4): 044501. doi: 10.7498/aps.61.044501
We study solitary wave propagation in periodic dimer granular chains of beads with the same material but different sizes by binary collision approximation. This kind of chain which is called N:1 dimer consists of pairs of N big beads and one small bead. First we present a method to map the actual chain into an effective chain, then use the binary collision approximation to obtain the transmitted solitary wave speed, the total time taken by the pulse to pass through the chain, and the frequency of oscillation of the small particle. Frequency of oscillation, which increases with the decrease of the radius of the small particle, is analytically obtained. And the results are in excellent agreement with numerical results. For the total time of the pulse passing through the chain, the results of theoretical analysis is in good agreement with numerical results when N 2. The relative error seems no change with the chain length but becomes larger with the increase of the value of N.
2012, 61 (4): 044701. doi: 10.7498/aps.61.044701
A dynamic simulation of fiber reinforced composite mold filling process in a complex cavity is presented based on the gas-solid-liquid three-phase model for mold filling and the finite volume method on non-staggered grids. The interface evolution and the information about physical quantities, such as velocity, stresses, pressure, etc, are given. The motions of fibers, including transformation and orientation, are obtained as well. Numerical results show that fiber orientation in the horizontal mid-plane is related to the shape of the cavity, which is quite different from the case of the skin-core-skin structure orientation along cavity thickness. In the inlet region, fibers encircle the cavity inlet, and fiber orientation is vertical to the incoming flow direction along the horizontal and the vertical regions of the cavity, while around the corners of the cavity, fibers point to the corners of the cavity.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2012, 61 (4): 045201. doi: 10.7498/aps.61.045201
In this paper, We measure the plasma parameters using a single probe to study the characteristics of our designed magnetic control direct current glow discharge experimental device. We can obtain the relationship between the electron density and pressure, the electron density distribution and the magnetic field type, the electron density distribution, the magnetic field strength, etc. In addition, magnetic field generated by coils is calculated by the finite element method , through which the distributions of two magnetic fields with different connection modes are simulated. In experiment the states of the plasma are controled partly by these two different types of magnetic field position, and this “control” role is consistent with existing theory.
Theory and simulation of laser pulse trapping and amplifying in the interaction with a thin foil and a solid target
2012, 61 (4): 045202. doi: 10.7498/aps.61.045202
In this paper we suggest a simple structure consisting of a thin foil and a solid target with specified gap width to achieve the laser-field amplification, and study the trapping process of a single pulse. Both the theoretical and simulation results show that the EM fields are trapped as standing waves between two layers of enhanced electron density. The laser energy can be accumulated like the accumulation of charges in capacitor, a stably trap is reached after several oscillations. The compressing and the expanding of the electron layer, generated by the incident light pressure, the trapped light pressure and the charge electrostatic fields, are the direct reason for the energy oscillations.
2012, 61 (4): 045203. doi: 10.7498/aps.61.045203
A strong pressure increase is produced by X-ray heating a foam-solid target in comparison with heating directly the solid target, due to impedance mismatch at the foam-solid interface. We study the pressure increase of CH foam-aluminum target, and find that only by shock present in foam can the ablative pressure in aluminum increase. In this paper an analytical model based on strong wave assumption is developed to study the shock impedance mismatching crossing the interface and the interaction of rarefaction with shock. We evaluate the shock pressure amplified in the aluminum layer as a function of density and adiabatic exponent, and show that the amplified pressure is a short high pressure pulse which is destroyed by rarefactions. This is in agreement with simulation results by MULTI code.
A three-dimensional particle-in-cell/Monte Carlo computer simulation based on negative hydrogen ion source
2012, 61 (4): 045204. doi: 10.7498/aps.61.045204
Based on the analysis of the plasma physics mechanism in negative hydrogen ion source, the particle-in-cell algorithm is studied and optimized and a high efficient storage method of particles is designed. Using the Monte Carlo collision model, considering the plasma potential and coulomb collisions between charged particles, the full three-dimensional particle-in-cell/ Monte Carlo algorithm (PIC/MCC) is developed. With the magnetic charge model, using the FDTD method, the line cusp magnetic field is calculated. With the negative hydrogen ion source JT-60U and considering the main reactions in the negative hydrogen ion source, the full three-dimensional PIC/MCC simulation algorithm is verified by simulation.
2012, 61 (4): 045205. doi: 10.7498/aps.61.045205
In order to investigate the mechanism of dielectric barrier atmospheric pressure glow discharge(APGD) in Ar, an one-dimensional multiple particle self-consistent coupled fluid model is proposed. And the finite-element method (FEM) is used in the numerical calculation model, so the periodic evolvement waveforms of gas voltage, barrier surface charge density and discharge current density are investigated. The spatio temporal distributions of electrons, ions, metastable particles density and space electrical field are also obtained. The simulation results show that the charges accumulated on the barrier dielectric surface play an important role in ignition and extinguishment of the discharge. With the increase of applied voltage amplitude, the DBD breakdown occurs ahead of time, and discharge current density and the surface charge density increase gradually, which indicate that the discharge process becomes fierce. Furthermore, with the increase of relative permittivity of dielectric material, the discharge current density also gradually increases. The spatio temporal distributions of the particles density and the space electrical field show that the DBD breakdown occurs every half the AC period and the discharge under conditions considered in this model is a typical atmospheric pressure glow discharge(APGD), having an obvious cathode fall region, a negative glow region, and a positive column region.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2012, 61 (4): 046101. doi: 10.7498/aps.61.046101
The cohesive energies, Fermi energies and local density of states (LDOS) are calculated by the first-principles based on pseudopotential plane wave method in this paper to investigate the physical nature of corrosion of Pb-Mg-Al alloy. The mechanism of electrochemical corrosion is analyzed according to the calculated electronic structure parameters. The results show that the stable phase in Pb-Mg-Al alloy is Mg17Al12>Mg2Pb>Mg. The Fermi energy (Ef) values of these phases with Ef(Mg)>Ef(Mg2Pb)>Ef (Mg17Al12) indicate that Mg is most likely to lose electrons while Mg17Al12 is difficult. LDOS result reveals that Mg and Mg2Pb phases are unstable compared with Mg17Al12 in the same external conditions, they are more likely to lose electrons and easier to corrod. The difference in Fermi energy between different phases in Pb-Mg-Al alloy forms the electrodynamic force of the electrochemical corrosion, which leads electrons to flow from the Mg and Mg2Pb phases with higher Fermi energy to Mg17Al12 phase with lower Fermi energy, further to corrode in Pb-Mg-Al alloy.
2012, 61 (4): 046102. doi: 10.7498/aps.61.046102
Metallic glasses, quasicrystals and many intermetallic compounds belong to complex alloy phases which are characterized by specific nearest-neighbor coordination polyhedral clusters representing the local atomic features of their parent phases. However, the nearest neighbor atoms are often distributed on multiple shells, which makes the cluster difficult to define exactly. For an alloy phase dominated by the clusters, there showed appear distinct differences between the cluster part and the average structure of the phase, especially with regards to the atomic density. The clusters should have the highest atomic density so that their presence is very pronounced in phase compared with the average structure. In this paper, radial distribution of atomic density (numbers of atoms contained in different spherical shells) is proposed to define the cluster in a clear manner. The spherical shell enclosing the highest atomic density and having dense-packed triangular facets is selected as the nearest-neighbor cluster. Al-Ni-Zr alloy phases are taken for example to illustrate the easy use of this method, and the relationship between cluster and metallic glass formation is also explained.
2012, 61 (4): 046103. doi: 10.7498/aps.61.046103
BaTiO3 is a kind of perovskite ferroelectric which has the advantages of ferroelectric property, piezoelectric property and radiation resistance. BaTiO3 thin films and devices have important applications in strong irradiation environment. The structure damage, especially the oxygen vacancy has a crucial influence on the response of ferroelectric under radiation. Molecular dynamics is used to simulate the formation process and the recovery process of defects in BaTiO3 under the impact of primary knock-on atom (PKA). The results show that the initial motion direction and energy of PKA have significant effects on the number of defects, and the averaged threshold displacement energies of Ba, O and Ti atom are 69 eV, 51 eV and 123 eV respectively. The calculated displacement energy is obviously larger than default value (25 eV) in SRIM code. Furthermore the SRIM code is used to simulate the proton irradiation damage in BaTiO3 thin film. The results show that the number of vacancy increases with the increase of proton energy, but the increase rate decreases, and the number of vacancy decreases obviously with the increase of incidence angle when it is more than 60°.
2012, 61 (4): 046104. doi: 10.7498/aps.61.046104
Stability of hydrogen (H) in tungsten (W) with carbon (C) impurity is investigated by using the first-principles method. In intrinsic W, C exhibits a week attractive interaction with H at a distance of ～2.5Å, and it is the most stable site of H in the bulk W with C. In the presence of the monovacancy in W, H prefers to bind onto an isosurface of the same charge density of 0.10 Å-3, due to the existence of C. Our research finds that the monovacancy in W can contain only ten H atoms and H molecule cannot be formed in comparison with the result without C, suggesting a strong effect of C on H stability in W. When two C atoms exit in the vacancy, the charge density of the isosurface that H binds onto is 0.13 Å-3.
2012, 61 (4): 046105. doi: 10.7498/aps.61.046105
Ion irradiation of semiconductors is a well understood method to tune the carrier concentration in a controlled manner. We show that the ferromagnetism in Ga0.94Mn0.06As films, known to be hole-mediated, can be modified by He ion irradiation. The coercivity can be increased by more than three times. The magnetization, Curie temperature and the saturation field along the out-of-plane hard axis all decrease as the fluence increases. The electrical and structural characterization of the irradiated Ga0.94Mn0.06As layers indicates that the controlled amending of magnetism results from a compensation of holes by generated electrical defects and not from a structural modification.
First-principles investigations on the electronic, elastic and thermodynamic properties of Cr2MC(M=Al, Ga)
2012, 61 (4): 046301. doi: 10.7498/aps.61.046301
We investigate the electronic, elastic and thermodynamic properties of nanolaminate Cr2MC(M=Al, Ga) by using the ab initio pseudopotential total energy method. Our results show that they have shown almost identical volume compressibilities. The axial compressibility investigations show that the c axis is always stiffer than a axis. The internal coordinate calculations revealed that the values of Cr atoms in Cr2AlC are always larger than those in Cr2GaC. The elastic constants calculations demonstrated the structural stability within 050 GPa. The obtained bulk moduli by quasi-harmonic Debye model observed that the bulk moduli of Cr2MC(M=Al, Ga)decrease with temperature at 0 GPa, but increase at 300 GPa. We also found that the Debye temperatures of Cr2GaC are always smaller than those of Cr2AlC at any conditions. However, the opposite cases can be found in thermal expansion coefficients, Grneisen parameter, entropy and heat capacity when comparing their respective counterparts between Cr2GaC and Cr2AlC. The electronic density of states and energy band distribution demonstrated that the Cr2MC(M=Al, Ga) have shown similar profiles with the only exception of the more localized s and p electrons in Cr2GaC than their respective counterparts in Cr2AlC.
Experimental study of thermal conductivity of polyethylene nanowire arrays fabricated by the nanoporous template wetting technique
2012, 61 (4): 046501. doi: 10.7498/aps.61.046501
We fabricate low-density polyethylene (LDPE) nanowire array with a diameter of 200 nm by using a nanoporous template wetting technique, and the thermal conductivity at 2080℃ is experimentally studied by a nanosecond laser flash method. The measured thermal conductivity of the fabricated nanowire array is about 2.2 W/mK at room temperature, which is about one order of magnitude higher than its bulk counterpart. The thermal conductivity is found to increase slightly with the increase of temperature. The estimated thermal conductivity of a single LDPE nanowire is as high as 5 W/mK at room temperature. The high orientation of chain of the LDPE nanowire may arise from the integrative effects of shear rate, vibrational perturbation, translocation, nanoconfinement and crystallization. Findings in this study provide a useful strategy for enhancing the intrinsic thermal properties of polymer nanostructures.
2012, 61 (4): 046801. doi: 10.7498/aps.61.046801
Intermolecular attractive forces lead to the adhesion problem in M/NEMS. The Van der Waals formula for the interaction between macroscopic objects can be used only in the situation with no deformation. As to the adhesive contact between elastic bodies it is still unknown how the attractive force contributes to the normal force on the interface. In this paper large-scale molecular dynamics simulation is performed to study the adhesive contact between a rigid spherical tip and an elastic flat substrate. We study the effect of atomic-scale surface roughness on the adhesive properties, including pull-off force between tips and substrate, the variation of adhesive force with applied load, and the distribution of contact stress. The results show that the adhesive force varies linearly with the applied load for the atomic-scale smooth contact. But for the atomic-scale rough contact the variation of adhesive force with applied load can be divided into two phases, which are distinguished by different increasing slops. Compared with the smooth contact, the rough contact has a small pull-off force, but exhibits a large adhesive force during the contacting process. Our simulations indicate that the pull-off force cannot characterize the contribution of attractive interaction to the normal force on the interface in the case of an elastic adhesion contacting.
2012, 61 (4): 046802. doi: 10.7498/aps.61.046802
In this study, CdTe(111) thin films were epitaxially grown on freshly cleaved BaF2 substrate using molecular beam epitaxy (MBE). In situ characterization of reflection high energy electron diffraction (RHEED) reveals the growth mode of transition from 2D to 3D. XRD analysis results verify the single crystalline property of the as-grown films. Theoretical method is adopted to fit the measured near infrared transmission spectrum, revealing a CdTe energy gap of 1.511 eV at room temperature.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2012, 61 (4): 047101. doi: 10.7498/aps.61.047101
The adsorption of atomic oxygen on the Nb (110) surface is systematically investigated through the first-principles method for oxygen coverage ranging from 0.25 to 1 monolayer (ML)．It is found that the hollow site is the most energetically favorable for the whole coverage range considered and that the long-bridge site takes the second place．The work function increases almost linearly with the increase of oxygen coverage for the long-bridge site adsorption, whereas for the hollow site adsorption the work function decreases when the coverage is 0.25 ML or 0.5 ML and increases when the coverage is 0.75 ML or 1 ML．Using the planar averaged charge density and the dipole moment change we can explain the complicated change of work function induced by atomic oxygen adsorption．In addition, the interaction between O and Nb is analyzed by the surface atomic structure and electronic density of states．
2012, 61 (4): 047102. doi: 10.7498/aps.61.047102
Using the density-functional theory, the electronic structures of pure and Li-N, Li-2N codoped wurtzite ZnO systems are explored. It is find that Li-N and Li-2N codoped wurtzite ZnO systems each cause the Fermi level to cross the top of the valence band and to form shallow acceptor level, which indicates that p-type ZnO system can be obtained by codoping Li and N. Moreover, the carrier concentration is enhanced in the Li-2N codoped system and this structure is more favorable for the formation of p-type ZnO.
First-principles study of structural stability and electronic properties of rhombohedral and tetragonal PbZr0.5Ti0.5O3
2012, 61 (4): 047103. doi: 10.7498/aps.61.047103
The energetic stability, the structural and the electronic properties of rhombohedral and tetragonal PbZr0.5Ti0.5O3 are systematically investigated by the first-principles plane-wave pseudopotential and the virtual crystal approximation (VCA) based on the density functional theory, within the frameworks of local density approximation (LDA) and generalized gradient approximation (GGA). Our calculation results show that the total energy of the rhombohedral phase is lower than that of the tetragonal phase, which suggests that the rhombohedral structure is more energetically stable than the tetragonal one. Furthermore, the structural parameters calculated in the GGA are well consistent with experimental values. From the analysis of electronic structure, we can find the strong hybridization between Ti/Zr d and O 2p both in two phases. Furthermore the hybridization between Ti-O is stronger than that between Zr-O; there also exists the hybridization between Pb s, d and O 2s, 2p. Moreover, the hybridization between Pb 5d and O 2s in the rhombohedral phase is stronger than that in the tetragonal phase, which indicates that the rhombohedral phase is more stable than the tetragonal phase.
2012, 61 (4): 047104. doi: 10.7498/aps.61.047104
N-doped Cu2O films are deposited at different temperatures by sputtering a CuO target in the mixture of Ar and N2. By the analysis of transmission spectra, it is found that the N-doped Cu2O films are changed into a direct allowed band-gap semiconductor and the optical band gap energy is enlarged to 2.52±0.03 eV for the films deposited at different temperatures. The first-principles calculations indicate that the energy band gap increase by 25%, which is in good agreement with the experimental result. The change from a direct forbidden band-gap transition to a direct allowed band-gap transition can be attributed to the occupation of 2p electrons of N at the top of valence band in the N-doped Cu2O film.
2012, 61 (4): 047105. doi: 10.7498/aps.61.047105
Understanding of the interaction between Nb and interstitial H is helpful for using Nb metallic membrane as H2 purification selective membrane. By first-principles calculations, the site occupation of H in the interstitials of the bcc Nb lattice is studied, and the relation between the site energy and the size of the interstitial is discussed. The interaction between interstitial H and Nb lattice is analyzed and the influence of the electronic structure on the interaction is discussed. The results show that in addition to the influence of the interstitial size on the H solution energy, strong bonding interaction between H-1s and Nb-3d is another important reason for the low H solution energy in Nb lattice. The H diffusion coefficient in Nb metal is evaluated and results show that it is approximately 7.8× 10-9 m2/s at 500 ℃, Which is in agreement with experimental observation.
2012, 61 (4): 047106. doi: 10.7498/aps.61.047106
Using first principle based on the density functional theory, we have studied the the electronic and magnetic properties of zigzag graphene nanoribbons (ZGNRs) doped by gold atoms in divacancy. Our calculations show that edge site is the most stable doping site for gold atom, and the magnetism of ZGNRs is inhibited by the introduction of impurities. However, in the case of large enough doping ratio, the magnetic moment of doped edge restores anomalously. The band structure characteristic of gold doped ZGNRs is sensitive to doping ratio. As the doping ratio increases, Au doped ZGNRs show semiconducting, half-metallic and metallic properties, respectively. Our calculations prove that gold atoms doped ZGNRs could modulate the magnetic and band structure character, guiding the follow-up experiments, and promoting the application of graphene in spintronics.
2012, 61 (4): 047201. doi: 10.7498/aps.61.047201
The Bi and the Te nano-powders are prepared by inert gas protected evaporation-condensation method; the n-type Bi2Te3 bulk materials with nano-layer and twin crystallite sub-structures are then synthesized by mechanical alloying and spark plasma sintering technique. The effects of grain size and microstructure on thermoelectric property are also systematically studied. The SEM and the TEM analyseis show that the nano-layered Bi2Te3 bulk materials with twin crystallite sub-structures could be fabricated by controlling the preparing procedures. The thermoelectric property result shows that the thermal conductivity is lowered compared with that of bulk material of coarse-grained starting powders. The thermal conductivity decreases from 1.80 W/mK to 1.19 W/mK at 423 K, the lattice thermal conductivity decreases from 1.16 W/mK to 0.61 W/mK at 423 K, indicating that the phonon scattering could be enhanced due to the coexistence of nano-layer and twin crystallite sub-structures, leading to reduced phonon thermal conductivity. The dimensionless figure of merit ZT reaches 0.74 at 423 K for bulk materials sintered at 693 K.
2012, 61 (4): 047202. doi: 10.7498/aps.61.047202
A new kind of electric-double-layer indium-tin-oxide (ITO) thin-film transistor (TFT) is fabricated on a paper substrate by one-shadow-mask process. The channel layer can be simultaneously self-assembled between ITO source/drain electrodes by only one shadow mask during RF magnetron sputtering deposition at room temperature. Base on this, we choose microporous SiO2 with electric double layer effect as a gate dielectric, and successfully develop the ultralow-voltage oxide TFT on a paper substrate. The TFT exhibits a good performance with an ultralow operation voltage of 1.5 V, a field-effect mobility of 20.1 cm2/Vs , a subthreshold swing of 188mV/decade, and a large on-off ratio of 5× 105. The full-room-temperature oxide TFT on the paper substrate by one-shadow-mask process shows a lot of advantages, such as low operation voltage, simple device process, low cost, etc. Such a TFT is very promising for the application of low-power and portable electronic products in the future.
2012, 61 (4): 047203. doi: 10.7498/aps.61.047203
Performance enhancement of strained Si material originates from the stress on it, which can be measured by Raman spectroscopy. A study of the theoretical model of strain-induced Raman spectrum frequency shift in strained Si material is of profound theoretical and practical significance. The Raman frequency shift of strained Si is significantly correlated with the stress intensity, the stress type and the crystal plane. However, the corresponding reports republished are lacking in integrality and systematization in the process of modeling. In this paper, according to the theory of Raman spectroscopy, based on Secular equation and Raman selection rules, quantitative relationships between strain tensor and Raman frequency shift for uniaxial and biaxial strained Si grown on (001), (101), and (111) SiGe substrates are achieved. On this basis, theoretical models of mechanical stress and Raman spectrum for uniaxial and biaxial strained Si materials grown on (001), (101), and (111) SiGe substrates are obtained using Hooke's law, respectively. The procedure for setling up these models is elaborate and systematic and the results obtained are comprehensive and quantificational, which can provide an important reference for the stress analysis in strained Si material.
2012, 61 (4): 047301. doi: 10.7498/aps.61.047301
Kink effect is analyzed in AlGaN/GaN devices primarily. A semiempirical model is given by analyzing the kink effect on AlGaN/GaN high electron mobility transistor and by considering the relationship between Vds,kink and gate voltage. Due to a little error between simulation results and measured data, this model can be used to identify the occurrence of kink effect and change in drain current. The analyses of experimental results and model simulation lead to a conclusion that impact ionization plays an important role in generating kink effect.
2012, 61 (4): 047302. doi: 10.7498/aps.61.047302
A new phenomenon is observed when a photoconductive InSb detector with 0.228eV band gap is irradiated by 10.6 μm laser, whose photon energy is 0.12 eV. The detector is heated by this out-band laser, due to the absorption of laser energy. However, a transformation temperature T0 exists in this process. When the temperature of the detector, T, is lower than T0, the number of carriers remains constant but the conductivity changes because of a change in mobility. The mobility decreases with the increase of temperature and varies as T-2.35. At T>T0, the concentration of thermally-activated carrier increases with temperature, which is proportional to exp (-Eg/2k0T). As a result, the influence of carrier concentration becomes more and more important. As a result, the output of the detector decreases. In a word, the output voltage of photoconductive detector results from the temperature dependence of mobility and concentration of carriers. This work provides an experimental basis for improving the carrier transport model.
2012, 61 (4): 047303. doi: 10.7498/aps.61.047303
A threshold voltage model is created by analyzing differents distributions of surface electric field and the condition of avalanche breakdown, based on the structure of a novel high speed semiconductor device p-IMOS in this paper. Model verification is carried out using the 2D device simulator ISE. By analyzing the model, the dependences of threshold voltage on drain-source voltage, Si layer thickness and gate length are studied. The results of the model are in good agreement with experimental results and ISE simulation results. The proposed model can also be easily used for the reasonable analysis and the design of p-IMOS.
2012, 61 (4): 047501. doi: 10.7498/aps.61.047501
The compound and the film of the critical charge-ordering Gd0.55Sr0.45MnO3 thin film are prepared using the solid state reaction technique and the pulsed laser deposition method respectively. The properties of the photoinduced relative change in the resistance of the film are investigated. Experimental results indicate that the film exhibits the semiconductive conduction and the charge-ordering temperature is about 70 K from the fitting of a variable-range hopping model. The maximum value of the photoinduced relative change in resistance is about 99.8% when the laser with a power density of 40 mW/mm2 irradiates the film, and the rise time is about 8s independent of temperature. The maximum value of the photoinduced relative change in resistance is about 44% at T=20 K when the laser with a power density of 6 mW/mm2 irradiates the film. The time constant is increased with the increase of temperature, which is attributed to the competition between photoinduced effect and thermal fluctuation.
2012, 61 (4): 047502. doi: 10.7498/aps.61.047502
Magnetic quantum-dot cellular automata (MQCA) function arrays are fabricated by electron beam lithography, thermal evaporation and lift-off technologies at room temperature. The effects of exposure dose and exposure time on MQCA function array patterns with three various spacings are experimentally investigated. The results show that the ideal pattern can only be obtained with 100 pA electron beam current and 0.38 μs exposure time. Magnetic force microscopy measurement on the fabricated inverter structure shows that the array demonstrates correct logic operation, which validates the feasibility of fabrication process for MQCA function arrays. Moreover, defect is observed in the experiments, simulations on the defective array show that missing nanomagnet defect in the array leads to signal inversion error.
Effect of surface plasmon polariton of Ag nanoparticles on the photoluminescence property of up-conversion materials
2012, 61 (4): 047801. doi: 10.7498/aps.61.047801
Silver nanoparticles are added into Yb3+ and Er3+ co-doped NaYF4 up-conversion materials using co-sintering process. Changes in diffraction intensity and surface morphology are inspected by X-ray diffraction and scanning electron microscope measurement, respectively. The optical absorption and Photoluminescence spectra are measured using UV/visible spectrophotometer and photoluminescence measurements. By optimizing the quantity of Ag nanoparticles, we obtain obvious enhancements of the photoluminescence intensities of Yb3+ and Er3+ co-doped NaYF4 materials, which shows a 28% enhancement in 300—800 nm wavelength range and a 55% enhancement at 544 nm. Different mechanisms are proposed for light quenching and surface plasmon enhanced absorption with Ag nanoparticles adjustment.
2012, 61 (4): 047802. doi: 10.7498/aps.61.047802
The local electric field components of the elliptical gold nanotube are calculated based on the finite difference time domain (FDTD) method. It is find that when the wavelength of the incident light is just at a resonant wavelength, the local field enhancement of the gold nanotube reaches a maximum. The increase of the semiminor axis of the ellipse makes the distribution of the local field change from a distribution that is high in both sides and low in the middle part of the nanotube into a distribution that is uniform around the tube. With the increase of the angle between the incident polarization and the semimajor axis, the local electric field components increase rapidly. The increases of the dielectric constants for both the core and the embedding medium cause the local field around the nanotube to decrease.
Theory research of negative dynamic conductivity in electrically pumped multiple graphene layer structures with split gates
2012, 61 (4): 047803. doi: 10.7498/aps.61.047803
The negative dynamic conductivity of graphene in THz range makes it to be a promise medium in THz radiation and amplification. This paper proposes electrically pumped multiple graphene layer structures with split gates, sets up the theory model of electrically induced n-i-p junction, calculates the ac conductivity associated with the interband and intraband transitions under the conditions of population inversion, discusses the bias voltage, gate voltage, number of graphene layers and the momentum relaxation time dependences of ac conductivity. It is shown that the real part of dynamic conductivity within terahertz range can be negative in certain conditions, namely, interband radiation is greater than the intraband absorption, which demonstrates the feasibility of taking electrically pumped multiple graphene layer structures with split gates as an active medium in radiating terahertz coherent source.
2012, 61 (4): 047901. doi: 10.7498/aps.61.047901
Molecular dynamics simulation is applied to the investigation of energy exchanges between single hydrogen and graphite (001). In addition to energy transfer efficiency, in this paper we analyse various kinds of possible processes, which are the absorption on the upside graphite surface, reflection, absorption on the downside graphite surface and penetration, during the course of a hydrogen atom bombarding the crystalline graphite containing four graphene sheets. The simulation results show that the interlayer interaction has a big influence on the reflection, especially when the incident energy is larger than 20.0 eV. The reflection coefficient increases evidently compared with that in single graphene sheet case. If the incident hydrogen has a kinetic energy more than 25.0 eV, it can penetrate the four- sheet graphite at some striking locations. When the incident energy is larger than 28.0 eV, the energy transferring to the first graphene sheet is more than to the second graphene sheet. These results will be helpful for understanding the chemical erosion of carbon based materials and the tritium retention occurring in fusion devices.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
The effect of substrate on terahertz transmission properties through metal subwavelength dual-ring structure
2012, 61 (4): 048101. doi: 10.7498/aps.61.048101
Terahertz waves through a split-ring resonator (SRR) can induce the resonant absorption, and this can be explained by using the theory of LC resonant circuit and the model of half wave resonance. However, in the dual-ring structure without the split gap, we still observe the resonant absorption in the THz frequency range. By analyzing the phenomenon, we think that it can be explained by using a the model of half wave resonance. Furthermore, it is found that if the structure is fabricated on quartz crystal substrate, we can obtain the angle-dependent terahertz waveforms using terahertz time-domain spectroscopy (TDS) when the sample is rotated in plane, as well as the frequency domain spectra. But this phenomenon does not exist in the silicon-based structure, which may be attributed to the birefringence effect of the quartz crystal substrate on the subwavelength metal dual-ring structure. The main purpose of this article is to explain the physical process of the effect.
2012, 61 (4): 048102. doi: 10.7498/aps.61.048102
Understanding the laws of grain growth in three dimensions is one of the classic problems of materials science. By considering the anisotropy in real polycrystalline structure and the relationship between the integral of surface mean curvature and the mean caliper diameter of a convex individual grain, three-dimensional von Neumann equation for accurate grain growth rate is studied. The result shows that accurate grain growth rate of a convex grain is related to the grain mean caliper diameter, the sum of the length of grain edges and the corresponding dihedral exterior angles. This result is verified by Kelvin tetrakaidecahedron and the only five convex regular polyhedra.
2012, 61 (4): 048103. doi: 10.7498/aps.61.048103
Void is one of the most common type of structure flaws existing in brittle materials, which dramatically affects the shock loading response of brittle materials. A quantitative discrete element method is employed in this work to study the fracture characteristics of porous isotropic brittle material under shock wave compression. Scenarios of isolated void, three types of simple distribution and random distribution of voids are computed, from which we find that shear fracture and local tensile fracture are two type of basic fracture modes for brittle material under shock wave compression. Coalescence of damage bands between voids can induce the collapse of voids at relatively low pressure, while stress relaxation caused by damage can shield fracture evolution in a certain zone. The combination of amplification and shielding effects of damage results in a unique pattern of alternate distribution of severe and mild damage zones. These simulation results present a basic physics picture for the understanding of evolution process and mechanism of fracture in porous brittle material under shock wave compression.
2012, 61 (4): 048401. doi: 10.7498/aps.61.048401
Bismuth sulfide (Bi2S3) polycrystalline bulks are fabricated by spark plasma sintering (SPS) combined with a mechanical alloying (MA) process. The electrical transport properties are investigated with a special emphasis on dry-milling speed, wet-milling time and mediums in MA process. The phase structure, the microstructure, and the electrical/thermal transport properties for the bulk samples are measured. The results suggest that the second Bi2O3 phase is formed because of the micro-oxidation when the wet medium is fixed to absolute ethyl alcohol, which refines the grain sizes and affects electrical transport properties of the bulk samples. Prolonging the wet-milling time in absolute ethyl alcohol causes the increase of resistivity and the decrease of the power factor. The electrical transport properties degrade due to the porous microstructure in bulk samples even without oxidization as the acetone is used as a milling medium. The largest ZT value 0.25 is obtained at 573 K for the samples dry-milled under 425 r/min for 15 h without wet-milling, which is the highest value in the reported values so far.
Determination of optical constants and thickness of photoactive layer in polymer oslar cells by single transmission measurement
2012, 61 (4): 048801. doi: 10.7498/aps.61.048801
We present a simple and accurate method of determininy the optical constant and physical thickness of the photoactive layer in a polymer solar cell. The applicabilities of the physics models including Forouhi-Bloomer and Lorentz-Oscillator models in transmission curve fitting are compared. This method is used to calculate the optical constants and film thicknesses of poly(3-hexylthiophene) (P3HT) /[6,6]-phenyl C61-butyric acid methyl ester (PCBM) and poly[2- methoxy-5-5(2'-ethyl-hexyloxy)-1,4-phenylenevinylene](MEH-PPV)/PCBM bulk heterojunction, The calculated transmission curves fit to the experimental ones well. The results accord with those reported in the literature and from the step profiler, and their error is less than 4%. The optical constant and the physical thickness of polymer solar cell after the optimization process including thermal annealing and adding high-boiling-point additive are studied, and the results are consistent with the voltage-current characteristics of the cell. This method is suited for bulk heterojunction films and can be used in polymer solar cell optimization and detection system.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2012, 61 (4): 049201. doi: 10.7498/aps.61.049201
Due to the fact that regularization method can overcome the ill-posed characteristic and obviously suppress noise and errors propagation in practical inverse problem, a new algorithm (genetic algorithm with regularization method) is proposed, combining fitness function of genetic algorithm with regularization term to retrieve ocean duct parameter. Simulation experiments show that retrieval accuracy of the new algorithm is better than that of traditional genetic algorithm. When the noise errors are less than 10%, the noise-immune performance is better. Finally, the inversion result from airborne radar clutter local date of sea surface in the Wallops island is compared with real refractivity profile, pointing out the efficiency of algorithm. The new method provides a new consideration for inverse research of ocean atmosphere duct.