Vol. 57, No. 4 (2008)
2008, 57 (4): 1991-1997. doi: 10.7498/aps.57.1991
A new model of the complex adaptive systems is established， and the theory developed by the author on the kinetic evolution and phase transition of complex adaptive systems is extended to the general case， with which some more realistic complex adaptive systems may be described- As an application of the new model， we compare the simulation results of winning the toss with the corresponding theoretical one given in this paper- The theoretical results turn out to be in good accordance with the simulation-
Differential equations of motion for constrained systems with respect to three kinds of nonholonomic variations
2008, 57 (4): 1998-2005. doi: 10.7498/aps.57.1998
Based on an analysis of three kinds of non-equivalent nonholonomic variations，i-e-，the Suslovs variation，Hlders variation and vakonomic variation， the method of Lagrange multipliers and stationary action principle are utilized to discuss the differential equations of motion for nonlinear nonholonomic constrained systems with respect to the three kinds of variations- The condition for the three kinds of equations to be equivalent is investigated- The equations for affine nonholonomic constrained systems are also obtained as special cases of the general nonholonomic systems- Two examples are given to illustrated the validity of the result-
2008, 57 (4): 2006-2010. doi: 10.7498/aps.57.2006
Mei symmetry and Mei conserved quantity of Nielsen equation with multipliers for a nonholonomic， non-conservative system of Chetaev's type are studied- The differential equations of motion of Nielsen equation with multipliers for the system，the definition and criterion of Mei symmetry，and the condition and the form of Mei conserved quantity deduced directly by Mei symmetry for the system are obtained- An example is given to illustrate the application of the results-
2008, 57 (4): 2011-2015. doi: 10.7498/aps.57.2011
A percolation model of random porous media， with its nonzero bonds obeying the uniform distribution， is researched in this article- The scaling exponents of the permeability are calculated at the percolation critical point- The simulation suggests that the exponents are not universal，but depending on the parameter of the uniform distribution- This indicates that even if the first negative moment of the probability density function of the nonzero bonds exists， the corresponding scaling exponent at the critical point will not be universal- Our simulation result is different from the conclusion reached by Sahimi-
2008, 57 (4): 2016-2020. doi: 10.7498/aps.57.2016
We propose a new remote state preparation process- After the sender measures her particles，adopting the new process，the receiver can greatly reduce the classical communication cost- With a partial entangled three-particle Greenberger-Horne-Zeilinger (GHZ) state and a partial entangled two-particle state as the quantum channel，we present a scheme for preparing remotely a three-particle GHZ state- Taking this scheme as an example， we show how to use this process in detail and present the application range of this process- Analysis shows: only one classical bit is required for remote preparation of the three-particle GHZ state by using this process-
2008, 57 (4): 2021-2025. doi: 10.7498/aps.57.2021
A kind of generalized Sine-Gordon equations with strong damping are studied，considering both viscosity effect and external damping- Firstly，by the aid of Galerkin method，under the initial value conditions u(x，t)∈H10 (Ω) ，ut∈L2 (Ω)，we prove the existence and uniqueness of a global weak solution u(x，t) for the initial boundary value problems and the constant dependenceof solution on the initial value- Secondly， under the initial value conditionsu(x，0)∈H10 (Ω)∩H2(Ω)，ut(x，0)∈H10 (Ω)，the course of proof of the existence of strong solution u(x，t) is also explained by using Galerkin method-
2008, 57 (4): 2026-2030. doi: 10.7498/aps.57.2026
We study the entanglement evolution of two three-level atoms in a fermionic environment. Our results show that the entanglement evolution depends not only on the strength of the interaction between the system and environment， but also on the structure of states. For the cases of concern， we find that the stronger the interaction between the system and environment is， the faster the entanglement of the system decreases. The entanglement of pure quantum states vanishes completely when the time goes to infinity; while for the mixed state，the entanglement will be completely destroyed by decoherence in finite time. The decoherence-free subspace SDF has been identified by using linear entropy to measure decoherence. A density matrix that only can be expanded by the elements of SDF does not perceive the presence of the environment and the entanglement and its linear entropy remain unchangeable. Our analysis will shed some light on the effect of a fermionic environment on the entanglement of bosonic systems.
The Gerdjikov-Ivanov equation which appears，in the fields of quanta field theory，weak nonlinear dispersive water wave，nonlinear optics， etc.， has been discussed. Nonlinear mathematical physics equation with higher order nonlinear terms is educed in the discussion of Gerdjikov-Ivanov equation. The Liénard equation is chosen as subsidiary ordinary differential equation, with the help of which and according to homogeneous balance principle，the Gerdjikov-Ivanov equation has been solved，and the envelope solitary wave solutions and envelope sinusoidal wave solutions have been obtained.
2008, 57 (4): 2035-2040. doi: 10.7498/aps.57.2035
The phenomenon of stochastic resonance (SR) in a bistable system subject to correlated multiplicative colored and additive white noises and a periodic rectangular signal with a constant component is investigated in the unified colored noise approximation and by applying the two-state theory. The expression of the signal-to-noise ratio (SNR) is obtained for arbitrary signal amplitude. The SNR is a non-monotonic function of intensities of multiplicative colored and additive white noises，correlation time of multiplicative colored noise and the strength of the coupling between noises， so SR appears in the bistable system. Meanwhile， it is more effective to control SR through adjusting the additive white noise intensity than adjusting the multiplicative colored noise intensity. Moreover，the effects of potential asymmetry and the strength of the coupling between noises on SNR are opposite.
2008, 57 (4): 2041-2047. doi: 10.7498/aps.57.2041
A feedback method is proposed to control the stochastic resonance by feeding back the output of bistable system to the input to re-act on the system. In a typical bistable system， the signal-to-noise ratio and spectral power amplification are considered as observable variables to evaluate the stochastic resonance effect. The stochastic resonance phenomenon caused by the linear or nonlinear feedback functions are studied respectively. The theoretical analysis and numerical simulation show that the stochastic resonance is controllable， and this method is especially suitable to the system of which the parameters are fixed or difficult to vary.
2008, 57 (4): 2048-2052. doi: 10.7498/aps.57.2048
Two bistable systems can become multistable through nonlinear coupling. The coupled system is more valuable than the single bistable system in theoretical research and practical application. The response characteristics of the coupled system subjecting to noise and periodic signal have been analytically analyzed to show the effect of coupled coefficient and bistable system parameters on stochastic resonance. The results show that the stochastic resonance is produced by the effect of zonal double-well potential. The schematic diagram of feedback coupled control principle has also been constructed. The method gives a reliable theoretical basis for artificially producing stochastic resonance or strengthening the effect of resonance in bistable system to control the stochastic resonance, and also for its application. The results of numerical simulation agree with that of theoretical analysis.
2008, 57 (4): 2053-2059. doi: 10.7498/aps.57.2053
The theory and simulation technique of weak signal detection using chaos state phase change is researched in this paper. The method of weak signal detection by using the Duffing oscillator's initial conditions sensitivity was firstly analyzed， the existing transition process would affect the detection capability. Then an improved method was presented. In order to compare these two methods， the input pseudo-noise expression and the calculating step selection in simulation were researched， and the simulation models were established. The new method was shown to have better capability through analyzing the signal detection experimental results under the condition of typical noise intensity background. The experiments showed that the Duffing oscillator in special critical chaos state is also sensitive to noise， which leads to the difficulty in exactly deciding the minimal detection limit by phase change， and the limitation of the phase change method was discussed.
2008, 57 (4): 2060-2067. doi: 10.7498/aps.57.2060
By means of techniques of state observer， the anti-synchronization of a class of chaotic neural networks with time delay is investigated. Compared with the anti-synchronization strategies used in other chaotic systems， the technique presented here is rather simple and the convergence rate can be adjusted by tuning the eigenvalues through the pole placement technique. Finally， two numerical examples and computational simulations are given to show the effectiveness of the proposed scheme.
2008, 57 (4): 2068-2072. doi: 10.7498/aps.57.2068
For the nonlinear vibration system，if the derived system can reach synchronization with the original system by linear coupling， a corresponding anticipated system can be formed to realize the anticipated synchronization. The response of anticipated system can synchronize with the future response of original system and realize the prediction for nonlinear response，even for chaos in the originalsystem. In order to realize a long-term prediction， a series of anticipated systems can be formed by the rule presented in this paper. For chaotic motion in Duffing system， a 6-level anticipated system is formed， which can predict the response of original Duffing system in about 3s.
2008, 57 (4): 2073-2080. doi: 10.7498/aps.57.2073
A novel adaptive synchronization method is proposed for a class of chaotic systems with uncertain parameters based on Lyapunov stability theory. Adaptive controller and updating law of parameters are obtained， and for specific error systems the controller can be simplified. This method is simple and systemic. A new chaotic system and the hyperchaotic Chen system are taken as examples to illustrate the effectiveness of the proposed method. Numerical simulation illustrates the feasibility of the technique.
2008, 57 (4): 2081-2091. doi: 10.7498/aps.57.2081
This paper further investigates the locally and globally adaptive synchronization of united complex dynamical networks based on a new method of network split. Time-delay has been introduced into networks to split and upon which a model of united complex dynamical networks with multi-links has been constructed. Several new locally and globally asymptotically stable network synchronization criteria are deduced by using Lyapunov stability theory and some hypotheses. The validity of the results was proved through numerical simulation of this paper.
The bifurcation analysis of a conjugate Chen chaotic system and the hyperchaos generation based on the system
2008, 57 (4): 2092-2099. doi: 10.7498/aps.57.2092
The Hopf bifurcation is analyzed for a new type of three-dimensional autonomous chaotic system，of which the chaotic attractor belongs to the category of conjugate Chen attractors. A new four-dimensional autonomous hyperchaotic system is constructed based on this chaotic system by introducing an additional controller. This hyperchaotic system with one parameter presents hyperchaotic behavior over a range of parameter values. The analysis of Lyapunov exponents and bifurcation diagram demonstrates the evolution process from periodic orbits， quasi-periodic orbits to chaos， and further to hyperchaos as the parameter increases.
2008, 57 (4): 2100-2106. doi: 10.7498/aps.57.2100
In this paper，methods of chaos-control of a semiconductor laser are studied using a photoelectric delayed negative-feedback scheme with additive photoelectric delayed control， and the maximum Laypunov exponent of the controlled system is analyzed. Three physical models of photoelectric delayed chaos-control of an injected semiconductor laser are presented. Firstly，by adjusting the delay time or the feedback photocurrent， the chaotic laser can be conducted into period-3 state， period-5 or other multi-periodic states. Secondly， by periodic modulation of the photocurrent into the laser， the chaotic laser can be brought into a period-8 states or a period-9 state. Lastly， we make use of the photoelectric delayed negative-feedback and an additive current modulation to perform chaos-control， the chaotic laser can be stabilized into a single-periodic state or multi-periodic state.
2008, 57 (4): 2107-2114. doi: 10.7498/aps.57.2107
The Chebyshev polynomial approximation is used to investigate the stochastic bifurcation of van der Pol system with two-sided barriers. The numerical results show that there is not only diverse phenomena of period-doubing bifurcation in the system，but also exist, the grazing bifurcation which is unique for the non-smooth system. The grazing bifurcation is investigated emphatically， and the influence of the random factor on the grazing bifurcation in the stochastic non-smooth dynamics system is analyzed. It is shown that the Chebyshev polynomial approximation is a valid method for the investigation of non-smooth systems.
The study of a cellular automaton traffic flow model with public transit，harbor-shaped bus stop and mixed different-maximum-speed vehicles on single lane
2008, 57 (4): 2115-2121. doi: 10.7498/aps.57.2115
Based on the Nagel-Schreckenberg models of traffic flow， a cellular automaton traffic model with different-maximum-speed vehicles mixed on single lane is proposed， which considers public transit and harbor-shaped bus stop. The influence of deceleration probability， the density of vehicles， the average stop time of public transit vehicles， the proportion of public transit vehicles occupied in the mixed vehicles and the bus stop spacing on mixed traffic flow are analyzed by computer simulation. The models traffic flow characters are presented and analyzed.
2008, 57 (4): 2122-2129. doi: 10.7498/aps.57.2122
Intelligent traffic systems can help road users make reasonable route choice by providing them real-time traffic information，thus alleviate congestion. Considering the existence of overlapping between routes， this paper applies the cellular automaton model to simulate and investigate the route choice behavior under two information service strategies， namely travel time feedback strategy and mean velocity feedback strategy. Our simulation results indicate that the mean velocity feedback strategy can reduce the route travel time more efficiently than the time feedback strategy， particularly in longer networks with very short or very long overlapping segments. Its advantage in improving the network utilization efficiency decreases with the route length and drops first and rises afterwards with the length of overlapping part.
THE PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
2008, 57 (4): 2130-2135. doi: 10.7498/aps.57.2130
Based on the coupled wave theory， this paper presents the design， numerical analysis， and simulation of two types of TE0n-TE0(n+1) ripple-wall waveguide mode converters: a uniform ripple-wall converter and a nonuniform ripple-wall waveguide converter. For the first type，17.17GHz，six-period TE02-TE03 mode converter and 34.30GHz，six-period TE01-TE02 mode converter were operative； and 34.30GHz，six-period TE01-TE02 for the nonuniform one was operative. The design study is focused on the maximization of the conversion efficiency and mode purity coupled with a broad bandwidth. The nonuniform one seems to have a significant advantage over the uniform one， its conversion efficiency reaches 99.5% at center frequency and has a conversion bandwidth of more than 1.5GHz. Simulation results agree well with the theoretical predictions.
2008, 57 (4): 2136-2140. doi: 10.7498/aps.57.2136
The medium effect of nucleon-nucleon cross section σmedNN(α) on the isoscaling parameter α is investigated for two couples of central nuclear center reactions 40Ca+48Ca and 60Ca+48Ca， 112Sn+112Sn and 124Sn+124Sn with isospin dependent quantum molecular dynamics at beam nucleon energies from 40 to 60MeV. It is found that there is very obvious medium effects of nucleon-nucleon cross section σmedNN(α) on the isoscaling parameters α. The mechanism of the above properties is also studied and discussed.
2008, 57 (4): 2141-2144. doi: 10.7498/aps.57.2141
In this paper, we describe the structure of Micromegas (micro mesh gaseous structure) detector which has been developed recently by our group. And its count plateau, gas gain and energy resolution in the condition of a 55Fe X-ray source and various Ar-CO2 mixtured gases are plotted as functions of mesh voltage. Based on the experimental results, we confirm that we have mastered the technique of fabricating this kind of detector with high energy resolution and excellent performance.
ATOMIC AND MOLECULAR PHYSICS
2008, 57 (4): 2145-2151. doi: 10.7498/aps.57.2145
The density function (B3LYP) method has been used to optimize the possible ground-state structures of AlO2 and Al2O molecules. The results show that the ground state of AlO2 molecule has D∞h symmetry and is in the X2Πu state. The parameters of structure are RAl-O=0.1661 nm and De=8.9217eV, respectively. The results also show that the ground state of Al2O molecule has D∞h symmetry and is in the X1Σg state. The parameters of structure are RO-Al=0.1731 nm and De=10.5269eV, respectively. The potential energy functions of AlO2 and Al2O have been derived from the many-body expansion theory. The contours describe correctly the configurations and the dissociation energies of the two ground-state molecules.
2008, 57 (4): 2152-2160. doi: 10.7498/aps.57.2152
Using multi-configuration Dirac-Fock method and the corresponding packages GRASP92 and RATIP, as well as the newly developed RERR06, we calculated inner- and outer-shell photoionization cross sections of configurations 1s2nl(n=2,3; l=s, p) of atomic lithium. In the calculations, the relaxation effect caused by photoionization of electrons was included. It was found that the relaxation effect plays an important role in the inner-shell photoionizational processes, contrary to the outer-shell photoionizational processes. Further, the effects of relaxation on inner-shell photoionizational processes from different states are not the same. It is more prominent for the excited states than the ground state, and for the higher excited states than the lower excited states.
2008, 57 (4): 2161-2164. doi: 10.7498/aps.57.2161
The (110) crystal surface of Si was bombarded by slow highly charged ions (Pbq+，Arq+) and the secondary particle emission was measured for different incident angles. Comparing the relationship between the sputtering yield and the incident angle, channeling effect was suggested. The channeling effect in interaction of highly charged ions with Si causes the sputtering yield to depend strongly on kinetic energy. Highly charged ions can enhance sputtering yield at smaller incident angles. At incident angles from 40° to 50°, the higher the potential energy of highly charged ion, the greater the sputtering yield.
2008, 57 (4): 2165-2173. doi: 10.7498/aps.57.2165
The geometries, total energies, and frequencies of ZrnCo (n=1—13) clusters have been systematically investigated by using density functional theory with the generalized gradient approximation. The equilibrium geometries, stabilities, gap and magnetism have been determined. The results show that the relative stabilities of Zr4Co, Zr7Co, Zr9Co and Zr12Co are stronger than other sized clusters, indicating that they are magic number clusters, and especially, the true ground state for Zr12Co cluster has icosahedral structure with Ih symmetry. Moreover, the stability of Zr12Co is the strongest of all the investigated clusters. The magnetic moment of ZrnCo clusters mainly comes from the localized d electron. The system magnetic moment may be divided into three stages along with the size change: n=1—3 corresponds to the stable magnetic moment, the magnetic moment of ZrnCo clusters starts to show vibration quenching from n=4, until n=8 when the magnetic moments is completely quenched. The charge transfer and the strong hybridization between s, p and d states might be one major reason for quenching of the magnetic moment of ZrnCo clusters. Meanwhile, the clusters which are composed of transition metals doped in different characteristic materials is worth further studying, for example, TMX12 and Zr13TM clusters have interestingly similar structures, stability and magnetism.
2008, 57 (4): 2174-2178. doi: 10.7498/aps.57.2174
The structure of surface layer of monocrystalline Si by implanting nitrogen cluster ions N+10 was transformed directly the nanocrystalline, which led to the change in optical properties of monocrystalline Si. Excited by the ultraviolet light of 250—320nm, the sample showed a clear luminescence band of 330—500nm and an extra intensive spectral peak with good monochromaticity around 360nm. The intensity of the peak were 5 times high as the intensity of the substrate or N+ implantation samples and 1.5 times high at the N+2 implantation samples, respectively. It was also found there were two other stable luminescence bands around 730nm in the visible region and around 830nm in the infrared region, respectively. The preliminary study indicated that an excellent photoluminescence material was formed in the implanted layer.
CLASSICAL AREA OF PHENOMENOLOGY
2008, 57 (4): 2179-2185. doi: 10.7498/aps.57.2179
The resonance property of the pair of left-handed metamaterial (LHM) and negative permittivity medium is analyzed in depth, and the results are applied to the design of an improved miniaturized cavity resonator (IMCR). The IMCR is numerically simulated, and the results show that the IMCR is resonant at 10.3GHz while its length and width is only 4.58 and 5.08mm, respectively, which are half that of the conventional one's. This research may contribute in theory to the miniaturization of resonance structures based on LHM.
The spatial distribution of optical field in random media with different filling densities of the same material particles
2008, 57 (4): 2186-2191. doi: 10.7498/aps.57.2186
The problem of light localization in two-dimensional random media with different filling densities of the same material particles is studied by use of the finite difference time domain method. Based on the experimental parameters of random media, the spatial distributions of optical field in the media with different filling densities of scattering particles are simulated. Results show that when the scattering mean free path is close to the wavelength， the optical fields are localized obviously in the random media. And with the particle filling density increasing, the localization becomes stronger. So with the same pump beam, it is easier to achieve laser radiation for random media with higher filling densities of scattering particles. The results agree with the experiments qualitatively.
2008, 57 (4): 2192-2198. doi: 10.7498/aps.57.2192
Temporal femtosecond laser speckles are investigated both theoretically and experimentally. It is theoretically predicted that the temporal femtosecond speckles is generated when a coherent femtosecond pulse propagates through a random media. Temporal speckles on femtosecond scale are observed experimentally. These temporal femtosecond speckles are a direct result of random interference between scattered waves. Statistical characteristics of temporal speckles are discussed, and experimental results are presented.
Influence of auxiliary violet light on holographic diffraction efficiency under different recording intensities in bacteriorhodopsin film
2008, 57 (4): 2199-2204. doi: 10.7498/aps.57.2199
The photochromic bacteriorhodopsin (BR) film can be used as a rewritable holographic recording medium. Due to the scattering and the reflecting of light from the BR film, the grating contrast of the hologram is diminished during the hologram recording. When the intensity of the recording light is weak, the influence of the scattering and reflection on the grating contrast is low enough to be neglected. But in the case of intense recording light, this influence should not be overlooked. It is found that the influence of the auxiliary violet light on the holographic diffraction efficiency kinetics is distinct under different recording intensities. At weak recording light intensities, the steady diffraction efficiency are increased and the peak diffraction efficiency is suppressed by the auxiliary violet light. But for intense recording light, the steady diffraction efficiency as well as the peak diffraction efficiency are both increased by the auxiliary violet light. Based on the two-level model of BR photochromism, we give a good theoretical explanation to the above phenomena.
2008, 57 (4): 2205-2211. doi: 10.7498/aps.57.2205
The entropy evolution properties of field (atom) in a moving cascade three-level atom interacting with a single-mode field are studied through two photon transitions. When the atom is initially in coherent state, the influences of the atomic motion, the field-mode structure parameter and the initial average photon number on the evolution of the entropy are discussed. The results show that the entropy evolution period of the field is dependent of the atomic motion and the field-mode structure parameter. However, the parameter of the initial average photon number will neither lengthen nor shorten the entropy evolution period of the field, but the numerical values of maximum and sub-minimum of entropy will be affected by it.
2008, 57 (4): 2212-2217. doi: 10.7498/aps.57.2212
A quantum key distribution (QKD) scheme using binary-modulated coherent states is proposed in this paper. Compared with Gaussian sources of QKD using Gaussian-modulated coherent states, the binary sources of the proposed scheme are the simplest sources, and the binary modulation is the most usual modulation format in the digital optical-fiber communication. The security of the proposed schemeagainst beam splitter attack is analyzed using Shannon information theory. The analytical expression of the secret information rate is given in terms of modulation-demodulation parameters and channel parameters. The quantum noise of coherent states guarantees the security of the proposed scheme.
2008, 57 (4): 2218-2221. doi: 10.7498/aps.57.2218
The quantum mechanics system with interaction between quasi-Λ type four-level atoms and multi-mode light fields is discussed. Under the mechanism of quantum interference, the relative dielectric permittivity and the relative magnetic permeability will change remarkably. In certain range of parameters, both of them will be negative. At this time, the left-handedness effect happens.
2008, 57 (4): 2222-2228. doi: 10.7498/aps.57.2222
The nonlinearities of a semiconductor optical amplifier (SOA) are exploited to realize various all-optical signal processing at ultrahigh speed. We demonstrate reconfigurable logic gates with XNOR, AND, NOR, OR, and NOT functions based on various nonlinearities of single SOA, including four-wave mixing, cross gain modulation and transient cross phase modulation. Since the modulation speed of the SOA is limited by the recovery time of carrier density, a detuning optical bandpass filter with 0.32nm-bandwidth follows the SOA to enhance the modulation speed. Multi-logic gates at 40Gbit/s are obtained with single SOA.
2008, 57 (4): 2229-2235. doi: 10.7498/aps.57.2229
The expression for the spectral density of polychromatic Gaussian and Gaussian-Schell model (GSM) beams propagating through double slits in Young's experiment are derived, which enables us to study the behavior of spectral switches in Young's experiment illuminated by fully and partially coherent polychromatic light beams and to judge if their spectral switches belong to the effect in singular optics. It is shown that the criterion for the effect in singular optics with polychromatic light field should be Smin=0 (Smin is the minimum of the spectral density at spectral switches) rather than Imin=0 (Imin is the minimum of the total intensity). In Young's experiment illuminated by polychromatic GSM beams the spectral switches in both near and far zones are not attributed to the effect in singular optics because Smin≠0. If the double slits in Young’s experiment are illuminated by polychromatic Gaussian beam, the spectral switches in the far zone belong to the effect in singular optics, whereas it is not the case in the near zone.
Microwave control of optical bistability and multistability in the presence of antirotating wave coupling
2008, 57 (4): 2236-2241. doi: 10.7498/aps.57.2236
We show the control of optical bistability and multistability via the atomic coherence induced by the microwave field in the presence of the antirotating wave coupling. Appearance or disappearance of bistability and multistability， manipulation of the hysteresis hoop widths and the threshold intensity are achieved by changing the initial phase of the microwave field. Physically， the rotating wave and antirotating wave couplings can be treated as the equivalent bichromatic excitation. Each of the coupled levels are split into an infinite set of sublevels. As a consequence， each of the bare state transitions is split into an infinite set of transitions of different frequencies. The coherent superposition of these split transitions， which depends on the initial phase of the microwave field， determines the nonlinear absorption and dispersion of the medium.
Research on group velocity slowdown in erbium-doped optical fiber under controllable absorption loss
2008, 57 (4): 2242-2247. doi: 10.7498/aps.57.2242
Group velocity of the optical pulse propagation would be reduced based on coherent population oscillation effect in an absorbing medium. According to the absorption properties of erbium-doped optical fiber, there would appear a sharp absorption loss, which brought more difficulties to the actual measurement of slow light and its application in communications. From the ground rate equation of erbium-ion, the delay of optical pulse was simulated under controllable absorption loss， and experiments were implemented to verify the theoretical results. Moreover, we researched how the different pump power and fiber length effect the signal loss and the optical pulse delay.
2008, 57 (4): 2248-2252. doi: 10.7498/aps.57.2248
The influences of the delay time, the cell length and the pump energy on the gain amplification of weak Stokes signal of stimulated Brillouin scattering (SBS) in water are investigated theoretically and experimentally. In the experiment a frequency-doubled seed injection Nd:YAG laser with a pulse duration of 7.2 ns was used. Results showed that the pump pulse should lag the Stokes signal pulse by an amount equal to pulse half-width to obtain the maximum gain. Depending on the pump energy the signal below 10-5 mJ is linearly amplified. By using CS2 as amplifier medium, a Stokes signal of 1pJ from water is amplified with high gain (7×107) in the regime below the pump's self-SBS threshold.
2008, 57 (4): 2253-2259. doi: 10.7498/aps.57.2253
The dynamic process of the small-scale self-focusing of ultrashort laser pulse is experimentally investigated. By using tabletop femtosecond laser, the pulse which is spatially modulated by the crossed-fiber diffraction passes through the carbon bisulfide. With different input power, the small-scale self-focusing of the pulse was obtained. By analyzing the contrast of the beam, we find that the small-scale self-focusing does not increase all along with the increasing of input power, but some new growth points appear in different places. They compete with each other, and finally lead to multiple splitting. The experimental results confirmed the numerical simulation.
2008, 57 (4): 2260-2265. doi: 10.7498/aps.57.2260
In this paper, we theoretically investigated the influence of nonlocality on (1+2)-dimensional spatial solitons in undoped nematic liquid crystals (NLCs). We confirmed that the nonlinear index coefficient and the general characteristic length of the nonlinear nonlocality for the NLC are dependent on the pretilt angle of the NLC molecules. Then the Schrdinger-type nonlinear equation in strong nonlocality was given and from the equation the analytical expressions of the single soliton and the critical power were respectively obtained. In experiment, we varied the degree of nonlocality by changing the bias voltage and indirectly determined the relation between the critical power and pretilt angle. At last, by comparison of analytical solutions with the numerical simulation and the experimental result, we verified that our expressions are precise.
Dynamics of semiconductor laser with optical feedback: Evolution from low-frequency fluctuations to chaos
2008, 57 (4): 2266-2272. doi: 10.7498/aps.57.2266
The low-frequency fluctuations and high-dimensional chaos with 12.2 correlation dimensions were generated experimentally by a semiconductor laser with optical feedback. Extensive experimental and numerical studies were performed to reveal the evolution process from low-frequency fluctuations to chaos. Our results showed that there exists a obvious critical point for the semiconductor laser's bias current. When the bias current Ib is set below 1.03Ith, the peak-to-peak value of the low-frequency fluctuations increases at first and then decreases with the feedback strength decreasing, while its average period keeps decreasing. In this process, the output of the semiconductor laser never goes chaos. However, when the bias current Ib is set beyond 1.03Ith, chaos appears and is persistent in this range. Moreover, the peak-to-peak value of the low-frequency fluctuations increases continuouslly and its duration decreases with the decrease of the feedback strength. Three stages experienced in the process leading from stable emission and low-frequency fluctuation to chaos are established. Numerical simulations are well consistent with the experimental results.
2008, 57 (4): 2273-2277. doi: 10.7498/aps.57.2273
In this paper, a kinetic model for the radiation-induced production of the intrinsic point defect E′ center is proposed. The relation of E′ center concentration changing with dose is obtained. The results show that the production of E′ center includes two processes, i.e. the creation of new defects and the activation of preexisting defects. The stained bonds (or oxygen replacement) in silica networks lead to the creation of new defects, whose concentration increases linearly with the dose. The preexisting defects produce the activation, which tends to saturation. The theoretical result is in good agreement with the experiment.
2008, 57 (4): 2278-2283. doi: 10.7498/aps.57.2278
A new type of holographic recording material using safranine T sensitized photopolymer based on polyvinyl-alcohol was fabricated and studied. And the material was specially exposed to the Ar+ laser of 514.5nm wavelength. The results indicate that the material has high diffraction efficiency, sensitivity and refractive index modulation. The maximum diffraction efficiency is nearly 40%, the sensitivity is 7.22×10-4cm2/mJ and the refractive index modulation is 3.65×10-4. But the Bragg-mismatch caused by the exposure must be considered during recording. Holograms were recorded in the material, and the images can be reconstructed clearly. So the photopolymer material is suitable for high-density volume holographic storage.
Research on the mechanism of reflection characteristics of laser irradiation on cat eye optical lens
2008, 57 (4): 2284-2294. doi: 10.7498/aps.57.2284
Using the theory of matrix optics and expanding the aperture function into a sum of finite complex Gaussian functions, this paper theoretically deduces the mechanism of cat eye effect generated by laser irradiation on the cat eye optical lens and analyzes the reflection law of cat eye effect through numerical computation. Then an experiment is operated to validate the cat eye effect by making 532nm laser irradiate on an optical zoom lens whose focusis ranged between 12 and 72mm. By using the above method, the law governing the variation of laser reflection characteristics with the parameters of the transmitting laser beam and the cat eye optical lens, including aperture, focus and focal shift etc, is established. The study reveals that the reflection characteristics of cat eye effect are intimately correlated with the parameters of the cat eye optical lens and transmitting laser beam, furthermore, the characteristics would change obviously when there is a large change in the distance between the waist of the incident beam and the cat eye optical lens. In particular, a reasonable choice of the positive focal shift would result in a higher peak intensity at the return place than that of no focal shift, when the distance between the waist of the incident beam and the cat eye optical lens varies, whereas the former conclusion based on geometry optics was that the highest peak intensity could be obtained when the optical lens has no focal shift.
Research on the optimum thickness of metallic thin film utilized to excite surface plasmon resonance
2008, 57 (4): 2295-2299. doi: 10.7498/aps.57.2295
The optimum thickness of metallic thin film utilized to support the surface plasmon resonance (SPR) has been investigated, based on the characteristics of electromagnetic field energy distributing in the film and its compound permittivity. It was shown that the optimum film thickness is related to the wavelength of exciting light and the refractive index (RI) of the metallic thin film. In this paper, a mathematical expression has been established to describe their relationship, and verified in the experiment. Our theoretical analysis is also consistent with previous experimental results. When the SPR sensor is employed in angular interrogation, in order to obtain highest sensitivity, the optimum thickness of metallic thin film can be deduced from the wavelength of the exciting light and the imaginary part of the film RI, or be obtained with the center wavelength of the exciting light and imaginary part of RI when the sensor is utilized in wavelength interrogation.
2008, 57 (4): 2300-2305. doi: 10.7498/aps.57.2300
A new phenomenon of granular segregation is observed, in which the binary granular mixture submitted to the vertically vibration behaves in cyclic segregation patterns which switches between the Brazil nut, reverse Brazil nut and sandwich patterns with a frequency that accelerates with time. The rule of cycing is investigated, and the phase diagram is depicted. The new phenomenon is qualitatively explained in terms of the collision, friction and air dissipation, and it was experimentally verified that the cycling of segregation patterns has close relation with the presence of air.
2008, 57 (4): 2306-2313. doi: 10.7498/aps.57.2306
The dual-mesh hybrid numerical method is used to investigate the salt finger convection, as well as the heat and mass transfer, in saturated porous media. It is found that at low normalized porosity of porous media, saline diffusion is much faster than thermal diffusion. The fingers grow straightly to the top and bottom walls, which would lessen the saline gradient of stratified system. Also, it is indicated that the hybrid method is perfect for dealing with small scale convection in porous media because of its great veracity, efficiency and stability.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2008, 57 (4): 2314-2319. doi: 10.7498/aps.57.2314
Time-resolved measurement of plasma emission spectra by pulsed XeCl excimer laser ablation of Al under different conditions is reported. The experimental results show that plasma emission spectra are composed of atomic spectrum, univalent ion spectrum and continuous radiation. The atomic spectrum intensity and duration is most significant, the next is the univalent ion spectrum, the continuous radiation is weakest. The detailed mechanism of species excitation is discussed. The continuum radiation comes from electron bremsstrahlung and recombination of electron and ion, the primary atomic and univalent ion excitation originate from recombination of ion and electron. The experimental results are qualitatively explained with the mechanism.
2008, 57 (4): 2320-2327. doi: 10.7498/aps.57.2320
Zakharov-Kuznetsov(ZK) equation, modified ZK equation and coupled ZK equation are derived for magnetized plasma containing both positively and negatively charged cold ions and hot electron by using the reductive perturbation method in this paper. We obtain a solitary wave solution, and also obtain the variation of solitary wave parameters, such as amplitude, width, velocity of soliton, with the mass ratio of negative ions to positive ions, the negatively charged ion number density and the magnetic field intensity.
2008, 57 (4): 2328-2333. doi: 10.7498/aps.57.2328
A high-intensity cold cathode based on field emission from multi-walled carbon nanotubes was fabricated by screen-printing. The high intensity was obtained when the cathode was subjected to a pulsed electric field. The emission properties of the cathode were investigated in both single-pulse and double-pulse mode experiments. Emission photos of the cathode surface were investigated, and the pulsed emission mechanism of the carbon nanotube was explosive electron emission. In the single-pulse mode, the field emission current density is 99 A/cm2 at an applied electric field of 16.7 V/μm. In the double-pulse mode, the emission current density can reach 267 A/cm2 at an applied electric field of 15.4 V/μm. This carbon nanotube cathode appears to be suitable for high-power microwave device applications.
Formations of conic surfaces on diamond films induced by hot filament assisted double-bias hydrogen plasma
2008, 57 (4): 2334-2339. doi: 10.7498/aps.57.2334
The hot filament assisted double-bias hydrogen has been employed for the surface fabrication of the as-formed chemical vapor deposited diamond films on the nanometer scale. Diamond cone arrays are successfully prepared through hydrogen plasma etching. The intrinsic columnar structure of the diamond films leads to the unevenly distributed ion etching rate, which plays an important role in the cone formation. Simultaneously, the carbon-containing species sputtered out may re-deposit on the surface. The evolvement of the characteristic surface is thus determined by the competition between the ion etching and the carbon deposition. The application of grid electrode influences the discharge characteristics at the substrate region. By controlling the grid current, the surface structure of diamond films may be significantly tuned. Moreover, by introducing small amounts of methane in the etching process, the increased concentration of carbon-containing species enhances the secondary diamond nucleation on the film surface, which further promotes the uniformity of the diamond cone arrays.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
Theoretical study on the reaction mechanism of Sr,Ti,O reactions in early growth of SrTiO3 thin films
2008, 57 (4): 2340-2346. doi: 10.7498/aps.57.2340
It is very important to learn the microscopic reaction and particle states of Sr,Ti,O atoms which are evaporated by laser molecular beam epitaxy in the early growth of SrTiO3 thin films. The preferential reaction processes of Sr,Ti,O atoms were investigated by generalized gradient approximation of density functional theory at the PW91/DNP level. The reaction mechanisms of SrO,TiO2 and SrTiO3 molecules were calculated and analyzed by highest occupied and lowest unoccupied molecular orbital theory, and several possible configurations of SrTiO3 molecules were calculated. We obtained stable configurations of SrTiO3 molecule, of which the geometric eigenvalue is similar with the SrTiO3 unit cell. These results indicate that the SrO,TiO2 and SrTiO3 molecules are the primary particle states in the early growth of SrTiO3 thin film.
2008, 57 (4): 2347-2351. doi: 10.7498/aps.57.2347
In this paper, reverse-impact method for measuring release sound velocities was improved through special treatment of the impacted face of the window, and using time-resolved velocity interferometer system for any reflector. Our experimental technique solved the problem that bulk and longitudinal sound velocities can not be obtained simultaneously using the original method reported in the literature, due to the effect of release behaviors in the buffer. With the modified reverse-impact method, sound velocities of tin were measured with higher accuracy (measuring errors of longitudinal and bulk sound velocities being about 2% and 5%, respectively) in the range of 37—80 GPa. The experimental results are consistent with thermodynamic theoretical values.
2008, 57 (4): 2352-2357. doi: 10.7498/aps.57.2352
A method to determine the effective shear modulus under shock loading conditions by using longitudinal and bulk sound velocities along the quasi-elastic release was introduced in present paper. The effective shear modulus was calculated for LY12 aluminum over shock pressures range of 20—70 GPa. Results show that the effective shear modulus decreases linearly with the stress, reaching to zero at the reverse yield point, and the effective shear modulus at first release can be depicted by the modified Steinberg-Cochran-Guinan model. Using the effective shear modulus data, simulation has been successful in reproducing the experimental data with quasi-elastic release behavior, showing smooth transition from elastic wave to fully plastic wave.
2008, 57 (4): 2358-2362. doi: 10.7498/aps.57.2358
Based on the composite oscillator model of atomic-scale wearless friction, a quantum mechanics model is proposed for analyzing the energy dissipation. The analysis indicates that the energy dissipation is discontinuous in a sliding process. The energy absorbing ability of an atom on a contact surface is discrete when the atom jumps from one equilibrium state to another. At the same time, the ability of a contact surface atom to absorbe the interfacial potential energy is stronger in a high energy state than in a low energy state. This will provide the theoretical basis for analyzing the periodical change in friction.
Characterization of Hall-Petch relation on microstructural parameter of softer sublayer and the hardness in nanomultilayers
2008, 57 (4): 2363-2367. doi: 10.7498/aps.57.2363
Scale dependent microstructures and plastic deformation in Ni/Al nanomultilayers with different modulated ratio has been investigated as a function of bilayer thickness by X-ray diffraction and nanoindentation. Experimental results show that the hardness and the related microstructural parameter in the softer sublayer exhibited the similar variation tendency, which indicated the sensitivity of plastic behaviors on the microstructural constraints in softer sublayer. Further investigation showed that in the nanometer regime, by adopting a new parameter r in softer sublayer (r=Lsub/d, where Lsub is the sublayer thickness and d is the grain size), the competing grain-boundary and interface-boundary strengthening mechanisms could be unified. Moreover, within the overall scale, the deformation of nanomultilayers could follow the Hall-Petch relation with decreasing characteristic length scales.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2008, 57 (4): 2368-2373. doi: 10.7498/aps.57.2368
The electronic structures of spin-Peierls (SP) compound GeCuO3 have been studied by first-principles calculations with plane wave pseudo-potential method. The calculated results indicate that the 3d orbital degree of freedom for the Cu2+ has been frozen, and the unpaired electrons occupy the dx2-y2 orbits. The exchange splitting of the spin-up and spin-down dx2-y2 orbitals leads to the insulating nature of the system. GeCuO3 belongs to the category of covalent insulators, since there are very strong hybridizations between the Cu 3d and O(2) 2p states around the Fermi level. These strong covalent interactions lead to not only the deviation of the ideal spin moment of the Cu2+, but also the stabilization of the one-dimensional antiferromagnetic (AFM) ordering. We fit the spin exchange coupling constants within the Noodleman's broken symmetry methods through the calculated total energy for the various spin ordered states of GeCuO3. The calculated results indicate that the one-dimensional AFM interactions along the c axis are very strong in GeCuO3, which is the essential reason that brings on the SP transition at low temperature.
2008, 57 (4): 2374-2379. doi: 10.7498/aps.57.2374
An ab initio method with the first-principles plane-wave pseudopotentials based on the density functional theory has been used to calculate the physical character and electrochemical performance of various alloy phases in Li-Sn alloy. The results show that Li5Sn2 alloy phase, with relatively small volume expansion ratio and large reversible capacity, is an ideal alloy electrode phase. On the other hand, many kinds of tin thin film electrodes were fabricated by direct current and radio frequency magnetic sputtering on copper foil collector as anode materials for lithium-ion batteries. The values of calculated Li intercalated potential using the CASTEP program are well consistent with the experimental values.
2008, 57 (4): 2380-2385. doi: 10.7498/aps.57.2380
The site preference of 3d transition metals in B2-NiAl intermetalics and its effects on bond characters are investigated by the pseudopotential plane wave method. The formation energy results show that Sc, Ti, V and Zn prefer to occupy the Al site, while Cr, Mn, Fe, Co and Cu prefer to occupy the Ni site in NiAl. By analyzing changes in crystal lattice constants, Mulliken population, overlap population and valence charge density, the crystal lattice distortion and bond characters are discussed. The results show that the lattice distortions of Sc, Ti, V and Zn doped NiAl play an important role in determining the alloy properties. The strong repulsion interaction and anti-bonds as well as charge transfer between the dopant atom and the nearest_neighbor Ni atoms are observed, which makes the fraction of covalent bonds to be decreased and the fraction of the metallic bonds to be increased by Sc, Ti, V and Zn substituting the Al sites of NiAl. The bond character changes a little by Cr, Mn, Fe, Co and Cu substituting the Ni sites of NiAl.
Influence of structure and doping concentration of AlxGa1-xN/GaN double quantum wells on wavelength and absorption coefficient of intersubband transitions
2008, 57 (4): 2386-2391. doi: 10.7498/aps.57.2386
By solving the Schrdinger and Poisson equations self-consistently, the central barrier height, central barrier width, well width, and doping concentration in the barriers of symmetric Al0.75Ga0.25N/GaN double quantum wells (DQWs) have been studied to investigate their influences on the wavelength and absorption coefficient of intersubband transitions (ISBTs). A smaller wavelength of the ISBT between the first odd and the second even order subbands (S1odd-S2even ISBT) in Al0.75Ga0.25N/GaN DQWs and a larger absorption coefficient of the S1odd-S2even ISBT were obtained with decreased central barrier height, when the central barrier height was larger than 0.62 eV. The wavelength of the S1odd-S2even ISBT decreases, and the absorption coefficient of the S1odd-S2even ISBT increases, when the width of the central barrier is reduced. On the other hand, decreasing the width of the well will result in smaller wavelength of the S1odd-S2even ISBT and larger absorption coefficient of the S1odd-S2even ISBT when the width of the well is narrower than 1.9 nm. When doping concentration in the barriers is smaller than 1018/cm3， the wavelength of the S1odd-S2even ISBT is unchanged, while the absorption coefficient of the S1odd-S2even ISBT increases with the doping concentration. These results provide useful guidance for realization of ultrafast two-color optoelectronic devices operating in the optical communication wavelength range.
2008, 57 (4): 2392-2398. doi: 10.7498/aps.57.2392
Many interesting long-time dynamic properties of solid interface, such as deep diffusion, pervasion and phase forming, cannot be simulated directly using traditional molecular dynamics (MD) because of nanosecond timescale limitations. Thus, a simpler bias potential form has been proposed within the Voter's hyper dynamics scheme. In this method, the potential energy wells are raised by adding a coefficient, which was defined as the accelerating factor, to the original potential. So, the escape rate from potential wells was enhanced, which extends the timescale by several orders of magnitude comparing to the traditional MD simulations. What's more important, the features of potential surface are reserved even without any in_advance knowledge of the location of either the potential energy wells or saddle points. We demonstrate this method by applying it to the mutual diffusion of atoms in Mg/Zn interface with different accelerating factors using a simple Lennard-Jones potential. The results showed that long-time MD simulation can be realized very easily by our approach.
2008, 57 (4): 2399-2403. doi: 10.7498/aps.57.2399
Kinetic Monte Carlo simulations are applied to study the growth of self-assembled vertical ordering InAs quantum dot superlattice on GaAs substrate. The study is focused on the influences of flux and interruption time at the initial stage when the first sub-monolayer is forming on the wetting layer. We demonstrate that uniform sized and regularly ordered island arrays can be obtained by controlling flux and interruption time, by means of studying the surface morphology, average island size, island size distribution and the standard deviation of island size distribution comprehensively. The size and order of island arrays will greatly affect the location and size of quantum dots in subsequent three-dimensional growth.
2008, 57 (4): 2404-2408. doi: 10.7498/aps.57.2404
The electronic structure parameters of Ti alloys, such as density of states, Fermi energy level, environment-sensitive embedding energy etc, have been calculated by recursion method. The larger environment-sensitive embedding energy of Pd in the bulk of Ti alloy than that on the surface of Ti alloy indicates that Pd is apt to segregate on the surface. The negative formation energy of Pd clusters suggests that the distribution of Pd atoms on the surface of Ti alloy is in the form of atom clusters. The range of the local density of states of Pd is narrow, while that of Ti is wide. So a peak appears in the total density of states of Ti alloys between -20 and -15 eV. This peak has a significant effect on the Fermi level. It makes the Fermi energy of the surface Ti alloys with more Pd atoms low. So the microcells form between the area with more Pd atoms and the area with fewer or without Pd atoms on the surface of Ti alloys. Ti dissolves preferentially in the corrosive media. This leads to form a layer of Pd atomic clusters on Ti alloy surface. The Pd atomic layer serves as the electrocatalytic surface, facilitating the passivation of Ti alloys and improving the corrosion resistance.
2008, 57 (4): 2409-2414. doi: 10.7498/aps.57.2409
There may be an antiferromagnetic quantum phase transition near x=1.0０ in NiS2-xSex system. The polycrystalline samples of NiS2-xSex (x=0.96, 0.98, 1.00, 1.05, 1.10 and 1.20) were prepared by the solid state reaction method, and the measurements of their structure, magnetization and resistivity were carried out. It is found that the temperature dependence of susceptibility is typical characteristic of a strongly correlated electron system. Similar to high Tc superconductors, the relationship between resistivity and temperature displays a linear behavior in a wide temperature range from 50 to 300 K. For NiS2-xSex(x=0.98 and 1.00) samples, its ρ(T) is proportional to T3/2 at lower temperatures (3—30 K), which shows a non-Fermi-liquid behavior. But for NiS2-xSex (x=1.10 and 1.20) samples, its ρ(T) is proooportional to T2 at lower temperatures (3—30 K), which shown the Fermi-liquid behavior. The antiferromagnetic quantum spin fluctuation related to quantum phase transition is discussed in detail.
Effect of disorder degree on the localization-delocalization transition in one-dimensional disordered system with long-range correlations
2008, 57 (4): 2415-2420. doi: 10.7498/aps.57.2415
In order to reveal the effect of disorder degree on the localization-delocalization transition in one-dimensional disordered system with long-range correlations, the long-range power-law correlated energy sequence was modified. By using the renormalization group method, the Lyapunov exponent characterizing the localization-delocalization transition was calculated. The results showed that, compared with the correlation exponent, the disorder degree plays an opposite role in this transition. When the correlation exponent was fixed but the disorder degree increased, the extended states, which appeared at the center of energy band due to the influence of long-range correlations, were gradually converted to localized states. When the disorder degree increased to a critical value Wc, the whole eigenstates of the system became localized states, and the critical value Wc increased with the increase of correlation exponent.
2008, 57 (4): 2421-2426. doi: 10.7498/aps.57.2421
Using the slave-boson mean-field approximation, we theoretically study the transport properties of the ground state of the T-shaped coupled quantum dots with two metallic leads by means of one-impurity Anderson Hamiltonian. It is found that when this system is in the equilibrium state, with the increase of the inter-dot coupling, the Kondo resonance is suppressed, and the single Kondo peak of the density of the Kondo quantum dot is split into two asymmetric Kondo peaks. When it is in the non-equilibrium state, when increasing the bias, this asymmetric Kondo splitting is enhanced.
The control of the crucial current in current-induced magnetization switching with an external magnetic field
2008, 57 (4): 2427-2431. doi: 10.7498/aps.57.2427
The current induced magnetization switching (CIMS) in a nano-scale pseudo-spin-valve (PSV) structure at low external magnetic filed is investigated using a macroscopic phenomenological model based on the magneto-dynamic equation. Magnetization reversal conditions and the corresponding critical currents are obtained by considering both the spin-dependent scattering at the ferromagnetic/nonmagnetic interfaces and the relaxation of spin accumulation in the ferromagnetic layer. The movement of the resistance-current hysteresis of the PSV structure at low external magnetic filed are explained by the calculation. The way to control the crucial current in the CIMS effect with an external magnetic field is also suggested.
2008, 57 (4): 2432-2437. doi: 10.7498/aps.57.2432
The CaCu3Ti4-xFexO12 (0≤x≤0.2) ceramics have been prepared by a standard solid-state reaction method, and the influence of Fe doping on the microstructure and dielectric properties of CaCu3Ti4-xFexO12 ceramics were investigated by the X-ray diffraction, scanning electron microscopy, dielectric spectroscopy and impedance spectroscopy. It has been found that complete solid solutions are formed for all of the compositions x. With the increase of Fe content, the semiconductivity of grain vanishes gradually and the dielectric constant decreases. For specimens with x≤0.04, the two dielectric relaxation process Ⅰ and Ⅱappeared in the frequency ranges of 106—108 and 103—104 Hz, respectively. These two dielectric relaxation process were considered to be associated with grain boundaries and interfacial polarization between the electrode and ceramic surface, respectively. In addition, the third dielectric relaxation Ⅲ was detected in the high-temperature dielectric spectroscopy of CaCu3Ti3.99Fe0.01O12 ceramic, which was caused by a hopping process of localized charge carriers. The activation energy of this thermally excited relaxation is 0.78 eV, as obtained by using the Arrhenius formula.
2008, 57 (4): 2438-2444. doi: 10.7498/aps.57.2438
According to the conclusion that the charge which passes through a low-temperature quantum conductor over fixed time has a binomial distribution, a shot noise model based on the physical implication of Landauer formula in the coherent and mesoscopic system is built, and time sequences of shot noise are produced by the Monte Carlo methods. It is well known that shot noise suppression in mesoscopic systems originates from the correlation in electron transport, and the bimodal probability distribution of the transmission eigenvalues in quantum chaos cavity and disordered metal leads to shot noise suppression, from which it is possible to correlate the distribution of the transmission eigenvalues and the correlation in electron transport.
Investigation on the formation mechanism and diffusion of the electrode metal of oxidized Au/Ni/p-GaN ohmic contact in different alloying time
2008, 57 (4): 2445-2449. doi: 10.7498/aps.57.2445
Using Rutherford backscattering (RBS)/channeling method, we investigated the microstructure evolution in oxydizing atmosphere of Ni/Au contact on p-GaN during annealing at constant temperature (500 ℃) but for different alloying time. The technique of RBS/channeling and simulation of RUMP program were used to investigate the diffusion of the electrode metal. It was found that Ni diffused to the interior of the sample, Au diffused to the surface of p-GaN after 60s alloying time and the epitaxial structures of Au also began to form on p-GaN at the same time. The O also diffused into the sample after 180 s alloying time. Considering a continuous decrease in the ρc, it is suggested that the epitaxial structures of Au on p-GaN and the formation of NiO play a critical role in forming low resistance ohmic contact to p-GaN. At 300 s alloying time at the same annealing temperature (500℃), the epitaxial structure of Au was impoved further and the O also diffused deeper into the sample, and the ohmic contact reached the optimal value.
Research on the current collapse in AlGaN/GaN high-electron-mobility transistors through the inverse piezoelectric polarization model
2008, 57 (4): 2450-2455. doi: 10.7498/aps.57.2450
Current collapse in AlGaN/GaN high-electron-mobility transistors was studied and the effect of effective external electrical field on the concentration of two-dimensional electron gas (2DEG) near the channel of hetero-junction of AlGaN/GaN was simulated through the self-consistent implementation of the one-dimensional Poisson-Schrdinger equations. An inverse piezoelectric polarization model was proposed to analyze the degradation of 2DEG and the current collapse. It was found that the density of 2DEG was strongly dependent on the inverse polarization. When the voltage was 0,10 and 15 V, the density was 1.53×1013cm-2, 1.04×1013cm-2 and 0.789×1013cm-2, respectively. It was clear that the 2DEG density dropped 48.4% with voltage increasing from 0 to 15V when the inverse piezoelectric polarization was considered. At last, the method of restraining current collapse was further discussed.
2008, 57 (4): 2456-2461. doi: 10.7498/aps.57.2456
Results of investigation on AlGaN/GaN field-plate high electron mobility transistor with different field-plate (FP) geometry are presented. The effect of the field-plate length LFP on the electric field distribution in the channel is thoroughly analyzed by establishing a simplified model. The simulation gives the following estimates: Both the FP length LFP and the thickness t of the insulator under the FP, can reshape the electric field distribution in the channel. If LFP is short, the breakdown voltage Vbr increases with LFP. When LFP increases to a certain extent, Vbr keeps invariable. After optimizing LFP in this paper, Vbr has been increased by 64%. Good agreement between experimental and simulation data is achieved.
2008, 57 (4): 2462-2467. doi: 10.7498/aps.57.2462
This paper realizes cellular neural networks using the characteristic of negative differential resistance of hybrid single electron transistor and complementary metallic oxide semiconductor field effect transistor structure. The main building blocks consisting of cell core circuit, A and B template circuits are designed. Then a cellular neural network (CNN) is built and its application in image processing is studied. The computer simulation shows that the designed circuits are suitable for CNN implementation owing to its simple structure, low power dissipation and fast response. It could be used to form CNN of various scales so as to further increase the density of integrated circuits.
Research on correlation of 1/fγ noise and hot carrier degradation in metal oxide semiconductor field effect transistor
2008, 57 (4): 2468-2475. doi: 10.7498/aps.57.2468
Metal oxide semiconductor field effect transistor (MOSFET) in high-, mid- and low-gate stresses of hot carrier degradation effect and its 1/fγ noise feature are studied. Based on the formation theories of interface traps and oxide traps in Si-SiO2 and the MOSFET 1/fγ noise mechanism, the process of exchange carrier between the defect in gate oxide and the free-carrier in the channel is simulated by the duo-phonon emission model. A unified physical model for hot carrier effect, material defect, electrical parameter and noise was built. Also, a method characterizing the MOSFET anti-hot carrier abilities with noise parameter Sfγ is presented. The model is testified by the experiment, which is designed based on the relation between hot carrier and noise. Experimental results well confirm the developed model.
Investigation on the improvement of the stability and uniformity of solution-based metal-induced crystallization poly-Si using surface-embellishment
2008, 57 (4): 2476-2480. doi: 10.7498/aps.57.2476
To obtain poly-Si with good uniformity and stability, a new method of solution-based metal-induced crystallization (S-MIC) with surface-embellishment wasproposed in this paper. A special solution that can embellish the surface of a-Si was employed to improve the conglutination between nickel and a-Si thin film. Before the nickel solution was spin coated, the special solution was spin coated on the a-Si sample at first. By controlling the nickel salt concentration, the poly-Si with grain size of 20—70 μm and good uniformity could be achieved. Furthermore, the nickel salt concentration is lower than that used in the traditional method of S-MIC by 1 or 2 orders of magnitude.
Electron transport properties of In0.53Ga0.47As/In0.52Al0.48As quantum wells with two occupied subbands
2008, 57 (4): 2481-2485. doi: 10.7498/aps.57.2481
Magnetotransport properties of two-dimensional electron gas have been investigated for three In0.53Ga0.47As/In0.52Al0.48As quantum well samples having two occupied subbands with different well widths. When the intersubband scattering is considered, we have obtained the subband density, transport scattering time, quantum scattering time and intersubband scattering time, respectively, by analyzing the result of fast Fourier transform of the first derivative of Shubnikov-de Haas oscillations. It is found that the main scattering mechanism is due to small-angle scattering, such as ionized impurity scattering, for the first subband electrons.
2008, 57 (4): 2486-2492. doi: 10.7498/aps.57.2486
The valence electron structure and magnetic properties of Nd2Fe14B intermetallics have been investigated using empirical electronic theory of solids and molecules. The calculations fit the experimental data well. It reveals that the magnetic properties of Nd2Fe14B are proportion to the 3d electrons of Fe atoms. The moments increase from the Fe at 4c site to the Fe at 16k2 site, which is due to the changes between the valence electrons, dumb pair electrons and 3d magnetic electrons. According to the calculation, 78% of dumb pair electrons and 18% of valence electrons in d orbit change to magnetic electrons, this causes the increase of moments from the Fe at 4c site to the Fe at 16k2 site. Both Curie temperature and moments are proportion to the coordination numbers of the Fe atom, however, the Curie temperature decrease with the weighted equal bond numbers Iσ. The effect on Curie temperature of non-magnetic elements Nd and B is derived from the change of the Fe—B and Fe—Nd bond distances.
2008, 57 (4): 2493-2499. doi: 10.7498/aps.57.2493
Nanoparticles of Prussian blue analogues Ni3［Fe(CN)6］·8.28H2O have been successfully synthesized by co-precipitation method. The nanoparticles were characterized by elemental analysis, X-ray diffraction, Fourier transform infrared spectrometry and transmission electron microscopy. Magnetic measurement was carried by superconducting quantum interference device magnetometer and the result showed the the transition temperature Tc=22.8K. The interaction between Ni(Ⅱ) and Fe(Ⅲ) was identified to be a kind of ferromagnetic coupling and was clearly explained by molecular orbital theory. The Mssbauer spectra have been used to explain the origin of magnetism.
2008, 57 (4): 2500-2503. doi: 10.7498/aps.57.2500
The Helmholtz and Kelvin forces for volume force density in magnetic fluids are valid only in some situations. Based on calculation of the volume force density in magnetic fluids, the origin for the Helmholtz and Kelvin forces in magnetic fluids is suggested, and the range of validity of them is analyzed in this paper. The calculation results show that the volume force density can be expressed in the form of Helmholtz force when the permeability of magnetic fluid is independent of the applied magnetic field, while the Kelvin force is valid when the average magnetic moment of magnetic particles in the magnetic fluid is independent of the specific volume of magnetic fluid. When the susceptibility is proportional to the density of magnetic fluid, the Helmholtz force is reduced to the Kelvin force.
2008, 57 (4): 2504-2508. doi: 10.7498/aps.57.2504
A series of synthetic antiferromagnets and spin valves with synthetic antiferrimagnets as free layers were fabricated by DC magnetron sputtering. It is found that inserting a thin Co90Fe10 layer of proper thickness between Ni81Fe19 and Cu is an effective means to strengthen the antiferromagnetic coupling in the synthetic antiferrimagnet, and obtain synthetic antiferromagnets with excellent magnetic properties, i.e., higher saturation magnetic field, lower saturation magnetization value and better thermal stability. In addition, spin valves using the synthetic antiferrimagnet as a free layer have better stability.
2008, 57 (4): 2509-2513. doi: 10.7498/aps.57.2509
The properties of dual-synthetic antiferromagnet (DSAF) structure Co90Fe10(5 nm)/Ru(x nm)/Co90Fe10(3 nm)/Ru(y nm)/Co90Fe10(5 nm) (in which x=0.45 y=0.45; x=0.45 y=1; x=1, y=1) were studied. The effect of DSAF structure on giant magnetoresistance (GMR) single spin-valve (SPV) structures was also studied. The study shows that the properties of DSAF are better than those of the conventional synthetic antiferromagnet Co90Fe10(5 nm)/Ru(0.5 nm)/Co90Fe10(3 nm). We demonstrated that the DSAF structure as a pinned layer dramatically enhances the GMR ratio in single SPV films at room temperature.
Soft magnetic properties and microwave permeability of multilayer nanogranular films with high resistivity
2008, 57 (4): 2514-2518. doi: 10.7498/aps.57.2514
The resistivity, soft magnetic properties and microwave permeability of multilayer nanogranular films suitable for high frequency applications have been investigated in this work. These films were prepared by repeated sequential depositions of thin layers of Co40Fe40B20 and SiO2. After vacuum annealing at 250℃ under a uniform in-plane magnetic field of 100kA/m for 2h, the Co40Fe40B20/SiO2 multilayer films have the hard axis coercivity of 210A/m, saturation magnetization of 838.75kA/m, resistivity of 2.06×103μΩcm. Moreover, the results show that the films have an in-plane uniaxial anisotropy of 2.78kA/m. As a consequence, the real part of measured permeability keeps constant at 125 up to 1.6GHz. The resonance frequency was about1.73GHz.
2008, 57 (4): 2519-2523. doi: 10.7498/aps.57.2519
The theory of stress-impedance effect in amorphous alloys was established by introducing stress-induced anisotropy and magnetic anisotropy and solving Maxwell's equations and Landau-Lifshitz equation with the Gilbert term. The established theory can be used to explain the stress-impedance effect, and the calculation results were in good agreement with the experimental results. We calculated the stress-impedance change with different anisotropy parameters, and reached important conclusions both in theory and for applications.
2008, 57 (4): 2524-2528. doi: 10.7498/aps.57.2524
The experiment result shows that under unipolar voltage stress, electrons tunneling into ultra-thin gate oxide during on-time will recombine with the trapped holes, and neutral electron traps would be generated. These electron traps would assist electrons tunneling into gate oxide. It is proposed that the characteristic time τ for the charge to be trapped in SiO2 is longer than the on-time ton of unipolar stress. During the on-time few charges are trapped and during the off-time some trapped charges will be detrapped, thus very few neutral electron traps are generated. With the stress time increasing, the density of neutral electron traps would reach a critical value, and then the gate oxide would break down abruptly. The increase in unipolar breakdown time tBD is attributed to fewer charge trapping during the on-time and charge detrapping during the off-time.
2008, 57 (4): 2529-2536. doi: 10.7498/aps.57.2529
The coherent interactions between one-dimensional discrete spatial optical solitons, derived by Petviashvili iteration method, are numerically investigated with the split-step beam propagation method in light-induced planar waveguide arrays. The influences of the initial phase difference, the soliton peak intensities, the periods and intensities of the writing beams for waveguide arrays, and the amplitudes of the externally bias fields on the interactions between two discrete solitons are analyzed in detail. It is found that the interactions between two parallel discrete solitons with different initial phase differences behave in a way similar to that encountered in continuous media, i.e., in-phase (out-of-phase) discrete solitons attract (repel) each other, and the intermediates are always accompanied with energy transfer. In addition, the interaction processes are influenced by the variations of soliton peak intensities, the configurations of the light-induced waveguide arrays, and the amplitudes of the external bias fields.
2008, 57 (4): 2537-2542. doi: 10.7498/aps.57.2537
ZnO and ZnO-SiO2 nano-scale thin films were deposited on glass substrates by the sol-gel method. The ultraviolet(UV)-visible spectrophotometer was used to analyze their optical properties. The UV-visible transmittance spectra of ZnO films showed that when the mole concentration of ZnO precursor decreased from 0.7 to 0.06 mol/L, besides the absorption edge at 380 nm, a new absorption edge around 320 nm appeared. Furthermore the decrease of mole concentration of ZnO precursor improved the transmittance in the range between 380 and 320 nm. However, ZnO-SiO2 nano-scale thin films only have one absorption edge at 310nm. The optical band gap of ZnO was enlarged when deposited in SiO2 matrix.
2008, 57 (4): 2543-2547. doi: 10.7498/aps.57.2543
The quasiparticle dynamics in the electron-doped high-Tc superconductor La2-xCexCuO4 (LCCO) was investigated by ultrafast time-resolved optical pump-probe technique. We present results in three temperature regions: namely the low temperature (TTc) region, that around Tc(0.7Tc≤T≤Tc) and the high temperature (T>Tc) region. In general, there was a slowdown of the relaxation as the temperature decreases. However, the quasiparticle lifetime was found to exhibit a quasi-divergence as the temperature approached Tc from below. The relaxation time in normal state was much longer than what was expected for metallic relaxation, indicating the presence of a T-independent pseudogap. We have also observed picosecond rise component of photoinduced change in reflectivity ΔR/R in the electron-doped LCCO, which is attributed to the Cooper pair-breaking dynamics. The rise time became shorter as the temperature increased. The experimental results are analyzed by Rothwarf-Taylor model.
2008, 57 (4): 2548-2553. doi: 10.7498/aps.57.2548
A new method to reduce the dark current of GaN based Schottky barrier ultraviolet photodetector is proposed. In comparision with conventional i-GaN/n＋-GaN structure, an additional thin p-GaN cap layer is introduced on the i-GaN(n--GaN) in the new structure. The simulation results showed that the additional layer makes the dark current to decrease in the photodetector due to the increase of the Schottky barrier height. The effects of thickness and carrier concentration of p-GaN layer on the dark current of the photodetector were also studied. It is suggested that the dark current of the new structure device could be better reduced by employing p-GaN with higher carrier concentration as the cap layer.
2008, 57 (4): 2554-2561. doi: 10.7498/aps.57.2554
Glasses with the compositions of (77-x)GeO2-xGa2O3-8Li2O-10BaO-5La2O3(x=4,8,12,16) doped with 8.0mol% of Tm3+ were fabricated. The spectroscopic properties and thermal stability were investigated experimentally with increasing Ga2O3 content from 4mol% to 16mol%. It is noted from differential thermal analysis curves that the thermal stability of the glasses are improved by increasing Ga2O3. According to Judd-Ofelt theory, the Judd-Ofelt strength parameters (Ω2，Ω4，Ω6) of Tm3+-doped GeO2-Ga2O3-Li2O-BaO-La2O3 were calculated, by which the radiative transition probabilities, fluorescence branching ratios and radiative lifetimes were obtained. The emission spectra of Tm3+-doped germanate glasses and the effects of Ga2O3 on the emission properties were investigated with 808nm laser diode excitation. The fluoroscopic intensity and the maximum stimulated emission cross section first decrease along with the increase of Ga2O3 content from 4mol% to 16mol%, and reach a minimum at Ga2O3 content of 12mol%, and then increase. The effects of Ga2O3 on the glass structure and the spectral parameters were also preliminarily investigated.
2008, 57 (4): 2562-2566. doi: 10.7498/aps.57.2562
Single crystals of sapphire (Al2O3) with (0001), (1010) and (1120) orientations were implanted at 623 K with 110 keV Ar ions to fluence of 9.5×1016 cm-2. The ion-implanted Al2O3 samples were annealed at 873, 1073, 1273 and 1373K for 60 min in vacuum and in air, respectively. Photoluminescence (PL) spectra of the as-implanted samples showed an emission band at 506 nm, with excitation wavelength at 300 nm. The PL peak intensity of (0001) and (1010) orientation samples were maximum after annealing at 1073K in both vacuum and in air. The annealing in air at 873, 1073 and 1273K lead to much higher PL peak intensity compared to annealing in vacuum. In all the samples the emission band disappeared after annealing at 1373K both in vacuum and in air. The experimental results indicate that annealing temperature, annealing atmosphere and crystal orientation play important roles for the PL peak intensity at 506nm of sapphire implanted with Ar ions. The PL peak at 506nm after Ar-implantation and annealing is related with the population of interstitial Al atoms introduced by the ion bombardment and the formation of argon gas bubbles and the resolution of Ar atoms during annealing.
2008, 57 (4): 2567-2573. doi: 10.7498/aps.57.2567
We propose models of the incoherent optical absorption and exciton transport to simulate the processes of absorption and exciton diffusion in organic photovoltaic devices according to the optical principle and diffusion theory. The functional relation of optical absorption, and the power conversion efficiency versus the thickness of the film has been obtained. We demonstrate that the developed model can be useful for the modelling of the optical absorption and exciton separation in the optimization of devices of organic photovoltaic cells with high power conversion efficiency.
A theoretical model is developed to study the optical transition and optical gain of PbTe/CdTe quantum dots. The model is based on the k·p envelope function approach, and anisotropic band structure characteristics of PbTe are taken into consideration. The relationships of optical gain of PbTe/CdTe quantum dots vs dot size and injection carrier density are given. The theoretical results suggest that PbTe/CdTe quantum dots with dot size of 15—20nm are promising materials for mid-infrared lasers, which may produce optical gain higher than 5000cm-1 when the injection carrier density ranges (0.3—3)×1018cm-3. The optical gain decreases with dot size increasing. However, higher injection carrier density is required for PbTe/CdTe quantum dots with smaller sizes (<15nm). Therefore the optimal PbTe quantum dot sizes are 15—20nm.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2008, 57 (4): 2582-2586. doi: 10.7498/aps.57.2582
Well aligned ZnO nanorod arrays on Si substrate previously coated with a thin buffer layer of ZnO seeds via radio-frequency magnetron sputtering, which were synthesized by the aqueous solution method. The sample was characterized by scanning electron microscopy, X-ray diffraction, high-resolution transmission electron microscopy and selected area electron diffraction. The results show that ZnO nanorods have good crystal quality， are highly oriented and grown along c-axis. The full width at half maximum of the (0002) peaks of the nanorod arrays is only 0.06°. Furthermore, a strong and narrow eigen peak centered at 378nm and very weak emission in visible region due to impurities or defects were observed in the photoluminescence spectra.
2008, 57 (4): 2587-2594. doi: 10.7498/aps.57.2587
The symbolic dynamics was introduced to analyze the complexity of cognitive event-related potentials (ERPs). This method was first applied to the case of the MIX(p) system, and then was compared with the approximate entropy which had been successfully used to analyze the biological and medical experimental data. The complexity of ERPs in the Oddball paradigm was calculated with the symbolic dynamics. It was found that the complexity of ERPs over the frontal, central, and parietal areas declined significantly during the processing of tasks, reached at the minimum corresponding to the latency of P300 component, and rose back after the response was performed. In addition, this method was sensitive to the discrimination between target stimuli and non-target stimuli. The results indicate that the complexity based on the symbolic dynamics can sensitively reflect the processing of the cognitive task, and will be a promising tool to analyze the neural activity associated with the cognitive task.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
KdV equation with a forcing term for the evolution of the amplitude of Rossby waves along a slowly changing topography
2008, 57 (4): 2595-2599. doi: 10.7498/aps.57.2595
Employing the weakly nonlinear method and perturbation method, the evolution ofthe amplitude of Rossby waves when the topography slowly changes is studied. AKdV equation with a forcing term governing this evolution is obtained from the quasi-geostrophic vorticity equation, and the forcing term is induced by the slowly changing topography.
Improved Q vector analyses from the perspective of field separation and its application in a torrential rain event
2008, 57 (4): 2600-2606. doi: 10.7498/aps.57.2600
According to the theory of separation and reconstruction of wind field, the Q vector is separated accordingly in limited area using the approach of harmonic-cosine series expansion. The rotational and divergent components of Q vector are introduced into diagnosing weather systems as an improvement in the traditional Q vector analyses. Through a case study of a torrential rain event, potential advantages of the improved Q analyses approach can be seen in dynamic identification of both the location and the position of heavy rain centers.
The influence of atmospheric turbulence on the spatial correlation property of partially coherent Hermite-Gaussian beams
2008, 57 (4): 2607-2613. doi: 10.7498/aps.57.2607
Based on the extended Huygens-Fresnel principle, the expression for the spectral degree of coherence of the partially coherent Hermite-Gaussian (H-G) beams propagating through atmospheric turbulence is derived by using the quadratic approximation of Rytov's phase structure function, and the influence of atmospheric turbulence on the spatial correlation is studied. It is shown that the oscillatory behavior and phase singularities of the degree of spectral coherence may appear when partially coherent H-G beams propagate through atmospheric turbulence, which is very different from the behavior of Gaussian-Schell-model beams. But, the oscillatory behavior becomes weaker with increasing turbulence, and even disappears when the turbulence is strong enough. The smaller the coherence parameter, the less the spatial correlation property of partially coherent H-G beams is affected by the turbulence. In addition, a comparison between the mean-squared width of the spectral degree of coherence and the mean-squared width of the spectral intensity is also given. Some interesting results are obtained, and are explained with regard to their physical nature.
Entropy of the scalar field in general accelerating non-stationary black holes with electric charge and magnetic charge
2008, 57 (4): 2614-2620. doi: 10.7498/aps.57.2614
Using tortoise coordinate transformation, Klein-Gordon field equation is simplified and thermal radiation temperature near the event horizon is obtained. Meanwhile, adopting thin film brick-wall model and regulating the cut-off parameter and the thin film's thickness properly, the entropy of thin film near the event horizon is acquired. The results show that the entropy of the black hole is proportional to the area of the event horizon.
2008, 57 (4): 2621-2626. doi: 10.7498/aps.57.2621
Based on the five-dimensional cosmological model with a big bounce, by using the forms of the effective equation of state for dark energy,the expression of the interesting function f(z) is given in this model.Furthermore,the evolution of some cosmological quantities with the redshift z is presented. In addition, it is shown that the evolution of the dimensionless density parameters Ωdm, Ωde and deceleration parameter q for the first case is the best fit one,that consists with the present observations.