中国物理B

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A polynomial approximation for the scattering wavefunction of an arbitrary spherically symmetric potential

黎培进, 鲍诚光

2002 (2): 105-108. doi: 10.1088/1009-1963/11/2/301

摘要 ( 1107 )

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A new variational approach is proposed to calculate scattering phase shifts. In this approach, the variational parameter is contained not explicitly in the trial function but in the boundary conditions that the scattering wavefunction should satisfy. It was found that a high accuracy could be reached. Furthermore, the scattering wavefunction appears as a polynomial of finite orders in the interior region. This expression is convenient for applications.

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How to control a geometric quantum gate

郝三如, 侯伯宇, 惠小强, 岳瑞宏

2002 (2): 109-114. doi: 10.1088/1009-1963/11/2/302

摘要 ( 1222 )

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In this paper, we detail the theoretical ideas which are used to explain the mechanism of the laser controlling the geometric quantum gates introduced in the work by Ekert et al. (Ekert A, Ericsson M, Hayden P, Inanori H, Jones J A, Oi D K L and Vedral V 2000 J. Mod. Opt. 47 2501). We have introduced a two-level Hamiltonian system, and directed to solve this system, and then obtained the probability distribution of this two-level system. We also show the relationships between the external laser fields and the transition of the qubit in the two-qubit controlled-phase gate, and how the transition of the qubit depends on the external laser fields and the states of the controlled qubit.

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Analytical method for yrast line states in the interacting two-component Bose-Einstein condensate

解炳昊, 景辉

2002 (2): 115-119. doi: 10.1088/1009-1963/11/2/303

摘要 ( 1324 )

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The yrast spectrum for the harmonically trapped two-component Bose-Einstein condensate (BEC), omitting the difference between the two components, has been studied using an analytical method. The energy eigenstates and eigenvalues for L=0,1,2,3 are given. We illustrate that there are different eigenstate behaviours between the even L and odd L cases for the two-component BEC in two dimensions. Except for symmetric states, there are antisymmetric states for the permutation of the two components, which cannot reduce to those in a single condensate case when the value of L is odd.

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The fine structure splitting of the level of lithium in Rydberg states

胡先权, 胡文江, 孔春阳

2002 (2): 120-125. doi: 10.1088/1009-1963/11/2/304

摘要 ( 1350 )

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The Hamiltonian of the four-body problem for a lithium atom is expanded in series. The level shift and level formula of a lithium atom in Rydberg states are achieved by means of the calculation of polarization of the atomic core (including the contribution of dipole, quadrupole and octupole components). We also consider the effect of relativity theory, the orbital angular momentum L and the spin angular momentum S coupling scheme (LS coupling) and high-order correction of the effective potential to the level shift. The fine structure splitting (N=5-12, L=4-9, J=L±1/2) and level intervals in Rydberg states have been calculated by the above-mentioned formula and compared with recent experimental data.

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Study on optical electron polarimeter and measurement of the relative Stokes parameters of weak light

阮存军, 庞文宁, 高君芳, 尚仁成

2002 (2): 126-131. doi: 10.1088/1009-1963/11/2/305

摘要 ( 1230 )

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In this paper, we present the research on an optical electron polarimeter, which is used to determine the polarization of an incident electron beam by measuring the relative Stokes parameters of the fluorescence emitted from the He gas following the impact excitation with the electron beam. The fundamental theory of the optical electron polarimeter is discussed with the 3³P→2³S transition of He. The structure and performance of the instrument are described, which are different in some aspects from previous works. The arrangement of the experiment for measuring the relative Stokes parameters of linearly polarized weak light is also investigated, which actually involves the same processes as the polarization measurement with the incident electron beam. The results obtained are in agreement with the theoretical prediction.

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Investigation of titanium nitride coating by broadband laser ultrasonic spectroscopy

高伟民, Christ Glorieux, Walter Lauriks, Jan Thoen

2002 (2): 132-138. doi: 10.1088/1009-1963/11/2/306

摘要 ( 1060 )

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We present a laser ultrasonic method to investigate a titanium nitride (TiN) coating specimen. The technique is based on the principle of surface acoustic wave (SAW) dispersion during acoustic propagation on a half-space with the presence of a thin layer. Due to the high efficiency of laser line-source excitation, we have been able to generate and detect a SAW with an excellent signal-to-noise ratio in a wide frequency band. An inverse fitting algorithm was employed to extract simultaneously the thickness and the elastic parameters of the TiN coating from the experimental SAW velocity dispersion curve.

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A Lennard-Jones embedded-atom potential and its application to the study of melting

王暾, 周富信, 刘曰武

2002 (2): 139-143. doi: 10.1088/1009-1963/11/2/307

摘要 ( 1026 )

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The nearest-neighbour Lennard-Jones potential from the embedded-atom method is extended to a form that includes more than nearest neighbours. The model has been applied to study melting with molecular dynamics. The calculated melting point, fractional volume change on melting, heat of fusion and linear coefficients of thermal expansion are in good agreement with experimental data. We have found that the second and third neighbours influence the melting point distinctly.

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Influences of nonlinear interactions of polaritons in uniaxial crystals

牛家胜, 罗莹, 马本堃

2002 (2): 144-149. doi: 10.1088/1009-1963/11/2/308

摘要 ( 1095 )

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We have analysed theoretically the polarization and dielectric constant of uniaxial ionic crystals, including the effect of the nonlinear interactions between ions. The spectrum of polaritons (coupling modes of photons and optical phonons) under nonlinear interactions was developed. A new branch of dispersion relations emerges in the original frequency gap.

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The study of the valence bond property in a two-different-quantum-dot molecule

王立民, 罗莹, 马本堃

2002 (2): 150-155. doi: 10.1088/1009-1963/11/2/309

摘要 ( 1258 )

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The electronic energy spectrum and wavefunction of a quantum-dot molecule are studied by means of the finite-element solution of the single electron Schr?dinger equation. We find that the nature of the coupling can be covalent, ionic, or "intermediate" new mixed states, depending on various parameters such as the separation distance between two dots, the height of potential barrier, matching of the energies and parities of the orbital localized on each dot. The bond property can be used to explain the experimental result obtained by Oosterkamp et al. (1998 Nature 395 873).

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Double threshold behaviour of I－V characteristics of CoSi₂/Si Schottky contacts

竺士炀, 茹国平, 屈新萍, 李炳宗, R.L.Van Meirhaeghe, C.Detavernier, F.Cardon

2002 (2): 156-162. doi: 10.1088/1009-1963/11/2/310

摘要 ( 1157 )

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The forward current－voltage (I－V) characteristics of polycrystalline CoSi₂/n-Si(100) Schottky contacts have been measured in a wide temperature range. At low temperatures (≤200K), a plateau-like section is observed in the I－V characteristics around 10^-4A·cm^-2. The current in the small bias region significantly exceeds that expected by the model based on thermionic emission (TE) and a Gaussian distribution of Schottky barrier height (SBH). Such a double threshold behaviour can be explained by the barrier height inhomogeneity, i.e. at low temperatures the current through some patches with low SBH dominates at small bias region. With increasing bias voltage, the Ohmic effect becomes important and the current through the whole junction area exceeds the patch current, thus resulting in a plateau-like section in the I－V curves at moderate bias. For the polycrystalline CoSi₂/Si contacts studied in this paper, the apparent ideality factor of the patch current is much larger than that calculated from the TE model taking the pinch-off effect into account. This suggests that the current flowing through these patches is of the tunnelling type, rather than the thermionic emission type. The experimental I－V characteristics can be fitted reasonably well in the whole temperature region using the model based on tunnelling and pinch-off.

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Time dependence of the average charge and current in a dissipative mesoscopic circuit

嵇英华, 雷敏生, 欧阳楚英

2002 (2): 163-166. doi: 10.1088/1009-1963/11/2/311

摘要 ( 1283 )

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Taking into consideration the interactions between electrons and phonons, we have studied the temporal evolution of the average charge and current in a dissipative mesoscopic RLC circuit. Our results show that a mesoscopic RLC circuit can be treated as an interactive system between an electromagnetic harmonic oscillator and many lattice harmonic oscillators; this is called the bathing of the harmonic oscillators. The results also show that the quantum equation of motion of the linear mesoscopic RLC circuit is identical in form to its classical equation of motion, the only difference between them being their respective meanings. In order to thoroughly study the quantum properties of a dissipative mesoscopic circuit, we have to consider not only the electromagnetic energy of the circuit, but also the crystal lattice vibration energy and the interactive energy between electrons and phonons.

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Investigation of ultra-thin ferromagnetic films with a simple cubic lattice

王怀玉, 周云松, 王崇愚, D. L. Lin

2002 (2): 167-173. doi: 10.1088/1009-1963/11/2/312

摘要 ( 1281 )

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Using Green's function method, we investigate ferromagnetic films with a simple cubic lattice containing up to ten monolayers. The Hamiltonian includes the Heisenberg exchange term, surface anisotropy (SA) and dipole interaction (DI). We calculate the magnetization as a function of temperature and film thickness, and we analyse the behaviour of spin canting. The result is in agreement with experiments. We calculate phase diagrams of SA versus DI to show the conditions under which spontaneous magnetization can occur. As a special case, we discuss the Heisenberg model without SA and DI.

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Structural and magnetic properties of Dy₂Co_17-xMn_x compounds

李云宝, 张立刚, 张绍英, 沈保根

2002 (2): 174-177. doi: 10.1088/1009-1963/11/2/313

摘要 ( 1050 )

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Using x-ray diffraction and magnetic measurements, we have studied the structural and magnetic properties of Dy₂Co_17-xMn_x (x=0∽5) compounds with a rhombohedral Th₂Zn₁₇-type structure. With an increasing Mn concentration x, the unit-cell volume V was found to increase linearly. The Curie temperature T_c decreases linearly, and the saturation magnetization M_s at 5K first increases slightly for x<1, then decreases rapidly for x>1 with a further increase of Mn concentration x. In compounds for x=1～3, a spin reorientation was found. A magnetic diagram of the compounds is given.

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Magnetoresistance and magnetic properties of Fe₃O₄ nanoparticle compacts

王海, 赵宏武, 王常生, 王荫君, 詹文山, 李凤英, 靳常青, 孟凡斌, 李养贤

2002 (2): 178-182. doi: 10.1088/1009-1963/11/2/314

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In this paper, we report on the magnetic properties of Fe₃O₄ nanoparticles with different grain sizes under different pressures. In all the samples, the saturated magnetization M_s shows a linear decrease with increasing pressure. The thickness of the magnetic dead layer on the nanoparticle surface under different pressures was roughly estimated, which also increases with increasing pressure. The transport measurements of the nanoparticle Fe₃O₄ compacts show that the low-field magnetoresistance (MR) value is insensitive to the grain size in a wide temperature range; however, the high-field MR value is dependent on grain size, especially at low temperatures. These experimental results can be attributed to the different surface states of the nanoparticles.

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Percolation-related magnetic coupling and magnetoresistance properties in Fe/Si_1-xAg_x multilayers

王海, 唐云俊, 李祥, 陈熹, 王荫君, 朱涛, 赵宏武, 詹文山

2002 (2): 183-187. doi: 10.1088/1009-1963/11/2/315

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In this paper, we present a study of the magnetic coupling and magnetoresistance (MR) properties in Fe/Si_1-xAg_x multilayers with a granular Si_1-xAg_x spacer layer. We have found that, with increasing silver content (x) in a silicon matrix, the magnetic state of multilayers changes from a nonmagnetic coupling state to weak antiferromagnetic around the percolation point of the ～2.4 nm thick spacer Si_1-xAg_x. The MR measurements also reveal an abrupt increase of MR near the same percolation point. These changes are ascribed to the formation of the percolation path in the granular spacer.

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New approach to enhance the up-conversion efficiency in the Ho³⁺-Yb³⁺ co-doped system

肖思国, 阳效良, 刘政威

2002 (2): 188-190. doi: 10.1088/1009-1963/11/2/316

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We investigate the up-conversion luminescence spectra of Ho³⁺-Yb³⁺ co-doped materials excited with two types of laser. It is found that the efficiency of green emission (⁵S₂－⁵I₈) can be enhanced by restraining the transition of ⁵S₂－⁵I₇ since the two types of radiation both start on the ⁵S₂ level. We propose an important approach to enhance an expected up-conversion emission.

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Small-angle x-ray scattering study of the formation and growth of δ' in binary Al－7.87at% Li alloy

柴志刚, 孟繁玲, 邹青, 王煜明

2002 (2): 191-193. doi: 10.1088/1009-1963/11/2/317

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The formation and growth of δ' particles in an alloy of Al－7.87at% Li have been investigated using the small angle x-ray scattering technique. It has been shown that a transition zone, which exists between the δ' phase and the matrix after ageing for 16 h, disappears after ageing for 32 h. The existence of a transition zone indicates that the δ' phase is precipitated in the process of spinodal decomposition. This result also indicates that the growth of δ' particles should be divided into two stages. During the first stage, the δ' phase increases eating up the transition zone before ageing for 32 h, and in the second stage it grows by swallowing up each other after ageing for 32 h. In the latter stage, the larger and smaller δ' particles can coexist in the alloy.

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