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Chin. Phys. B  
  Chin. Phys. B--2002, Vol.11, No.2
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A polynomial approximation for the scattering wavefunction of an arbitrary spherically symmetric potential

Li Pei-Jin, Bao Cheng-Guang
Chin. Phys. B 2002, 11 (2): 0105;  doi: 10.1088/1009-1963/11/2/001
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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.

How to control a geometric quantum gate

Hao San-Ru, Hou Bo-Yu, Xi Xiao-Qiang, Yue Rui-Hong
Chin. Phys. B 2002, 11 (2): 0109;  doi: 10.1088/1009-1963/11/2/002
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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.

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

Xie Bing-Hao, Jing Hui
Chin. Phys. B 2002, 11 (2): 0115;  doi: 10.1088/1009-1963/11/2/003
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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.

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

Hu Xian-Quan, Hu Wen-Jiang, Kong Chun-Yang
Chin. Phys. B 2002, 11 (2): 0120;  doi: 10.1088/1009-1963/11/2/004
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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.

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

Ruan Cun-Jun, Pang Wen-Ning, Gao Jun-Fang, Shang Ren-Cheng
Chin. Phys. B 2002, 11 (2): 0126;  doi: 10.1088/1009-1963/11/2/005
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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.

Investigation of titanium nitride coating by broadband laser ultrasonic spectroscopy

Gao Wei-Min, Christ Glorieux, Walter Lauriks, Jan Thoen
Chin. Phys. B 2002, 11 (2): 0132;  doi: 10.1088/1009-1963/11/2/006
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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.

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

Wang Tun, Zhou Fu-Xin, Liu Yue-Wu
Chin. Phys. B 2002, 11 (2): 0139;  doi: 10.1088/1009-1963/11/2/007
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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.

Influences of nonlinear interactions of polaritons in uniaxial crystals

Niu Jia-Sheng, Luo Ying, Ma Ben-Kun
Chin. Phys. B 2002, 11 (2): 0144;  doi: 10.1088/1009-1963/11/2/008
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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.

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

Wang Li-Min, Luo Ying, Ma Ben-Kun
Chin. Phys. B 2002, 11 (2): 0150;  doi: 10.1088/1009-1963/11/2/009
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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).

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

Zhu Shi-Yang, Ru Guo-Ping, Qu Xin-Ping, Li Bing-Zong, R.L.Van Meirhaeghe, C.Detavernier, F.Cardon
Chin. Phys. B 2002, 11 (2): 0156;  doi: 10.1088/1009-1963/11/2/010
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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.

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

Ji Ying-Hua, Lei Min-Sheng, Ouyang Chu-Ying
Chin. Phys. B 2002, 11 (2): 0163;  doi: 10.1088/1009-1963/11/2/011
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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.

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

Wang Huai-Yu, Zhou Yun-Song, Wang Chong-Yu, D. L. Lin
Chin. Phys. B 2002, 11 (2): 0167;  doi: 10.1088/1009-1963/11/2/012
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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.

Structural and magnetic properties of Dy2Co17-xMnx compounds

Li Yun-Bao, Zhang Li-Gang, Zhang Shao-Ying, Shen Bao-Gen
Chin. Phys. B 2002, 11 (2): 0174;  doi: 10.1088/1009-1963/11/2/013
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Using x-ray diffraction and magnetic measurements, we have studied the structural and magnetic properties of Dy2Co17-xMnx (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 Tc decreases linearly, and the saturation magnetization Ms 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.

Magnetoresistance and magnetic properties of Fe3O4 nanoparticle compacts

Wang Hai, Zhao Hong-Wu, Wang Chang-Sheng, Wang Yin-Jun, Zhan Wen-Shan, Li Feng-Ying, Jin Chang-Qing, Meng Fan-Bin, Li Yang-Xian
Chin. Phys. B 2002, 11 (2): 0178;  doi: 10.1088/1009-1963/11/2/014
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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 Ms 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.

Percolation-related magnetic coupling and magnetoresistance properties in Fe/Si1-xAgx multilayers

Wang Hai, Tang Yun-Jun, Li Xiang, Chen Xi, Wang Yin-Jun, Zhu Tao, Zhao Hong-Wu, Zhan Wen-Shan
Chin. Phys. B 2002, 11 (2): 0183;  doi: 10.1088/1009-1963/11/2/015
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In this paper, we present a study of the magnetic coupling and magnetoresistance (MR) properties in Fe/Si1-xAgx multilayers with a granular Si1-xAgx 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 Si1-xAgx. 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.

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

Xiao Si-Guo, Yang Xiao-Liang, Liu Zheng-Wei
Chin. Phys. B 2002, 11 (2): 0188;  doi: 10.1088/1009-1963/11/2/016
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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 (5S25I8) can be enhanced by restraining the transition of 5S25I7 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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