中国物理B

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EXCHANGE EFFECTS AND COLLISION MECHANISMS IN (e, 2e) PROCESSES

陈长进, 倪致祥, 施启存, 徐克尊

1998 (7): 481-487. doi: 10.1088/1004-423X/7/7/001

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In this work the triple differential cross sections for electron impact ionization of helium at an incident energy of 64.6 eV is considered in the coplanar symmetric energy-sharing and fixed relative angles of the two out-going electrons kinematics. A new collision process called triple-binary collision is identified. It has been shown that the ordinary double-binary collision process is excluded from the collision kinematics considered here. It has also been shown how the exchange effects symmetrically contribute to the peaks in the cross sections.

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PREPARATION OF TWO-MODE NONCLASSICAL STATES IN AN ION TRAP BY A QUANTIZED LASER BEAM

冯芒, 罗学立, 朱熙文, 施磊, 颜旻

1998 (7): 488-493. doi: 10.1088/1004-423X/7/7/002

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Based on the former quantum-mechanical study on the effective harmonic trap confining an ultracold ion, we make a quantum-mechanical treatment for the radiation field of the laser. Thus some two-mode nonclassical states, i.e., an approximate two-mode squeezed state and different two-mode entangled Fock states can be prepared in the red- and blue-detuning cases, respectively, within Lamb-Dicke regime and strong confinement limit.

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THEORETICAL STUDIES ON HIGH POWERED RADIO-FREQUENCY TRANSVERSE-EXCITED WAVEGUIDE ATOMIC GAS LASERS

孙逊, 辛建国

1998 (7): 494-503. doi: 10.1088/1004-423X/7/7/003

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By solving the extended Boltzmann equation and the electron number balance equation, we obtained the electron energy distribution functions (EEDFs) and the electronic excitation rates in the discharge of the radio-frequency (RF) transverse-excited intermediate pressure waveguide heliumneon laser. The population densities of the upper-laser-level Ne 3S₂ for 0.6328μm lasing and 3.39μm lasing have been determined by solving the rate euqations. The results indicate that under the proper operating conditions the population densities of Ne 3S₂ of the RF transverse-excited waveguide He-Ne laser can be increased by almost one order compared to that of a traditional longitudinal-excited capillary discharge helium-neon laser under typical working conditions. The increase of input power density and active discharge area (area scaling technique) has been proved to be reliable way to increase the output power of the RF transverse-excited waveguide atomic lasers.

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EFFECT OF TEMPERATURE ON ELECTRICAL RESISTIVITY OF A HYDROGENATED LaNi₅ THIN FILM

朱开贵, 石建中, 张立德

1998 (7): 504-509. doi: 10.1088/1004-423X/7/7/004

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The electrical resistivity of a hydrogenated LaNi₅ thin film has been investigated as a function of temperature in vacuum and in hydrogen. While the film was heated in vacuum for the first time, the change in resistivity exhibited different characteristics during different ranges of temperatures due to the competition of two effects owing to the lattice scattering of conductive electrons and the number of them. The resistivity had a sharp drop near 600 K, which originates from the formation of high conducting lanthanum hydride and nickel due to a reaction between the dissolved hydrogen and LaNi₅. The change in resistivity was not repeatable during the successive heating and cooling processes. When the film was heated under a hydrogen atmosphere, a drop in resistivity occurred near 700 K due to the reaction between LaNi₅ and the hydrogen atmosphere. The film showed a linear temperature dependence of receptivity with completeness of the reaction. It was found that the reaction was irreversible. The film lost the ability of hydrogen absorption after the reaction, and it had a phase change from LaNi₅ to LaH and Ni. This result was supported by X-ray diffraction patterns.

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PATH INTEGRAL CALCULATION OF QUANTUM TUNNELING FOR CUBIC POTENTIAL AT FINITE TEMPERATURE

张云波, 梁九卿, 蒲富恪

1998 (7): 510-517. doi: 10.1088/1004-423X/7/7/005

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The periodic instanton method is used to study the decay rates of metastable ground state and excited states of the cubic potential. The imaginary part of the energy is calculated through the standard procedure in the path-integral scheme. A formula of the decay rate valid for the entire region of energy is obtained. This formula provides a linkage between classical thermal activation at high temperatures and purely quantum tunneling at zero temperature. It is shown that in the low energy limit this more general result reduces exactly to the vacuum result. The temperature dependence of the decay rate agrees with earlier works in the literature.

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INTERLAYER EXCHANGE COUPLING IN COMPLEX MAGNETIC MULTILAYERS

张向东, 李列明, 李伯臧, 蒲富恪

1998 (7): 518-528. doi: 10.1088/1004-423X/7/7/006

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We extend the hole confinement model of Edwards et al. to the problem of two kinds of complex magnetic sandwich structures. One is the magnetic sandwich covered on both sides by nonmagnetic films(case 1) and the other is that covered by magnetic films(case 2). The interlayer exchange coupling and the angular dependence of coupling energy in the two cases are investigated systematically. For case 1, our results show that the magnetic and outer nonmagnetic films influence significantly the oscillation behavior of exchange coupling and the appearance of noncollinear exchange coupling is very sensitive to the thickness of magnetic and outer nonmagnetic layers. Our results also show that the nonoscillatory component of the coupling generally varies with the thickness of magnetic(outer nonmagnetic) films and the results in the case where the thickness of both magnetic(outer nonmagnetic) films vary simultaneously are significantly different from that in the case where the thickness of one of the two magnetic(outer nonmagnetic) films is fixed while the other is varied, which is qualitatively in agreement with the experimental measurements. For case 2, the exponential dependence of exchange coupling on the thickness of the intermagnetic layer has been obtained, similar to the Parkin's experimental results for giant magnetoresistance.

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EXCHANGE COUPLING BETWEEN TWO MAGNETIZED LAYERS IN A METAL

王少峰, 汪仲诚, 邹世昌

1998 (7): 529-540. doi: 10.1088/1004-423X/7/7/007

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The exchange coupling energy for two magnetized monolayers embedded symmetrically in a metal and polarized in an arbitrary direction has been investigated in contact interaction approximation. Since the model can be solved exactly in particular for free-electron case, the coupling energy contributed from both extended state electrons and bound state electrons is calculated rigorously. For weak interaction, it is found that the leading term in the power-series expansion of density of states can give a correct coupling energy compared with rigorous one while extended state electrons give a much larger coupling energy. Furthermore, the relevant problems such as phase shift, 90°coupling and lattice effects have been discussed; an asymptotic expression of the interlayer coupling has been derived in a different way and used to calculate the exchange energy between magnetic layers in copper with Fermi surface obtained from de Haas-van Alphen effect.

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XRD AND EXAFS INVESTIGATION OF THE DECOMPOSITION OF Co-Cu SOLID SOLUTION PREPARED BY MELT SPINING

刘涛, 胡天斗, 谢亚宁, 蒋海音, 王德武, 王文东, 李华飚

1998 (7): 541-546. doi: 10.1088/1004-423X/7/7/008

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High solid-solubility Co₁₅Cu₈₅ alloys were prepared by melt spining and then submitted to isothermal and anisothermal annealing to obtain granular alloys. The synchrotron radiation X-ray diffraction patterns and extended X-ray absorption fine structure (EXAFS) spectra were measured to investigate the decomposition of supersaturated solid solution upon annealing. Experimental results indicated the precipitation of ultrafine Co-rich particles occurred at about 500℃ and the annealing had a great influence on the middle-range order of Co atoms. While the fitted EXAFS spectra showed that the interfaces of Co precipitates seem to be enriched with vacancy-type defects, leading to distorted interfaces.

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