中国物理B

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CHEMICAL POTENTIAL QUANTIZATION AND BOSE-EINSTEIN CONDENSATION

郑久仁

1999 (10): 721-732. doi: 10.1088/1004-423X/8/10/001

摘要 ( 1225 )

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In this paper, first of all, we proved if the ideal Bose gas with a finite volume and number of particles has a non-degenerate single-particle energy level ε_n, the chemical potential μ can take the value μ_n=ε_n and there is a phase transition temperature T_p,n, where n=0,1,2… Taking ε₀≤ε_n<ε_n+1, then T_p,0≥T_p,n>T_p,n+1. When the temperature T>T_p,n or T≤T_p,n+1, μ≠ε_n and the most probable occupation number N_n=0. In the temperature interval T_p,n≥ T>T_p,n+1,μ=ε_n and 0≤N_n=N-Σ_jN_j<～supN_n, where N_j is the most probable occupation number in the degenerate level j. Thus, if the finite ideal Bose gas has some non-degenerate single-particle levels, there exists a characteristic temperature T_p=T_p,0. The chemical potential μ is quantized when T≤T_p, and this leads to the creation of a macroscopic quantum state (pure state) or Bose-Einstein condensation phase. T_p=T_p,0 is a first-order phase transition point, T_p,n≠0 is a zero-order phase transition point. Next, we obtained a new expression of the most probable distribution of the finite ideal Bose gas. In this expression N_j is directly proportional to g_j-1, where g_j and N_j are, respectively, the degeneracy and the most probable occupation number in the degenerate level j. This property agrees with what chemical potential can be quantized if there is a non-degenerate level for the finite ideal Bose gas. Finally, using this expression, we defined a micro-partition function M, obtained the statistical expressions of some thermodynamical quantities.

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CONTROLLING CHAOS WITH OPTICAL FEEDBACK IN A Q SWITCHING CO₂ LASER

顾春明, 沈柯

1999 (10): 733-738. doi: 10.1088/1004-423X/8/10/002

摘要 ( 1207 )

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A method of chaos control with optical feedback in a Q switching CO₂ laser was proposed and investigated theoretically. The results of the computer simulation show that to control the chaos to the stable states could be realized by adjusting the feedback coefficient or delay-time.

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INTERACTION BETWEEN A MOVING ATOM AND AN ELECTROMAGNETIC WAVE

张敬涛, 陈朝阳, 徐至展

1999 (10): 739-745. doi: 10.1088/1004-423X/8/10/003

摘要 ( 1073 )

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Dynamics of a two-level atom moving in an electromagnetic field is studied. The atomic motion gives rise to a momentum-dependent detuning which holds back the atomic transition, and leads to a momentum-dependent Rabi oscillation which causes an overlapping among different Rabi oscillations. When the field is in a Fock state, the atomic population and the mean momentum of the atom exhibit damping oscillation, the damping rate is related to the momentum distribution; the collapse-revival phenomena of the atomic population and the mean momentum will occur if the atomic momentum has some special distribution. When the field is in a superposition state, the collapse-revival phenomena are modified by the atomic momentum distribution and disappear for the wider atomic momentum wavepackets. We also find that each atomic level will split into two sublevels with the same energy difference when the field is in a Fock state and the atom has a definite momentum.

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CALCULATION OF DIFFUSION BARRIERS FOR HELIUM ATOM IN METALS

龙德顺, 徐会忠, 王炎森, 赵国庆, 彭述明, 赵鹏骥, 徐志垒

1999 (10): 746-753. doi: 10.1088/1004-423X/8/10/004

摘要 ( 1091 )

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The diffusion barriers for the single helium atom in 3d transition metals are systematically studied by effective medium theory without any adjustable parameters. In the calculation, the relaxiation effects of lattice are taken into account. The comparison of our calculated results with the available experimental data and other theoretical values shows good agreement.

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LINEAR AMPLIFIER VIA THE SQUEEZED COHERENT STATES SUPERPOSITIONS

1999 (10): 754-767. doi: 10.1088/1004-423X/8/10/005

摘要 ( 1128 )

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Much attention is given for the squeezed coherent states (SCS's) superposition. The s-parameterized charactristic function (CF) for the output field with the superposition of SCS's as input field is given. The s-parameterized quasiprobabilty distribution function (QDF) for the output field with superposition of SCS's as input state are investigated. Various moments are calculated by using the s-parameterized CF for that field. The Glauber second-order coherence function is calculated. The quadrature squeezing for the output field are discussed. Some QDF's of the output fields are plotted as functions of the interaction time. Phase properties of the superpostion of SCS's are studied. The s-parameterized phase distributions obtained by integrating the s-parameterized QDF over radial variable are illustrated.

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NEUTRON DETECTOR FOR FUSION BURN HISTORY MEASUREMENTS

杨建伦, 王根兴, 朱启华, 温树槐, 刘忠礼, 杨洪琼, 唐正元, 吴时彬, 伍凡

1999 (10): 768-775. doi: 10.1088/1004-423X/8/10/006

摘要 ( 1537 )

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We have designed a fast, sensitive neutron detector for recording the fusion history of inertial confinement fusion experiments. With a response time of <40 ps, it was for burn history measurements for deuterium/tritium-filled targets producing as few as ～10⁸ neutrons/shot. The detector is based on the fast rise-time (<20 ps) of BC422 plastic scintillator which, shaped in thin cylinder sheet or curved (in a geometry compensating way) plate, acts as a neutron-to-light converter in a Pb shielding. The Pb shielding shields the scintillator from target X-ray, scattered light and target debris and allows the scintillator to be positioned within 3 cm from the target. The scintillator emits light with wavelengths from 350 to 450 nm. A group of achromatic lens relays the scintillator image along a 1 m optical path to the S20 photocathode of a streak camera outside the chamber. Lens coupling was chosen to give acceptable temporal dispersion. In the design phase, a computer code was programmed to calculate and improve the physical parameters of the optical system, such as light collection efficiency, time dispersion, image position, intensity distribution on the image plane, etc. Some of these parameters were finally measured using a deuterium lamp and a piece of BC422 scintillator activated by X-ray or 0.35 μm laser pulse. The measured results agree well with the prediction of the computer code.

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GROUND STATE AND ELEMENTARY EXCITATIONS OF THE PEIERLS-HUBBARD MODEL

石云龙, 章豫梅, 陈鸿, 吴翔

1999 (10): 776-782. doi: 10.1088/1004-423X/8/10/007

摘要 ( 1441 )

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The ground state and elementary excitations of the Peierls-Hubbard model are studied by using Gaussian wave functional method. The results show that the charge and spin degrees of freedom couple to each other due to the simultaneous existance of the dimerization and Hubbard repulsions. In the region of β²_s>2π the spin gap m_s is still present. Also the influence of Hubbard repulsions on the dimerization is derived from the critical behavior of ground state energy.

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POSITRON ANNIHILATION IN CARBON NANOTUBE POWDERS

马兴坤, 陈宏, 何元金, Y. Nagashima, H. Saito, T. Hyodo

1999 (10): 783-786. doi: 10.1088/1004-423X/8/10/008

摘要 ( 1227 )

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Positron lifetime spectra have been measured in two kinds of carbon nanotube powders as a function of temperature range between 32 and 296 K. It has been found that all spectra are essentially temperature-independent in the above temperature range. The results of analysis show that there are three components in the powders of carbon nanotube with an average diameter of 30 nm, and four components in the powders of carbon nanotube with typical diameters of around 15 nm. The average values of lifetime components obtained at various temperatures are about 220, 390 ps, and 2.0 ns for the former, and about 140, 300, 650 ps and 6.4 ns for the latter.

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