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 ε_{0}≤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_{j}N_{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.

A method of chaos control with optical feedback in a Q switching CO_{2} 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.

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.

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.

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.

PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES

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 ～10^{8} 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.

CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

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 β^{2}_{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.

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.