The plane symmetric general solution to the Einstein-Maxwell equations in D =n+2 dimensions is presented. In addition to the n(n+1)/2 spacelike Killing vector fields characterizing the higher dimensional plane symmetry, there is also an extra Killing vector field in the solution, suggesting that the generalized Birkhoff theorem proved for 4-dimensional spacetimes might also be valid in higher dimensions.

The processes of excitation transfer from the 8^{2}S_{1/2} state to the 6^{2}D state of rubidium in Rb(8^{2}S_{1/2})-Rb(5^{2}S_{1/2}) and Rb(8^{2}S_{1/2})-H_{2} collisions have been studied experimentally. During irradiating the Rb vapor, mixed with H_{2}, by two light beams for selective stepwise excitation, the Rb 8^{2}S_{1/2}→5^{2}P_{3/2} direct fluorescence and the Rb 6^{2}D_{3/2}→5^{2}P_{1/2} sensitized fluorescence have been measured as a function of H_{2} gas pressure. The measurements yielded the cross-sections σ(8^{2}S_{1/2}→6^{2}D) and σ^{*}(8^{2}S_{1/2}→6^{2}D) of Rb 8^{2}S_{1/2}→6^{2}D excitation tranfer induced by collisions with 5^{2}S_{1/2} atom and H_{2} molecules respectively. The results are discussed in detail.

Using rate equation formalism, we got an analytic expression describing the optical pumping of an alkali atomic beam in strong megnetic fields. By comparing with the traditional description in single velocity approximation, the influence of velocity distribution in a practical alkali atomic beam on the pumping effect was quantitatively discussed. It turned out that the rate equation treatment in velocity approximation might result in an error within twenty percent. Laser optical pumping of ^{7}Li atomic beam in a strong magnetic field (0.15T) was experimentally studied, and the results are in agreement with the theory in which the velocity distribution is taken into account.

We report the optical bistability in CdS_{x}Se_{1-x}-doped glass channel waveguide with rise and fall times of about 24 and 30ps, respectively, in bistable switching by means of fiber coupling input with a power of about 3mW. The third-order nonlinear susceptibility (x^{(3)}) of CdS_{x}Se_{1-x}-doped glass is estimated experimentally to be 1.8×10^{-9} esu. The high speed switching and the value of x^{(1)} show that the optical bistability is caused by an optical nonlinearity which can be attributed to the band-filling effect.

CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES

Chemisorption of atomic hydrogen on the Si(100)2×1 surface has been investigated in detail by using a field ion-scanning tunneling microscope (FI-STM). The results showed that the adsorption geometry changed from the 2×1 monohydride phase to the 1×1 dihydride phase with increasing exposure of hydrogen. The data of desorption of the hydrogen-saturated Si surface showed that on annealing at 670K the surface becomes highly disordered: the 1×1 dihydride structure is eliminated and the 2×1 reconstructed monohydride is also hardly to identify. When the temperature rises to as high as 730 K, the surface is domi-nated by the 2×1 structure with missing dimer rows, and some adatom chains occur on the Si substrate ler-races. We attribute the formation of these atomic chains to an epitaxial growth of Si atoms which are formed by the dissociation of SiH_{x} (x= 1, 2, 3 or 4) compounds on the Si surface.

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

Ab initio calculations of optical-phonon deformation potentials (ODP's), i.e., d_{0}, d_{30}, d_{10} (val) and d_{10} (con) for sixteen semiconductors were carried out systematically. The calculations are based on the LMTO-ASA band-structure method within the framework of the frozen-phonon approximation model, in which the displacement of empty sphere is considered to match its atomic sphere partners, We have compared the d_{0} values obtained by several different theoretical calculation methods and studied the main factors affecting them. It is pointed out that the two different models (rhombohedral strain model and frozen-phonon model) for calculations will lead to different results of d_{0}.

High quality epitaxial YBa_{2}Cu_{3}O_{7-x} thin films have been succcessfully prepared by dc magnetron sputtering deposition, on (100) and (110) aligned SrTiO_{3}, LaAlO_{3} and yttria-stabilized zirconia (YSZ) substrates. The films showed zero resistance around 90 K and had a J_{c} (at 77 K, H=0) over 10^{6}A/cm^{2}. It was found that superconducting properties and structures of the films were strongly dependent on oxygen pressure and substrate temperature. The epitaxial structure of the films have been studied by X-ray diffraction. Rutherford backscattering and channeling spectroscopy, X-ray double-crystal diffraction and transmission election microscopy. The experimental results demonstrated that the epitaxial YBa_{2}Cu_{3}O_{7-x} films had excellent superconducting properties and quite perfect structure.

The positron annihilation lifetime spectra and their temperature dependence for the YBa_{2}Cu_{3}O_{y} superconducting ceramics doped with Sn have been studied. The resutts of analyses indicate that the element Sn is substituted for Cu(1) sites, and this leads to the ordering of the oxygen atoms as well as the increase of local electronic density in the Cu(1)-O layer where Cu(1) is substituted by Sn.

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