Energie-Relaxation warmer Ladungsträger in Germanium und Silizium

Experimental data on warm carrier energy relaxation times in germanium and silicon are reported. Measurements were done by the method of harmonic mixing of microwaves 1 in the temperature range between 77 and 200 °K. Relaxation times obtained forn-type germanium are in agreement with data reported byMorgan 2 as well as that ofSeeger 3 andGibson et al. 4. Measured energy relaxation times forp-type germanium as well as forn- andp-type silicon are reported here for the first time. A qualitative interpretation is given in terms of scattering by acoustic and optical phonons.     

Calculation of electric-field gradients based on higher-order generalized Douglas-Kroll transformations

In this paper, the calculation of electric-field-like properties based on higher-order Douglas-Kroll-Hess (DKH) transformations is discussed. The electric-field gradient calculated within the Hartree-Fock self-consistent field framework is used as a representative property. The properties are expressed as an analytic first derivative of the four-component Dirac energy and the nth-order DKH energy, respectively. The differences between a "forward" transformation of the relativistic energy or the "back transformation" of the wave function is discussed in some detail. Detailed test calculations were carried out on the electric-field gradient at the halogen nucleus in the series HX (X=F,Cl,Br,I,At) for which extensive reference data are available. The DKH method is shown to reproduce (spin-free) four-component Dirac-Fock results to an accuracy of better than 99% which is significantly closer than previous DKH studies. The calculations of both the Hamiltonian and the property operator are shown to be essentially converged after the second-order transformation, even for elements as heavy as At. In addition, we have obtained results within the density-functional framework using the DKHZ and zeroth-order regular approximation (ZORA) methods. The latter results included picture-change effects at the scalar relativistic variant of the ZORA-4 level and were shown to be in quantitative agreement with earlier results obtained by van Lenthe and Baerends. The picture-change effects are somewhat smaller for the ZORA method compared to DKH. For heavier elements significant differences in the field gradients predicted by the two methods were found. Based on comparison with four-component Dirac-Kohn-Sham calculations, the DKH results are more accurate. Compared to the spin-free Dirac-Kohn-Sham reference values, the ZORA-4 formalism did not improve the results of the ZORA calculations.     

p(↑)+p(↑) → π+d at 515 and 578 MeV

The polarization asymmetries A_0y and A_y0, and the spin correlation parameter A_yy have been measured at SIN at 515 and 578 MeV using a transversally polarized proton beam and target. The results are compared with various theoretical models.     

The exciton in recombination in AlxGa1−xAs-GaAs quantum-well heterostructures

Low temperature (4.2–77 K) photoluminescence and laser data on metal-organic chemical vapor deposited Al_xGa_1?xAs-GaAs quantum-well heterostructures are presented that exhibit emission peaks between the lowest energy confined-carrier transitions and their phonon sidebands. These data indicate the involvement of the two-dimensional exciton in recombination in quantum-well heterostructures.     

On the aromaticity of annulenones

Application of our method of predicting aromaticity¹ shows that the behavior of the annulenones is intermediate between that of the fulvenes (a nonaromatic series) and the annulenes (a series with strong alternation between aromatic and antiaromatic).     

Viscoelasticity of a Fermi liquid

Infinite homogeneous Fermi systems in the degenerate regime are described by the Uehling-Uhlenbeck equation. The eigenvalue problem associated with the linearized collision operator is solved analytically. Initial value problems are studied with the help of the spectral representation of the time evolution operator. The dynamic transport coefficients of the system can then be calculated in the framework of linear response theory. As an example the viscoelastic behaviour of the Fermi liquid is related to the relaxation of a quadrupole deformation in momentum space. In this connection also the coupling of the driving field to 2p-2h excitations will be discussed. The theory is applied to normal liquid³He and to nuclear matter.     

Decay of bcc-structure and of bond-orientational order in a fluid

The decay of an initially prepared bcc structure and the bond-orientational order (anisotropy of the first coordination shell) are studied in a non-equilibrium molecular dynamics simulation for a fluid of 1024 particles interacting with a repulsive r^-12 potential. Data are presented for pair-correlation functions and order parameters associated with an angle dependence described by cubic harmonics of the ranks 4, 6 and 8. These cubic pair-correlation functions and cubic order parameters are defined by the expansion of the pair-correlation function and of the bond orientational distribution function with respect to Cartesian tensors. The relaxation of the local anisotropy shows a pretransformational slowing down for densities approaching the freezing point.     

On the theory of irreversible processes in molecular liquids and liquid crystals, nonequilibrium phenomena associated with the second and fourth rank alignment tensors

A theory for nonequilibrium phenomena primarily associated with the molecular orientation in liquids and liquid crystals is developed within the framework of irreversible thermodynamics. The alignment tensors of rank 2 and 4 are taken into consideration as additional macroscopic variables. With appropriate assumptions concerning the dependence of the internal energy, specific volume and the entropy on the alignment, a dynamic Landau theory is obtained. Nonlinear relaxation equations are derived for the alignment tensors. They possess zero and nonzero stable stationary solutions corresponding to the isotropic and nematic phases of a liquid crystal when the temperature is above or below the transition temperature. Some further applications of the relaxation equations are discussed, in particular fluctuations of the intensity of the depolarized component of scattered light.