Table of contents

The thermodynamic approach is applied to the enhanced diffusional behaviour of one-dimensional maps on a grid of unit cells. The characteristic entropy functions are calculated and compared with normally diffusive cases. A picture of the diffusive process arises in which the relation between the dynamical instability of the map and the velocity on the grid is emphasized.

We present the first application of optically trapped atoms as targets for a scattering experiment. We report absolute total electron scattering cross-sections for ground-state Rb atoms in the range of 7-500 eV.

A systematic study of the dipole plasmons and polarizabilities of potassium clusters within the pseudojellium model and using the TDLDA is presented. We explicitly show that effects of core electrons can explain the discrepancy between the jellium model and experiments.

The surface potential of a liquid-crystal 4'-n-octyl-4-cyanobiphenyl (8CB) film at the air-water interface has been measured by a vibrating plate condenser. Phase transitions during multilayer formation could be observed by this method, which were inaccessible by other methods before.

A volatile liquid wetting a substrate forms a thick, uniform film in equilibrium with a saturated vapor phase. We show that under evaporating conditions such a liquid is effectively non-wetting, i.e. stable as a droplet on top of a thin film. This is in agreement with recent observations. Here we calculate the contact angle of the droplet as a function of the liquid-vapor surface tension and the partial pressure. This effect will be useful as a means of probing the microscopic interfacial forces involved in wetting via macroscopic measurements.

Compton scattering on transition metals provides direct evidence for the strength of electron correlations. A homogeneous-electron-gas-like treatment of these correlations turns out to be unsatisfactory. The lacking corrections can be well understood to arise from atomic correlations which are complementary to those within the electron gas. They are quantitatively obtained from results for a generalized Hubbard model Hamiltonian for the 3d electrons that was adapted to other ground-state properties.

Using angle-resolved photoemission, dramatic changes in the electronic structure and the associated in-plane electron effective mass have been observed with increasing Mg coverages for Mg overlayers on Mo(112). There is clear evidence of a gradual evolution of the band structure and effective mass across the nonmetal-to-metal transition in the Mg overlayer. The results are in very good agreement with the relative change of the screening parameter determined by resonant photoemission.

The persistent current of small disordered 2D cylinders is calculated by an exact numerical diagonalization of a tight-binding interacting model Hamiltonian. A significant enhancement in the ensemble-averaged persistent current amplitude occurs for a certain range of electron-electron interaction strength. For similar 1D and quasi-1D geometries no significant enhancement is observed. This current enhancement is due to the shift of the onset point of the Mott-Hubbard metal insulator transition. Our results may provide an explanation to a longstanding problem in mesoscopic physics, namely, the high values of persistent current measured in experiment.

The conductance of a 20 μm long quantum wire in an AlGaAs/GaAs heterostructure is investigated as a function of a perpendicular magnetic field and the temperature. Quantization in units of 2e²/h occurs at moderate field strengths and sufficiently low temperatures. For higher magnetic fields an additional conductance plateau at e²/h occurs due to an exchange-interaction-induced Zeeman splitting of the corresponding subband. With increasing temperature, the conductance plateaus are partially reduced, which is characteristic for the interaction between the electrons and three-dimensional phonons. The interpretation is based on an enhancement of the electron-phonon scattering as soon as the Fermi velocity within a subband exceeds the sound velocity. Within the region of strong scattering the temperature dependence of the conductance changes from T³ to T.

I show that a double-layer fractional quantum Hall state (kkk) with filling fraction 1/k possesses a nondissipative-drag effect, namely, a Hall voltage/current applied to one layer induces a dragged Hall voltage/current in the second layer. This effect is a subtle consequence of a recently discovered neutral gapless mode with linear dispersion (Wen X. G. and Zee A., Phys. Rev. Lett., 69 (1992) 1811), and it should be observable experimentally.

The temperature dependence of the magnetoresistance (MR) of BSCCO 2:2:1:2 single-crystal whiskers in the normal state is shown to violate the Kohler rule above T_c up to room temperatures. The appropriate magnetoconductance data prove that the origin of the MR is due to the suppression of two-dimensional superconducting fluctuations rather than resulting from two different scattering times in the normal state.

Muon spin rotation/relaxation (μSR) data for the antiferromagnetic Kondo metal CePtSn are presented, especially for the region around its magnetic transition. We find above 8.8 K spectra typical for a paramagnet with fast spin fluctuations and below 7.5 K spectra corresponding to the presence of long-range magnetic order. The behavior in between, however, is unusual. There the spectral shape shows that the system of Ce magnetic moments enters slightly above the Néel temperature into a quasi-static state with the randomness of the paramagnetic spin orientations frozen in. The frozen moments are substantially reduced compared to the effective paramagnetic moment at high temperatures and also to the ordered moment in the antiferromagnetic states.

We present data of a numerical simulation which indicate that the small frustrated exchange interaction present in Fe_xZn_1-xF₂ is the mechanism underlying the observed spin-glass (SG) phase for x = 0.25.