Dispersive anisotropic ballistic phonon propagation in [100] GaAs

We report experimental observations of the phonon focusing pattern in [100] GaAs using low temperature electron beam scanning for phonon generation. The typical dispersive effects for high-frequency phonons expected from the calculations by Tamura have clearly been observed using PbIn tunnel junctions for phonon detection. The quantitative comparison of our experimental results with the frequency dependent calculations by Tamura allowed to determine the dominant phonon frequencies contributing to the detector signal in our different experiments. Above the temperature of the -point the dominant phonon frequencies appear to be shifted considerably to lower values, which could be explained by a heating effect in the liquid-He layer adjacent to the tunnel junction detector. By comparing the observed magnitude of the detector signal with different theoretical treatments of the detector response, we have found satisfactory agreement for a model where the perturbation due to the high-frequency phonons is restricted to the base electrode of the detector reached first by the phonons following their passage through the crystal.     

Imaging of spatial structures in crystals with ballistic phonons

Ballistic phonon propagation in single crystals has been used for imaging defect structures in bulk material. We have realized this principle by means of low-temperature scanning electron microscopy. As model cases we have imaged laser drilled holes in single-crystalline sapphire and -quartz samples. Comparing the acoustic images with those obtained by optical transmission microscopy, we found that fine structure seen optically was well reproduced acoustically. In addition strain fields due to plastic deformations could clearly be imaged acoustically. The spatial resolution of our imaging technique is estimated to be a few micrometer. By using two or more phonon detectors, three-dimensional imaging of spatial inhomogeneities becomes possible.     

Vortex core physics in the cuprate superconductor Nd2−xCexCuOy

Flux-flow voltage steps and negative differential resistivity in the cuprate Nd_2−xCe_xCuO_y are explained in terms of subbands between the Fermi energy and the energy gap affecting the quasiparticle dynamics.     

Characterization of single-crystalline GaAs by imaging with ballistic phonons

Ballistic phonon propagation in single-crystalline [001]-oriented gallium arsenide has been studied using low-temperature scanning electron microscopy for imaging. Deviations in the phonon focusing pattern due to dispersion effects were found by comparing the phonon images to theoretical calculations of the long-wavelength limit. The phonon propagation behavior in, samples cut from differently prepared wafers has been investigated. For highly impure crystals we found a pronounced increase of the diffusive signal component at the expense of the ballistic one. Samples with varying dislocation densities also showed a sensitive dependence, of the ballistic phonon propagation on these crystal defects. For focusing calculations considering elastic scattering processes the diffusivity of the phonons could be determined as a function of the mean scattering length. We have found phonon mean free paths of 0.35 mm to 0.80 mm for the various GaAs crystals.     

Experimental progress in the nonlinear behavior of semiconductors

During the past five years spontaneous oscillations and chaotic behavior have been observed in the electronic transport of many semiconductors. Whereas the temperature range of some experiments extends up to room temperature, the majority of the measurements have been performed at liquid helium temperatures. We summarize these experimental developments and discuss their impact on the field of nonlinear dynamics which is rapidly progressing at present.     

Damping of the vortex motion in the cuprate superconductors

We have investigated the flux-flow resistivity in different cuprate superconductors. The subjects summarized in the following revealed unexpected novel features and were of particular interest.