Kurt Lassmann

Phonon spectroscopy of the low energy vibrations of interstitial oxygen in germanium

In oxygen doped germanium we find by phonon spectroscopy with superconducting tunnelling junctions [1] (Fig.1) a series of states from 0.18 meV up to 4.08 meV above the ground state (Fig.3). The sequence can be approximated by a free rotation of the interstitial oxygen atom (Fig.2) corresponding to \(E = {{\left( {\hbar l} \right)}^{2}}/\left( {2mr_{0}^{2}} \right).\hbar l = 0,\pm 1,\pm 2, \ldots \) is the angular momentum for the rotation around the [111]-axis, m the reduced mass of the system and r 0 the average distance of the oxygen to the [111]-axis.

TRANSITION OF 280-GHz PHONONS FROM SUPERCONDUCTING TUNNELING JUNCTIONS INTO LIQUID HELIUM AND SILICON.

Comparative phonon emission experiments carries out in liquid helium and vacuum with superconducting tin-tin tunneling junctions evaporated on silicon crystals, show a transmission into helium about three times higher than the transmission into silicon.

Intrinsic and experimental quasiparticle recombination times in superconducting films

Experimental quasiparticle recombination lifetime data for superconducting Al, Sn, and Pb films are compared with calculations based on a ray acoustic model taking account of the film thickness dependence of the reabsorption of recombination phonons. Information on the true or intrinsic quasiparticle recombination lifetime obtained from these and other data is discussed.

Ultrasonic Attenuation Due to the Neutral Acceptor Indium in Silicon

We have measured the temperature and frequency dependencies of the u.s.-attenuation of a longitudinal u.s. wave in [111] excited by an evaporated CdS-transducer in the frequency range 0.38 to 4 GHz in silicon doped with the deep acceptor indium (p = 5. 1015 cm−3) for comparison with the results of the attenuation measurements in GaAs due to the deep acceptor Mn /1/ /2/. There we have seen that this deep acceptor ground state may be characterized by a distribution of splittings δ of the two Kramers levels due to random local fields and with a level 3 meV above, which may be connected to a dynamic Jahn Teller effect.

Effect of laser annealing on specular and diffuse scattering of 285 GHz phonons at polished silicon surfaces

Abstract We have investigated the time resolved backscattering of high frequency phonons (⩾ 285 GHz) at laser annealed silicon surfaces at low temperatures. It is found that the scattering off the free surface becomes predominantly specular up to frequencies well above 285 GHz and that the anomalous transmission into liquid helium (Kapitza effect) is strongly reduced.

Quasiparticle recombination time in superconducting tin and normal electronic density of states at the Fermi surface from tunnel junction experiments

We have measured the temperature dependence of the effective quasiparticle recombination time in superconducting tin tunnel junctions by current and laser pulse excitation. The experimental times show satisfactory agreement with calculations based on the ray acoustic lifetime model of Eisenmenger et al. taking into account the film thickness dependence of the phonon reabsorption, 2Δ-phonon volume loss processes and phonon transmission from the junction into the substrate and liquid helium. On the basis of the BCS density of thermally excited quasiparticles and simplified rate equations for quasiparticle recombination, and from the analysis of measurements of decaying excess quasiparticle concentrations we obtain a mean valueN0=(2.73±0.03) 1022 eV−1 cm−3 for the electronic density of states at the Fermi Surface in thin, evaporated tin films. This value differs less than 5% from that obtained from the experimental electronic heat-capacity coefficient of the bulk material.

Frequency dependence of the specular and diffuse phonon scattering from silicon surfaces

Phonon backscattering experiments with polished silicon surfaces show that there is no Kapitza anomaly at frequencies corresponding to the aluminum junction detector threshold (∼80 GHz), whereas at higher frequencies the anomalous transmission into liquid helium or solid nitrogen increases (reduced backscattering by the coverage), closely related to the increase of the diffuse scattering at the uncovered surface.

Transmission of heater generated high frequency phonons through a sapphire-He II boundary

We report on a new method to measure the absolute transmission coefficient through a solid-He II-boundary by determining absolutely the temperature amplitude of the second sound pulses in the liquid.

Resonant phonon scattering by calcium colloids in electron irradiated calcium fluoride

High frequency phonons with wavelengths in the nm-range should be scattered by mesoscopic defects in single crystals due to excitation of elastic resonances. The technique of thermal conductivity has been applied to investigate such an interaction in the case of Ag-colloids in NaCl [1] and γ-irradiated LiF [2]. Much better spectral resolution (~ 1 GHz) is possible by phonon spectroscopy with superconducting tunnelling junctions [3] whence more detailed information on the interaction with these defects is to be expected. Here we report on such an investigation of Ca-colloids produced by e−-irradiation in CaF2. We find a good correlation of the observed phonon scattering (and its variation with thermal treatment) with a resonance peak in the optical Mie scattering [4].

Depopulation effects in the phonoconduction response of A+ -states in silicon and germanium

We show here that deviations from these assumptions are effective in the experiment and responsible in particular for the observed illumination dependence of the phonoconduction signal. The phonon spectrum of an Al - STJ at bias V = eU > 2ΔAl consists of a continuum with a sharp threshold at Ώm = V - 2ΔAl. In the differentiated spectrum Ώm becomes the most prominent bias-tunable feature, namely the quasimonochromatic line of phonon spectroscopy used for the approximate analysis of sharp phonon scattering resonances such as the OI line of Fig. 1a (which shows inversion of the center due to strong scattering).