V. Narayanamurti

Ballistic Phonons and the Transition to Second Sound in Solid 3He

We report a study of heat pulse propagation in solid 3He as a function of temperature (between .05K and .5K) and molar volume (24.5 cm3/mole to 23.7 cm3/mole). We use the fast heat pulse technique(1) and present data which shows the transition from ballistic flow to second sound. We present evidence for unusual behaviour in the temperature dependence of the velocity and intensity of the slow transverse phonon (propagating close to [110]). The implication of these measurements for specific heat data(2) are discussed.

Anisotropic phonon generation in GaAs epilayers and pn junctions

Abstract Phonon generation in n -GaAs epilayers and pn junctions is studied by means of a superconducting bolometer and time of flight techniques. Phonon emission is found to depend markedly on propagation direction, carrier concentration and defect density. Results obtained with n -layers are compared to theoretical estimates of the generation rate due to screened piezoelectric and deformation potential interactions. The data provide a vivid demonstration of theoretical selection rules.

The time dependent specific heat of dielectric glasses

Abstract Measurements on the time dependence of the specific heat exist now both at short and long timescales. Below 0.3 K the short time (}10 μs) specific heat of all the materials studied is smaller than the long time specific heat, but larger than the Debye value. Above 0.2 K most of the specific heat is coupled to the phonons already at 10 μs. However, measurements at very long timescales reveal that the specific heat has a component with a logarithmic time dependence, as proposed by the tunneling model, although only part of the total specific heat can be ascribed to it.

Symmetry of a Donor Related (DX) Center in AℓxGa1-xAs

The phenomena of low temperature persistent impurity photoconductivity in compound semiconductors has attracted considerable recent attention.1,2 For the case of AlxGa1-xAs it involves a non-effective mass like donor-complex labeled a “DX” center. The unoccupied DX center is believed to be resonant with the conduction band, yet sufficiently localized to produce a large lattice relaxation1 (see top of Fig. 1 which shows the configuration coordinate diagram proposed by Lang et al). In this figure the defect activation energy, Eo (∿ 0.1 eV), is small compared to the optical activation energy, Eop (∿ 1.5 eV). When kT<Eo, persistent photoconductivity is possible. Also shown in Fig. 1 is a crystal model of GaAs showing possible distortions of As site donors (such as Te) and Ga site donors (such as Sn).

Phonon-Quasiparticle Scattering in Dilute 3He/4He Mixtures

Phonon-3He quasiparticle scattering times in dilute mixtures of 3He in liquid 4He have been determined using the fast-heat-pulse technique. These scattering times were found to be in reasonably good numerical agreement with those theoretically calculated for Rayleigh-like scattering by Baym and Ebner. The scattering is shown to exhibit a strong pressure dependence.