Phonon fluorescence in superconductors and the propagation characteristics of high-frequency phonons in Ge: Sb and Al₂O₃: V³⁺

Abstract

We present a detailed account of phonon generation by superconducting tunnel junctions (Sn-I-Sn) and by Sn and Pb_0.5Tl_0.5 films pumped by a heat pulse. The spectrum and propagation characteristics of the generated phonons is studied through the resonance absorption by Sb-donor levels in uniaxially compressed Ge. At all values of generator power (up to a few watts) the emitted phonons show a large density at a value of energy equal to the superconducting energy gap (2Δ). In contrast, the spectrum emitted by a constantan heater is shown to be in quantitative agreement with the blackbody-radiation model. The experiments show that the mean free path of longitudinal and transverse phonons in the superconductor changes discontinuously when hω=2Δ. Theoretical calculations show that significant reabsorption of phonons of energy 2Δ results in a nonlinear buildup of the intensity of the 2Δ phonons. The propagation characteristics of these phonons in Ge: Sb show a frequency and polarization dependence in excellent agreement with the Griffin-Carruthers theory of resonance-fluorescence phonon scattering by donor levels. The magnetic field tunability of the generated 2Δ phonons in Sn is utilized to study the ground state of V³⁺ in Al₂O₃. Transverse phonons of energy 1.02 meV propagating along the c axis are resonantly absorbed as expected according to theory.