Rotary Phonon Echoes in Silica Glass

Abstract

Acoustic resonance experiments have played a central role in developing a quantitative picture of the low-temperature properties of glasses [1,2]. The strong coupling between phonons and two-level tunneling systems, the two intrinsic low energy excitations of disordered insulators, has stimulated the development of novel acoustic propagation methods. Among these, coherent phonon echo studies [2,3] have provided an abundance of detailed information on phonon — two-level system couplings and on relaxation times. At temperatures below 0.1 K, where coherence persists for relatively long times (≳10−6 sec), two and three pulse phonon echoes, the acoustic analogs of magnetic spin echoes, have been used to study tunneling systems in silica glass [2,3], and neutron-irradiated quartz [4].