Low-temperature acoustic properties of metallic glasses

The dynamics of two-level systems (TLS) are substantially different in metallic glasses than in insulators because the TLS are coupled to conduction electrons in addition to phonons. This coupling does not merely cause a broadening of the energy eigenstates of a TLS; it can alter the fundamental nature of the tunneling. These effects are reflected in the low-temperature acoustic-attenuation and velocity changes of these materials. We compare theoretical calculations of ultrasonic properties of a distribution of TLS in a metal to previouly unexplained 1-GHz acoustic measurements of the sound velocity and attenuation of the metallic glass ${\mathrm{Pd}}_{0.775}$ ${\mathrm{Si}}_{0.165}$ ${\mathrm{Cu}}_{0.06}$ below 1 K.