Low-temperature anomalies in disordered solids: A cold case of contested relics?

Vassiliy Lubchenko

Published 2018-08-07Version 1

Amorphous solids manifest puzzling effects of mysterious degrees of freedom often referred to as "two-level systems." These degrees of freedom give rise to a heat capacity and phonon scattering that are in great excess over what would be expected for a solid that has a unique vibrational ground state. Of particular conceptual importance is the apparent near universality of phonon scattering in amorphous solids made by quenching a liquid. To rationalise this universality, scale-free scenarios have been proposed that either hinge on there being long-range interactions between bare structural degrees of freedom or that invoke long-range criticality stemming from the emergence of marginally stable vibrational modes. In a contrasting, local scenario, the puzzling low-temperature degrees of freedom are, instead, weakly-interacting, strongly anharmonic degrees of freedom each of which involves the motion of a few hundred particles. These anharmonic degrees of freedom correspond to the low-barrier subset of the cooperative activated events responsible for mass transport near the glass transition temperature $T_g$. In this scenario, the universality of phonon scattering comes about because the characteristic energy scale of the local anharmonic resonances and the strength of their interaction with phonons are both set by $T_g$, while their concentration is set by a cooperativity size $\xi$ for dynamics at the glass transition temperature. Despite their stylistic differences, all these scenarios attempt to describe the gradual freezing-out of translational degrees of freedom as an aperiodic assembly of atoms is cooled toward absolute zero. The local scenario is distinct in explicitly predicting qualitatively new physics at the length scale $\xi$. A key signature of this lengthscale is the vibrational excitations of the locally-activated configurations ... See article for the rest of Abstract.