Temperature crossovers in the specific heat of amorphous magnets

Abstract

It is argued that the specific heat of amorphous solids at low temperatures can be understood to arise from a single branch of collective modes. The idea is illustrated in a model of a correlated spin glass for which magnetic anisotropies are present but they are completely frustrated by disorder. The low-energy spectrum is dominated by soft modes corresponding to propagating Halperin-Saslow spin waves at short wavelengths, evolving into a relaxation and a diffuson mode at the long wavelengths. The latter gives rise to an anomalous temperature behavior of the specific heat at $T\ll T_{*}$: $C\sim T$ in $d=3$ solids, $C\sim T\ln(1/T)$ in $d=2$. The temperature scale $T_{*}$ has a non-trivial dependence on the damping coefficient describing magnetic friction, which can be related to fluctuations of the spin-torque operator via a Green-Kubo formula. Halperin-Saslow modes can also become diffusive due to disclination motion (plastic flow). Extensions of these ideas to other systems (including disordered phases of correlated electrons in cuprate superconductors and of moir\'e superlattices) are discussed