A fermion phenomenology of low-temperature strongly-noncrystalline solids

TL;DR

This paper proposes that low-temperature excitations in strongly-noncrystalline solids are fermions, providing a unified phenomenological framework that explains their thermal and nonlinear response properties through a Fermi glass model.

Contribution

It introduces a fermionic phenomenological theory for low-energy excitations in SNSs, challenging the traditional two-level systems approach and explaining various experimental observations.

Findings

Fermions are localized and weakly interacting in SNSs.

Nearly constant density of states near Fermi energy leads to linear specific heat.

Parameters of Landau-Fermi glass are consistent with experimental data.

Abstract

Insisting on the relevance of spin-statistics theorem, I propose that anomalous low-energy excitations of strongly-noncrystalline solids (SNSs), observed at low temperatures T < 1 K, are fermions, which are localized and weakly interacting. This phenomenological theory rationalizes all low-energy quantum many-body states of SNSs as Goldstone bosons-phonons and half-integer-spin fermions. Fermi glass, rather than isolated two-level systems (TLSs), appears to be a consistent theory of various thermal and nonlinear response properties of SNSs. A robust consequence of this theory is nearly constant-in-energy density of fermion states near Fermi energy, which in turn implies linear-in-temperature specific heat and frequency-independent sound internal friction. Consistent parameters of Landau-Fermi glass are calculated on theoretical grounds as well as from experimental data.