Deformations, relaxation and broken symmetries in liquids, solids and glasses: a unified topological field theory

TL;DR

This paper develops a unified topological field theory combining hydrodynamics and field theory to explain phonons, deformations, and relaxation phenomena in liquids, solids, and glasses based on symmetry and topological defect considerations.

Contribution

It introduces a novel formalism linking higher-form symmetries and topological defects to collective excitations and relaxation times in amorphous and crystalline materials.

Findings

Predicts propagating shear waves in liquids above a critical wave-vector.

Provides a theoretical explanation for positive sound dispersion.

Identifies the Burgers vector as a topological order parameter distinguishing phases.

Abstract

We combine hydrodynamic and field theoretic methods to develop a general theory of phonons as Goldstone bosons in crystals, glasses and liquids based on non-affine displacements and the consequent Goldstones phase relaxation. We relate the conservation, or lack thereof, of specific higher-form currents with properties of the underlying deformation field -- non-affinity -- which dictates how molecules move under an applied stress or deformation. In particular, the single-valuedness of the deformation field is associated with conservation of higher-form charges that count the number of topological defects. Our formalism predicts, from first principles, the presence of propagating shear waves above a critical wave-vector in liquids, thus giving the first formal derivation of the phenomenon in terms of fundamental symmetries. The same picture provides also a theoretical explanation of the corresponding "positive sound dispersion" phenomenon for longitudinal sound. Importantly, accordingly to our theory, the main collective relaxation timescale of a liquid or a glass (known as the $\alpha$ relaxation for the latter) is given by the phase relaxation time, which is not necessarily related to the Maxwell time. Finally, we build a non-equilibrium effective action using the in-in formalism defined on the Schwinger-Keldysh contour, that further supports the emerging picture. In summary, our work suggests that the fundamental difference between solids, fluids and glasses has to be identified with the associated generalized higher-from global symmetries and their topological structure, and that the Burgers vector for the displacement fields serves as a suitable topological order parameter distinguishing the solid (ordered) phase and the amorphous ones (fluids, glasses).