Soft Manifold Dynamics Behind Negative Thermal Expansion

TL;DR

This paper introduces minimal models to understand the origin of large negative thermal expansion in under-constrained systems, revealing how low-energy modes driven by underconstraint cause NTE and are evidenced by experimental measurements.

Contribution

The paper presents a theoretical framework linking underconstraint to negative thermal expansion through the dynamics of low-energy modes, supported by experimental evidence.

Findings

Underconstraint organizes extensive low-energy modes causing NTE.

Mixing of twist and translation modes is a signature of underconstraint.

Model predictions align with infrared and neutron scattering data.

Abstract

Minimal models are developed to examine the origin of large negative thermal expansion (NTE) in under-constrained systems. The dynamics of these models reveals how underconstraint can organize a thermodynamically extensive manifold of low-energy modes which not only drives NTE but extends across the Brillioun zone. Mixing of twist and translation in the eigenvectors of these modes, for which in ZrW2O8 there is evidence from infrared and neutron scattering measurements, emerges naturally in our model as a signature of the dynamics of underconstraint.