Candidate Elastic Quantum Critical Point in LaCu$_{6-x}$Au$_x$

TL;DR

This study investigates how substituting Au in LaCu$_{6-x}$Au$_x$ suppresses a structural phase transition, revealing a potential elastic quantum critical point characterized by residual structural instability and negligible electronic coupling.

Contribution

It provides experimental and theoretical evidence for an elastic quantum critical point in LaCu$_{6-x}$Au$_x$, highlighting the suppression of a ferroelastic transition with Au substitution.

Findings

Linear suppression of the orthorhombic-monoclinic transition with Au content.

Residual structural instability persists at the critical composition.

Negligible coupling between structural instability and electronic states.

Abstract

The structural properties of LaCu$_{6-x}$Au$_x$ have been studied using neutron diffraction, x-ray diffraction, and heat capacity measurements. The continuous orthorhombic-monoclinic structural phase transition in LaCu$_{6}$ is suppressed linearly with Au substitution until a complete suppression of the structural phase transition occurs at the critical composition, $x_{c}$ = 0.3. Heat capacity measurements at low temperatures indicate residual structural instability at $x_c$ that extends well into the orthorhombic phase. The instability is ferroelastic in nature, with density functional theory (DFT) calculations showing negligible coupling to electronic states near the Fermi level. The data and calculations presented here are consistent with the zero temperature termination of a continuous structural phase transition suggesting that the LaCu$_{6-x}$Au$_x$ series hosts an elastic quantum critical point.