Electronuclear Quantum Criticality

TL;DR

This paper reports a rare example of electronuclear quantum criticality in a metal, where entangled electronuclear states drive quantum critical fluctuations, evidenced by anomalous specific heat behavior.

Contribution

It introduces a new type of quantum criticality involving entangled electronuclear states in a metal with weak Kondo and RKKY interactions.

Findings

Quantum critical fluctuations originate from entangled electronuclear states.

Strong spin fluctuations prevent magnetic ordering.

Anomalous temperature and field dependence of specific heat observed.

Abstract

We present here a rare example of electronuclear quantum criticality in a metal. The compound YbCu4.6Au0.4 is located at an unconventional quantum critical point (QCP). In this material the relevant Kondo and RKKY exchange interactions are very weak, of the order of 1 K. Furthermore, there is strong competition between antiferromagnetic and ferromagnetic correlations, possibly due to geometrical frustration within the fcc Yb sublattice. This causes strong spin fluctuations which prevent the system to order magnetically. Because of the very low Kondo temperature the Yb3+ 4f-electrons couple weakly with the conduction electrons allowing the coupling to the nuclear moments of the 171Yb and 173Yb isotopes to become important. Thus, the quantum critical fluctuations observed at the QCP do not originate from purely electronic states but from entangled electronuclear states. This is evidenced by the anomalous temperature and field dependence of the specific heat at low temperatures.