Signatures of a Quantum Critical Endpoint in the Kitaev Candidate Na$_2$Co$_2$TeO$_6$

TL;DR

This study identifies a quantum critical endpoint in Na$_2$Co$_2$TeO$_6$, a candidate Kitaev material, through high-resolution measurements revealing entropy accumulation and tunable criticality near 6 Tesla.

Contribution

It provides the first experimental evidence of a quantum critical endpoint in a honeycomb antiferromagnet with potential Kitaev interactions, expanding understanding of quantum criticality in such materials.

Findings

Sign change in out-of-plane thermal expansion at 6 T

Entropy accumulation at the critical field

Quantum criticality tunable by magnetic field and pressure

Abstract

The putative Kitaev material Na$_2$Co$_2$TeO$_6$ has recently been proposed to enter a quantum spin disordered state when magnetic fields are applied in parallel to the honeycomb layers. In this report we uncover signatures of a quantum critical endpoint (QCEP) associated with the assumed order-disorder transition by means of high-resolution capacitance dilatometry. At the critical field $B_\mathrm{C} \simeq 6$~T , a sign change of the out-of-plane thermal expansion coefficient $\alpha_c$ indicates accumulation of entropy upon crossing the phase boundary. The proportional relationship between isothermal magnetisation and magnetostriction signals that the QCEP can be tuned by magnetic field and pressure simultaneously. The presented results expand the material classes that exhibit metamagnetic quantum criticality to honeycomb antiferromagnets with possible Kitaev interactions.