The Role of Quantum Metastability and the Perspective of Quantum Glassiness in Kitaev Fractional Spin Dynamics: An NMR Study

TL;DR

This study uses NMR measurements on Na2Co2TeO6 to explore quantum spin dynamics, revealing a quantum spin glass state with metastability and fractionalization, challenging the traditional homogeneous quantum spin liquid view.

Contribution

It provides experimental evidence of a quantum spin glass state with fractionalization features in a Kitaev-like material, highlighting quantum metastability and heterogeneity.

Findings

Detection of a quantum spin glass state below 10 K.

Observation of fractional spin excitations consistent with Kitaev models.

Evidence of quantum metastability affecting spin dynamics at low temperatures.

Abstract

The suppression of magnetic order and the detection of a half-quantized thermal Hall effect in {\alpha}-RuCl3 under an external magnetic field have sparked significant debate, whether these phenomena point to spin fractionalization, as posited by the Kitaev quantum spin liquid (QSL) model, or if they arise from a more conventional mechanism in an antiferromagnetically ordered spin state. Here, through 23Na NMR relaxation measurements on the layered cobaltate Na2Co2TeO6 at two distinct magnetic fields (4.7 and 9.4 Tesla), we provide compelling evidence supporting a variant interpretation. While upon cooling, the NMR relaxation times align with the temperature dependence predicted by Kitaev fractional spin excitations, below 10 K, a dynamically heterogeneous state is detected with Quantum Spin Glass (QSG) characteristics. In this state, the spin fractionalization dynamics unfold over markedly different time scales, and are characterized by quantum metastability, inducing prolonged equilibration as T->0. This observation challenges the prevailing view of a primarily homogeneous QSL state, as the ground state of tangible Kitaev systems.