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

The suppression of magnetic order and the detection of a half-quantized thermal Hall effect in α-RuCl ₃ 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 ²³ Na NMR relaxation measurements on the layered cobaltate Na ₂ Co ₂ TeO ₆ 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’s 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.