Disorder Induced Quantum Griffiths Phase in the Proximate Kitaev Quantum Spin Liquid Na2Co2TeO6
Abstract
Abstract Disorder is generally expected to destabilize a Kitaev quantum spin liquid, yet it can also generate new quantum phases. Na2Co2TeO6, which contains substantial intrinsic Na-site disorder, provides a promising platform to examine how randomness reshapes spin dynamics. Here, by employing 23Na NMR, we reveal a disorder induced softening of the spin excitations. Between 10 K and 100 K, the temperature dependence of the NMR relaxation times is consistent with the presence of gapped fractional spin excitations. Below 10 K, however, they bifurcate into two distinct channels: a slow-relaxing channel, preserving the gapped excitations character, and a fast-relaxing channel that follows a Korringa-like law, indicating the emergence of nearly gapless spin excitations. This two-channels regime is accompanied by ultraslow, microsecond scale spin fluctuations following an Arrhenius activation law. Besides, ab initio NMR lineshape calculations show that a quantum spin liquid-like model reproduces the low-temperature NMR spectra sufficiently better than magnetically ordered models. Together, our findings indicate transition upon cooling to a disorder-driven quantum spin liquid Griffiths phase, comprising slowly fluctuating gapped regions within a matrix of nearly gapless low-energy spin excitations.
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