Effect of quenched disorder on a quantum spin liquid state of triangular-lattice antiferromagnet 1T-TaS${}_2$

Abstract

A quantum spin liquid (QSL) is an exotic state of matter characterized by quantum entanglement and the absence of any broken symmetry. A long-standing open problem, which is a key for fundamental understanding the mysterious QSL states, is how the quantum fluctuations respond to randomness due to quenched disorder. Transition metal dichalcogenide 1T-TaS${}_2$ is a candidate material that hosts a QSL ground state with spin-1/2 on the two-dimensional perfect triangular lattice. Here, we performed systematic studies of low-temperature heat capacity and thermal conductivity on pure, Se-substituted and electron irradiated crystals of 1T-TaS${}_2$. In pure 1T-TaS${}_2$, the linear temperature term of the heat capacity $\gamma T$ and the finite residual linear term of the thermal conductivity in the zero-temperature limit ${\kappa }_0/T\equiv \kappa /T(T\to 0)$ are clearly resolved, consistent with the presence of gapless spinons with a Fermi surface. Moreover, while the strong magnetic field slightly enhances ${\kappa }_0/T$, it strongly suppresses $\gamma$. These unusual contrasting responses to magnetic field imply the coexistence of two types of gapless excitations with itinerant and localized characters. Introduction of additional weak random exchange disorder in 1T-Ta(S${}_{1-x}$Se${}_x$)${}_2$ leads to vanishing of ${\kappa }_0/T$, indicating that the itinerant gapless excitations are sensitive to the disorder. On the other hand, in both pure and Se-substituted systems, the magnetic contribution of the heat capacity obeys a universal scaling relation, which is consistent with a theory that assumes the presence of localized orphan spins forming random singlets. Electron irradiation in pure 1T-TaS${}_2$ largely enhances $\gamma$ and changes the scaling function dramatically, suggesting a possible new state of spin liquid.

10 pages, 9 figures. arXiv admin note: text overlap with arXiv:1803.06100