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

International audienceA quantum spin liquid (QSL) is an exotic state of matter characterized by quantum entanglement and theabsence of any broken symmetry. A longstanding open problem, which is key for fundamental understandingthe mysterious QSL states, is how the quantum fluctuations respond to randomness due to quenched disorder.Transition metal dichalcogenide 1T-TaS2 is a candidate material that hosts a QSL ground state with spin-1/2 onthe two-dimensional perfect triangular lattice. Here, we performed systematic studies of low-temperature heatcapacity and thermal conductivity on pure, Se-substituted, and electron-irradiated crystals of 1T-TaS2, wherethe substitution of S by Se induces weak disorder and electron irradiation induces strong quenched disorder.In pure 1T-TaS2, the linear temperature term of the heat capacity γ T and the finite residual linear term of thethermal conductivity in the zero-temperature limit κ0/T ≡ κ/T (T → 0) are clearly resolved, consistent with thepresence of gapless spinons with a Fermi surface. Moreover, while the strong magnetic field slightly enhancesκ0/T , it strongly suppresses γ . These unusual contrasting responses to magnetic field imply the coexistenceof two types of gapless excitations with itinerant and localized characters. Introduction of additional weakrandom exchange disorder in 1T-Ta(S1−xSex )2 leads to vanishing of κ0/T , indicating that the itinerant gaplessexcitations are sensitive to the disorder. On the other hand, in both pure and Se-substituted systems, the magneticcontribution of the heat capacity obeys a universal scaling relation, which is consistent with a theory that assumesthe presence of localized orphan spins forming random singlets. These results appear to capture an essentialfeature of the QSL state of 1T-TaS2; localized orphan spins induced by disorder form random valence bonds andare surrounded by a QSL phase with spinon Fermi surface. Electron irradiation in pure 1T-TaS2 largely enhancesγ and changes the scaling function dramatically, suggesting a possible new state of spin liquid