Effect of quenched disorder on the quantum spin liquid state of the triangular-lattice antiferromagnet 1T−TaS2

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-TaS2 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-TaS2, where the 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 γT and the finite residual linear term of the thermal conductivity in the zero-temperature limit κ0/T ≡ κ/T (T → 0) are clearly resolved, consistent with the presence of gapless spinons with a Fermi surface. Moreover, while the strong magnetic field slightly enhances κ0/T , it strongly suppresses γ. 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(S1−xSex)2 leads to vanishing of κ0/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. These results appear to capture an essential feature of the QSL state of 1T-TaS2; localized orphan spins induced by disorder form random valence bonds and are surrounded by a QSL phase with spinon Fermi surface. Electron irradiation in pure 1T-TaS2 largely enhances γ and changes the scaling function dramatically, suggesting a possible new state of spin liquid.

Survival of itinerant excitations and quantum spin state transitions in YbMgGaO4 with chemical disorder

“…In reality, it is very rare to observe this nonzero κ 0 / T term in the studied QSL candidates. So far only the organic EtMe 3 Sb[Pd(dmit) 2 ] 2 5 , 33 and the inorganic 1T-TaS 2 40 exhibit a nonzero κ 0 / T term, both of which are spin-1/2 TAFs. But it is also notable that some recent studies reported a zero κ 0 / T term in these two materials 41 , 42 .…”

Possible itinerant excitations and quantum spin state transitions in the effective spin-1/2 triangular-lattice antiferromagnet Na2BaCo(PO4)2

“…The uncertainty of κ 0 / T caused by the fitting is ~2%. The κ 0 / T value of Na 2 BaCo(PO 4 ) 2 is ~30 times smaller than that of EtMe 3 Sb[Pd(dmit) 2 ] 2 and ~10 times smaller than that of 1T-TaS 2 18 – 20 . One may ask whether this value is too small to be resolved by the κ measurement at ultralow temperatures.…”

“…While most of the suggested 2D gapless QSLs exhibit the first two hallmarks, they do not exhibit the third one. In reality, so far only the organic EtMe 3 Sb[Pd(dmit) 2 ] 2 reported by Yamashita et al 18,19 and the inorganic 1T-TaS 2 reported by Murayama et al 20 exhibit a non-zero κ 0 /T term, both of which are spin-1/2 TAFs. However, some other groups also reported a zero κ 0 /T term in these two materials, raising a controversy [21][22][23] .…”

Mottness versus unit-cell doubling as the driver of the insulating state in 1T-TaS2

“…The remaining orbital, according to band theory, should form a half-filled band, and the experimentally observed insulating behaviour is usually attributed to its localisation at the SD centre by strong electron-electron (e-e) interactions 10,11 . From this foundation it has been suggested that, since a Mott state in 1T-TaS 2 realises a triangular lattice of localised S = ½ spins, it might host a quantum spin liquid (QSL), an unusual phase of quantum electronic matter in which, due to geometric frustration and quantum fluctuations, the spins refuse to magnetically order even down to T = 0 K [12][13][14][15][16] .…”