The interplay between disorder and superconductivity is a subtle and fascinating phenomenon in quantum many body physics. The conventional superconductors are insensitive to dilute nonmagnetic impurities, known as the Anderson's theorem 1 .Destruction of superconductivity and even superconductor-insulator transitions 2-10 occur in the regime of strong disorder. Hence disorder-enhanced superconductivity is rare and has only been observed in some alloys or granular states 11-17 . Because of the entanglement of various effects, the mechanism of enhancement is still under debate.Here we report well-controlled disorder effect in the recently discovered monolayer
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.
Non-Fermi liquids are strange metals whose physical properties deviate qualitatively from those of conventional metals due to strong quantum fluctuations. In this paper, we report transport measurements on the FeSe 1−x S x superconductor, which has a quantum critical point of a nematic order without accompanying antiferromagnetism. We find that in addition to a linear-in-temperature resistivity ρ xx ∝ T , which is close to the Planckian limit, the Hall angle varies as cot θ H ∝ T 2 and the low-field magnetoresistance is well scaled as ρ xx /ρ xx ∝ tan 2 θ H in the vicinity of the nematic quantum critical point. This set of anomalous charge transport properties show striking resemblance with those reported in cuprate, iron-pnictide, and heavy fermion superconductors, demonstrating that the critical fluctuations of a nematic order with q ≈ 0 can also lead to a breakdown of the Fermi liquid description.
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