Superconductivity emerging upon Se doping of the quantum spin liquid 1T-TaS2

Paradigmatic quantum spin liquid has been in the past closely linked to the high-temperature superconductivity. Here we focus on a layered 1T-TaS2, which has recently emerged as a surprising candidate for a quantum spin liquid on a triangular lattice. Upon doping 1T-TaS2 with Se, the Se-77 nuclear magnetic resonance and Ta-181 nuclear quadrupole resonance data show a remarkable robustness of the quantum spin liquid at low Se doping levels despite the growth of inhomogeneities in the electronic state. A dramatic change in the low-energy electronic excitations are observed in samples with a moderate Se content where the Mott insulating state gives way to a correlated metal and a low-temperature superconductivity. The effects of antiferromagnetic correlations can still be traced in the normal state above the superconducting critical temperature T-c = 3.5 K and may, together with the underlying disorder, account for the large superconducting gap-to-T-c ratio 2 Delta/k(B)T(c) approximate to 7 deduced from the scanning tunneling microscopy.