Experimental status of an organic quantum-spin-liquid candidate and its doped version

Quantum spin liquid (QSL) is a sought-after quantum matter, in which spins are strongly correlated and entangled but do not order. The family of organic materials with quasi-triangular lattices, κ-(BEDT-TTF)₂X, include promising candidates of the QSL and the doped QSL. In this talk, I will review and critically discuss diverse experimental data including updated results for the organic QSL candidate, κ-(BEDT-TTF)₂Cu₂(CN)_3, and also report experimental progress on the doped QSL candidate, κ-(BEDT-TTF)₄Hg_2.89Br₈. For the former, a focus is given on the anomalous magnetic properties arising associated with the enigmatic 6-K anomaly that appears in spin and lattice degrees of freedom. By doping, this phase is replaced by BEC-like superconductivity and a quantum-critical non-Fermi liquid above T_c . I will discuss the nature of the possible QSL and the interrelationship between the undoped and doped systems.