Chasing the spin gap through the phase diagram of a frustrated Mott insulator

The Mott insulator κ -(BEDT-TTF)_2Cu_2(CN)_3 has been a strong candidate for a gapless quantum spin liquid, but recent experiments suggested a spin-gapped phase below 6 K. Pustogow et al. study the entropy of this phase by driving the system through the metal-insulator transition with a strain engineering approach. The quest for entangled spin excitations has stimulated intense research on frustrated magnetic systems. For almost two decades, the triangular-lattice Mott insulator κ -(BEDT-TTF)_2Cu_2(CN)_3 has been one of the hottest candidates for a g a p l e s s quantum spin liquid with itinerant spinons. Very recently, however, this scenario was overturned as electron-spin-resonance (ESR) studies unveiled a spin gap, calling for reevaluation of the magnetic ground state. Here we achieve a precise mapping of this spin-gapped phase through the Mott transition by ultrahigh-resolution strain tuning. Our transport experiments reveal a reentrance of charge localization below T ^⋆ = 6 K associated with a gap size of 30–50 K. The negative slope of the insulator-metal boundary, d T ^⋆/ d p < 0, evidences the low-entropy nature of the spin-singlet ground state. By tuning the enigmatic ‘6K anomaly’ through the phase diagram of κ -(BEDT-TTF)_2Cu_2(CN)_3, we identify it as the transition to a valence-bond-solid phase, in agreement with previous thermal expansion and magnetic resonance studies. This spin-gapped insulating state persists at T → 0 until unconventional superconductivity and metallic transport proliferate.