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

Understand

The quest for entangled spin excitations has stimulated intense research on frustrated magnetic systems.

• For almost two decades, the triangular-lattice Mott insulator $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ has been the hottest candidate for a $gapless$ 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.