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

Abstract

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. Our transport experiments reveal a reentrance of charge localization below $T^{\star }=6$ K associated with a gap size of 30-50 K. The negative slope of the insulator-metal boundary, $d{T}^{\star }/dp<0$, evidences the low-entropy nature of the spin-singlet ground state. By tuning the enigmatic '6 K anomaly' through the phase diagram of $\kappa$-(BEDT-TTF)${}_2$Cu${}_2$(CN)${}_3$, we identify it as the transition to a valence-bond-solid phase, with typical magnetic and structural fingerprints, that persists at $T\to 0$ until unconventional superconductivity and metallic transport proliferate.

7 pages total, 4 pages main text, 4 figures

References in corpus (12)

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• Absence of charge order in the dimerized κ-phase BEDT-TTF salts
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• Inverse Occam's razor
• Charge disproportionation in the spin-liquid candidate $\kappa$-(ET)${}_2$Cu${}_2$(CN)${}_3$ at 6 K revealed by ${}^{63}$Cu NQR measurements
• Valence bond phases of herbertsmithite and related copper kagome materials
• Simultaneous Control of Bandfilling and Bandwidth in Electric Double-Layer Transistor Based on Organic Mott Insulator $\kappa$-(BEDT-TTF)${}_2$Cu[N(CN)${}_2$]Cl

Cited by in corpus (4)

• Ingredients for Generalized Models of $\kappa$-Phase Organic Charge-Transfer Salts: A Review
• Superconductivity and Mottness in Organic Charge Transfer Materials
• Photoinduced Frustration Modulation in $\kappa$-type Quantum Spin Liquid Candidates
• Gapped magnetic ground state in the spin-liquid candidate $\kappa$-(BEDT-TTF)${}_2$Ag${}_2$(CN)${}_3$ suggested by magnetic spectroscopy