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.

arXiv comment7 pages total, 4 pages main text, 4 figures

References in corpus (32)

• Spin Liquid State in an Organic Mott Insulator with Triangular Lattice
• Quantum Spin Liquids
• Mott transition, antiferromagnetism, and unconventional superconductivity in layered organic superconductors
• Mott Transition from a Spin Liquid to a Fermi Liquid in the Spin-Frustrated Organic Conductor kappa-(ET)2Cu2(CN)3
• Mott transition and transport crossovers in the organic compound $\kappa -(BEDT-TTF{)}_{2}Cu[N(CN{)}_2]Cl$
• Revision of model parameters for kappa-type charge transfer salts: an ab initio study
• Ab initio Derivation of Low-Energy Model for $\kappa$-ET Type Organic Conductors
• Quantum spin-liquid behavior in the spin-1/2 random Heisenberg antiferromagnet on the triangular lattice
• Quantum Critical Transport Near the Mott Transition
• Lattice effects and entropy release at the low-temperature phase transition in the spin-liquid candidate $\kappa$-(BEDT-TTF)${}_2$Cu${}_2$(CN)${}_3$
• Absence of charge order in the dimerized κ-phase BEDT-TTF salts
• Finite temperature crossovers and the quantum Widom line near the Mott transition
• Pressure-Tuned Exchange Coupling of a Quantum Spin Liquid in the Molecular Triangular Lattice $\kappa$-(ET)${}_2$Ag${}_2$(CN)${}_3$
• Four-Spin Terms and the Origin of the Chiral Spin Liquid in Mott Insulators on the Triangular Lattice
• Gapped magnetic ground state in quantum-spin-liquid candidate $\kappa$-(BEDT-TTF)${}_2$-Cu${}_2$(CN)${}_3$
• Quantum Spin Liquids Unveil the Genuine Mott State
• Mott Transition in a Valence Bond Solid Insulator with a Triangular Lattice
• Temperature dependence of structural and electronic properties of the spin-liquid candidate kappa-(BEDT-TTF)2Cu2(CN)3
• Critical Spin Liquid versus Valence Bond Glass in Triangular Lattice Organic $\kappa$-(ET)${}_2$Cu${}_2$(CN)${}_3$
• Collective excitation of electric dipole on molecular dimer in organic dimer-Mott insulator
• Quantum Spin Liquid with Emergent Chiral Order in the Triangular-lattice Hubbard Model
• Rise and Fall of Landau's Quasiparticles While Approaching the Mott Transition
• Quasi-continuous transition from a Fermi liquid to a spin liquid
• Spin-Peierls Instability of Three-Dimensional Spin Liquids with Majorana Fermi Surfaces
• Low-Temperature Dielectric Anomalies at the Mott Insulator-Metal Transition
• Two-dimensional ground-state mapping of a Mott-Hubbard system in a flexible field-effect device
• Inverse Occam's razor
• Site-specific ${}^{13}$C NMR study on locally distorted triangular lattice of organic conductor $\kappa$-(BEDT-TTF)${}_2$Cu${}_2$(CN)${}_3$
• 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
• Lattice effects in the quasi-two-dimensional valence-bond-solid Mott insulator EtMe${}_3$P[Pd(dmit)${}_2$]${}_2$

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