Gapped magnetic ground state in quantum-spin-liquid candidate $\kappa$-(BEDT-TTF)${}_2$-Cu${}_2$(CN)${}_3$

Abstract

Geometrical frustration, quantum entanglement and disorder may prevent long-range order of localized spins with strong exchange interactions, resulting in a novel state of matter. $\kappa$-(BEDT-TTF)${}_2$-Cu${}_2$(CN)${}_3$ is considered the best approximation of this elusive quantum-spin-liquid state, but its ground-state properties remain puzzling. Here we present a multi-frequency electron-spin resonance study down to millikelvin temperatures, revealing a rapid drop of the spin susceptibility at $T^{\ast }=6\mathrm{K}$. This opening of a spin gap, accompanied by structural modifications, suggests the enigmatic `$6\mathrm{K}$-anomaly' as the transition to a valence-bond-solid ground state. We identify an impurity contribution that becomes dominant when the intrinsic spins form singlets. Only probing the electrons directly manifests the pivotal role of defects for the low-energy properties of quantum-spin systems without magnetic order.

34 pages, 17 figures

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• Metallic coplanar resonators optimized for low-temperature measurements
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• Cryogenic frequency-domain electron spin resonance spectrometer based on coplanar waveguides and field modulation
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• Angle-dependent electron spin resonance of YbRh${}_2$Si${}_2$ measured with planar microwave resonators and in-situ rotation

Cited by in corpus (5)

• Molecular Quantum Materials: Electronic Phases and Charge Dynamics in Two-Dimensional Organic Solids
• Mott-driven BEC-BCS crossover in a doped spin liquid candidate, kappa-(BEDT-TTF)4Hg2.89Br8
• Microscopic probe of magnetic polarons in antiferromagnetic Eu${}_5$In${}_2$Sb${}_6$
• Spin Vortex Crystal Order in Organic Triangular Lattice Compound
• Persistence of fermionic spin excitations through a genuine Mott transition in $\kappa$-type organics