Gapped magnetic ground state in quantum spin liquid candidate κ-(BEDT-TTF)2Cu2(CN)3

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Science  16 Apr 2021:
Vol. 372, Issue 6539, pp. 276-279
DOI: 10.1126/science.abc6363

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A gapped spin liquid

Quantum spin liquids avoid conventional magnetic ordering down to the lowest temperatures. Among the candidates for this state of matter, organic salts such as κ-(BEDT-TTF)2Cu2(CN)3 have been prominent. Miksch et al. studied this material using electron spin resonance to elucidate the nature of its ground state. Instead of the expected gapless state, the temperature dependence of spin susceptibility suggests the formation of a spin gap.

Science, this issue p. 276


Geometrical frustration, quantum entanglement, and disorder may prevent long-range ordering of localized spins with strong exchange interactions, resulting in an exotic state of matter. κ-(BEDT-TTF)2Cu2(CN)3 is considered the prime candidate for this elusive quantum spin liquid state, but its ground-state properties remain puzzling. We present a multifrequency electron spin resonance (ESR) study down to millikelvin temperatures, revealing a rapid drop of the spin susceptibility at 6 kelvin. This opening of a spin gap, accompanied by structural modifications, is consistent with the formation of a valence bond solid ground state. We identify an impurity contribution to the ESR response that becomes dominant when the intrinsic spins form singlets. Probing the electrons directly manifests the pivotal role of defects for the low-energy properties of quantum spin systems without magnetic order.

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