Novel Pauli-paramagnetic quantum phase in a Mott insulator

Abstract

In Mott insulators, the strong electron-electron Coulomb repulsion prevents metallicity and charge excitations are gapped. In dimensions greater than one, their spins are usually ordered antiferromagnetically at low temperatures. Geometrical frustrations can destroy this long-range order, leading to exotic quantum spin liquid (QSL) states. However, their magnetic ground states have been a long-standing mystery. Here we show that a QSL state in the organic Mott insulator EtMe${}_3$Sb[Pd(dmit)${}_2$]${}_2$ with two-dimensional triangular lattice has Pauli-paramagnetic-like low-energy excitations, which are a hallmark of itinerant fermions. Our torque magnetometry down to low temperatures (30 mK) up to high fields (32 T) reveal distinct residual paramagnetic susceptibility comparable to that in a half-filled two-dimensional metal. This demonstrates that the system is in a magnetically gapless ground state, a critical state with infinite magnetic correlation length. Moreover, our results are robust against deuteration, pointing toward the emergence of an extended `quantum critical phase', in which low-energy spin excitations behave as in paramagnetic metals with Fermi surface, despite the frozen charge degree of freedom.

7 pages, 3 figures. Published in Nature communication

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• Boundary-limited and glassy-like phonon thermal conduction in EtMe${}_3$Sb[Pd(dmit)${}_2$]${}_2$
• Quasi-one-dimensional Spin Dynamics in a Molecular Spin Liquid System
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• An ARPES Study of the Electronic Structure of the Quantum Spin Liquid EtMe3Sb[Pd(dmit)2]2
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