Novel Pauli-paramagnetic quantum phase in a Mott insulator

TL;DR

This paper reports the discovery of a quantum spin liquid state in a Mott insulator exhibiting Pauli-paramagnetic-like excitations, indicating a gapless, critical magnetic ground state with itinerant fermion behavior.

Contribution

It provides evidence for a novel quantum spin liquid with metallic-like spin excitations in a two-dimensional organic Mott insulator, challenging traditional magnetic ground state expectations.

Findings

Observation of Pauli-paramagnetic susceptibility in the insulator

Identification of a gapless, critical magnetic ground state

Robustness of the state against deuteration

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.