Heat Transport in Herbertsmithite: Can a Quantum Spin Liquid Survive Disorder?

TL;DR

This study uses heat transport measurements to investigate whether quantum spin liquids in herbertsmithite survive disorder, finding no evidence of spin excitations and suggesting disorder localizes gapless states.

Contribution

It provides experimental constraints on the nature of the ground state in kagome lattice QSL candidates, highlighting the impact of disorder on gapless spin excitations.

Findings

No spin excitation contribution detected in thermal conductivity.

Disorder likely localizes gapless spin excitations.

Results are compatible with gapped QSL or quantum paramagnetic states.

Abstract

Arguably the most favorable situation for spins to enter the long-sought quantum spin liquid (QSL) state is when they sit on a kagome lattice. No consensus has been reached in theory regarding the true ground state of this promising platform. The experimental efforts, relying mostly on one archetypal material ZnCu$_3$(OH)$_6$Cl$_2$, have also led to diverse possibilities. Apart from subtle interactions in the Hamiltonian, there is the additional degree of complexity associated with disorder in the real material ZnCu$_3$(OH)$_6$Cl$_2$ that haunts most experimental probes. Here we resort to heat transport measurement, a cleaner probe in which instead of contributing directly, the disorder only impacts the signal from the kagome spins. For ZnCu$_3$(OH)$_6$Cl$_2$ and a related QSL candidate Cu$_3$Zn(OH)$_6$FBr, we observed no contribution by any spin excitation nor any field-induced change to the thermal conductivity. These results impose different constraints on various scenarios about the ground state of these two kagome compounds: while a gapped QSL, or certain quantum paramagnetic state other than a QSL, is compatible with our results, a gapless QSL must be dramatically modified by the disorder so that gapless spin excitations are localized.