Field-Induced Instability of a Gapless Spin Liquid with a Spinon Fermi Surface

TL;DR

This study reveals that the gapless spin liquid state with a spinon Fermi surface in a kagome antiferromagnet becomes unstable under small magnetic fields, leading to a gapped state, highlighting a universal destabilization mechanism.

Contribution

It demonstrates the field-induced instability of a gapless spin liquid with a spinon Fermi surface in a kagome antiferromagnet using nuclear magnetic resonance measurements.

Findings

Magnetic fields induce a transition from gapless to gapped spin liquid.

The instability occurs with infinitesimal magnetic fields.

Similar behavior observed in organic triangular-lattice antiferromagnets.

Abstract

The ground state of the quantum kagome antiferromagnet Zn-brochantite, ZnCu$_3$(OH)$_6$SO$_4$, which is one of only a few known spin-liquid (SL) realizations in two or three dimensions, has been described as a gapless SL with a spinon Fermi surface. Employing nuclear magnetic resonance in a broad magnetic-field range down to millikelvin temperatures, we show that in applied magnetic fields this enigmatic state is intrinsically unstable against a SL with a full or a partial gap. A similar instability of the gapless Fermi-surface SL was previously encountered in an organic triangular-lattice antiferromagnet, suggesting a common destabilization mechanism that most likely arises from spinon pairing. A salient property of this instability is that an infinitesimal field suffices to induce it, as predicted theoretically for some other types of gapless SL's.