Thermodynamic evidence for field-angle dependent Majorana gap in a Kitaev spin liquid

TL;DR

This study provides thermodynamic evidence of a field-angle dependent Majorana gap in a Kitaev spin liquid candidate, linking bulk excitations with edge current signatures and revealing a possible nematic quantum spin liquid at high fields.

Contribution

It demonstrates the angle-dependent Majorana gap in -RuCl3 and establishes the bulk-edge correspondence in a Kitaev quantum spin liquid through thermodynamic measurements.

Findings

Angle-dependent low-energy excitations with sextuple node structure

Gap amplitude increases with magnetic field, consistent with Majorana fermions

Possible emergence of a nematic quantum spin liquid at higher fields

Abstract

The exactly-solvable Kitaev model of two-dimensional honeycomb magnet leads to a quantum spin liquid (QSL) characterized by Majorana fermions, relevant for fault-tolerant topological quantum computations. In the high-field paramagnetic state of $\alpha$-RuCl3, half-integer quantization of thermal Hall conductivity has been reported as a signature of edge current, but the bulk nature of this state remains elusive. Here, from high-resolution heat capacity measurements under in-plane field rotation, we find strongly angle-dependent low-energy excitations in bulk $\alpha$-RuCl3. The excitation gap has a sextuple node structure, and the gap amplitude increases with field, exactly as expected for itinerant Majorana fermions in the Kitaev model. Our thermodynamic results are fully linked with the transport quantization properties, providing the first demonstration of the bulk-edge correspondence in a Kitaev QSL. Moreover, at higher fields where the quantum thermal Hall effect vanishes, we find the possible emergence of a novel nematic QSL state with two-fold rotational symmetry.