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

Abstract

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