Session 2-1

Abstract

Quantum spin liquid (QSL) is an exotic state of matter that lacks long-range magnetic order down to zero temperature, while exhibiting distinctive patterns of quantum mechanical entanglement. A central challenge in experimentally identifying the QSLs lies in detecting fractionalized excitations, which serve as hallmarks of the underlying topological order. Recently, the Kitaev model for insulating magnets on a two-dimensional (2D) honeycomb lattice has attracted considerable attention, as it hosts a QSL with fractionalized excitations in the form of Majorana fermions and non-Abelian anyons [1]. Under a magnetic field, the emergence of Majorana edge current and non-Abelian anyons in the bulk is predicted to give rise to the half-integer quantized thermal Hall (HIQTH) effect.

Here, we report the observation of the HIQTH effect on α-RuCl₃, a prime candidate material for the Kitaev QSL [2]. Remarkably, the HIQTH effect is observed even when the magnetic field is applied parallel to the in-plane thermal current. The angular dependence of the HIQTH effect exhibits the same sign structure as the topological Chern number predicted for the Kitaev QSL [3]. Furthermore, when the magnetic field is rotated across the b-axis, we observe both a sign reversal of the HIQTH effect and the closing of an energy gap, resulting in the formation of a Dirac cone. These results provide compelling evidence that the observed HIQTH effect originates from Majorana fermions [4].