Magnon bound states versus anyonic Majorana excitations in the Kitaev honeycomb magnet \u03b1-RuCl3

Dirk Wulferding,Youngsu Choi,Seung-Hwan Do,Kwang-Yong Choi,Yann Gallais,Peter Lemmens,Chan Hyeon Lee,Clément Faugeras

doi:10.1038/s41467-020-15370-1

Abstract

The pure Kitaev honeycomb model harbors a quantum spin liquid in zero magnetic fields, while applying finite magnetic fields induces a topological spin liquid with non-Abelian anyonic excitations. This latter phase has been much sought after in Kitaev candidate materials, such as α-RuCl3. Currently, two competing scenarios exist for the intermediate field phase of this compound (B = 7 − 10 T), based on experimental as well as theoretical results: (i) conventional multiparticle magnetic excitations of integer quantum number vs. (ii) Majorana fermionic excitations of possibly non-Abelian nature with a fractional quantum number. To discriminate between these scenarios a detailed investigation of excitations over a wide field-temperature phase diagram is essential. Here, we present Raman spectroscopic data revealing low-energy quasiparticles emerging out of a continuum of fractionalized excitations at intermediate fields, which are contrasted by conventional spin-wave excitations. The temperature evolution of these quasiparticles suggests the formation of bound states out of fractionalized excitations.

Highlights

The pure Kitaev honeycomb model harbors a quantum spin liquid in zero magnetic fields, while applying finite magnetic fields induces a topological spin liquid with non-Abelian anyonic excitations
The green-shaded continuum C agrees with observations in several previous Raman scattering studies[12,25,26] and has been identified as Majorana fermionic excitations stemming from a fractionalization of spin degrees of freedom in the Kitaev honeycomb model[17]
In the Kitaev model, Majorana bound states are created through flux pairs combined with Majorana fermions in a ΔSz = ±1

Summary

Introduction

The pure Kitaev honeycomb model harbors a quantum spin liquid in zero magnetic fields, while applying finite magnetic fields induces a topological spin liquid with non-Abelian anyonic excitations This latter phase has been much sought after in Kitaev candidate materials, such as α-RuCl3. In the intermediate field range of 7–10 T, the magnetic order melts into a quantum disordered phase, in which the half-integer quantized thermal Hall conductance is reported[13] This remarkable finding may be taken as evidence for a field-induced topological spin liquid with chiral Majorana edge states and the central charge q = ν/2 (Chern number ν = 1). In the original Kitaev honeycomb model, non-Abelian Majorana excitations are created upon breaking time-reversal symmetry, e.g., through applying a magnetic field[17] These composite quasiparticles correspond to bound states of localized fluxes and itinerant Majorana fermions[18].

Methods

Results

Conclusion