Fermionic response from fractionalization in an insulating two-dimensional magnet

TL;DR

This paper presents experimental evidence of fermionic excitations in a two-dimensional magnetic insulator, supporting the presence of Majorana fermions in quantum spin liquids, which are typically elusive to observe.

Contribution

It provides the first systematic experimental demonstration of fermionic quasiparticles in a 2D quantum spin liquid using Raman scattering data.

Findings

Fermionic excitations observed in $ ext{α}$-RuCl$_3$ across broad energy and temperature ranges.

Experimental evidence supports the existence of Majorana fermions in a 2D magnetic insulator.

Highlights the role of excited states in detecting topological quantum spin liquids.

Abstract

Conventionally ordered magnets possess bosonic elementary excitations, called magnons. By contrast, no magnetic insulators in more than one dimension are known whose excitations are not bosons but fermions. Theoretically, some quantum spin liquids (QSLs) -- new topological phases which can occur when quantum fluctuations preclude an ordered state -- are known to exhibit Majorana fermions as quasiparticles arising from fractionalization of spins. Alas, despite much searching, their experimental observation remains elusive. Here, we show that fermionic excitations are remarkably directly evident in experimental Raman scattering data across a broad energy and temperature range in the two-dimensional material $\alpha$-RuCl$_3$. This shows the importance of magnetic materials as hosts of Majorana fermions. In turn, this first systematic evaluation of the dynamics of a QSL at finite temperature emphasizes the role of excited states for detecting such exotic properties associated with otherwise hard-to-identify topological QSLs.