High-Field Quantum Disordered State in $\alpha$-RuCl3: Spin Flips, Bound States, and a Multi-Particle Continuum

TL;DR

This study investigates the high-field quantum disordered phase in $ ext{α-RuCl}_3$, revealing a multi-particle continuum, bound states, and the importance of Kitaev interactions through Raman spectroscopy and theoretical calculations.

Contribution

It provides experimental and theoretical evidence for a quantum disordered state with multi-particle excitations in high magnetic fields in $ ext{α-RuCl}_3$, highlighting the role of Kitaev and off-diagonal exchanges.

Findings

Maximized continuum as long-range order is suppressed by magnetic field

Emergence of a two-particle bound state above the quantum phase transition

Identification of a gapped multi-particle continuum with a single-particle excitation

Abstract

Layered $\alpha$-RuCl3 has been discussed as a proximate Kitaev spin liquid compound. Raman and THz spectroscopy of magnetic excitations confirm that the low-temperature antiferromagnetic ordered phase features a broad Raman continuum, together with two magnon-like excitations at 2.7 and 3.6 meV, respectively. The continuum strength is maximized as long-range order is suppressed by an external magnetic field. The state above the field-induced quantum phase transition around 7.5 T is characterized by a gapped multi-particle continuum out of which a two-particle bound state emerges, together with a well-defined single-particle excitation at lower energy. Exact diagonalization calculations demonstrate that Kitaev and off-diagonal exchange terms in the Fleury-Loudon operator are crucial for the occurrence of these features in the Raman spectra. Our study firmly establishes the partially-polarized quantum disordered character of the high-field phase.