Continuum of spin excitations in an ordered magnet

TL;DR

This paper demonstrates that a magnetically ordered state in Na$_2$BaCo(PO$_4$)$_2$ can exhibit a spin excitation continuum typically associated with quantum spin liquids, challenging previous assumptions.

Contribution

It shows that ordered magnets can host a spin excitation continuum due to quantum fluctuations, complicating the interpretation of such continua as exclusive evidence of quantum spin liquids.

Findings

Observation of a spin excitation continuum in an ordered magnet

Quantum fluctuations induce continuum in Na$_2$BaCo(PO$_4$)$_2$

Challenges the use of continua as definitive quantum spin liquid signatures

Abstract

Continuum of spin excitations observed in inelastic neutron scattering experiments are often considered as a strong evidence of quantum spin liquid formation. When quantum spin liquid is indeed the ground state of a disorder-free magnetic compound, the elementary excitation is no longer the conventional spin waves (magnons). Instead, the magnons fractionalize into spinons, leaving only a two-spinon continuum detectable in inelastic neutron scattering experiments. For a clean ordered antiferromagnet, it was unclear if we can observe a continuous spectrum similar to the ones in a quantum spin liquid state. Here we show that the magnetically ordered state in Na$_2$BaCo(PO$_4$)$_2$ is able to host a spin excitation continuum induced by strong quantum fluctuations. Thus, a second thought is necessary when concluding such continuum as signature of quantum spin liquid in new material explorations.