Continuum of spin excitations in an ordered magnet

Public summary

*Inelastic neutron scattering experiments performed across all low-temperature phases in Na2BaCo(PO4)2.

*Density matrix renormalization group calculations agree quantitatively with neutron scattering results.

*Spin excitation continua induced by quantum fluctuations, even in the presence of magnetic ordered states.

*Spin excitation continuum alone should not be used as “smoking gun evidence” of quantum spin liquids.

Abstract

Spin excitation continua observed in neutron scattering studies are often considered to be strong evidence of quantum spin liquid formation. In a disorder-free magnetic compound with a quantum spin liquid ground state, the elementary excitation is no longer the conventional spin waves (magnons). Instead, the magnons fractionalize into spinons, producing a characteristic two-spinon continuum. However, it remained uncertain whether a clean, ordered antiferromagnet could exhibit a continuous spectrum similar to that of a quantum spin liquid. This paper presents evidence of a spin excitation continuum in the magnetically ordered state of Na2BaCo(PO4)2, where free spinons are absent. This challenges the interpretation of such a continuum as a definitive signature of a quantum spin liquid in new material studies.