Electron spin resonance in a dilute magnon gas as a probe of magnon scattering resonances

TL;DR

This paper investigates how electron spin resonance can detect magnon scattering resonances in a dilute magnon gas, revealing universal behaviors and potential for exploring strongly correlated quantum phenomena like Efimov states.

Contribution

It introduces a quantum cluster expansion approach to identify signatures of magnon scattering resonances via ESR measurements, linking to phenomena in ultracold atomic systems.

Findings

Frequency shift changes sign at resonance

Linewidth peaks at the scattering resonance

Potential observation of Efimov states in quantum magnets

Abstract

We study the electron spin resonance in a dilute magnon gas that is realized in a ferromagnetic spin system at low temperature. A quantum cluster expansion is developed to show that the frequency shift of the single-magnon peak changes its sign and the linewidth reaches its maximum across a scattering resonance between magnons. Such characteristic behaviors are universal and can be used to experimentally locate the two-magnon resonance when an external parameter such as pressure is varied. Future achievement of the two-magnon resonance may have an impact comparable to the Feshbach resonance in ultracold atoms and will open up a rich variety of strongly correlated physics such as the recently proposed Efimov effect in quantum magnets. We also suggest how the emergence of an Efimov state of three magnons and its binding energy may be observed with the electron spin resonance.