Brownian Motion and Quantum Dynamics of Magnetic Monopoles in Spin Ice

TL;DR

This paper investigates the quantum dynamics of magnetic monopoles in spin ice, revealing Brownian diffusion driven by spin tunnelling, collective interactions, and uncovering novel phenomena like monopole plasma oscillations and critical behavior.

Contribution

It demonstrates how high-frequency magnetic susceptibility reveals quantum spin dynamics and monopole behavior in spin ice, resolving previous contradictions and highlighting adiabatic susceptibility as a key characteristic.

Findings

Monopole diffusion is strictly Brownian but influenced by spin tunnelling.

Evidence of monopole plasma oscillations in weak fields.

Unconventional critical behavior observed in strong fields.

Abstract

Spin ice illustrates many unusual magnetic properties, including zero point entropy, emergent monopoles and a quasi liquid-gas transition. To reveal the quantum spin dynamics that underpin these phenomena is an experimental challenge. Here we show how crucial information is contained in the frequency dependence of the magnetic susceptibility and in its high frequency or adiabatic limit. These measures indicate that monopole diffusion is strictly Brownian but is underpinned by spin tunnelling and is influenced by collective monopole interactions. We also find evidence of driven monopole plasma oscillations in weak applied field, and unconventional critical behaviour in strong applied field. Our results resolve contradictions in the present understanding of spin ice, reveal unexpected physics and establish adiabatic susceptibility as a revealing characteristic of exotic spin systems.