Monopolar and dipolar relaxation in spin ice Ho$_2$Ti$_2$O$_7$

TL;DR

This study investigates magnetic relaxation in Ho$_2$Ti$_2$O$_7$ spin ice, revealing two distinct temperature-dependent processes involving monopole dynamics and spin fractionalization, advancing understanding of non-equilibrium magnetic behavior.

Contribution

It provides a comprehensive experimental analysis of relaxation processes in Ho$_2$Ti$_2$O$_7$, highlighting the roles of monopole motion and bound state reorientation across different temperature regimes.

Findings

Low-temperature relaxation linked to monopole motion along the field.

High-temperature relaxation involves reorientation of monopolar bound states.

Monopole conductivity decreases with cooling, similar to semiconductors.

Abstract

When degenerate states are separated by large energy barriers, the approach to thermal equilibrium can be slow enough that physical properties are defined by the thermalization process rather than the equilibrium. The exploration of thermalization pushes experimental boundaries and provides refreshing insights into atomic scale correlations and processes that impact steady state dynamics and prospects for realizing solid state quantum entanglement. We present a comprehensive study of magnetic relaxation in Ho$_2$Ti$_2$O$_7$ based on frequency-dependent susceptibility measurements and neutron diffraction studies of the real-time atomic-scale response to field quenches. Covering nearly ten decades in time scales, these experiments uncover two distinct relaxation processes that dominate in different temperature regimes. At low temperatures (0.6K<T<1K) magnetic relaxation is associated with monopole motion along the applied field direction through the spin-ice vacuum. The increase of the relaxation time upon cooling indicates reduced monopole conductivity driven by decreasing monopole concentration and mobility as in a semiconductor. At higher temperatures (1K<T<2K) magnetic relaxation is associated with the reorientation of monopolar bound states as the system approaches the single-spin tunneling regime. Spin fractionalization is thus directly exposed in the relaxation dynamics.