Critical speeding-up near the monopole liquid-gas transition in magnetoelectric spin-ice

TL;DR

This study reveals critical dielectric behavior and a surprising speeding-up of monopole dynamics near the liquid-gas transition in spin-ice, highlighting the magnetoelectric nature of emergent monopoles and their critical dynamics.

Contribution

First broadband spectroscopic measurements of dielectric response in spin-ice near the monopole transition, demonstrating critical signatures and dynamic speeding-up.

Findings

Critical signatures in dielectric response at monopole transition

Critical speeding-up of dielectric relaxation time near the critical temperature

Evidence of magnetoelectric character of monopole excitations

Abstract

Competing interactions in the so-called spin-ice compounds stabilize a frustrated ground-state with finite zero-point entropy and, interestingly, emergent magnetic monopole excitations. The properties of these monopoles are at the focus of recent research with particular emphasis on their quantum dynamics. It is predicted that each monopole also possesses an electric dipole moment, which allows to investigate their dynamics via the dielectric function \epsilon(\nu). Here, we report on broadband spectroscopic measurements of \epsilon(\nu) in Dy2Ti2O7 down to temperatures of 200mK with a specific focus on the critical endpoint present for a magnetic field along the crystallographic [111] direction. Clear critical signatures are revealed in the dielectric response when, similarly as in the liquid-gas transition, the density of monopoles changes in a critical manner. Surprisingly, the dielectric relaxation time \tau\ exhibits a critical speeding-up with a significant enhancement of 1/\tau\ as the temperature is lowered towards the critical temperature. Besides demonstrating the magnetoelectric character of the emergent monopole excitations, our results reveal unique critical dynamics near the monopole condensation transition.