Possible Quantum Diffusion of Polaronic Muons in Dy$_2$Ti$_2$O$_7$ Spin Ice

TL;DR

This paper proposes that the low-temperature muon relaxation in Dy$_2$Ti$_2$O$_7$ is due to quantum diffusion of muons via tunneling in a disordered spin environment, contrasting with magnetic fluctuation explanations.

Contribution

It introduces a quantum diffusion model for muon relaxation in Dy$_2$Ti$_2$O$_7$, highlighting tunneling as the dominant process at low temperatures.

Findings

Plateau in relaxation rate below 7 K explained by quantum tunneling.

Crossover at 50 K between tunneling and incoherent hopping.

Model fits experimental data better than classical diffusion models.

Abstract

We interpret recent measurements of the zero field muon relaxation rate in the frustrated magnetic pyrochlore Dy$_2$Ti$_2$O$_7$ as resulting from the quantum diffusion of muons in the substance. In this scenario, the plateau observed at low temperature ($<7$ K) in the relaxation rate is due to coherent tunneling of the muons through a spatially disordered spin state and not to any magnetic fluctuations persisting at low temperature. Two further regimes either side of a maximum relaxation rate at $T^* = 50$ K correspond to a crossover between tunnelling and incoherent activated hopping motion of the muon. Our fit of the experimental data is compared with the case of muonium diffusion in KCl.