Experimental signature of the attractive Coulomb force between positive and negative magnetic monopoles in spin ice

TL;DR

This paper demonstrates the magnetic analogue of the Poole-Frenkel effect in spin ice, confirming the Coulomb attraction between emergent magnetic monopoles through exponential-root field growth of magnetic current.

Contribution

It provides experimental evidence of Coulomb forces between magnetic monopoles in spin ice, extending the universality of the exponential-root field response to magnetic systems.

Findings

Magnetic current exhibits exponential-root field growth under applied magnetic field.

Confirmation of Coulomb attraction between magnetic monopoles in spin ice.

Transition from non-Ohmic to Wien effect with increasing temperature.

Abstract

A non-Ohmic current that grows exponentially with the square root of applied electric field is well known from thermionic field emission (the Schottky effect), electrolytes (the second Wien effect) and semiconductors (the Poole-Frenkel effect). It is a universal signature of the attractive Coulomb force between positive and negative electrical charges, which is revealed as the charges are driven in opposite directions by the force of an applied electric field. Here we apply thermal quenches to spin ice to prepare metastable populations of bound pairs of positive and negative emergent magnetic monopoles at millikelvin temperatures. We find that the application of a magnetic field results in a universal exponential-root field growth of magnetic current, thus confirming the microscopic Coulomb force between the magnetic monopole quasiparticles and establishing a magnetic analogue of the Poole-Frenkel effect. At temperatures above 300 mK, gradual restoration of kinetic monopole equilibria causes the non-Ohmic current to smoothly evolve into the high field Wien effect for magnetic monopoles, as confirmed by comparison to a recent and rigorous theory of the Wien effect in spin ice. Our results extend the universality of the exponential-root field form into magnetism and illustrate the power of emergent particle kinetics to describe far-from equilibrium response in complex systems.