Evidence for the Confinement of Magnetic Monopoles in Quantum Spin Ice

TL;DR

This study provides spectroscopic evidence that magnetic monopoles in quantum spin ice are confined by a linear potential, contrasting with the deconfined monopoles previously hypothesized in spin ice materials.

Contribution

It reports the first direct spectroscopic evidence of magnetic monopole confinement in a quantum spin ice material, using high-resolution neutron spectroscopy.

Findings

Detection of unequally-spaced magnetic excitations

Modeling with a linear potential for monopole pairs

Evidence for monopole confinement in Pr2Sn2O7

Abstract

Magnetic monopoles are hypothesised elementary particles connected by Dirac strings that behave like infinitely thin solenoids. Despite decades of searches, free magnetic monopoles and their Dirac strings have eluded experimental detection, although there is substantial evidence for deconfined magnetic monopole quasiparticles in spin ice materials. Here we report the detection of a hierarchy of unequally-spaced magnetic excitations \emph{via} high resolution inelastic neutron spectroscopic measurements on the quantum spin ice candidate Pr$_{2}$Sn$_{2}$O$_{7}$. These excitations are well-described by a simple model of monopole pairs bound by a linear potential with an effective tension of 0.642(8) K~$\cdot$\AA$^{-1}$ at 1.65~K. The success of the linear potential model suggests that these low energy magnetic excitations are direct spectroscopic evidence for the confinement of magnetic monopole quasiparticles in the quantum spin ice candidate Pr$_{2}$Sn$_{2}$O$_{7}$.