Thermal conductivity of quantum magnetic monopoles in the frustrated pyrochlore Yb2Ti2O7

TL;DR

This study investigates the thermal conductivity of Yb2Ti2O7, revealing quantum monopoles as highly mobile, heavy bosonic excitations with suppressed energy, distinct from classical monopoles, in a frustrated quantum spin liquid.

Contribution

It provides the first evidence of quantum monopoles with long mean free paths and suppressed excitation energies in a three-dimensional quantum spin liquid.

Findings

Quantum monopoles have suppressed excitation energies compared to classical monopoles.

Quantum monopoles exhibit very long mean free paths, indicating high mobility.

Thermal conductivity shows nonmonotonic magnetic field dependence explained by quantum monopole excitations.

Abstract

We report low-temperature thermal conductivity $\kappa$ of pyrochlore Yb$_2$Ti$_2$O$_7$, which contains frustrated spin-ice correlations with significant quantum fluctuations. In the disordered spin-liquid regime, $\kappa(H)$ exhibits a nonmonotonic magnetic field dependence, which is well explained by the strong spin-phonon scattering and quantum monopole excitations. We show that the excitation energy of quantum monopoles is strongly suppressed from that of dispersionless classical monopoles. Moreover, in stark contrast to the diffusive classical monopoles, the quantum monopoles have a very long mean free path. We infer that the quantum monopole is a novel heavy particle, presumably boson, which is highly mobile in a three-dimensional spin liquid.