Possible observation of highly itinerant quantum magnetic monopoles in the frustrated pyrochlore Yb2Ti2O7.

Y Tokiwa,Y Yasui,T Shibauchi,M Udagawa,S Kittaka,Y Matsuda,T Terashima,D Terazawa,Y Shimoyama,T Yamashita,T Sakakibara

doi:10.1038/ncomms10807

Abstract

The low-energy elementary excitations in frustrated quantum magnets have fascinated researchers for decades. In frustrated Ising magnets on a pyrochlore lattice possessing macroscopically degenerate spin-ice ground states, the excitations have been discussed in terms of classical magnetic monopoles, which do not contain quantum fluctuations. Here we report unusual behaviours of magneto-thermal conductivity in the disordered spin-liquid regime of pyrochlore Yb2Ti2O7, which hosts frustrated spin-ice correlations with large quantum fluctuations owing to pseudospin-1/2 of Yb ions. The analysis of the temperature and magnetic field dependencies shows the presence of gapped elementary excitations. We find that the gap energy is largely suppressed from that expected in classical monopoles. Moreover, these excitations propagate a long distance without being scattered, in contrast to the diffusive nature of classical monopoles. These results suggests the emergence of highly itinerant quantum magnetic monopole, which is a heavy quasiparticle that propagates coherently in three-dimensional spin liquids.

Highlights

The low-energy elementary excitations in frustrated quantum magnets have fascinated researchers for decades
To study the elementary excitations in the spin-liquid state of Yb2Ti2O7, we measured the thermal conductivity, which is a powerful probe for low-energy excitations at low temperatures, providing a sensitive measurement of a flow of entropy conducted by magnetic excitations and phonons
Our analysis shows the evidence of the substantial heat transport by quantum magnetic monoples, whose excitation energy is significantly suppressed from that of classical monopoles

Summary

Introduction

The low-energy elementary excitations in frustrated quantum magnets have fascinated researchers for decades. The monopole thermal conductivity can be well separated from the phonon contribution, which obeys magnetic field/temperature (H/T) scaling. Our analysis shows the evidence of the substantial heat transport by quantum magnetic monoples, whose excitation energy is significantly suppressed from that of classical monopoles.

Results

Conclusion