Experimental identification of electric dipoles induced by magnetic monopoles in Tb2Ti2O7

TL;DR

This study provides experimental evidence that magnetic monopoles in the spin ice material Tb2Ti2O7 induce electric dipoles, revealing a new aspect of magnetoelectric coupling in frustrated magnetic systems.

Contribution

It demonstrates the first clear experimental observation of electric dipoles induced by magnetic monopoles in a spin ice compound, confirming theoretical predictions.

Findings

Magnetic monopoles induce electric dipoles in Tb2Ti2O7.

Structural phase transition occurs at Hc~2.3 T under magnetic field.

Non-linear phonon splitting indicates oxygen displacement.

Abstract

The fundamental principles of electrodynamics allow an electron carrying both electric monopole (charge) and magnetic dipole (spin) but prohibit its magnetic counterpart. Recently it was predicted that the magnetic "monopoles" carrying emergent magnetic charges in spin ice systems can induce electric dipoles. The inspiring prediction offers a novel way to study magnetic monopole excitations and magnetoelectric coupling. However, no clear example has been identified up to now. Here, we report the experimental evidence for electric dipoles induced by magnetic monopoles in spin frustrated Tb2Ti2O7. The magnetic field applied to pyrochlore Tb2Ti2O7 along [111] direction, brings out a "3-in-1-out" magnetic monopole configuration, and then induces a subtle structural phase transition at Hc~2.3 T. The transition is evidenced by the non-linear phonon splitting under magnetic fields and the anomalous crystal-field excitations of Tb3+ ions. The observations consistently point to the displacement of the oxygen O" anions along [111] axis which gives rise to the formation of electric dipoles. The finding demonstrates that the scenario of magnetic monopole having both magnetic charge and electric dipole is realized in Tb2Ti2O7 and sheds light into the coupling between electricity and magnetism of magnetic monopoles in spin frustrated systems.