Olga Petrova
Junior Research Chair
Department of Physics, Ecole Normale Superieure, Paris

Magnetic monopoles in quantum spin ice

The quest for quantum spin liquids is an important enterprise in strongly correlated physics, yet candidate materials are still few and far between. Meanwhile, the classical front has had far more success, epitomized by the exceptional agreement between theory and experiment for a class of materials called spin ices. It is therefore natural to introduce quantum fluctuations into this well-established classical spin liquid model, in the hopes of obtaining a fully quantum spin liquid state.

The spin-flip excitations in spin ice fractionalize into pairs of magnetic monopoles carrying opposite effective charge. Quantum fluctuations have a parametrically larger effect on monopole motion than on the spin ice ground states. Therefore, the leading manifestations of quantum behavior appear in the dynamics of the monopoles. I will first discuss a particularly crisp phenomenon that we found in the presence of weak dilution with nonmagnetic ions: the emergence of hydrogenic excited states in which a magnetic monopole is bound to a vacancy at various distances. I will show how this vacancy problem can be approximated by an exactly solvable model of a single particle hopping on the Bethe lattice. This allows us to obtain an approximate expression for the dynamic structure factor for the weakly diluted quantum spin ice, which can be directly measured in neutron scattering experiments.

In the more general, unperturbed case we find that the classical degeneracy of the excited states is only partially lifted by quantum fluctuations in the form of a transverse field term. I will discuss a family of extensively degenerate excited states that make up an approximately flat band at the first excited classical energy level. These states are exact up to the splitting of the spin ice ground state manifold.