Spin ice, with its magnetic monopole excitations, is perhaps the outstanding example a classical, topological spin liquid. Nonetheless, the role of quantum effects in spin-ice materials remains poorly understood. This question gain fresh urgency from studies of "quantum spin-ice" materials such as Yb2Ti2O7 [1,2] and Pr2Zr2O7 , and recent experiments which suggest that the spin ice Dy2Ti2O7 may undergo a phase transition at very low temperature .
In this talk, we explore some of the new phenomena which can arise as a result of quantum fluctuations in a spin-ice material. We show how quantum tunnelling between different spin-ice configurations can convert spin-ice into a quantum spin liquid with photon-like excitations , review the numerical evidence that such a state exists [6-9], and discuss how it might be identified in experiment [8,9].
We also consider the nature of the quantum ground state in a realistic model of spin ice, directly motivated by Dy2Ti2O7. We identify the principles which govern magnetic order in the presence of long-range dipolar interactions, and use quantum Monte Carlo simulation to show that only a very small amount of quantum tunnelling is needed to convert these ordered states into a quantum spin liquid .
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