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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 [3], and recent
experiments which suggest that the spin ice Dy2Ti2O7 may undergo a phase
transition at very low temperature [4].

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 [5],
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 [10].

[1] K. Ross et al., Phys. Rev. X 1, 021002 (2012).
[2] L.-J. Chang et al., Nature Commun. 3, 992 (2012)
[3] K. Kimura et al., Nature Commun. 4, 1934 (2013)
[4] D. Pomaranski et al., Nature Phys. 9, 353 (2013).
[5] M. Hermele et al., Phys. Rev. B 69, 064404 (2004).
[6] A. Banerjee et al., Phys. Rev. Lett. 100, 047208 (2008)
[7] N. Shannon et al., Phys. Rev. Lett. 108, 067204 (2012).
[8] O. Benton et al., Phys. Rev. B 86, 075154 (2012).
[9] Y. Kato et al., arXiv:1411.1918
[10] P. McClarty et al., arXiv:1410.0451

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