Neutrons for Science and Industry

Quantum liquids in rare-earth pyrochlore oxides Romain Sibille

by Dr Romain Sibille (Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institut)

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Magnetic systems with competing interactions often adopt exotic ground states, which can be relevant to study new physics in quantum matter [1]. A recurrent ingredient to stabilize such phases is frustration, such as in pyrochlore oxides where rare-earth magnetic moments decorate a lattice of corner-sharing tetrahedra. An unusual spin liquid appears for example in the pyrochlore Ho2Ti2O7, which features a classical ‘spin ice’ short-range correlated state [2,3]. A local constraint – the 2-in-2-out ‘ice rule’ acting on each tetrahedron – leads to a manifold of degenerate ground states in which the spin correlations give rise to emergent magnetostatics [4]. Spin flips violating the ice rule generate magnetic monopole excitations [5], a mobile magnetic charge regarded as a quasiparticle carrying half of the dipole moment. A quantum analogue of the spin ice state is predicted to be a special type of quantum spin liquid formed through the coherent superposition of spin ice configurations [6,7]. Remarkably, the low-energy physics of this quantum spin ice state is predicted to be a lattice analogue of quantum electrodynamics. I review recent results on tracking the experimental signatures of quantum mechanical ground states in the pyrochlores Pr2Hf2O7 [8-9] and Ce2Sn2O7 [10-11].

[1] Balents, L. Nature 464, 199 (2010).
[2] Bramwell, S. T. et al. Phys. Rev. Lett. 87, 047205 (2001).
[3] Castelnovo, C., Moessner, R. & Sondhi, S. L. Annu. Rev. Condens. Matter Phys. 3, 35–55 (2012).
[4] Fennell, T. et al. Science 326, 415 (2009).
[5] Castelnovo, C., Moessner, R. & Sondhi, S. L. Nature 451, 42–45 (2008).
[6] Hermele, M., Fisher, M. P. A. & Balents, L. Phys. Rev. B 69, 064404 (2004).
[7] Gingras, M. J. P. & McClarty, P. A. Rep. Prog. Phys. 77, 056501 (2014).
[8] Sibille, R. et al. Phys. Rev. B 94, 024436 (2016).
[9] Sibille, R. et al. Nature Physics 14, 711-715 (2018).
[10] Sibille, R. et al. Phys. Rev. Lett. 115, 097202 (2015).
[11] Sibille, R. et al. Nature Physics 16, 546-552 (2020).

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Dr. Jitae Park
Dr. Dominic Hayward

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