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Magnetic monopoles in spin ice

Abstract

Electrically charged particles, such as the electron, are ubiquitous. In contrast, no elementary particles with a net magnetic charge have ever been observed, despite intensive and prolonged searches (see ref. 1 for example). We pursue an alternative strategy, namely that of realizing them not as elementary but rather as emergent particles—that is, as manifestations of the correlations present in a strongly interacting many-body system. The most prominent examples of emergent quasiparticles are the ones with fractional electric charge e/3 in quantum Hall physics2. Here we propose that magnetic monopoles emerge in a class of exotic magnets known collectively as spin ice3,4,5: the dipole moment of the underlying electronic degrees of freedom fractionalises into monopoles. This would account for a mysterious phase transition observed experimentally in spin ice in a magnetic field6,7, which is a liquid–gas transition of the magnetic monopoles. These monopoles can also be detected by other means, for example, in an experiment modelled after the Stanford magnetic monopole search8.

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Figure 1: The pyrochlore and diamond lattices.
Figure 2: Mapping from dipoles to dumbbells.
Figure 3: Monopole interaction.
Figure 4: Phase diagram of spin ice in a [111] field.

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Acknowledgements

We thank S. Bramwell, J. Chalker, C. Chamon and S. Kivelson (especially for pointing out ref. 8) for discussions. This work is supported in part by EPSRC (CC), and NSF (SLS). We also thank A. Canossa for support with the graphics.

Author Contributions All authors contributed equally to the manuscript.

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Correspondence to C. Castelnovo.

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Castelnovo, C., Moessner, R. & Sondhi, S. Magnetic monopoles in spin ice. Nature 451, 42–45 (2008). https://doi.org/10.1038/nature06433

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