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Séminaire Magnétisme non conventionnel

Jeudi 8 Février à 14h00
Salle Remy Lemaire, K223

Orateur : Valentin RABAN (ENS Lyon)
"Magnetic monopoles in spin ice and the fluctuation-dissipation relation"


Over the last decade the magnetic materials known as spin ice have been at the centre of attention in the magnetism community. This follows the development of a theory for the low energy spectrum of excitations above the ground state manifold in terms of quasi-particles carrying isolated magnetic charge – a so called magnetic monopole [1]. As a consequence, this remarkable system can be very accurately modelled by a lattice Coulomb gas with kinetic constraints that restricts particle movement to the magnetic configurations - the dumbbell model [2,3,4,5].

During my thesis, we planned to take spin ice in a new direction, which has itself been of great interest in recent years – the study of its non-equilibrium properties in the form of the fluctuation dissipation theorem. Indeed some of the most significant progresses in non-equilibrium statistical mechanics in the last twenty years have come from the study of the fluctuation dissipation ratio as one moves out of equilibrium. We focused first on the behaviour of the magnetic monopoles after a thermal quench, where it has been shown that they can be trapped in neutral pairs [6] which remain out-of-equilibrium for a macroscopically long time. In doing so, we were able to define effective temperatures for this regime which show a clear thermodynamical separation of the spin and monopole degrees of freedom.

Then, in on-going work we investigates the phase diagram of the dumbbell model in the three dimensional space of chemical potential, temperature and staggered chemical potential that breaks the Z_2 symmetry of the diamond lattice. The rich phase diagram contains 2D phase boundaries between monopole fluid and crystal phases, terminating in lines of critical end points. As in [7], Kibble-Zurek dynamical scaling is expected near these points. The resulting slow, out-of-equilibrium dynamics could be studied using the previously developed tools for the generalised fluctuation-dissipation theorem.

References :

[1] Castelnovo, C., Moessner, R. & Sondhi S., Nature 451, 42-45 (2008).
[2] L. Jaubert and P. C. W. Holdsworth, Nature Physics 5, 258 - 261 (2009).
[3] L. D. C. Jaubert, M. J. Harris, T. Fennell, R. G. Melko, S. T. Bramwell, and P. C. W. Holdsworth, Phys. Rev. X 3, 011014, (2013) (open source).
[4] M.E. Brooks-Bartlett, S.T. Banks, L.D.C. Jaubert, A. Harman-Clarke, and P.C.W. Holdsworth, Phys. Rev. X, 4, 011007 (2014) (open source).
[5] V. Kaiser, S. T. Bramwell, P. C. W. Holdsworth, R. Moessner, Phys. Rev. Lett. 115, 037201 (2015).
[6] C. Castelnovo, R. Moessner, and S. L. Sondhi, Phys. Rev. Lett. 104, 107201 (2010)
[7] J. Hamp, A. Chandran, R. Moessner, and C. Castelnovo, Phys. Rev. B 92, 075142 (2015)

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