Groupe de Physique Statistique

Equipe 106, Institut Jean Lamour

par Orateurs
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Séminaire de groupe

Structure of dynamical correlations developing on top of an entropically designed frustrated manifold
Mathieu Taillefumier
ITP, Goethe university, Frankfurt
jeudi 20 décembre 2012 , 10h25
Salle de séminaire du groupe de Physique Statistique

By combining monte carlo and spin dynamics simulations, we investigate the precessionnal dynamics of the classical kagome antiferromagnet through the calculation of the dynamical structure factor $S({\bf Q}, t)$. Recently, evidences for spin wave like excitations in the two distincts low temperature regimes − whose temperature ranges are given by the entropically driven onset of spin plane coplanarity at $T_0/J\approx 5\ 10^{-3}$ has been given [1]. However, only a little is known about the longer time scales describing the fluctuations around the ground-state manifold. Here, to capture how such propagative excitations arise in spite of short-ranged static correlations, we give more insight about this relaxationnal dynamics and establish in particular the temperature and wave-vector dependence of the lifetime of locally ordered states. Although the infinite components spins model [2] qualitatively reproduce the dynamical properties in the cooperative paramagnetic regime, we show at lower temperature that the entropic selection (i) leads to strongly different dynamical correlations for the in-plane and out-of-plane spin components below the transition (the out-of-plane fluctuations being governed by weathervane-modes), and (ii) almost suppresses the diffusive behavior observed in the cooperative regime in favor of mainly propagative spin transfers [3]. We furthermore show that the powder averaged spectrum is overwhelmed by these spin fluctuations, pointing out that the propagative spin waves evidenced in [1] would be hardly discernible in powder samples down to very low temperatures, as observed experimentally. [1] J. Robert, B. Canals, V. Simonet and R. Ballou, Physical Review Letters , (2008). [2] Conlon and Chalker, Physical Review Letters (2009) [3] M. Taillefumier, J. Robert, B. Canals, C. Henley and R. Moessner in preparation (2012).

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