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Séminaire Magnétisme non conventionnel

Jeudi 6 Octobre à 14h00,
Salle Remy Lemaire, K223

Oratrice : Diane Lançon (LQM - EPF Lausanne & ILL Grenoble) "Correlated hopping and correlated decay of excitations in the 2D quantum magnet SrCu2(BO3)2"


SrCu2(BO3)2 is a good realization of the Shastry-Sutherland model with S=1/2 spins, which is of particular interest in the context of frustrated quantum magnetism. The inelastic spectrum of SrCu2(BO3)2 (SCBO) exhibits an almost dispersion-less spin gap of about 3 meV, and a bound-state of two triplets form at 5 meV. The unusual size and dispersion-less nature of the gap is an effect of the frustration which prevents triplets from hopping up to sixth order. Above the single triplet mode, correlated hopping is predicted to lead to a dispersive multi-magnon continuum [1]. Perfect frustration is weakly lifted by an anisotropic Dzyaloshinsky-Moriya (DM) interaction, creating a small dispersion at zero field, and has been predicted to be responsible for a Dirac dispersion at a critical field [2]. We used inelastic neutron scattering to study the excitation spectra of SCBO at both the energies of the single triplet mode and of the multi-excitation modes, as well as the effect of temperature and applied field.
Peculiar temperature damping of the single triplet mode at finite temperature has been observed in SCBO and explained by a real space correlated-decay model [3]. Lifting the degeneracy of the single triplet mode with an applied magnetic field, we find a Q-dependence of the fast damping process and a damping width which also depends on the excited triplet. Complete mapping of the triplet dispersion as a function of field confirmed the predicted Dirac-dispersion at a critical field. Finally, the neutron scattering results of the multi-excitation part of the spectra bear more resemblance to a series of dispersion-less band than to a continuum, and the extracted structure factor for each of the bands will be discussed.

Figure : (left) Projection of the crystal structure of SrCu2(BO3)2 along the c-axis, showing the magnetic CuBO3 plane and corresponding Shastry-Sutherland model. (right) LET data at 1.5K and zero field, showing the weak dispersion of each the 3 triplet modes due to the Dzyaloshinskii-Moriya interaction. Data integrated along QL .

[1] Miyahara, S., & Ueda, K. (2003). Theory of the orthogonal dimer Heisenberg spin model for SrCu2 (BO3) 2. Journal of Physics : Condensed Matter, 15(9), R327.
[2] Romhányi, J., Penc, K., & Ganesh, R. (2015). Hall effect of triplons in a dimerized quantum magnet. Nature communications, 6.
[3] Zayed, M. E., Rüegg, C., Strässle, T., Stuhr, U., Roessli, B., Ay, M., ... & Conder, K. (2014). Correlated Decay of Triplet Excitations in the Shastry-Sutherland Compound SrCu 2 (BO 3) 2. Physical review letters, 113(6), 067201.

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