Séminaire du lundi 21 novembre 2011 à 10h30

IMPMC, Université P. et M. Curie, 4, Place Jussieu, 75005 Paris

Salle de Conférence (401)

4ème étage

Tour 23

Couloir 22-23

Phonon softening and charge density wave formation in NbSe₂ and TiSe₂

Frank WEBER (Karlsruher Institut für Technologie Institut für Festkörperphysik)

Invite par M. D’ASTUTO matteo.dastuto@impmc.upmc.fr

 We report [1] inelastic x-ray scattering measurements of the temperature dependence of phonon dispersion in the prototypical charge-density-wave (CDW) compounds NbSe₂ and TiSe₂. Surprisingly, acoustic phonons in NbSe₂ soften to zero frequency and become overdamped over an extended region around the CDW wavevector. This extended phonon collapse is dramatically different from the sharp cusp in the phonon dispersion expected from Fermi surface nesting. Instead, our experiments combined with ab initio calculations, show that it is the wavevector dependence of the electron-phonon coupling that drives the CDW formation in NbSe2 and determines its periodicity. For TiSe₂ [2], we found theoretically that the above described electron-phonon coupling scenario is able to stabilize a CDW ground state as well. Calculations of the lattice dynamical properties are in good agreement with our experimental results for the soft phonon dispersion and linewidth. A more detailed analysis of the calculations yield wavevector dependent matrix elements, strong enough to stabilize the CDW order in combination with an enhanced electronic joint density of states at the ordering wavevector.
1) F.Weber et al. Phys. Rev. Letters 107(10),107403(2011).
2) F.Weber et al http://arXiv.org/abs/1105.3292(2011)

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