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Séminaire MCBT

Mardi 11 décembre à 11h00,
Salle Louis Weil, E424

Orateur : Maria Hellgren (IMPMC, Paris)
"First-principle study of excitonic charge density waves in low-dimensional systems"


Charge density wave (CDW) instabilities are common in low-dimensional systems and are often detected in the vibrational spectrum as a softening of a particular phonon mode. In some unusual cases the phonon softening is accompanied by a soft electronic mode. The CDW transition in TiSe2 is expected to have this double character and decades of intense research has tried to reveal which mechanism dominates. This would, e.g., provide important clues for understanding the interplay between the CDW and the superconducting phase that emerges with the application of pressure or doping.

In this talk I will introduce a first-principle theoretical framework for describing excitonic instabilities and their coupling to the lattice. The CDW transitions in two paradigmatic cases are studied : TiSe2 and metallic carbon nanotubes. In the case of TiSe2 the CDW instability is predicted to be predominantly driven by a phonon instability strongly enhanced by the presence of excitonic correlations [1]. This is supported by the good agreement between our numerical results and experiments for both electronic and vibrational properties. Purely excitonic instabilities are found in metallic carbon nanotubes and I will show that a universal gap-opening mechanism can be formulated within a unified picture of excitonic and Peierls instabilities [2].

[1] M. Hellgren, J. Baima, R. Bianco, M. Calandra, F. Mauri, L. Wirtz, Phys. Rev. Lett. 119, 176401 (2017)
[2] M. Hellgren, J. Baima, A. Acheche, Phys. Rev. B 98, 201103(R) (2018)

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