"Electronic and vibrational properties of TiSe2 in the charge-density wave phase from first principles "

Who: Raffaello Bianco (Université Pierre et Marie Curie and CNRS)

Place: Donostia International Physics Center

Date: Monday, 4 May 2015, 12:00

Electronic and vibrational properties of TiSe2 in the charge-density wave phase from first principles

Raffaello Bianco
CNRS, UPMC Univ Paris 06, Paris, France

The charge-density wave instability in TiSe2 has recently received considerable attentions,
both for conceptual reasons and techonological applications, since in this system the
charge-density wave phase competes with superconductivity and its origin remains
controversial. In fact, several mechanisms have been proposed for this structural transition
depending on the driving role played either by the electron-electron or by the electron-lattice
interaction [1-4].

We present a first-principle study of the charge-density wave phase in TiSe2.
We considered several local functionals and both experimental and theoretical cell
parameters. We show that, if the cell parameters used are in agreement with the experiment,
density-functional calculations based only on the electron-phonon coupling are able to
reproduce not only the structural instability of TiSe2 but also the effective distortion observed
in the experiments. The results obtained are robust with respect to the local functional used.
We also present a study of the electronic structure evolution of the system under the
charge-density wave deformation. In particular, we apply the unfolding techinque in order to
compare the calculated energy bands for the distorted structure with angle-resolved
photoemission spectroscopy (ARPES) data taken at low temperature.
In order to obtain a better agreement between ARPES and calculated bands, both at high and
low temperature, we investigated the effect of the correlation on the electrons of the localized
Ti-d orbitals by using the LDA+U method. We show that within this approximation
the electronic bands for both the undistorted and distorted structure are in
very good agreement with ARPES. On the other hand, the U eliminates the phonon instability
of the system. Some possible explanations for this counter intuitive result are proposed.

[1] F.J. Di Salvo, D. E. Moncton and J. V. Waszczak, Phys. Rev. B 14, 4321 (1976)
[2] H. Cercellier et al., Phys. Rev. Lett. 99, 146403 (2007)
[3] M. Calandra and F. Mauri, Phys. Rev. Lett. 106, 196406 (2011)
[4] Porer et al., Nature Materials 13, 857 (2014)

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