Triple-q charge-density-wave state in TiSe2: Exciton-phonon cooperative scenario

Date

Mar 8, 2018

Time

10:30 AM - 11:30 AM

Speaker

Tatsuya Kaneko, PhD

Affiliation

Computational Condensed Matter Physics Laboratory, RIKEN

Language

en

Main Topic

Materialien

Other Topics

Materialien, Physik

Host

Grit Rötzer

Description

Transition-metal dichalcogenides (TMDs) are representative materials that show the charge-density-wave CDW) states. The majority of group-IV TMDs are simple d0 semiconductors, in which the Fermi level is located between the valence chalcogen p and conduction transition-metal d bands [1]. However, 1T-TiSe2 is the only material that shows the CDW transition among the group-IV TMDs [2]. Because the electronic band structure of TiSe2 is located near the semimetal-semiconductor phase boundary, its CDW state has been investigated as a candidate for the excitonic ordering state induced by the interband Coulomb interaction [3]. On the other hand, the phononic mechanism of the CDW has also been suggested in TiSe2,where the CDW is explained simply by the electron-phonon coupling [4]. Thus, the two different driving forces for the CDW have been suggested in TiSe2, of which the determination is still controversial.Here, we investigate the mechanism of the CDW in TiSe2 on the basis of a realistic multi-orbital d-p model with the electron-phonon coupling and inter-site Coulomb interaction, which enables us to treat both the phononic and excitonic mechanisms of the CDW transition. Using the effective model, we discuss the stability of the triple-q CDW in which the phonon modes at the M1, M2, and M3 points of the Brillouin zone are frozen simultaneously. Here, we show that the excitonic interaction favors to further stabilize the triple-q CDW state caused by the phononic mechanism. We thus demonstrate that the electron-phonon and excitonic interactions cooperatively stabilize the triple-q CDW state in TiSe2. In addition, to elucidate the electronic structure in the triple-q CDW, we calculate the local charge density distribution in TiSe2. We show that the local bonding orbital of the Ti d and Se p orbitals is the essence in the triple-q CDW, which induces a vortex-like antiferroelectric electron polarization. [1] M. Chhowalla et al., Nature Chem. 5, 263 (2013). [2] F. J. Di Salvo, D. E. Moncton, and J. V. Waszczak, Phys. Rev. B 14, 4321 (1976). [3] H. Cercellier et al., Phys. Rev. Lett. 99, 146403 (2007). [4] N. Suzuki, A. Yamamoto, and K. Motizuki, J. Phys. Soc. Jpn. 54, 4668 (1985). [5] T. Kaneko, Y. Ohta, and S. Yunoki, arXiv:1711.08547. .

Links

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Last modified: Mar 8, 2018, 8:39:47 AM