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Charge density wave hampers exciton condensation in 1TTiSe2

Chao Lian, Zulfikhar A. Ali, and Bryan M. Wong
Phys. Rev. B 100, 205423 – Published 25 November 2019

Abstract

The Bose-Einstein condensation of excitons continues to garner immense attention as a prototypical example for observing emergent properties from many-body quantum effects. In particular, titanium diselenide (TiSe2) is a promising candidate for realizing exciton condensation and was experimentally observed only very recently [Kogar et al., Science 358, 1314 (2017)]. Surprisingly, the condensate was experimentally characterized by a soft plasmon mode that only exists near the transition temperature Tc of the charge density wave (CDW). Here, we characterize and analyze the experimental spectra using linear-response time-dependent density functional theory and find that the soft mode can be attributed to interband electronic transitions. At the CDW state below Tc, the periodic lattice distortions hamper the spontaneous formation of the exciton by introducing a CDW gap. The band gap raises the soft mode and merges it into the regular plasmon. Our surprising results contradict previous simplistic analytical models commonly used in the scientific literature. In addition, we find that a finite electronic temperature Te introduces a dissipation channel and prevents the condensation above Tc. The combined effect of the CDW and Te explains the fragile temperature dependence of the exciton condensation. Taken together, our work provides the first ab initio atomic-level framework for rationalizing recent experiments and further manipulating exciton condensates in TiSe2.

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  • Received 12 July 2019
  • Revised 19 October 2019

DOI:https://doi.org/10.1103/PhysRevB.100.205423

©2019 American Physical Society

Physics Subject Headings (PhySH)

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Chao Lian, Zulfikhar A. Ali, and Bryan M. Wong*

  • Department of Chemical and Environmental Engineering, Materials Science and Engineering Program, and Department of Physics and Astronomy, University of California-Riverside, Riverside, California 92521, USA

  • *bryan.wong@ucr.edu

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Issue

Vol. 100, Iss. 20 — 15 November 2019

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