| 09:00-10:40 | We1: E-ph coupling in novel 2D materials (I) |
| 10:40-11:20 | Coffee break |
| 11:20-13:00 | We2: E-ph coupling in novel 2D materials (II) |
| 13:00-15:30 | Poster session B |
| Afternoon off - some suggested activities can be found here | |
| 19:30-20:30 | Public lecture by Polanyi in Kutxa Sala Andia (city center) |
Chair: J. Wells, Trondheim, Norway
Contributed talk
Calculation of strongly anharmonic phonons in transition metal dichalcogenides
1Donostia International Physics Center (DIPC), Donostia, Basque Country, Spain
2CNRS, UPMC, IMPMC, Paris, France
Many transition metal dichalcogenides (TMDs) crystallize in the bulk with layered structures that can be exfoliated down to a monolayer [1]. Both in the bulk and the monolayer, TMDs show a large variaty of remarkable physical phenomena, including superconductivity and the formation of charge-density waves (CDWs) [2]. Therefore, calculating from first-principles their electronic and vibrational properties becomes crucial to characterize and understand these phenomena.
In many TMDs the standard harmonic approximation for calculating vibrational properties breaks down as it predicts that the experimentally observed structures have imaginary phonon modes and, thus, should be dynamically unstable. This is particularly remarkable in TMDs that undergo a CDW transition. Interestingly, the instabilities are larger in the monolayer than in the bulk [3].
The fact that the CDW transition is driven by the softening with temperature of a phonon mode [4] indicates that anharmonic effects play a crucial role in many TMDs. Here we present fully first-principles calculations of anharmonic phonon spectra in NbSe2, a prototypical TMD with a CDW, making use of our recently developed stochastic self-consistent harmonic approximation (SSCHA) [5,6]. The SSCHA is a variational method valid to calculate vibrational properties in strongly anharmonic crystals. We show that with the SSCHA we can understand the dynamical stability of NbSe2, calculate the transition temperature of the CDW, and understand the superconducting mechanism.
[1] K. S. Novoselov et al., Proc. Natl. Acad. Sci. USA 102, 10451 (2005).
[2] Y. Cao et al., arXiv:1502.03755 (2015)
[3] M. Calandra et al., Phys. Rev. B 80, 241108(R) (2009)
[4] F. Weber et al., Phys. Rev. Lett. 107, 107403 (2011)
[5] I. Errea et al., Phys. Rev. Lett. 111, 177002 (2013)
[6] I. Errea et al., Phys. Rev. B 89, 064302 (2014)