Hunting excitonic instabilities in low dimensional materials | Dr. Daniele Varsano
Colloquium Series in Theoretical and Computational Physics at the Physics Department of the University of Trieste, Miramare Campus.
Dr. Daniele Varsano (CNR-NANO, Modena)
Spontaneous condensation of excitons is a long-sought phenomenon analogous to the condensation of Cooper pairs in a superconductor. It is expected to occur in a semiconductor at thermodynamic equilibrium if the binding energy of the excitons—electron (e) and hole (h) pairs interacting by Coulomb force—overcomes the band gap, giving rise to a new phase: the “excitonic insulator” (EI) [1].
Low dimensional systems are excellent candidates for the EI realization because of reduced Coulomb screening, and indeed a structural phase transition was observed in few-layer systems. However, previous work could not disentangle to which extent the origin of the transition was in the formation of bound excitons or in the softening of a phonon.
In this talk I will discuss our theoretical predictions, based on the combination of first-principles and model approaches concerning carbon nanotubes [2], monolayer TMD in the T’ phase (MoS2, WTe2) [3,4] showing that in this system topological and excitonic order cooperatively enhance the bulk gap by breaking the crystal inversion symmetry. Moreover I will also focus on bulk MoS2 demonstrating theoretically that at high pressure it is prone to the condensation of genuine excitons of finite momentum, whereas the phonon dispersion remains regular.
References
[1]L. V. Keldysh, Y. V. Kopaev, Sov. Phys. Sol. State 6, 2219 (1965) ; W. Kohn, “Metals and insulators” in Many-Body Physics, C. 1967
[2] D. Varsano, S. Sorella, D. Sangalli, M. Barborini, S. Corni, E. Molinari and M. Rontani et al., Nat. Commun. 8, 1461 (2017).
[3] D. Varsano, M. Palummo, E. Molinari, M. Rontani Nat. Nanotechnol. 15, 367–372 (2020) ; B. Sun et al, Nat. Physics 18, 94 (2022)
[4] S. Ataei, D. Varsano, E. Molinari, M. Rontani, PNAS, 118, e2010110118 (2021)