Who: Dr. Massimo Rontani, Cnr-Nano, Modena, Italy
Place: Donostia International Physics Center
Date: Monday, 17 July 2017, 15:00
Carbon nanotubes as excitonic insulators
Daniele Varsano1, Sandro Sorella2, Davide Sangalli3, Matteo Barborini1, Stefano Corni1, Elisa Molinari1,4, and Massimo Rontani1
1Cnr-Nano, Modena, Italy
2SISSA & Cnr-Iom Democritos, Trieste, Italy
3Cnr-Ism, Roma, Italy
4FIM, University of Modena, Modena, Italy
Fifty years ago Walter Kohn speculated  that a zero-gap semiconductor might be unstable against the spontaneous generation of excitons?electron-hole pairs bound together by Coulomb attraction. The reconstructed ground state would then open a gap breaking the symmetry of the underlying lattice, a genuine consequence of electronic correlations. Here we show  that this 'excitonic insulator' is realized in zero-gap armchair carbon nanotubes by performing first-principles calculations through many-body perturbation theory as well as quantum Monte Carlo. The excitonic order modulates the charge between the two carbon sublattices opening an experimentally observable gap, which scales as the inverse of the tube radius and weakly depends on the axial magnetic field. Our findings confute the Luttinger liquid paradigm for nanotubes and provide tests to experimentally discriminate between excitonic and Mott insulator.
 D. Sherrington and W. Kohn, Rev. Mod. Phys. 40, 767 (1968).
 D. Varsano, S. Sorella, D. Sangalli, M. Barborini, S. Corni, E. Molinari, and M. Rontani, arXiv:1703.09235.