ICTP-EAIFR is pleased to welcome Dr. Fatema Mohamed, from the Theroretical spectroscopy grooup at LSI, Ecole Polytechnique (France). Dr. Fatema Mohamed has kindly offered to give a seminar entitled: "Excitons in Helium under pressure".

Date and time: Thursday, 16th November 2023 at 10:00

Physical Venue: ICTP-EAIFR’s conference hall (fifth floor), EINSTEIN Block, former “KIST2” Building, UR-CST, Nyarugenge Campus, University of Rwanda, Kigali.

Abstract:

The absorption and the electron energy loss spectra of solid helium at different pressure are calculated using the first principles approach based on the Bethe-Salpeter equation of many-body perturbation theory. The results are used to explain the effect of pressure on on HCP helium that has been measured by spectroscopy experiments [1,2]. We found a well defined exciton peak, showing linear pressure dependence, both in the peak position and intensity. We calculated the excitonic dispersion of the loss function along the Γ-M direction for momentum transfers beyond the first Brillouin zone. We found strong parabolic like dispersion of the spectra, dominated by the screened Coulomb interaction. Near Bragg reflection points we found anomalous angular dependence of of the exciton peak, due to the crystal local field effects, analogously to the plasmon in graphite [3].

References:
1- M.-L. David, et al. Appl. Phys. Lett. 98, 171903, 2011.
2- H. K. Mao, et al. Phys. Rev. Lett. 105, 186404, 2010. 3- R. Hambach, et al. Phys. Rev. Lett. 101, 266406, 2008.