Coherent Phonons

This work focuses on an investigation of electron-phonon coupling in Bismuth. We calculate from first principles the dependence of the total energy on the degree of electon-hole excitation in the system in conjuntion with the position of the atoms within the unit cell (see figure). This is done by constraining the band occupations in a standard DFT calculation [1,2]. Using this total energy surface, we can calculate information such as the softening of the phonon frequencies and the change on the equilibrium atomic positions under ultra-fast laser excitation [3].

This work is also done in collaboration with an experimental group based in the FOCUS centre at University of Michigan, who have confirmed many of our results using experiments giving femtosecond resolution utilising the X-ray source at SLAC. We also use this approach together with DFPT in order to calculate the softening of modes throughout the Brillouin zone under excitation. Our current work is concentrated on a calculation of the change in phonon lifetime with increase in excited electron-hole carrier density and is part of a collaboration with members of a group based at Lawrence Livermore National Laboratory.

References
[1] P. Tangney and S. Fahy, Phys. Rev. B 65, 54302 (2002).
[2] P. Tangney P and S. Fahy, Phys. Rev. Lett. 82, 4340-4343 MAY 24 1999
[3] E. D. Murray, D. M. Fritz, J. K. Wahlstrand, S. Fahy and D. A. Reis, Phys. Rev. B 72, 060301 (2005)

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