Time: 4:30 p.m.
Place: UHW 11/1102
The seminar is shifted to 4:00 p.m.!
Xavier Gonze, ETSF / IMCN, Université Catholique de Louvain, Louvain-la-Neuve, Belgium
The energy bands of semiconductors exhibit significant shifts with temperature at constant volume. Formulas derived by Allen, Heine and Cardona [1-3] in a semiempirical context cannot be transposed to Density Functional Theory or to Many-Body Perturbation Theory  without critical reexamination. Indeed, the rigid-ion approximation was appropriate in this semi-empirical context, while the complete formulation includes an extra term, the non-site-diagonal Debye-Waller term. The importance of this extra term was examined for diatomic molecules and found to be of a size similar to the standard Debye-Waller and Fan terms. This might explain the discrepancy found between previous theory and experiment for solids. Furthermore, the slow convergence of the sum over unoccupied states of the Allen-Heine-Cardona approach can be avoided in a new formalism proposed here, based on Density- Functional Perturbation Theory, leading to a dramatic decrease of calculation times.
 P.B. Allen, V. Heine, J. Phys. C 9, 2305 (1976).
 P.B. Allen, M. Cardona, Phys. Rev. B 24, 7479 (1981).
 P.B. Allen, M. Cardona, Phys. Rev. B 27, 4760 (1983).
 A. Marini, Phys. Rev. Lett. 101, 106405 (2008).