Fermi surface nesting and phonon instabilities in simple cubic calcium

Abstract

Phonon instabilities and Fermi surface nesting are studied in the high-pressure simple cubic phase of calcium by means of ab initio calculations. We have focused on nesting along Γ X, which could be responsible for some of the anomalies observed in the phonon spectrum. Phonon frequencies calculated with the density functional perturbation theory are imaginary at several Brillouin-zone points (e.g. at M). However, including anharmonic contributions to the potential might be crucial to stabilize simple cubic calcium, as solving the Schrödinger equation associated to the transversal unstable mode at M gives a positive frequency.

Keywords:

• simple cubic calcium
• fermi surface nesting
• phonon instabilities
• anharmonicity
• high pressure

Acknowledgements

The research presented herein was performed under project BFM2003-04428, funded by the Spanish Ministry of Education and Science. I.E. would like to thank the Basque Department of Education, Universities and Research for financial help, and M.M.C. is also thankful to the Spanish Ministry of Education and Science for economic support and grant BES-2005-8057. Finally, the authors are also thankful to SGI-IZO SGIker UPV EHU for the allocation of computational resources, as these have made the present work a reality. A.R.O. gratefully acknowledges funding from the Swiss National Science Foundation (grants 200021-111847/1 and 200021-116219) and access to the Skif MSU supercomputer (Moscow State University) and supercomputers at the Swiss Supercomputer Centre (CSCS, Manno).