Phonon softening and metallization of a narrow-gap semiconductor by thermal disorder

O. Delaire,1 K. Marty,1 M. B. Stone,1 P. R. C. Kent,1 M. S. Lucas,2 D. L. Abernathy,1 D. Mandrus,1 B. C. Sales1

1- Oak Ridge National Laboratory, Oak Ridge, TN 37831
2-Air Force Research Laboratory, Wright-Patterson AFB, OH 45433

Achievement

We have shown how, in some materials, there can be a surprisingly strong coupling between certain features of the electronic structure and the way the atoms in a solid vibrate. This insight should help us understand better how heat is transported in a solid. Inelastic neutron scattering measurements of Fe1-xCoxSi alloys were combined with quantum mechanics based calculations to show why the alloys exhibit unusual softening as the temperature is increased. Our results show that for alloys with a rapidly changing concentration of electrons near the chemical potential, there are likely to be strong temperature-dependent interactions between the atom vibrations and electrons.

(Left) Phonon dispersions of FeSi measured via time-of-flight inelastic neutron scattering, compared with calculations (light blue lines) provides clear evidence of the unusual softening of atomic motion with increasing temperature. (Right) Phonon density of states for FeSi and CoSi from ab initio molecular dynamics simulations. A large phonon softening in FeSi between 300K and 1200K is predicted.

Significance

By combining extensive neutron scattering based analysis with the results of first principles molecular dynamics calculations, we have clearly demonstrated a strong coupling between the phonon and electron states when there are sharp electronic features around the Fermi level. These effects are likely to be common to many narrow gap materials including some superconductors, heavy-Fermion compounds, and many thermoelectric materials. Our results demonstrate the importance of including these effects in predicting or optimizing heat flow in these materials.

Credit

This work was published in Proceedings of the National Academy of Sciences (7 March 2011, 2010-14869RR, doi: 10.1073/pnas.1014869108). Research at ORNL’s Spallation Neutron Source, High Flux Isotope Reactor, and Center for Nanophase Materials Sciences (PRCK) was sponsored by the U. S. Department of Energy Scientific User Facilities Division. Computations by PRCK used resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science, U.S. Department of Energy.