Electron-phonon beyond Fröhlich: dynamical quadrupoles in polar and covalent solids

IMCN

Electron-phonon (e-ph) interactions play an important role in various physical phenomena such as the conventional phonon-mediated superconductivity, the phonon-assisted light absorption, the dependence on temperature of band structures, the zero-point renormalization of the band gap in semiconductors, and the thermal and electrical conductivities. In polar semiconductors, electrical (Fröhlich) dipoles are created by the displacement of the charged ions around their equilibrium position. These dipoles create a very long-range potential that typically extends over multiple unit cells of the crystal. They have thus a large impact on the behavior of electrons: for instance, the electrical conductivity is limited by strong dipoles that scatter the electrons.

The current state-of-the-art computational approaches take these dipoles into account for obtaining e-ph related physical properties. However, besides dipoles, dynamical quadrupoles are also created by the ionic displacements, even in nonpolar semiconductors such as silicon (see Figure). So far, these have been disregarded by the e-ph community because they were thought to bring a much smaller contribution to the total scattering of electrons. In our work, we show that they are far from negligible and should therefore be taken into account: without dynamical quadrupoles, computations of the electrical conductivity can be as far as 30% off. Previously published theoretical results should therefore be critically reconsidered in the light of our findings.

In addition to these new physical results, we have implemented a new and efficient ab initio method for computing the phonon-limited transport properties of semiconductors. These new developments are available in ABINIT, a software used by thousands of researchers around the world and whose development is coordinated at the MODL division of the IMCN.

This research has been the object of two joint publications co-authored by IMCN researchers.

References:

G. Brunin, H. P. C. Miranda, M. Giantomassi, M. Royo, M. Stengel, M. J. Verstraete, X. Gonze, G.-M. Rignanese, and G. Hautier, Electron-phonon beyond Fröhlich: dynamical quadrupoles in polar and covalent solids, Phys. Rev. Lett. 125, 136601 (2020) [weblink]

G. Brunin, H. P. C. Miranda, M. Giantomassi, M. Royo, M. Stengel, M. J. Verstraete, X. Gonze, G.-M. Rignanese, and G. Hautier, Phonon-limited electron mobility in Si, GaAs, and GaP with exact treatment of dynamical quadrupoles, companion paper Phys. Rev. B 102, 094308 (2020) [weblink]