6 pages, 3 figures. Supplemental material: 10 pages, 11 figures
In insulators, Born effective charges describe the electrical polarization induced by the displacement of individual atomic sublattices. Such a physical property is at first sight irrelevant for metals and doped semiconductors, where the macroscopic polarization is ill defined. Here we show that, in clean conductors, going beyond the adiabatic approximation results in nonadiabatic Born effective charges that are well defined in the low-frequency limit. In addition, we find that the sublattice sum of the nonadiabatic Born effective charges does not vanish as it does in the insulating case, but instead is proportional to the Drude weight. We demonstrate these formal results with density functional perturbation theory calculations of Al and electron-doped SnS_{2} and SrTiO_{3}.
We thank D. Vanderbilt and R. Resta for discussions and insightful comments on the manuscript. C. E. D. acknowledges support from the National Science Foundation under Grant No. DMR-1918455. S. C. acknowledges support from the National Science Foundation under Grant No. DMR-1848074. M. S. acknowledges support from the European Research Council (ERC) through Grant “MULTIFLEXO” No. 724529; from Ministerio de Economia, Industria y Competitividad (MINECO-Spain) through Grants No. PID2019–108573GB-C22 and No. CEX2019-000917-S; and from Generalitat de Catalunya (Grant No. 2017 SGR1506). The Flatiron Institute is a division of the Simons Foundation.
With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).