Dynamical Born effective charges

We study the electronic linear-response function induced by nuclear motion in solids by extending the concept of the Born effective charge to the dynamical regime. The dynamical Born effective charge is defined as the Fourier transform of the macroscopic electronic current divided by the Fourier transform of the velocity of a moving nucleus that induces the current. Using time-dependent density functional theory, we calculate the dynamical Born effective charges for selected materials and find rich spectra with pronounced resonance peaks. In the zero-frequency limit, the conventional static Born effective charges are recovered. Our results establish the dynamical Born effective charge as a natural observable for probing non-adiabatic electron–nuclear coupling in solids.