Electronic excitations and their effect on the interionic forces in simulations of radiationdamage in metals

Abstract

Using time-dependent tight-binding simulations of radiation damage cascades in amodel metal we directly investigate the nature of the excitations of a system ofquantum mechanical electrons in response to the motion of a set of classical ions. Wefurthermore investigate the effect of these excitations on the attractive electronic forcesbetween the ions. We find that the electronic excitations are well described by aFermi–Dirac distribution at some elevated temperature, even in the absence of the directelectron–electron interactions that would be required in order to thermalize anon-equilibrium distribution. We explain this result in terms of the spectrum ofcharacteristic frequencies of the ionic motion. Decomposing the electronic forceinto four well-defined components within the basis of instantaneous electroniceigenstates, we find that the effect of accumulated excitations in weakening theinterionic bonds is mostly (95%) accounted for by a thermal model for the electronicexcitations. This result justifies the use of the simplifying assumption of a thermalizedelectron system in simulations of radiation damage with an electronic temperaturedependence and in the development of temperature-dependent classical potentials.