We analyze beyond linear-response theory the energy loss of a slow dipole moving inside a free-electron gas. The energy loss is obtained from a nonlinear treatment of the scattering of electrons at the dipole-induced potential. This potential and the total electronic density are calculated with density-functional theory. We focus on the interference effects, i.e., the difference between the energy loss of a dipole and that of the isolated charges forming it. Comparison of our results to those obtained in linear-response theory shows that a nonlinear treatment of the screening is required to accurately describe the energy loss of slow dipoles.

This work was supported in part by the Basque Departamento de Educación, Universidades e Investigación, the University of the Basque Country UPV/EHU ~Grant No. 9/UPV 00206.215-13639/2001!, and the Spanish Ministerio de Ciencia y Tecnología ~Grant Nos. MAT2001-0946 and BFM2001-0076!. R.D.M. acknowledges financial support by the Gipuzkoako Foru Aldundia.

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