Left panel: Plasmon linewidth dispersion obtained upon keeping 3 (triangles) and 6 (squares) valence bands in Kohn-Sham density response function. Inset: LDA band structure of K; the arrow indicates the value of plasmon energy at zero wave vector. Right panel: Calculated density of states (DOS) for potassium — total DOS and contributions from states of s, p, and d symmetry; the zero of energy is the Fermi level.

Adolfo Eguiluz, Oak Ridge National Laboratory
James M. Sullivan and Wei Ku, University of Tennessee and Oak Ridge National Laboratory

Research Objectives
Our research is devoted to the development of many-body techniques for the ab initio study of electron dynamics in strongly correlated materials, including rare-earth metal hydrides, transition metals and transition-metal oxides, narrow-band metals such as Zn and Cd, semiconductors, and insulators. New schemes are being developed for the ab initio evaluation of charge- and spin-density response and quasiparticle states in the presence of strong correlations. Two theoretical frameworks are being utilized: time-dependent density-functional theory (TDDFT), and many-body perturbation theory, implemented within the Baym-Kadanoff (BK) method of conserving approximations.

Computational Approach
Our code new.chig.exe calculates the charge and transverse spin response of bulk materials within TDDFT, using realistic all-electron wave functions and band structures. mpi_expSandwich has been used to calculate the charge- and spin-density response of transition metals with shallow semi-core levels, again within TDDFT. PW_GW_jjdiag computes self-consistently the electron self-energy and Green’s function within the screened interaction approximation. Two newer codes, pw_SICOEP.exe and genBZ_EXX.exe, evaluate, via the optimized effective potential method, the self-interaction-free potentials which occur in the Kohn-Sham version of DFT. A new code, genBZ_EXX_fxw.exe, is being developed to augment ground state results with the corresponding dynamical exchange-correlation kernel.

 
Accomplishments
We have developed a novel technique to perform many-body calculations on the Matsubara time axis, using a non-uniform “power” mesh which allows us to perform fast non-linear interpolation to a uniform mesh. This technique has been implemented to treat, for the first time, the effects of the core electrons on the quasiparticle states in the valence region. We have found novel and important results traced to the effect of the core electrons on the Fock diagram. For example, the impact of this effect on the band gap of Si is of the order of 1 eV, which agrees quite well with experiment.

We have addressed the intriguing lineshape observed in recent electron energy loss experiments on Zn. Our results highlight the enormous impact of d band location on the loss spectrum. We have offered a new interpretation of the experimental spectra, identifying the threshold feature as a subtle coherent effect involving d electron excitation.

Significance
TDDFT is a rapidly developing field; its impact on electron dynamics may eventually rival the enormous effect which ground-state DFT has had on materials theory. Most of the recent advances refer to atoms and molecules; our program aims at developing orbital-dependent methods for extended systems.

Publications
W. Ku and A. G. Eguiluz, “Plasmon lifetime in K: A case study of correlated electrons in solids amenable to ab initio theory,” Phys. Rev. Lett. 82, 2350 (1999).

A. G. Eguiluz, W. Ku, and J. M. Sullivan, "Dynamical response of correlated electrons in solids probed by inelastic scattering experiments: An ab initio theoretical perspective," J. Phys. Chem. Solids 61, 383 (2000).

A. G. Eguiluz and W. Ku, "Ab initio studies of electronic excitations in solids," in Electron Correlations and Materials Properties, edited by A. Gonis, N. Kioussis, and M. Ciftan (Plenum, New York, 1999). Available also as cond-mat/9903032.