Electronic Structure of Clusters: Applications to High-T(c) Superconductors.

Abstract

We present a method of studying the electronic structure of clusters by ab initio, self-consistent, local spin density functional calculations. The linear combination of Gaussian orbitals (LCGO) method has been adopted in developing a general cluster program. A variational charge fitting procedure is used in the evaluation of the matrix elements of the electrostatic potential, and a separate least-squares fitting procedure is used in regard to the exchange-correlation potential. Self-consistent solutions have been obtained using this program for the Ba$\sb4$Cu$\sb2$O$\sb7$ clusters as a portion of the Cu-O chain in the high-T$\sb{c}$ superconductor YBa$\sb2$Cu$\sb3$O$\sb7$. Four different charge states of Ba$\sb4$Cu$\sb2$O$\sb7$ are considered. The spin-polarized calculation predicts zero local moment on Cu sites. The cluster ground state shows strong covalent bonding between the copper and oxygen atoms. Transition state calculations are made to determine electronic relaxation occurring in photo-emission and to evaluate the Hubbard interaction parameters $U\sb{dd}$ and $U\sb{pp}$. We have derived a complete formula to calculate the photoemission spectrum using the LCGO wave functions. Ultraviolet photoelectron spectra of the YBa$\sb2$Cu$\sb3$O$\sb7$ crystal have been calculated. Our results are compared with experiment. Effects of fluorine and nitrogen substitutions to high-T$\sb{c}$ superconductors are investigated by studying the electronic structure of Ba$\sb4$Cu$\sb2$O$\sb6$N and Ba$\sb4$Cu$\sb2$O$\sb6$F clusters. Results for other interesting systems such as the Fe$\sb2$ molecule, the planar CuO$\sb6$ and Cu$\sb4$O$\sb4$ clusters are also discussed in this work.