Transfer Integrals and Band Structures in (Et) 2 X Salts
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TRANSFER INTEGRALS AND BAND STRUCTURES IN (ET) 2X SALTS OLIVER H. LEBLANC, JR., MARGARET L. BLOHM, and RICHARD P. MESSMER, General Electric Corporate Research & Development, Schenectady, NY 12301 ABSTRACT Transfer integrals (tij) between pairs of nearest neighbor ET molecules were calculated by an ab initio method. Tight-binding one-electron energy bands constructed from the tij are similar to those previously calculated by Mori and by Whangbo and their coworkers by semi-empirical, extended Hiickel methods, but quite different from those found by Kuibler et al. in 13-(ET) 213 using the augmented spherical wave (ASW) method. However, all these band models are suspect. The Hubbard on-site repulsion parameter U is estimated to be about twice the band widths, indicating that a full treatment of the Hubbard hamiltonian is needed. Also, polaron effects appear to control transport except at very low temperature. INTRODUCTION The (ET)2 X salts [ET: bis(ethylenedithio)tetrathiafulvalene] containing ET molecules, ET+ radical cations, and closed shell X- anions exhibit a variety of interesting electrical and magnetic properties, including superconductivity, which are not entirely understood theoretically[I]. Indeed, a controversy has arisen as how best to approach these solids theoretically. We describe new calculations that may shed light on some of these issues. The crystals have structures in which the ETs are tightly packed together in planar layers sandwiched between parallel layers of X- anions. Experimental evidence suggests that electrons on the anions occupy filled orbitals well below the Fermi level, so that the electrical and magnetic properties are associated primarily with the ET's and not with the anions. Thus, the conductivity is orders of magnitude higher parallel to the ET layers than perpendicular to them, and it and other properties are more sensitive to the detailed way the ET's pack together in the layers than to the nature of the anion. The anions are therefore usually ignored in theoretical analyses. The most extensive calculations have been carried out by Mori[2] and by Whangbo[3] and their coworkers who used the semi-empirical extended Hiickel approximation to construct tight-binding one-electron bands based upon a single molecular state, the highest occupied molecular orbital (HOMO) of ET. Mixing with other ET orbitals and with orbitals on the closed shell X- anions was ignored. They have had fair success in accounting for many of the properties of (ET)2X salts with these band models. Our calculations proceed in the same spirit, but are more elaborate. In the extended Hiickel method, only overlap integrals between pairs of atomic basis functions (bfs) are actually computed; hamiltonian matrix elements are estimated as needed by multiplying the respective overlap integrals by semi-empirical constants. We have used instead an ab intio approach to calculate transfer integrals, tij's, between nearest neighbor ET's; the required hamiltonian matrix elements are calculated exactly from the atomic bfs, with
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