Ab initio two-dimensional multiband low-energy models of EtMe3Sb[Pd(dmit)(2)](2) and kappa-(BEDT-TTF)(2)Cu(NCS)(2) with comparisons to single-band models

作者：Nakamura Kazuma^*; Yoshimoto Yoshihide; Imada Masatoshi

来源：Physical Review B, 2012, 86(20): 205117.

DOI：10.1103/PhysRevB.86.205117

We present ab initio two-dimensional extended Hubbard-type multiband models for EtMe3Sb[Pd(dmit)(2)](2) (where dmit is 1,3-dithiole-2-thione-4,5-dithiolate) and kappa-(BEDT-TTF)(2)Cu(NCS)(2) [where BEDT-TTF is bis(ethylenedithio)-tetrathiafulvalene] after a downfolding scheme based on the constrained random-phase approximation (cRPA) and maximally localized Wannier orbitals, together with the dimensional downfolding. In the Pd(dmit)(2) salt, the antibonding state of the highest occupied molecular orbital (HOMO) and the bonding/antibonding states of the lowest unoccupied molecular orbital (LUMO) are considered to be the orbital degrees of freedom, while, in the kappa-BEDT-TTF salt, the HOMO-antibonding/bonding states are considered. Accordingly, a three-band model for the Pd(dmit)(2) salt and a two-band model for the kappa-(BEDT-TTF) salt are derived. We derive single-band models for the HOMO-antibonding state for both of the compounds as well. The HOMO antibonding band of the Pd(dmit)(2) salt has a triangular structure of the transfers with a one-dimensional anisotropy, in contrast to the nearly equilateral triangular structure predicted in the extended Huuckel results. The ratio of the larger interchain transfer t(b) to the intrachain transfer t(a) is around t(b)/t(a) similar to 0.82. Our calculated screened onsite interaction U and the largest offsite interaction V are similar to 0.7 and similar to 0.23 eV, respectively, for EtMe3Sb[Pd(dmit)(2)](2) and similar to 0.8 and similar to 0.2 eV for kappa-(BEDT-TTF)(2)Cu(NCS)(2). These values are large enough compared to transfers t as similar to 55 meV for the Pd(dmit)(2) salt and similar to 65 meV for the kappa-BEDT-TTF one, and the resulting large correlation strength (U-V)/t similar to 10 indicates that the present compounds are classified as the strongly correlated electron systems. In addition, the validity whether the present multiband model can be reduced to the single-band model for the HOMO-antibonding state, widely accepted in the literature, is discussed. For this purpose, we estimated the order of vertex corrections ignored in the cRPA downfolding to the single-band model, which is given by W%26apos;/D, where W%26apos; is a full-screened-interaction matrix element between the HOMO-antibonding and other bands away from the Fermi level (namely, HOMO-bonding or LUMO-bonding/antibonding bands), whereas D is the energy distance between the Fermi level and the bands away from the Fermi level. In the present materials, W%26apos;/D estimated as 0.3-0.5 signals a substantial correction and thus the exchange process between the low-energy HOMO-antibonding and other bands away from the Fermi level may play a key role to the low-energy ground state. This supports that the minimal models to describe the low-energy phenomena of the organic compounds are the multiband models and may not be reduced to the single-band model.

• 出版日期2012-11-16