Influence of molecular conformations on the electronic structure of organic charge transfer salts

Abstract

We report ab-initio calculations for the electronic structure of organic charge transfer salts κ-(ET)2Cu[N(CN)2]Br, κ-(ET)2Cu[N(CN)2]I, κ -(ET)2Cu[N(CN)2]Cl and κ-(ET)2Cu2(CN)3. These materials show an ordering of the relative orientation of terminal ethylene groups in the BEDT-TTF molecules at finite temperature and our calculations correctly predict the experimentally observed ground state molecular conformations (eclipsed or staggered). Further, it was recently demonstrated that the ethylene endgroup relative orientations can be used to reversibly tune κ-(ET)2Cu[N(CN)2]Br through a metal-insulator transition. Using a tight-binding analysis, we show that the molecular conformations of ethylene endgroups are intimately connected to the electronic structure and significantly influence hopping and Hubbard repulsion parameters. Our results place κ-(ET)2Cu[N(CN)2]Br in eclipsed and staggered configurations on opposite sides of the metal-insulator transition.PACS numbers: 71.10. Fd, 71.15.Mb, 71.20.Rv, 74.70.Kn Quasi-two dimensional charge transfer salts κ-(BEDT-TTF) 2 X, where BEDT-TTF stands for bisethylenedithio-tetrathiafulvalene, often abbreviated as ET, constitute a fascinating family of materials due to their rich phase diagrams comprising metallic, superconducting, Mott insulating and spin-liquid phases [1][2][3][4].These electronic properties of ET-based materials are very sensitive to disorder. Irradiation experiments have shown that lattice disorder lowers the T c of κ-(ET) 2 Cu(SCN) 2 [5] and causes electron localization in κ-(ET) 2 Cu[N(CN) 2 ]Br [6]. Early on it was also realized that ET molecules have intramolecular degrees of freedom, namely the configurations of their two ethylene endgroups (see Fig. 3), which can either be aligned parallel (eclipsed, E) or canted (staggered, S) [7][8][9][10][11][12][13][14][15][16][17]. The energetically favorable configuration is not universal for different anions X and packing motifs. For some materials a glass-like freezing of the ethylene endgroups upon cooling has been observed [7,8,[15][16][17][18][19].Especially the first ambient pressure ET-based superconductor β-(ET) 2 I 3 attracted a lot of interest, because its T c can be enhanced from 1.5 K to 8 K by forcing the ET molecules, which are endgroup disordered at ambient pressure, to assume staggered configuration through application of shear and pressure [7-11, 20, 21]. Recently, it was shown that ethylene endgroup disorder can be used to reversibly tune κ-(ET) 2 Cu[N(CN) 2 ]Br through a metal to insulator transition [17,22,23].It is believed that materials κ-(ET) 2 X have a common phase diagram, which is mainly controlled by the value of the on-site Coulomb repulsion U over the electronic bandwidth [24]. Changes in physical properties in the presence of ethylene endgroup disorder have so far been interpreted as a consequence of lattice disorder, with the exception of recent scanning tunneling spectroscopy experiments [25]. Surprisingly, the effect of different ethylene endgroup configurations on the ele...

Importance of spin-orbit coupling in layered organic salts

“…The final hopping integrals are then given as the entries of H . For the organics, this method has proved reliable, and agrees very well with hopping integrals extracted from solid-state DFT calculations in terms of a Wannier function basis [37][38][39]. Using the obtained hopping integrals, the spin-orbit hopping elements λ ij were computed in the first order approximation:…”

“…1 (c)). Without SOC, these bands would be degenerate at the Brillouin zone boundary [37][38][39], but SOC splits the bands. This splitting may be directly observed in quantum oscillation experiments.…”

Near-degeneracy of extendeds+dx2−y2anddxy

“…Although the triangular lattice Hubbard model has been remarkably successful in explaining the overall phase diagram 1,41,42 and also some more subtle physics 43 of κ-(ET) 2 X materials, the recent discovery of multiferroicity 44 in antiferromagnetic κ-(ET) 2 Cu[N(CN) 2 ]Cl and the still unresolved problem of superconducting pairing-symmetry 45,46 call for alternative approaches. In particular, the dimer model on the anisotropic triangular lattice is only an approximation with a priori unclear range of applicability to the real lattice structure of κ-(ET) 2 X charge transfer salts.…”

Investigation of Many‐Body Effects in the Quasi‐Two‐Dimensional Electronic System of Organic Charge‐Transfer Salts

“… discusses ab initio density functional theory (DFT) calculations within an all‐electron full‐potential local orbital (FPLO) basis of the electronic structure of κ ‐Br. Subsequently, a tight‐binding model is derived from projective molecular orbital Wannier functions . The resulting low‐energy Hamiltonian H 0 comprises four bands arising from the highest occupied molecular orbitals of the individual BEDT‐TTF molecules in the unit cell and is 3/4‐filled.…”

“…Subsequently, a tight-binding model is derived from projective molecular orbital Wannier functions. [79,80] The resulting low-energy Hamiltonian H 0 comprises four bands arising from the highest occupied molecular orbitals of the individual BEDT-TTF molecules in the unit cell and is 3/4filled.…”