The dimer model for κ-phase organic superconductors

Abstract

We prove that the upper electronic bands of κ-phase BEDT-TTF salts are adequately modeled by an half-filled tight-binding lattice with one site per cell. The band parameters are derived from recent ab initio calculations, getting a very simple but extremely accurate one-electron picture. This picture allows us to solve the BCS gap equation adopting a real-space pairing potential. Comparison of the calculated superconducting properties with the experimental data points to isotropic s0-pairing. Residual many-body or phonon-mediated interactions offer a plausible explanation of the large variety of physical properties observed in κ-phase BEDT-TTF salts.PACS numbers: 71.15.Fv, 74.20.Fg, 74.70.Kn, Superconductivity (SC) in organic charge transfer (CT) salts has been discovered more than 15 years ago. At present, κ-phase BEDT-TTF (ET) salts are the most promising organic superconductors (OSC) [1]. OSC have similar features to cuprate superconductors: they exhibit highly bidimensional structures, narrow electronic bands, low carrier densities and possibly strong electronic correlations. Superconducting properties are also similar in organics and cuprates, exhibiting singlet pairing [2], very low coherence lengths, high magnetic penetration depths and critical fields [1]. Perhaps the most characteristic feature of cuprate superconductors is the competition between SC and antiferromagnetism (AFM). This competition shows up also in OSC, as demonstrated by the presence of antiferromagnetic spin fluctuations in the metallic state of several compounds [2], or even by reentrant SC in the presence of magnetic order. In this respect the different behavior of κ-ET 2 Cu[N(CN) 2 ]X (ET-X) salts where X=Cl, Br is noteworthy: the two compounds are isostructural, but at ambient pressure the Br-compound is a superconductor with T c = 11.6 K, whereas the Clcompound is a Mott insulator with (possibly commensurate) antiferromagnetic ordering [2]. On the other hand, even a very small external pressure (270 bar), turns ETCl to the superconducting ground state (T c = 12.8 K) [1]. Experimental studies of normal and superconducting state properties in OSC are still scanty and often inconclusive, due to the extreme sensitivity of the material to pressure, radiation and sample preparation. At the same time, the amount of theoretical work on OSC is scarce if compared with that developed for cuprates, possibly due to the apparent complexity of the OSC structures. In the following we will show that, by a proper choice of the basis functions, the electronic structure of κ-phase ET salts can be accurately described in terms of a rectangular lattice with nearest and next-nearest neighbor interactions. Similar one-band models are often adopted in the study 1

Quasi-two-dimensional organic superconductors: A review

“…References [59,127] and references therein for a review and discussion) to derive values λ ∼ 0.3. Similarly, infrared measurements [128] have been taken to suggest that "intramolecular vibrations are not directly involved in the superconductivity mechanism" and that the electron-phonon coupling constant is small; others found "no evidence that higher values of T c are achieved through a softer lattice" (i.e. that intermolecular vibrations are not involved in a simple, BCS-like manner) and that the superconductivity "is likely to be a much Normalised frequency of the…”

Structures and electronic phases of the bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) clusters and κ-(BEDT-TTF) salts: A theoretical study based on ab initio molecular orbital methods

“…[5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] From the first-principles approach, Xu et al calculated the Fermi surfaces of -͑BEDT-TTF͒ 2 Cu͑NCS͒ 2 using the local density approximation ͑LDA͒ based on the density functional theory ͑DFT͒. 5 From the ab initio molecular orbital ͑MO͒ theory, Demiralp and Goddard optimized the BEDT-TTF monomer at HF/6-31G** and studied the physical properties using the 2-D Hubbard model within the Hartree-Fock ͑HF͒ approximation.…”

“…[11][12][13] Fortunelli and Painelli described the ab initio evaluation of Hubbard parameters for the BEDT-TTF dimer unit of the -͑BEDT-TTF͒ 2 Cu͓N͑CN͒ 2 ͔Br salt. [14][15][16][17] Okuno and Fukutome showed an effective Hamiltonian of ␤-and -phases and concluded that the electron correlation is very strong. 18 The nesting and pressure effects of the Fermi surface were discussed in Refs.…”

Millimeter-Wave Magneto-optical Determination of the Anisotropy of the Superconducting Order Parameter in the Molecular Superconductorκ-(BEDT−TTF)2Schrama1, Rzepniewski2, Edwards3 et al. 1999Phys. Rev. Lett.603452View full textAdd to dashboardCite

“…1). Furthermore, it has been possible to fit experimental Fermi-surface-topology data to a simplified model of the tight-binding band structure (the so-called effective dimer model) to a good degree of accuracy [5,[9][10][11].The combination of unconventional superconductivity and a tractable analytical representation of the band structure makes the k-͑BEDT-TTF͒ 2 X superconductors attractive for theoretical studies, and a number of authors [9,10,12,13] have explored the possibility of d-wave superconductivity mediated by spin fluctuations. Such approaches predict that the superconducting order parameter will have four, roughly perpendicular, gap nodes, the exact orientation of the nodes depending on the underlying details of the Fermi surface [10,12].…”