Theory of triangular lattice quasi-one-dimensional charge-transfer solids

Abstract

Recent investigations of the magnetic properties and the discovery of superconductivity in quasione-dimensional triangular lattice organic charge-transfer solids have indicated the severe limitations of the effective 1 2 -filled band Hubbard model for these and related systems. We present computational studies within the 1 4 -filled band Hubbard model for these highly anisotropic systems. Individual organic monomer molecules, and not their dimers, constitute the sites of the Hamiltonian within our theory. We find enhancement of the long-range component of superconducting pairing correlations by the Hubbard repulsive interaction U for band parameters corresponding to κ-(BEDT-TTF)2CF3SO3, which is superconducting under moderate pressure. We find significantly weaker superconducting pairing at realistic values of U in κ-(BEDT-TTF)2B(CN)4, and we ascribe the experimentally observed transition to a spin-gapped insulator to the formation of a paired-electron crystal. We make the testable prediction that the spin gap will be accompanied by charge-ordering and period doubling in two lattice directions. The weaker tendency to superconductivity in κ-(BEDT-TTF)2B(CN)4 compared to κ-(BEDT-TTF)2CF3SO3 is consistent with the more one-dimensional character of the former. Pressure-induced superconductivity is, however, conceivable. The overall results support a valence bond theory of superconductivity we have proposed recently.

Superconductivity due to cooperation of electron-electron and electron-phonon interactions at quarter filling

“…In all lattices we find that as a function of λ,P reaches a broad peak at an intermediate value of λ before beginning to decrease. A similar broad maximum inP is seen as a function of U in zero temperature λ = 0 calculations at ρ = 0.5 [16,17]. We expect the decrease inP at larger λ and/or U is caused by the increasing effective mass of pairs.…”

“…4) is very different from ρ = 1. At ρ ∼ 0.5 zero-temperature calculations find thatP increases with U [15][16][17]. Here, for all the lattices we find thatP increases with increasing U and λ at the same densities where T = 0 calculations find enhancement by U alone.…”

Absence of Superconductivity in the Hubbard Dimer Model for κ-(BEDT-TTF)2X

“…The U dependence of Figure 4e,f are harder to interpret. We believe that the unusual U dependence here is due to shorter range correlations, for example as in Figure 4b where P ⊥ (r) changes discontinuously at small U at r ∼ 2 [42]. As in Reference [41], as distances r < 3 are excluded, we again find a continuous decrease ofP ⊥ with increasing U.…”

“…Pressure increases frustration, causing the spin singlets to be mobile. Calculations on an anisotropic triangular lattice [41] as well as on κ-lattices [42,57] have indeed found that superconducting pair correlations are enhanced by the Hubbard U uniquely at 1 4 -filling. DMRG calculations on a width-three triangular lattice found evidence for p-wave and d-wave SC in the density range 0.40∼0.65 [58], which would appear to agree with the conclusion from the previous work [41].…”

Absence of superconductivity in the Hubbard dimer model for kappa-(BEDT-TTF)_2X

“…We believe that the unusual U dependence here is due to shorter range correlations, for example as in Fig. 4(b) where P ⊥ (r) changes discontinuously at small U at r ∼ 2 [41]. If distances r < 3 are excluded, we again find a continuous decrease of P⊥ with increasing U. Figs.…”