Superconductivity and a Mott Transition in a Hubbard Model on an Anisotropic Triangular Lattice

Abstract

A half-filled-band Hubbard model on an anisotropic triangular lattice (t in two bond directions and t ′ in the other) is studied using an optimization variational Monte Carlo method, to consider a Mott transition and superconductivity arising in κ-(BEDT-TTF)2X. Adopting wave functions with doublon-holon binding factors, we reveal that a first-order Mott (conductorto-nonmagnetic insulator) transition takes place at U = Uc, approximately of the bandwidth, for a wide t ′ /t range. This transition is not directly connected to magnetism. Robust d-wave superconductivity appears in a restricted parameter range: immediately below Uc and the moderate strength of frustration (0.4 < ∼ t ′ /t < ∼ 0.7), where short-range antiferromagnetic correlation sufficiently develops but does not come to a long-range order. The relevance to experiments is also discussed.

Spin-liquid phase due to competing classical orders in the semiclassical theory of the Heisenberg model with ring exchange on an anisotropic triangular lattice

“…It is well known that the perfect nesting of the square lattice means that is insulating for arbitrarily small U/t. For t = t, it is found, numerically, that the Mott transition occurs at around U/t = 10-15, depending on the method used [3,[15][16][17][18][19][20][21]79]. The metal-insulator transition line in Fig.…”

“…This model contains three parameters: U the effective on-site Coulomb repulsion, t the nearest-neighbor hopping integral, and t the next-nearest-neighbor hopping integral along one diagonal only. The Hubbard model on the anisotropic triangular lattice has been studied via a number of approaches [15][16][17][18][19][20][21]. Some methods have suggested that a spin liquid is realized in the insulating phase.…”

Staggered Flux State in Two-Dimensional Hubbard Models

“…E is symmetric with respect to t ′ /t = 0 owing to the electron-hole symmetry. 35 Similarly to the case on the frustrated square lattice, E(FS) and E(d−SC) (if the band renormalization is considered 115,116 ) are constant. For the ground state, we find from Fig.…”

Paired electron crystal: Order from frustration in the quarter-filled band

“…What is interesting though is that the proposed scenario can give a unified approach to SC in all organic CTS, irrespective of whether the insulating state proximate to SC is AFM 102 , CO 67,103 or VBS 82 . Recall that within existing mean field theories the AFM-to-SC transition is driven by spin fluctuations [13][14][15][16][17][18][19][20][21][22][23] , while the CO-to-SC transition is driven by charge fluctuations 65 . Even if we ignore that recent precise numerical calculations [25][26][27] have demonstrated the absence of SC within the proposed spin-fluctuation models in this context (thereby raising doubts also about mean-field theory of charge-fluctuation mediated superconductivity), different mechanisms of SC for structurally related materials with identical or nearidentical molecular components appear to be unrealistic.…”

“…An extension of the RVB theory of dopant-induced SC in ρ = 1 is the proposal that frustration-induced SC occurs in the anisotropic triangular lattice within the simple Hubbard model even for ρ exactly 1, where a narrow superconducting phase is straddled on both sides by broader PM and AFM insulator phases [13][14][15][16][17][18][19][20][21][22][23] . It has been claimed that this transition explains the SC in the CTS [13][14][15][16][17][18][19][20][21][22][23][24] . Recent numerical work by us and others, however, have determined that SC is absent within the ρ = 1 triangular lattice Hubbard model [25][26][27] and the earlier results are artifacts of mean-field approximations.…”