A pairing-correlation scheme due to tensor force in fermion systems with quasi two-dimensional structure is given. We have obtained two types of the spin-triplet pairings with two-dimensional mixed s, d- and pure d-waves which are characteristic of tensor force. A possible application to high-Tc superconductivity is briefly discussed.
We address the problem of superconductivity in a system where the only many-body interactions are repulsive. The Eliashberg equations have been generalized to account for possible pairing correlations in the two-dimensional Hubbard model. Details of the two-dimensional band structure have been explicitly taken into account when considering the symmetry of the superconducting state. The pairing kernels have been discussed at low doping and at temperatures close to the superconducting transition temperature. We have proved that local Coulomb repulsion leads to attractive pairing correlations in the d-wave channel. Extended s-wave superconductivity is less likely to occur within this purely electronic model, at least in the physically interesting region of doping.
A coupled spin-fermion system is investigated as a model of high- T c superconductivity. By the exact diagonalization technique, the binding energy of two fermions and several correlation functions in the ground state are calculated to study possible pairings of holes in orbitals of p π , p σ , d 3 z 2 - r 2 etc. The bound states of fermions are suggested in two regions in the parameter space. In the small transfer region, it is caused by local defects induced in the antiferromagnet at the fermion sites. The larger transfer region is characterized by the relatively extended disturbance of the antiferomagnetic correlation of the substrate spins, where the fermion forms singlet-like cloud around it. The latter is a characteristic feature of the antiferromagnetic fermion-spin coupling. The relevance of the latter region to high- T c superconductivity is discussed. The possibilities of bound states of fermions in Kondo lattices are also investigated.
We demonstrate the possibility of triplet p-wave pairing at low electron density a large number of models such as 3D and 2D Fermi-gas models with hard-core repulsion, 3D and 2D Hubbard models, and the Shubin-Vonsovsky model. The critical temperature for p-wave pairing can be considerably higher in the spin-polarized case or even in a two-band situation at low density and can reach experimentally observable values of 1–5 K. We also discuss briefly the d-wave pairing and high-Tc superconductivity with Tc ∼ 100 K which arise in the extended Hubbard model and in the generalized t-J model when close to half-filling.
The search for superconductivity with unconventional pairing symmetry has been a central focus in the study of strongly correlated electron systems. In this work, we report a numerically exact study of the spin-triplet pairing in a two-dimensional Hubbard model with repulsive interactions, employing the determinant quantum Monte Carlo method. The model includes next-nearest-neighbor and third-nearest-neighbor hopping terms and maintains spin balance. By tuning the fermion filling close to a type-II Van Hove singularity (VHS) in the model, we numerically investigate the ordering tendencies of several possible pairing channels with different symmetries. Our numerical results provide clear evidence of the spin-triplet p-wave pairing instability approaching low temperatures, as revealed by the vertex contribution to the pairing susceptibility. This signature becomes increasingly pronounced as the interaction strength increases in the weak to intermediate regime. We further find that, near the type-II VHS, the dominant spin-spin correlations in the system are ferromagnetic, suggesting its close relation to the spin-triplet pairing instability. Our findings offer a reliable approach to realize spin-triplet p-wave superfluidity in the repulsive Hubbard model from an unbiased numerical perspective.