| Mechanism of spin-triplet superconductivity has been one of the major issues in the field of strongly correlated electron systems. Here, the ferromagnetic interaction between electrons is believed to play an essential role in the occurrence of triplet superconductivity. In this paper, we study the simplest model in this context: i.e., the model of two Hubbard chains coupled with Heisenberg-type ferromagnetic exchange interaction J. This model may be regarded as the degenerate two-band Hubbard model with the on-site Hund's rule coupling in transition-metal oxides. This model may also be regarded as the Hubbard chains with the interchain ferromagnetic interaction which may come from the ring-exchange mechanism in quasi-one-dimensional organic materials [1].
We use the density-matrix renormalization group (DMRG) method and exact-diagonalization technique on small clusters to calculate the charge gap, spin gap, binding energy, pair correlation functions, etc., as well as the anomalous Green's function of the model. We thereby show that the model contains the state of mobile `rung-triplet' pairs, i.e., spin-triplet superconductivity, in the wide parameter and filling region. The binding energy, e.g., scales well with J when J is large. We also show that the spin gap corresponding to th Haldane gap for the spin-1 Heisenberg chain opens at half filling, the size of which becomes small away from half filling.
[1] Y. Ohta et al., Phys. Rev. B 72, 012503 (2005).
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