Two-layer Hubbard Model Research Articles

Pairing susceptibility of iron-based superconductors within a two-layer Hubbard model

By using the determinant quantum Monte Carlo method, we studied the dominant pairing susceptibility of iron-based superconductors within an extended Hubbard model, which describes the underlying electronic structure of both iron pnictides and iron chalcogenides. The extended Hubbard model is constructed by two iron layers, each of which forms two sublattices on a square structure. Although the coupling between the two layers has different effects on the behavior of pairings in iron pnictides and iron chalcogenides, our non-biased numerical simulations reveal that the pairing with Sxy symmetry dominates over the studied parameter for both materials.

Gate-induced gap in bilayer graphene suppressed by Coulomb repulsion

We investigate the effect of on-site Coulomb repulsion $U$ on the band gap of the electrically gated bilayer graphene by employing coherent potential approximation in the paramagnetic state, based on an ionic two-layer Hubbard model. We find that, while either the on-site Coulomb repulsion $U$ or the external perpendicular electric field $E$ alone will favor a gapped state in the bilayer graphene, competition between them will surprisingly lead to a suppression of the gap amplitude. Our results can be applied to understand the discrepancies of gap size reported from optical and transport measurements, as well as the puzzling features observed in angular resolved photoemission spectroscopic study.

Possible half-metallic phase in bilayer graphene: Calculations based on mean-field theory applied to a two-layer Hubbard model

Charge neutral bilayer graphene has a gapped ground state as transport experiments demonstrate. One of the plausible such ground states is layered antiferromagnetic spin density wave (LAF) state, where the spins in top and bottom layers have same magnitude with opposite directions. We propose that lightly charged bilayer graphene in an electric field perpendicular to the graphene plane may be a half metal as a consequence of the inversion and particle-hole symmetry broken in the LAF state. We show this explicitly by using a mean field theory on a 2-layer Hubbard model for the bilayer graphene.

Spin magnetic susceptibility in the two-layer Hubbard model

The two-layer Hubbard model is studied by using the composite operator method. Magnetic properties of the model in the normal state are analyzed by studying the uniform static spin magnetic susceptibility as a function of doping and temperature. The relevance of the model to describe the physical behavior of two-layered cuprate high- T c superconductors is discussed.

Superconductivity Within the Pair-Tunneling Model Close to the Metal-Insulator Transition

We have considered the problem of s-wave and d-wave superconductivity in the two-layer Hubbard model close to the metal-insulator transition. To mimic the formation of the insulating gap, the Coulomb correlations have been taken into account within the Hubbard I approximation. The interlayer momentum-conserving Josephson tunneling between the layers has been included on the mean-field level. We demonstrate that the interlayer tunneling may contribute to the occurrence of mixed d + s wave symmetry of the superconducting state with a dominating d-wave component at a low concentration of holes. The problem of the validity of the pair-tunneling model is also briefly discussed.

Interlayer tunneling in a strongly correlated electron-phonon system.

We discuss the role of interlayer tunneling for superconducting properties of strongly correlated (U\ensuremath{\rightarrow}\ensuremath{\infty} limit) two-layer Hubbard model coupled to phonons. Strong correlations are taken into account within the mean-field approximation for auxiliary boson fields. To consider phonon-mediated and interlayer tunneling contribution to superconductivity on equal footing we incorporate the tunneling term into the generalized Eliashberg equations. This leads to the modification of the phonon-induced pairing kernel and implies a pronounced enhancement of the superconducting transition temperature in the d-wave channel for moderate doping. In numerical calculations the two-dimensional band structure has been explicitly taken into account. The relevance of our results for high-temperature superconductors is briefly discussed. \textcopyright{} 1996 The American Physical Society.

Pairing correlations in a two-layer Hubbard model.

Pairing correlations in a two-layer Hubbard model.

Nodeless d-wave pairing in a two-layer Hubbard model.

Pairing correlations in a two-layer Hubbard model are studied using (1) an Eliashberg-like approximation for the exchange of spin fluctuations and (2) quantum Monte Carlo simulations. A possible pair structure in which the gap has one sign on the bonding and the opposite sign on the antibonding Fermi surface is found. Some implications of this for the transport properties of the cuprate superconductors are discussed.