Fermion-magnon bound states in strong magnetic field: Analysis of the extended Hubbard model

Aki Odawara, Hideo Yoshioka

Agnieszka Cichy

The aim of this Ph.D. thesis was to investigate superconducting properties in the presence of Zeeman magnetic field in systems with local fermion pairing on the lattice. The study also concerned the evolution from the weak coupling (BCS-like) limit to the strong coupling limit of tightly bound local pairs (BEC) with increasing attraction, both in the ground state and at finite temperatures, within the spin-polarized extended Hubbard model. The analysis was also extended to the case of spin dependent hopping integrals (mass imbalance), with special attention paid to the BCS-BEC crossover physics in the ground state. The methods used included: the mean field approximation (BCS-Stoner type) and the estimation of the phase coherence temperature within the Kosterlitz-Thouless (KT) scenario in two dimensions. The BCS-BEC crossover was also analyzed in three dimensions, at finite temperatures, within the spin polarized Attractive Hubbard Model (AHM), going beyond the mean field approximation. In this case, the critical temperatures of the superconducting transition were determined within the self-consistent T-matrix method. The strong coupling expansion was applied to map the spin polarized AHM onto the model of hard-core bosons on the lattice.

V. Ya. Krivnov, Alexander A. Ovchinnikov, Vladyslav O. Cheranovskii

Vladyslav O. Cheranovskii, E. V. Ezerskaya, İ. Özkan

Abstract We studied the energy spectrum of the 1‐D extended Hubbard model with spin‐dependent hopping and related spin ladder system formed by two coupled XXZ spin 1/2 chains with the interchain interaction of Ising type. It was proved that the model excitation spectrum has no gap excepting some special values of z‐projection of the ground‐state total spin. The thorough analytic consideration of two‐magnon states was given. The existence up to five bound states at specified value of quasimomentum of the pair of inverted spins was shown. We also present the results of density matrix renormalization group calculations that showed nonadequacy of the pair approximation for n‐magnon bound states of the extended model with the strong electron–electron interactions. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002

F. Mancini, F. P. Mancini

Mukesh Eswaran, John C. Kimball

Interactions between strongly correlated electrons (or holes) and spin waves are investigated with the aid of a simple physical model. The model is the large-interaction limit of the Hubbard model with an additional ferromagnetic Heisenberg interaction between nearest-neighbor singly occupied sites. We find that even though there is no obvious electron-magnon attraction in the model, electron-magnon bound states exist for some crystal structures.