Interplay and Competition Between Superconductivity and Charge Orderings in the Zero-Bandwidth Limit of the Extended Hubbard Model with Pair Hopping and on-Site Attraction

Abstract

We present studies of an effective model which is a simple generalization of the standard model of a local pair superconductor with on-site pairing (i.e., the model of hard core bosons on a lattice) to the case of finite pair binding energy. The tight binding Hamiltonian consists of (i) the effective on-site interaction U , (ii) the intersite density-density interactions W between nearest-neighbors, and (iii) the intersite charge exchange term I , determining the hopping of electron pairs between nearest-neighbor sites. In the analysis of the phase diagrams and thermodynamic properties of this model, we treat the intersite interactions within the mean-field approximation. Our investigations of the U < 0 and W > 0 case show that, depending on the values of interaction parameters, the system can exhibit three homogeneous phases: superconducting (SS), charge-ordered (CO), and nonordered (NO) as well as the phase separated SS-CO state.

Superconductivity, Charge Orderings, Magnetism, and Their Phase Separations in the Ground State of Lattice Models of Superconductor with Very Short Coherence Length

“…Names of transition orders used are consistent with our earlier works [7][8][9][10][11][12][13][14][33][34][35][36]. In particular, a transition between a homogeneous phase and the PS state is symbolically named as a "third-order" transition.…”

“…1b. The I1 phase and the PS:SC/CO state are degenerated in the whole ranges of their occurrences, but at T > 0, this degeneration is removed and only the PS:SC/CO state exists [13]. The SC-I1 and I1-CO transitions are second-order, whereas the SC-PS:SC/CO and PS:SC/CO-CO transitions are "third-order".…”

“…refs. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] and references therein). Moreover, the phase separation (PS) has been evidenced (by various microscopy techniques such as STM, MFM, etc.)…”

“…The effective model considered has a form of single-band extended Hubbard model with pair hopping at the atomic limit (t = 0) [4][5][6][7][8][9][10][11][12][13][14][15][16][17]: …”

“…The analysis of the ground state of model (1) has been performed by means of a variational approach (VA) in the grand canonical ensemble [6][7][8][9][10][11][12][13][14], which treats on-site U term exactly and the intersite interactions within the mean-field approximation (MFA). The MFA is a rigorous treatment of the intersite terms in the limit of infinite dimensions d → +∞ (or large coordination number z); thus, the result obtained in this paper for model (1) are also exact in the limit d → +∞.…”