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physica status solidi (b)

On the BCS‐to‐Bose crossover in 2D anisotropic superconductors

B. Tobijaszewska, R. Micnas

Published Jan 1, 2006• 3 citations
Publisher

Abstract

Abstract We analyze the competing pairings and crossover from BCS to local pair (LP) superconductivity for the d ‐wave pairing symmetry in the ground state. The phase diagrams for the extended Hubbard model with intersite electron pairing and hard‐core repulsion (U = ∞) are obtained for the 2D square lattice. The influence of the nnn hopping (t 2) on the crossover is discussed. For appropriate values of the parameters (nnn repulsion) and |t 2/t | < 0.5 the crossover is simultaneous with the vanishing of nodal points in the quasiparticle energy. The vanishing of nodal points is clearly reflected in the evolution of the presented superconducting characteristics. Results for d pairing are compared with those for the crossover in the s*‐wave channel and the role of chiral px + i py states is also discussed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

DOI
10.1002/pssb.200562516

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Abstract We analyze the competing pairings and crossover from BCS to local pair (LP) superconductivity for the d ‐wave pairing symmetry in the ground state. The phase diagrams for the extended Hubbard model with intersite electron pairing and hard‐core repulsion (U = ∞) are obtained for the 2D square lattice. The influence of the nnn hopping (t 2) on the crossover is discussed. For appropriate values of the parameters (nnn repulsion) and |t 2/t | < 0.5 the crossover is simultaneous with the vanishing of nodal points in the quasiparticle energy. The vanishing of nodal points is clearly reflected in the evolution of the presented superconducting characteristics. Results for d pairing are compared with those for the crossover in the s*‐wave channel and the role of chiral px + i py states is also discussed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Abstract A refined variational wave function for the two‐dimensional repulsive Hubbard model is studied numerically, with the aim of approaching the difficult crossover regime of intermediate values of U. The issue of a superconducting ground state with d‐wave symmetry is investigated for an average electron density n ≈ 0.8 and for U = 8t. Due to finite‐size effects a clear‐cut answer to this fundamental question has not yet been reached. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Abstract We analyze a competition between d‐wave superconductivity (SC) and antiferromagnetism (AF) and show the influence of these phenomena on the superfluid stiffness. We deal with the extended Hubbard model on a 2D square lattice with the nn and nnn hopping (t–t′–U–W model) in the BCS‐Hartree–Fock approximation. Thermodynamic analysis shows that both, second or first order phase transition between the AF, SC and mixed AF + SC phases are possible. The homogeneous mixed phase or phase separation can be stabilized depending on interaction parameters, band structure and band filling. The corresponding temperature phase diagrams are determined. In the coexistence region we investigate the influence of the antiferromagnetism on the superfluid properties. In the mixed AF + SC ground state the superfluid density is reduced with growing antiferromagnetic ordering and vanishes at half–filling. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
We study the effects of phase fluctuations in 2D short-coherence length superconductors on the basis of the extended Hubbard model with intersite attraction.In the Kosterlitz-Thouless scenario the critical temperatures for anisotropic pairings were determined and compared with the ones from BCS Hartree-Fock approximation.The Uemura-type plots, i.e. the critical temperature vs. zero temperature phase stiffness, were obtained for extended s-wave and d x 2y 2-wave pairings.We also discuss the crossover from BCS to local pair superconductivity for d-wave pairing.
We study the superfluid properties of short-coherence-length superconductors described by the extended Hubbard model with on-site repulsive and intersite attractive interaction. We have analysed the superfluid stiffness and thermodynamic properties of the model versus electron concentration and pairing interaction on a two-dimensional square lattice with nearest- and next-nearest-neighbour hopping. The effects of phase fluctuations on anisotropic superconductivity of extended s-and d x 2 − y 2 -wave type were examined within the Kosterlitz–Thouless (KT) scenario. The KT critical temperatures were determined and compared with those from the BCS–Hartree–Fock approximation. The Uemura-type plots, i.e. the critical temperature versus zero-temperature phase stiffness, were obtained for both extended s- and d-wave pairings. The states with mixed (s ∗ and d x 2 − y 2 ) symmetry, and in particular the time-reversal breaking state of s + id symmetry, were analysed and the phase diagrams taking into account these states are presented. We also briefly discuss the crossover from BCS to local pair superconductivity for the d x 2 − y 2 -wave pairing. A comparison of the theoretical results with the experimental data, in particular those of T c versus λ −2 (0), for high- T c cuprate superconductors is made.
The two component model of coexisting local electron pairs and itinerant fermions coupled via charge exchange mechanism, which mutually induces superconductivity in both subsystems, is discussed. The cases of isotropic s-wave and anisotropic pairing of extended s and d x 2 −y 2-wave symmetries are analyzed for a 2D square lattice within the BCS-mean field approximation and the Kosterlitz-Thouless theory. We determined the phase diagrams and superconducting characteristics as a function of the position of the local pair (LP) level and the total electron concentration. The model exhibits several types of interesting crossovers from BCS like behavior to that of LP’s. Some of our results are discussed in connection with a two-component scenario of preformed pairs and unpaired electrons for exotic superconductors. Key words: Boson-Fermion model, Phase fluctuations A system of interacting charged bosons (bound electron pairs) and electrons can show features which are intermediate between those of local pair superconductors and those of classical BCS systems. Such a two component (bosonfermion) model is of relevance for high temperature superconductors (HTS) and other exotic superconductors [1,2,3,4,5,6,7,8,9,10]. Recently, we have studied a generalization of this model to the case of anisotropic pairing [9,10]. Here, we shortly outline the study and present further results concerning the phase diagrams and superconducting properties of such a system in the case of isotropic s-wave pairing as well as for anisotropic pairings of d- and extended s-wave symmetries, for a 2D square lattice.
We considered the BCS-BEC crossover at T = 0 in the framework of a lattice fermion model (the extended Hubbard model).We analyzed the case of on-site hard core repulsion (U = ∞) and intersite attraction (W < 0) in a 3D system with extended s-wave pairing symmetry.The mechanisms of density-driven and attraction-strength-driven crossover were analyzed.We obtained the ground state phase diagram.The analyzed quantities are: chemical potential, order parameter, coherence length, and fraction of condensed particles.