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Ukrainian Journal of Physics

A Model for dx2 – y2 Superconductivity in the Strongly Correlated Fermionic System

Irina Bariakhtar, Alexander Nazarenko

Published May 1, 2014• 1 citation
Publisher

Abstract

Based on the known phenomenology of high-Tc cuprates and the available numerical calculations of the t − J model, a two-dimensional effective fermionic model with the nearest neighbor attraction is proposed. Numerical calculations suggest that the model has the dx2−y2 superconductivity (SC) in the ground state at a low fermionic density. We argue that this model captures the important physics of the dx2−y2 superconducting correlations found earlier in the t − J model by the exact diagonalization approach. Within a self-consistent RPA diagrammatic study, the density and the coupling strength dependence of the critical temperature is calculated. We also investigate the influence of the impurities on our results and show that the suppression of the superconductivity is insignificant, when the retardation effects are accounted for as opposed to the Hartree–Fock approximation.

DOI
10.15407/ujpe59.05.0487

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Based on the known phenomenology of high-Tc cuprates and the available numerical calculations of the t − J model, a two-dimensional effective fermionic model with the nearest neighbor attraction is proposed. Numerical calculations suggest that the model has the dx2−y2 superconductivity (SC) in the ground state at a low fermionic density. We argue that this model captures the important physics of the dx2−y2 superconducting correlations found earlier in the t − J model by the exact diagonalization approach. Within a self-consistent RPA diagrammatic study, the density and the coupling strength dependence of the critical temperature is calculated. We also investigate the influence of the impurities on our results and show that the suppression of the superconductivity is insignificant, when the retardation effects are accounted for as opposed to the Hartree–Fock approximation.
A two-dimensional fermionic model with attractive interactions is discussed. Its form is motivated by the phenomenology of the underdoped high-Tc{T}_{c}Tc​ cuprates, as discussed in previous literature. The exact solution to the two-particle problem leads to a bound state in the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ subspace. Numerical techniques show that the model has strong ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ pairing correlations in the ground state at low fermionic density. Within a self-consistent random phase approximation diagrammatic study, the model has a ddd-wave superconducting (SC) phase. The density dependence of the critical temperature is calculated. We argue that in the context of ddd-wave SC this model fulfills the role that the attractive on-site Hubbard model has played for sss-wave SC. We also show that another candidate, the attractive $t\ensuremath{-}U\ensuremath{-}V$ model, is actually not useful as a toy model for ddd-wave SC for a variety of reasons.
A Model of Correlated Fermions with dx2−y2d_{x^2-y^2}dx2−y2​ Superconductivity
Motivated by the phenomenology of the high-Tc cuprates, a two dimensional fermionic model with attractive interactions is here discussed. The exact solution to the two particle problem leads to a bound state in the dx2−y2d_{x^2 - y^2}dx2−y2​ subspace. Numerical techniques suggest that the model has dx2−y2d_{x^2 - y^2}dx2−y2​ superconductivity (SC) in the ground state at low fermionic density. Within a self-consistent RPA diagrammatic study, the density dependence of the critical temperature is calculated. We argue that in the context of d-wave SC this model fulfills the role that the attractive on-site Hubbard model has played for s-wave SC. We also show that another candidate, the attractive ``t-U-V'' model, which has d-wave SC at the mean-field level is actually not useful as a realization of this family of condensates for a variety of reasons.
We consider an extended Hubbard model with nearest-neighbor correlated hopping and next-nearest-neighbor hopping ${t}^{\ensuremath{'}}$ obtained as an effective model for cuprate superconductors. Using a generalized Hartree-Fock BCS approximation, we find that for high enough ${t}^{\ensuremath{'}}$ and doping, antiferromagnetism is destroyed and the system exhibits d-wave superconductivity. Near optimal doping we consider the effect of antiferromagnetic spin fluctuations on the normal self-energy using a phenomenological susceptibility. The resulting superconducting critical temperature as a function of doping is in good agreement with experiment.
We examine the competition and relationship between an antiferromagnetic (AF) Mott insulating state and a ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ superconducting (SC) state in two dimensions using semianalytical, i.e., diagrammatic calculations of the t-U-W model. The AF Mott insulator is described by the ground state of the half filled Hubbard model on a square lattice with on-site Coulomb repulsion U and nearest-neighbor single-particle hopping t. To this model, an extra term W is added, which depends upon the square of the single-particle nearest-neighbor hopping. Staying at half band filling and a constant value of U, it was previously shown with quantum Monte Carlo (QMC) simulations that one can generate a quantum transition as a function of the coupling strength W between an AF Mott insulating state and a ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}\mathrm{SC}$ state. Here we complement these earlier QMC simulations with physically more transparent diagrammatic calculations. We start with a standard Hartree-Fock (HF) calculation to capture the ``high-energy'' physics of the t-U-W model. Then, we derive and solve the Bethe-Salpeter equation, i.e., we account for fluctuation effects within the time-dependent HF or generalized random-phase approximation scheme. Spin and charge susceptibility as well as the effective interaction vertex are calculated and systematically compared with QMC data. Finally, the corresponding BCS gap equation obtained for this effective interaction is solved.
Supercomputing correlations in the two dimensional t-J model at zero temperature are evaluated using numerical techniques. At the fermionic density 〈n〉\ensuremath{\sim}1/2, strong signals of dx2{\mathit{d}}_{\mathit{x}}^{2}dx2​-y2{\mathit{y}}^{2}y2 superconductivity were observed in the ground state. These conclusions are based on a study of static pairing correlations, the Meissner effect, and flux quantization as indicators of superconductivity. A tentative phase diagram of the two dimensional t-j model is presented.
It is rigorously shown that the two-dimensional Hubbard model with narrow bands (including next-nearest-neighbor hopping, etc.) does not exhibit ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave pairing long-range order at any nonzero temperature. This kind of pairing long-range order will also be excluded at zero temperature if an excited energy gap opens in the charge excitation spectrum of the system. These results hold true for both repulsive and attractive Coulomb interactions and for any electron fillings, and are consistent with quantum Monto Carlo calculations.