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Chinese Physics Letters

The η-Pairing Superconductivity in Spin-Density Wave Background

X M Qiu, Z J Wang

Published Oct 1, 1993• 2 citations
Publisher

Abstract

In this letter, we propose a modified attractive Hubbard model at half filling that can exhibit superconductivity through η -pairing mechanism in spin-density wave representation and derive a concise relationship between the energy disparity and the single-particle energy spectrum. This relationship, in the two limits of U , clearly shows that the system does not display superconductivity for very low doping concentration. but demonstrates superconductivity when the doping concentration exceeds a certain critical value. This conclusion is in qualitative agreement with the familiar experiments on high- T c superconductivity.

DOI
10.1088/0256-307x/10/10/012

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In this letter, we propose a modified attractive Hubbard model at half filling that can exhibit superconductivity through η -pairing mechanism in spin-density wave representation and derive a concise relationship between the energy disparity and the single-particle energy spectrum. This relationship, in the two limits of U , clearly shows that the system does not display superconductivity for very low doping concentration. but demonstrates superconductivity when the doping concentration exceeds a certain critical value. This conclusion is in qualitative agreement with the familiar experiments on high- T c superconductivity.
We present recent theoretical results on superconductivity in correlated-electron systems, especially in the two-dimensional Hubbard model and the three-band d-p model. The mechanism of superconductivity in high-temperature superconductors has been extensively studied on the basis of various electronic models and also electron-phonon models. In this study, we investigate the properties of superconductivity in correlated-electron systems by using numerical methods such as the variational Monte Carlo method and the quantum Monte Carlomethod. The Hubbard model is one of basic models for strongly correlated electron systems, and is regarded as the model of cuprate high temperature superconductors. The d-p model is more realistic model for cuprates. The superconducting condensation energy obtained by adopting the Gutzwiller ansatz is in reasonable agreement with the condensation energy estimated for YBa2Cu3O7. We show the phase diagram of the ground state using this method. We have further investigated the stability of striped and checkerboard states in the under-doped region. Holes doped in a half-filled square lattice lead to an incommensurate spin and charge density wave. The relationship of the hole density x and incommensurability δ, δ~x, is satisfied in the lower doping region, as indicated by the variationalMonte Carlocalculations for the two-dimensional Hubbard model. A checkerboard-like charge-density modulation with a roughly period has also been observed by scanning tunneling microscopy experiments in Bi2212 and Na-CCOC compounds. We have performed a variational Monte Carlo simulation on a two-dimensional t-t′-t″- U Hubbard model with a Bi-2212 type band structure and found that the period checkerboard spin modulation, that is characterized by multi Q vectors, is indeed stabilized. We have further performed an investigation by using a quantumMonte Carlomethod, which is a numerical method that can be used to simulate the behavior of correlated electron systems. We present a new algorithm of the quantum Monte Carlo diagonalization that is a method for the evaluation of expectation value without the negative sign problem. We compute pair correlation functions and show that pair correlation is indeed enhanced with hole doping.
The Penson-Kolb-Hubbard model, i.e. the Hubbard model with the pair-hopping interaction J is studied.We focus on the properties of the superconducting state with the Cooper-pair center-of-mass momentum q = Q (η-phase).The transition into the η-phase, which is favorized by the repulsive J (J < 0) is found to occur only above some critical value |J c |, dependent on band filling, on-site interaction U and band structure, and the system never exhibits standard BCS-like features.This is in obvious contrast with the properties of the isotropic s-wave state, stabilized by the attractive J and attractive U , which exhibit at T = 0 a smooth crossover from the BCS-like limit to that of tightly bound pairs with increasing pairing strength.
We demonstrate the possibility of triplet p-wave pairing at low electron density a large number of models such as 3D and 2D Fermi-gas models with hard-core repulsion, 3D and 2D Hubbard models, and the Shubin-Vonsovsky model. The critical temperature for p-wave pairing can be considerably higher in the spin-polarized case or even in a two-band situation at low density and can reach experimentally observable values of 1–5 K. We also discuss briefly the d-wave pairing and high-Tc superconductivity with Tc ∼ 100 K which arise in the extended Hubbard model and in the generalized t-J model when close to half-filling.
Understanding superconductivity emerging from repulsive fermions remains a major challenge in condensed matter physics. In this paper, we investigate the pairing tendencies in a one-dimensional, three component repulsive Hubbard model, using the density matrix renormalization group method. At half-filling, the system exhibits density wave ground state due to strong Hubbard repulsions. Upon doping, we find that Cooper pairs can emerge, whose fluctuations predominate the long-range physics in the system across a wide parameter range. The effective attractions between Cooper pairs are mediated by the particle-hole fluctuations in the third non-pairing component, resembling an excitonic mechanism of superconductivity. The coexistence of multiple density waves and superconductivity at different fermion fillings is explored. We also present an analytical study of the pairing mechanism in both weak and strong coupling limits. Our results provide a new perspective for understanding and exploring unconventional superconductivities in strongly correlated fermionic systems.
We investigate the possibility of superconductivity in the large-UUU limit of the Hubbard model near half filling, using a slave-boson representation. The local constraint is treated in a novel way, incorporating short-range fluctuations in addition to the global constraint. We find slave-boson pairing and a superconducting extended sss-wave phase at hole concentrations $\ensuremath{\delta}$-above 4% and below 32%.