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Chinese Physics Letters

Cooper Pairing Near the Boundary in the - U Hubbard Model

Xu Yuan, Xin-Zhong Yan

Published Sep 1, 1997• 1 citation
Publisher

Abstract

The Cooper pairing near the boundary of a superconductor is analyzed by using the linear-response theory. The system is described by the square lattice ( t , - U ) Hubbard model occupying half space. At half filling, the pairing also corresponds to the antiferromagnetic ordering in the repulsive Hubbard model. The numerical result clearly shows that, at weak to intermediate coupling ( U / t ⩽ 1), the boundary exerts strong influence on the order parameter; the weaker the coupling, the deeper the order parameter is suppressed near the boundary. The present scheme is valid in the whole temperature regime of Cooper pairing.

DOI
10.1088/0256-307x/14/9/016

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The Cooper pairing near the boundary of a superconductor is analyzed by using the linear-response theory. The system is described by the square lattice ( t , - U ) Hubbard model occupying half space. At half filling, the pairing also corresponds to the antiferromagnetic ordering in the repulsive Hubbard model. The numerical result clearly shows that, at weak to intermediate coupling ( U / t ⩽ 1), the boundary exerts strong influence on the order parameter; the weaker the coupling, the deeper the order parameter is suppressed near the boundary. The present scheme is valid in the whole temperature regime of Cooper pairing.
A variational ground state of the repulsive Hubbard model on a square lattice is investigated numerically for an intermediate coupling strength (U=8t)(U=8t)(U=8t) and for moderate sizes (from $6\ifmmode\times\else\texttimes\fi{}6$ to $10\ifmmode\times\else\texttimes\fi{}10$). Our ansatz is superior to other widely used variational wave functions. The results for order parameters and correlation functions provide insight into the antiferromagnetic state at half-filling as well as strong evidence for a superconducting phase away from half-filling.
We obtain a complete and numerically exact in the weak-coupling limit $(U\ensuremath{\rightarrow}0)$ ground-state phase diagram of the repulsive fermionic Hubbard model on the square lattice for filling factors 0<n<20<n<20<n<2 and next-nearest-neighbor hopping amplitudes $0\ensuremath{\le}{t}^{\ensuremath{'}}\ensuremath{\le}0.5$. Phases are distinguished by the symmetry and the number of nodes of the superfluid order parameter. The phase diagram is richer than may be expected and typically features states with a high---higher than that of the fundamental mode of the corresponding irreducible representation---number of nodes. The effective coupling strength in the Cooper channel $\ensuremath{\lambda}$, which determines the critical temperature Tc{T}_{c}Tc​ of the superfluid transition, is calculated in the whole parameter space and regions with high values of $\ensuremath{\lambda}$ are identified. It is shown that besides the expected increase of $\ensuremath{\lambda}$ near the Van Hove singularity line, joining the ferromagnetic and antiferromagnetic points, another region with high values of $\ensuremath{\lambda}$ can be found at quarter filling and ${t}^{\ensuremath{'}}=0.5$ due to the presence of a line of nesting at ${t}^{\ensuremath{'}}\ensuremath{\ge}0.5$. The results can serve as benchmarks for controlled nonperturbative methods and guide the ongoing search for high-Tc{T}_{c}Tc​ superconductivity in the Hubbard model.
Lattice Hamiltonians with on-site interaction WWW have W=0 solutions, that is, many-body {\em singlet} eigenstates without double occupation. In particular, W=0 pairs give a clue to understand the pairing force in repulsive Hubbard models. These eigenstates are found in systems with high enough symmetry, like the square, hexagonal or triangular lattices. By a general theorem, we propose a systematic way to construct all the W=0 pairs of a given Hamiltonian. We also introduce a canonical transformation to calculate the effective interaction between the particles of such pairs. In geometries appropriate for the CuO2_{2}2​ planes of cuprate superconductors, armchair Carbon nanotubes or Cobalt Oxides planes, the dressed pair becomes a bound state in a physically relevant range of parameters. We also show that W=0 pairs quantize the magnetic flux like superconducting pairs do. The pairing mechanism breaks down in the presence of strong distortions. The W=0 pairs are also the building blocks for the antiferromagnetic ground state of the half-filled Hubbard model at weak coupling. Our analytical results for the 4×44\times 44×4 Hubbard square lattice, compared to available numerical data, demonstrate that the method, besides providing intuitive grasp on pairing, also has quantitative predictive power. We also consider including phonon effects in this scenario. Preliminary calculations with small clusters indicate that vector phonons hinder pairing while half-breathing modes are synergic with the W=0 pairing mechanism both at weak coupling and in the polaronic regime.
We investigate the Hubbard model on a two-dimensional square lattice by the perturbation expansion to the fourth order in the on-site Coulomb repulsion U. Numerically calculating all diagrams up to the fourth order in self-energy, we examine the convergence of perturbation series in the lattice system. We indicate that the coefficient of each order term rapidly decreases as in the impurity Anderson model for T > 0.1t in the half-filled case, but it holds in the doped case even at lower temperatures. Thus, we can expect that the convergence of perturbation expansion in U is very good in a wide parameter region also in the lattice system, except for T < 0.1t in the half-filled case. We next calculate the density of states in the fourth-order perturbation. In the half-filled case, the shape in a moderate correlation regime is quite different from the three peak structure in the second-order perturbation. Remarkable upper and lower Hubbard bands locate at w = +(-)U/2, and a pseudogap appears at the Fermi level w=0. This is considered as the precursor of the Mott-Hubbard antiferromagnetic structure. In the doped case, quasiparticles with very heavy mass are formed at the Fermi level. Thus, we conclude that the fourth-order perturbation theory overall well explain the asymptotic behaviors in a strong correlation regime.
The weak-coupling form of the effective interaction between two holes in the antiferromagnetic state of the two-dimensional Hubbard model near half-filling is derived. It is shown to predict a Cooper instability in the singlet ddd-wave channel, with binding temperature Tc>100{T}_{c}>100Tc​>100 K. Our results differ from those of the "spin-bag" theory of Schrieffer, Wen, and Zhang.
In order to investigate possible nonphonon mechanisms for superconductivity in the copper oxide compounds, the two-dimensional Hubbard model is studied in the weak-coupling limit. A scaling theory is developed which allows a consistent treatment of the coupled ln 2 ( T )-singularities arising from density wave and Cooper pairing fluctuations. For a half-filled band a spin-density-wave (antiferromagnetic) phase transition occurs, whereas for small deviations from half-filling spin fluctuations lead to a d -type superconducting phase. The method generalizes straightforwardly to finite-range interactions. The possible relevance to oxide superconductors and experimental tests are briefly discussed.