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arXiv:cond-mat/0505618 · Journal of the Physical Society of Japan

Development of Superconducting Correlation at Low Temperatures in the Two-dimensionalt–JModel

Takashi Koretsune, Masao Ogata

Published May 15, 2005• 9 citations
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Abstract

The equal-time pairing correlation function of the two-dimensional t-J model on a square lattice is studied using a high-temperature expansion method. The sum of the pairing correlation, its spatial dependence, and the correlation length are obtained as functions of temperature down to $T \simeq 0.2 t$. By comparison of single-particle contributions in the correlation functions, we find an effective attractive interaction between quasi-particles in $d_{x^2-y^2}$-wave pairings. It is shown that d-wave correlation grows rapidly at low temperatures for 0.5 < n < 0.9, with n being the electron density. The temperature for this growth is roughly scaled by J/2. This is in sharp contrast to the Hubbard model in a weak or intermediate coupling region, where there is no numerical evidence of superconductivity.

DOI
10.1143/jpsj.74.1390

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The equal-time pairing correlation function of the two-dimensional t-J model on a square lattice is studied using a high-temperature expansion method. The sum of the pairing correlation, its spatial dependence, and the correlation length are obtained as functions of temperature down to T≃0.2tT \simeq 0.2 tT≃0.2t. By comparison of single-particle contributions in the correlation functions, we find an effective attractive interaction between quasi-particles in dx2−y2d_{x^2-y^2}dx2−y2​-wave pairings. It is shown that d-wave correlation grows rapidly at low temperatures for 0.5 < n < 0.9, with n being the electron density. The temperature for this growth is roughly scaled by J/2. This is in sharp contrast to the Hubbard model in a weak or intermediate coupling region, where there is no numerical evidence of superconductivity.
We study the ``t-J'' model with strongly correlated electrons (infinite-U Hubbard-model kinetic energy plus superexchange J) by exact diagonalization on small two-dimensional clusters of six, eight, and twelve sites on a square lattice with periodic boundary conditions. We find an attractive pairing potential for two holes when J/t\ensuremath{\ge}(J/t)c{)}_{\mathit{c}})c​\ensuremath{\simeq}0.2, with t the hopping matrix element. Ground-state calculations show that extended s-wave and d-wave pairing susceptibilities and equal-time pairing correlations grow weakly with increasing J and system size at fixed filling n. Pairing correlations may be enhanced for (J/t)>(J/t)c{)}_{\mathit{c}})c​\ensuremath{\approxeq}0.2. We compare equal-time pairing correlations to a mean-field theory for J=0 and note potential difficulties with interpreting numerical results for such small systems. The widths of one- and two-hole energy bands in low-spin states are of order J; the one-hole effective mass is larger than the two-hole effective mass in such states. Spin-charge correlation functions for a single hole in the eight- and twelve-site lattices are consistent with the spin-charge decoupling scenario for resonating-valence-bond states.
The pairing susceptibility of the one-dimensional t-J model is examined numerically by the quantum transfer matrix method. The temperature dependence of the pairing susceptibility χ s shows logarithmic behavior at low temperatures as χ s ∼αlog T for T being the temperature. The coefficient α is determined as a function of J / t and the fillingness for J and t being the exchange interaction and the transfer. Beyond that of noninteracting fermions, α grows rapidly with the increase of | J |.
We utilize numerical linked-cluster expansions (NLCEs) and the determinantal quantum Monte Carlo algorithm to study pairing correlations in the square-lattice Hubbard model. To benchmark the NLCE, we first locate the finite-temperature phase transition of the attractive model to a superconducting state away from half filling. We then explore the superconducting properties of the repulsive model for the ddd-wave and extended sss-wave pairing symmetries. The pairing structure factor shows a strong tendency to ddd-wave pairing and peaks at an interaction strength comparable to the bandwidth. The extended sss-wave structure factor and correlation length are larger at higher temperatures but clearly saturate as temperature is lowered, whereas the ddd-wave counterparts, which start off lower at high temperatures, continue to rise near half filling. This rise is even more dramatic in the ddd-wave susceptibility. The convergence of NLCEs breaks down as the susceptibilities and correlation lengths become large, so we are unable to determine the onset of long-range order. However, our results extend the conclusion, previously restricted to only magnetic and charge correlations, that NLCEs offer a unique window into pairing in the Hubbard model at strong coupling.
We study the t-J model in one and two dimensions. It is shown that superconducting correlations at large distances are enhanced at doping 〈n〉=1/2, and near the region of phase separation. Including a nearest-neighbor density-density repulsion term of strength V, and working at large V/t and J/t, our results can be explained using a previously discussed variational state where electrons are paired in singlets without cluster formation, in analogy to the attractive Hubbard model. We argue that the physics of the t-J-V model is basically independent of the dimension and that a smooth connection exists between small and large V/t. Then, we conjecture that the two-dimensional t-J model should also present strong superconducting correlations near the region of phase separation, at 〈n〉\ensuremath{\sim}1/2. Numerical results both in one and two dimensions support our claims.
The mechanism of fermionic pairing is the key to understanding various phenomena such as high-temperature superconductivity and the pseudogap phase in cuprate materials. We study the pair correlations in the attractive Hubbard model using ultracold fermions in a two-dimensional optical lattice. By combining the fluctuation-dissipation theorem and the compressibility equation of state, we extract the interacting pair correlation functions and deduce a characteristic length scale of pairs as a function of interaction and density filling. At sufficiently low filling and weak on-site interaction, we observe that the pair correlations extend over a few lattice sites even at temperatures above the superfluid transition temperature.
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.