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Abstract: The interplay between antiferromagnetism (AF) and superconductivity (SC) in cuprates is studied for the two-dimensional Hubbard model with a diagonal transfer t′, using a variational Monte Carlo method. Optimizing an improved function for strongly correlated values of U/t, we construct phase diagrams in the δ (doping rate)-t′/t space. It is found that the stable state is sensitive to the value of model parameters: For the extremely large values of U/t, a coexisting state is realized for t′/t ≳−0.15, whose range of doping rate extends as t′/t increases. In contrast, for t′/t =−0.3, AF and SC states are mutually exclusive, and a coexisting state does not appear. As U/t decreases, the area of pure AF extends, and that of coexisting state shrinks. As a result, the coexisting state disappears for t′/t =−0.15 and U/t =12, probable values for hole-doped cuprates. Compared with the t–J model, the Hubbard model has richer phases. [ABSTRACT FROM AUTHOR]

Abstract: Coexistence of and competition between antiferromagnetism (AF) and d-wave superconductivity (SC) are studied for a Hubbard model on the square lattice with a diagonal transfer , using a variational Monte Carlo method. The following improvements are introduced into the trial function: (1) Coexistence of AF and d-wave singlet gaps that allows a continuous description of their interplay, (2) band renormalization effect, and (3) refined doublon–holon correlation factors. Optimizing this function for a strongly correlated value of , we construct a phase diagram in the (doping rate)- space, and find that for a coexisting state is realized, whose range of extends as increases. In contrast, for , AF and SC states are mutually exclusive, and a coexisting state does not appear. In connection with the “two-gap” problem, we confirm even for the present refined function that the gradient of momentum distribution function at the antinodal point mainly dominates the magnitude of the d-wave SC correlation function. [Copyright &y& Elsevier]

Abstract: We extend the doublon–holon binding wave function, which was recently often used to consider Mott transitions and -wave superconductivity, for the half-filled-band Hubbard model on square lattices with the diagonal hopping t′. We introduce two new features: (1) In the doublon–holon binding factor, more accurate configuration correlations are included. (2) The band renormalization effect owing to the electron correlation is introduced within the third-neighbor hopping, including the anisotropy between x- and y-directions in the nearest-neighbor hopping. Using an optimization variational Monte Carlo technique, we draw some definite answers to the topics concerning the above features. [Copyright &y& Elsevier]

Abstract: As a model of the novel superconductor Na x CoO2 · yH2O, we study a triangular Hubbard model with doubly degenerate atomic orbitals, using an optimization variational Monte Carlo method. It is found that when the isolated Fermi surface overlaps the van Hove singularity points, and d+id-wave superconducting states, among various pairing symmetries, are appreciably stabilized by large Coulomb interactions. The pairing is originating from the potential energy gain induced dominantly by the enhancement of antiferromagnetic spin correlations. The possibility of triplet pairing is very slight in this model. [Copyright &y& Elsevier]

Abstract: The interplay between antiferromagnetism (AF) and superconductivity (SC) in cuprate is studied for the two-dimensional Hubbard model with a diagonal transfer t′, using a variational Monte Carlo method. Optimizing an improved wave-function for strongly correlated values of U/t, we construct a phase diagram. It is found that the stable state is sensitive to the value of model parameters: For t′/t ≳−0.15 a coexisting state is realized, whose range of doping rate extends as t′/t increases. In contrast, for t′/t =−0.3, AF and SC states are mutually exclusive, and the area of pure AF shrinks as U/t increases. [ABSTRACT FROM AUTHOR]

Abstract: A strongly correlated Hubbard (t–t ′–U) model is studied using a variational Monte Carlo method. The magnitude of momentum distribution function varies slightly near the nodal quasi-Fermi surface , but varies outstandingly in an antinodal part [near ], as the value of varies, which is sensitive to the strength of superconductivity. Furthermore, the behavior of the slope of around coincides well with that of the d-wave superconducting correlation function. It follows the electrons near the antinode play a leading role to control the strength of superconductivity. [ABSTRACT FROM AUTHOR]

Abstract: To elucidate systematically the interplay between antiferromagnetism (AF) and superconductivity (SC) in High- cuprates, we study the effect of the long-range transfers, and , by applying a variational Monte Carlo method to a model. In the trial wave function, coexistence of SC and AF phases and contribution of long-range Cooper pairs, namely the higher harmonic components of the SC gap, are considered. Our result exhibits marked electron–hole asymmetry; for the hole-(electron-)doped case, the SC state is very stable (unstable), while the AF state is irrelevant (predominant). The coexisting phase always appears in the under-doped region, both in the hole-doped and electron-doped cases. Furthermore, the optimized long-range pairing components successfully yield the asymmetry of the gap form observed in the hole-doped and electron-doped cuprates. [ABSTRACT FROM AUTHOR]