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We implement a Quantum Monte Carlo calculation for a repulsive Hubbard In the present QMC study of a generalized HM (which includes next-nearest neighbor hopping interactions as well as the usual nearest-neighbor ones), we present evidence for a superconducting tendency in the The SP feature may be incorporated into the Hubbard model within the metallic regime by introducing a next-nearest neighbor interaction. This allows the SP to lie at the Fermi level at a doping of, say 15-25%, while the insulating point, at which the antiferromagnetic instability occurs, lies at 0% doping. These features are characteristic of real cuprate materials [13], [14]. It is in the former situation (15-25% doping) that the model is found to support superconductivity.The model is specified as followsIn (1), U is the repulsive on-site Coulomb interaction, t is the nearest-neighbor hopping integral ( ij denotes nearest neighbor interactions), t ′ is the next-nearest neighbor hopping integral ( ij denotes next-nearest neighbor interactions); t and t ′ are defined to be positive. 2The noninteracting band structure of the tt ′ -Hubbard model (1) has saddle points at energy −4t ′ , and at k = (0, π) and (π, 0). If we take the hole doping x to be of order x ≈ t ′ , then the saddle points in the noninteracting band structure lie near E F (see Fig. 1). In the absence of t ′ , the required doping would be zero, making the sample insulating. The electronic effective mass below the SP's is heavier than the mass above, the ratio being (t + 2t where Θ is a projection parameter and |Ψ 0 a single determinant taken as the ground state of the noninteracting band structure of 1.The exponent in (2) demonstrated [7] in models such as the attractive Hubbard model and simplified models with electronphonon coupling, where a pairing tendency is anticipated to occur. While studying finite systems one has always to extrapolate to the thermodynamic limit. Due to the unsystematic finite level structure of the finite Hubbard model this has turned out to be very difficult. We use two different approaches: a) we are able to analyse the plateau in terms of the effective pairing interaction J, and hence deduce a value for T c in the infinite-sample limit. b) we find analogous finite size scaling behaviour for the superconducting correlations for the repulsive and the attractive Hubbard model.The condition as to whether the finite cluster of size L is 'superconducting' (correlation length ξ > L), or whether superconducitivity is suppressed by finite size effects (ξ < L), does not enter into these considerations and is not relevant for this paper. to derive the single-band U formally from a multi-band model [19] give a value of order 6eV, i.e. 6t, in the case t = 1 eV. We have observed clear evidence for the superconducting tendency for U in the range 0.5t < U < 3t; for larger U-values unnacceptable error bars are obtained.The superconducting correlation function χ(R j ) for d x 2 −y 2 symmetry is defined aswhereHere, p implies a sum over nearest neighbors...

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We propose an experimental technique for an easy to control realization of a binary dipolar monolayer where the two components have oppositely oriented dipole moments constrained perpendicular to the plane of motion without the application of an external field. The experimental setup ensures that hydrodynamic effects do not play a crucial role in the structure formation, the particles move deterministically due to the dipole-dipole interaction. At low concentrations, cluster-cluster aggregation occurs with chainlike morphologies, while at high concentration the particles self-assemble into various types of binary crystal lattices, in good agreement with the theoretical predictions. The structures formed by the particles are found to be sensitive to external perturbations due to the central interparticle forces, however, static friction arising at the contact surface of particles can increase the stability compared to systems with only viscous friction.