T. A. Maier

Systematic Study of d-Wave Superconductivity in the 2D Repulsive Hubbard Model

The cluster size dependence of superconductivity in the conventional two-dimensional Hubbard model, commonly believed to describe high-temperature superconductors, is systematically studied using the dynamical cluster approximation and quantum Monte Carlo simulations as a cluster solver. Because of the nonlocality of the d-wave superconducting order parameter, the results on small clusters show large size and geometry effects. In large enough clusters, the results are independent of the cluster size and display a finite temperature instability to d-wave superconductivity.

d -wave pairing from spin fluctuations in the K x Fe 2 − y Se 2 superconductors

Angle-resolved photoemission spectroscopy measurements on the recently discovered superconduc- tors in the KFe2Se2 family with critical temperatures up to - 33K suggest that no Fermi pockets of hole character centered on the {\Gamma} point of the Brillouin zone are present, in contrast to all other known ferropnictide and ferrochalcogenide superconductors. Using a fluctuation exchange approximation and a 5-orbital tight-binding description of the band structure, we calculate the effective pairing interaction. We find that the pairing state in this system is most likely to have d-wave symmetry due to pair scattering between the remaining electron Fermi pockets at wave vector q - ({\pi}, {\pi}), but without any symmetry-imposed nodes for the given Fermi surface. We propose experimental tests of this result, including the form of the resonance spectrum probed by inelastic neutron scattering.

Quantum Monte Carlo algorithm for nonlocal corrections to the dynamical mean-field approximation

We present the algorithmic details of the dynamical cluster approximation (DCA), with a quantum Monte Carlo (QMC) method used to solve the effective cluster problem. The DCA is a fully-causal approach which systematically restores non-local correlations to the dynamical mean field approximation (DMFA) while preserving the lattice symmetries. The DCA becomes exact for an infinite cluster size, while reducing to the DMFA for a cluster size of unity. We present a generalization of the Hirsch-Fye QMC algorithm for the solution of the embedded cluster problem. We use the two-dimensional Hubbard model to illustrate the performance of the DCA technique. At half-filling, we show that the DCA drives the spurious finite-temperature antiferromagnetic transition found in the DMFA slowly towards zero temperature as the cluster size increases, in conformity with the Mermin-Wagner theorem. Moreover, we find that there is a finite temperature metal to insulator transition which persists into the weak-coupling regime. This suggests that the magnetism of the model is Heisenberg like for all non-zero interactions. Away from half-filling, we find that the sign problem that arises in QMC simulations is significantly less severe in the context of DCA. Hence, we were able to obtain good statistics for small clusters. For these clusters, the DCA results show evidence of non-Fermi liquid behavior and superconductivity near half-filling.

Lifting of nodes by disorder in extended- s -state superconductors: Application to ferropnictides

We show, using a simple model, how ordinary disorder can gap an extended-

Neutron scattering resonance and the iron-pnictide superconducting gap

s

Origin of gap anisotropy in spin fluctuation models of the iron pnictides

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A non-crossing approximation for the study of intersite correlations

({A}_{1g})

Angle-resolved photoemission spectra of the Hubbard model

symmetry superconducting state with nodes. The concomitant crossover of thermodynamic properties, particularly the

Phase diagram of the Hubbard model: Beyond the dynamical mean field

T

Efficient calculation of the antiferromagnetic phase diagram of the three-dimensional hubbard model

dependence of the superfluid density, from pure power-law behavior to an activated one is exhibited. We discuss applications of this scenario to experiments on the ferropnictide superconductors.