Evidence that the pseudogap (PG) in a near-optimally doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+delta}$ sample destroys the BCS logarithmic pairing instability [1] raises again the question of the role of the PG in the high-temperature superconducting cuprates
[2]. The elimination of the BCS instability is consistent with the view that the PG competes with superconductivity. However, as noted in [1], the onset of superconductivity with a $T_c sim 90$ K suggests an alternative scenario in which the PG reflects the formation of short range pairing correlations. Here, we report results obtained from a dynamic cluster quantum Monte Carlo approximation (DCA) for a 2D Hubbard model and conclude that (1) the PG, like the superconductivity, arises due to short-range antiferromagnetic correlations and (2) contrary to the usual case in which the pairing instability arises from the Cooper instability, here, the strength of the spin-fluctuations increases as the temperature decreases leading to the pairing instability.
Recent experiments on the alkali-intercalated iron selenides have raised questions about the symmetry of the superconducting phase. Random phase approximation calculations of the leading pairing eigenstate for a tight- binding 5-orbital Hubbard-Hund
model of AFe2Se2 find that a d-wave (B1g) state evolves into an extended s{pm} (A1g) state as the system is hole-doped. However, over a range of doping these two states are nearly degenerate. Here, we calculate the imaginary part of the magnetic spin susceptibility chi(q,{omega}) for these gaps and discuss how the evolution of neutron scattering resonances can distinguish between them.
Using a dynamical cluster quantum Monte Carlo approximation, we investigate the effect of local disorder on the stability of d-wave superconductivity including the effect of electronic correlations in both particle-particle and particle-hole channels
. With increasing impurity potential, we find an initial rise of the critical temperature due to an enhancement of anti-ferromagnetic spin correlations, followed by a decrease of Tc due to scattering from impurity-induced moments and ordinary pairbreaking. We discuss the weak initial dependence of Tc on impurity concentration found in comparison to experiments on cuprates.
Inelastic neutron scattering provides a probe for studying the spin and momentum structure of the superconducting gap. Here, using a two-orbital model for the Fe-pnicitide superconductors and an RPA-BCS approximation for the dynamic spin susceptibili
ty, we explore the scattering response for various gaps that have been proposed.
The question of whether one should speak of a pairing glue in the Hubbard and t-J models is basically a question about the dynamics of the pairing interaction. If the dynamics of the pairing interaction arises from virtual states, whose energies corr
espond to the Mott gap, and give rise to the exchange coupling J, the interaction is instantaneous on the relative time scales of interest. In this case, while one might speak of an instantaneous glue, this interaction differs from the traditional picture of a retarded pairing interaction. However, if the energies correspond to the spectrum seen in the dynamic spin susceptibility, then the interaction is retarded and one speaks of a spin-fluctuation glue which mediates the d-wave pairing. Here we present results from numerical studies which provide insight into this question.
A dynamic cluster quantum Monte Carlo algorithm is used to study a spin susceptibility representation of the pairing interaction for the two-dimensional Hubbard model with an on-site Coulomb interaction equal to the bandwidth for various doping level
s. We find that the pairing interaction is well approximated by ${3/2}Ub(T)^2chi(K-K)$ with an effective temperature and doping dependent coupling $Ub(T)$ and the numerically calculated spin susceptibility $chi(K-K)$. We show that at low temperatures, $Ub$ may be accurately determined from a corresponding spin susceptibility based calculation of the single-particle self-energy. We conclude that the strength of the d-wave pairing interaction, characterized by the mean-field transition temperature, can be determined from a knowledge of the dressed spin susceptibility and the nodal quasiparticle spectral weight. This has important implications with respect to the questions of whether spin fluctuations are responsible for pairing in the high-T$_c$ cuprates.
