Neutron scattering as a probe of the Fe-pnicitide superconducting gap

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 susceptibility, we explore the scattering response for various gaps that have been proposed.Comment: 5 pages, 4 figure

The Effects of Weak Spatiotemporal Noise on a Bistable One-Dimensional System

We treat analytically a model that captures several features of the phenomenon of spatially inhomogeneous reversal of an order parameter. The model is a classical Ginzburg-Landau field theory restricted to a bounded one-dimensional spatial domain, perturbed by weak spatiotemporal noise having a flat power spectrum in time and space. Our analysis extends the Kramers theory of noise-induced transitions to the case when the system acted on by the noise has nonzero spatial extent, and the noise itself is spatially dependent. By extending the Langer-Coleman theory of the noise-induced decay of a metastable state, we determine the dependence of the activation barrier and the Kramers reversal rate prefactor on the size of the spatial domain. As this is increased from zero and passes through a certain critical value, a transition between activation regimes occurs, at which the rate prefactor diverges. Beyond the transition, reversal preferentially takes place in a spatially inhomogeneous rather than in a homogeneous way. Transitions of this sort were not discovered by Langer or Coleman, since they treated only the infinite-volume limit. Our analysis uses higher transcendental functions to handle the case of finite volume. Similar transitions between activation regimes should occur in other models of metastable systems with nonzero spatial extent, perturbed by weak noise, as the size of the spatial domain is varied.Comment: 16 page

Systematic analysis of a spin-susceptibility representation of the pairing interaction in the 2D Hubbard model

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 levels. We find that the pairing interaction is well approximated by {3/2}\Ub(T)^2\chi(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.Comment: 5 pages, 5 figure

Evolution of the neutron resonances in AFe2Se2

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