Scaling of nascent nodes in extended s-wave superconductors

We analyze the low-energy properties of superconductors near the onset of accidental nodes, i.e. zeroes of the gap function not enforced by symmetry. The existence of such nodes has been motivated by recent experiments suggesting a transition between nodeless and nodal superconductivity in iron-based compounds. We find that the low-temperature behavior of the penetration depth, the specific heat, and the NMR spin-lattice relaxation rate are determined by the scaling properties of a quantum critical point associated with the nascent nodes. Although the power-law exponents are insensitive to weak short-range electronic interactions, they can be significantly altered by the curvature of the Fermi surface or by the three-dimensional character of the gap. Consequently, the behavior of macroscopic quantities near the onset of nodes can be used as a criterion to determine the nodal structure of the gap function.Comment: minor changes; version accepted for publicatio

Manifestations of nematic degrees of freedom in the magnetic, elastic, and superconducting properties of the iron pnictides

We investigate how emergent nematic order and nematic fluctuations affect several macroscopic properties of both the normal and superconducting states of the iron pnictides. Due to its magnetic origin, long-range nematic order enhances magnetic fluctuations, leaving distinctive signatures in the spin-lattice relaxation rate, the spin-spin correlation function, and the uniform magnetic susceptibility. This enhancement of magnetic excitations is also manifested in the electronic spectral function, where a pseudogap can open at the hot spots of the Fermi surface. In the nematic phase, electrons are scattered by magnetic fluctuations that are anisotropic in momentum space, giving rise to a non-zero resistivity anisotropy whose sign changes between electron-doped and hole-doped compounds. We also show that due to the magneto-elastic coupling, nematic fluctuations soften the shear modulus in the normal state, but harden it in the superconducting state. The latter effect is an indirect consequence of the competition between magnetism and superconductivity, and also causes a suppression of the orthorhombic distortion below T_{c}. We also demonstrate that ferro-orbital fluctuations enhance the nematic susceptibility, cooperatively promoting an electronic tetragonal symmetry-breaking. Finally, we argue that T_{c} in the iron pnictides might be enhanced due to nematic fluctuations of magnetic origin.Comment: 17 pages, revised version to appear in Superconductor Science Technology focus issue on Fe-based superconductor

The whole mesh Deformation Model for 2D and 3D image segmentation

In this paper we present a novel approach for image segmentation using Active Nets and Active Volumes. Those solutions are based on the Deformable Models, with slight difference in the method for describing the shapes of interests - instead of using a contour or a surface they represented the segmented objects with a mesh structure, which allows to describe not only the surface of the objects but also to model their interiors. This is obtained by dividing the nodes of the mesh in two categories, namely internal and external ones, which will be responsible for two different tasks. In our new approach we propose to negate this separation and use only one type of nodes. Using that assumption we manage to significantly shorten the time of segmentation while maintaining its quality

Nematicity as a probe of superconducting pairing in iron-based superconductors

In several families of iron-based superconducting materials, a d-wave pairing instability may compete with the leading s-wave instability. Here we show that when both states have comparable free energies, superconducting and nematic degrees of freedom are strongly coupled. While nematic order causes a sharp non-analytic increase in $T_{c}$, nematic fluctuations can change the character of the s-wave to d-wave transition, favoring an intermediate state that does not break time-reversal symmetry but does break tetragonal symmetry. The coupling between superconductivity and nematicity is also manifested in the strong softening of the shear modulus across the superconducting transition. Our results show that nematicity can be used as a diagnostic tool to search for unconventional pairing states in iron pnictides and chalcogenides.Comment: revised version; supplementary material include

Anisotropic in-plane resistivity in the nematic phase of the iron pnictides

We show that the interference between scattering by impurities and by critical spin fluctuations gives rise to anisotropic transport in the Ising-nematic state of the iron pnictides. The effect is closely related to the non-Fermi liquid behavior of the resistivity near an antiferromagnetic quantum critical point. Our theory not only explains the observed sign of the resistivity anisotropy $\Delta\rho$ in electron doped systems, but also predicts a sign change of $\Delta\rho$ upon sufficient hole doping. Furthermore, our model naturally addresses the changes in $\Delta\rho$ upon sample annealing and alkaline-earth substitution.Comment: revised version accepted in PRL; supplemental material include