We have reexamined the problem of disorder in two-band superconductors, and shown within the framework of the T -matrix approximation, that the suppression of Tc can be described by a single parameter depending on the intraband and interband impurity scattering rates. Tc is shown to be more robust against nonmagnetic impurities than would be predicted in the trivial extension of Abrikosov-Gor'kov theory. We find a disorder-induced transition from the s± state to a gapless and then to a fully gapped s++ state, controlled by a single parameter -the sign of the average coupling constant λ . We argue that this transition has strong implications for experiments.

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Materials with a 5d 4 electronic configuration are generally considered to have a nonmagnetic ground state (J = 0). Interestingly, Sr2YIrO6 (Ir 5+ having 5d 4 electronic configuration) was recently reported to exhibit long-range magnetic order at low temperature and the distorted IrO6 octahedra were discussed to cause the magnetism in this material. Hence, a comparison of structurally distorted Sr2YIrO6 with cubic Ba2YIrO6 may shed light on the source of magnetism in such Ir 5+ materials with 5d 4 configuration. Besides, Ir 5+ materials having 5d 4 are also interesting in the context of recently predicted excitonic types of magnetism. Here we report a single-crystal-based analysis of the structural, magnetic, and thermodynamic properties of Ba2YIrO6. We observe that in Ba2YIrO6 for temperatures down to 0.4 K, long-range magnetic order is absent but at the same time correlated magnetic moments are present. We show that these moments are absent in fully relativistic ab initio band-structure calculations; hence, their origin is presently unclear.

Transitions between topologically distinct electronic states have been predicted in different classes of materials and observed in some. A major goal is the identification of measurable properties that directly expose the topological nature of such transitions. Here we focus on the giant-Rashba material bismuth tellurium iodine (BiTeI) which exhibits a pressure-driven phase transition between topological and trivial insulators in threedimensions. We demonstrate that this transition, which proceeds through an intermediate Weyl semi-metallic state, is accompanied by a giant enhancement of the Berry curvature dipole which can be probed in transport and optoelectronic experiments. From first-principles calculations, we show that the Berrry-dipole -a vector along the polar axis of this material-has opposite orientations in the trivial and topological insulating phases and peaks at the insulator-to-Weyl critical points, at which the nonlinear Hall conductivity can increase by over two orders of magnitude.

We analyze the interplay between superconductivity and the formation of bound pairs of fermions (BCS-BEC crossover) in a 2D model of interacting fermions with small Fermi energy EF and weak attractive interaction, which extends to energies well above EF . The 2D case is special because two-particle bound state forms at arbitrary weak interaction, and already at weak coupling one has to distinguish between bound state formation and superconductivity. We briefly review the situation in the one-band model and then consider two different two-band models -one with one hole band and one electron band and another with two hole or two electron bands. In each case we obtain the bound state energy 2E0 for two fermions in a vacuum and solve the set of coupled equations for the pairing gaps and the chemical potentials to obtain the onset temperature of the pairing, Tins and the quasiparticle dispersion at T = 0. We then compute the superfluid stiffness ρs(T = 0) and obtain the actual Tc. For definiteness, we set EF in one band to be near zero and consider different ratios of E0 and EF in the other band. We show that, at EF E0, the behavior of both two-band models is BCS-like in the sense that Tc ≈ Tins EF and ∆ ∼ Tc. At EF < E0, the two models behave differently: in the model with two hole/two electron bands, Tins ∼ E0/ log1/2 , and Tc ∼ EF , like in the one-band model. In between Tins and Tc the system displays preformed pair behavior. In the model with one hole and one electron band, Tc remains of order Tins, and both remain finite at EF = 0 and of order E0. The preformed pair behavior still does exist in this model because Tc is numerically smaller than Tins. For both models we re-express Tins in terms of the fully renormalized two-particle scattering amplitude by extending to two-band case the method pioneered by Gorkov and Melik-Barkhudarov back in 1961. We apply our results for the model with a hole and an electron band to Fe-pnictides and Fe-chalcogenides in which superconducting gap has been detected on the bands which do not cross the Fermi level, and to FeSe, in which superconducting gap is comparable to the Fermi energy. We apply the results for the model with two electron bands to Nb-doped SrTiO3 and argue that our theory explains rapid increase of Tc when both bands start crossing the Fermi level.