The new alpha-Fe(Te,Se) superconductors share the common iron building block and ferminology with the LaFeAsO and BaFe(2)As(2) families of superconductors. In contrast with the predicted commensurate spin-density-wave order at the nesting wave vector (pi, 0), a completely different magnetic order with a composition tunable propagation vector (deltapi, deltapi) was determined for the parent compound Fe_{1+y}Te in this powder and single-crystal neutron diffraction study. The new antiferromagnetic order survives as a short-range one even in the highest T_{C} sample. An alternative to the prevailing nesting Fermi surface mechanism is required to understand the latest family of ferrous superconductors.

We report our study of the evolution of superconductivity and the phase diagram of the ternary Fe(Se 1-x Te x ) 0.82 (0 ≤ x ≤ 1.0) system. We discovered a new superconducting phase with T c , max = 14 K in the 0.3 < x < 1.0 range. This superconducting phase is suppressed when the sample composition approaches the end member FeTe 0.82 , which exhibits an incommensurate antiferromagnetic order. We discuss the relationship between the superconductivity and magnetism of this material system in terms of recent results from neutron scattering measurements. Our results and analyses suggest that superconductivity in this new class of Fe-based compounds is associated with magnetic fluctuations, and therefore may be unconventional in nature.

Magnetization switching at the interface between ferromagnetic and paramagnetic metals, controlled by current-induced torques, could be exploited in magnetic memory technologies. Compelling questions arise regarding the role played in the switching by the spin Hall effect in the paramagnet and by the spin-orbit torque originating from the broken inversion symmetry at the interface. Of particular importance are the antidamping components of these current-induced torques acting against the equilibrium-restoring Gilbert damping of the magnetization dynamics. Here, we report the observation of an antidamping spin-orbit torque that stems from the Berry curvature, in analogy to the origin of the intrinsic spin Hall effect. We chose the ferromagnetic semiconductor (Ga,Mn)As as a material system because its crystal inversion asymmetry allows us to measure bare ferromagnetic films, rather than ferromagnetic-paramagnetic heterostructures, eliminating by design any spin Hall effect contribution. We provide an intuitive picture of the Berry curvature origin of this antidamping spin-orbit torque as well as its microscopic modelling. We expect the Berry curvature spin-orbit torque to be of comparable strength to the spin-Hall-effect-driven antidamping torque in ferromagnets interfaced with paramagnets with strong intrinsic spin Hall effect. In one interpretation discussed in the literature so far, currentinduced switching at ferromagnet/paramagnet interfaces 1,2 originates from an antidamping component of the spin-orbit torque (SOT) 1, at the broken space-inversion-symmetry interface, while in another 2,23,25 , the spin Hall effect (SHE) [26][27][28][29][30][31][32] in the paramagnet combines with the antidamping spin-transfer torque (STT) [33][34][35][36] in the ferromagnet. Because, so far, the theories have considered a scattering-related SOT with an antidamping component that is expected to be relatively weak compared with the field-like SOT component 18,19 , much attention has been focused on the SHE-STT interpretation, in which the large SHE originates from the Berry curvature in the band structure of a clean crystal 2,28,29,37 . The focus of the present work is on a large antidamping SOT that stems from a Berry curvature origin analogous to intrinsic SHE.In conventional semiclassical transport theory, the linear response of the carrier system to the applied electric field is described by the non-equilibrium distribution function of carrier eigenstates, which are considered to be unperturbed by the electric field. The form of the non-equilibrium distribution function is obtained by accounting for the combined effects of the carrier acceleration in the field and of scattering. For the SOT, the non-equilibrium distribution function can be used to evaluate the currentinduced carrier spin density, which then exerts the torque on the magnetization via carrier-magnetic moment exchange coupling. The field-like component of the SOT reported in previous theoretical and experimental studies in (Ga,Mn)As films 4,8,9,11,24 and pred...

We have investigated the effect of Fe nonstoichiometry on properties of the Fe 1+y (Te, Se) superconductor system by means of resistivity, Hall coefficient, magnetic susceptibility, and specific heat measurements. We find that the excess Fe at interstitial sites of the (Te, Se) layers not only suppresses superconductivity, but also results in a weakly localized electronic state. We argue that these effects originate from the magnetic coupling between the excess Fe and the adjacent Fe square planar sheets, which favors a short-range magnetic order.