B. Qian

Superconductivity close to magnetic instability in Fe(Se 1-xTex)0.82

We report our study of the evolution of superconductivity and the phase diagram of the ternary Fe(Se1-xTex)0.82 (0≤x≤1.0) system. We discovered a 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 class of Fe-based compounds is associated with magnetic fluctuations and therefore may be unconventional in nature.

Charge-carrier localization induced by excess Fe in the superconductor Fe1+yTe1−xSex

We have investigated the effect of Fe nonstoichiometry on properties of the Fe1+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.

Spin Gap and Resonance at the Nesting Wave Vector in Superconducting FeSe0:4Te0:6

Neutron scattering is used to probe magnetic excitations in FeSe_{0.4}Te_{0.6} (T_{c} = 14 K). Low energy spin fluctuations are found with a characteristic wave vector (1/21/2L) that corresponds to Fermi surface nesting and differs from Q_{m} = (delta01/2) for magnetic ordering in Fe_{1+y}Te. A spin resonance with variant Plancks over 2piOmega_{0} = 6.51(4) meV approximately 5.3k_{B}T_{c} and variant Plancks over 2piGamma = 1.25(5) meV develops in the superconducting state from a normal state continuum. We show that the resonance is consistent with a bound state associated with s_{+/-} superconductivity and imperfect quasi-2D Fermi surface nesting.

From (π,0) magnetic order to superconductivity with (π,π) magnetic resonance in Fe1.02Te1−xSex

The iron chalcogenide Fe(1+y)(Te(1-x)Se(x)) is structurally the simplest of the Fe-based superconductors. Although the Fermi surface is similar to iron pnictides, the parent compound Fe(1+y)Te exhibits antiferromagnetic order with an in-plane magnetic wave vector (pi,0) (ref. 6). This contrasts the pnictide parent compounds where the magnetic order has an in-plane magnetic wave vector (pi,pi) that connects hole and electron parts of the Fermi surface. Despite these differences, both the pnictide and chalcogenide Fe superconductors exhibit a superconducting spin resonance around (pi,pi) (refs 9, 10, 11). A central question in this burgeoning field is therefore how (pi,pi) superconductivity can emerge from a (pi,0) magnetic instability. Here, we report that the magnetic soft mode evolving from the (pi,0)-type magnetic long-range order is associated with weak charge carrier localization. Bulk superconductivity occurs as magnetic correlations at (pi,0) are suppressed and the mode at (pi, pi) becomes dominant for x>0.29. Our results suggest a common magnetic origin for superconductivity in iron chalcogenide and pnictide superconductors.

Weak anisotropy of the superconducting upper critical field in Fe 1.11 Te 0.6 Se 0.4 single crystals

We have determined the resistive upper critical field Hc2 for single crystals of the superconductor Fe1.11Te0.6Se0.4 using pulsed magnetic fields of up to 60T. A rather high zero-temperature upper critical field of mu0Hc2(0) approx 47T is obtained, in spite of the relatively low superconducting transition temperature (Tc approx 14K). Moreover, Hc2 follows an unusual temperature dependence, becoming almost independent of the magnetic field orientation as the temperature T=0. We suggest that the isotropic superconductivity in Fe1.11Te0.6Se0.4 is a consequence of its three-dimensional Fermi-surface topology. An analogous result was obtained for (Ba,K)Fe2As2, indicating that all layered iron-based superconductors exhibit generic behavior that is significantly different from that of the high-Tc cuprates.

Tunable (δπ, δπ)-type antiferromagnetic order in α-Fe(Te,Se) superconductors

Magnetic spin fluctuations is one candidate to produce the bosonic modes that mediate the superconductivity in the ferrous superconductors. Up until now, all of the LaOFeAs and BaFe2As2 structure types have simple commensurate magnetic ground states, as result of nesting Fermi surfaces. This type of spin-density-wave (SDW) magnetic order is known to be vulnerable to shifts in the Fermi surface when electronic densities are altered at the superconducting compositions. Superconductivity has more recently been discovered in alpha-Fe(Te,Se), whose electronically active antifluorite planes are isostructural to the FeAs layers found in the previous ferrous superconductors and share with them the same quasi-two-dimensional electronic structure. Here we report neutron scattering studies that reveal a unique complex incommensurate antiferromagnetic order in the parent compound alpha-FeTe. When the long-range magnetic order is suppressed by the isovalent substitution of Te with Se, short-range correlations survive in the superconducting phase.

Incommensurate itinerant antiferromagnetic excitations and spin resonance in the FeTe0.6Se0.4 superconductor

We report on inelastic neutron-scattering measurements that find itinerantlike incommensurate magnetic excitations in the normal state of superconducting

Magnetostructural transformation and magnetocaloric effect in MnNiGe1-xGax alloys

{\text{FeTe}}_{0.6}{\text{Se}}_{0.4}

Friedel-Like Oscillations from Interstitial Iron in Superconducting Fe1+yTe0.62Se0.38

({T}_{c}=14\text{ }\text{K})