The iron chalcogenide Fe1+y(Te1–xSex) is structurally the simplest of the Fe-based superconductors. Although the Fermi surface is similar to iron pnictides, the parent compound Fe1+yTe exhibits antiferromagnetic order with an in-plane magnetic wave vector (p,0) . This contrasts the pnictide parent compounds where the magnetic order has an in-plane magnetic wave vector (p,p) 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 (p,p). A central question in this burgeoning field is therefore how (p,p) superconductivity can emerge from a (p,0) magnetic instability. In this article, we report that the magnetic soft mode evolving from the (p,0)-type magnetic long-range order is associated with weak charge carrier localization. Bulk superconductivity occurs as magnetic correlations at (p,0) are suppressed and the mode at (p,p) becomes dominant for x>0.29. Our results suggest a common magnetic origin for superconductivity in iron chalcogenide and pnictide superconductors.
DOI:https://doi.org/10.1103/PhysRevB.78.224503, Editor’s Suggestion, cited 695+ times (Google Scholar)
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 Tc,max=14 K in the 0.3<x<1.0 range. This superconducting phase is suppressed when the sample composition approaches the end member FeTe0.82, 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.
In addition, we also conducted neutron scattering studies on the Fe1+y(Te,Se) superconductor system in collaboration with Bao et al . Their work revealed that the antiferromagnetic (AFM) order in the non-superconducting parent compound Fe1+yTe propagates along the diagonal direction of the Fe square lattice and can be tuned from commensurate to incommensurate one by changing Fe stoimechiometry (W. Bao & Z.Q. Mao et al., Phys. Rev. Lett. 102, 247001 (2009).), in sharp contrast with the commensurate Fermi surface nesting-driven AFM order along the edge of the Fe square lattice seen in iron pnictides. This is also a highly cited work (cited +770 times, Google Scholar).
DOI:https://doi.org/10.1103/PhysRevB.80.174509, cited 260+ times (Google Scholar)
In this article, 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.