Review of annealing effects and superconductivity in Fe1+yTe1−xSex superconductors

Abstract

Among iron-based superconductors, Fe1+yTe1−xSex is unique in its structural simplicity, consisting of only FeTe/Se layers, which is favorable for probing the mechanism of superconductivity. Recently, a topological surface superconductivity as well as Majorana fermions has been observed, which makes Fe1+yTe1−xSex the first high-temperature topological superconductor. Thus, Fe1+yTe1−xSex is unique for the study of both high temperature and topological superconductivities. Since the large size single crystal of Fe1+yTe1−xSex can be easily grown, much research has been performed. However, a large number of the reported results are controversial, or differ between groups, including those related to basic properties such as resistivity, susceptibility, the Hall effect, gap structure, phase diagram, etc. These controversies are believed to stem from the sample-dependent Fe nonstoichiometries, which originate from the partial occupation of the second Fe site (the excess Fe site, expressed as y in the formula of Fe1+yTe1−xSex) in the Te/Se layer. The excess Fe with valence near Fe+ will provide electron doping into the system. Meanwhile, excess Fe is also strongly magnetic, which provides local moments that interact with the adjacent Fe layers. The magnetic moment from the excess Fe will act as a paring breaker and also localize the charge carriers. Thus, the existence of the excess Fe complicates the study of Fe1+yTe1−xSex from the superconducting to normal state properties. Removing excess Fe is essential in order to probe the intrinsic properties and mechanism of superconductivity, as well as for the applications of Fe1+yTe1−xSex compounds. In this topical review, we present an overview of the reported annealing methods, and conclude the effective approaches to remove the excess Fe in Fe1+yTe1−xSex. Furthermore, we discuss the mechanism of annealing based on the evolutions of structure, composition, and morphology. We also review the annealing effects on the normal state and superconducting properties, including the magnetism, transport properties, band structure, Tc, phase diagram, upper critical field, anisotropy, critical current density, gap structure, and superconducting pairing. This review presents not only the optimal way to prepare crystals without excess Fe, but also the intrinsic properties of Fe1+yTe1−xSex without the influence of excess Fe.