Review of annealing effects and superconductivity in Fe_1+<i>y</i>Te_1−<i>x</i>Se_<i>x</i> superconductors

Abstract

Fe$_{1+y}$Te$_{1-x}$Se$_x$ is unique in their structural simplicity,\nconsisting of only FeTe/Se layers, which is favorable for probing the mechanism\nof superconductivity. Recently, a topological surface superconductivity as well\nas the Majorana Fermions has been observed, which makes\nFe$_{1+y}$Te$_{1-x}$Se$_x$ the first high temperature topological\nsuperconductor. Since large size single crystal of Fe$_{1+y}$Te$_{1-x}$Se$_x$\ncan be easily grown, many researches have been performed. However, a large part\nof the reported results are under controversy, including the resistivity,\nsusceptibility, Hall effect, gap structure, phase diagram, etc. These\ncontroversies are believed to come from the sample-dependent Fe\nnonstoichiometries, which originate from the partial occupation of the second\nFe site (excess Fe) in the Te/Se layer. The excess Fe with valence near Fe$^+$\nwill provide electron doping into the system. Meanwhile, the excess Fe is also\nstrongly magnetic, which will act as a paring breaker and also localize the\ncharge carriers. Removing the excess Fe is essential to probe the intrinsic\nproperties and mechanism of superconductivity of Fe$_{1+y}$Te$_{1-x}$Se$_x$\ncompounds. In this topical review, we propose the effective approaches to\nremove excess Fe in Fe$_{1+y}$Te$_{1-x}$Se$_x$. Furthermore, we discuss the\nmechanism of annealing based on the evolutions of structure, composition, and\nmorphology with annealing. Moreover, we also review the annealing effects on\nthe normal state and superconducting properties, including the magnetism,\ntransport properties, band structure, $T_{\\rm{c}}$, phase diagram, upper\ncritical field, anisotropy, critical current density, gap structure, and\nsuperconducting pairing. This review presents not only the optimal way to\nprepare crystals without excess Fe, but also the intrinsic properties of\nFe$_{1+y}$Te$_{1-x}$Se$_x$ without the influence of excess Fe.\n