Recent advances in β-FeSe(1−x) and related superconductors

It has been more than four years since the discovery of β-FeSe(1−x) superconductors. Through the efforts of many outstanding research groups, unprecedented advances in the field have been achieved. High-quality single crystals of β-FeSe(1−x) and related compounds have been prepared by various techniques, allowing us to explore in detail the physical properties of this class of materials. Detailed characterizations of the structure and properties of these crystals have helped us to understand the origin of superconductivity in β-FeSe(1−x). The occurrence of superconductivity is associated with the low-temperature structure distortion, which is accompanied by several anomalies. Recent measurements on quasiparticle and acoustic phonon dynamics with respect to the orbital modification in β-FeSe(1−x) suggest the opening of an energy gap below 130–140 K, accompanied by a coincident transfer of optical spectral weight in the visible range and alterations in transport properties. These observations provide convincing evidence that the modification of the electronic structure occurs prior to the lattice distortion. They further suggest that the high-temperature gap and the lattice symmetry breaking are driven by short-range orbital and/or charge orders.