Discrete superconducting phases in FeSe-derived superconductors. Phys. Rev. Lett. 121, 207003 (2018)
A general feature of unconventional superconductors is the existence of a superconducting dome in the phase diagram, i.e., a continuous increase, maximum, and then decrease of the transition temperature Tc, with charge carrier doping, applied external pressure, or isovalent doping. Such an SC dome is a common feature shared by other unconventional superconductors, such as cuprate, heavy-fermion, and organic superconductors. Iron selenide has recently attracted great interest. However, attempts to build a phase diagram have been hampered by the limited control over the intercalation of metals and sample inhomogeneity.
In this work, by performing developed in-situ solid ionic gating technique, Tianping Yin et al. continuously intercalate alkali metal into FeSe thin flake with a precision of 1%. Based on the experiments, three main findings are summarized. 1. The discreteness of superconducting phase diagram in FeSe-derived superconductors is universal, which contrasts sharply to other unconventional superconductors. 2. A new superconducting phase of Li0.17FeSe with Tc at 36 K is discovered for the first time in the underdoped region. 3. We discovered the importance of the Fe site for the observation of the discrete superconducting phases. These discrete SC phases are easily destroyed by a small amount of copper substitution at the iron site, but is insensitive to sulfur substitution at the selenium site. These results offer an insight into the mechanism of superconductivity in FeSe-derived superconductors.
