Superconductivity at 46 K in FeSe - Dr. Mahmoud Abdel-Hafiez

FeSe and doped FeSe exhibit intriguing and distinctive properties, which are currently the research focus in the field of high temperature superconductors. Undoped FeSe possesses a nematic order below 90 K and superconductivity of 8 K. The most mysterious property here is not even the pressure or strain induced Tc increase, but a giant enhancement of the superconductivity at the FeSe/SrTiO₃ interface, where SrTiO₃ (STO) has nothing in common with magnetic interaction. It seems that STO provides phonons that enhance superconductivity in single-layered FeSe. International collaborations including HPSTAR physicist, Dr. Mahmoud Abdel-Hafiez, gave a systematic study in doping controlled FeSe system, proposing that the enhanced superconductivity is independent of the thickness of FeSe while intrinsic to FeSe. However, it is under heated debate whether the nematic order, below 90 K in FeSe, is driven by spin or orbital fluctuations. These issues in FeSe superconductors have been published this year in Nat. Mater. 15, 159 Nat. Mater. 15, 159 (2016), recently in Nat. Comm. 7, 10840 (2016), and last year in Phys. Rev. B 91, 165109 (2015).

Iron-based superconductor with layered structure is currently the family with the second highest critical temperature, behind the cuprates. The observation of Tc from 65 K to 109 K in FeSe monolayer grown on SrTiO₃ (STO) substrates has attracted considerable attention and interest since 2012. Such a high Tc is unexpected in the FeSe system, because the bulk FeSe exhibits a Tc of only 8 K at ambient pressure. This discovery could be thought as a break-through in FeSe system in recent five years. These work proposed that interface between two different materials provided a potential pathway to high-Tc superconductivity. Also electron doping is considered as the other crucial factor for increasing Tc. The origin of high-temperature superconductivity in two-dimensional FeSe system is crucial for probing the superconductivity mechanism in the Fe-based superconductors, and thus providing insights on how to boost high-temperature superconductivity in general.

Caption: Phase diagram of electron doped FeSe.

The researchers used photoemission spectroscopy to directly probe the electron worlds of K doped thick FeSe films and FeSe0.93S0.07 bulk crystals, establishing the phase diagram of FeSe as a function of electron doping. They found that the correlation strength remarkably increases with increasing doping, while an insulating phase emerges in the heavily overdoped regime. Between the nematic phase and the insulating phase, a dome of enhanced superconductivity is observed, with the maximum superconducting transition temperature of about 46 K.

“The enhanced superconductivity is independent of the thickness of FeSe while intrinsic to K-dosed FeSe without the contribution from interface,” presented in the paper. This is inconsistent with the statement that interface plays a major role in enhanced superconductivity in doped FeSe film.

“It seems there is long way to clarify the debate between electron doping and interfacial effects,” said Dr. Abdel-Hafiez.