paper

Enhanced superconductivity accompanying a Lifshitz transition in electron-doped FeSe monolayer

arXiv:1606.01470 · doi:10.1038/ncomms14988

Abstract

The origin of enhanced superconductivity over 50 K in the recently discovered FeSe monolayer films grown on SrTiO $_{3}$ (STO), as compared to 8 K in bulk FeSe, is intensely debated. As with the ferrochalcogenides A $_{x}$ Fe $_{2 - y}$ Se $_{2}$ and potassium doped FeSe, which also have a relatively high superconducting critical temperature ( $T_{c}$ ), the Fermi surface (FS) of the FeSe/STO monolayer films is free of hole-like FS, suggesting that a Lifshitz transition by which these hole FSs vanish may help increasing $T_{c}$ . However, the fundamental reasons explaining this increase of $T_{c}$ remain unclear. Here we report a 15 K jump of $T_{c}$ accompanying a second Lifshitz transition, characterized by the emergence of an electron pocket at the Brillouin zone (BZ) centre, that is triggered by high electron doping following in-situ deposition of potassium on FeSe/STO monolayer films. Our results suggest that the pairing interactions are orbital-dependent with the $d_{x y}$ orbital playing a determining role in generating enhanced superconductivity in FeSe.

20 pages, 8 figures