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

X Shi,X-X Wu,P Richard,J P Hu,S C Wang,H Ding,Z-Q Han,X-L Peng,Y-J Sun,T Qian,M-W Qiu

doi:10.1038/ncomms14988

Abstract

The origin of enhanced superconductivity over 50 K in the recently discovered FeSe monolayer films grown on SrTiO3 (STO), as compared to 8 K in bulk FeSe, is intensely debated. As with the ferrochalcogenides AxFe2−ySe2 and potassium-doped FeSe, which also have a relatively high-superconducting critical temperature (Tc), 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 Tc. However, the fundamental reasons explaining this increase of Tc remain unclear. Here we report a 15 K jump of Tc accompanying a second Lifshitz transition characterized by the emergence of an electron pocket at the Brillouin zone centre, which 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 in generating enhanced superconductivity in FeSe.

Highlights

The origin of enhanced superconductivity over 50 K in the recently discovered FeSe monolayer films grown on SrTiO3 (STO), as compared to 8 K in bulk FeSe, is intensely debated
As determined in previous angle-resolved photoemission spectroscopy (ARPES) studies[2,3] and in our current experiment, the Fermi surface (FS) topology of FeSe/STO(001) monolayer films shown in Fig. 1a consists of nearly doubly degenerate electron-like pockets centred at the M point, in contrast to FeSe bulk crystals[16] and most of the ferropnictide superconductors[17]
The dxz/dyz character of the orbitals sinking below EF across the first Lifshitz transition (accompanying a jump of Tc) is different from the pz/dxy orbital character emerging at G across the second Lifshitz transition

Summary

Introduction

The origin of enhanced superconductivity over 50 K in the recently discovered FeSe monolayer films grown on SrTiO3 (STO), as compared to 8 K in bulk FeSe, is intensely debated. FeSe-based materials with relatively high Tc0s share one common key point in their Fermi surface (FS) topology: the absence of hole pockets at the Brillouin zone (BZ) centre[9,10,11]. The importance of this FS topology to superconductivity has been further supported by doping electron carriers on the surface of FeSe films or crystals using potassium deposition[12,13,14], or in their bulk using liquidgating technique[15]. Starting from a sample annealed at 350 °C for 20 h, which already transfers a relatively high-electron concentration into the samples, as confirmed by large electron FS pockets, we deposit K atoms in situ onto the surface and achieve a higher doping level

Methods

Results

Conclusion