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

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.

FeTe1−Se monolayer films: towards the realization of high-temperature connate topological superconductivity

“…Strong intensity appears at the BZ center as the Se decreases. We will explain below that this originates from an electron band in the unoccupied side, similar to the case of K deposited FeSe/STO monolayer [12] and K deposited bulk FeTe 1-x Se x [23].…”

“…The FeSe/STO monolayer films possess a simple FS topology characterized by large electronlike pockets at the M point and the absence of hole-like pockets at Γ [8,9,12], which differs from that of FeSe bulk materials and most of the iron-pnictide superconductors [19]. We show in Fig.…”

“…Some iron-based superconductors have been proposed to fulfill this type of realization. For example, the monolayers of FeSe grown on SrTiO 3 (STO) substrates, which have a maximum T c higher than 50 K [7,8,9,10,11,12,13], may have a nontrivial topology due to band inversion at the Brillouin zone boundary (M point) [14,15].…”

“…Monolayer films of FeTe 1-x Se x were epitaxially grown on 0.05wt% Nb-doped STO(001) substrates by the same process as FeSe/STO(001) described in our previous work [12]. We have grown samples with different Se concentrations by tuning the flux ratio between Se (99.999%) and Te (99.99%).…”

Role of Electric Fields on Enhanced Electron Correlation in Surface-Doped FeSe

“…Observations of a superconducting T c as high as 100 K in a monolayer (ML) FeSe/SrTiO 3 (STO) system [1][2][3] have intensified interest in electron-doped FeSe systems [4][5][6]. In the FeSe/STO system, electron transfer from the STO substrate to FeSe appears to be a key ingredient for realizing superconductivity [2,7], and additional electron doping to the system by potassium (K) dosing increases T c [8]. Moreover, surface doping experiments by K dosing [9][10][11][12], Na dosing [13], and liquid gating [14] have shown that electron doping can also increase T c for bulk, thick-film, and multilayer FeSe.…”