High-temperature superconductivity and its robustness against magnetic polarization in monolayer FeSe on EuTiO3

Abstract

Spin degree of freedom generally plays an important role in unconventional superconductivity. In many of the iron-based compounds, superconductivity is found in close proximity to long-range antiferromagnetic order, whereas monolayer FeSe grown on SrTiO3, with enhanced superconductivity, exhibits no magnetic or nematic ordering. Here we grow monolayer and multilayer FeSe on antiferromagnetic EuTiO3(001) layers, in an effort to introduce a spin polarization in proximity to the superconductivity of FeSe. By X-ray magnetic dichroism, we observe an antiferromagnet–ferromagnet switching on Eu and Ti sites in EuTiO3 driven by the applied magnetic field, with no concomitant spin polarization on the Fe site of FeSe. Transport measurements show enhanced superconductivity of monolayer FeSe on EuTiO3 with a transition temperature of ~30 K. The band structure revealed by photoemission spectroscopy is analogous to that of FeSe/SrTiO3. Our work creates a platform for the interplay of spin and unconventional superconductivity in the two-dimensional limit.

Modulation doping of the FeSe monolayer on SrTiO$_3$

“…In light of the very small number of electrons demanded by the FeSe monolayer, namely σ 0 ∼ 0.1 electrons per Fe, the STO substrate preannealed in UHV or doped with Nb (La) with a large number of free electrons ba-sically serves as an "infinite" charge reservoir. This also applies to other reported FeSe/Ti oxide interfaces [29][30][31][32][33][34] where the equilibrium charge transfer always occurs. The tetravalent Ti in these substrate surfaces induces similar polarization changes at the interface Se, which, together with the similar experimental work functions [44,53,54], results in nearly equal potential differences.…”

“…Vacuum level However, varying the modulation doping at FeSe/oxide interfaces has been challenging, and an outstanding puzzle from the past decade research is the universal FeSe doping about 0.1 electrons per Fe on a range of Ti oxide substrates [7,[29][30][31][32][33][34]. Although other mechanisms for the FeSe doping were suggested [34,35], we will concentrate on the study of charge transfer between the FeSe monolayer and STO using density functional theory (DFT) in order to find the crucial parameter that limits the monolayer doping.…”

“…It in turn leads to almost substrate independent FeSe doping, 0.1±0.02 electrons per Fe. The about 20% doping variation [29][30][31][32][33][34] is attributed to some differences in the work function and the details of the Ti oxide surface layers resulting from different crystal orientations and atomic structures of these titanates. Recently, the universal doping is expanded to the FeSe monolayer on Fe oxide substrates which have the work function ∼4.6 eV [55].…”

Nanomaterials for Quantum Information Science and Engineering

“…In this regard, 2D materials provide a notable advantage and research opportunity due to their atomically controlled thicknesses and dangling bond free surfaces. 2D superconductors demonstrate the potential for eventual use in superconducting qubits: TMD superconductors such as NbSe2 have a large critical field due to Ising SOC [345,360,590] , and FeSe monolayers epitaxially grown on oxide substrates is a novel high critical temperature superconductor [352,353,591] . All-2D JJs with highly transparent interfaces have been demonstrated [358,361] and remain promising.…”

“…2D transition metal carbides (TMCs) such as Mo2C have been fabricated and exhibit superconductivity in few-layer films [350] . Monolayer FeSe grown on SrTiO3 (STO) [351] and other oxide substrates [352,353] has emerged as a high-𝑇𝑇 𝐶𝐶 superconductor, with a 𝑇𝑇 𝐶𝐶 exceeding 100 K in some experiments. Unconventional superconductivity was discovered in Moiré superlattices of graphene (and graphene with hBN) arising from flat minibands [354][355][356] .…”