Enhanced superconductivity in monolayer FeSe films on SrTiO3(001) via metallic δ-doping

Interface engineering has been proven to be effective in discovering new quantum states, such as topological states, superconductivity, charge density waves, magnetism, etc., which require atomic-scale heterostructure fabrication. Monolayer FeSe on SrTiO₃ substrates has attracted intense interest owing to its remarkable interface-enhanced superconductivity (Wang et al.，Chin. Phys. Lett. 29, 037402 (2012); Zhang et al.，Chin. Phys. Lett. 31, 017401 (2014)). Previous experimental investigations disclosed significant interfacial electron transfer to the FeSe monolayer, from the TiO_2-δ charge reservoir layer with oxygen vacancies as the intrinsic donors. Moreover, the monolayer FeSe exhibits additionally enlarged gap magnitudes than other electron-doped FeSe (i.e., 15-20 meV vs. 12 meV), which has been attributed to the cooperative contribution from electron-phonon coupling with specific longitudinal optical phonon modes from TiO_2-δ surfaces (Lee et al.，Nature 515, 245 (2014)). Incoherent Cooper pairing and pseudogap have been claimed because of the remarkably lower zero-resistance temperature achieved hitherto than the gap-opening temperature (65-83 K) (He et al.，Nat. Mater. 12, 605 (2013)；Xu et al.，Nat. Commun. 12, 2840 (2021)). Previous scanning tunneling microscopy/spectroscopy characterization disclosed dense domains in the monolayer FeSe films, and the superconducting gaps are suppressed around domain boundaries and even vanished in nanometer-scales domains (Jiao et al.，Phys. Rev. M 6, 064803 (2022)). The domains originate from the low-temperature (105 K) antiferrodistortive phase transition in bulk-SrTiO₃. Previous efforts on improving the uniformity in the monolayer FeSe are usually at the expense of weakening the interface coupling, or vice versa.

Qi-Kun Xue and Lili Wang from Tsinghua University and Minghu Pan from Shaanxi Normal University reported enhanced interface coupling as well as improved spatial uniformity in monolayer FeSe films on SrTiO₃(001) via metallic δ-doping (Au and Al atoms), and hence, enhanced superconductivity. The Al and Eu atoms with higher oxygen affinity than Ti scavenge oxygen from the TiO_2-δ surface, thus increasing the density of the surface oxygen vacancies but preventing their clustering, as revealed by lowered work functions with reduced electronic variation. Monolayer FeSe films on such δ-doped SrTiO₃(001) exhibit reduced intensity of domains and generally enlarged superconducting gaps, indicating the strengthened Cooper pairing with improved electronic homogeneity. Consequently, the temperature-dependent resistivity measurements disclosed an onset transition temperature of 53 K and a zero-resistance temperature of 27 K. This work has been published in the 4^th issue of the National Science Review in 2024, titled “Significantly enhanced superconductivity in monolayer FeSe films on SrTiO3(001) via metallic δ-doping”. Xiaotong Jiao from Shaanxi Normal University), Wenfeng Dong, Dr. Mingxia Shi and Dr. Heng Wang from Tsinghua University contributed equally to this work.

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