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

Superconductivity transition temperature (T_c) marks the inception of a macroscopic quantum phase-coherent paired state in fermionic systems. For 2D superconductivity, the paired electrons condense into a coherent superfluid state at T_c, which is usually lower than the pairing temperature, between which intrinsic physics including Berezinskii-Kosterlitz-Thouless transition and pseudogap state are hotly debated. In the case of monolayer FeSe superconducting films on SrTiO₃(001), although the pairing temperature (T_p) is revealed to be 65-83 K by using spectroscopy characterization, the measured zero-resistance temperature ([Formula: see text]) is limited to 20 K. Here, we report significantly enhanced superconductivity in monolayer FeSe films by δ-doping of Eu or Al on SrTiO₃(001) surface, in which [Formula: see text] is enhanced by 12 K with a narrowed transition width ΔT_c ∼ 8 K, compared with non-doped samples. Using scanning tunneling microscopy/spectroscopy measurements, we demonstrate lowered work function of the δ-doped SrTiO₃(001) surface and enlarged superconducting gaps in the monolayer FeSe with improved morphology/electronic homogeneity. Our work provides a practical route to enhance 2D superconductivity by using interface engineering.