Zefeng Lin^1*, Yuting Zhang^1,2*, Kaibing Wen^1,2,Qihong Chen^1,2†, Kui Jin^{1, 2†}

1. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

2. School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China

FeSe-based superconductors have attracted extensive attention because of their simple crystal structure, rich physical properties, and highly tunable superconducting transition temperature (Tc). Although FeSe exhibits a Tc of only about 8 K at ambient pressure, its superconducting transition temperature can be significantly enhanced to about 40 K by various approaches, including pressure, ionic-liquid gating, surface deposition, and intercalation.Beyond these tuning methods, interfacial effects can further enhance superconductivity in FeSe-based systems, as exemplified by monolayer FeSe/SrTiO3 grown by molecular beam epitaxy (MBE), which was reported to exhibit superconducting gap opening above 65 K. The Tc enhancement is widely believed to originate from interfacial effects, especially electron doping from SrTiO3 and electron-phonon coupling with SrTiO3 phonons. Compared with MBE, pulsed laser deposition (PLD) offers unique advantages for the growth of protective capping layers, complex interfaces and superlattices, thus providing a promising route for systematically investigating interfacial superconductivity in the FeSe/SrTiO3 system.

In this work, a series of high-quality FeSe/SrTiO3 ultrathin films with tunable thicknesses of approximately 4–18 nm were grown by PLD through systematic optimization of the growth conditions, including the laser pulse frequency and laser energy. Clear reflection high-energy electron diffraction (RHEED) intensity oscillations were observed during growth, indicating a layer-by-layer growth mode. The RHEED oscillation behavior is comparable to that reported for MBE-grown FeSe/SrTiO3 films, suggesting a similar level of epitaxial quality. X-ray diffraction (XRD) measurements show that, apart from the diffraction peaks of the SrTiO3 substrate, only the FeSe (00l) reflections are observed, with no detectable impurity phases, indicating high phase purity and preferred c-axis orientation. Pronounced Laue oscillations around the FeSe (001) reflection, together with smooth and well-defined oscillations in X-ray reflectivity (XRR), further confirm the flat surface and uniform thickness of the films. As the film thickness increases, the diffraction peaks gradually become narrower, the Laue oscillation period decreases.

Transport measurements show that all films exhibit metallic behavior, with Tc,onset above 30 K, significantly higher than that of bulk FeSe under ambient pressure. The present samples exhibit higher Tc values than previously reported PLD-grown FeSe/SrTiO3 films of comparable thickness.At a thickness of 8 unit cells (UC), the films exhibit Tc,onset of ∼52 K and Tc,0∼30 K, representing the highest values reported so far for PLD-grown FeSe films and approaching the superconducting performance of MBE-grown monolayer FeSe. Further analysis reveals a clear thickness dependence of superconductivity: as the thickness decreases, Tc,onset increases, and the zero-resistance transition temperature (Tc,0) also shows an overall upward trend. This behavior is particularly pronounced for films thinner than about 33 unit cells, whereas Tc becomes much less sensitive to thickness above this range. This thickness dependence is consistent with previous results for MBE-grown FeSe/SrTiO3 films, but differs markedly from that of conventional superconducting thin films, suggesting that interfacial effects play an important role in the superconducting properties of FeSe/SrTiO3 films.

This work not only demonstrates the effectiveness of PLD for preparing high-quality, high-Tc FeSe/SrTiO3 ultrathin films, but also provides an experimental basis for the future fabrication of FeSe/SrTiO3 superlattices and for further enhancement of Tc through multi-interface engineering.