Direct Observation of High-Temperature Superconductivity in One-Unit-Cell FeSe Films

TLDR

Interface engineering of heterostructures has enabled discovery of new superconductors and raised Tc, as shown by one‑unit‑cell FeSe on SrTiO₃ where STM/STS revealed a ~20 meV gap and ARPES found an isotropic ~15 meV gap closing near 65 K, suggesting the thinnest high‑Tc superconductor. This study aims to prepare 1‑UC FeSe films on insulating SrTiO₃ with FeTe capping to enable direct transport measurements. The authors grew 1‑UC FeSe on insulating SrTiO₃ substrates and protected the films with non‑superconducting FeTe layers to allow transport and magnetic characterization. Transport and magnetic measurements show high‑temperature superconductivity in the 1‑UC FeSe films, with an onset Tc above 40 K and a critical current density of ~1.7×10⁶ A cm⁻² at 2 K, demonstrating the effectiveness of interface engineering.

Abstract

Heterostructure based interface engineering has been proved an effective method for finding new superconducting systems and raising superconductivity transition temperature (TC). In previous work on one unit-cell (UC) thick FeSe films on SrTiO3 (STO) substrate, a superconducting-like energy gap as large as 20 meV, was revealed by in situ scanning tunneling microscopy/spectroscopy (STM/STS). Angle resolved photoemission spectroscopy (ARPES) further revealed a nearly isotropic gap of above 15 meV, which closes at a temperature of ~ 65 K. If this transition is indeed the superconducting transition, then the 1-UC FeSe represents the thinnest high TC superconductor discovered so far. However, up to date direct transport measurement of the 1-UC FeSe films has not been reported, mainly because growth of large scale 1-UC FeSe films is challenging and the 1-UC FeSe films are too thin to survive in atmosphere. In this work, we successfully prepared 1-UC FeSe films on insulating STO substrates with non-superconducting FeTe protection layers. By direct transport and magnetic measurements, we provide definitive evidence for high temperature superconductivity in the 1-UC FeSe films with an onset TC above 40 K and a extremely large critical current density JC ~ 1.7*106 A/cm2 at 2 K. Our work may pave the way to enhancing and tailoring superconductivity by interface engineering.