Recently, the Topological Quantum Computation Group and Low-dimensional Quantum Materials Group at Beijing Academy of Quantum Information Sciences (BAQIS) collaborated with the Department of Physics at Tsinghua University, to establish the direct relationships between superconducting gaps and electrical transport properties in one-unit-cell FeSe films and provide new experimental evidence for understanding the mechanism of its interface superconductivity, by using a homemade molecular beam epitaxy, scanning tunneling microscope and in-situ electrical transport measurement combined system. The research results were published in Nature Communications entitled “Electronic inhomogeneity and phase fluctuation in one-unit-cell FeSe films” on April 20, 2024.

The interface-enhanced superconductivity in one-unit-cell FeSe films on SrTiO₃ substrates is an important discovery in the area of high-temperature superconductivity in recent years. However, one-unit-cell FeSe films are extremely air-sensitive, impeding its ex-situ transport measurements. To overcome this difficulty, researchers designed and developed an ultrahigh vacuum molecular beam epitaxy, scanning tunneling microscope, and in-situ electrical transport measurement combined system (Rev. Sci. Instrum. 91, 063902 (2020)). Within such a system, researchers could in-situ study the electrical properties and scanning tunneling spectroscopy of microscale one-unit-cell FeSe films simultaneously in ultrahigh vacuum, without the influence of atmosphere and inhomogeneity in large scale samples.

(a) Schematic of the measurement setup. (b) The resistance and the gap magnitude as a function of temperature in one-unit-cell FeSe films. (c) A two-step nonlinear V(I) behavior revealed by the microscale electrical transport measurement.

The experimental findings present that one-unit-cell FeSe films with twin boundaries can be regarded as a percolative system consisting of two superconducting phases. The superconducting gaps are about 15 meV and 10 meV, respectively. The electrical transport measurements show a two-step nonlinear V(I) behavior. Moreover, the onset transition temperature and zero-resistance temperature are consistent with the pairing temperature of dominated 15 meV phase and BKT transition temperature, respectively. The results indicate the broadened superconducting transition in FeSe/SrTiO₃ is related to intrinsic electronic inhomogeneity due to distinct two-gap features and phase fluctuations of two-dimensional superconductivity. This study could help understand the superconducting behavior and superconducting mechanism of this system.

The co-first authors of this paper are Dapeng Zhao, Wenqiang Cui, assistant researchers at BAQIS, and Yaowu Liu from Tsinghua University. The corresponding authors are Professor Qi-Kun Xue, the president of BAQIS, associate researcher Lili Wang from Tsinghua University, and Professor Yilin Wang from Shandong University. Liguo Zhang, Yunyi Zang, associate researchers of BAQIS and Ding Zhang, Ke He, adjunct scientists of BAQIS, also participate in this work. This work is supported by the National Natural Science Foundation of China, the Innovation Program for Quantum Science and Technology and Beijing Natural Science Foundation.

Link to the article: https://www.nature.com/articles/s41467-024-47350-0#Sec7