The thinnest high-temperature superconductor

Superconductivity is one of the most important topics in physics and materials science. The superconducting transition temperature (TC) of a quasi-2D superconductor, a superconductor with one dimension smaller than the coherence length of Cooper pairs, is normally descending with decreasing sample thickness since thermal and quantum fluctuations are expected to destroy the superconducting phase coherence even at low temperatures. The question of how thin a superconductor can be is not only crucial for understanding superconductivity in low-dimensional systems but also important for technological application in low-dissipation quantum devices. Nowadays, with the development of molecular beam epitaxy (MBE) technique, the fabrication of high-quality crystalline metallic films with thickness down to one unit-cell (1-UC) on semiconducting or insulating substrates becomes reality and makes it possible for the study of superconductivity at 2D limit. Different from traditional theoretical understanding of 2D superconductivity for free-standing films, in such film/substrate heterostructures the interface may dramatically affect the superconductivity and even possibly enhance TC compared with thicker film or bulk case. Recently, a research team led by Professor Qi-Kun Xue successfully grew 1-UC-thick and millimeterto centimeter-sized homogeneous FeSe films on SrTiO3 (STO) substrate by MBE, where a superconducting-like gap was discovered by in situ scanning tunnelingmicroscopy/spectroscopy (STM/ STS) study [1]. Surprisingly, the detected gap is 20 meV and almost 10 times larger than the gap of bulk FeSe. Soon after that, angle resolved photoemission spectroscopy experiments further revealed a nearly isotropic gap of above 15 meV, which closes at a temperature of around 65 K [2,3]. However, the question is whether the observed gap is a superconducting gap. Direct evidence of superconductivity in 1-UC FeSe films, such as zero resistance and Meissner effect, is highly desired. Very recently, for the first time Professor Jian Wang and Professor Qi-Kun Xue’s collaboration team