Bekaert J, Bignardi L, Aperis A, van Abswoude P, Mattevi C, Gorovikov S, Petaccia L, Goldoni A, Partoens B, Oppeneer P M, Peeters F M, Milošević M V, Rudolf P, Cepek C
Department of Physics, University of Antwerp, Groenenborgerlaan 171, B-2020, Antwerp, Belgium.
Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747AG, Groningen, The Netherlands.
Sci Rep. 2017 Oct 31;7(1):14458. doi: 10.1038/s41598-017-13913-z.
Two-dimensional materials are known to harbour properties very different from those of their bulk counterparts. Recent years have seen the rise of atomically thin superconductors, with a caveat that superconductivity is strongly depleted unless enhanced by specific substrates, intercalants or adatoms. Surprisingly, the role in superconductivity of electronic states originating from simple free surfaces of two-dimensional materials has remained elusive to date. Here, based on first-principles calculations, anisotropic Eliashberg theory, and angle-resolved photoemission spectroscopy (ARPES), we show that surface states in few-monolayer MgB make a major contribution to the superconducting gap spectrum and density of states, clearly distinct from the widely known, bulk-like σ- and π-gaps. As a proof of principle, we predict and measure the gap opening on the magnesium-based surface band up to a critical temperature as high as ~30 K for merely six monolayers thick MgB. These findings establish free surfaces as an unavoidable ingredient in understanding and further tailoring of superconductivity in atomically thin materials.
众所周知，二维材料具有与其体相材料截然不同的特性。近年来，原子级超薄超导体不断涌现，但有一个限制条件，即除非通过特定的衬底、插层剂或吸附原子来增强，否则超导电性会严重减弱。令人惊讶的是，迄今为止，源自二维材料简单自由表面的电子态在超导性中所起的作用仍不明确。在此，基于第一性原理计算、各向异性埃利亚什贝格理论和角分辨光电子能谱（ARPES），我们表明，少层MgB中的表面态对超导能隙谱和态密度有主要贡献，这与广为人知的类体相σ-和π-能隙明显不同。作为原理验证，对于仅六个单层厚的MgB，我们预测并测量了镁基表面带上高达约30 K的临界温度时的能隙开启情况。这些发现表明，自由表面是理解和进一步调控原子级超薄材料中超导电性时不可避免的一个因素。
