For nearly a decade, scientists have investigated the remarkable high-temperature superconductivity of an iron-based thin film consisting of a single layer of iron selenide (FeSe) on a substrate of strontium titanate (SrTiO₃). A single monolayer of FeSe/SrTiO₃ becomes superconducting at a temperature much higher than found for bulk FeSe. Past research indicated that the interface between the FeSe monolayer and its substrate plays a crucial role in the material's superconducting properties. In spite of the importance of this interface, there has been uncertainty about many of its fundamental features, such as the distance between the FeSe monolayer and the substrate. To determine this and other key features, researchers in this study employed high-brightness x-rays at the U.S. Department of Energy’s Advanced Photon Source (APS) to help resolve the interfacial structure of monolayer FeSe/SrTiO₃ at the picoscale, equivalent to one-thousandth of a nanometer. It is anticipated that the new data about the interfacial structure, published in the journal Science Advances, will guide development of refined theoretical models, not only of superconductivity in the FeSe/SrTiO₃ system but of thin-film superconductivity in general. Understanding the exact mechanisms responsible for thin-film superconductivity is essential for applying these materials to practical high-temperature superconducting devices and developing better performing superconductors.

The temperature at which a material begins to superconduct is called the superconducting transition temperature, or T_C. Transition temperatures for FeSe/SrTiO₃ can be as high as 70 to 109 kelvin (70–109 K). The variation in T_C is due to the different ways that individual FeSe/SrTiO₃ samples are created and processed. One aim of this research was to identify the processing techniques yielding the highest transition temperatures for the FeSe/SrTiO₃ material.

The FeSe/SrTiO₃ system is grown as an epitaxial film. In this process, a highly pure SrTiO₃ substrate surface is prepared first. Then iron and selenium atoms are deposited on the substrate to form a single atomic layer of FeSe, which aligns with the substrate's crystalline lattice. This means that the conjoined FeSe and SrTiO₃ compounds share a common crystallographic relationship, or registry. Although the lattices of the two compounds possess similar structural symmetries, subtle variations often occur that can have a substantial impact on properties. For instance, the angles formed by the bonds between the Fe and Se atoms can be strongly influenced by forces applied by the substrate.

Changes observed in the registry for FeSe thicker than a monolayer are one reason for the observed variation in their superconducting transition temperatures. Other reasons for reduced T_C include improper annealing, which involves mildly heating the sample to reduce distortions, and capping the FeSe monolayer with a protective thin film.

In the search for the registry yielding the highest T_C, the scientists were guided by previous research that identified two candidates exhibiting the highest symmetry between the FeSe monolayer and the substrate. In situ Surface x-ray diffraction (SXRD) experiments conducted at the X-ray Science Division Surface Scattering & Microdiffraction Group’s 33-ID-E x-ray beamline at the APS (an Office of Science user facility at Argonne National Laboratory) established which of the two candidate registries actually existed in the highest quality FeSe/SrTiO₃ samples. This determination relied on SXRD diffraction patterns called crystal truncation rods (CTRs). Only one of the two candidate registries fit the CTR patterns generated by the SXRD experiments (Fig. 1).

After identifying the proper registry, the researchers could then calculate the distances between each layer of atoms in the FeSe/SrTiO₃ system. The results reveal an asymmetry in the FeSe monolayer (upper portion of Fig. 1). The distance between the Fe layer (brown spheres in Fig. 1) and the upper and lower Se layers are 1.43 angstroms (1.43 Å) and 1.35 Å, respectively. Additionally, the distance between the entire FeSe monolayer and the top layers of the substrate (blue and red spheres in Fig. 1) is nearly 3 Å.

The SXRD measurements are considered highly reliable, partly because they were performed in situ, meaning that the data was collected from thin film samples grown in a ultra high vacuum x-ray scattering system. In contrast, previous studies mainly relied on ex situ measurements that probed cross sections of segmented samples monolayer films protected by thick films of Se or FeTe. Ex situ methods can distort the thin-film system and is a potential reason why previous experiments assigned different values for the interlayer distances.

The accurate determination of the registry and atomic-layer distances within FeSe/SrTiO₃ allowed the researchers to calculate other important interfacial parameters, such as the bond angles between the iron and selenium atoms in the FeSe monolayer. This new picoscale knowledge of FeSe/SrTiO₃ will help scientists improve their models of how superconductivity operates in this and other thin-film systems. ― Philip Koth

See: Rui Peng^1,2*, Ke Zou^1‡, M. G. Han⁴, Stephen D. Albright¹, Hawoong Hong⁴, Claudia Lau¹, H. C. Xu^1,2, Yimei Zhu³, F. J. Walker¹**, and C. H. Ahn¹***, “Picoscale structural insight into superconductivity of monolayer FeSe/SrTiO₃” Sci. Adv. 6, eaay4517 (8 April 2020). DOI: 10.1126/sciadv.aay4517

Author affiliations: ¹Yale University, ²Fudan University, ³Brookhaven National Laboratory, ⁴Argonne National Laboratory ^‡Present address: University of British Columbia

Correspondence: * [email protected], ** [email protected],*** [email protected]

This work was supported by AFOSR grant FA9550-15-1-0472. The work at Brookhaven National Laboratory was supported by the U.S. Department of Energy (DOE) Office of Science-Basic Energy Sciences, under contract no.DESC0012704. TEM sample preparation using FIB was performed at the Center for Functional Nanomaterials, Brookhaven National Laboratory. This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357.

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