Abstract
Binary Iron selenide (FeSe) thin films have been widely studied for years to unveil the high temperature superconductivity in iron-based superconductors. However, the origin of superconducting transition in this unconventional system is still under debate and worth deep investigations. In the present work, the transition from insulator to superconductor was achieved in non-superconducting FeSe ultrathin films (~8 nm) grown on calcium fluoride substrates via a simple in-situ Mg-coating by a pulsed laser deposition technique. The Mg-coated FeSe film with an optimized amount of Mg exhibited a superconducting critical temperature as 9.7 K and an upper critical field as 30.9 T. Through systematic characterizations on phase identification, carrier transport behavior and high-resolution microstructural features, the revival of superconductivity in FeSe ultrathin films is mostly attributed to the highly crystallized FeSe and extra electron doping received from external Mg-coating process. Although the top few FeSe layers are incorporated with Mg, most FeSe layers are intact and protected by a stable magnesium oxide layer. This work provides a new strategy to induce superconductivity in FeSe films with non-superconducting behavior, which might contribute to a more comprehensive understanding of iron-based superconductivity and the benefit to downstream applications such as magnetic resonance imaging, high-field magnets and electrical cables.
Highlights
Among the family of electrical materials, high-temperature superconducting material always attracts considerable attention considering its huge potential in high-efficiency electric transport and high-field magnets, and due to the probable complement in condensed matter physics [1]
Iron-based superconductors [2,3,4,5] are considered one of the promising candidates that might unveil the mechanism of high-temperature superconductivity (HTS) since dramatic enhancement in critical temperature (Tc ) has been repeatedly achieved in binary iron selenide (FeSe) composite through a variety of strategies, including elemental substitution [6,7,8]/intercalation [9,10], pressurization [11,12], liquidgate [13], and the substrate-induced heavy electron doping into FeSe unit cells [14,15,16]
Typical XRD θ-2θ results for the pristine FeSe ultrathin film (#UFM0) and Mg-coated
Summary
Among the family of electrical materials, high-temperature superconducting material always attracts considerable attention considering its huge potential in high-efficiency electric transport and high-field magnets, and due to the probable complement in condensed matter physics [1]. Iron-based superconductors [2,3,4,5] are considered one of the promising candidates that might unveil the mechanism of high-temperature superconductivity (HTS) since dramatic enhancement in critical temperature (Tc ) has been repeatedly achieved in binary iron selenide (FeSe) composite through a variety of strategies, including elemental substitution [6,7,8]/intercalation [9,10], pressurization [11,12], liquidgate [13], and the substrate-induced heavy electron doping into FeSe unit cells [14,15,16]. It is well accepted that the limited electron dopants transferred from STO substrate is not sufficient to drive the HTS in the second and beyond unit-cells [14,17,18,19]
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