Research on superconductivity of epitaxial FeSe films

Abstract

The newly discovered iron-based superconductors in 2008 have become the second class of high temperature superconductor besides the cuprates. Among all these iron-based compounds, β-FeSe has the simplest chemical structure and can be an archetype system for unraveling the mechanism of superconductivity. By using state-of-the-art molecular beam epitaxy (MBE) technique, high-quality superconducting FeSe single crystalline films with finely controllable topography and composition have been successfully prepared, with their superconducting properties extensively investigated. The low-temperature scanning tunneling spectra reveal that the superconducting gap in the quasiparticle density of states is visible down to two unit cells of FeSe films, and evidence for a gap function with nodal lines. Electron pairing with two-fold symmetry has also been demonstrated by direct imaging of quasiparticle excitations in the vicinity of magnetic vortex cores, Fe adatoms and Se vacancies. The two-fold pairing symmetry is supported by our observation of striped electronic nanostructures in the slightly Se-doped samples, primarily due to the orbital ordering. Twin boundaries run at approximately 45° to the Fe-Fe bond directions, and noticeably suppress the superconducting gap and pin magnetic vortices. This is likely caused by the increased Se height in the vicinity of twin boundaries, providing the first local evidence for the importance of this height to the mechanism of superconductivity. Furthermore, we reveal signatures of a bosonic mode in the local quasiparticle density of states of superconducting FeSe films, whose energy reduces with decreasing gap magnitude Ω . Recently, the growth recipe has been further extended to grow FeSe film on SrTiO3(001) substrate, leading to the high- T c superconductivity in the very FeSe/SrTiO3 interface. A superconducting gap as large as 20 meV and the magnetic field induced vortex state revealed by in situ scanning tunneling microscopy suggest that the superconductivity of single-unit-cell FeSe films could occur around 77 K. Our subsequent transport and measurements of the one-unit-cell thick FeSe films on insulating SrTiO3 substrates with non-superconducting FeTe protection layers provide definitive evidence for high temperature superconductivity with an onset T c above 40 K which are much higher than T c~9.4 K for bulk FeSe, respectively. The finding provides a completely new avenue and method to reveal the pairing mechanism of high- T c superconductors, and to further enhance T c, stirring up worldwide increasing attention in the field of high- T c superconductors and material sciences.