Dumbbell Defects in FeSe Films: A Scanning Tunneling Microscopy and First-Principles Investigation

Abstract

The properties of iron-based superconductors (Fe-SCs) can be varied dramatically with the introduction of dopants and atomic defects. As a pressing example, FeSe, parent phase of the highest-$T_c$ Fe-SC, exhibits prevalent defects with atomic-scale "dumbbell" signatures as imaged by scanning tunneling microscopy (STM). These defects spoil superconductivity when their concentration exceeds 2.5%. Resolving their chemical identity is prerequisite to applications such as nanoscale patterning of superconducting/nonsuperconducting regions in FeSe, as well as fundamental questions such as the mechanism of superconductivity and the path by which the defects destroy it. We use STM and density functional theory to characterize and identify the dumbbell defects. In contrast to previous speculations about Se adsorbates or substitutions, we find that an Fe-site vacancy is the most energetically favorable defect in Se-rich conditions, and reproduces our observed STM signature. Our calculations shed light more generally on the nature of Se capping, the removal of Fe vacancies via annealing, and their ordering into a $\sqrt{5}$$\times$$\sqrt{5}$ superstructure in FeSe and related alkali-doped compounds.

arXiv comment7 pages, 5 figures

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• Discovery of Orbital-Selective Cooper Pairing in FeSe
• High-temperature superconductivity in one-unit-cell FeSe films
• Incoherent Cooper pairing and pseudogap behavior in single-layer FeSe/SrTiO${}_3$
• Superconducting gap structure of FeSe
• Impact of Iron-site defects on Superconductivity in LiFeAs
• An extensive impurity-scattering study on the pairing symmetry of monolayer FeSe films on SrTiO3
• Tuning stoichiometry and its impact on superconductivity of monolayer and multilayer FeSe on SrTiO3
• Absence of superconductivity in ultra-thin layers of FeSe synthesized on a topological insulator
• Impurity-induced bound states inside the superconducting gap of FeSe
• Visualization of local magnetic moments emerging from impurities in the Hund's metal states of FeSe
• Charge order driven by multiple-Q spin fluctuations in heavily electron-doped iron selenide superconductors
• Stoichiometry and defect superstructures in epitaxial FeSe films on SrTiO3
• The study of intrinsic defect state of FeSe with scanning tunneling microscopy
• Checkerboard order state in superconducting FeSe/SrTiO3(001) monolayer
• Modulation doping of the FeSe monolayer on SrTiO${}_3$
• High-Resolution Spectroscopy of the Intermediate Impurity States near a Quantum Phase Transition
• Compressed Sensing for STM imaging of defects and disorder
• Atomic-scale analysis of disorder by similarity learning from tunneling spectroscopy
• Inferior interfacial superconductivity in 1 UC FeSe/SrVO${}_3$/SrTiO${}_3$ with screened interfacial electron-phonon coupling
• Impact of atomic defects in the electronic states of FeSe${}_{1-x}$S${}_x$ superconducting crystals
• Fragile pressure-induced magnetism in FeSe superconductors with a thickness reduction