Asymmetrically optimized structure in a high-T _c single unit-cell FeSe superconductor

Area of Science:

• Condensed Matter Physics
• Materials Science
• Superconductivity

Background:

• Iron-based superconductors are a significant area of research due to their high critical temperatures (Tc).
• Single unit-cell (UC) FeSe grown on SrTiO3(001) substrates has demonstrated record-breaking Tc values among these materials.
• Understanding the structural factors influencing superconductivity in these thin films is crucial for further advancements.

Purpose of the Study:

• To investigate the atomic structure of single UC FeSe on SrTiO3(001).
• To correlate structural properties with the observed high superconducting transition temperature (Tc).
• To explore the implications of structural asymmetry on electronic properties and superconductivity.

Main Methods:

• Total-reflection high-energy positron diffraction was employed to precisely measure atomic heights.
• Intensity analysis based on dynamical diffraction theory was used for structural determination.
• First-principles calculations were performed to understand the electronic band structure and doping effects.

Main Results:

• Asymmetric Se heights of 1.44 ± 0.03 Å and 1.33 ± 0.03 Å were observed, with an average of 1.39 ± 0.04 Å.
• The average Se-Fe-Se bond angle was found to be 109.3 ± 1.6°, close to the ideal tetrahedral angle.
• First-principles calculations indicated that structural asymmetry splits electron bands at the M point without altering bandwidth.

Conclusions:

• The single UC FeSe on SrTiO3(001) possesses an asymmetrically optimized structure crucial for its high Tc.
• Structural asymmetry is predicted to induce non-centrosymmetric superconductivity at low electron doping.
• At higher doping levels, the average anion height becomes the dominant factor for achieving high Tc.