Signatures of Amorphous Shiba State in FeTe_0.55Se_0.45

Area of Science:

• Condensed Matter Physics
• Materials Science
• Superconductivity

Background:

• Iron-based superconductors like FeTe0.55Se0.45 exhibit complex electronic properties, including surface Dirac states and potential topological superconductivity.
• The superconducting state in these materials is typically understood to involve multiple gapped bands and spatially extended Bogoliubov excitations.

Purpose of the Study:

• To investigate the nature of Bogoliubov excitations in the iron-based superconductor FeTe0.55Se0.45.
• To characterize the observed density of states peaks and determine their origin.

Main Methods:

• Scanning tunneling spectroscopy (STS) was employed to probe the electronic density of states.
• Analysis of the spatial and junction-resistance dependence of STS data.
• Comparison of experimental results with theoretical simulations.

Main Results:

• STS data revealed multiple peaks in the density of states above a primary superconducting gap.
• These peaks were determined not to be coherence peaks from different bands.
• Simulations indicated that the peaks originate from spatially overlapping generalized Shiba states.

Conclusions:

• The observed phenomena suggest an amorphous state of Bogoliubov quasiparticles, distinct from conventional excitations.
• This amorphous state is reminiscent of impurity bands found in semiconductors.
• The findings necessitate a re-evaluation of the nature of superconducting excitations in certain iron-based superconductors.