Quantum Vortex Core and Missing Pseudogap in the Multiband BCS-BEC Crossover Superconductor FeSe.

Area of Science:

• Condensed Matter Physics
• Materials Science
• Superconductivity

Background:

• Iron selenide (FeSe) is a superconductor near the Bardeen-Cooper-Schrieffer (BCS) and Bose-Einstein Condensation (BEC) crossover regime.
• In this regime, superconducting properties like gap size and transition temperature (Tc) are comparable to Fermi energy.
• Theoretical predictions include quantized vortex bound states and a pseudogap from preformed Cooper pairs above Tc.

Purpose of the Study:

• To experimentally search for quantized vortex bound states and pseudogap features in FeSe.
• To investigate the implications of these findings for understanding FeSe superconductivity.

Main Methods:

• Spectroscopic-imaging scanning tunneling microscopy (SI-STM) was employed.
• SI-STM allows for atomic-resolution imaging and local electronic density of states measurements.

Main Results:

• Friedel-like oscillations, indicative of quantized vortex bound states, were observed near vortices.
• A pseudogap in the quasiparticle-excitation spectrum above the superconducting transition temperature was not detected.

Conclusions:

• The observation of quantized vortex states aligns with theoretical expectations for the BCS-BEC crossover regime.
• The absence of a detectable pseudogap, despite theoretical predictions, suggests a more complex scenario.
• The multiband nature of FeSe is proposed as a potential explanation for these seemingly contradictory experimental findings.