# Quantum vortex core and missing pseudogap in the multi-band BCS-BEC-crossover superconductor FeSe

## TL;DR

This study investigates the vortex core states and pseudogap phenomena in FeSe, a multi-band superconductor in the BCS-BEC crossover regime, using spectroscopic-imaging STM, revealing quantized vortex states but no pseudogap.

## Contribution

First direct observation of vortex bound state quantization in FeSe, highlighting the complex multi-band effects on vortex and pseudogap phenomena.

## Key findings

- Friedel-like oscillations indicate quantized vortex bound states
- No pseudogap detected above Tc in FeSe
- Multi-band effects may influence vortex and pseudogap behavior

## Abstract

FeSe is argued as a superconductor in the Bardeen-Cooper-Schrieffer Bose-Einstein-condensation crossover regime where the superconducting-gap size and the superconducting transition temperature Tc are comparable to the Fermi energy. In this regime, vortex bound states should be well quantized and the preformed pairs above Tc may yield a pseudogap in the quasiparticle-excitation spectrum. We performed spectroscopic-imaging scanning tunneling microscopy to search for these features. We found Friedel-like oscillations near the vortex, which manifest the quantized levels, whereas the pseudogap was not detected. These apparently conflicting observations may be related to the multi-band nature of FeSe.