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

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.

5 pages, 4 figures. To appear in PRL. Supplemental Material is available at http://www.riken.jp/epmrt/Hanaguri/SI/FeSe_Vortex_Tdep_SM.pdf

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• Field-induced superconducting phase of FeSe in the BCS-BEC cross-over
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• Emergence of the nematic electronic state in FeSe
• Observation of pseudogap behavior in a strongly interacting Fermi gas
• BCS-BEC Crossover and the Unitary Fermi Gas
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• Giant superconducting fluctuations in the compensated semimetal FeSe at the BCS-BEC crossover
• Imaging superconducting vortex core and lattice with the scanning tunneling microscope
• Two distinct superconducting pairing states divided by the nematic end point in FeSe${}_{1-x}$S${}_x$
• Superconductivity vs bound state formation in a two-band superconductor with small Fermi energy -- applications to Fe-pnictides/chalcogenides and doped SrTiO3
• Effects of gap anisotropy upon the electronic structure around a superconducting vortex
• BCS-like critical fluctuations with limited overlap of Cooper pairs in FeSe
• Pseudogap Behavior of the Nuclear Spin-lattice Relaxation Rate in FeSe Probed by ${}^{77}$Se-NMR

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