Observation of Discrete Conventional Caroli–de Gennes–Matricon States in the Vortex Core of Single-Layer <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>FeSe</mml:mi><mml:mo>/</mml:mo><mml:msub><mml:mrow><mml:mi>SrTiO</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>
Chen Chen, Qin Liu, Wei-Cheng Bao, Y. J. Yan, Qiang-Hua Wang, Tong Zhang, Donglai Feng
Abstract
Using low-temperature scanning tunneling microscopy (STM), we studied the vortex states of single-layer FeSe film on a ${\mathrm{SrTiO}}_{3}$ (100) substrate, and the local behaviors of superconductivity at sample boundaries. We clearly observed multiple discrete Caroli--de Gennes--Matricon states in the vortex core, and quantitative analysis shows their energies well follow the formula: $E{=\ensuremath{\mu}\mathrm{\ensuremath{\Delta}}}^{2}/{E}_{F}$, where $\ensuremath{\mu}$ is a half integer ($\ifmmode\pm\else\textpm\fi{}1/2,\ifmmode\pm\else\textpm\fi{}3/2,\ifmmode\pm\else\textpm\fi{}5/2\dots{}$) and $\mathrm{\ensuremath{\Delta}}$ is the mean superconducting gap over the Fermi surface. Meanwhile, a fully gapped spectrum without states near zero bias is observed at the ${[110]}_{\mathrm{Fe}}$ oriented boundary of 1 and 2 ML FeSe films, and atomic step edge of 1 ML FeSe. Accompanied with theoretical calculations, our results indicate an $s$-wave pairing without sign change in the high-${T}_{C}$ $\mathrm{FeSe}/{\mathrm{SrTiO}}_{3}$ superconductor.