Discrete energy levels of Caroli-de Gennes-Martricon states in quantum limit due to small Fermi energy in FeTe$_{0.55}$Se$_{0.45}$

TL;DR

This study reports the direct observation of discrete Caroli-de Gennes-Martricon (CdGM) states in FeTe$_{0.55}$Se$_{0.45}$, confirming quantum limit predictions and indicating a very small Fermi energy in this superconductor.

Contribution

First experimental detection of discrete CdGM energy levels in FeTe$_{0.55}$Se$_{0.45}$, validating quantum limit theory in a superconductor with small Fermi energy.

Findings

Discrete CdGM energy levels observed in FeTe$_{0.55}$Se$_{0.45}$

Fermi energy in the system is very small

CdGM states are stable versus space

Abstract

Caroli-de Gennes-Martricon (CdGM) states were predicted in 1964 as low energy excitations within vortex cores of type-II superconductors. In the quantum limit, namely $T/T_\mathrm{c} \ll \Delta/E_\mathrm{F}$, the energy levels of these states were predicted to be discrete with the basic levels at $E_\mu = \pm \mu \Delta^2/E_\mathrm{F}$ ($\mu = 1/2$, $3/2$, $5/2$, ...). However, due to the small ratio of $\Delta/E_\mathrm{F}$ in most type-II superconductors, it is very difficult to observe the discrete CdGM states, but rather a symmetric peak appears at zero-bias at the vortex center. Here we report the clear observation of these discrete energy levels of CdGM states in FeTe$_{0.55}$Se$_{0.45}$. The rather stable energies of these states versus space clearly validates our conclusion. Analysis based on the energies of these CdGM states indicates that the Fermi energy in the present system is very small.