Andreev reflection in topological nodal-line semimetals superconductor junction

Abstract

Andreev reflection is an important quantum tunneling phenomenon in the conductor-superconductor junction. The Andreev reflection coefficients T _AR of a hybrid system with s -wave superconductor connected by topological nodal-line semimetals (TNLSMs-SC junction system) is calculated theoretically by using the Landauer–Büttiker formula combined with the nonequilibrium Green’s function method. The results show that when the direction of the boundary state electron and the incident electron are the same, only the bulk states of the TNLSMs involve the Andreev reflection of the hybrid system, and the Andreev reflection coefficients T _AR enhance with the increase of the Fermi energy $E_\textrm{F}$ . We also study the effect of on-site energy ε _z and mass term m on the Andreev reflection and find that the Andreev reflection in the system decreases rapidly with the increase of on-site energy ε _z and mass term m . Moreover, we find that only in the presence of a mass term m , the Andreev reflection coefficients T _AR of the system changes with the rise of the Fermi energy E _F . When a perpendicular magnetic field is applied in the system, the Andreev reflection coefficients T _AR in the superconducting gap will appear a series of oscillating peaks. For a hybrid system with the large perpendicular magnetic field applied, we find that the maximum Andreev reflection coefficients $T_{\textrm{AR}} = 7.2$ at the Fermi energy $E_\textrm{F} = 0.0$ and the incident electron energy $E = \pm0.1$ . The Andreev reflection coefficients T _AR is gradually enhanced in the superconducting gap (incident energy $|E|\unicode{x2A7D}0.2$ ) when a disorder is applied to the superconductor region of the system. However, the symmetry of the Andreev reflection coefficients T _AR is broken when the perpendicular magnetic field is applied to the system. These peculiar transport properties of the TNLSMs-SC junction system are expected to provide theoretical guidance for future applications.

Keywords

• topological nodal-line semimetals
• superconductor
• Andreev reflection
• nonequilibrium Green’s function