Tunneling conductance of graphene ferromagnet-insulator-superconductor junctions

Abstract

We study the transport properties of a graphene ferromagnet-insulator superconductor (FIS) junction within the Blonder-Tinkham-Klapwijk formalism by solving spin-polarized Dirac-Bogoliubov-de-Gennes equation. We find that the retro and specular Andreev reflections in the graphene FIS junction are drastically modified in the presence of exchange interaction and that the spin-polarization ($P_T$) of tunneling current can be tuned from the positive to negative value by bias voltage ($V$). In the thin-barrier limit, the conductance $G$ of a graphene FIS junction oscillates as a function of barrier strength $\chi$. Both the amplitude and phase of the conductance oscillation varies with the exchange energy $E_{ex}$. For $E_{ex}<E_F$ (Fermi energy), the amplitude of oscillation decreases with $E_{ex}$. For $E_{ex}^c>E_{ex}>E_F$, the amplitude of oscillation increases with $E_{ex}$, where $E_{ex}^c=2E_F+U_0$ ($U_0$ is the applied electrostatic potential on the superconducting segment of the junction). For $E_{ex}>E_{ex}^c$, the amplitude of oscillation decreases with $E_{ex}$ again. Interestingly, a universal phase difference of $\pi /2$ in $\chi$ exists between the $G-\chi$ curves for $E_{ex}>E_F$ and $E_{ex}<E_F$. Finally, we find that the transitions between retro and specular Andreev reflections occur at $eV=|E_F-E_{ex}|$ and $eV=E_{ex}+E_F$, and hence the singular behavior of the conductance near these bias voltages results from the difference in transport properties between specular and retro Andreev reflections.

Accepted for publication in Physical Review B

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• Anomalous integer quantum Hall effect in AA-stacked bilayer graphene
• Signatures of d-Wave Symmetry on Thermal Dirac Fermions in Graphene-Based F/I/d Junctions
• Effect of transport-induced charge inhomogeneity on point-contact Andreev reflection spectra at ferromagnet-superconductor interfaces
• Revisiting Andreev processes in superconductor-graphene-superconductor (SGS) Josephson junctions: Comparison with experimental results