Gate-voltage control of equal-spin Andreev reflection in half-metal/semiconductor/superconductor junctions

• We study the equal-spin Andreev reflection in half-metal/semiconductor/superconductor (HM/Sm/SC) junctions.
• The equal-spin Andreev reflection appearance when inserting a ferromagnetic insulator at the Sm/SC interface.
• The finite in-gap conductance is attributed to the emergence of the equal-spin Andreev reflection.
• The finite in-gap conductance shows a nonmonotonic dependence on Rashba spin–orbit interaction.
• The finite in-gap conductance can be controlled by applying an external gate voltage.

With the Blonder–Tinkham–Klapwijk (BTK) approach, we investigate conductance spectrum in Ferromagnet/Semiconductor/Superconductor (FM/Sm/SC) double tunnel junctions where strong Rashba spin–orbit interaction (RSOI) is taken into account in semiconductors. For the half-metal limit, we find that the in-gap conductance becomes finite except at zero voltage when inserting a ferromagnetic insulator (FI) at the Sm/SC interface, which means that the appearance of a long-range triplet states in the half-metal. This is because of the emergence of the unconventional equal-spin Andreev reflection (ESAR). When the FI locates at the FM/Sm interface, however, we find the vanishing in-gap conductance due to the absence of the ESAR. Moreover, the non-zero in-gap conductance shows a nonmonotonic dependence on RSOI which can be controlled by applying an external gate voltage. Our results can be used to generate and manipulate the long-range spin triplet correlation in the nascent field of superconducting spintronics.