Andreev Bound States in Confined Superconducting Semiconductor Nanowires*

The hybridization of superconductors (SC) and semiconducting nanowires leads to a variety of interesting phenomena including nanoscale superconductivity and Majorana fermion physics. Andreev bound states (ABS), which are a discrete energy spectrum as a result of Andreev reflections of electrons and holes between two normal-SC interfaces, are predicted to support Majorana bound states under certain conditions. The LaAlO$_3$/SrTiO$_3$ interface possesses native superconductivity and strong spin-orbit coupling and is thus a promising platform for observing signatures of Majorana fermions. Here we investigate a superconducting nanowire quantum dot created by reversible ``write'' and ``erase'' processes using a conductive atomic force microscope tip. Transport studies show that electrons can travel in different regimes dominated by resonant pair tunneling and Andreev reflection in a single device.