Seminar: Electrically controllable nonreciprocal Josephson effect in planar tunnel junctions

Nonreciprocal Josephson effect (NJE), known also as the Josephson diode effect (JDE), in which zero-voltage supercurrents flow preferably in one direction, is a consequence of time-reversal symmetry breaking. Although NJE or JDE has been found in various Josephson junction, electrically controllable, nonreciprocal Josephson tunnel junctions (as opposed to superconducting weak links) prepared by techniques compatible with complementary-metal-oxide-semiconductor (CMOS) technology in microelectronics are yet to be demonstrated. We fabricated high-quality planar Josephson tunnel junctions featuring a composite tunnel barrier of Al₂O₃ and Hf_1-xZr_xO₂ using CMOS compatible atomic layer deposition techniques. These JTJs were found, unexpectedly, to show NJE in zero magnetic fields with the nonreciprocity controlled by an electric training current well above the critical current in zero magnetic fields. The quasiparticle tunneling spectra were found to show features associated with superconducting energy gaps of the two electrodes but no nonreciprocal effects. I will offer a tentative explanation of the observed effects in a picture involving the presence of both the conventional positive and unconventional negative Josephson couplings that leads to the spontaneous breaking of the time-reversal symmetry.