Non-reciprocal superconductivity and the field free Josephson Diode*

Nonreciprocal transport is incredibly important in technology; for example, asymmetry in the current-voltage response in semiconductor junctions has been the cornerstone of computing technology for half a century. The “diode” effect is a very basic demonstration of nonreciprocity. Nonreciprocal superconductivity, however, proved elusive, and only in 2020 was the superconducting diode effect (superconducting in one direction while normal conducting in the other) discovered for the first time in a bulk alloy of V/Nb/Ta. By breaking both inversion and time reversal symmetry (using an applied magnetic field), a difference in the critical superconducting current (I) for positive vs negative voltages (V) was seen. Recently, we demonstrated a Josephson diode (JD), created in a quantum material Josephson junction (QMJJ, a junction made up of two superconductors separated by a barrier comprised of a quantum material). A diodic effect was seen without an applied magnetic field; a puzzling result for theoretical physicists but an important advance for potential technological application as nanoscale field control and manipulation remains a challenge. Using an inversion symmetry breaking heterostructure of NbSe2/Nb3Br8/NbSe2, half-wave rectification of a square-wave excitation was achieved with low switching current density, high rectification ratio, and high robustness. Following that work, a plethora of nonreciprocal QMJJ and other architectures have been investigated and shown to demonstrate nonreciprocal superconductivity. Future directions for optimizations and novel explorations, as well as a broader impact of using other quantum materials will be discussed.