AC Josephson Signatures of the Superconducting Higgs/Amplitude Mode

The Higgs/amplitude collective mode in superconductors corresponds to oscillations of the amplitude of the order parameter. While its detection typically relies on optical techniques with an external electromagnetic field resonant with the Higgs mode, we present a purely transport-based setup wherein it is excited in a voltage biased Josephson junction. Demonstrating the importance of the order parameter dynamics, we find that in highly transparent junctions featuring single-band s-wave superconductors, the interplay of the Higgs resonance and the Josephson physics enhances the second harmonic Josephson current oscillating at twice the Josephson frequency. If the leads have unequal equilibrium superconducting gaps, this second harmonic component may even eclipse its usual first harmonic counterpart, thus furnishing a unique hallmark of the Higgs oscillations.