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The scanning tunneling microscope (STM) is a powerful tool for atomic-scale spectroscopy of normal and superconducting materials. By using superconducting tips, Josephson junctions can be formed when approaching a superconducting substrate. A fingerprint of the Josephson junction is a zero-bias peak in voltage-biased differential-conductance spectroscopy. However, an applied bias destroys the phase coherence of the junction.
Replacing the conventional voltage bias with an effective current bias, phase coherence can be longer preserved in the superconducting state. Here, we first investigate the phase dynamics in Pb-Pb junctions. We identify switching and retrapping currents that mark the transitions between superconducting and normal states. Adding magnetic adatoms to the surface of a superconductor leads to Yu-Shiba-Rusinov states inside the superconducting gap. Using Josephson spectroscopy, we find that the switching currents are significantly reduced compared to the pristine junction, indicating a local reduction of the superconducting order parameter. Even more interestingly, the retrapping current shows an asymmetric behavior with respect to the biasing direction [1]. This implies that a supercurrent can flow without dissipation in one direction while experiencing resistance in the opposite direction. We attribute this diode-like effect to the electron-hole asymmetry of Yu-Shiba-Rusinov states [1,2]. When then expose the junctions to high-frequency radiation where we find Shapiro steps, signifying the coherent absorption of photons, although phase diffusion is enhanced at the same time [3].
[1] M. Trahms, L. Melischek, J. F. Steiner, B. Mahendru, I. Tamir, N. Bogdanoff, O. Peters, G. Reecht, C. B. Winkelmann, F. von Oppen, K. J. Franke, Diode effect in Josephson junctions with a single magnetic atom, Nature 615, 628 (2023).
[2] J. F. Steiner, L. Melischek, M. Trahms, K. J. Franke, F. von Oppen, Diode effects in current-biased Josephson junctions, Phys. Rev. Lett. 130, 177002 (2023).
[3] M. Trahms, B. Mahendru, C. B. Winkelmann, K. J. Franke, From Shapiro steps to photon-assisted tunneling in microwave-driven atomic-scale Josephson junctions with a single (magnetic) adatom, arXiv:2509.26228 (2025).