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Abstract: Atomic scale Josephson junctions are formed in a scanning tunneling microscope (STM) between a superconducting tip and sample. The investigated junctions show hysteretic behavior with respect to the current sweep direction. In the presence of single magnetic adatoms non-reciprocal features emerge in the VI-characteristics. Magnetic adatoms on superconducting surfaces couple to the superconducting condensate and induce Yu-Shiba-Rusinov (YSR) states. YSR states can be probed by either electrons or holes with different tunneling probabilities resulting in asymmetric quasiparticle currents inside the superconducting gap. We find that this electron-hole asymmetry causes the non-reciprocity of the STM Josephson junctions. Furthermore, the phase coherence of the Cooper-pair tunneling processes is investigated by exposing the pristine Pb-Pb junction to high frequency irradiation. Phase diffusion is enhanced in the presence of the irradiation. Additionally, step-like features emerge as energy is absorbed by the tunneling Cooper pairs. The observed steps experience hysteretic behavior depending on the bias sweep direction. Ref: Diode effect in Josephson junctions with a single magnetic atom, M. Trahms et al., Nature 615, 628 (2023).
