Point contact spectroscopy of superconductors via nanometer scale point contacts formed by resistive switching

Point contact spectroscopy is commonly used to investigate electronic properties of superconductors. Here we show that nanometer scale point contacts, which enable to study the superconductor properties locally, can be created by means of the resistive switching phenomenon. Our experiments were performed on sandwiched MgB2/Al/TiO2/Pt structures, where multiple bipolar resistive switching cycles were conducted. The differential conductance as a function of voltage was measured at temperatures below the critical temperature of the MgB2 superconductor. In the low-resistance state the MgB2 and Pt electrodes are connected by an ultrathin metallic filament which creates at the MgB2 electrode the Sharvin point contact with diameter below 10 nm. In this case the differential conductance data demonstrate the Andreev reflections due to the carrier transport between the superconducting MgB2 electrode and filament. From these data the two-gap superconductivity of MgB2 is clearly visible which also confirms the fit by the Blonder-Tinkham-Klapwijk model. If the bottom electrode is made of a superconductor with known gap, our approach allows us to estimate from the Andreev reflection spectrum the resistance of both the filament and point contact. We can then determine from the Sharvin formula the cross-section size of the point contact and thus also the filament cross-section size. In the high resistance state when the filament is ruptured, the differential conductance data demonstrate the spectrum typical for tunneling between two normal metals, with a zero-bias anomaly due to the Altshuler-Aronov effect. This suggests that the filament is not ruptured at the superconducting MgB2 electrode but elsewhere.