Enhanced superconductivity at the interface of W/Sr₂RuO₄ point contacts

Differential resistance measurements are conducted for point contacts (PCs) between the Sr₂RuO₄ (SRO) single crystal and the tungsten tip approaching along the c axis direction of the crystal. Since the contact is made at liquid helium temperature and the tungsten tip is hard enough to penetrate through the surface layer, consistent superconducting features are observed. First, with the tip pushed towards the crystal, the zero-bias conductance peak (ZBCP) due to Andreev reflection at the normal-superconducting interface increases from 3% to more than 20%, much larger than previously reported, and extends to temperatures higher than the bulk transition temperature. Reproducible ZBCP within 0.2 mV may also help determine the gap value of SRO, on which no consensus has been reached. Second, the logarithmic background can be fitted with the Altshuler-Aronov theory of electron-electron interaction for tunneling into quasi-two-dimensional electron systems. Feasibility of such fitting confirms that spectroscopic information such as density of states is probed, and electronic temperature retrieved from such fitting can be important to analyze the PC spectra. Third, at bias much higher than 0.2 mV there are conductance dips due to the critical current effect. These dips persist up to 6.2 K, possibly due to enhanced superconductivity under uniaxial pressure.