Smooth NdBa2Cu3O7-delta thin films and ramp Josephson junctions

We report on c-axis oriented NdBa2Cu3O7-delta (NBCO) superconducting thin films grown on SrTiO3 substrates using pulsed laser deposition. The transition temperature of these NBCO films was around 89.5 K and the root mean square (RMS) surface roughness for 150 nm thick films was 0.75 nm. Insulating layers of PrBa2Cu3O7-delta/SrTiO3/PrBa2Cu3O7-delta grown on top of the NBCO superconducting films result in superconductor/insulator multilayers of about 400 nm in total thickness and an RMS surface roughness of 2.4 nm. Smooth ramps with angles of about 20 degrees are patterned in the multilayers using a photoresist reflow process and Ar ion milling. 15-25 nm thick Ga-doped PrBa2Cu3O7-delta barriers and NBCO counter electrodes are deposited on the ramp forming Josephson junctions. Current-voltage (I - V) curves of the obtained ramp Josephson junctions were studied at 4.2 K. Multiple Shapiro steps were observed when the junctions were irradiated at frequencies around 10 GHz. The amplitudes of these steps oscillate with microwave power in agreement with the resistively shunted Josephson junction (RSJ) model