Ramp Josephson junctions based on NdBa2Cu3O7-δ electrodes and PrBa2Cu3O7-δ barriers

Licentiate thesis, 2006

We report on smooth high quality c-axis oriented NdBa2Cu3O7-δ (NBCO) superconducting thin films grown on (001) 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 was 0.75 nm for 150 nm thick films. Insulating layers of PrBa2Cu3O7-δ/SrTiO3/PrBa2Cu3O7-δ grown in-situ 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° are patterned in the multilayers using a photoresist reflow process and Ar+-ion milling. A study of the conduction mechanism in PrBa2Cu3O7-δ with different Ga doping levels has been done. We found that the conductivity was governed by variable range hopping conductivity at higher temperatures (>200 K). At lower temperatures (<200 K) the conductivity can be described by Glazman & Matveev theory for hopping through a few number of localized states. A Ga-doped PrBa2Cu3O7-δ insulting barrier around 20 nm thick and NBCO counter electrode are deposited on the ramp forming a Josephson junction. Current-voltage curves of the obtained ramp Josephson junctions were studied at 4.2 K. Multiple Shapiro steps were observed when the junctions were irradiated at a frequency of 9.7 GHz, despite the fact that they have a large excess current. The amplitudes of these steps oscillate with microwave power in agreement with the resistively shunted Josephson junction (RSJ) model.