Effects of low-temperature ozone annealing on current voltage and low-frequency noise characteristics of 100-tilt YBa_2Cu_3O_(7-x) grain-boundary Josephson junctions

摘要

The dc and noise characteristics of YBa_2Cu_3O_(7-x) [100]-tilt bicrystal Josephson junctions have been studied before and after annealing in ozone. Junctions with a misorientation angle 2x14° have been characterized in the temperature range from 7 K to 75 K, and their I_cR_n-product values were equal up to 5.4 mV at 7 K. Ozone annealing of the junctions at 140°C results in a two-fold decrease of the normal-state resistances R_n. The values of the I_cR_n-product have been increased not more than 7% after annealing. The spectral densities S_v(f) of the voltage fluctuations measured at various currents were of 1/f ~α -type with α= 0.8-1.0. The normalized spectral densities S_r(f)=S_R(f)/R_n~2 and S_i(f)=S_(Ic)(f)/I_c~2 of normal-state resistance fluctuations δR_n and critical current fluctuations δI_c, respectively, plus a coefficient k of the correlation between δR_n and δI_c fluctuations have been derived from the measured spectra S_v(f) before and after annealing. The modification of transport and noise characteristics were consistent with a junction model of the [100]-tilt junction, where the supercurrent and quasiparticle current flow through the same junction area, the annealing in ozone results in a decrease of the barrier thickness and the barrier is free from localized states. We suggest that the low-frequency noise arises at the interface between superconductor and the barrier.