Proximity effects and pair currents in cuprate junctions

Abstract

Proximity effects and pair currents were measured in epitaxial trilayer \textit{c-axis} junctions comprise of a $PrBa_2Cu_3O_{7-\delta}$ barrier sandwiched in between an overdoped $Y_{0.94}Ca_{0.06}Ba_2Cu_3O_{7-\delta}$ and underdoped $YBa_2Cu_{2.7}Co_{0.3}O_y$ layers. These junctions had two $\rm T_c$ values of $\rm T_c(high)=84-86$ K and $\rm T_c(low)=50-55$ K, allowing investigation when both electrodes are superconducting, or when only one is superconducting while the other is in its pseudogap regime. For T below $\rm T_c(high)$ but much above $\rm T_c(low)$, two distinct proximity effect transitions were observed in the resistance at two temperature regimes, between 80 and 84 K, and 76 to 80 K. The first is a conventional proximity effect with the $\rm T_c(high)$ electrode, while the second is a second-order proximity effect of this electrode with uncorrelated pairs in the pseudogap regime. Conductance spectra measured between 2 and 86 K showed four different $I_c$ pair currents which were attributed to coherent pairs tunneling through the barrier below 42 K, to fluctuating pairs current up to $\sim$77 K, and to proximity pairs current between 77 and 84 K. All pair currents were suppressed under magnetic fields, with two distinct decay parameters that originated in the two different electrodes, with a significant suppression observed in the pseudogap regime.