Temporal evolution of electric transport properties of YBCO Josephson junctions produced by focused Helium ion beam irradiation

Abstract

Using a $30\mathrm{keV}$ focused He ion beam (He-FIB) with a wide range of irradiation doses $D=100$ to $1000ions/nm$ we fabricated Josephson and resistive barriers within microbridges of epitaxially grown single crystalline YBCO thin films and investigated the change of their electric transport properties with time. One set of samples (#1A) was simply stored at room temperature under nitrogen atmosphere. A second set (#2D) was post-annealed at $90^{\circ }\mathrm{C}$ using high oxygen pressures and a third set (#2E) at low oxygen pressures. We found that for #1A the critical current density $j_c$ at $4.2\mathrm{K}$ changes as $j_c\propto \exp (-\sqrt{t/\ddot{I}„})$ with time $t$, where the relaxation times $\ddot{I}„$ increases exponentially with $D$, which can be described within a limited diffusion based model. In order to increase the diffusion rate we annealed the junctions from #2D at $90^{\circ }\mathrm{C}$ for $30\min$ in oxygen environment. Directly after annealing the critical current density $j_c$ increased, while the normal state resistance $R_n$ decreased. Repeated measurements showed that within a week the junctions relaxed to a quasi-stable state, in which the time scale for junction parameter variations increased to several weeks, making this a feasible option to achieve temporal stability of parameters of He-FIB Josephson junctions in YBCO.