Temporal evolution of electric transport properties of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:msub><mml:mi>YBa</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>Cu</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mrow><mml:mn>7</mml:mn><mml:mo>−</mml:mo><mml:mi>δ</mml:mi></mml:mrow></mml:msub></mml:math> Josephson junctions produced by focused-helium-ion-beam irradiation
Max Karrer, K. Wurster, Julian Linek, M. Meichsner, R. Kleiner, E. Goldobin, D. Koelle
Abstract
We examined the temporal evolution of Josephson and resistive barriers created by a 30-keV focused helium ion beam in microbridges of epitaxially grown single-crystal ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ thin films. Repeated electric transport measurements at 4.2 K within 300 days after irradiation revealed an increase in the critical current density ${j}_{\mathrm{c}}$ for devices stored at room temperature under nitrogen atmosphere. This behavior can be described by a diffusion-based model of displaced chain oxygen moving back to original lattice sites, thus healing the barrier and partially restoring critical current. We find that ${j}_{\mathrm{c}}\ensuremath{\propto}\mathrm{exp}(\ensuremath{-}\sqrt{t/\ensuremath{\tau}})$ with time $t$. The relaxation time $\ensuremath{\tau}$ increases exponentially with helium irradiation dose and can exceed several hundred days for high-quality Josephson junctions. To achieve higher diffusion rates and thus shorter relaxation times, we annealed some devices in different oxygen partial pressures, right after irradiation. Within a week, those junctions relaxed to a quasistable state, making this a feasible option to achieve temporal stability of device parameters.