Oxygen ordering in untwinned YBa2Cu3O7−δ films driven by electrothermal stress

We experimentally investigate the displacement of oxygen vacancies at high current densities in highly untwinned ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ films grown on top of MgO substrates. Transport bridges oriented along the ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ crystallographic directions [100] ($a$ axis), [010] ($b$ axis), and [110] $({45}^{\ensuremath{\circ}}$ from principal axes) reveal that the onset of vacancy migration is mainly determined by the local temperature (or equivalently by the dissipated power) rather than the associated activation energy. Exceeding this threshold value, a clear directional migration proceeds as evidenced by optical microscopy. Concomitant electrotransport measurements show that an intermediate phase, characterized by a decrease in resistivity, precedes long-range migration of vacancies. We numerically demonstrate that this intermediate phase consists of a homogenization of the oxygen distribution along the transport bridge, a phenomenon strongly dependent on the activation energy and the initial degree of disorder. These findings provide some important clues to determine the level of order/disorder in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ films based on electric transport measurements.