S-shaped Nonlinearities in the I(V) Characteristic of YBa$_2$Cu$_3$O$_{7-\delta}$ Microstrips Due to the Presence of Phase Slip Lines.

Optimally doped high-$T_c$ superconducting YBa$_2$Cu$_3$O$_{7-\delta}$ microstrips were fabricated using a chemical-free technique based on selective epitaxial growth. Pulsed I(V) measurements of the YBa$_2$Cu$_3$O$_{7-\delta}$ microstrips exhibit steps under current biasing and an s-shaped nonlinearity under voltage biasing. Similar features have been seen in narrow superconductors, where $w < \xi$, and are explained by the formation of phase slip centers. The presence of these features in YBa$_2$Cu$_3$O$_{7-\delta}$ microstrips are indicative of the formation of phase slip lines, 2D analogs of phase slip centers. The evolution of the s-shaped nonlinearity was studied as a function of temperature and applied magnetic field and will be discussed with respect to the stiffness of the phase and the amplitude of the superconducting order parameter in the high-$T_c$ cuprates.