Non-equilibrium resistive states of superconducting NbN microstrips in a transverse magnetic field

Applying current in type-II superconducting sub-micron size structures often leads to a discontinuity of the current-voltage characteristic as the instability current is reached. Also voltage-bias mode allows to avoid such an abrupt switch to the normal state, and gives a relevant outlook of the involved resistive phenomenons. Current and voltage-bias measurements on NbN microbridges with a constriction are reported here as a function of temperature and transverse magnetic field. Starting from a 75nm-thick sputtered film, 160nm-wide microstrips were patterned with an half-wide section constriction in their middle. From the current-bias curves it appears that the discontinuities are due to a flux flow instability that rapidly evolves in a hot-spot. Moreover applying the voltage reveals several dissipative states that leads to successive S-shaped current oscillations in a small current range. Formation of vortex channels can be considered to explain these features.