Topological Excitations in Superconducting Nanostripes: Resistive States and Noise

We investigate competition between one- and two-dimensional topological excitations - phase slips and vortices - in formation of resistive states and noise generation in ultrathin superconducting NbN nanostripes in a wide temperature range below the mean-field transition temperature $T_{C0}$. The widths $w$ = 100 nm of our ultrathin NbN samples is substantially larger than the Ginzburg-Landau coherence length $\xi$ = 4nm and the fluctuation resistivity above $T_{C0}$ has a two-dimensional character. However, our data shows that the resistivity below $T_{C0}$ is produced by one-dimensional excitations, - thermally activated phase slip strips (PSSs) overlapping the sample cross-section. We determine the scaling phase diagram, which shows that even in wider samples the PSS contribution dominates over vortices in a substantial region of current/temperature variations [1]. The above fluctuations generated by topological excitations also provide a noise limit to superconducting detectors operating in a resistive state, e.g. for dark counts in single-photon detectors. [1] M. Bell et al., Phys. Rev. B 76, 094521 (2007).