We investigate competition between one- and two-dimensional topological excitations—phase slips and vortices—in the formation of resistive states in quasi-two-dimensional superconductors in a wide temperature range below the mean-field transition temperature $T_{C0}$. The widths $w=100\mathrm{nm}$ of our ultrathin NbN samples are substantially larger than the Ginzburg-Landau coherence length $\xi =4\mathrm{nm}$, and the fluctuation resistivity above $T_{C0}$ has a two-dimensional character. However, our data show that the resistivity below $T_{C0}$ is produced by one-dimensional excitations—thermally activated phase slip strips (PSSs) overlapping the sample cross section. We also determine the scaling phase diagram, which shows that even in wider samples the PSS contribution dominates over vortices in a substantial region of current and/or temperature variations. Measuring the resistivity within 7 orders of magnitude, we find that the quantum phase slips can only be essential below this level.

• Received 1 August 2007

DOI:https://doi.org/10.1103/PhysRevB.76.094521