Topological excitations and fluctuations in superconducting nanostripes
Bell, Matthew Thomas
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Described in this dissertation is the investigation of 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 C 0 . The widths w = 100 nm of our ultra-thin Niobium Nitride (NbN) samples studied are substantially larger than the Ginzburg-Landau coherence length ξ = 4 nm, and the fluctuation resistivity observed above T C 0 has a two-dimensional character. However, our results show that the resistivity below T C 0 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 and/or temperature variations. The above fluctuations generated by topological excitations provide a sensitivity limit to superconducting detectors operating in a resistive state, e.g. for dark counts in single-photon nanostripe-based detectors. Our universal phase diagram represents an entire picture of the competition between electron heating, vortices, regular phase slips and PSSs within a broad range of temperatures and bias currents in nanostripe devices.