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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 nm of our ultrathin NbN samples are substantially larger than the Ginzburg-Landau coherence length = 4 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 ͑PSSs͒ 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.The nature of the resistive state in superconductors attracts much attention from the physics community, since it involves fundamental phenomena and advanced concepts 1,2 such as the mechanisms of high-T c superconductivity, 3 thermal fluctuations, 4,5 macroscopic quantum tunneling, 6-8 coherence, 9 topological excitations, 10 and phase disordering. 11 Resistive states are used in a number of quantum nanodevices, such as logic elements, 12 ultrasensitive detectors of radiation, single-photon counters, and nanocalorimeters. 13,14 Understanding resistive states in nanoscale superconductors is critical for the advancement of fundamental science and the development of novel applications.Phase slips and vortices are elementary topological excitations which create resistive states. 1,2,10 Wires with radius less than the coherence length or stripes with w Ͻ are one-dimensional ͑1D͒ superconductors. In 1D structures, the resistive state is produced by phase slips and is well described by the Langer-Ambegaokar-McCumber-Halperin ͑LAMH͒ theory of thermally activated phase slips ͑TAPSs͒. 1,2,4,5 At low enough temperatures, the quantum phase slips ͑QPSs͒ should be important, 6 but the magnitude of this effect and the characteristic resistance at the transition from TAPSs to QPSs are still under debate. 7,8 In two-dimensional ͑2D͒ superconductors, the resistive state is formed by moving vortices. Above the BerezinskiiKosterlitz-Thouless ͑BKT͒ transition temperature T C , there is a nonzero concentration of free vortices due to thermal unbinding of vortex-antivortex pairs ͑VAPs͒. 15,16 Below T C in an infinite 2D superconductor, VAPs are tightly bound and only a significant bias current can unbind the pairs, resulting in a nonlinear flux flow resistance. In finite size samples, free vortices can exist below T C and produce a linear resistance at low bias currents. 15 It is commonly believed that the transition from 1D phase slip excitations to 2D vortex physics takes place at w / ϳ 1. 1,2,7,12,17 However, despite thorough studies of phase slip and vortex mechanisms, an in...

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Document VersionPublisher's PDF, also known as Version of Record (includes final page, issue and volume numbers)Please check the document version of this publication:• A submitted manuscript is the author's version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website.• The final author version and the galley proof are versions of the publication after peer review.• The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication Citation for published version (APA):Zinoni, C., Alloing, B., Li, L., Marsili, F., Fiore, A., Lunghi, L., ... Gol'tsman, G. N. (2007). Single-photon experiments at telecommunication wavelengths using nanowire superconducting detectors. Applied Physics Letters, 91(3), 031106-1/3. [031106]. DOI: 10.1063/1.2752108 General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. The authors report fiber-coupled superconducting single-photon detectors with specifications that exceed those of avalanche photodiodes, operating at telecommunication wavelength, in sensitivity, temporal resolution, and repetition frequency. The improved performance is demonstrated by measuring the intensity correlation function g ͑2͒ ͑͒ of single-photon states at 1300 nm produced by single semiconductor quantum dots.

The authors have realized NbN ͑100͒ nanofilms on a 3C-SiC ͑100͒/Si͑100͒ substrate by dc reactive magnetron sputtering at 800°C. High-resolution transmission electron microscopy ͑HRTEM͒ is used to characterize the films, showing a monocrystalline structure and confirming epitaxial growth on the 3C-SiC layer. A film ranging in thickness from 3.4 to 4.1 nm shows a superconducting transition temperature of 11.8 K, which is the highest reported for NbN films of comparable thickness. The NbN nano-films on 3C-SiC offer a promising alternative to improve terahertz detectors. For comparison, NbN nanofilms grown directly on Si substrates are also studied by HRTEM. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2766963͔The ability to grow superconducting NbN films of several nanometer thick is of significant importance to the development of modern photon detector technology. Superconducting hot electron bolometer ͑HEB͒ mixers based on such nanofilms are the only sensitive heterodyne detectors for high-resolution spectroscopy at frequencies between 1.5 and 6 THz.1-4 These detectors will be used on the Herschel space telescope 5 and are required in various future conceptual space missions. 6 Another type of detector, the superconducting single photon detector ͑SSPD͒, 7 is based on similar films and is ultrafast and sensitive for the detection of both visible and infrared photons. SSPDs can perform high speed photon counting which has many applications, for example, optical communications and quantum information.To date, HEB mixers are based on ultrathin NbN films grown primarily on substrates such as Si with its native oxide, 2-4 MgO, 8 and Si with a buffering MgO film. 9 SSPDs are based on NbN films grown on sapphire substrates. For films with an intended thickness of 3.5 nm ͑not directly measured͒, the highest superconducting transition temperatures ͑T c ͒ are reported to be 9.5-11 K.9 Among them, NbN films on MgO, MgO buffer layers and sapphire substrates have higher T c than NbN films on Si substrates. These substrates allow for epitaxial growth of the NbN films, 8-10 resulting in a monocrystalline structure. For HEB mixers, Si is a preferred substrate because of its low loss at terahertz frequencies, well-established processing technology, and inherent reliability. However, the drawback to using Si in this case is the limited intermediate frequency bandwidth, which is set by the thermal time constant.In this letter, we demonstrate superconducting NbN nanofilms on a 3C-SiC buffered Si substrate. The films were characterized by high-resolution transmission electron microscopy ͑HRTEM͒. In addition, the superconducting properties were measured.The 3C-SiC buffer layers were heteroepitaxially grown on Si ͑100͒ substrates by atmospheric pressure chemical vapor deposition at 1280°C using a process described in detail elsewhere.11 To ensure reasonably good crystal quality near the top surface of the 3C-SiC layer given its lattice mismatch with Si, we choose a thickness of 1 m for 3C-SiC layer. As reported previously, 11 t...