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| Author | Beck, Matthias; Leiderer, Paul; Kabanov, Viktor V.; Gol'tsman, Gregory; Helm, Manfred; Demsar, Jure | ||||
| Title | Energy-gap dynamics of a superconductor NbN studied by time-resolved terahertz spectroscopy | Type | Abstract | ||
| Year | 2012 | Publication | INIS | Abbreviated Journal | INIS |
| Volume | 45 | Issue | 12 | Pages | 1-3 |
| Keywords | NbN energy gap | ||||
Abstract  |
Using time-resolved terahertz (THz) spectroscopy we performed direct studies of the photoinduced suppression and recovery of the SC gap in a conventional SC NbN. Both processes are found to be strongly temperature and excitation density dependent. The analysis of the data with the established phenomenological Rothwarf-Taylor model enabled us to determine the important microscopic constants: the Cooper pair-breaking rate via phonon absorption and the bare quasiparticle recombination rate. From the latter we were able to extract the dimensionless electron-phonon coupling constant, λ=1.1±0.1, in excellent agreement with theoretical estimates. The technique also allowed us to determine the absorbed energy required to suppress SC, which in NbN equals the thermodynamic condensation energy (in cuprates the two differ by an order of magnitude). Finally, we present the first studies of dynamics following resonant excitation with intense narrow band THz pulses tuned to above and below the superconducting gap. These suggest an additional process, particularly pronounced near Tc, that could be attributed to amplification of SC via effective quasiparticle cooling. | ||||
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| Notes | Approved | no | |||
| Call Number | Serial | 1383 | |||
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| Author | Bulaevskii, L. N.; Graf, Matthias J.; Kogan, V. G. | ||||
| Title | Vortex-assisted photon counts and their magnetic field dependence in single-photon superconducting detectors | Type | Journal Article | ||
| Year | 2012 | Publication | Phys. Rev. B | Abbreviated Journal | Phys. Rev. B |
| Volume | 85 | Issue | 1 | Pages | 9 |
| Keywords | SSPD; SNSPD; single-vortex crossing; normal-state belt | ||||
| Abstract |
We argue that photon counts in a superconducting nanowire single-photon detector (SNSPD) are caused by the transition from a current-biased metastable superconducting state to the normal state. Such a transition is triggered by vortices crossing the thin and narrow superconducting strip from one edge to another due to the Lorentz force. Detector counts in SNSPDs may be caused by three processes: (a) a single incident photon with sufficient energy to break enough Cooper pairs to create a normal-state belt across the entire width of the strip (direct photon count), (b) thermally induced single-vortex crossing in the absence of photons (dark count), which at high-bias currents releases the energy sufficient to trigger the transition to the normal state in a belt across the whole width of the strip, and (c) a single incident photon of insufficient energy to create a normal-state belt but initiating a subsequent single-vortex crossing, which provides the rest of the energy needed to create the normal-state belt (vortex-assisted single-photon count). We derive the current dependence of the rate of vortex-assisted photon counts. The resulting photon count rate has a plateau at high currents close to the critical current and drops as a power law with high exponent at lower currents. While the magnetic field perpendicular to the film plane does not affect the formation of hot spots by photons, it causes the rate of vortex crossings (with or without photons) to increase. We show that by applying a magnetic field one may characterize the energy barrier for vortex crossings and identify the origin of dark counts and vortex-assisted photon counts. | ||||
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| Notes | Approved | no | |||
| Call Number | RPLAB @ gujma @ | Serial | 733 | ||
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| Author | Semenov, A. V.; Devyatov, I. A.; Korneev, A. A.; Smirnov, K. V.; Goltsman, G. N.; Melnikov, A. P. | ||||
| Title | Derivation of expression for thermodynamic potential of “dirty” superconductor | Type | Journal Article | ||
| Year | 2012 | Publication | Rus. J. Radio Electron. | Abbreviated Journal | Rus. J. Radio Electron. |
| Volume | Issue | 4 | Pages | ||
| Keywords | dirty superconductor, Usadel theory, thermodynamic potential | ||||
| Abstract |
We derive a formula for thermodynamic potential of dirty superconductor which express it via isotropic quasiclassical Green functions of Usadel theory. Our result allows unify description of dynamic processes and fluctuations in superconducting nano-electronic devices. | ||||
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| Language | Russian | Summary Language | Original Title | ||
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| Notes | 7 pages | Approved | no | ||
| Call Number | Serial | 1824 | |||
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| Author | Minaeva, O.; Fraine, A.; Korneev, A.; Divochiy, A.; Goltsman, G.; Sergienko, A. | ||||
| Title | High resolution optical time-domain reflectometry using superconducting single-photon detectors | Type | Conference Article | ||
| Year | 2012 | Publication | Frontiers in Opt. 2012/Laser Sci. XXVIII | Abbreviated Journal | Frontiers in Opt. 2012/Laser Sci. XXVIII |
| Volume | Issue | Pages | Fw3a.39 | ||
| Keywords | SSPD, SNSPD, Photodetectors; Fiber characterization; Light beams; Optical time domain reflectometry; Photon counting; Single mode fibers; Single photon detectors; Superconductors | ||||
| Abstract |
We discuss the advantages and limitations of single-photon optical time-domain reflectometry with superconducting single-photon detectors. The higher two-point resolution can be achieved due to superior timing performance of SSPDs in comparison with InGaAs APDs. | ||||
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| Publisher | Optical Society of America | Place of Publication | Editor | ||
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| Notes | Approved | no | |||
| Call Number | Serial | 1237 | |||
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| Author | Julia Toussaint, Roman Grüner, Marco Schubert, Torsten May, Hans-Georg Meyer, Benjamin Dietzek, Jürgen Popp, Matthias Hofherr, Matthias Arndt, Dagmar Henrich, Konstantin Il'in, and Michael Siegel | ||||
| Title | Superconducting single-photon counting system for optical experiments requiring time-resolution in the picosecond range | Type | Journal Article | ||
| Year | 2012 | Publication | Abbreviated Journal | AIP REVIEW OF SCIENTIFIC INSTRUMENTS | |
| Volume | 83 | Issue | Pages | ||
| Keywords | SSPD, picosecond, time-resolution | ||||
| Abstract |
We have developed a cryogenic measurement system for single-photon counting, which can be used in optical experiments requiring high time resolution in the picosecond range. The system utilizes niobium nitride superconducting nanowire single-photon detectors which are integrated in a timecorrelated single-photon counting (TCSPC) setup. In this work, we describe details of the mechanical design, the electrical setup, and the cryogenic optical components. The performance of the complete system in TCSPC mode is tentatively benchmarked using 140 fs long laser pulses at a repetition frequency of 75MHz. Due to the high temporal stability of these pulses, the measured time resolution of 35 ps (FWHM) is limited by the timing jitter of the measurement system. The result was crosschecked in a Coherent Anti-stokes Raman Scattering (CARS) setup, where scattered pulses from a β-barium borate crystal have been detected with the same time resolution. |
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| Notes | Approved | no | |||
| Call Number | RPLAB @ seleznev @ | Serial | 885 | ||
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