Influence of thickness, width and temperature on critical current density of Nb thin film structures

Ilin, K.; Rall, D.; Siegel, M.; Engel, A.; Schilling, Andreas; Semenov, A. D.; Huebers, Heinz-Wilhelm

doi:10.1016/j.physc.2010.02.042

Cited by 37 publications

(32 citation statements)

References 15 publications

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“…I c follows the temperature dependence of the GL depairing current. temperature dependence, which is consistent with the result found on Nb bridges [35]. The key result from Fig.…”

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confidence: 92%

Experimental Test of Theories of the Detection Mechanism in a Nanowire Superconducting Single Photon Detector

et al. 2014

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We report an experimental test of the photodetection mechanism in a nanowire superconducting single photon detector. Detector tomography allows us to explore the 0.8-8 eV energy range via multiphoton excitations. High accuracy results enable a detailed comparison of the experimental data with theories for the mechanism of photon detection. We show that the temperature dependence of the efficiency of the superconducting single photon detector is determined not by the critical current but by the current associated with vortex unbinding. We find that both quasiparticle diffusion and vortices play a role in the detection event. DOI: 10.1103/PhysRevLett.112.117604 PACS numbers: 79.20.Ws, 03.65.Wj, 85.25.Oj Nanowire superconducting single photon detectors (SSPDs or SNSPDs) [1,2] are currently the most promising detection systems in the infrared, achieving detection efficiencies of up to 93% at 1550 nm [3]. Despite these technological advances, the fundamentals of the working principle of these detectors are poorly understood and under active investigation, both theoretically [4][5][6][7][8][9][10][11] and experimentally [12][13][14][15][16][17][18][19][20][21][22].A typical SSPD consists of a few nm thin film of a superconducting material such as NbN or WSi, nanofabricated into a meandering wire geometry. When biased sufficiently close to the critical current of the superconductor, the energy of one or several photons can be enough to trigger a local transition to the resistive state, resulting in a detection event. The energy of the absorbed photon is distributed through an avalanchelike process, creating a nonequlibrium population of quasiparticles. This quasiparticle population then disrupts the supercurrent flow, resulting eventually in a detection event.In this Letter, we address the nature of this disruption, which lies at the heart of the photodetection mechanism in SSPDs. At present, there are three important open questions. First, it is unknown whether the detection event occurs when the energy of the incident photon causes a cylindrical volume inside the wire to transition to the normal state [see Fig. 1(a)] [1], or whether it is enough for the superconductivity to be weakened but not destroyed by the depletion of Cooper pairs over a more extended region [see Fig. 1The second open question is whether magnetic vortices play any role in the detection mechanism. There are two varieties of vortex-based models. The first is an extension of the normal-core model, where, a vortex-antivortex pair forms at the point where the photon is absorbed [ Fig. 1(c)] [5]. In the second, the weakening of superconductivity lowers the energy barrier for either a vortex crossing [6,23] or a vortex-antivortex pair crossing [ Fig. 1(d)].The last open question pertains to the temperature dependence of the photoresponse of SSPDs. Intuitively, one would expect the SSPD to be less efficient at lower temperatures, as the detector works by breaking superconductivity and the energy gap of a superconductor decreases with increasing temp...

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“…I c follows the temperature dependence of the GL depairing current. temperature dependence, which is consistent with the result found on Nb bridges [35]. The key result from Fig.…”

Section: Fig 4 (Color Online)supporting

confidence: 92%

Experimental Test of Theories of the Detection Mechanism in a Nanowire Superconducting Single Photon Detector

et al. 2014

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“…The critical temperature T c was defined as the lowest temperature at which a non-zero resistance could be measured. We found T c ≈ 12 K for all our samples with a variations from sample to sample of less than 0.3 K. Samples with a smaller strip width typically have a lower critical temperature 6 [15,16]. The current-voltage (CV) characteristics of the samples were measured in the current-bias mode at 4.2 K. The critical current I c of the spiral structures was associated with the well pronounced jump in the voltage from zero to a finite value corresponding to the normal state.…”

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confidence: 64%

Asymmetry in the effect of magnetic field on photon detection and dark counts in bended nanostrips

et al. 2015

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Keywords: bended superconducting nanostrips, current crowding, photon detection, dark counts, magnetic field asymmetry PACS number(s): 74.25. Ha, 85.25.Oj, 78.67.Uh Current crowding in the bends of superconducting nano-structures not only restricts measurable critical current in such structures but also redistributes local probabilities for dark and light counts to appear. Using structures from strips in the form of a square spiral which contain bends with the very same curvature with respect to the directions of bias current and external magnetic field, we have shown that dark counts as well as light counts at small photon energies originate from areas around the bends. The minimum in the rate of dark counts reproduces the asymmetry of the maximum critical current density as function of the magnetic field. Contrary, the minimum in the rate of light counts demonstrate opposite asymmetry. The rate of light counts become symmetric at large currents and fields. Comparing locally computed absorption probabilities for photons and the simulated threshold detection current we found the approximate locations of areas near bends which deliver asymmetric light counts. Any asymmetry is absent in Archimedean spiral structures without bends.2

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“…This is caused by a larger amount of defects in the crystalline structure in comparison with the bulk, that reduces the electron mean-free path l and causes peculiarities in the electron-electron [1] and electronphonon [2] interactions. If a specimen is made of a superconductor, like in the case of Nb, its normal-state and mixed-state properties become very sensitive to the structural quality [3], purity [4], and the film thickness d [5,6].…”

Section: Introductionmentioning

confidence: 99%

“…The correlation between the structural and superconducting properties in thin films of Nb represents a widely studied problem [3][4][5][6][7][8][9][10][11][12][13][14][15]. In particular, the influence of the lattice parameter a, grain size D, and surface roughness Δd on the critical temperature T c and the normal-state resistivity ρ n has been a matter of thorough experimental investigations on nano-granular disordered [7][8][9], high-purity [10], and dirty [11] Nb films.…”

Section: Introductionmentioning

confidence: 99%

“…In particular, the influence of the lattice parameter a, grain size D, and surface roughness Δd on the critical temperature T c and the normal-state resistivity ρ n has been a matter of thorough experimental investigations on nano-granular disordered [7][8][9], high-purity [10], and dirty [11] Nb films. Different growth techniques have been used in these works [3][4][5][6][7][8][9][10][11][12][13][14][15]. For the growth of high-purity thin Nb films revealing outstanding structural quality, the molecular beam epitaxy (MBE) method has been shown to be the most efficient as compared to others [12][13][14], see also [15] for a topical review.…”

Section: Introductionmentioning

confidence: 99%

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Crossover from dirty to clean superconducting limit in dc magnetron-sputtered thin Nb films

Dobrovolskiy

Huth

2012

Thin Solid Films

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High-quality Nb (110) thin films with residual resistance ratios up to 60 and critical temperatures T c ≈ 9.27 K have been prepared by conventional dc-magnetron sputtering on α-Al 2 O 3 by careful selection of the sputtering conditions. This allowed for a systematic study of the influence of the growth rate on the structural quality and the superconducting properties of the films. The optimized growth conditions were revealed at the substrate temperature T s = 850°C, Ar pressure P s = 0.4 Pa, and the growth rate g ≃ 0.5 nm/s. The results of the films' structural characterization by X-ray diffraction, reflection high-energy electron diffraction, and atomic force microscopy are presented. In terms of the electron mean free path l and the superconducting coherence length ξ, deduced from the magneto-resistivity data, the clean superconducting limit (l > ξ) is realized in the high-purity films. For comparison, in impure Nb films sputtered at room temperature while keeping the rest of the sputtering parameters unvaried, the opposite dirty limit (ξ ≳ l) ensues. The merits of these findings are discussed in the context of the demands of present-day fluxonics devices regarding the normal-state and flux-flow properties of superconducting films they are made of.

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Influence of thickness, width and temperature on critical current density of Nb thin film structures

Cited by 37 publications

References 15 publications

Experimental Test of Theories of the Detection Mechanism in a Nanowire Superconducting Single Photon Detector

Experimental Test of Theories of the Detection Mechanism in a Nanowire Superconducting Single Photon Detector

Asymmetry in the effect of magnetic field on photon detection and dark counts in bended nanostrips

Crossover from dirty to clean superconducting limit in dc magnetron-sputtered thin Nb films

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