Peculiarities of the thickness dependence of the superconducting properties of thin Nb films

Ilin, K.; Vitusevich, S. А.; Jin, Biao; Gubin, A. I.; Klein, N.; Siegel, M.

doi:10.1016/j.physc.2004.03.118

Cited by 13 publications

(8 citation statements)

References 6 publications

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“…To consistently discuss our experimental results we first compare the size of our bridges with the effective penetration depth and the coherence length at zero temperature. Using the value of λ that was reported in [15], we estimated for our Nb films the values of λ eff equal to 6.6, 2.1 and 0.845 µm for thickness 8, 12 and 20 nm, respectively. NbN films are usually characterised by a much larger penetration depth.…”

Section: Discussionmentioning

confidence: 99%

Critical current of Nb and NbN thin‐film structures: The cross‐section dependence

Ilin

Siegel

Semenov³

et al. 2005

phys. stat. sol. (c)

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We present results of a systematic study of the critical current in superconducting Nb and NbN thin film bridges. The bridges with a width from 50 nm to 10 µm were patterned from thin superconducting films by means of electron-beam lithography and ion milling. For both materials the nominal critical current density extrapolated to zero temperature varied with the bridge width and thickness. We attribute these variations to a fabrication-enhanced reduction of the effective, superconducting cross-section of the bridges with respect to their geometric cross-section and to an uneven distribution of the super-current over the superconducting core of the bridge. In very thin bridges, i.e. 5 nm and 8 nm for NbN and Nb, respectively, the nominal current density increased drastically when the bridge width became smaller than 500 nm. We associate the enhancement of the critical current density in narrow bridges with the crossover from depinning of magnetic vortices to either their nucleation or breaking of Cooper pairs. 1 Introduction Modern technologies allow a reproducible fabrication of superconducting thin-film structures with a size of only few nanometers. The availability of superconducting nano-structures triggers observations and detail studies of phenomena not attainable on a larger scale. One of them is the Cooper-pair breaking current (depairing current) that is the ultimate limit of the current-carrying capacity of a superconductor. The difficulty of experimental observation of the depairing current lies in the fact that the superconducting structure must have a width less than both the effective superconducting penetration depth λ eff and the Ginzburg-Landau superconducting coherence length ξ GL . The first condition ensures uniform distribution of the current over the cross-section of the strip. The second condition eliminates vortex nucleation and flow, which would gives rise to dissipation before the value of the depairing current is reached. Likharev [1] predicted that the vortex flow will occur at about one-half of the depairing current. He also suggested that no vortices can exist/nucleate in a superconducting strip line if it has a width smaller than 4.4ξ GL . In the following we will call this condition the Likharev's criteria.In superconducting strip lines with a width larger than λ eff and ξ GL , the critical current is usually determined by depinning of magnetic vortices. In other words, a voltage appears when pinned magnetic vortices start to move across the strip under the influence of the Lorentz force. The value of the depinning critical current density j pin is determined by the concentration, type and size of the pinning sites. Thus j pin should be strongly influenced by the film deposition technique. Patterning additionally influences the value of j pin and hampers observation of the depairing current since quality of the strip edges and their roughness (being about ξ GL ) might play a key role in nucleation and pinning of magnetic vortices.

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Section: Discussionmentioning

confidence: 99%

Critical current of Nb and NbN thin‐film structures: The cross‐section dependence

Ilin

Siegel

Semenov³

et al. 2005

phys. stat. sol. (c)

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“…The two gaps can indeed be different for many reasons. For example, the two superconductors may be made of different materials [96], or the two superconducting films might have different thicknesses [97][98][99], or the two S-TI interfaces might be made of a different quality [100,101]. Moreover, in realistic experimental conditions, the finite temperature difference in the nonlinear regime may induce the superconducting gaps to take different values on the two sides [96].…”

Section: System and Modelmentioning

confidence: 99%

Nonlocal thermoelectric engines in hybrid topological Josephson junctions

et al. 2021

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The thermoelectric performance of a topological Josephson nonlocal heat engine is thoroughly investigated. The nonlocal response is obtained by using a normal metal probe coupled to only one of the proximized helical edges in the middle of the junction. In this configuration, we investigate how the flux and phase biases trigger the nonlocal thermoelectric effects under the application of a thermal difference between the superconducting terminals. Possible experimental nonidealities such as asymmetric proximized superconducting gaps are considered, showing how the nonlocal response can be affected. The interplay between Doppler shift, which tends to close gaps, and Andreev interferometry, which affects particle-hole resonant transport, are clearly identified for different operating regimes. Finally, we discuss the power and the efficiency of the topological thermoelectric engine which reaches maximum power at maximal efficiency for a well-coupled normal probe. We find quite high nonlocal Seebeck coefficients of the order of tenths of μV/K at a few Kelvin, a signal that would also be clearly detectable against any spurious local effects even with moderate asymmetry of the gaps.

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“…The two gaps can indeed be different for many reasons. For example, the two superconductors may be made of different materials 92 , or the two superconducting films might have different thicknesses [93][94][95] , or the two S-TI interfaces might be made of different quality 96,97 . Moreover, in realistic experimental conditions, the finite temperature difference in the nonlinear regime may induce the superconducting gaps to take different values on the two sides 92 .…”

Section: System and Modelmentioning

confidence: 99%

Nonlocal thermoelectric engines in hybrid topological Josephson junctions

Blasi,

Taddei,

Arrachea

et al. 2021

Preprint

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Peculiarities of the thickness dependence of the superconducting properties of thin Nb films

Cited by 13 publications

References 6 publications

Critical current of Nb and NbN thin‐film structures: The cross‐section dependence

Critical current of Nb and NbN thin‐film structures: The cross‐section dependence

Nonlocal thermoelectric engines in hybrid topological Josephson junctions

Nonlocal thermoelectric engines in hybrid topological Josephson junctions

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