High-frequency filtering for low-temperature thermal transport studies in nanostructures

Filtering of external unwanted RF-noise and thermal noise generated at the high-temperature parts of the measuring circuit is essential for successful measurements of thermal transport of nanostructures at low temperatures. This is because of thermal decoupling of the systems, i.e. the extreme weakness of thermal conduction at sub-Kelvin temperatures, leading easily to overheating even with excess power in sub pW range. We have started to improve the noise filtering in our cryogenic dilution refrigerators, which can reach a base temperature of ~ 50 mK. The miniature low-pass filters were made from special RF sealing compound Eccosorb CR124, stainless steel powder of grain size 50 micrometers and a typical low-temperature epoxy (Stycast). Measured cut-off frequency is at 1 MHz, low enough to cut most of the radiated RF power

Normal-Metal–Insulator–Superconductor Tunnel Junction With Atomic-Layer-Deposited Titanium Nitride as Superconductor

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High-quality superconducting titanium nitride thin film growth using infra-red pulsed laser deposition

Superconducting titanium nitride (TiN) thin films were deposited on magnesium oxide, sapphire and silicon nitride substrates at 700 °C, using a pulsed laser deposition (PLD) technique, where infrared (1064 nm) pulses from a solid-state laser were used for the ablation from a titanium target in a nitrogen atmosphere. Structural studies performed with x-ray diffraction showed the best epitaxial crystallinity for films deposited on MgO. In the best films, superconducting transition temperatures, T C, as high as 4.8 K were observed, higher than in most previous superconducting TiN thin films deposited with reactive sputtering. A room temperature resistivity down to ~17 μΩ cm and residual resistivity ratio up to 3 were observed in the best films, approaching reported single crystal film values, demonstrating that PLD is a good alternative to reactive sputtering for superconducting TiN film deposition. For less than ideal samples, the suppression of the film properties were correlated mostly with the unintended incorporation of oxygen (5–10 at%) in the film, and for high oxygen content films, vacuum annealing was also shown to increase the T C. On the other hand, superconducting properties were surprisingly insensitive to the nitrogen content, with high quality films achieved even in the highly nitrogen rich, Ti:N = 40/60 limit. Measures to limit oxygen exposure during deposition must be taken to guarantee the best superconducting film properties, a fact that needs to be taken into account with other deposition methods, as well.peerReviewe

Energy of dendritic avalanches in thin-film superconductors

A method for calculating stored magnetic energy in a thin superconducting film based on quantitative magneto-optical imaging is developed. Energy and magnetic moment are determined with these calculations for full hysteresis loops in a thin film of the superconductor NbN. Huge losses in energy are observed when dendritic avalanches occur. Magnetic energy, magnetic moment, sheet current and magnetic flux distributions, all extracted from the same calibrated magneto-optical images, are analyzed and discussed. Dissipated energy and the loss in moment when dendritic avalanches occur are related to each other. Calculating these losses for specific spatially-resolved flux avalanches is a great advantage, because of their unpredictable and non-reproducible nature. The relative losses in energy are much higher than the relative losses in moment