Dimensional crossover and incipient quantum size effects in superconducting niobium nanofilms

Superconducting and normal state properties of sputtered Niobium nanofilms have been systematically investigated, as a function of film thickness in a d=9-90 nm range, on different substrates. The width of the superconducting-to-normal transition for all films remained in few tens of mK, thus remarkably narrow, confirming their high quality. We found that the superconducting critical current density exhibits a pronounced maximum, three times larger than its bulk value, for film thickness around 25 nm, marking the 3D-to-2D crossover. The extracted magnetic penetration depth shows a sizeable enhancement for the thinnest films, aside the usual demagnetization effects. Additional amplification effects of the superconducting properties have been obtained in the case of sapphire substrates or squeezing the lateral size of the nanofilms. For thickness close to 20 nm we also measured a doubled perpendicular critical magnetic field compared to its saturation value for d>33 nm, indicating shortening of the correlation length and the formation of small Cooper pairs in the condensate. Our data analysis evidences an exciting interplay between quantum-size and proximity effects together with strong-coupling effects and importance of disorder in the thinnest films, locating the ones with optimally enhanced critical properties close to the BCS-BEC crossover regime

Tests of SNIS Josephson Arrays Cryocooler Operation

Cryogen-free operation of is essential to spread applications of superconductivity and is indeed unavoidable in some cases. In electrical metrology applications, higher temperature operation to reduce the refrigerator size and complexity is not yet possible, since arrays of Josephson junctions for voltage standard applications made with high-temperature superconductors are not yet available. The superconductor-normal metal-insulator-superconductor (SNIS) technology developed at INRIM uses low temperature superconductors, but allows operation well above liquid helium temperature. It is thus interesting for application to a compact cryocooled standard. We studied SNIS devices cooled with a closed-cycle refrigerator, both in DC and under RF irradiation. Issues related to thermal design of the apparatus are analyzed. The dependence of RF steps on the number of junctions observed is discussed in detail and interpreted as a consequence of power dissipated inside the chip

Analysis of Internally Shunted Josephson Junctions

partially_open11sìFollowing the ever-rising demand for new functionalities and novel materials in superconducting circuitry, we provide a complete view on the self-shunting problem in Josephson junctions, relating it to specific features of a multichannel weak link between electrodes where averaging over the channels yields a bimodal distribution of transparencies with maxima near unity and zero. We provide two examples of such internally shunted devices, namely, four-layered Nb/Al-Al oxide-Nb junctions with strongly disordered nanometer-thick insulating layers where stochastic distribution of transparencies takes place on a local rather than a global scale and MoRe/W-doped Si-Si-MoRe devices with strongly inhomogeneous silicon interlayers partly doped by metallic nanoclusters where the main charge transport occurs across resonance-percolating trajectories. We show how the predicted universal distribution function of transmission coefficients can be verified experimentally without any fitting parameters and analyze some old and new experimental data from this perspective. We believe that our results can form a base for novel four-layered Josephson junctions with enhanced superconducting properties and, at the same time, well-separated metallic electrodes.partially_openLacquaniti, Vincenzo; Cassiago, Cristina; De Leo, Natascia; Fretto, Matteo; Sosso, Andrea; Febvre, Pascal; Shaternik, Volodymyr; Shapovalov, Andrii; Suvorov, Olexandr; Belogolovskii, Mikhail; Seidel, PaulLacquaniti, Vincenzo; Cassiago, Cristina; DE LEO, Maria; Fretto, MATTEO ANDREA; Sosso, Andrea; Febvre, Pascal; Shaternik, Volodymyr; Shapovalov, Andrii; Suvorov, Olexandr; Belogolovskii, Mikhail; Seidel, Pau

Superconductivity in primary voltage metrology at INRiM

Primary electrical Metrology is among the first and, to date, one of the most active and successful research field for superconducting devices. Voltage standards based on the Josephson effect are in use since few years after it discovery and have enormously reduced the uncertainty of DC voltage calibrations. INRIM the Italian National Metrology Institute has a long tradition in research on the application of superconductivity to electrical standards. This paper is an overview of main results and ongoing activities

Cryogen-Free Operation of SNIS for AC Quantum Voltage Standards

We report on our recent and ongoing activities on helium-free operation with ac Josephson standards based on programmable superconductor-normal conductor-insulator-superconductor (SNIS) arrays. Liquid helium-free operation provides ease of use, which is a wider number of applications and users. Moreover, it allows to reduce cable loading, which is crucial to overcome frequency-related limitations to the accuracy. Thermalization problems not faced with helium cooling are still challenging, in particular with programmable standards. To identify the effect of He-free cooling on electrical behavior, we compared the array steps in cryocooler a liquid helium. Results show that SNIS arrays can be operated in cryocooler and are interesting for application to programmable voltage standard

Improved Characteristics of Intrinsically Shunted Nb/Al - AlO(x) - Nb Josephson Junctions

Josephson junctions with non-hysteretic current-voltage characteristics form the basis of most superconducting electronic circuits including RSFQ logic and programmable Josephson voltage standards. In contrast to conventional SIS devices, Nb/Al AlOx - Nb (SNIS) junctions with an additional comparatively thick Al interlayer and a comparatively thin barrier AlOx are intrinsically shunted at T >= 4.2 K. In this contribution, we provide experimental and theoretical arguments proving that this finding is mainly explained by a broad distribution of highly-conductive barrier transparencies with a significant effect from nearly ballistic modes. An additional advantage of the proposed SNIS junctions is possibility to tune the critical voltage value by modifying Nb and/or Al film thicknesses. With observations of wide Shapiro steps up to 1.25 V at 6.3 K we show that this type of Josephson junctions can be successfully used at temperatures above 4.2 K. The presence of well-developed quantized voltage features even at 7.2 K means that Nb/Al - AlOx- Nb devices can successfully operate far above the liquid helium temperature and, in principle, are compatible with two-stage cryocoolers