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We present a proof-of-concept nanoelectronics device with a negative differential conductance, an attractive from the applied viewpoint functionality. The device, characterized by the decreasing current with increasing voltage in a certain voltage region above a threshold bias of about several hundred millivolts, consists of two superconducting electrodes with an amorphous 10-nm-thick silicon interlayer doped by tungsten nano-inclusions. We show that small changes in the W content radically modify the shape of the trilayer current–voltage dependence and identify sudden conductance switching at a threshold voltage as an effect of Andreev fluctuators. The latter entities are two-level systems at the superconductor-doped silicon interface where a Cooper pair tunnels from a superconductor and occupies a pair of localized electronic states. We argue that in contrast to previously proposed devices, our samples permit very large-scale integration and are practically feasible.

We have studied thin-film superconductor - semiconductor (with quantum dots)—superconductor MоRe–Si(W)—MoRe junctions, where electrons are tunneling through a single or several quantum dots within the Si(W) barrier. Current–voltage characteristics (CVCs) of the samples have been measured in a wide voltage range from –900 to 900 mV at temperatures from 4.2 to 8 K. At relatively high tungsten content in the barrier, we have observed emergence of the Josephson effect. Characteristic voltages I $_{c}$ R $_{N}$ , the product of the critical Josephson supercurrent I $_{c}$ and the normal-state resistance R $_{N}$ , of the samples were unusually high. Simultaneously, we have observed large excess quasiparticle currents I exc in the dissipative part of CVCs, which is a strong evidence of intensive electron-to-hole Andreev reflections in the junctions studied. When the W content in the barrier was decreased, the Josephson current disappeared, and we have observed resonant current peaks in the CVCs at bias voltages from 40 to 300 mV, which were symmetrical for positive and negative voltages. In the studied heterostructures, metal clusters inside the barrier behave as quasi-one-dimensional quantum dots; hence, the charge transport can be adequately described by scattering matrices within the quantum model of one-dimensional charge transport.

poster 3EPo2C-01; International audience

session: Nanoscale Physics; International audience

session poster 1EP2-08; International audience

International audience; Following 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.

We discuss an universal effect in tunnel characteristics of layered metal - insulator – metal structures, where the dielectric barrier is formed by a nanoscale disordered oxide film: a universal distribution of the insulator layer transparencies, which does not depend on specific microscopic characteristics of it. Experimental results for superconducting three- and four-layered structures with inhomogeneous tunnel barriers, confirming the existence of a universal distribution of transparencies, are given jointly with a simple theoretical interpretation, based on the equipartition hypothesis of a product of the barrier height on the way which an electron passes within it.

We have fabricated trilayered sandwiches consisting of superconducting electrodes made of MoRe alloy with a critical temperature of about 10 K and a silicon interlayer of thicknesses up to 20 nm doped by tungsten with atomic concentrations up to 10 at%. For concentrations below 5 at%, measurements of transport characteristics have revealed the presence of charging energy in ultra-small dopant granules (the Coulomb-blockade effect) without any supercurrent through the junction. Unexpectedly, a persistent current at zero voltage bias has exposed itself at higher W concentrations and even for the thickest W:Si layers. Microwave-radiation experiments have proven that in this case we are dealing with a Josephson current. The observation is explained as the fingerprint of 'open' channels in the charge transmission due to resonance-percolating trajectories inside the strongly inhomogeneous silicon interlayer with metallic nanoclusters. We have calculated the ratio of super- and excess currents using a universal distribution function for randomly arranged localized states and found good agreement with our experimental data without any adjustable parameters. The novel functionalities due to the disorder in doped semiconducting films make it possible to fabricate trilayered junctions with enhanced conductance properties and, at the same time, with well separated metallic electrodes.

page 137; International audience

International audience

session Electronics - Junctions, circuit design and fabrication (2A-E-O1); International audience; Following 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, four-layered Nb/Al-Al oxide-Nb junctions with strongly disordered nm-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.