Volodymyr E. Shaternik

Analysis of Internally Shunted Josephson Junctions

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.

Tunneling through localized barrier states in superconducting heterostructures

Thin film heterostructures composed of superconducting electrodes (molybdenum rhenium alloy) and a nanoscale silicon layer doped with tungsten, have been designed and experimentally studied. The current-voltage characteristics of junctions exhibiting local maxima of the current against the background of abrupt current increases for the first time, were measured in the voltage range of −800 to 800 mV, at temperatures of 4.2–8 K. The positions of these singularities, which are symmetrical with respect to zero voltage, varied from sample to sample within the range of 40–300 mV. With increasing temperature, they became blurred and completely vanished with the disappearance of superconductivity in the electrodes. The nature of the observed singularities is associated with the properties of electron tunneling through the impurity states localized in the semiconducting barrier. The use of a superconducting electrode enhances the interaction of the localized electron with the conduction electrons thanks to the ro...

Charge Transport in Hybrid Tunnel Superconductor—Quantum Dot—Superconductor Junctions

We have studied thin-film superconductor - semiconductor (with quantum dots)-superconductor MoRe-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 IcRN, the product of the critical Josephson supercurrent Ic and the normal-state resistance RN, of the samples were unusually high. Simultaneously, we have observed large excess quasiparticle currents Iexc 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.

Charge transport in superconducting MoRe–Si(W)–MoRe heterostructures with hybrid semiconductor barrier containing metal nanoclusters

Thin-film MoRe–Si(W)–MoRe heterostructures consisting of superconducting electrodes (molybdenum-rhenium alloy) and a hybrid semiconductor tunnel barrier consisting of a nanosized silicon layer with tungsten nanoclusters were fabricated and experimentally studied. Current-voltage characteristics of the heterostructures were measured in a wide voltage range from −900 to 900 mV and at temperatures from 4.2 to 8 K, under applied magnetic fields and microwave irradiation. We argue that the temperature dependences of the superconducting critical current and normal-state resistance of the heterostructures might indicate the presence of Coulomb blockade regime, resonant tunneling and resonant-percolation transport mechanism in the junctions, depending on the tungsten content in the hybrid barrier and the applied bias voltage. The measured characteristics suggest that for the superconducting current exceeding some critical value, the phase-slip centers of the superconducting order parameter are formed in the tungs...

MoRe-based and MgB2 -based tunnel junctions and their characteristics

Perspective Josephson Mo-Re alloy-oxide-Pb, MgB2- oxide – Mo-Re alloy and Mo-Re alloy-normal metal-oxide- normal metal-Mo-Re alloy junctions have been fabricated and investigated. Thin (~50-100 nm) MoRe superconducting films are deposited on Al2O3 substrates by using a dc magnetron sputtering of MoRe target. Thin (~50-100 nm) MgB2 superconducting films are deposited on Al2O3 substrates by using e-beam evaporation of boron and thermal coevaporation of magnesium. To investigate a transparency spread for the fabricated junctions barriers the computer simulation of the measured quasiparticle I–V curves have been done in framework of the model of multiple Andreev reflections in double-barrier junction interfaces. Its demonstrated the investigated junctions can be described as highly asymmetric double-barrier Josephson junctions with great difference between the two barrier transparencies. Results of computer simulation of quasiparticles I–V curves of junctions are presented and discussed. The IC(T) characteristics, measured for Josephson heterostructures with different thickness of metal layer s and exposure dose E, essentially deviate from an Ambegaokar- Baratoff (A&B) IC(T) behavior and Kulik-Omelianchuck (K&O) curves, because of proximity effect caused by the comparatively high value of s (up to 100 nm).

Dissipation effects in superconducting heterostructures with tungsten nanorods as weak links

V.E. Shaternik, A.P. Shapovalov, O.Yu. Suvorov, E.S. Zhitlukhina, M.A. Belogolovskii, P. Febvre, and A.A. Kordyuk G.V. Kurdyumov Institute for Metal Physics, National Academy of Sciences of Ukraine 36 Academician Vernadsky Blvd., Kyiv 03142, Ukraine E-mail: shat@imp.kiev.ua V.N. Bakul Institute for Superhard Materials, National Academy of Sciences of Ukraine 2 Avtozavodskaya Str., Kyiv 04074, Ukraine O.O. Galkin Donetsk Institute for Physics and Engineering, National Academy of Sciences of Ukraine 46 Nauki Ave., Kyiv 03028, Ukraine Vasyl’ Stus Donetsk National University, 21 600-richya Str., Vinnytsia 21021, Ukraine Superconducting Electronics Group, IMEP-LAHC – CNRS UMR 5130, Université Savoie Mont Blanc Le Bourget du Lac Cedex 73376, France

Negative differential conductance in doped-silicon nanoscale devices with superconducting electrodes

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.

Structure and Transport Characteristics of Tunnel Junctions with Hybrid Semiconductor Barriers with Quantum Dots

Structure and Transport Characteristics of Tunnel Junctions with Hybrid Semiconductor Barriers with Quantum Dots V.E. Shaternik, A.P. Shapovalovb,∗, T.A. Prikhna, O.Yu. Suvorov, M.A. Skorik, A.V. Shaternik, V.I. Bondarchuk and E.E. Zubov G.V. Kurdyumov Institute for Metal Physics, Kyiv 03142, Ukraine V. Bakul Institute for Superhard Materials, Kyiv 07074, Ukraine Vasyl’ Stus Donetsk National University, Vinnytsia 21021, Ukraine

Structure and Properties of MgB 2 Bulks, Thin Films, and Wires

This paper describes the established correlations between the characteristics of MgB₂-based superconducting materials and their structural features, in particular the oxygen distribution and content. As indicated by SEM and Auger analysis, a sizeable amount of oxygen is usually present in superconducting MgB₂ -based materials (bulks, thin films, and wires). The matrix phase of bulk MgB₂ contains rather small amounts of oxygen, but contains a high amount of dispersed inclusions or areas with compositions close to MgBO. X-ray phase analysis with Rietveld refinement (using the FullProf Suite program package) of several highly dense MgB₂ -based bulk samples (with high critical current densities) showed that the superconducting phase had a composition within the range MgB_1.68O_0.26 and MgB_1.74O_0.32 instead of pure MgB₂. Besides, a small amount of a phase with MgO structure was observed in the materials by X-rays as well. The calculation of the enthalpy of formation confirms the possibility of oxygen solubility in MgB₂ and shows that the formation of MgB_1.75B_0.25 is most favorable. The results of <italic>ab initio</italic> calculations of the electronic structure and stability of the MgB₂ compounds with partial oxygen substitution for boron show that it is energetically preferable for oxygen atoms to replace boron pairwise.

Structure and properties of oxygen-containing thin films and bulk MgB2

A structural Auger spectroscopy study of MgB2 thin (~140 nm) oxygen-containing polycrystalline films produced by magnetron sputtering and 99% dense MgB2 bulks synthesized at 2 GPa allows us to conclude that jc of MgB2 depends to a high extent on the amount and distribution of oxygen in the material matrix. jc reached 7.8-2.7 MA/cm2 below 1T at 20 K in the films and 0.3-0.9 MA/cm2 (depending on the boron used) in the bulks. The higher jc in MgB2 thin films can be associated with finer oxygen-enriched Mg-B-O inclusions and their higher density in the film structure compared to the bulk. Calculations of the total electron density of states (DOS) in MgB2, MgB1.75O0.25, MgB1.5O0.5 and MgBO showed that all the compounds are conductors with metal-like behaviour. The DOS is even higher in MgB1.5O0 5 than in MgB2 and the binding energy is similar. So, the experimentally found presence of some dissolved oxygen in MgB2 does not contradict its high SC performance. The introduction of a high amount of oxygen into the MgB2 structure does not dramatically reduce the materials Tc and allows obtaining highjc as observed in our MgB2 films and bulks.