A. P. Shapovalov

Effect of higher borides and inhomogeneity of oxygen distribution on critical current density of undoped and doped magnesium diboride

The effect of doping with Ti, Ta, SiC in complex with synthesis temperature on the amount and distribution of structural inhomogeneities in MgB2 matrix of high-pressure-synthesized-materials (2 GPa) which can influence pinning: higher borides (MgB12) and oxygen-enriched Mg-B-O inclusions, was established and a mechanism of doping effect on jc increase different from the generally accepted was proposed. Near theoretically dense SiC-doped material exhibited jc= 106 A/cm2 in 1T field and Hirr =8.5 T at 20 K. The highest jc in fields above 9, 6, and 4 T at 10, 20, and 25 K, respectively, was demonstrated by materials synthesized at 2 GPa, 600 °C from Mg and B without additions (at 20 K jc= 102 A/cm2 in 10 T field). Materials synthesized from Mg and B taken up to 1:20 ratio were superconductive. The highest jc (6×104 A/cm2 at 20 K in zero field, Hirr= 5 T) and the amount of SC phase (95.3% of shielding fraction), Tc being 37 K were demonstrated by materials having near MgB12 composition of the matrix. The materials with MgB12 matrix had a doubled microhardness of that with MgB2 matrix (25±1.1 GPa and 13.08±1.07 GPa, at a load of 4.9 N, respectively).

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.

Formation of nanostructure in magnesium diboride based materials with high superconducting characteristics

The paper presents an analysis of the properties of bulk superconducting magnesium diboride-based materials obtained by heating at high quasi-hydrostatic pressures (1–2 GPa), hot pressing (30 MPa), spark plasma sintering (16–96 MPa) and loose powder sintering. It is shown that the optimization of impurity distribution in MgB2 can be achieved by varying the synthesis conditions and introducing dopants. In particular, polycrystalline MgB2 materials synthesized at 2 GPa and containing a high amount of impurity oxygen demonstrates high critical current densities (106 and 103 A/cm2 at 20 K in magnetic fields of 1 and 8.5 T, respectively). It is found that the oxygen impurities are mainly localized in nanolayers or nanoinclusions, homogeneously distributed in the matrix. They act as pinning centers, while the MgB2 matrix also contains small amounts of dissolved oxygen. Impurity or intentionally added carbon entering the magnesium diboride structure leads to an increase in the critical magnetic fields up to Bc2 ...

The effect of size of the SiC inclusions in the AlN–SiC composite structure on its electrophysical properties

AlN–SiC–Y3Al5O12 composite materials with a high absorption of microwave frequency (27–65 dB/cm) produced by pressureless sintering of mixtures consisting of AlN(2H), Y2O3, and SiC (6H) in 46, 4, 50 wt %, respectively, have been studied. The SiC components of the mixtures were used in sizes of 1, 5, and 50 μm. It has been shown that the resistivity of the developed materials depends essentially on the materials structures: sizes of SiC inclusions, distances between them, and state of the interfaces. It has been found that the increase of the SiC inclusions sizes in the material structure from 3 to 7 μm results in the decrease of the resistivity from 104 to 90 Ω·m, and at the decrease of the SiC inclusions sizes from 3 to 0.5 μm there forms a SiC uninterrupted skeleton, which also decreases the resistivity to 210 Ω·m. Thus, composite materials that contain 50 wt % SiC (inclusions sizes of 3 μm) are the most efficient in producing absorbers of microwave radiation. Interlayers of yttrium aluminum garnet, which are located at the SiC grains boundaries, prevent the forming of AlN(2H)–SiC(6H) solid solutions and thus, make it possible to keep high dielectric characteristics of a composite material based on aluminum nitride and afford a high absorption of a microwave radiation.

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...

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