Mikhail Belogolovskii

Analysis of the Temperature Stability of Overdamped

The temperature stability is one of the most important factors determining the successful application of the Josephson effect to the ac voltage standard or to analog and digital electronics. Whereas SIS hysteretic junctions with an Ambegaokar-Baratoff (A&B) IC(T) behavior exhibit a reduced drift around the working temperature of the liquid helium, metallic-barrier SNS junctions follow a Kulik-Omelianchuck (K&O) behavior with a sharp temperature dependence of Ic in this region. The objective of our research is to study this aspect for overdamped Nb/Al-AlOx/Nb heterostructures with current densities of 10-75 kA/cm2 and characteristic voltages from 100 to more than 500 muV, that have been fabricated at INRiM. The IC(T) characteristics, measured for Josephson heterostructures with different thickness, s, and exposure, E, essentially deviate from A&B and K&O curves, because of proximity effect caused by the comparatively high value of s (up to 100 nm). We study theoretically two extreme limits: the clean and the dirty limit for the interlayer between the superconducting electrodes and discuss the temperature stability of the junctions characterizing it with the temperature derivative dIC/dT. The combined experimental and theoretical analysis of the problem provides a way for understanding, controlling and improving the design of the Nb/Al-AlOx/Nb junctions in order to enhance their reliability.

{\hbox{Nb}}/{\hbox{Al-}}{\hbox{AlO}}_{x}/{\hbox{Nb}}

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.

Josephson Junctions

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...

Analysis of Internally Shunted Josephson Junctions

Perspective for qubits Josephson Mo-Re alloy-oxide–Pb, Mo-Re alloy-normal metal–oxide–MoRe alloy, and NbN -normal metal–oxide–Pb junctions have been fabricated and investigated.

Tunneling through localized barrier states in superconducting heterostructures

Josephson-junction devices such as suitably designed nanoSQUIDs can provide a sensing element for micro and nanoelectronics. In this paper, the properties of such device must be tailored in order to have an optimal response. In particular, it relates its temperature stability, and a comparatively weak dependence of the critical supercurrent <italic>I</italic>_c on temperature <italic>T</italic> in the working range. To realize it, a concave upward <italic>I</italic>_c-versus-<italic>T</italic> curve is required. The aim of this paper was to study conditions when such temperature dependence can be realized in four-layered S/N-I-S Josephson junctions, mantaining the overdamped behavior. We show how the shape of the <italic>I</italic>_c-versus-<italic>T</italic> dependence can provide important information about the strength of the S/N proximity coupling.

MoRe-BASED AND NbN-BASED TUNNEL JUNCTIONS AND THEIR CHARACTERISTICS

We have studied transport characteristics of mesoscopic multiple-mode superconducting contacts formed between two grains in bulk two-gap magnesium diboride. The experimental setup was realized by driving a normal-metal tip into MgB2 polycrystalline sample and proved to be extremely stable, providing possibility to perform pressure experiments at low temperatures. It is argued that in our procedure a small piece of the superconducting electrode is captured by the tip apex and, as a result, two junctions in series are formed: a junction between a tip and MgB2 grain and a mesoscopic disordered contact between two superconducting pellets. Although the relative weight of the first junction resistance was considerably less, its contribution is shown to be important for the comparison of measured data with expected gap values. Two hallmarks of multiple Andreev reflections inside the MgB2–c–MgB2 contact (c stands for a high-transparent constriction), a zero-bias 1/ |V|-like singularity of the dc differential cond...

Superconducting and Dissipative Characteristics of Overdamped SNIS Josephson Junctions for Sensing Applications

Submicron Josephson devices are of a particular interest to the emerging field of quantum information, nanosized superconducting quantum interference devices (nano-SQUID) fabrication, AC voltage synthesis circuits for quantum metrology, etc. In this paper, we report on the development of our technology for producing non-hysteretic Nb/A-AlOx-Nb four-layered junctions with a diverse range of submicron dimensions, based on the focused ion beam (FIB) sculpting technique. We observed essential modifications of the main electrical characteristics as a result of the shrinking of the junction area, and relate them to the changes of the buried Nb/Al interface. We show that the electrical parameters-versus-temperature dependence can provide information on the quality of the junction interfaces. This aspect could be useful to better control of the entire fabrication process, allowing a further large-scale integration of Josephson nanodevices with improved transport characteristics.

Tracing the evolution of the two energy gaps in magnesium diboride under pressure

A study of the density of quasiparticle states of a lead film, which is a conventional superconductor with spin-singlet electron pairing, as a function of the nanoscale ferromagnetic nickel layer thickness that is in direct contact with the lead (inverse proximity effect). It is found that the penetration depth of superconducting correlations into the ferromagnetic nickel is of the same order of magnitude as in contacts involving lead and a normal metal. This behavior can be explained by the appearance of an inhomogeneous exchange field at the interface, which leads to the effective conversion of spin-singlet (rapidly decaying in a ferromagnet) Cooper pairs into spin-triplet pairs, which are stable with respect to exchange interaction.

Controlling the Interface Properties of Submicrometric Nb/A–

Emergence of phase-slip lines is one of the main mechanisms for dissipation in current-carrying superconducting thin films with lateral dimensions larger than the superconducting coherence length. We show that effective control of the wide resistive region between dissipation-free and completely ohmic transport regime can be achieved by evaporating a half-metallic strip, made from a Co2CrAl compound in our case, directly onto a type-II superconducting film, a Mo56Re44 alloy in our case, and tuning the spin-polarized injection current. We have revealed steps-like features in current-voltage characteristics of the Mo-Re films, even without any injection, and explained it within a “self-injection” scenario in contacts formed by ferromagnetic (F) and superconducting (S) layers. At certain injection levels, a set of resistive metastable states following the main criteria for the phase-slip events was observed. We have found that low magnetic fields, even less than the lower critical field of the Mo-Re alloy, have a significant impact on the resistive transition region of the S film probably due to their effect on the magnetic ordering in the F strip, thus producing a strong coupling between transport characteristics of proximized S and F layers.

\hbox{AlO}_{\rm x}

Theoretical and experimental analysis of electron transport across ultra-thin, homogeneously disordered oxide layers is presented with particular regard to the question of how much the effects are universal. We show that (i) distribution of transparencies across dirty subnanometer-thick insulating films is bimodal and (ii) conductance-voltage characteristics of oxide layers with thicknesses increased up to several nanometers are power functions with an index near 1.3. The universality of transport properties is explained as an effect of strong local barrier-height fluctuations generated by the presence of oxygen vacancies.