Quantum coherent transport in a three-arm beam splitter and a Braess paradox

The Braess paradox encountered in classical networks is a counterintuitive phenomenon when the flow in a road network can be impeded by adding a new road or, more generally, the overall net performance can degrade after addition of an extra available choice. In this work, we discuss the possibility of a similar effect in a phase-coherent quantum transport and demonstrate it by example of a simple Y-shaped metallic fork. To reveal the Braess-like partial suppression of the charge flow in such device, it is proposed to transfer two outgoing arms into a superconducting state. We show that the differential conductance-vs-voltage spectrum of the hybrid fork structure varies considerably when the extra link between the two superconducting leads is added and it can serve as an indicator of quantum correlations which manifest themselves in the quantum Braess paradox.Comment: 9 pages, 3 figures, the author version presented at the Quantum 2017 Workshop (Torino, Italy, 7-13 May 2017) and submitted to the International Journal of Quantum Information; v2: reference 9 added and the introduction extende

Nonequilibrium 1/f Noise in Low-doped Manganite Single Crystals

1/f noise in current biased La0.82Ca0.18MnO3 crystals has been investigated. The temperature dependence of the noise follows the resistivity changes with temperature suggesting that resistivity fluctuations constitute a fixed fraction of the total resistivity, independently of the dissipation mechanism and magnetic state of the system. The noise scales as a square of the current as expected for equilibrium resistivity fluctuations. However, at 77 K at bias exceeding some threshold, the noise intensity starts to decrease with increasing bias. The appearance of nonequilibrium noise is interpreted in terms of bias dependent multi-step indirect tunneling.Comment: 4pages, 3figures,APL accepte

Bias Dependent 1/f Conductivity Fluctuations in Low-Doped La1−x_{1-x}Cax_{x}MnO3_3 Manganite Single Crystals

Low frequency noise in current biased La$_{0.82}$Ca$_{0.18}$MnO$_{3}$ single crystals has been investigated in a wide temperature range from 79 K to 290 K. Despite pronounced changes in magnetic properties and dissipation mechanisms of the sample with changing temperature, the noise spectra were found to be always of the 1/f type and their intensity (except the lowest temperature studied) scaled as a square of the bias. At liquid nitrogen temperatures and under bias exceeding some threshold value, the behavior of the noise deviates from the quasi-equilibrium modulation noise and starts to depend in a non monotonic way on bias. It has been verified that the observed noise obeys Dutta and Horn model of 1/f noise in solids. The appearance of nonequilibrium 1/f noise and its dependence on bias have been associated with changes in the distribution of activation energies in the underlying energy landscape. These changes have been correlated with bias induced changes in the intrinsic tunneling mechanism dominating dissipation in La$_{0.82}$Ca$_{0.18}$MnO$_{3}$ at low temperatures.Comment: Accepted for publication in the Journal of Applied Physic

Universality of transport properties of ultra-thin oxide films

We report low-temperature measurements of current-voltage characteristics for highly conductive Nb/Al-AlOx-Nb junctions with thicknesses of the Al interlayer ranging from 40 to 150 nm and ultra-thin barriers formed by diffusive oxidation of the Al surface. In the superconducting state these devices have revealed a strong subgap current leakage. Analyzing Cooper-pair and quasiparticle currents across the devices, we conclude that the strong suppression of the subgap resistance comparing with conventional tunnel junctions originates from a universal bimodal distribution of transparencies across the Al-oxide barrier proposed earlier by Schep and Bauer. We suggest a simple physical explanation of its source in the nanometer-thick oxide films relating it to strong local barrier-height fluctuations which are generated by oxygen vacancies in thin aluminum oxide tunnel barriers formed by thermal oxidation.Comment: revised text and a new figur

Studies of resistance switching effects in metal/YBa2Cu3O7-x interface junctions

Current-voltage characteristics of planar junctions formed by an epitaxial c-axis oriented YBa2Cu3O7-x thin film micro-bridge and Ag counter-electrode were measured in the temperature range from 4.2 K to 300 K. A hysteretic behavior related to switching of the junction resistance from a high-resistive to a low-resistive state and vice-versa was observed and analyzed in terms of the maximal current bias and temperature dependence. The same effects were observed on a sub-micrometer scale YBa2Cu3O7-x thin film - PtIr point contact junctions using Scanning Tunneling Microscope. These phenomena are discussed within a diffusion model, describing an oxygen vacancy drift in YBa2Cu3O7-x films in the nano-scale vicinity of the junction interface under applied electrical fields.Comment: To be published in Applied Surface Science

Submicron-sized MoRe-doped Si-MoRe Josephson junctions with a low specific capacitance

Abstract We start with a short look at the problem of low-capacitance Josephson junctions, its history, and actual state-of-the-art. It is argued that such devices are important for applications requiring nonhysteretic current-voltage characteristics since reduction of capacitance by several times makes it possible to increase the device resistance by the same amount while keeping the McCumber-Stewart damping parameter unaltered. Moreover, at very high frequencies the capacitance in the RCSJ circuit with a parallel connection starts to shunt the superconducting current component due to reduction of the corresponding reactance inversely proportional to C. Hence, to extend the operating frequency range of a Josephson junction its capacitance should be as small as possible. As a solution of a new type of the Josephson device, less resistive and with smaller capacitance, we propose and realize a submicron-sized trilayer with tens nm-thick Si interlayer doped by metallic ultra-small inclusions and superconducting Mo-Re alloy electrodes

Effect of crystallographic anisotropy on the resistance switching phenomenon in perovskites

Resistance switching effects in metal/perovskite contacts based on epitaxial c-axis oriented Y-Ba-Cu-O (YBCO) thin films with different crystallographic orientations have been studied. Three types of Ag/YBCO junctions with the contact restricted to (i) c-axis direction, (ii) ab-plane direction, and (iii) both were designed and fabricated, and their current-voltage characteristics have been measured. The type (i) junctions exhibited conventional bipolar resistance switching behavior, whereas in other two types the low-resistance state was unsteady and their resistance quickly relaxed to the initial high-resistance state. Physical mechanism based on the oxygen diffusion scenario, explaining such behavior, is discussed.Comment: The final version was published in Journal of Applied Physics (2012

Interaction corrections to tunneling conductance in ballistic superconductors

It is known that in the two-dimensional disordered superconductors electron-electron interactions in the Cooper channel lead to the negative logarithmic in temperature correction to the tunneling conductance above the critical temperature. Physically this result appears due to the density of states suppression by superconductive fluctuations near the Fermi level. It is interesting that the other correction, which accounts for the Maki-Thompson-type interaction of fluctuations, is positive and exhibits strong power law, which dominates the logarithmic term in the immediate vicinity of the critical temperature. An interplay between these two contributions determines the zero-bias anomaly in fluctuating superconductors. This paper is devoted to the fate of such interaction corrections in the ballistic superconductors. It turns out that ballistic dynamic fluctuations perturb single-particle density of states near the Fermi level at the energy scale which is different from that in the diffusive case. As the consequence, fluctuation region becomes much broader. In this regime we confirm that correction to the tunneling conductance remains negative and logarithmic not too close to the critical temperature while in the immediate vicinity of the transition we find novel power law for the Maki-Thompson contribution. It is suggested that peculiar non-monotonous temperature dependence of the tunneling conductance may be probed via magneto-tunnel experiments.Comment: 4 pages, 2 figure