68 research outputs found
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 LaCaMnO Manganite Single Crystals
Low frequency noise in current biased LaCaMnO 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 LaCaMnO 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
Tracing the evolution of the two energy gaps in magnesium diboride under pressure
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 conductance and peaks connected to the two gap values, have been revealed. Finally, we report results of a hydrostatic compression experiment showing the evolution of the MgB₂ gap values with pressure. In contrast to the theoretical expectations, we have observed an increase of the smaller gap ∆π whereas the larger gap ∆σ decreased with increasing pressure as it should be for the electron–phonon pairing mechanism. We argue that the so-called separable model of anisotropy effects is insufficient to describe such changes and only improved two-band versions are capable to reproduce the pressure effect on the energy gaps in magnesium diboride
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