Dendritic flux avalanches in superconducting films

Thermomagnetic instability in general, and dendritic flux avalanches in particular, have attracted considerable attention of both scientists and engineers working on superconductor applications. Though being harmful for the performance of many superconducting devices, the avalanches provide a fruitful playground for experimental and theoretical studies of complex dynamics of the vortex matter. In this paper, we report on the progress in understanding the mechanisms responsible for the development of the giant magnetic avalanches. We review recent results on magneto-optical imaging of the fingering instability in superconducting films and analyze them basing on the recent theoretical model that establishes criteria for onset of the dendritic avalanches

Magneto-optical investigations of Ag-sheathed Bi-2223 tapes with ferromagnetic shielding

An increase in the critical current and suppression of AC losses in superconducting wires and tapes with soft magnetic sheath have been predicted theoretically and confirmed experimentally. In this work we present the results of magneto-optical investigations on a series of Ag-sheathed Bi-2223 tapes with Ni coating. We visualize distributions of magnetic field at increasing external field and different temperatures, demonstrating a difference between the flux propagation in the superconductor with Ni rims and a reference sample without Ni coating.Comment: 2 page

Thermoelectric effects in superconducting proximity structures

Attaching a superconductor in good contact with a normal metal makes rise to a proximity effect where the superconducting correlations leak into the normal metal. An additional contact close to the first one makes it possible to carry a supercurrent through the metal. Forcing this supercurrent flow along with an additional quasiparticle current from one or many normal-metal reservoirs makes rise to many interesting effects. The supercurrent can be used to tune the local energy distribution function of the electrons. This mechanism also leads to finite thermoelectric effects even in the presence of electron-hole symmetry. Here we review these effects and discuss to which extent the existing observations of thermoelectric effects in metallic samples can be explained through the use of the dirty-limit quasiclassical theory.Comment: 14 pages, 10 figures. 374th WE-Heraus seminar: Spin physics of superconducting heterostructures, Bad Honnef, 200

First-principles quantum transport modeling of thermoelectricity in single-molecule nanojunctions with graphene nanoribbon electrodes

We overview nonequilibrium Green function combined with density functional theory (NEGF-DFT) modeling of independent electron and phonon transport in nanojunctions with applications focused on a new class of thermoelectric devices where a single molecule is attached to two metallic zigzag graphene nanoribbons (ZGNRs) via highly transparent contacts. Such contacts make possible injection of evanescent wavefunctions from ZGNRs, so that their overlap within the molecular region generates a peak in the electronic transmission. Additionally, the spatial symmetry properties of the transverse propagating states in the ZGNR electrodes suppress hole-like contributions to the thermopower. Thus optimized thermopower, together with diminished phonon conductance through a ZGNR/molecule/ZGNR inhomogeneous structure, yields the thermoelectric figure of merit ZT~0.5 at room temperature and 0.5<ZT<2.5 below liquid nitrogen temperature. The reliance on evanescent mode transport and symmetry of propagating states in the electrodes makes the electronic-transport-determined power factor in this class of devices largely insensitive to the type of sufficiently short conjugated organic molecule, which we demonstrate by showing that both 18-annulene and C10 molecule sandwiched by the two ZGNR electrodes yield similar thermopower. Thus, one can search for molecules that will further reduce the phonon thermal conductance (in the denominator of ZT) while keeping the electronic power factor (in the nominator of ZT) optimized. We also show how often employed Brenner empirical interatomic potential for hydrocarbon systems fails to describe phonon transport in our single-molecule nanojunctions when contrasted with first-principles results obtained via NEGF-DFT methodology.Comment: 20 pages, 6 figures; mini-review article prepared for the special issue of the Journal of Computational Electronics on "Simulation of Thermal, Thermoelectric, and Electrothermal Phenomena in Nanostructures", edited by I. Knezevic and Z. Aksamij

Nucleation and propagation of thermomagnetic avalanches in thin-film superconductors

Stability of the vortex matter — magnetic flux lines penetrating into the material — in type-II superconductor films is crucially important for their application. If some vortices get detached from pinning centres, the energy dissipated by their motion will facilitate further depinning, and may trigger an electromagnetic breakdown. In this paper, we review recent theoretical and experimental results on development of the above mentioned thermomagnetic instability. Starting from linear stability analysis for the initial critical-state flux distribution we then discuss a numerical procedure allowing to analyze developed flux avalanches. As an example of this approach we consider ultra-fast dendritic flux avalanches in thin superconducting disks. At the initial stage the flux front corresponding to the dendrite’s trunk moves with velocity up to 100 km/s. At later stage the almost constant velocity leads to a specific propagation regime similar to ray optics. We discuss this regime observed in superconducting films coated by normal strips. Finally, we discuss dramatic enhancement of the anisotropy of the flux patterns due to specific dynamics. In this way we demonstrate that the combination of the linear stability analysis with the numerical approach provides an efficient framework for understanding the ultra-fast coupled nonlocal dynamics of electromagnetic fields and dissipation in superconductor films

Dephasing and dissipation in qubit thermodynamics

VK: Low Temperature LaboratoryWe analyze the stochastic evolution and dephasing of a qubit within the quantum jump approach. It allows one to treat individual realizations of inelastic processes, and in this way it provides solutions, for instance, to problems in quantum thermodynamics and distributions in statistical mechanics. We demonstrate that dephasing and relaxation of the qubit render the Jarzynski and Crooks fluctuation relations (FRs) of nonequilibrium thermodynamics intact. On the contrary, the standard two-measurement protocol, taking into account only the fluctuations of the internal energy U, leads to deviations in FRs under the same conditions. We relate the average ⟨e−βU⟩ (where β is the inverse temperature) with the qubit's relaxation and dephasing rates in the weak dissipation limit and discuss this relationship for different mechanisms of decoherence.Peer reviewe

Shuttle transport in nanostructures

The coupling between mechanical deformations and electronic charge transport in nanostructuresand in composite materials with nanoscale components gives rise to a new class of phenomena |nanoelectromechanical transport | and opens up a new route in nanotechnology. The interplaybetween the electronic and mechanical degrees of freedom is especially important in nanocompositesconsisting of materials with very di\uaeerent elastic properties. Mechanical degrees of freedom takeon a primary role in the charge transfer process in many single-electron devices, where transport iscontrolled by quantum-mechanical tunnelling and Coulomb interactions, but where tunnel barrierscan be modi\uafed as a result of mechanical motion. A typical system of this kind is a single-electrontransistor (SET) with deformable tunnel barriers, a so called Nano-Electro-Mechanical SET (NEM-SET). The new kind of electron transport in this and other types of nanodevices is referred to as"shuttle transport" of electrons, which implies that electrons is transferred between metallic leadsvia a movable small-sized cluster. The present review is devoted to the fundamental aspects ofshuttle transport and to a description of major developments in the theoretical and experimentalresearch in the \uafeld. Prospective applications of this exciting phenomenon that remarkably combinestraditional mechanics of materials with the most advanced e\uaeects of quantum physics, will also betouched upon

Nanomechanical shuttle transfer of electrons

Small metal particles embedded in a material subject to an external electric field can contribute to the conductance by mechanically transporting charge. This was already demonstrated in Millikan\u27s pioneering experiment that proved that charge is quantized. While the effect of charge quantization can be pronounced for submicron conducting particles and lead to Coulomb blockade of W tunnelling, the nanomechanical aspect of single-electron tunnelling becomes prominent only in 5 nanometer-size self-assembled structures. The coupling between mechanical deformations and LU electronic charge transport in composite materials with nanoscale components gives rise to a new class of phenomena-nanoelectromechanical transport-and opens up a new route in nanotechnology. The interplay between the electronic and mechanical degrees of freedom is especially important in nanocomposites consisting of materials with very different elastic properties. A typical system of this kind is a single-electron transistor (SET) with cleformable tunnel barriers, a so called Nano-Electro-Mechanic.al SET (NEM-SET). The new kind of electron transport in this and other types of nanodevices is referred to as "shuttle transport" of electrons, which implies that electrons is transferred between metallic leads via a movable small-sized cluster. The present review is devoted to the fundamental aspects of shuttle transport and to a description of major developments in the theoretical and experimental research in the field. Prospective applications of this exciting phenomenon that remarkably combines traditional mechanics of materials with the most advanced effects of quantum physics, will also be touched upon

Nanomechanical shuttle transfer of electrons

Small metal particles embedded in a material subject to an external electric field can contribute to the conductance by mechanically transporting charge. This was already demonstrated in Millikan\u27s pioneering experiment that proved that charge is quantized. While the effect of charge quantization can be pronounced for submicron conducting particles and lead to Coulomb blockade of W tunnelling, the nanomechanical aspect of single-electron tunnelling becomes prominent only in 5 nanometer-size self-assembled structures. The coupling between mechanical deformations and LU electronic charge transport in composite materials with nanoscale components gives rise to a new class of phenomena-nanoelectromechanical transport-and opens up a new route in nanotechnology. The interplay between the electronic and mechanical degrees of freedom is especially important in nanocomposites consisting of materials with very different elastic properties. A typical system of this kind is a single-electron transistor (SET) with cleformable tunnel barriers, a so called Nano-Electro-Mechanic.al SET (NEM-SET). The new kind of electron transport in this and other types of nanodevices is referred to as "shuttle transport" of electrons, which implies that electrons is transferred between metallic leads via a movable small-sized cluster. The present review is devoted to the fundamental aspects of shuttle transport and to a description of major developments in the theoretical and experimental research in the field. Prospective applications of this exciting phenomenon that remarkably combines traditional mechanics of materials with the most advanced effects of quantum physics, will also be touched upon

Shuttle transport in nanostructures

The coupling between mechanical deformations and electronic charge transport in nanostructures and in composite materials with nanoscale components gives rise to a new class of phenomena | nanoelectromechanical transport | and opens up a new route in nanotechnology. The interplay between the electronic and mechanical degrees of freedom is especially important in nanocomposites consisting of materials with very di®erent elastic properties. Mechanical degrees of freedom take on a primary role in the charge transfer process in many single-electron devices, where transport is controlled by quantum-mechanical tunnelling and Coulomb interactions, but where tunnel barriers can be modi¯ed as a result of mechanical motion. A typical system of this kind is a single-electron transistor (SET) with deformable tunnel barriers, a so called Nano-Electro-Mechanical SET (NEM- SET). The new kind of electron transport in this and other types of nanodevices is referred to as "shuttle transport" of electrons, which implies that electrons is transferred between metallic leads via a movable small-sized cluster. The present review is devoted to the fundamental aspects of shuttle transport and to a description of major developments in the theoretical and experimental research in the ¯eld. Prospective applications of this exciting phenomenon that remarkably combines traditional mechanics of materials with the most advanced e®ects of quantum physics, will also be touched upon