Presence Of Transport Current Research Articles

Sensitivity of SQUID-on-tip (SOT) to a magnetic nano-particle

Within the state-of-the-art 3D Ginzburg–Landau (GL) formalism, we investigate the static features of the most sensitive SQUID-on-tip (SOT) device placed on the top of a nano-particle. The SOT sensor’s free energy, Cooper pair density, and screening current density in the loop are investigated for the nano-particle’s various placements and sizes. The dynamic aspects of the device, such as its voltage–current characteristics, are evaluated in the presence of transport current using the two-dimensional (2D) time-dependent GL formalism. The sensitivity of the SOT is examined using quantum oscillations of the device’s depairing current as a function of the nano-particle’s size and location. The location of the nano-particle at the device’s center and closer to the device’s loop is required for optimal SOT sensitivity.

Effect of transport current on suppression of superconductivity with ultrashort laser pulse

We study the suppression of superconductivity with ultrashort laser pulse in the presence of transport current. The theoretical model is based on the Bardeen–Cooper–Schrieffer relations for the superconducting state coupled with kinetic equations for nonequilibrium Bogoliubov quasiparticles and phonons. The results of numerical simulation for picosecond and femtosecond laser pulses of optical and infrared ranges are given. We discuss the effects of main problem parameters, including the current density.

Accessing the Spectral Function in a Current-Carrying Device.

The presence of an electrical transport current in a material is one of the simplest and most important realizations of nonequilibrium physics. The current density breaks the crystalline symmetry and can give rise to dramatic phenomena, such as sliding charge density waves, insulator-to-metal transitions, or gap openings in topologically protected states. Almost nothing is known about how a current influences the electron spectral function, which characterizes most of the solid's electronic, optical, and chemical properties. Here we show that angle-resolved photoemission spectroscopy with a nanoscale light spot provides not only a wealth of information on local equilibrium properties, but also opens the possibility to access the local nonequilibrium spectral function in the presence of a transport current. Unifying spectroscopic and transport measurements in this way allows simultaneous noninvasive local measurements of the composition, structure, many-body effects, and carrier mobility in the presence of high current densities. In the particular case of our graphene-based device, we are able to correlate the presence of structural defects with locally reduced carrier lifetimes in the spectral function and a locally reduced mobility with a spatial resolution of 500nm.

Study of the equation for the Abrikosov vortex pinning on a linear defect in a superconducting wafer

An equation for an Abrikosov vortex bound state on a linear defect in a superconducting thin film in the presence of transport current is studied. A theoretical analysis of the corresponding boundary problem has been carried out and an estimate of the density of the critical current resulting in the vortex string depinning is obtained. The shape of an elastic vortex string is calculated. The instability threshold of the vortex-bound state due to the increase in the current, which results in the vortex string depinning, is found using numerical modeling. The results agree qualitatively with the data of the experiments carried out on high-temperature superconductors.

Mechanical Behaviors of a Cylindrical Superconducting Composite with Transport Current

The stress, displacement, and magnetostriction induced by flux pinning for a superconducting composite cylinder, which is consisted of two concentric shells with different critical current densities, are calculated analytically in the presence of transport current. Firstly, both the flux and current distributions are given for increasing and decreasing transport current cases by adopting the Bean model. Then the elastic solutions to stress, displacement and magnetostriction are obtained by using plane strain approach. Finally, lots of numerical results on these mechanical behaviors (including the cases of inner superconducting shell being replaced by an elastomer and/or by a hole) are displayed graphically and analyzed in detail. Numerical results show that, among others, during the transport current reduction, tensile stress especially radial tensile stress will occur in the outer region of the composite, and that in general, displacement is always negative in the superconducting composite with transport current. In addition, different to homogeneous superconducting cylinder, as the applied maximal transport current exceeds outer-cylinder critical current, the hysteresis loop of the magnetostriction exists for the full cycle of the transport current. Moreover, different to superconducting composite (including the case of the inner shell being an elastomer), the inner hole has significant effects on both the stress and displacement distributions of the outer superconducting shell, and for a hollow superconductor, the magnetostriction loop still exists even if the maximal transport current does not exceeds the its critical current.

Tunable Generation of Correlated Vortices in Open Superconductor Tubes

As shown theoretically, geometry determines the dynamics of vortices in the presence of transport currents in open superconductor micro- and nanotubes subject to a magnetic field orthogonal to the axis. In low magnetic fields, vortices nucleate periodically at one edge of the tube, subsequently move along the tube under the action of the Lorentz force and denucleate at the opposite edge of the tube. In high magnetic fields, vortices pass along rows closest to the slit. Intervortex correlations lead to an attraction between vortices moving at opposite sides of a tube. Open superconductor nanotubes provide a tunable generator of superconducting vortices for fluxon-based quantum computing.

Stress distribution in a flat superconducting strip with transport current

The stress and magnetostriction induced by flux pinning for a flat superconducting strip of a type II superconductor are calculated analytically in the presence of transport current. The plane stress approach is used to find the exact solutions. By assuming that the current density is magnetic field independent, the body force and normal stress distributions for increasing and decreasing transport currents are given. In addition, the pinning induced magnetostriction is calculated. The results show that, during the transport current reduction, tensile stress may occur. It is worth pointing out that, in the flat superconducting strip with transport current, the stresses are mainly negative. The hysteresis loop of the magnetostriction cannot be observed for the full cycle of the transport current.

Friedel oscillations in the presence of transport currents in a nanowire

We investigate the Friedel oscillations in a nanowire coupled to two macroscopic electrodes of different potentials. We show that the wave-length of the density oscillations monotonically increases with the bias voltage, whereas the amplitude and the spatial decay exponent of the oscillations remain intact. Using the nonequilibrium Keldysh Green functions, we derive an explicit formula that describes voltage dependence of the wave-length of the Friedel oscillations.

Features in evanescent Aharonov-Bohm interferometry

In this work we analyze an Aharonov-Bohm interferometer in the tunneling regime. In this regime, a current magnification effect, which arises in presence of transport currents, is absent. A slight modification in the form of a quantum well incorporated in one of the arms leads to the revival of the current magnification. Systematics in magnetoconductance oscillations are observed in this evanescent wave geometry. In this framework we also see absence of Fano line shapes in transmission resonances but once again one can recover these if the direct path supports propagating modes.

Current magnification effect in mesoscopic systems at equilibrium

We study the current magnification effect and associated circulating currents in mesoscopic systems at equilibrium. Earlier studies have revealed that in the presence of transport current(non-equilibrium situation), circulating currents can flow in a ring even in the absence of magnetic field. This was attributed to current magnification which is quantum mechanical in origin. We have shown that the same effect can be obtained in equilibrium systems, however, in the presence of magnetic flux. For this we have considered an one-dimensional open mesoscopic ring connected to a bubble, and the system is in contact with a single reservoir. We have considered a special case where bubble does not enclose magnetic flux, yet circulating currents can flow in it due to current magnification.

Vortex dynamics near zero field in 3D and 2D superconductors

We have performed voltage noise S V measurements for thick (100 nm) and thin (6 nm) films of amorphous (a-)Mo x Si 1− x to the study vortex dynamics in the presence of transport current at and near zero field ( B=0) below the transition temperature T c. Irrespective of the film thickness, the large 1/ f-type noise appears at B=0 in the low current region where the current–voltage ( I– V) characteristics show strong nonlinearity. As the flux lines penetrate the film by applying a field higher than the lower critical field H c1, the noise decreases abruptly. These results are consistent with the view that large noise at B=0 is due to density fluctuations of thermally excited and subsequently grown vortex loops and dissociated vortex–antivortex pairs for thick and thin films, respectively.

Propagation of magnetostatic waves in a ferrite plus high-T c superconductor structure in the presence of transport current in the superconductor

A thin-film structure consisting of a ferrite and a high-Tc superconductor was used to investigate the effct of the transport current in the superconductor on the amplitude-frequency characteristic and dispersion of surface magnetostatic waves (MSWs) in a ferrite film. It was found that the nature of energy transfer between the MSWs and the superconducting film undergoes a significant change as the transport current is varied. In particular, in one of the current ranges, energy can be transferred both from the MSWs to the superconductor and back again, whereas in another range it can only be efficently transferred to the superconductor.

Flux-flow phenomena and current-voltage characteristics of Josephson-junction arrays with inductances.

We compute current-voltage characteristics of an array of resistively shunted Josephson junctions with screening effects treated self-consistently by introducing inductance coefficients. The flux distribution and flux motion through the system is analyzed in the presence of transport currents and of an external magnetic field. We also discuss the applicability of our model to describe inhomogeneous high-${\mathit{T}}_{\mathit{c}}$ superconductors.

Non-uniform distribution of normal zone in composite superconductors with contact resistance

A region of resistive domains is found in composite superconductors with thermal and electrical contact resistance between the normal metal matrix and the superconductor. The V-I characteristic of a sample with resistive domains is found. It is shown that the existence of resistive domains leads to the emergence of hysteresis upon decay and recovery of superconductivity in the presence of transport current. It is shown that the degradation of composite superconductors with contact resistance may be attributed to the formation of resistive domains.

Magnetization Studies in Nb 3Sn and Ti–Nb Alloys

Magnetization measurements were performed on commercial Nb3Sn and Ti–Nb alloy samples to determine hysteretic loss (magnetization vs magnetic field) and flux-jump stability at 4.2°K in magnetic fields up to 9.3 Wb/m2. The aforementioned superconducting properties were also observed and recorded with various geometries, field sweep rates and thermal environments. The samples consisted of ribbons and/or wires ranging in size from 1.0 to 0.1 cm wide and 0.1 to 0.0001 cm thick. The transport currents ranged from a few amperes to greater than a thousand amperes. These data were obtained utilizing an experimental setup similar to that described by Fietz. The shape and magnitude of the magnetization curves with and without the presence of transport current was not a function of frequency in the range employed, 0.012 to 3.0 Wb/m2·min, except for the multifilament composites. The magnetization and flux-jump stability was found to be a function of the sample dimensions, as would be expected with the earlier expressions given by Wipf, Hancox, Chester, and Stekly. The transport current density was not a function of the dimensions. These data on magnetization and transport current effects are in accord with earlier work by LeBlanc and co-workers on Nb–Zr. There is a first principle derivation of the flux-pinning effects that uses a Josephson junction tunneling model and results in the Kim-Anderson expression. The internal field distribution could be characterized by Hint(x)=(Ha−Hs exp(−x/λ)+Hs, λ=(Hs−Ha)/Jc(Ha). The exponential expression for Jc(Ha) vs Ha of Fietz et al. fits the sample data; however, the Kim-Anderson expression that would be the first term in the expansion of the exponential would also fit the observed data within the experimental error. The data concerning transport current and magnetization can be accurately accounted for (in the infinite sheet configuration) by expressions derived by Yasukochi. The thin plate geometry, with the proper demagnetization factor, is in agreement as well.