Browsing by Author "Bullard, Thomas J. III"

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    Dynamics of magnetic flux lines in the presence of correlated disorder
    Bullard, Thomas J. III; Das, Jayajit; Täuber, Uwe C. (2013-07-28)
    We investigate the dynamics of interacting magnetic flux lines driven by an external current in the presence of linear pinning centers, arranged either in a periodic square lattice or placed randomly in space, by means of three-dimensional Monte Carlo simulations. Compared to the non-interacting case, the repulsive forces between the vortices reduce the critical current J_c, as determined from the depinning threshold in the current-voltage (I-V) characteristics. Near the depinning current J_c, the voltage power spectrum S(w) reveals broad-band noise, characterized by a $1/w^a$ power law decay with a <= 2. At larger currents the flux lines move with an average velocity v_{cm}. For a periodic arrangement of columnar pins with a lattice constant $d$ and just above J_c, distinct peaks appear in the voltage noise spectrum S(w) at w ~ v_{cm}/d which we interpret as the signature of stick-slip flux line motion.
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    Nonequilibrium steady states of driven magnetic flux lines in disordered type-II superconductors
    Klongcheongsan, T.; Bullard, Thomas J. III; Täuber, Uwe C. (IOP, 2010-02-01)
    We investigate driven magnetic flux lines in layered type-II superconductors subject to various configurations of strong point or columnar pinning centers by means of a three-dimensional elastic line model and Metropolis Monte Carlo simulations. We characterize the resulting nonequilibrium steady states by means of the force-velocity / current-voltage curve, static structure factor, mean vortex radius of gyration, number of double-kink and half-loop excitations, and velocity / voltage noise spectrum. We compare the results for the above observables for randomly distributed point and columnar defects, and demonstrate that the three-dimensional flux line structures and their fluctuations lead to a remarkable variety of complex phenomena in the steady-state transport properties of bulk superconductors.
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    A Study of Narrowband Noise Characteristics Associated with Vortex Motion in High Temperature Superconductors
    Bullard, Thomas J. III (Virginia Tech, 2005-05-11)
    Vortex motion plays an important role in the transport properties of high Tc superconductors. In the presence of a sufficiently large applied current vortices will drift creating an ohmic resistance in the material, while defects in the material will tend to inhibit their motion. Some types of material defects are more effective at pinning then others, and therefore, above the depinning threshold, may effect the motion of vortices differently. To investigate their motion, voltage noise generated by moving vortices is studied for different material defect types using a nonequilibrium Metropolis Monte Carlo simulation. The current-voltage (I-V) characteristics obtained from the simulation for various vortex densities and defect types show features similar to those obtained in experiments. The power spectra generated for point and columnar disorder are then compared for increasing vortex density. Above, but near the depinning threshold, broadband noise associated with plastic vortex flow is observed for columnar defects at low vortex densities, while for higher densities a triangular lattice is obtained along with a washboard signal and higher harmonics. For point defects a washboard signal with higher harmonics is always observed in the region investigated. These results suggest that power spectra for both point and columnar defects are qualitatively similar for higher vortex densities (larger magnetic fields). A second comparison is made by observing, on the one hand, the power spectra for finite linear defects increasing in length and, on the other hand, increasing point defect strength. Power spectra and structure factor results are very similar for these results as well. Both show a trend from an ordered to a disordered system with a washboard peak first increasing and then decreasing in power with increasing pinning efficiency. For both defect types the power spectrum is eventually dominated by broadband noise indicating the approach to the pinned glassy phases.
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    Vortex transport and voltage noise in disordered superconductors
    Das, Jayajit; Bullard, Thomas J. III; Täuber, Uwe C. (Elsevier, 2003-02-01)
    We study, by means of three-dimensional Monte Carlo simulations, the current–voltage (I–V) characteristics and the voltage noise spectrum at low temperatures of driven magnetic flux lines interacting with randomly placed point or columnar defects, as well as with periodically arranged linear pinning centers. Near the depinning current Jc, the voltage noise spectrum S(ω) universally follows a 1/ωα power law. For currents J>Jc, distinct peaks appear in S(ω) which are considerably more pronounced for extended as compared to point defects, and reflect the spatial distribution of the correlated pinning centers.
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    Vortex washboard voltage noise in type-II superconductors
    Bullard, Thomas J. III; Das, Jayajit; Daquila, George L.; Täuber, Uwe C. (Springer, 2008-10-01)
    In order to characterize flux flow through disordered type-II superconductors, we investigate the effects of columnar and point defects on the vortex velocity / voltage power spectrum in the driven non-equilibrium steady state. We employ three-dimensional Metropolis Monte Carlo simulations to measure relevant physical observables including the force-velocity / current-voltage (I-V) characteristics, vortex spatial arrangement and structure factor, and mean flux line radius of gyration. Our simulation results compare well to earlier findings and physical intuition. We focus specifically on the voltage noise power spectra in conjunction with the vortex structure factor in the presence of weak columnar and point pinning centers. We investigate the vortex washboard noise peak and associated higher harmonics, and show that the intensity ratios of the washboard harmonics are determined by the strength of the material defects rather than the type of pins present. Through varying columnar defect lengths and pinning strengths as well as magnetic flux density we further explore the effect of the material defects on vortex transport. It is demonstrated that the radius of gyration displays quantitatively unique features that depend characteristically on the type of material defects present in the sample.