Session B9: Superconductivity: Currents and Vortex Dynamics

Critical current density distribution in YBCO coated conductors measured with a ``magnetic knife''

We present a simple and straightforward tool for investigating the spatial $J_{c}$ distribution in YBCO coated conductors. The samples were prepared by co-evaporation and PLD on IBAD and RABiTS templates. The tool we have used is a ``magnetic knife'' [1] consisting of several Nd$_{2}$Fe$_{14}$B permanent magnets in close proximity to the superconducting tape. A 200 $\mu $m wide zone of low magnetic field $B$ is embedded in a background field of around 600~mT. This region is scanned across the sample, in the direction perpendicular to the direction of tape current flow. The critical current for each position is measured with a four-point technique at 75 K (liquid N$_{2}$ at ambient pressure). The raw data are deconvoluted with a Fourier inversion method. Several techniques for obtaining the magnetic field distribution in the magnetic knife (simulations and measurements) will be compared and discussed. Finally, differences in the current distributions for different coated conductor samples will be discussed. [1] ten Haken et al., Physica C 334, 163 (2000)   

Testing the limits for critical currents in YBa$_{2}$Cu$_{3}$O$_{7}$ films

Vortex pinning in YBa$_{2}$Cu$_{3}$O$_{7}$ films can be very strong. At low temperatures and in the absence of applied magnetic field (\textbf{H}), critical current densities J$_{c}$ of about 20{\%} of the depairing limit have been obtained. This is as high as the best achieved in commercial Nb-based superconducting wires after decades of optimization. Remarkably, similar J$_{c}$s are attained in YBa$_{2}$Cu$_{3}$O$_{7}$ films grown by various methods that produce vastly different nanostructures, suggesting that perhaps we are close to an effective J$_{c}$ limit regardless of the details of the pinning mechanisms. In contrast, the different types of pinning centers (either naturally occurring or artificially introduced by material nanoengineering) produce distinctively different J$_{c}$ behavior as a function of \textbf{H} strength and orientation. I will present a comparison of pinning mechanisms in YBa$_{2}$Cu$_{3}$O$_{7}$ films and will analyze the possibilities of further improvements.   

Irreversibility line of YBa$_{2}$Cu$_{3}$O$_{7}$ films as a function of angle and field up to 50 Tesla

Studying the irreversibility line (resistivity=0) in high T$_{c}$ superconductors is scientifically and technologically relevant because the critical current drops to zero at this vortex solid-liquid transition. We have used low current transport measurements to study the irreversibility line of YBa$_{2}$Cu$_{3}$O$_{7}$ films in fields up to 50 T. Electronic mass anisotropy can describe most of the angular dependence, but fails to account for deviations along the crystalline axes. Correlated pinning causes a large increase in the irreversibility field along the $a-b$ planes, and a small $c$-axis peak. Inclusion of BaZrO$_{3}$ not only adds $c$-axis correlated defects, but increases the overall irreversibility field and alters the shape of the resistivity vs. magnetic field curve in the liquid state. We will discuss the results in terms of vortex pinning, the corresponding types of phase transitions, micro-structural analysis, and information obtained from critical current measurements.   

AC current driven vortex dynamics in YBCO thin films and coated conductors

The effect of an AC current and a static magnetic field on the vortex dynamics in YBa$_{2}$Cu$_{3}$O$_{7-x}$ (YBCO) thin films and coated conductors is studied by time-resolved magneto-optical imaging. Our measurements show that the AC current enables the vortex lattice in the YBCO thin film to reorganize into two coexisting states with different characteristics: a quasi-static state in the sample interior and a dynamic state near the edges. Vortices and anti-vortices, induced by the current during the cycle, penetrate from the edges into the sample and interact with the pinned vortices altering the flux lattice. We compare the AC current driven vortex dynamics in YBCO thin films and coated conductors with particular focus on the AC loss characteristics of multifilamentary samples. Finite-element method (FEM) calculations adopting a recently developed method, are used to compute current density, field profiles and AC losses during the cycle. The model assumes a thermal activation of the magnetic flux that leads to a nonlinear dependence of the electric field and current density. The FEM calculations show a very good agreement with the measured data.   

Influence of disorder on the vortex pinning and cutting of YBa$_2$Cu$_3$O$_7$ films

Flux cutting and recombination has been used to explain high critical current densities ($J_c$) observed in experiments done in Force-Free (FF) and Variable Lorentz Force(VLF) configurations \textit{i.e.}, when the current $I$ is totally or partially aligned to the applied magnetic field $H$. However, the effect of different pinning centers has not been studied. We present angular and field $J_c$ studies in FF and VLF configurations and study the effects of random, correlated and extended defects on the $J_c$ of YBa$_2$Cu$_3$O$_7$ films. Results are analyzed in terms of vortex pinning at different defects and vortex cutting mechanism. We show that pinning greatly influences $J_c$ in FF and VLF, up to the point of obtaining $J_c$ higher than $J_c(H=0) $ up to magnetic fields as high as 3T   

Evolution of the vortex-solid to vortex-liquid melting line inY$_{1-x}$Pr$_{x}$Ba$_{2}$Cu$_{3}$O$_{6.96}$ and YBa$_{2}$Cu$_{3}$O$_{6.5}$ to 45 tesla

By extending magneto-transport measurements to fields up to 45 tesla, we have been able to examine the vortex glass melting line of Y$_{1-x}$Pr$_{x}$Ba$_{2}$Cu$_{3}$O$_{6.96}$ (x = 0 - 0.4) thin film samples and that of an oxygen deficient YBa$_{2}$Cu$_{3}$O$_{6.5}$ single crystal over an extended temperature range, 0.03~$T_{c}~\leq~T~\leq~T_{c}$, larger than heretofore reported. The melting lines are analyzed in the context of the model of Blatter \& Ivlev (BI) [PRL $\bf{70}$, 2621 (1993)] with temperature dependent parameters, $\xi$, $\lambda$, etc. The temperature dependence of the relaxation time of a single vortex flux line, displaced by quantum/thermal fluctuations, is deduced such that the \textit{entire} melting line of each sample can be fit to smoothly by the modified expression of BI, implying that the physical mechanism responsible for the manner and conditions of the melting of the vortex solid can be described smoothly over the entire temperature $-$ field range. This research was sponsored by the DOE under Research Grant No. DE-FG02-04ER46105. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by NSF Cooperative Agreement No. DMR-0084173, by the State of Florida, and by the DOE.   

Vortex pinning in single crystal CaC$_{6}$

Crystals of the new graphite intercalation superconductor CaC$_{6}$ were synthesized in a liquid transport process in which graphite single crystals are exposed to an eutectic Ca-Li melt at 350 $^{\circ}$C. The resulting samples displayed a sharp superconducting transition at 11.6 K. X-ray diffraction reveals the rhombohedral CaC$_{6}$ structure with no indication of graphite second phases. The phase diagram and the vortex pinning properties were determined using magnetization and Hall magnetometry measurements. The irreversibility line for fields applied along the c-axis lies close to the upper critical field and displays down to temperatures of 2 K a linear temperature dependence with a slope of about -230 G/K. An analysis based on the Bean critical state model yields a critical current density of 10$^{4}$ A/cm$^{2}$ at 4.5 K and zero field. The effect of particle irradiation on the flux pinning properties of CaC$_{6}$ will be presented. This work was supported by the US Department of Energy, BES-Materials Sciences, under Contract DE-AC02-06CH11357.   

Magnetic relaxation and critical currents of NdBCO coated conductors

A magnetic study of the critical current density $J_c$ and magnetic relaxation (`creep') effects in thin NdBa$_2$Cu$_3$O$_{7-\delta}$ (NdBCO) superconducting films of thicknesses 0.7 and 2.1 $\mu$m was conducted. These films, doped with BaZrO$_3$, were deposited by a PLD process on `IBAD' substrates. The $J_c$ values display broad peaks near the c-axis. This is associated with densely spaced columnar defects distributed about this axis. We analyzed the magnetic relaxation data using the Maley expression for the activation energy $U$ as a function of current density $J$. The data are described fairly well by the collective creep interpolation formula $U=U_0\left[\left(J_{c0}/J\right)^\mu-1\right]$ with $U_0$, $J_{c0}$, and $\mu$ treated as fitting parameters.   

Vortex Phases in Mesoscopic Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ Single Crystals in Magnetic Fields Near \textit{ab}-plane

In order to study the vortex matter in the layered superconductors in magnetic fields parallel to the \textit{ab}-plane, we performed the in-plane resistivity and the $c$-axis resistivity measurements on the bulk and mesoscopic Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ single crystals. In the bulk samples, a boundary between the strong pinning phase and the weak pinning vortex phase was found at about 3$^{o}$ away from the \textit{ab}-plane, indicating possible crossover from the vortex chain + lattice phase into tilted (vortex chain) phase. The vortex phase in the parallel magnetic fields, exhibited a strongly non-Ohmic behavior, indicating the possible two-stage melting phase transition. It was found that the vortex lock-in transition in mesocopic crystals, becomes considerably broad in high magnetic fields, while exhibiting the sharp features in low magnetic field region. The first penetration field of vortex pancakes demonstrates a nontrivial field dependence.   

Peak Effect in Polycrystalline Vortex Matter

The peak effect (PE) in $J_{c}$ in weakly-disordered type-II superconductors is believed to mark the transition between a disordered vortex state and a quasi-ordered Bragg glass regime. Some strongly-disordered type-II superconductors, such as the binary alloy V-21at.\%Ti, also exhibit ``peak effect'' at temperatures close to $H_{c2}$, despite lack of atomic long-range order and presence of sample composition inhomogeneities. SANS field-cooled measurements on a V-21at.\%Ti sample show that both deep in the mixed state and close to the PE transition, there exist no long-range orientationally-ordered vortex lattices (VL's). The neutron scattering data analysis shows that the diffraction radial widths do not change significantly as a function of field, suggesting that VL states ordered on the scale of $\mu m$ exist. We conjecture that the ``peak effect'' in V-21at.\%Ti corresponds to the disordering of ordered VL Larkin domains. The V-21at.\%Ti peak effect phase diagram is mapped via ac susceptometry. This measurement reveals that the peak effect disappears below a certain field, as has been reported in other superconductors.   

Magnetocaloric Studies of the Peak Effect in Nb

We report a magnetocaloric study of the peak effect and Bragg glass transition in a Nb single crystal. The thermomagnetic effects due to vortex flow into and out of the sample are measured. The magnetocaloric signature of the peak effect anomaly is identified. It is found that the peak effect disappears in magnetocaloric measurements at fields significantly higher than those reported in previous ac-susceptometry measurements. Investigation of the superconducting to normal transition reveals that the disappearance of the bulk peak effect is related to inhomogeneity broadening of the superconducting transition. The emerging picture also explains the concurrent disappearance of the peak effect and surface superconductivity, which was reported previously in the sample under investigation. Based on our findings we discuss the possibilities of multicriticality associated with the disappearance of the peak effect.   

Collapse of the critical state in superconducting niobium

Giant abrupt changes in the magnetic flux distribution in niobium foils were studied by using magneto-optical visualization, thermal and magnetic measurements. Uniform flux jumps and sometimes almost total catastrophic collapse of the critical state are reported. Results are discussed in terms of thermomagnetic instability mechanism with different heat removal channels. Video figures are available at: http://www.cmpgroup.ameslab.gov/supermaglab/video/Nb.html   

Analysis of collective pinning and depinning of the flux line lattice in pristine 2H-NbSe$_{2}$

Larkin and Ovchinnikov predicted collective pinning of the flux line lattice (FLL) in type II superconductors several decades ago. The collective pinning results from the interplay between strong vortex-vortex interaction and randomly distributed weak pinning centers in a media. The evidence of collective pinning was previously observed at a magnetic field, H, close to H$_{c2}$ in current-driven transport experiments on the macroscopic scale. But there still exists a lack of understanding of collective pinning on the microscopic level. In this talk, we show collective pinning and depinning of the FLL in pristine 2H-NbSe$_{2}$ in a long time series (15 days), measured by a low temperature scanning tunneling microscope. We observed the motion of the FLL within an area of 400 nm $\times $ 400 nm, with an initial magnetic field of 0.5 T. The motion was caused by the very slow decay of magnetic field ($\sim $ 5 nT/s) in a defective superconducting magnet. The average speed of FLL was $\sim $ 2.5 pm/s, lower than previously reported. Using highly time resolved data, we will further discuss the average direction of motion, the strength of pinning centers in pristine 2H-NbSe$_{2}$, flux line mass, and the difference between current-driven and field-driven FLL motions.   

Paramagnetic effect in nano-opal-lead structure

We report magnetic studies of the paramagnetic effect observed in the superconducting nano-structured opal-lead system. Positive magnetization is clearly observed when the sample is cooled in field. The paramagnetic effect is strongly dependent on the applied field, cooling rate and the background magnetization. The results suggest that the paramagnetic moment is due to flux trapping and the competition between the positive and negative moments due to the temperature dependence of penetration depth.   

A concept of the upper threshold field for dendritic vortex avalanches in superconductors.

Dendritic vortex avalanches in superconductors (SC), associated with thermo-magnetic instabilities, are known to destroy a metastable critical state and severely detriment performance of SC applications. Several thermo-magnetic models have been proposed to determine conditions under which a superconductor is to undergo a dendritic magnetic avalanche. However, there remains an omnipresent experimental fact that has not been given a theoretical explanation: instabilities always disappear above some upper threshold field. Our recently developed model [Phys. Rev. Lett. 97, 077002 (2006)] predicts divergence of the threshold field at low values of the critical current j$_{c}$. In its turn, at increasing magnetic field j$_{c}$ becomes strongly suppressed by the field. We explain disappearance of the dendritic avalanches in terms of the divergence of the upper threshold field due to a strong dependence of the critical current on magnetic field j$_{c}$(B) and support it by the results of our recent magneto-optical investigations of dendritic flux avalanches in NbN thin films. We then verify the model in a range of controllably varied j$_{c}$ values.