Collective pinning of imperfect vortex lattices by material line defects in extreme type-II superconductors

The critical current density shown by a superconductor at the extreme type-II limit is predicted to follow an inverse square-root power law with external magnetic field if the vortex lattice is weakly pinned by material line defects. It acquires an additional inverse dependence with thickness along the line direction once pinning of the interstitial vortex lines by material point defects is included. Moderate quantitative agreement with the critical current density shown by second-generation wires of high-temperature superconductors in kG magnetic fields is achieved at liquid-nitrogen temperature.Comment: 10 pages, 3 figures, 2 tables. To appear in Physical Review

Strong Enhancement of the Critical Current at the Antiferromagnetic Transition in ErNi2B2C Single Crystals

We report on transport and magnetization measurements of the critical current density Jc in ErNi2B2C single crystals that show strongly enhanced vortex pinning at the Neel temperature TN and low applied fields. The height of the observed Jc peak decreases with increasing magnetic field in clear contrast with that of the peak effect found at the upper critical field. We also performed the first angular transport measurements of Jc ever conducted on this compound. They reveal the correlated nature of this pinning enhancement, which we attribute to the formation of antiphase boundaries at TN.Comment: 3 figure

Comment on `Strong Vortex Liquid Correlation' from Multiterminal Measurements on Untwinned YBa2_2Cu3_3O7−δ_{7-\delta} Single Crystals'

A.Rydh and \"O.Rapp [Phys. Rev. Lett. {\bf 86}, 1873 (2001).] claim that the vortex liquid in untwinned YBa$_2$Cu$_3$O$_{7-\delta}$ crystals is correlated above the melting transition, in striking contrast to previous work [D.L\'opez {\it et al.}, Phys. Rev. Lett. {\bf 76}, 4034 (1996).]. In this Comment we present new measurements using the same experimental technique on twinned and untwinned YBa$_2$Cu$_3$O$_{7-\delta}$ crystals with similar overall characteristics as those reported by Rydh and Rapp . The comparison of the vortex correlation response in both cases indicates that the central conclusion of their work is not correct. Our results reconfirm the work by L\'opez {\it et al.} and points on the origin of the misinterpretation in the work of Rydh and Rapp.Comment: comment on A.Rydh and \"O.Rapp, Phys. Rev. Lett. {\bf 86}, 1873 (2001). accepted in Phys. Rev. Let

Upper critical field and thermally activated flux flow in single crystalline Tl0.58_{0.58}Rb0.42_{0.42}Fe1.72_{1.72}Se2_2

The upper critical field $\mu_0H_{c2}(T_c)$ of Tl$_{0.58}$Rb$_{0.42}$Fe$_{1.72}$Se$_2$ single crystals has been determined by means of measuring the electrical resistivity in both a pulsed magnetic field ($\sim$60T) and a DC magnetic field ($\sim$14T). It is found that $H_{c2}$ linearly increases with decreasing temperature for $\textbf{H}$$\parallel$$c$, reaching $\mu_0H_{c2}^{\textbf{H}\parallel c}(0\textrm{K})\simeq60$ T. On the other hand, a larger $\mu_0H_{c2}(0\textrm{K})$ with a strong convex curvature is observed for $\textbf{H}$$\perp$$c$ ($\mu_0H_{c2}^{\textbf{H}\perp c}$(18K)$\simeq$60T). This compound shows a moderate anisotropy of the upper critical field around $T_c$, but decreases with decreasing temperature. Analysis of the upper critical field based on the Werthamer-Helfand-Hohenberg (WHH) method indicates that $\mu_0H_{c2}(0\textrm{K})$ is orbitally limited for $\textbf{H}$$\parallel$$c$, but the effect of spin paramagnetism may play an important role on the pair breaking for $\textbf{H}$$\perp$$c$. All these experimental observations remarkably resemble those of the iron pnictide superconductors, suggesting a unified scenario for the iron-based superconductors. Moreover, the superconducting transition is significantly broadened upon applying a magnetic field, indicating strong thermal fluctuation effects in the superconducting state of Tl$_{0.58}$Rb$_{0.42}$Fe$_{1.72}$Se$_2$. The derived thermal activation energy for vortex motion is compatible with those of the 1111-type iron pnictides.Comment: 7 pages, 6 figure

Psuedo-isotropic upper critical field in cobalt-doped SrFe2As2 epitaxial films

The temperature and angular dependence of the upper critical field (Hc2) is reported for cobalt-doped SrFe2As2 epitaxial films between Tc and 0.5 K in pulsed magnetic fields up to 50 T. For H parallel c, Hc2 is close to a linear function of temperature, while in the perpendicular direction there is significant downward curvature that results in an Hc2 ratio (gamma = Hc2(perpendicular)/Hc2(parallel) that decreases nearly linearly with temperature, approaching gamma = 1 at low temperature with Hc2(0) = 47 T. We measure the complete upper-critical field phase diagram including angular dependence and model the data using a two band theory allowing us to determine the anisotropy of both bands, their relative diffusivities, and the relationship between BCS coupling constant matrix elements. We find an unusual relationship between the diffusivities of the two bands, with two anisotropic and opposite bands. This relationship is supported by the observation of a local maximum for Hc2(parallel) at low temperature

Correlated enhancement of Hc2 and Jc in carbon nanotube-doped MgB2

The use of MgB2 in superconducting applications still awaits for the development of a MgB2-based material where both current-carrying performance and critical magnetic field are optimized simultaneously. We achieved this by doping MgB2 with double-wall carbon nanotubes (DWCNT) as a source of carbon in polycrystalline samples. The optimum nominal DWCNT content for increasing the critical current density, Jc is in the range 2.5-10%at depending on field and temperature. Record values of the upper critical field, Hc2(4K) = 41.9 T (with extrapolated Hc2(0) ~ 44.4 T) are reached in a bulk sample with 10%at DWCNT content. The measured Hc2 vs T in all samples are successfully described using a theoretical model for a two-gap superconductor in the dirty limit first proposed by Gurevich et al.Comment: 12 pages, 3 figure

Elastic-to-plastic crossover below the peak effect in the vortex solid of YBa2Cu3O7 single crystals

We report on transport and ac susceptibility studies below the peak effect in twinned YBa2Cu3O7 single crystals. We find that disorder generated at the peak effect can be partially inhibited by forcing vortices to move with an ac driving current. The vortex system can be additionally ordered below a well-defined temperature where elastic interactions between vortices overcome pinning-generated stress and a plastic to elastic crossover seems to occur. The combined effect of these two processes results in vortex structures with different mobilities that give place to history effects.Comment: 4 pages, 4 figures. Published in PRB Rapid Comm., February 1, 200