Influence of nonlocal electrodynamics on the anisotropic vortex pinning in YNi2B2CYNi_2B_2C

We have studied the pinning force density Fp of YNi_2B_2C superconductors for various field orientations. We observe anisotropies both between the c-axis and the basal plane and within the plane, that cannot be explained by usual mass anisotropy. For magnetic field $H \parallel c$, the reorientation structural transition in the vortex lattice due to nonlocality, which occurs at a field $H_1 \sim 1kOe$, manifests itself as a kink in Fp(H). When $H \bot c$, Fp is much larger and has a quite different H dependence, indicating that other pinning mechanisms are present. In this case the signature of nonlocal effects is the presence of a fourfold periodicity of Fp within the basal plane.Comment: 4 pages, 3 figure

Irreversible Magnetization Under Rotating Fields And Lock-in Effect On A Erba2cu3o7-δ Single Crystal With Columnar Defects

We have measured the irreversible magnetization (Mi) of an ErBa2Cu3O7-δ single crystal with columnar defects (CD), using a technique based on sample rotation under a fixed magnetic field H. This method is valid for samples whose magnetization vector remains perpendicular to the sample surface over a wide angle range - which is the case for platelets and thin films - and presents several advantages over measurements of ML(H) loops at fixed angles. The resulting Mi(Θ) curves for several temperatures show a peak in the CD direction at high fields. At lower fields, a very well defined plateau indicative of the vortex lock-in to the CD develops. The H dependence of the lock-in angle ΦL follows the H-1 theoretical prediction, while the temperature dependence is in agreement with entropic smearing effects corresponding to long range vortex-defects interactions.641414450211445028Civale, L., Marwick, A.D., Worthington, T.K., Kirk, M.A., Thompson, J.R., Krusin-Elbaum, L., Sun, Y., Holtzberg, F., (1991) Phys. Rev. Lett., 67, p. 648Silhanek, A., Civale, L., Candia, S., Nieva, G., Pasquini, G., Lanza, H., (1999) Phys. Rev. B, 59, p. 13620Nelson, D.R., Vinokur, V.M., (1992) Phys. Rev. Lett., 68, p. 2398(1993) Phys. Rev. B, 48, p. 13060Blatter, G., Feigel'man, M.V., Geshkenbein, V.B., Larkin, A.I., Vinokur, V.M., (1994) Rev. Mod. Phys., 66, p. 1125Silhanek, A., Niebieskikwiat, D., Civale, L., Avila, M.A., Billoni, O., Casa, D., (1999) Phys. Rev. B, 60, p. 13189Avila, M.A., (2001), Ph. D. thesis, University of Campinas (UNICAMP), BrazilCandia, S., Civale, L., (1999) Supercond. Sci. Technol., 12, p. 192Clem, J.R., Sanchez, A., (1994) Phys. Rev. B, 50, p. 9355Prozorov, R., Poddar, A., Sheriff, E., Shaulov, A., Yeshurun, Y., (1996) Physica C, 264, p. 27Zhukov, A.A., Perkins, G.K., Bugoslavsky, Yu.V., Caplin, A.D., (1997) Phys. Rev. B, 56, p. 2809Hasanain, S.K., Ahmad, I., Semerad, R., (1999) Supercond. Sci. Technol., 12, p. 633Clem, J.R., (1982) Phys. Rev. B, 26, p. 2463Clem, J.R., Perez-Gonzalez, A., (1986) Phys. Rev. B, 33, p. 1601Perez-Gonzalez, A., Clem, J.R., (1990) Phys. Rev. B, 42, p. 4100Goeckner, H.P., Kouvel, J.S., (1994) Phys. Rev. B, 50, p. 3435Hasanain, S.K., Manzoor, S., Aftab, M., (1996) Physica C, 272, p. 43Hasan, M.K., Kouvel, J.S., (1997) Physica C, 276, p. 289Obaidat, I.M., Goeckner, H.P., Kouvel, J.S., (1997) Physica C, 291, p. 8Vlasko-Vlasov, V.K., Welp, U., Crabtree, G.W., Gunter, D., Kabanov, V.V., Nikitenko, V.I., Paulius, L.M., (1998) Phys. Rev. B, 58, p. 3446Obaidat, I.M., Park, S.J., Kouvel, J.S., (1998) Physica C, 308, p. 185Hasan, M.K., Shobaki, J., Al-Omari, I.A., Albiss, B.A., Al-Akhras, M.A., Azez, K.A., El-Qisari, A.K., Kouvel, J.S., (1999) Supercond. Sci. Technol., 12, p. 606Niebieskikwiat, D., Silhanek, A., Civale, Nieva, G., Levy, P., Krusin Elbaum, L., (2001) Phys. Rev. B, 63, p. 144504Krusin-Elbaum, L., Civale, L., Thompson, J.R., Feild, C., (1996) Phys. Rev. B, 53, p. 11744Civale, L., Pasquini, G., Levy, P., Nieva, G., Casa, D., Lanza, H., (1996) Physica C, 263, p. 389Zhukov, A.A., Perkins, G.K., Thomas, J.V., Caplin, A.D., Kupfer, H., Wolf, T., (1997) Phys. Rev. B, 56, p. 348

Sign reversal of the Hall resistance in the mixed-state of La1.89_{1.89} Ce0.11_{0.11}CuO4_{4} and La1.89_{1.89}Ce0.11_{0.11}(Cu0.99_{0.99}Co0.01_{0.01})O4_{4} thin films

The transport properties of La$_{1.89}$Ce$_{0.11}$CuO$_{4}$(LCCO) and La$_{1.89}$Ce$_{0.11}$(Cu$_{0.99}$Co$_{0.01}$)O$_{4}$ (LCCO:Co) superconducting thin films are investigated. When the external field $\bf H$ is applied along the crystallographic c-axis, a double sign reversal of the Hall voltage in the mixed state of LCCO:Co thin films is observed whereas a single sign reversal is detected in LCCO. A double sign reversal of the Hall signal in LCCO can be recovered if the magnetic field is tilted away from the plane of the film. We find that the transition from one to two of the Hall sign reversal coincides with the change in the pinning from strong to weak. This temperature/field induced transition is caused either by the magnetic impurities in LCCO:Co or by the coupling between the pancake vortices and the in-plane Josephson vortices in LCCO. These results are in agreement with early theoretical and numerical predictions.Comment: 6 pages, 4 figures, the proceedings of VORTEX VII in Physica

Scanning Hall probe microscopy of unconventional vortex patterns in the two-gap <tex>MgB_{2}$</tex> superconductor

peer reviewedThe low magnetic field vortex patterns nucleation and evolution in a high-quality two-gap superconductor MgB2 single crystal have been investigated by low-temperature scanning Hall probe microscopy. Large areas have been imaged with single-vortex resolution while changing systematically the thermodynamic parameters for field and temperature. The obtained patterns have been studied and compared with those of a reference 2H-NbSe2 single crystal. We found that the observed vortex patterns in MgB2 (e.g., stripes, clusters) appear due to competing vortex-vortex interactions as suggested by the theory of type-1.5 superconductivity

Morphology of Flux Avalanches in Patterned Superconducting Films

It is well known that under certain circumstances, magnetic fields applied perpendicularly to the plane of superconducting films can trigger flux avalanches. In such cases the penetration has a tree-like profile. However, in samples where a regular array of antidots, ADs, is present, the avalanches follow the rows of ADs as if they were guiding lines for the abrupt penetration. In this work we used the magneto-optical imaging technique to study the morphology of flux avalanches in two Nb films with a square lattice of square ADs, each one with a different lateral size, and a plain film for reference. We show that the morphology of the flux avalanches is greatly influenced by the size of the interstitial region