Proton-induced magnetic order in carbon: SQUID measurements

In this work we have studied systematically the changes in the magnetic behavior of highly oriented pyrolytic graphite (HOPG) samples after proton irradiation in the MeV energy range. Superconducting quantum interferometer device (SQUID) results obtained from samples with thousands of localized spots of micrometer size as well on samples irradiated with a broad beam confirm previously reported results. Both, the para- and ferromagnetic contributions depend strongly on the irradiation details. The results indicate that the magnetic moment at saturation of spots of micrometer size is of the order of $10^{-10}$ emu.Comment: Invited contribution at ICACS2006 to be published in Nucl. Instr. and Meth. B. 8 pages and 6 figure

Vortex Lattice Depinning vs. Vortex Lattice Melting: a pinning-based explanation of the equilibrium magnetization jump

In this communication we argue that the Vortex Lattice Melting scenario fails to explain several key experimental results published in the literature. From a careful analysis of these results we conclude that the Flux Line Lattice (FLL) does not melt along a material- and sample-dependent boundary $H_j(T)$ but the opposite, it de-couples from the superconducting matrix becoming more ordered. When the FLL depinning is sharp, the difference between the equilibrium magnetization $M_{eq}(T,H)$ of the pinned and unpinned FLL leads to the observed step-like change $\Delta M_{eq}(T,H)$. We demonstrate that the experimentally obtained $\Delta M_{eq}(T,H)$ can be well accounted for by a variation of the pinning efficiency of vortices along the $H_j(T)$ boundary.Comment: 8 pages, 1 Figur

Influence of temperature on the nodal properties of the longitudinal thermal conductivity of YBa2_2Cu3_3O7−x_{7-x}

The angle dependence at different temperatures of the longitudinal thermal conductivity $\kappa_{xx}(\theta)$ in the presence of a planar magnetic field is presented. In order to study the influence of the gap symmetry on the thermal transport angular scans were measured up to a few Kelvin below the critical temperature $T_c$. We found that the four-fold oscillation of $\kappa_{xx}(\theta)$ vanishes at $T > 20$K and transforms into a one-fold oscillation with maximum conductivity for a field of 8 T applied parallel to the heat current. Nevertheless, the results indicate that the d-wave pairing symmetry is the main pairing symmetry of the order parameter up to $T_c$. Numerical results of the thermal conductivity using an Andreev reflection model for the scattering of quasiparticles by supercurrents under the assumption of d-wave symmetry provide a semiquantitative description of the overall results.Comment: 7 figures, EPJ B (in press, 2003

The Labusch Parameter of a Driven Flux Line Lattice in YBa2_2Cu3_3O7_7 Superconducting Films

We have investigated the influence of a driving force on the elastic coupling (Labusch parameter) of the field-cooled state of the flux line lattice (FLL) in 400 nm thick YBa$_2$Cu$_3$O$_7$ superconducting films. We found that the FLL of a field-cooled state without driving forces is not in an equilibrium state. Results obtained for magnetic fields applied at $0^\circ$ and 30$^\circ$ relative to CuO$_2$ planes, show an enhancement of the elastic coupling of the films at driving current densities several orders of magnitude smaller than the critical one. Our results indicate that the FLL appears to be in a relatively ordered, metastable state after field cooling without driving forces.Comment: 4 Figure