R.A. Ribeiro

Anisotropic superconducting properties of aligned MgB2 crystallites

Samples of aligned MgB(2) crystallites have been prepared, allowing for the first time the direct identification of an upper critical field anisotropy H(ab)(c2)/H(c)(c2) = xi(ab)/xi(c) approximately 1.7, with xi(o,ab) approximately 70 A, xi(o,c) approximately 40 A, and a mass anisotropy ratio m(ab)/m(c) approximately 0.3. A ferromagnetic background signal was identified, possibly related to the raw materials purity.

Angular dependence of the bulk nucleation fieldHc2of alignedMgB2crystallites

Studies on the new MgB2 superconductor, with a critical temperature Tc ~ 39 K, have evidenced its potential for applications although intense magnetic relaxation effects limit the critical current density, Jc, at high magnetic fields. This means that effective pinning centers must be added into the material microstructure, in order to halt dissipative flux movements. Concerning the basic microscopic mechanism to explain the superconductivity in MgB2, several experimental and theoretical works have pointed to the relevance of a phonon-mediated interaction, in the framework of the BCS theory. Questions have been raised about the relevant phonon modes, and the gap and Fermi surface anisotropies, in an effort to interpret spectroscopic and thermal data that give values between 2.4 and 4.5 for the gap energy ratio. Preliminary results on the anisotropy of Hc2 have shown a ratio, between the in-plane and perpendicular directions, around 1.7 for aligned MgB2 crystallites and 1.8 for epitaxial thin films. Here we show a study on the angular dependence of Hc2 pointing to a Fermi velocity anisotropy around 2.5. This anisotropy certainly implies the use of texturization techniques to optimize Jc in MgB2 wires and other polycrystalline components.

Scaling of nonlinear complex susceptibility applied to the study of type-II superconductors

Abstract Different χ measurements can be collapsed into a single universal curve if plotted against the scaling variable h( ω /ω) 1/σ or, more generally, h / Jd , where J ( T , B , ω ) is a frequency dependent shielding current density, d is the sample half-width and h is the AC field amplitude. We have tested these scaling laws for two Nb samples, one melt textured YBCO sample, and one Er123 single crystal. Further, we found that J ( T , B , ω ) for Er123 displays a behavior closely related to the second magnetization peak ( fishtail effect ), observed in M × H curves. Our results are satisfactorily interpreted in terms of vortex dynamic models.

Irreversible magnetization under rotating fields and lock-in effect on a ErBa2Cu3O7-delta single crystal with columnar defects

We have measured the irreversible magnetization M_i of an ErBa_2Cu_3O_7 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 M_L(H) loops at fixed angles. The resulting M_i(\Theta) 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 \phi_L follows the H^{-1} theoretical prediction, while the temperature dependence is in agreement with entropic smearing effects corresponding to short range vortex-defects interactions.

Scaling analysis of magnetization curves based on collective flux creep for YBCO

Abstract We have explored a new way of interpreting relaxation effects in magnetization curves taken as a function of temperature, for several fixed magnetic fields, in field-cooling–warming experiments using YBa 2 Cu 3 O 7− δ sample. The thermally activated motion of vortices was studied, through a scaling analysis based on collective pinning theory. An intriguing monotonic increase of diamagnetism, while the sample is heated under an applied field, was explained by a relaxation process such that flux exit prevails over flux entry into the sample.

Josephson coupling between superconducting clusters in high-Tc materials

Abstract Diamagnetic moment for Bi 2 Sr 2 CaCu 2 O 8+ δ (Bi:2212) and YBa 2 Cu 3 O 7− δ (Y:123) crystals were measured at different fields H and temperatures. For the higher fields two distinct transition temperatures T g and T J are identified, with T g > T J . By increasing H the line T g ( H ) shifts very slowly while T J ( H ) shifts much faster to lower temperatures, displaying an upward curvature well described by a theory based on Josephson coupling between superconducting clusters. Here, we show data mainly for the Bi:2212 crystals, where sample dependent T J ( H ) lines are clearly observed.

Critical current study using ac susceptibility measurements in ErBa2Cu3O7-δ crystal showing the fishtail effect

Abstract Different χ ac measurements can be collapsed into a single curve if plotted against the ac penetration depth δ = h/Jd . Further, using this scaling law, it is possible to obtain the frequency dependence of the shielding current density J (ν, B, T ). In this work, this scaling relation was tested for a ErBa 2 Cu 3 O 7−δ single crystal. The frequency dependence of J (ν, B, T ) displays a behavior closely related with the second magnetization peak (“fishtail effect”) observed in M × H curves. This result supports a dynamical interpretation for the fihtail effect.

Diamagnetic peaks in magnetization versus temperature curves caused by flux trapped relaxation observed in YBa2Cu3O7−δ samples

Copyright (c) 1997 Elsevier Science B.V. All rights reserved. We present the results and interpretations for two types of diamagnetic peaks observed in magnetization versus temperature curves, taken for field cooled on warming (FCW) experiments in YBa 2 Cu 3 O 7−δ samples. The high field data (2.0 < H < 4.5 T), for a high quality twinned crystal, presents a universal behavior described by the collective flux creep theory.