Anisotropy of perpendicular field penetration into high-Tc superconductors induced by strong longitudinal field

Abstract

Anisotropy of perpendicular field penetration into high- General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. AbstractThe geometry of penetration of a magnetic field oriented perpendicular to the surface of a superconductor is changed by the presence of a longitudinal field. This is studied directly by means of the magneto-optical technique in different high-Tc materials. In YBa2Cu307 single crystals Brandt's mode of magnetization is observed, namely, penetration of an AC field only along the longitudinal field even if the corresponding dimension of the sample is much longer than the other two. This anisotropy is the evidence that the force-free configuration of current and vortices cannot be destroyed by cutting and reconnection of the vortices in this 3D superconductor. On the contrary, the absence of the induced anisotropy in Bi2Sr2CaCu2Os single crystals reveals the independence of pancake vortices in CuO layers from longitudinal field which makes a realization of the force-free configuration impossible in the layered 2D superconductor. An intermediate effect in thin films indicates that vortices are almost normal to the film surface even if the applied field is nearly parallel to it. The described experiment can be used as a new tool to study the internal vortex structure in superconductors.

Crossing fields in thin films of isotropic superconductors

“…Direct flux imaging revealed pronounced anisotropic vortex dynamics with preferential vortex motion along H || and formation of a sharp flux front, delaying the entry of new vortices across H || . We associated the novel observed flux patterns, similar to those reported earlier in [3], with the layered structure of YBCO, forcing the motion of the incoming pancake vortices along the Josephson-like, in-plane flux lines, and with the cutting processes of the in-plane and new entering vortices.. The simulations of the crossing process in [2] revealed time variations of the vortex shape and allowed us to conclude that defects can increase the flux crossing barrier due to the pinning of strongly bent segments of intersecting vortices, and hence enhance currents along the crossing flux fronts.…”

“…Albeit, our recent simulations show [25,26] that the vortex cutting can effectively delay the penetration of crossing vortices into the sample. Previous reports of crossing-fields in thin YBCO plates [2,3], and the anisotropy of the vortex dynamics in the thicker Nb film can be an experimental confirmation of this conjecture. …”

Flux cutting in high-Tcsuperconductors

“…The easy penetration in the direction of H || and delayed entry of B ⊥ across H || correspond to much smaller current density perpendicular to the in-plane field and increased current parallel to the field. A similar anisotropic B ⊥ distribution was reported in [19][20] for normal magnetization of a YBCO plate in the presence of in-plane field. Our well defined sample geometry and higher resolution shows more details of the vortex dynamics and clearly reveals the enhanced current near the long edges of the sample.…”

“…Moreover, there are no consistent microscopic observations of the vortex behavior in the general critical state, although some peculiar flux structures that emerge under crossing fields were imaged and reported [19][20][21][22]. Campbell [16] admits that "we are a long way from even a qualitative understanding of flux cutting and longitudinal currents".…”

Longitudinal magnetic field increases critical current in superconducting strip

“…Note that this critical state problem is similar to the situations investigated in Refs. [3,4]. But in those experiments an initial perpendicular magnetic field H 0 z was not applied, and this led to flux-line cutting.…”