ORAL SESSION 4B5: Imaging Vortex Behavior

Thursday, Feb. 24, 10:00 a.m. – 12:15 p.m., General Assembly Hall A (GRB)

Chair: A. de Lozanne (U Texas-Austin), I.K. Schuller (U of California-San Diego)

4B5.1 Vortex cores in cuprates observed by STS

Øystein Fischer 1, Bart Hoogenboom 1, Martin Kugler 1, Maggio-Aprile 1, and Christophe Renner 2. 1 Department of Condensed Matter Physics, University of Geneva, 24 Quai Ernest Ansermet, 1211 Geneva 4, Switzerland. 2 NEC Research Institute, 4 Independence Way, Princeton, NJ 08540, USA.

Presenting Author: O. Fischer

The vortex core has an internal electronic structure which depends on the microscopic nature of the superconductor. Scanning tunneling spectroscopy (STS) allows to probe this structure and is therefore a very sensitive probe of the superconducting state. Investigations of vortex cores in high temperature superconductors have revealed spectra which are very different from the expected behavior of a d-wave BCS superconductor. Two well developed peaks were observed in YBa₂Cu₃O₇, suggesting only two localized states, one above and one below the Fermi level. In Bi₂Sr₂CaCu₂O_8+d no prominent features are present, and the overall spectrum in the vortex core could be directly related to the zero field pseudogap spectrum as seen above the critical temperature. At low energy weak structures can be observed. These are reminiscent of the ones seen in YBa₂Cu₃O₇, but their weight in the spectrum is much smaller and their origin remains to be determined. The vortex cores in Bi₂Sr₂CaCu₂O_8+d are extremely small indicating a very small coherence length. They often appear as having two or several parts, suggesting a delocalization of the vortex core. We shall discuss the implications of these results.

4B5.2 STM study of Effects on Superconductivity of Individual Zinc Impurity Atoms in BSCCO

J.C. Seamus Davis 1, E.W. Hudson 1, K.M. Lang 1, H. Eisaki 2, S. Uchida 2, and S.H. Pan 3. 1 Physics Dept., University of California, Berkeley, CA 94720. 2 Dept. of Superconductivity, U. of Tokyo,Tokyo, Japan. 3 Boston University, Boston, MA.

Presenting Author: J.C. Seamus Davis

I will describe recent studies, using a high resolution very low temperature STM, of the high-T_c superconductor BiSrCaCuO [1], in which single impurity atoms are used to probe the structure of the order parameter [2].

Single impurity atoms generate local quasiparticle 'clouds' at the atomic scale. Imaging of the 'clouds' associated with individual Zn dopant atoms [2] substituted at the Cu site in the CuO₂ plane of BSCCO shows direct real-space consequences of the d-wave nature of the order parameter. Quasi-particle scattering resonances[1]are observed by tunneling spectroscopy at these sites, and their spectra are in good agreement with theories of single impurity atom scattering in a d-wave superconductor. LDOS imaging at the resonance energy shows a highly localized quasi-particle cloud which has a clear four-fold symmetry aligned with the d-wave gap nodes, in qualitative agreement with theory. The destruction of superconductivity within 1 nm of the Zn sites is directly demonstrated. New phenomena are also reported which are not described by existing theories and can thus provide new information on the physics of HTSC at the atomic-scale.

[1] "Quasi-Particle Scattering Resonances in Bi₂Sr₂CaCu₂O_8+d ", E.W. Hudson, S.H. Pan, A.K. Gupta, K-W Ng, and J.C. Davis, Science 285, 88 (1999).

[2] "Imaging the Effects of Individual Zinc Impurity Atoms on Superconductivity in Bi₂Sr₂CaCu₂O_8+d " S.H. Pan, E.W. Hudson, K.M. Lang, H. Eisaki, S. Uchida, and J.C. Davis, submitted.

4B5.3 Coherent and Incoherent Vortex Motion

P. Kes, J. Aarts, R. Besseling, R. Niggebrugge, and A. Troyanovski, Kamerlingh Onnes Laboratory, Leiden University, POB 9504, 2300 RA Leiden, The Netherlands

Presenting Author: P. Kes

A numerical and analytical study of vortex flow behavior in weak pinning channels has revealed that, for a channel in a perfect lattice, incommensurability between channel width and the vortex lattice constanat cuases (point) defects in the channel with a nearly vanishing barrier fc. The transport characteristics exhibit a crossover from low mobility (defect motion) to full flux flow (coherent motion). f_c versus commensurability parameter is a discontinuous function with sharp peaks at integer values. Disorder in the channel edges smears the sharp C-I transitions due to defect pinning at incommensurability. Also it lowers the commensurate f_c due to the generation of defect pairs. These observations agree with experiments we performed on weak pinning vortex channels in strong pinning films.

In addition, we observed and studied the motion of flux line lattices in various single crystals of NbSe₂ using fast scanning tunneling microscopy. In crystals with intrinsic weak point disorder the VL creeps coherently as a large bundle along one of the principal axes of the VL. However, we observe striking and for the creep regime unanticipated velocity modulations in the direction of motion, related to the VL periodicity, i.e. with the "washboard" frequency. In the transverse direction the fluctuations of the center of mass displacements are random. In crystals with strong line disorder (correlated pinning) caused by columnar defects due to swift ion irradiation collective pinning is observed in fields up to almost double the dose-equivalent field B while interesting plastic flow patterns are detected at 2B.

4B5.4 Images of Interlayer Josephson Vortices in Single-Layer Cuprates

Kathryn A. Moler, Department of Applied Physics, Stanford University, Stanford, CA 94305-4045, and John R. Kirtley, IBM T.J. Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598

Presenting Author: K.A. Moler

A Superconducting QUantum Interference Device (SQUID) microscope was used to image interlayer Josephson vortices, which are vortices oriented parallel to the planes in layered superconductors. The extent of such vortices along the planes is given by the interlayer penetration depth, a measure of the interlayer coupling in the superconducting state. In various near-optimally doped cuprate materials we find the following values for the interlayer penetration depth: 17 microns in Tl-2201, 8 microns in Hg-1201, and 5 microns in LSCO. In Tl-2201 and Hg-1201, these values are much longer than predicted by Anderson from the condensation energy in the context of the Inter-Layer Tunneling (ILT) model as a candidate mechanism for superconductivity.

4B5.5 Scanning Hall Probe Microscopy of Nanostructured Superconductors

Simon J. Bending 1, Alexander N Grigorenko 1, Gareth D. Howells 1, Richard G. Humphreys 2, Joost Bekaert 3, Margriet Van Bael 3, K. Temst 3, L. Van Look 3, Victor V. Moshchalkov 3, Yvan Bruynseraede 3, and G. Borghs 4. 1 Department of Physics, University of Bath, Bath BA2 7AY, United Kingdom. 2 DERA Electronics Division, St Andrew's Road, Malvern, Worcs. WR14 3PS, United Kingdom. 3 Laboratorium voor Vaste-Stoffysica en Magnetisme, Katholieke Universiteit Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium. 4IMEC vzw, Kapeldreef 75, 3001 Leuven, Belgium.

Presenting Author: S.J. Bending

High resolution scanning Hall probe microscopy has been used to study flux structures in various nanostructured superconducting samples. The first system to be considered consists of an array of 5mm square disks patterned in a YBCO epitaxial thin film. Close to the transition temperature (>0.85T_c) broad, symmetric Giant Vortex (GV) structures are imaged, regardless of the magnetic history, which fill almost the entire YBCO disks. Measurements of the local induction above the disks as a function of applied field in this regime show very sharp jumps which appear to correspond to changes in winding number of the GV state. At lower temperatures a crossover to a multi-vortex state can be traced characterised by much more localised and asymmetric flux distributions. Results will be compared with calculations of field profiles using Ginzburg Landau theory.

We have also investigated thin films containing various types of pinning array. Clear evidence for the existence of commensurate 'strongly pinned' flux line structures at certain fractional filling factors will be presented. We also directly demonstrate pole-selective pinning when an array of single domain ferromagnetic dots is used. Finally the possibility of the formation of a multi-terrace critical state during flux penetration will be explored.

4B5.6 Current-gradient anisotropy enhancement in layered superconductors

Boris Khaykovich 1, Eli Zeldov 1, Dan T. Fuchs 1, Katya Teitelbaum 1,2, Youri Myasoedov 1, Tsuyoshi Tamegai 3, S. Ooi 3, Marcin Konczykowski 4, Richard A. Doyle 5, and Steve F.W.R. Rycroft 5. 1 Dept. Condensed Matter Physics, Weizmann Institute of Science, Rehovot 76100, Israel. 2 Department of Physics, Kazan State University, Kazan, Russia. 3 Dept. Applied Physics, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. 4 CNRS, UMR 7246, Laboratoire des Solides Irradies, Ecole Polytechnique, 91128 Palaiseau, France. 5 Research Center in Superconductivity, University of Cambridge, Cambridge CB3 0HE, United Kingdom.

Presenting Author: E. Zeldov

Simultaneous transport and magnetization measurements in Bi₂Sr₂CaCu₂O₈ crystals are reported. The transport current is injected at the top surface, while microscopic arrays of Hall sensors measure the local magnetic field across the crystals at the bottom. Surprisingly, at elevated currents we find finite resistivity well below the magnetic irreversibility line. The simultaneous magnetization measurements show large hysteretic Bean profiles which correspond to critical current of about two orders of magnitude larger than the applied transport current, implying that the resistivity should be zero. The paradoxical finite resistivity is found to be non-monotonic with temperature and extremely non-linear. The new observations are described in terms of a shear-induced decoupling, in which the pancake vortices flow only in the top few layers and are decoupled from the pinned vortices in the rest of the crystal. The resulting dissipation cannot be described by conventional electrodynamics, and an additional dominant term has to be added in which the c-axis resistivity and the effective anisotropy are dramatically enhanced by the c-axis gradient of the in-plane current density, dj_ab/dz. The large c-axis resistivity induced by the pancake shear process prevents shunting of the current flowing in the top layers by the bottom part of the sample.

4B5.7 Study of Formation of the Quasi-Ordered Vortex Phase in Bi₂Sr₂CaCu₂O_8+d Using High-Temporal Resolution Magneto-Optical System

D. Giller 1, A. Shaulov 1, L. Dorosinskii 2, T. Tamegai 3, and Y. Yeshurun 1. 1 Institute of Superconductivity, Department of Physics, Bar-Ilan University, Ramat-Gan, Israel. 2 National Metrology Institute, P.K. 21, 41470 Gebze-Kocaeli, Turkey. 3 Department of Applied Physics, University of Tokyo, Hongo 7-3-1, Bunkyo-ku Tokyo 113-8656, Japan.

Presenting Author: Y. Yeshurun

Flux lines are injected into a Bi₂Sr₂CaCu₂O_8+d crystal by a sudden application (rise-time < 40 ms) of a steady magnetic field H. The field H and the temperature T of the experiment correspond to the quasi-ordered vortex state in the equilibrium H-T vortex phase diagram. Utilizing a high temporal resolution magneto-optical method for direct flux visualization, we followed the time evolution of the vortex structure in time intervals of 40 ms. Initially, Bean-like profiles are observed, indicating high bulk currents. The Bean profiles gradually evolve into dome-shaped profiles, characteristic of vanishing bulk currents at the equilibrium state. The entire relaxation process, from the moment of application of the field until equilibrium, depends on field and temperature and it is typically of order of seconds around 20 K. During this period of time the relaxation is characterized by an exponential decay, implying a finite pinning potential U as the bulk current density j approaches zero.