2PO1-100 Josephson Plasma Emission from a BSCCO Intrinsic Junction by Quasiparticle Injection

Kiejin Lee 1, Ienari Iguchi 2, Masashi Tachiki 3, Kazuto Hirata 3, and Takashi Mochiku 3. 1 Department of Physics, Sogang University, C.P.O. Box 1142, Seoul 121-742, Korea. 2 Department of Applied Physics and CREST, Tokyo Institute of Technology, Okayama, Meguro, Tokyo 152-8551, Japan. 3 National Research Institute for Metal, 1-2-1 Sengen, Tsukuba 305, Japan.

Presenting Author: K. Lee

We report the observation of the Josephson plasma emission due to quasiparticle injection into the c-axis direction of a BSCCO(2212) single crystal. The quasiparticles are injected into the c-axis direction of the BSCCO single crystal. Three different modes of microwave emissions are observed by biasing the current in the hysteretic curve of the BSCCO intrinsic junctions at the receiver frequencies 11.6 GHz and 47 GHz. At a low bias voltage, we observed the Josephson self-emission which can be understood as the Josephson phase-locked self-emission of series connected junction array. At a high bias voltage, we observed two different microwave emission due to nonequilibrium effects: incoherent broadband emissions at quasiparticle branches and a sharp coherent emission in a negative resistance region at the gap edge.

2PO1-102 Dynamic structure selection of moving Josephson lattice in layered superconductors

Alexei Koshelev and Igor Aranson, Materials Science Division, Argonne National Laboratory, Argonne, IL 60439

Presenting Author: A.E. Koshelev

We investigate dynamics of the Josephson lattice in the high temperature superconductors at high magnetic fields. Starting from coupled equations for superconducting phases and magnetic field we have derived equations for the phase shifts between moving Josephson structures in different layers. These equations reveal two families of steady state solutions: lattices with constant phase shifts between neighboring layers, starting from zero for the rectangular configurations to p for the triangular configuration, and double-periodic lattices. Every lattice has the resonant velocity given by the corresponding plasma wave velocity. These lattices exhibit three kinds of instabilities depending on lattice structure and the velocity: long wave length shear instability, short wave length instability with respect to alternating deformation, and negative differential viscosity instability. A specific lattice structure at given velocity is selected uniquely by the boundary conditions. Numerical investigation shows that at small velocities the lattice experiences smooth evolution of structure with the phase shift between neighboring layers decreases from p at zero velocity to p /2. However, when the the phase shift reaches p /2 the lattice bifurcates to the double periodic state. At higher velocities the periodic lattice with the phase shift smaller than p /2 is restored. The structure continue to evolve smoothly towards the rectangular configurations with increase of velocity. The current-voltage characteristic is non-monotonic in the region of the transitions.

2PO1-103 A simulation of the behavior of flux quanta in layered superconductors.

R. Vaccarone, I.N.F.N. - Sezione di Genova, Via Dodecaneso 33, 16146 Genoa, Italy

Presenting Author: R. Vaccarone

Highly anisotropic superconductors can be described by superconducting planes weakly connected by Josephson currents crossing perpendicularly the gap between them. The presence of fluxons in the interlayer space has been envisaged years ago. Recently these fluxons as been visualized by using a Scanning Magnetic Microscope. In this work, I describe the layered superconductor as an assembly of SC layers, much thinner then their penetration depth, coupled by Josephson junctions.

In this system, the magnetic flux is quantized, creating a vortex which extends over many layers.

By the numerical simulation, I can examine the interaction between vortices in the same junction or vortices in different junctions. I examine also the fluxon distribution in a finite sample subjected

to an externally applied field. A peculiar behaviour is seen, because the fluxons are free to move in the junction, being at the same time strongly confined in the junction itself.

2PO1-105 Vortex-Line-Lattices in Anisotropic Superconductors in an Applied Field

W.A.M. Morgado 1, M.M. Doria 1, G.M. Carneiro 1, and I.G. de Oliveira 2. 1 Instituto de Fisica, Universidade Federal do Rio de Janiero, C.P. 68528 Rio de Janeiro 21945-970 RJ, Brazil. 2 Universidade Iguacu - UNIG, Nova Iguacu 26260-100 RJ, Brazil.

Presenting Author: W.A.M. Morgado

Sudboe, Brandt, and Huse (Phys. Rev. Lett. 71, 1451 (1993)) have shown that for non-interacting flux lines (FL), and strong uniaxial anisotropy, the coexistence of multiple orientations of rigid FL is possible. They found two minima of the Gibbs energy, G, each associated to a different orientation of the FL. Motivated by these results, we generalize their analysis to the case of interacting FL. First we develop a method to minimize G in a given applied field of magnitude H and of arbitrary orientation , considering straight FL in the London approximation. Assuming a FL lattice (FLL) with one FL per unit cell, we develop a simulated annealing algorithm to find the unit cell shape and FL orientation with respect to the crystal axis. This algorithim is made fast and accurate by the use of a rapidly converging series to calculate the lattice sums entering the FL-FL interaction energy. Next we apply the method to low fields, that is for H close to the lower critical field H_c1. The self-energy drives the FL orthogonal to the c-axis and the interaction energy to another direction, as it is well known that in this low field region a pair of tilted FL has a minimum, that yields the so-called chain state. Here we report preliminary results obtained by our method that aim at understanding how the FLL that minimizes G depends on the magnitude and orientation of the applied field H in this low field region and whether or not there are coexisting FLL for interacting FL.

2PO1-106 The voltage noise anisotropy near the transition region in high quality YBa₂Cu₃O_7-d thin film

A. Taoufik 1, S. Senoussi 2, A. Tirbiyine 1, and A. Ramzi 1. 1 Equipe des Matériaux Supraconducteurs à Haute Température Critique, Département de Physique, Faculté des Sciences, Université Ibn Zohr, B. P:28/S, Agadir, Maroc. 2 Laboratoire de Physique des Solides (associé au CNRS. URA. 0002), Université Paris Sud, Bâtiment 510, 91405 Orsay Cedex, France.

Presenting Author: A. Taoufik

We have investigated the noise anisotropy in c-axis-up YBa₂Cu₃O_7-d thin film. The studied sample is a high quality single crystal YBa₂Cu₃O_7-d thin film deposited by the ablation laser on the surface (100) of SrTiO₃ substrate. The resistance vanishes in zero magnetic field at T_c = 90 K.

The voltage noise was measured near the transition temperature region using a DC four-probe method. For each value of applied magnetic field, we have observed a voltage noise peak when the temperature closes the transition temperature.

In this contribution, we present the voltage noise anisotropy in this material. We have studied the voltage noise variations as a function of the angle q between the applied magnetic field direction (0.3 - 5 T) and the c-axis. To do that, we fixed the temperature at the value where the maximal voltage noise is reached (in case of the figure this temperature is 84.2 K) at the given field after we varied the angle q and measured the voltage noise for each {theta} value.

Our results show a strong dependence between the voltage noise and the applied magnetic field direction (angle q ).

2PO1-107 Influence of the Microstructure on the Anisotropy Behavior of a-Axis Oriented Systems in the Vortex Liquid Phase

E.M. Gonzalez 1, J.L. Vicent 1, and J.M. Gonzalez 2. 1 Dpto. Física de Materiales, Facultad de Ciencias Físicas, Universidad Complutense, 28040 Madrid, Spain. 2 Dpto. Propiedades Ópticas, Magnéticas y de Transporte, Instituto de Ciencia de Materiales de Madrid (C.S.I.C.), Cantoblanco, 28049 Madrid, Spain.

Presenting Author: J.L. Vicent

High temperature superconducting systems (single films of EuBa₂Cu₃O₇ and EuBa₂Cu₃O₇/PrBa₂Cu₃O₇ multilayers) with a-axis orientation (Cu-O planes perpendicular to the substrate plane) have been grown by dc magnetron sputtering from stoichiometric targets on cubic (100) SrTiO₃ substrates. The influence of the peculiar microstructure (consisting on microdomains separated by 90° boundaries) is investigated in the linear region of the vortex liquid phase by means of resistivity measurements. In this sense, the anisotropy of our single films shows a deviation from the 3D effective mass model; a deviation that is not observed in the case of in-plane-aligned a-axis films. Only when the thickness of the EuBa₂Cu₃O₇ superconducting layers is reduced, the 3D behavior is recovered. To understand these behaviors we will have to take into account important factors such as the high temperature range, the microdomain sizes, the intervortex distances.

2PO1-108 Melting of regular and decoupled vortex lattices in Bi₂Sr₂CaCu₂O₈ crystals

M. Konczykowski 1, C.J. Van Der Beek 1, M.V. Indenbom 1,2, and E. Zeldov 3. 1 Laboratoire des Solides Irradies, Ecole Polytechnique, 91128 Palaiseau, France. 2 Institute of Solid State Physics, 142432 Chernoglovka, Russia. 3 Weizmann Institute of Science, Rehovot, Israel.

Presenting Author: M. Konczykowski

The dependence of the vortex lattice phase diagram in Bi₂Sr₂CaCu₂O₈ crystals on the angle between the applied magnetic field and the crystalline c-axis was investigated by a low frequency ac shielding technique (with the ac field // c). We performed a novel type of measurement in which the static field-components parallel to ab- and c- (H_ab and H_c) are varied independently. Three regimes of Hab are identified. At small H_ab, the shielding current in the ab-plane is enhanced because pancake vortices are trapped or guided by the Josephson vortices induced by the parallel field. At moderate H_ab, the decrease of the first-order phase transition field H_cFOT (identified by the paramagnetic peak) is observed. This is the result of the decreasing Josephson coupling and the perturbation of the pancake vortex lattice by Josephson vortices. Finally, we show the existence of a new transition at a critical value of H_ab. This transition is marked by the abrupt drop of the ab-plane shielding current. At the transition, the first order transition field becomes independent of the ab-plane field and the system behaves in a two-dimensional fashion. We tentatively attribute this transition to the melting of the Josephson vortex lattice.

2PO1-109 Conduction Noise in Bi₂Sr₂CaCu₂O_y

Yoshihiko Togawa 1, Ryuichi Abiru 1, Katsuya Iwaya 1, Haruhisa Kitano 1, and Atsutaka Maeda 2. 1 Department of Basic Science, University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan. 2 CREST, Japan Science and Technology Corporation (JST), 4-1-8 Honcho, Kawaguchi, 332-0012, Japan.

Presenting Author: Y. Togawa

We measured the conduction noise of Bi₂Sr₂CaCu₂O_y in the vortex state to investigate dynamics of vortices. The measured noise showed two characteristic features around the vortex-lattice phase transition; one is the broadband noise (BBN) and the other is narrow-band noise (NBN). BBN appeared in low frequency region before the resistivity onset. On the other hand, NBN came out and shifted to higher frequency region rapidly with increasing filed. Some of their behaviors are similar to those of the noise of local magnetization that was reported so far by ourselves [1].

The conduction noise is generated mainly from the fluctuation of vortex-velocity, d n and the local magnetization noise is from the fluctuation of vortex-density, d n. The comparison of both noise spectra gives us valuable information on the feature of moving vortices and the dynamic phase diagram. Detailed results will be presented in the paper.

[1] T. Tsuboi et al., Phys. Rev. Lett. 80, 4550 (1998).