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Pancake vortices  [PDF]
John R. Clem
Physics , 2004, DOI: 10.1007/s10948-004-0774-z
Abstract: I describe the magnetic-field and current-density distributions generated by two-dimensional (2D) pancake vortices in infinite, semi-infinite, and finite-thickness stacks of Josephson-decoupled superconducting layers. Arrays of such vortices have been used to model the magnetic structure in highly anisotropic layered cuprate high-temperature superconductors. I show how the electromagnetic forces between pancake vortices can be calculatated, and I briefly discuss the effects of interlayer Josephson coupling.
Microwave Response of Bi2Sr2CaCu2O8+δ Surface Intrinsic Josephson Junctions with Bending Pancake Vortices
中国物理快报 , 2007,
Abstract: We study the microwave response of surface intrinsic Josephson junctions on Bi2Sr2 CaCu2O8+δ, in which bending pancake vortex lines are introduced in a controllable way. It is found that the bending vortices can greatly influence the response. In some cases, typical Shapiro steps that lie far above the quasiparticle branch are observed, with the step interval satisfying the Josephson relation and their amplitude versus the square root of microwave power following the Bessel function behaviour. In the other cases, current steps that lie on the quasiparticle branch are observed, but only one or two steps appear at the same time under the variation of the microwave power.
Decoration of Josephson Vortices by Pancake Vortices in Bi2Sr2CaCu2Ox  [PDF]
V. K. Vlasko-Vlasov,A. Koshelev,U. Welp,G. W. Crabtree,K. Kadowaki
Physics , 2002, DOI: 10.1103/PhysRevB.66.014523
Abstract: Josephson vortices are imaged magneto-optically due to their decoration with pancake vortices in $Bi_{2}Sr_{2}CaCu_{2}O_{8+d}$ single crystals. Peculiarities of interaction between the pancake and Josephson vortices (JV) depending on the values of crossing fields and temperature are studied based on the observations of these decoration patterns. Evidences of the period-doubling in the decoration patterns compared to the JV stack period, migration of JV lines between neighboring stacks, and transitions between different JV configurations are reported. Imaging of the pancake/Josephson vortex decoration patterns over large areas is shown to be a sensitive tool for detecting local variations of the anisotropy and mapping imperfections in layered HTS samples.
Slowing down Josephson vortex lattice in Bi_2Sr_2CaCu_2O_{8+d} with pancake vortices  [PDF]
A. E. Koshelev,Yu. I. Latyshev,M. Konczykowski
Physics , 2006, DOI: 10.1103/PhysRevB.74.104509
Abstract: We study theoretically and experimentally influence of pancake vortices on motion of the Josephson vortex lattice in layered high-temperature superconductors. Mobility of the Josephson vortices in layered superconductors is strongly suppressed by small amount of pancake-vortex stacks. Moving Josephson vortex lattice forces oscillating zigzag deformation of the pancake-vortex stacks contributing to damping. The salient feature of this contribution is its nonmonotonic dependence on the lattice velocity and the corresponding voltage. Maximum pancake effect is realized when the Josephson frequency matches the relaxation frequency of the stacks. The pancake-vortex damping is strongly suppressed by thermal fluctuations of the pancake vortices. This theoretical picture was qualitatively confirmed by experiments on two mesas prepared out of Bi_2Sr_2CaCu_2O_{8+d} whiskers. We found that the Josephson-vortex flux-flow voltage is very sensitive to small c-axis magnetic field. The pancake-vortex contribution to the current indeed nonmonotonically depends on voltage and decreases with increasing temperature and in-plane magnetic field. We also found that irradiation with heavy ions has no noticeable direct influence on motion of the Josephson vortices but dramatically reduces the pancake-vortex contribution to the damping of the Josephson vortex lattice at low temperatures.
Fluctuating pancake vortices revealed by dissipation of Josephson vortex lattice  [PDF]
A. E. Koshelev,A. I. Buzdin,I. Kakeya,T. Yamamoto,K. Kadowaki
Physics , 2011, DOI: 10.1103/PhysRevB.83.224515
Abstract: In strongly anisotropic layered superconductors in tilted magnetic fields the Josephson vortex lattice coexists with the lattice of pancake vortices. Due to the interaction between them, the dissipation of the Josephson-vortex lattice occurs to be very sensitive to the presence of the pancake vortices. If the c-axis magnetic field is smaller then the corresponding lower critical field, the pancake stacks are not formed but the individual pancakes may exist in the fluctuational regime either near surface in large-size samples or in the central region for small-size mesas. We calculate the contribution of such fluctuating pancake vortices to the c-axis conductivity of the Josephson vortex lattice and compare the theoretical results with measurements on small mesas fabricated out of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ crystals. A fingerprint of fluctuating pancakes is characteristic exponential dependence of the c-axis conductivity observed experimentally. Our results provide strong evidence of the existence of the fluctuating pancakes and their influence on the Josephson-vortex-lattice dissipation.
Interactions between Josephson and pancake vortices revealed by second harmonic Hall measurements  [PDF]
Mauro Tesei,Lesley F Cohen
Physics , 2009,
Abstract: The dynamic interactions between Josephson and pancake vortices in highly anisotropic superconductors are probed by shaking the former vortices with an ac magnetic field and measuring the second harmonic Hall response of the latter vortices with the sensor positioned above the sample. We demonstrate the efficiency of the proposed measurement method that aims to probe the coupling between the two vortex species. New experimental data obtained on Bi2Sr2CaCu2O8 single crystals reveal interesting features and the possibility to determine the phase diagram of vortex matter in such superconductors.
Josephson vortices and solitons inside pancake vortex lattice in layered superconductors  [PDF]
Alexei E. Koshelev
Physics , 2003, DOI: 10.1103/PhysRevB.68.094520
Abstract: In very anisotropic layered superconductors a tilted magnetic field generates crossing vortex lattices of pancake and Josephson vortices (JVs). We study the properties of an isolated JV in the lattice of pancake vortices. JV induces deformations in the pancake vortex crystal, which, in turn, substantially modify the JV structure. The phase field of the JV is composed of two types of phase deformations: the regular phase and vortex phase. The phase deformations with smaller stiffness dominate. The contribution from the vortex phase smoothly takes over with increasing magnetic field. We find that the structure of the cores experiences a smooth yet qualitative evolution with decrease of the anisotropy. At large anisotropies pancakes have only small deformations with respect to position of the ideal crystal while at smaller anisotropies the pancake stacks in the central row smoothly transfer between the neighboring lattice positions forming a solitonlike structure. We also find that even at high anisotropies pancake vortices strongly pin JVs and strongly increase their viscous friction.
Hot-spot formation in stacks of intrinsic Josephson junctions in Bi2Sr2CaCu2O8  [PDF]
B. Gross,S. Guenon,J. Yuan,M. Y. Li,J. Li,A. Iishi,R. G. Mints,T. Hatano,P. H. Wu,D. Koelle,H. B. Wang,R. Kleiner
Physics , 2012, DOI: 10.1103/PhysRevB.86.094524
Abstract: We have studied experimentally and numerically temperature profiles and the formation of hot spots in intrinsic Josephson junction stacks in Bi2Sr2CaCu2O8 (BSCCO). The superconducting stacks are biased in a state where all junctions are resistive. The formation of hot spots in this system is shown to arise mainly from the strongly negative temperature coefficient of the c-axis resistivity of BSCCO at low temperatures. This leads to situations where the maximum temperature in the hot spot can be below or above the superconducting transition temperature Tc. The numerical simulations are in good agreement with the experimental observations.
Hot spots and waves in Bi2Sr2CaCu2O8 intrinsic Josephson junction stacks -a study by Low Temperature Scanning Laser Microscopy  [PDF]
H. B. Wang,S. Guénon,J. Yuan,A. Iishi,S. Arisawa,T. Hatano,T. Yamashita,D. Koelle,R. Kleiner
Physics , 2008, DOI: 10.1103/PhysRevLett.102.017006
Abstract: Recently, it has been shown that large stacks of intrinsic Josephson junctions in Bi2Sr2CaCu2O8 emit synchronous THz radiation, the synchronization presumably triggered by a cavity resonance. To investigate this effect we use Low Temperature Scanning Laser Microscopy to image electric field distributions. Apart from verifying the appearance of cavity modes at low bias we find that, in a high input power regime, standing-wave patterns are created through interactions with a hot spot, possibly pointing to a new mode of generating synchronized radiation in intrinsic Josephson junction stacks.
AC loss in ReBCO pancake coils and stacks of them: modelling and measurement  [PDF]
E. Pardo,J. Souc,J. Kovac
Physics , 2011, DOI: 10.1088/0953-2048/25/3/035003
Abstract: Many applications of ReBCO coated conductors contain stacks of pancake coils. In order to reduce their high AC loss, it is necessary to understand the loss mechanisms. In this article, we measure and simulate the AC loss and the critical current, I_c, in stacks of pancake coils ("pancakes"). We construct stacks of up to 4 pancakes and we measure them by electrical means. We also obtain the anisotropic field dependence of J_c from I_c measurements of the tape. This J_c is the only input to the simulations, together with the coil dimensions. After validating our computations with the measurements, we simulate stacks of many pancakes, up to 32. We found that the AC loss in a stack of (four) pancakes is very high, two orders of magnitude larger than for a single tape. A double pancake behaves as a single one with double width but a stack of more pancakes is very different. Finally, we found that a 2-strand Roebel cable reduces the AC loss in a stack of pancakes but not in a single pancake. In conclusion, the AC loss in stacks of pancakes is too high. However, our simulations are useful to predict the AC loss and optimise the coil design, reducing the AC loss.
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