Influence of the voltage taps position on the self-field DC and AC transport characterization of HTS superconducting tapes

The current-voltage (I-V) curve is the basic characteristic of a superconducting wire or tape. Measuring I-V curves is generally problematic when samples have poor stabilization. Soldering voltage taps to an active part of the conductor affects the effectiveness of the local cooling and/or can be difficult to do in certain devices such as fault current limiters and cables where the tapes are closely packed. In order to overcome these problems, voltage taps can be placed outside the active area of the superconductor. We proved both by simulations and experiments that this arrangement leads to the same results as the standard four point method and it provides more detailed information for sample protection. The same arrangement can also be used for AC transport loss measurement. However in this case particular care has to be taken because the eddy current loss in the current leads contributes to the total measured loss. We used numerical simulations to evaluate the contribution of the eddy current loss to the measured AC loss. With help of simulations one can determine whether the contribution of the eddy current loss is significant and possibly optimize the current leads to reduce that loss contribution

Fermi surface dichotomy on systems with fluctuating order

We investigate the effect of a dynamical collective mode coupled with quasiparticles at specific wavevectors only. This coupling describes the incipient tendency to order and produces shadow spectral features at high energies, while leaving essentially untouched the low energy quasiparticles. This allows to interpret seemingly contradictory experiments on underdoped cuprates, where many converging evidences indicate the presence of charge (stripe or checkerboard) order, which remains instead elusive in the Fermi surface obtained from angle-resolved photoemission experiments.Comment: 11 pages, 10 figure

Cooperation, competition and the emergence of criticality in communities of adaptive systems

The hypothesis that living systems can benefit from operating at the vicinity of critical points has gained momentum in recent years. Criticality may confer an optimal balance between exceedingly ordered and too noisy states. We here present a model, based on information theory and statistical mechanics, illustrating how and why a community of agents aimed at understanding and communicating with each other converges to a globally coherent state in which all individuals are close to an internal critical state, i.e. at the borderline between order and disorder. We study --both analytically and computationally-- the circumstances under which criticality is the best possible outcome of the dynamical process, confirming the convergence to critical points under very generic conditions. Finally, we analyze the effect of cooperation (agents try to enhance not only their fitness, but also that of other individuals) and competition (agents try to improve their own fitness and to diminish those of competitors) within our setting. The conclusion is that, while competition fosters criticality, cooperation hinders it and can lead to more ordered or more disordered consensual solutions.Comment: 20 pages, 5 figures. Supplementary Material: 8 page

Investigation of AC Loss in HTS Cross-Conductor Cables for Electrical Power Transmission

This paper presents the alternating current (AC) loss analysis on high-temperature superconductor (HTS) Cross-Conductor (CroCo) cables, in order to evaluate whether they could be utilized for electrical power transmission. The modeling of HTS CroCo cables was based on a cable assembled at the Karlsruhe Institute of Technology (KIT) and the AC loss calculation was based on the H-formulation model implemented in the finite-element method (FEM) software package COMSOL Multiphysics. The AC loss calculations have been carried out for isolated single-phase CroCo cable and three-phase CroCo cables. The AC loss angular dependence of a particular phase of CroCo cables during three phase operation has been studied. The current distributions of individual tapes within CroCo cables have been investigated

The Electron-Phonon Interaction in the Presence of Strong Correlations

We investigate the effect of strong electron-electron repulsion on the electron-phonon interaction from a Fermi-liquid point of view: the strong interaction is responsible for vertex corrections, which are strongly dependent on the $v_Fq/\omega$ ratio. These corrections generically lead to a strong suppression of the effective coupling between quasiparticles mediated by a single phonon exchange in the $v_Fq/\omega \gg 1$ limit. However, such effect is not present when $v_Fq/\omega \ll 1$. Analyzing the Landau stability criterion, we show that a sizable electron-phonon interaction can push the system towards a phase-separation instability. A detailed analysis is then carried out using a slave-boson approach for the infinite-U three-band Hubbard model. In the presence of a coupling between the local hole density and a dispersionless optical phonon, we explicitly confirm the strong dependence of the hole-phonon coupling on the transferred momentum versus frequency ratio. We also find that the exchange of phonons leads to an unstable phase with negative compressibility already at small values of the bare hole-phonon coupling. Close to the unstable region, we detect Cooper instabilities both in s- and d-wave channels supporting a possible connection between phase separation and superconductivity in strongly correlated systems.Comment: LateX 3.14, 04.11.1994 Preprint no.101

Effect of mesoscopic inhomogeneities on local tunnelling density of states

We carry out a theoretical analysis of the momentum dependence of the Fourier-transformed local density of states (LDOS) in the superconducting cuprates within a model considering the interference of quasiparticles scattering on quenched impurities. The impurities introduce an external scattering potential, which is either nearly local in space or it can acquire a substantial momentum dependence due to a possible strong momentum dependence of the electronic screening near a charge modulation instability. The key new effect that we introduce is an additional mesoscopic disorder aiming to reproduce the inhomogeneities experimentally observed in scanning tunnelling microscopy. The crucial effect of this mesoscopic disorder is to give rise to point-like spectroscopic features, to be contrasted with the curve-like shape of the spectra previously calculated within the interfering-quasiparticle schemes. It is also found that stripe-like charge modulations play a relevant role to correctly reproduce all the spectral features of the experiments.Comment: 11 pages and 5 figure

Charge inhomogeneity coexisting with large Fermi surfaces

We discuss how stripes in cuprates can be compatible with a Fermi-liquid-like Fermi surface and, at the same time, they give rise to a one-dimensional-like pseudo Fermi surface in the momentum distribution function.Comment: Proceedings of the M2S conference, July 2006, Dresden; 2 pages, 1 figure to appear on Phisica

Effective medium theory for superconducting layers: A systematic analysis including space correlation effects

We investigate the effects of mesoscopic inhomogeneities on the metal-superconductor transition occurring in several two-dimensional electron systems. Specifically, as a model of systems with mesoscopic inhomogeneities, we consider a random-resistor network, which we solve both with an exact numerical approach and by the effective medium theory. We find that the width of the transition in these two-dimensional superconductors is mainly ruled by disorder rather than by fluctuations. We also find that "tail" features in resistivity curves of interfaces between LaAlO3 or LaTiO3 and SrTiO3 can arise from a bimodal distribution of mesoscopic local Tc's and/or substantial space correlations between the mesoscopic domains.Comment: 12 pages, 10 figure