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[en] We report Raman measurements on Bi2Sr2CaCu2O8+δ single crystals that allow us to quantitatively evaluate the doping dependence of the density of Cooper pairs in the superconducting state. We show that the drastic loss of Cooper pairs in the antinodal region as the doping level is reduced is concomitant with a deep alteration of the quasiparticles dynamic above Tc and consistent with a pseudogap that competes with superconductivity. Our data also reveal that the overall density of Cooper pairs evolves with doping, distinctly from the superfluid density above the doping level pc = 0.2.
[en] The authors study a model of the Cooper pair in superconductivity whose basic mechanism is due to nonlinear-nonlocal and non-Hamiltonian interactions. The authors show the capability of the model for representing available data while possessing intriguing predictive capacities, and the authors propose its experimental verification. 16 refs., 3 figs., 1 tab
[en] Conventional superconductivity is well-explained in the framework of BCS theory by the formation of spin-singlet Cooper pairs. However, other exotic types of superconductivity involving, e.g., spin-triplet pairs exist as well. In general, superconducting correlations can be characterized by a nonvanishing pair amplitude which has a definite symmetry in spin, momentum and time or frequency. While the spin and momentum symmetry have been probed experimentally for different classes of superconductivity, the odd-frequency nature of certain superconducting correlations has so far been probed only indirectly. Here, we propose the thermopower as an unambiguous way to assess odd-frequency superconductivity. This is possible since the thermoelectric coefficient given by Andreev-like processes is only finite in the presence of odd-frequency superconductivity. We illustrate our general findings with a simple example of a superconductor-quantum dot-ferromagnet hybrid.
[en] We report hole-doping dependence of the in-plane resistivity ρab in a cuprate superconductor La2-xSrxCuO4, carefully examined using a series of high-quality single crystals. Our detailed measurements find a tendency towards charge ordering at particular rational hole doping fractions of 1/16, 3/32, 1/8, and 3/16. This observation appears to suggest a specific form of charge order and is most consistent with the recent theoretical prediction of the checkerboard-type ordering of the Cooper pairs at rational doping fractions x = (2m + 1)/2n, with integers m and n.
[en] We present an experimental study of transport properties of a large two-dimensional array of superconductor-normal-metal-superconductor (SNS) junctions comprised of the nanopatterned superconducting film, ensuring that NS interfaces of our SNS junctions are highly transparent. We find the anomalously high charge transmission at certain applied voltages commensurate with the magnitude of the gap in superconducting islands. This indicates the nonlocal nature of the charge transfer in multiply connected SNS systems. We propose the mechanism of the correlated transmission of Cooper pairs in large arrays of SNS junctions based on the combined action of the proximity effect and the simultaneous Andreev conversion processes at many NS-interfaces.
[en] Allowing with in a simple statistical theory for Cooper pairs with non-zero center-of-mass momentum (excluded in BCS theory) leads to a critical temperature that does not vanish in the limit of zero coupling, for all dimensions d>1. Thus, all observed superconductors-ranging from the quasi-one-dimensional organo-metallic materials, through the quasi-two-dimensional organics and cuprates, to the conventional and exotic three-dimensional ones, can in principle be accounted for. (Author) 24 refs
[en] We address the question of coexistence of superconductivity and ferromagnetism. Using a field theoretical approach we study a one-fermion effective model of a ferromagnetic superconductor in which the quasiparticles responsible for the ferromagnetism form the Cooper pairs as well. For the first time we solve self-consistently the mean-field equations for the superconducting gap and the spontaneous magnetization. We discuss the physical features which are different in this model and the standard BCS model and consider their experimental consequences
[en] Starting from the linearized BdG (Bogoliubov-de-Gennes) equation we make the simple observation that pairing can occur between particles with total momenta different from zero, e.g., with equal momentum and opposite spin, in cases of an effective interaction acting in the center-of-mass coordinates. (authors)
[en] The difference between a pure state, a statistically mixed state and the importance of interference in pure states are emphasized. Examples of quantum states are given where location, momentum or energy of a particle are not determined. As a model of a microscopic system a SQID is considered. Legget and Garg have derived inequalities analogous to the Bell's to decide experimentally if in a SQID at absolute zero temperature the magnetic flux is in a pure, i.e. indetermined state, or a statistically mixed state. Even if quantum mechanics is valid in macroscopic systems 'realism' need not to be abolished. (qui)