J. Mosqueira

On the consequences of the uncertainty principle on the superconducting fluctuations well inside the normal state

We first argue that the collective behaviour of the Cooper pairs created by thermal fluctuations well above the superconducting transition temperature, Tc, is dominated by the uncertainty principle which, in particular, leads to a well-defined temperature, Tc, above which the superconducting coherence vanishes. On the grounds of the BCS approach, the corresponding reduced temperature, ec ≡ ln (TcTc, is estimated to be around 0.55, i.e., above Tc 1.7 Tc coherent Cooper pairs cannot exist. The implications of these proposals on the superfluid density are then examined using the Gaussian-Ginzburg-Landau approximation. Then we present new measurements of the thermal fluctuation effects on the electrical conductivity and on the magnetization in different low- and high-Tc superconductors with different dopings which are in excellent agreement with these proposals and that demonstrate the universality of ec.

Resistivity anomalies above the superconducting transition in Y1Ba2Cu3O7−δ crystals and non-uniformly distributed critical-temperature inhomogeneities

Abstract We report the observation of a sharp peak (20% of the normal resistivity), just above the superconducting transition, in the curve of the ab plane electrical resistivity versus temperature,ϱab(T), of some non-fully oxygenated Y1Ba2Cu3O7−δ crystals, with transition temperatures, Tc, below 90 K. The peak amplitude decreases in the presence of an applied magnetic field perpendicular to the ab plane and it is also affected by the electrical-contact configuration or by further re-oxygenation of the crystals. This resistivity anomaly may easily be explained by assuming the presence of PTc inhomogeneities associated with small stoichiometric (oxygen-content) inhomogeneities, at long length scales, non-uniformly distributed in the crystals. Also, we show that depending on their spatial distribution, the Tc inhomogeneities may just deform (but without a peak) the resistivity versus temperature curve above Tc, very close to the transition, without appreciably broadening the measured resistive transition. These results may help to the understanding of the anomalies observed near Tc in different transport properties and materials by other authors.

Measurements of pool boiling heat transfer from ceramic Y1Ba2Cu3O7- delta superconductors to liquid nitrogen

We present measurements of pool boiling heat transfer from ceramic Y1Ba2Cu3O7- delta to liquid nitrogen in the range of heat flux density up to qe=35 W cm-2. Along the convective pool boiling regime, a linear relationship between the interface overtemperature, Tw, and qe, has been observed, with a value of the heat transfer coefficient h=1.3 W cm-2 K-1. In the nucleate regime the qe dependence on Tw follows quite well a power law, with an exponent close to 0.7. In the low heat flux regime qe<or approximately= 5 W cm-2, the heat transfer curve shows a double hysteresis, which seems to be closely related to the porosity of the samples.

Electric and magnetic characterization of NbSe2 single crystals: Anisotropic superconducting fluctuations above TC

Abstract Electric and magnetic characterization of NbSe 2 single crystals is first presented in detail. Then, some preliminary measurements of the fluctuation-diamagnetism (FD) above the transition temperature T C are presented. The moderate uniaxial anisotropy of this compound allowed us to observe the fluctuation effects for magnetic fields H applied in the two main crystallographic orientations. The superconducting parameters resulting from the characterization suggest that it is possible to do a reliable analysis of the FD in terms of the Ginzburg–Landau (GL) theory.

Measurements of the superconducting fluctuations in optimally doped BaFe2−xNixAs2 under high magnetic fields: probing the 3D-anisotropic Ginzburg–Landau approach

The superconducting fluctuations well inside the normal state of Fe-based superconductors were experimentally studied through the in-plane paraconductivity in several high-quality, optimally doped BaFe2-xNixAs2 crystals. These measurements were performed in magnetic fields with amplitudes up to 14 T, and different orientations relative to the c-axis of the crystals (theta = 0 degrees, 53 degrees, and 90 degrees). The results allowed a stringent check of the applicability of a recently proposed Ginzburg-Landau approach for the fluctuating electrical conductivity of three-dimensional (3D) anisotropic materials in the presence of finite applied magnetic fields.

Anomalous precursor diamagnetism at low reduced magnetic fields and the role of T C inhomogeneities in the superconductors Pb 55 In 45 and underdoped La 1.9 Sr 0.1 Cu O 4

The magnetic field dependence of the magnetization was measured above the superconducting transition in a high-

Measurements of the fluctuation-induced in-plane magnetoconductivity at high reduced temperatures and magnetic fields in the iron arsenide BaFe2−xNixAs2

{T}_{C}

Observation of enhanced fluctuation diamagnetism in lanthanum superconductors with dilute magnetic impurities

underdoped cuprate

Fluctuation induced diamagnetism in La1.9Sr0.1CuO4 superconductors well inside the high-reduced temperature and the finite magnetic field regimes

({\mathrm{La}}_{1.9}{\mathrm{Sr}}_{0.1}\mathrm{Cu}{\mathrm{O}}_{4})

Observation of anisotropic diamagnetism above the superconducting transition in iron pnictide Ba 1 − x K x Fe 2 As 2 single crystals due to thermodynamic fluctuations

and in a low-