Carlos Carballeira

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.

Ginzburg–Landau description of confinement and quantization effects in mesoscopic superconductors

An approach to the Ginzburg–Landau problem for superconducting regular polygons is developed making use of an analytical gauge transformation for the vector potential A which gives An=0 for the normal component along the boundary line of different symmetric polygons. As a result the corresponding linearized Ginzburg–Landau equation reduces to an eigenvalue problem in the basis set of functions obeying Neumann boundary condition. Such basis sets are found analytically for several symmetric structures. The proposed approach allows for accurate calculations of the order parameter distributions at low calculational cost (small basis sets) for moderate applied magnetic fields. This is illustrated by considering the nucleation of superconductivity in squares, equilateral triangles and rectangles, where vortex patterns containing antivortices are obtained on the Tc–H phase boundary. The calculated phase boundaries are compared with the experimental Tc(H) curves measured for squares, triangles, disks, rectangles,...

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.

Interannual changes in δ15N values in Fucus vesiculosus L.

The natural abundance of (15)N (δ(15)N) has been widely used to detect anthropogenically derived N loads in environmental impact studies. The present study involved retrospective analysis of subsamples of Fucus vesiculosus L. collected during a period of three years (2008-2010) from two sites: a control site, within a coastal reference area, and an area affected by the effluents of a marine land-based fish farm. The isotopic signal in different subsamples of the macroalgae thalli (tissue that has grown during the same period) varied depending on the age of the tissue. Moreover, the isotopic signal decreased significantly with the age of the frond to within a certain range. The δ(15)N of F. vesiculosus is temporally unstable; therefore, measurement of the δ(15)N of macroalgal tissues does not allow reliable retrospective biomonitoring of environmental pollution. Further knowledge about the growth and other biological aspects of this species is required.

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

The superconducting fluctuations well inside the normal state of Fe-based superconductors were studied through measurements of the in-plane paraconductivity and magnetoconductivity in high quality BaFe2-xNixAs2 crystals with doping levels from the optimal level (x = 0.10) up to highly overdoped (x = 0.20). These measurements, performed in magnetic fields up to 9 T perpendicular to the ab (Fe) layers, allowed a reliable check of the applicability to iron-based superconductors of Ginzburg-Landau approaches for 3D anisotropic compounds, even at high reduced temperatures and magnetic fields. Our results also allowed us to gain valuable insight into the dependence on the doping level of some central superconducting parameters (coherence lengths and anisotropy factor).

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

Abstract By using two grain oriented La1.9Sr0.1CuO4 (LaSCO) samples, with masses as big as 48 and 110 mg, the fluctuation induced diamagnetism (FD) above the superconducting transition temperature, Tc0, has been measured in the reduced temperature region bounded by 3×10−2≲ϵ≲0.5 and under reduced magnetic fields to within 8×10−3≲h≲0.2. Here ϵ≡ln(T/Tc0) and h≡H/Hc2(0), where Tc0 and Hc2(0) are, respectively, the mean field normal–superconducting transition temperature at zero applied magnetic field and the upper critical magnetic field amplitude. These measurements deeply penetrate, then, in both the Schmidt and Schmid (or zero magnetic field)-limit (h/ϵ≪1) and the Prange regime (h/ϵ≳1). For the first time in a high-temperature cuprate superconductor (HTSC), these FD data in both fluctuation regimes cover all the reduced temperatures not too close to Tco, including the high-reduced temperature region (ϵ≳0.1), where the contribution of the short-wavelength fluctuations may be particularly important. Then, the existing FD calculations on the grounds of the Gaussian–Ginzburg–Landau (GGL) approach for 2D-layered superconductors are extended to the short-wavelength regime by introducing two possible cutoff conditions: The conventional kinetic energy (or momentum) cutoff and the total energy cutoff, this last taking also into account the quantum localization energy contribution associated with the shrinkage of the superconducting wavefunction. The differences between both cutoff conditions are expected to be appreciable mainly in the high-reduced temperature region, where the superconducting coherence length, ξ(T), becomes of the order of ξ(0), the coherence length extrapolated to T=0 K. The analyses of our experimental data on the grounds of these phenomenological approaches show that the different FD regimes may be explained in terms of the GGL theory for layered superconductors only under a total energy cutoff. In contrast, when regularized through a momentum cutoff condition the GGL approach fails to explain the high-reduced temperature region in both fluctuation regimes (Schmidt and Schmid-limit and Prange regime). The corresponding cutoff amplitude is found to be of the order of 0.6, in excellent agreement with our previous FD results obtained in the finite magnetic field regime but at low-reduced temperatures (ϵ≲0.1). This cutoff amplitude is defined in units of ℏ 2 /2m ∗ ξ 2 ab (0) , where m ∗ is the effective mass of the superconducting carriers, ℏ is the reduced Planck constant and ξab(0) is the in-plane superconducting coherence length amplitude. These results strongly support, then, the adequacy of the mean field like GGL approach, regularized through a total energy cutoff, to explain the thermal fluctuations in HTSC at all the temperatures not too close to Tc0 (above ϵ≃3×10−2), including the high-reduced temperature region (above typically ϵ≃0.1). These results also show that, in contrast with the low-reduced temperature superconductors (LTSC), due to the shortness of their superconducting coherence length amplitudes the FD in the HTSC is not appreciably affected by non-local electrodynamic effects, at least for reduced magnetic fields up to 0.2. In addition, our present findings suggest that the presence of a normal-state pseudogap in underdoped LaSCO superconductors does not appreciably affect the behaviour of the fluctuation induced diamagnetism in all the accessible reduced temperatures above the superconducting transition.

In-plane and transverse superconducting fluctuation diamagnetism in the presence of charge-density waves in 2 H − Nb Se 2 single crystals

The fluctuation-diamagnetism (FD) above the superconducting transition was measured in 2H-NbSe2 single crystals. The moderate uniaxial anisotropy of this compound, and some experimental improvements, allowed to measure the superconducting fluctuation effects in the two main crystallographic directions. These results reveal that the nonlocal electrodynamic effects on the FD are highly anisotropic, and they also discard a possible contribution to the FD coming from the charge-density waves (CDW) appearing below

Fluctuation-induced diamagnetism in bulk isotropic superconductors at high reduced temperatures

Tcdw > Tc in 2H-NbSe2, in agreement with a phenomenological estimate.

Full-critical behaviour of the heat capacity around Tc and inhomogeneities in Y-123

The short-wavelength effects on the fluctuation-induced diamagnetism (FD) in bulk isotropic three-dimensional (3D) superconductors are taken into account by introducing in the Gaussian-Ginzburg-Landau approach different cut-off conditions. These calculations, which extend to the 3D case our previous results on layered superconductors, are then used to briefly analyse the FD data measured for the low-temperature superconducting alloy Pb-8 at.% In. These analyses confirm the adequacy of a total-energy cut-off for explaining, for low-temperature 3D superconductors also, the thermal fluctuation effects in the high-reduced-temperature region. These results thus provide further support to the recent proposal that, due to the localization energy, the size of the effective fluctuations cannot be appreciably smaller than the superconducting coherence length amplitude extrapolated to T = 0 K.