Manuel V. Ramallo

Measurements of the paraconductivity in the a-direction of untwinned Y1Ba2Cu3O7−δ single crystals

Abstract We report detailed experimental results of the paraconductivity in the a -direction, Δσ a , of two high-quality Y 1 ba 2 Cu 3 O 7- δ single crystals having a large (∼ 0.3 mm 2 ) untwinned region in their centers, and with transition temperatures of 90.0 K and 90.8 K. The exceptional sharpness of their resistive transitions (their upper half widths are less than 0.15 K) allows us to obtain quantitative information of Δσ a (ϵ), which is not affected by the CuO chains, up to reduced temperatures, ϵ, of the order of 10 -3 . The general scenario compatible with our experiments consists of the assumption of (1): A mean field-like region (MFR) up to ϵ≈2×10 -2 , where Δσ a (ϵ) may be explained in terms of the Lawrence-Doniach-like (LD) approach by taking into account the existence of two Josephson-coupled CuO 2 superconducting planes per unit cell. The characteristic superconducting length amplitudes in this MFR are ς ab (0) = (1.1±0.2) nm, and ς c (0) = (0.12±0.02) nm. (2) In agreement with our earlier qualitative resul ts, below ϵ≈2×10 -2 the measured ϱ a ( T ) separates from the mean-field behavior. Although so close to the transition we cannot exclude the influence of small stoichiometric inhomogeneities, these Δσ a (ϵ) data for ϵ⪅2×10 -2 may be explained, also on a quantitative level, on the grounds of the 3D-XY model, with a critical exponent of x = - 1 3 for Δσ a (ϵ), and x = - 2 3 for fluctuation-induced diamagnetism. This scenario correponds to one complex component order parameter, i.e., conventional 1 s 0 -wave pairing or one complex component unconventional pairing.

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.

Fluctuation-induced diamagnetism in biperiodic layered superconductors in the weak magnetic field regime

Abstract By using a generalization of the Lawrence-Doniach free-energy functional, we calculate in this work the fluctuation-induced diamagnetism above the superconducting transition, Δ χ , in a layered superconductor having two different interlayer distances, d 1 and d 2 , in the superconducting layer periodicity length, s = d 1 + d 2 , and two different Josephson-coupling strengths, γ 1 and γ 2 , between adjacent superconducting layers. The calculations are performed in the weak magnetic field limit. We include in our treatment the possibility of an unconventional superconducting pairing by considering in each plane a superconducting wave function of g complex components. For the first time, Δ χ in a multiperiodic layered superconductor is obtained explicitly, as a function of the coherence length amplitudes, of g and of γ 1 and γ 2 . Our results show that by introducing an effective number, N e , of independent fluctuating layers in the layer periodicity length, it is possible to express Δ χ in terms of the fluctuation-induced diamagnetism for one-single periodicity layered superconductor, with the same periodicity length s = d 1 + d 2 . As an example of their interest, these theoretical results for the superconducting-order parameter fluctuations above T c are used to quantitatively analyze the existing experimental Δ χ ( T ) data in the mean-field0like region, measured in the weak magnetic field limit in single crystals of two different copper oxide systems having two Josephson coupled superconducting CuO 2 planes in s : The Y 1 Ba 2 Cu 3 O 7− δ and the Bi 2 Sr 2 Ca 1 Cu 2 O 8 compounds.

Measurements of the magnetic susceptibility above the superconducting transition of YBa2Cu3O7−δ single crystals in the weak magnetic field limit

Abstract We report detailed experimental results of the magnetic susceptibility of high quality YBa 2 Cu 3 O 7−δ single crystals for the applied magnetic field, H , parallel (χ ab ) and perpendicular (χ c ) to the c direction. The data were obtained from 5 K to 300 K, and with μ 0 H ≤ 0.4 T, for which χ ab and χ c around the superconducting transition correspond to the so-called weak magnetic field limit, even for reduced temperatures of the order of 10 −2 . The excess (or para-) diamagnetism above but near the superconducting transition, extracted from these data following consistent procedures, was analyzed in terms of the existing approaches for independent gaussian fluctuations of the superconducting order parameter amplitude in layered materials. A scenario compatible with our experimental results is characterized by values of the correlation length amplitudes in the ab plane and in the c direction of, respectively, (1.0 ± 0.2) nm and (0.12 ± 0.03) nm, an effective number of superconducting planes in the unit cell length varying in the mean field-like region between 1 and 1.6, and by conventional 1 s 0 -wave pairing or one complex component unconventional pairing.

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

The fluctuation-induced diamagnetism ΔM, associated with the presence of precursor Cooper pairs in the normal state, has been measured in lanthanum with dilute magnetic (Pr) and nonmagnetic (Lu) impurities. It is found that while for pure La and La-Lu alloys ΔM agrees, as expected, with the predictions based on the conventional Gaussian-Ginzburg-Landau approach, it is much larger for La-Pr alloys (around a factor 5 for La-2 at.%Pr). These results suggest the existence of an indirect contribution to ΔM arising from the interaction between fluctuating Cooper pairs and magnetic impurities.

Measurements of the fluctuation-induced magnetoconductivity in the a-direction of an untwinned Y1Ba2Cu3O7−δ single crystal in the weak magnetic field limit

Abstract Measurements of the fluctuation-induced magnetoconductivity in the a -direction of an untwinned Y 1 Ba 2 Cu 3 O 7−δ single crystal, not affected by the presence of CuO chains, are presented. The measurements were performed with the applied magnetic field, H , perpendicular to the ab -planes, and with H ≤1 T, which corresponds to the weak magnetic field limit even for reduced temperatures, ϵ, of the order of 10 −2 . In the mean-field region, i.e., for 10 −2 ⪅ϵ⪅10 −1 , these data may be explained in terms of direct thermal order-parameter fluctuations, in agreement with our previous paraconductivity and fluctuation-induced dia-magnetism measurements in the same sample. These results suggest that the pair-breaking parameter, δ, is less than 5×10 −2 at T =100 K. This upper limit not only leads to the absence of appreciable Maki-Thompson effects in cooper oxide superconductors (HTSCs), but also has implications in other central open HTSC problems. By using the BCS-like expressions to relate δ and the Cooper-pair relaxation time, τ φ , we obtain, at T =100 K, τ φ ⪅10 -−15 s in the clean limit and τφ⪅10 −14 s in the dirty limit. This last value is comparable with the expected electronic-scattering time.

Effects of critical temperature inhomogeneities on the voltage–current characteristics of a planar superconductor near the Berezinskii–Kosterlitz–Thouless transition

We analyze numerically how the voltage?current (V ?I) characteristics near the so-called Berezinskii?Kosterlitz?Thouless (BKT) transition of 2D superconductors are affected by a Gaussian distribution of critical temperature inhomogeneities, randomly located in space and with long characteristic lengths (much larger than the in-plane superconducting coherence length amplitude). Our simulations allow us to quantify the broadening around the average BKT transition temperature of both the exponent ? in and of the resistance V/I. These calculations reveal that strong spatial redistributions of the local current will occur around the transition as either I or the temperature T are varied. Our results also support that the condition ? = 3 provides a good estimate for the location of the average BKT transition temperature , and that extrapolating to the ?(T) behavior well below the transition provides a good estimate for the average mean-field critical temperature .

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

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.

An effective-charge model for the trapping of impurities of fluids in channels with nanostructured walls.

We present model equations for the trapping and accumulation of particles in a cylindrical channel with nanostructured inner walls when a fluid passes through, carrying a moderate load of impurities. The basic ingredient of the model is the introduction of a phenomenological ‘effective-charge density’ of the walls, related to the electrical charges exposed in the nanotexture. The effective charge is gradually reduced as the flow runs through the channel and the trapped impurities cover the internal walls. Based on the proposed equations, the position and time dependence of the areal density of trapped impurities, and the filtration performance, may be calculated. It is proposed that experimentally testing these results may help to understand the enhanced trapping capability observed in many diverse nanotextured channel structures.

Thermal fluctuations near a phase transition probed through the electrical resistivity of high-temperature superconductors

We present a simple and inexpensive experimental setup that can determine the thermal fluctuations near a superconducting transition. By using equipment common in undergraduate laboratories, the in-plane dc electrical resistivity of a cuprate superconductor as a function of temperature is measured with resolution sufficient to analyze the fluctuation conductivity above the superconducting critical temperature, including the values of the critical exponents. We also present a simple calculation of the fluctuation conductivity, including its dependence on the layered structure of the material, within the Gaussian–Ginzburg–Landau formalism.