Fluctuations Of Superconducting Order Parameter Research Articles

Resistivity, Hall effect, and anisotropic superconducting coherence lengths of HgBa2CaCu2O6 thin films with different morphology

Thin films of the high-temperature superconductor HgBa2CaCu2O6 have been prepared on SrTiO3 substrates by pulsed-laser deposition of precursor films and subsequent annealing in mercury-vapor atmosphere. The microstructural properties of such films can vary considerably and have been analyzed by x-ray analysis and atomic force microscopy. Whereas the resistivity is significantly enhanced in samples with coarse-grained structure, the Hall effect shows little variation. This disparity is discussed based on models for transport properties in granular materials. We find that, despite of the morphological variation, all samples have similar superconducting properties. The critical temperatures T c ∼ 121.2 K …122.0 K, resistivity, and Hall data indicate that the samples are optimally doped. The analyses of superconducting order parameter fluctuations in zero and finite magnetic fields yield the in-plane ξ ab (0) ∼ 2.3 nm …2.8 nm and out-of-plane ξ c (0) ∼ 0.17 nm …0.24 nm Ginzburg–Landau coherence lengths at zero temperature. Hall measurements provide estimates of carrier scattering defects in the normal state and vortex pinning properties in the superconducting state inside the grains.

Size effects in electrical and magnetic properties of quasi-one-dimensional tin wires in asbestos

Bulk composites have been prepared based on one-dimensional fibers of natural chrisothil-asbestos with various internal diameters (d = 6–2.5 nm) filled with tin. The electrical and magnetic properties of quasi-one-dimensional Sn wires have been studied at low temperatures. The electrical properties have been measured at T = 300 K at a pressure P = 10 kbar. It has been found that the superconducting (SC) characteristics of the nanocomposites (critical temperature T c and critical magnetic field H c) increase as the Sn filament diameter decreases. The temperature spreading of the resistive SC transition also increases as the Sn filament diameter decreases, which is explained by the SC order parameter fluctuations. The size effects (the increase in critical temperature T c and transition width ΔT c) in Sn nanofilaments are well described by the independent Aslamazov–Larkin and Langer–Ambegaokara fluctuation theories, which makes it possible to find the dependence of T c of the diffuse SC transition on the nanowire diameter. Using the temperature and magnetic-field dependences of the magnetic moment M(T, H), it has been found that the superconductor–normal metal phase diagram of the Sn–asbestos nanocomposite has a wider region of the SC state in T and H as compared to the data for bulk Sn. The magnetic properties of chrisotil-asbestos fibers unfilled with Sn have been studied. It has been found that the Curie law is fulfilled and that the superparamagnetism is absent in such samples. The obtained results indicate the absence of magnetically ordered impurities (magnetite) in the chrisotil-asbestos matrix, which allowed one to not consider the problem of the interaction of the magnetic subsystem of the asbestos matrix and the superconducting subsystem of Sn nanowires.

Doping dependence of the critical fluctuation regime in the Fe-based superconductorBa1−xKxFe2As2

We investigate the importance of superconducting order parameter fluctuations in the 122 family of Fe-based superconductors, using high-resolution specific heat and thermal expansion data of various Ba$_{1-x}$K$_x$Fe$_2$As$_2$ single crystals covering a large range of the phase diagram from the strongly underdoped to the overdoped regime. By applying scaling relations of the 3d-XY and the 3d-Lowest-Landau-Level (3d-LLL) fluctuation models to data measured in different magnetic fields, we demonstrate that a strong increase of the critical fluctuation regime is responsible for the transition broadening in magnetic fields, which is a direct consequence of a magnetic-field-induced finite size effect due to a reduction of the effective dimensionality by a decreasing magnetic length scale related to the mean vortex separation and the confinement of quasiparticles in low Landau levels. The fluctuations are stronger in the underdoped and overdoped regimes and appear to be weakest at optimal doping.

Quantum fluctuations in Larkin-Ovchinnikov-Fulde-Ferrell superconductors

We study the superconducting order parameter fluctuations near the phase transition into the Larkin-Ovchinnikov-Fulde-Ferrell state in the clean limit at zero temperature. In contrast to the usual normal metal-to-uniform superconductor phase transition, the fluctuation corrections are dominated by the modes with the wave vectors away from the origin. We find that the superconducting fluctuations lead to a divergent spin susceptibility and a breakdown of the Fermi-liquid behavior at the quantum critical point.

High electric field study of the superconducting transition ofY Ba2Cu3O7−x

The superconducting transitions ofY Ba2Cu3O7−x (YBCO) films of different thicknesses in high current densities and high electric fields areinvestigated. A pulsed-current technique allows for true four-probe measurements with a pulseduration as short as 50 ns, resulting in a significant reduction of self-heating. Electric fields up to900 V cm−1 can be applied without sample damage. The data are compared to a theoreticalmodel that describes the superconducting transition by the Hartree approximationof the Lawrence–Doniach model for a layered superconductor, together with atemperature correction in order to account for the self-heating. Although this modelfits the electric-field dependence of the resistivity of a 330 nm thick YBCO filmwell, it fails for a 36 nm ultra-thin film, where self-heating is much smaller. Thisobservation is a direct indication of intrinsic non-ohmic behaviour, resulting fromthe suppression of superconducting order parameter fluctuations in high electricfields.

Positive in-plane and negative out-of-plane magnetoresistance in the overdoped high-temperature superconductorBi2Sr2CaCu2O8+x

The in-plane and out-of-plane magnetoresistances of moderately overdoped ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+x}$ single crystals have been studied from ${T}_{c}+5 \mathrm{K}$ to ${T}_{c}+100 \mathrm{K}$ with the magnetic field oriented perpendicular and parallel to the current, respectively. The in-plane magnetoresistance is positive, the out-of-plane magnetoresistance negative, and their temperature variations depend on the orientation of the magnetic field. A recent theory for superconducting order parameter fluctuations is extended for the interaction of carrier spins with the magnetic field. The anomalous, anisotropic magnetoresistance can be described well by a unique set of physical parameters, which were found to correspond to estimates from the normal-state transport properties.

Sign Change of the Out-of-plane Magnetoresistance in Oxygen-deficient YBCO

The out-of-plane magnetoresistance in optimally doped (Tc ≈ 90 K) and oxygen-deficient (Tc ≈ 58 K) thin films of YBa2Cu3O7−δgrown on vicinal SrTiO3substrates was investigated. The magnetoresistance of both compounds changes sign above the critical temperature. The results are compared to theoretical predictions for the magnetic-field suppression of superconducting order parameter fluctuations in anisotropic materials. It is suggested that the competition of Aslamazov-Larkin and density-of-states fluctuations, which contribute with different sign to the magnetoresistance, are responsible for the observed sign change.

Anisotropic magnetoresistance in the normal state of oxygen-deficient YBa2Cu3O7?? thin films induced by superconducting fluctuations

We have investigated both the transverse and the longitudinal magnetoresistance of oxygen-deficient YBa2Cu3O7−δ thin films above their critical temperatureT c =55 K. The magnetoresistance is solely caused by the magnetic-field suppression of superconducting orderparameter fluctuations, existing in the films up to 143 K, i.e. 2.6T c . The fluctuation effect provides a reliable determination of the Ginzburg-Landau coherence lengths, ξ ab = 2.5 nm and ξ c = 0.09 nm, with the anisotropy enhanced by oxygen depletion. No signature of the Maki-Thompson fluctuation process or a magnetoresistance resulting from the cyclotron motion of the normalstate quasiparticles was found.

Effect of superconducting fluctuations on the nmr relaxation rate of high-T c superconductors

The effect of superconducting order parameter fluctuations on the nuclear-spin relaxation rate, 1/T 1, is studied for clean two-dimensional systems by calculating the three Maki-Thomson-type diagrams which represent the lowest-order fluctuation contributions to the transverse susceptibility. For Gaussian fluctuations and for temperatures near the mean field transition temperature,T c0, we employ a weak-coupling theory in which the pair-fluctuation propagator can also include pair-breaking effects. We also go beyond the Gaussian theory and take into account the interactions between Cooper-pair fluctuations corresponding to the fourth-order Ginzburg-Landau fluctuation terms. We compare our results with previous results in the dirty limit and in 3D. We obtain a pronounced peak in 1/T1 atT c and briefly discuss possible reasons why this peak is not observed.

Critical-temperature inhomogeneities and resistivity rounding in copper oxide superconductors.

By using effective-medium approaches, we obtain the onset of the electrical-resistivity rounding, above the normal-superconducting transition, associated with inhomogeneities of the mean-field critical temperature {ital T}{sub {ital c}0} at scales larger than the superconducting correlation length. These results are compared with available data in single-crystal and single-phase (to within 4%) polycrystalline YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} samples. This comparison shows that the measured resistivity rounding cannot be explained by these types of local {ital T}{sub {ital c}0} inhomogeneities. Complementarily, our calculations allow us to check some proposals on {ital T}{sub {ital c}0} inhomogeneities associated with local sample strains or oxygen-content variations. The interplay between {ital T}{sub {ital c}0} inhomogeneities and superconducting order-parameter fluctuations (SCOPF) leads to the conclusion that in the mean-field-like region (MFR) above the superconducting transition, the {ital T}{sub {ital c}0} inhomogeneity contribution to the measured resistivity rounding in high-quality (single-phase) cuprate oxide superconductors is negligible. In contrast, our analysis confirms that in the MFR these effects may be explained quantitatively on the grounds of the Lawrence-Doniach theory for SCOPF.

Characterization of the superconducting Bi 2Ca 3 − sSr sCu 2O 8 + x ( s = 1, 1.5, 2, 2.5) oxides

Four monophasic Bi 2Ca 3 − s Sr s Cu 2O 8 + x oxides have been prepared from mixtures of analytical grade Bi 2O 3, CaCO 3, SrCO 3 and CuO, which were ground and heated in air to 1133 K ( s = 1), 1153 K ( s = 1.5 and 2) and 1173 K ( s = 2.5). X-ray powder diffraction showed that the four oxides are isomorphous and orthorhombic, and gave unit cell parameters ( a, b, c and V) which vary linearly with the strontium content. Resistivity measurements show that the sample for which s = 1 is superconducting, with superconducting order parameter fluctuations in two dimensions.

Rounding of the electrical resistivity near Tc of various high-temperature superconductors

Measurements of the rounding of the resistivity above T c in ceramic samples of three of the main CuO-based high-temperature superconductor systems are presented and analyzed in terms of superconducting order parameter fluctuations (SCOPF). Our results for the critical exponent of the excess conductivity in the mean-field-like regions are compatible with an order parameter of two components fluctuating in three dimensions (for the ABaCu0 system, with A ≡ Y or Re, and for the LaMCuO system, with M ≡ Sr or Ba) or two dimensions (for the BiSrCa CuO system). The results for the excess conductivity amplitude, which is found to be proportional to the measured conductivity near T c , seem to be associated with strong inhomogeneities of the local current density in ceramic samples.

Superconducting properties of TaN and VN films

Thin superconducting TaN and VN films produced by reactive cathode sputtering have been investigated. Superconducting-phase-transition smearing in applied field seems to be due to spatial fluctuations of the electron diffusion coefficient as well as dimensional crossover of dynamical superconducting order parameter fluctuations. Flux pinning occurs at grain boundaries by electron scattering mechanism. Transition to a dissipative state is induced by flux line separation from the pins but not by flux line lattice plastic shear.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Paraconductivity of ceramic YBa 2Cu 3O 7−δ in the mean-field-like region

A simple and unified phenomenological picture of the rounding of the electrical resistivity, ρ( T ), of ceramic HTSC above the critical temperature of individual grains is proposed. This approach is centered here on the mean-field-like region of Y 1 Ba 2 Cu 3 O 7−δ samples in zero-applied magnetic field. The approach combines the current phenomenological ideas on the electrical conductivity in granular superconductors, together with the empirical features of the Anderson-Zou proposal for the background conductivity, and with the Lawrence-Doniach treatment of the superconducting order parameter fluctuations in layered superconductors. To check this approach we present new experimental data on ρ( T ) in zero-applied magnetic field in different YBCO ceramic compounds, all with the same nominal composition but with important differences in their granular structure. These new experimental data strongly confirm the adequacy of that approach. In particular, the amplitude of the excess conductivity in the mean-field-like region (MFR) is explained quantitatively, to our knowledge for the first time, in ceramic superconductors. Our results also confirm that the paraconductivity in ceramic samples is essentially associated with the in-plane conductivity of the individual grains. Also, our results suggest that in the MFR the critical exponents of the excess conductivity are almost independent of the granular nature of the samples, provided that the correlation length amplitude is smaller than the characteristic dimensions of the grains. In the case of YBCO samples, our results confirm the 3D nature of SCOPF in most of the MFR in zero-applied magnetic field, and for the average amplitude of the Ginzburg-Landau correlation length in the c direction we find, ξ 1 (0) = 2.2 A.