Fluctuation Magnetoconductivity Research Articles

Fluctuation magnetoconductivity in superconducting Ca10(Pt4As8)(Fe2As2)5 single crystal

Fluctuation-induced conductivity up to μ0H = 14 T for superconducting Ca10(Pt4As8)(Fe2As2)5 single crystal was evaluated from the measured in-plane electrical resistivity, and it was analyzed by Aslamazov-Larkin (AL) theory for zero magnetic field and Ullah and Dorsey (UD) scaling method for magnetic field. A dimensional crossover of superconducting fluctuations from low-temperature three-dimension to high-temperature two-dimension was observed. When the dimensional crossover occurs, a temperature region in which two-dimensional and three-dimensional fluctuations occur simultaneously was found in the intermediate temperature region.

Quasi-two-dimensional behavior of 112-type iron-based superconductors

Fluctuation magnetoconductivity and magnetization above the superconducting transition temperature (Tc) are measured on the recently discovered 112 family of iron-based superconductors (IBS), Ca$_{1-x}$La$_x$Fe$_{1-y}$Ni$_y$As$_2$, which presents an extra As-As chain spacer-layer. The analysis in terms of a generalization of the Lawrence-Doniach (LD) approach to finite applied magnetic fields indicates that these compounds are among the most anisotropic IBS ($\gamma$ up to ~30), and provides a compelling evidence of a quasi-two-dimensional behavior for doping levels near the optimal one.

Fluctuation induced magneto-conductivity of Y3Ba5Cu8O18±x and YBa2Cu3O7−d

A comparative study of the fluctuation magneto-conductivity in [Formula: see text] (noted [Formula: see text]) and [Formula: see text] (noted [Formula: see text]) polycrystalline samples was carried out. Samples were synthesized in oxygen atmosphere using a standard solid state reaction technique. Phases and microstructure have been systematically investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The magneto-conductivity was measured in a magnetic field ranging from 0 T to 7 T applied perpendicular to the current path direction. The magneto-conductivity data were analyzed in terms of the temperature derivative of the resistivity and the logarithmic temperature derivative of the conductivity [Formula: see text], where [Formula: see text] is the fluctuation conductivity. Analyses of the magneto-conductivity data reveal that [Formula: see text] possesses a better quality of intrinsic Josephson junction compared to [Formula: see text] one while in the former one the depression in superconducting temperature is more pronounced. The applied magnetic field narrowed substantially the three dimensions (3D) Gaussian regime and for [Formula: see text] sample this regime is vanished for high magnetic field ([Formula: see text][Formula: see text]1 T). Results are discussed in relation with the difference in the structure and microstructure [Formula: see text] and [Formula: see text]. The upper critical and the irreversibility magnetic fields were estimated and were found to be higher in the [Formula: see text] sample.

High-field fluctuation magnetoconductivity and Hall reversal response in the Hg(Re)Ba2Ca2Cu3O8+δ superconductor

Systematic experiments of high-field (up to 50 kOe) fluctuation magnetoconductivity and Hall magnetoresistivity in Hg1-x Rex Ba2 CaCu3O8+d (x=0.18) polycrystalline samples growth by means the quartz tube technique are reported. The analysis of the experimental data was performed by using the recognized Kouvel-Fisher method, which is frequently applied to study of critical phenomena. Very close to the critical temperature TC, a genuinely critical regime of fluctuations characterized by the critical exponent λc =0.32±0.01 was identified in absence of magnetic fields. This result is consistent with the full dynamic 3D-XY universality class predicted by the model E of Hohenberg-Halperin with a dynamic critical exponent z = 3/2. The genuine critical regime become be unstable on the application of external magnetic fields H≈0.1 kOe. Near above the critical temperature TC, the determined exponent λG3=0.52±0.02 was interpreted as corresponding to homogeneous fluctuations, which develop in a space with three-dimensional geometry. This region is destroyed upon the application of magnetic fields above 0.5 kOe. Increasing the temperature, evidences of a homogeneous two-dimensional behavior are observed by means the identification of a λG2=1.02±0.04. Applied fields H>20 kOe destroy this fluctuation regime. Far above TC , effects of disorder and planar anisotropy produce a fluctuation spectrum characterized by a fractal topology with a critical exponent λG2-G1=1.32±0.04. At last, very far TC, a temperature region with λG1=1.52±0.04 was experimentally identified. This corresponds to the confinement of the quasi-particles into the Lowest-Landau-Level, due to the quantization of the electronic states around the axe of application of the external field. Measurements of Hall were performed. In the normal phase, the Hall resistivity is hole-like and inversely proportional to the temperature. In the mixed phase and when the applied field is below μ0H = 2 T, the Hall resistivity shows a double sign reversal. For fields above 2 T, the Hall resistivity remains positive, although qualitatively showing the trends observed at low fields. We attribute this behavior to two independent contributions with opposite sign. A negative term due to thermal fluctuations is relevant near TC, whereas a positive contribution related to vortex motion dominates at lower temperatures. Near the zero resistance state, the Hall resistivity varies as a power law of the longitudinal resistivity, with a field independent exponent β=1.41. PACS: 74.40.+k; 74.25.Bt; 74.60.Ec; 74.72.Bk. © 2014. Acad. Colomb. Cienc. Ex. Fis. Nat. All rights reserved.

High-field studies of superconducting fluctuations in high-Tccuprates: Evidence for a small gap distinct from the large pseudogap

We have used large pulsed magnetic fields up to 60 T to suppress the contribution of superconducting fluctuations (SCFs) to the $ab$-plane conductivity above ${T}_{c}$ in a series of YBa${}_{2}$Cu${}_{3}$O${}_{6+x}$ from the deep pseudogapped state to slight overdoping. Accurate determinations of the SCF contribution to the conductivity versus temperature and magnetic field have been achieved. Their joint quantitative analyses with respect to Nernst data allow us to establish that thermal fluctuations following the Ginzburg-Landau scheme are dominant for nearly optimally doped samples. The deduced coherence length $\ensuremath{\xi}(T)$ is in perfect agreement with a Gaussian (Aslamazov-Larkin) contribution for $1.01{T}_{c}\ensuremath{\lesssim}T\ensuremath{\lesssim}1.2{T}_{c}$. A phase-fluctuation contribution might be invoked for the most underdoped samples in a $T$ range which increases when controlled disorder is introduced by electron irradiation. For all dopings we evidence that the fluctuations are highly damped when increasing $T$ or $H$. This behavior does not follow the Ginzburg-Landau approach, which should be independent of the microscopic specificities of the superconducting state. The data permits us to define a field ${H}_{c}^{\ensuremath{'}}(T)$ and a temperature ${T}_{c}^{\ensuremath{'}}$ above which the SCFs are fully suppressed. The analysis of the fluctuation magnetoconductance in the Ginzburg-Landau approach allows us to determine the critical field ${H}_{c2}(0)$. The actual values of ${H}_{c}^{\ensuremath{'}}(0)$ and ${H}_{c2}(0)$ are found to be quite similar and both increase with hole doping. These depairing fields, which are directly connected to the magnitude of the superconducting gap, do therefore follow the ${T}_{c}$ variation which is at odds with the sharp decrease of the pseudogap ${T}^{*}$ with increasing hole doping. This is on line with our previous evidence that ${T}^{*}$ is not the onset of pairing. So the large gap seen by spectroscopic experiments in the underdoped regime has to be associated with the pseudogap. We finally propose here a three-dimensional phase diagram including a disorder axis, which makes it possible to explain most peculiar observations done so far on the diverse cuprate families.

The physical properties of borocarbide superconductor Y1−xSmxNi2B2C (x≦0.25) and Y1−xSmxPd5B3C0.4 (x≦0.4)

We have investigated the magnetic and transport properties of borocarbide superconductors YNi2B2C and YPd5B3C0.4 with Yttrium partially substituted by Samarium. The upper critical fields HC2 are determined by the scaling analysis of the thermal fluctuation magnetoconductivity. Around the transition region, the thermal fluctuation magnetoconductivity can be scaled by a universal function for all applied magnetic fields. The formula HC2(T)=HC2(0)[1−(T/TC)3/2]3/2 of a narrow-band pairing mechanism gives an excellent fit to the value of upper critical field HC2(0)=7.6T in the Y0.8Sm0.2Pd5B3C0.4 compound. The superconducting coherence length ξ is determined to be 6.58nm, the Ginzburg–Landau parameter κ is 29 and the penetration depth λ is 191nm.

Coherence transition in polycrystalline Y 0.95Tb 0.05Ba 2Cu 3O 7−δ superconductors

We report on fluctuation magnetoconductivity measurements in polycrystalline samples of Y 0.95Tb 0.05Ba 2Cu 3O 7− δ . The measurements were performed using low magnetic fields up to 50 Oe, applied parallel to the current. The results show that the resistive transition proceeds in two stages as seen by the temperature derivative of the resistivity near T C. The pairing transition occurs at a higher temperature, where superconductivity is stabilized within small and homogeneous regions of the sample, generically called grains. In the regime of near zero resistance state, the occurrence of a coherence transition was evidenced which was related to the connective nature of the granular samples. Our measurements show that Tb-doping causes an increase in the granularity on YBa 2Cu 3O 7− δ systems and a shift of the whole grain coupling phenomenology to lower temperatures.

Fluctuation Magneto-Conductivity in La–Ba–Ca–Cu–O Superconductors

Temperature variation of magneto-resistivity measurements have been carried out at 1 T magnetic field on (La1.5Gd0.5)Ba2CaCu5Oz and, on pure and 5 wt.% Ag doped (La1.6Y0.4)Ba2Ca0.8Cu4.8Oz superconductors. These data were analyzed in terms of fluctuation magneto-conductivity by using Aslamazov—Larkin and Maki—Thompson models for layered superconductors. These materials exhibit strong pair breaking effect and the typical phase breaking time at 100 K is found to be in the order of 10−16 s. The typical values of coherence lengths along ab plane and along c-direction at 0 K are found to be 16.6 and 0.5 A.

Excess conductivity and magneto-conductivity studies in pure and Ag-doped(La1−xYx)2Ba2CaCu5Oz superconductors

The temperature variations of electrical resistivity have been measured on pure and 5 wt% Ag-doped(La1−xYx)2Ba2CaCu5Oz superconductors in the absence and presence of 1 T magnetic field. The electrical resistivitydata in the absence of magnetic field were analysed in terms of fluctuation-inducedexcess conductivity in the mean-field regime by using the Aslamazov–Larkin (AL),Lawrence–Doniach (LD) and Maki–Thompson (MT) models. The fluctuations in theamplitude of order parameter are found to be two dimensional in nature. Themagneto-resistivity data were analysed in terms of fluctuation magneto-conductivity byusing the AL and MT models for layered superconductors and by taking into accountboth orbital and Zeeman terms. These materials exhibit a strong pair breakingeffect and the typical phase breaking time at 100 K is found to be of the order of10−16 s. The typical values of coherence lengths along theab-plane andalong the c-direction at 0 K are found to be 28.5 and 0.4 Å.

Magnetotransport properties of untwinned YBa 2Cu 3O y single crystals: novel 60-K-phase anomalies in the charge transport

We present the result of our accurate measurements of the a- and b-axis resistivity ( ρ a and ρ b ), magnetoconductivity Δ σ/ σ, Hall coefficient R H, and the a-axis thermopower S a in untwinned YBa 2Cu 3O y single crystals in a wide range of doping (6.45≤ y≤7.0). The systematics of our data reveals a number of novel 60-K-phase anomalies in the charge transport: (i) temperature dependences of ρ a show anomalous overlap below ∼130 K for 6.65≤ y≤6.80, (ii) Hall mobility μ H shows an enhancement near y≃6.65, which is reflected in an anomalous y dependence of σ xy , (iii) with decreasing temperature R H shows a marked drop upon approaching T c only in samples with 6.70≤ y≤6.85, (iv) superconducting fluctuation magnetoconductivity is anomalously enhanced near y≃6.7, and (v) H c2 is anomalously reduced near y≃6.70. We discuss that the fluctuating charge stripes might be responsible for these anomalies in the charge transport.

Magnetoresistance of untwinned YBa(2)Cu(3)O(y) single crystals in a wide range of doping: anomalous hole-doping dependence of the coherence length.

Magnetoresistance (MR) in the a-axis resistivity of untwinned YBa(2)Cu(3)O(y) single crystals is measured for a wide range of doping ( y = 6.45-7.0). The y dependence of the in-plane coherence length xi(ab) estimated from the fluctuation magnetoconductance indicates that the superconductivity is anomalously weakened in the 60-K phase; this observation, together with the Hall coefficient and the a-axis thermopower data which suggest the hole doping to be 12% for y approximately equal to 6.65, gives evidence that the origin of the 60-K plateau is the 1/8 anomaly. At high temperatures, the normal-state MR data show signatures of the Zeeman effect on the pseudogap in underdoped samples.

Thermal fluctuation and upper critical field of MgB 2 superconductor

We have investigated the magnetic and transport properties of MgB 2 superconductor over wide temperature and magnetic ranges. The upper critical fields H c2 are determined by the scaling analysis of the thermal fluctuation magnetoconductivity. Around the transition region, the thermal fluctuation magnetoconductivity can be scaled by a universal function for all applied magnetic fields. The formula H c2( T)= H c2(0)[1−( T/ T c) 3/2] 3/2 of a narrow-band pairing mechanism gives an excellent fit to the data. Based on the value of upper critical field H c2 (0)=155 kOe, the superconducting coherence length is determined to be 4.6 nm. The Ginzburg–Landau parameter κ is 50 in the MgB 2 superconducting system.

Fluctuation magnetoconductivity inYBa2Cu3O7:Gaussian, three-dimensionalXY,beyond three-dimensionalXY,and lowest-Landau-level scaling

Systematic measurements of the in-plane fluctuation magnetoconductivity in a ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ single crystal are presented. Fields between 0 and 14 T were applied either parallel or perpendicular to the Cu-O atomic planes. The data reveal the occurrence of a large Gaussian regime in the normal phase. Far above ${T}_{c},$ the mean-field fluctuation spectrum is effectively two dimensional. Decreasing the temperature towards ${T}_{c},$ a crossover to a three-dimensional (3D) Gaussian regime is seen at low applied fields. The analysis of these results allows the estimation of the coherence length perpendicular and parallel to the Cu-O planes, and reveals that superconductivity in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ is characterized by a double planar periodicity. The c lattice parameter is the relevant periodicity length of the 2D behavior, whereas the smallest distance between the double Cu-O layers plays an important role in the 3D fluctuation spectrum. In fields above 5 T applied parallel to the c axis, the fluctuation magnetoconductivity scales as predicted by the 3D lowest-Landau-level approximation of the Ginzburg-Landau theory. Very close to ${T}_{c},$ and for quite low values of the applied field, the results clearly show the occurrence of a genuine critical regime, where the exponent is consistent with the predictions of the full dynamic 3D $\mathrm{XY}$ universality class. Still closer to ${T}_{c},$ evidence is found for a fluctuation regime beyond 3D $\mathrm{XY}$ scaling. This new scaling raises the interesting possibility for the ultimate weakly first-order character of the superconducting transition in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}.$ The whole set of data is condensed on $H\ensuremath{-}T$ diagrams, which display the regions of stability for the observed fluctuation regimes, in fields oriented parallel or perpendicular to the Cu-O layers.

Fluctuation magnetoconductivity of BSCCO-2212 films in parallel magnetic field

The in-plane (δ σ ab ( T, H)) and out-of-plane (δ σ c ( T, H)) components of the magnetoconductivity tensor of Bi 2Sr 2CaCu 2O 8+ x films have been simultaneously measured in magnetic fields parallel to the film conducting planes (B⩽1 T) . A negative c-axis magnetoresistance is measured for the first time in this geometry. Both δ σ ab and δ σ c are well described by the fluctuation theory in the temperature range T c< T< T c+20 K, using a single set of microscopical parameters for both directions. The negative c-axis magnetoresistance in the field parallel to conducting layers is shown to be induced by the density of states Zeeman fluctuation contribution.

The effects of Sr and Ca on the magnetic irreversibility and fluctuation conductivity of YBCO-123

We have measured the magnetic irreversibility of the YBa 2−x D x Cu 3 O 7−δ ( D= Sr or Ca and x=0.5 or 1.0) polycrystalline superconductors in fields up to 7 kOe and of an oxygen depleted YBa 1.75Sr 0.25Cu 3O 6.85 polycrystalline sample in fields up to 40 kOe as well as the fluctuation magnetoconductivity of YBa 1.5 D 0.5 Cu 3 O 7−δ ( D= Sr or Ca ) up to 300 Oe. These partial substitutions of Ba as well as oxygen depletion cause reduction of T c proportional to the impurity concentration and an increase of the granular superconducting character, maintaining almost unchanged the overall aspect of the magnetic irreversibility and fluctuation conductivity. Our results show that in a low field region the flux dynamics is dominated by the intergranular Josephson flux, that is strongly marked by the conflicting and frustrated grain coupling physics, the magnetic irreversibility totally resembling that of the spin glasses. But in fields above 10 kOe the flux dynamics is more conventional.

Pairing and coherence transitions in granular GdBa 2Cu 3O 7− δ

We report on fluctuation magnetoconductivity measurements in a granular sample of GdBa 2Cu 3O 7− δ . The experiments show that the resistive transition is a two-stage process in this sample. The pairing transition occurs at a higher temperature, where superconductivity is established in the grains. The critical thermodynamics for this transition is that of the 3D–XY universality class, with dynamic exponent z=1.5. At a lower temperature occurs the coherence transition, where the fluctuating phases of individual grains become long-range ordered and a zero resistance state sets in.

Evidence of critical fluctuations from the magnetoconductivity data in Bi2Sr2Ca2Cu3O10+x phase

The fluctuation-induced magnetoconductivity of the Bi2Sr2Ca2Cu3O10+x phase is studied above zero-field critical temperature Tc(0) and for moderate magnetic fields. It is found that the Gaussian approximation for superconducting fluctuations underestimates the negative fluctuation magnetoconductance drastically in the Tc(0) < T < Tc(0) + 20 K temperature range. Taking into account the critical fluctuation contribution on the base of self-consistent Hartree approximation makes it possible to explain the data quantitatively in terms of the only Aslamazov-Larkin contribution for different magnetic fields and temperatures, consistently with the zero field data.

Magnetoconductivity measurements in Bi(2223) tape: evidence of critical fluctuations

Fluctuation-induced magnetoconductivity is studied on Bi(2223) phase. The Gaussian approximation drastically underestimates the negative fluctuation magnetoconductance in the temperature range of 20 K above the critical temperature Taking into account the critical fluctuation contribution on the base self-consistent Hartree approximation it is possible to explain quantitatively the data in terms of Aslamazov-Larkin contribution only.

Gaussian and Three-Dimensional-XY Scalings and Effects of Disorder in the Fluctuation Magnetoconductivity of Polycrystalline DyBa2Cu3O7-δ

Systematic measurements of the fluctuation magnetoconductivity in granular DyBa2Cu3O7–δ in magnetic fields up to 50 kOe are reported. Gaussian and critical scalings are observed in the normal state. In the Gaussian regimes, large intervals corresponding to low-dimensional fluctuations are evidenced. Below Tc, in the regime describing the approach to the zero-resistivity state, the fluctuation conductivity diverges as expected in the coherence transition of mesoscopic granular superconductors in low magnetic fields.

Gaussian and Three-Dimensional-XY Scalings and Effects of Disorder in the Fluctuation Magnetoconductivity of Polycrystalline DyBa2Cu3O7

Gaussian and Three-Dimensional-XY Scalings and Effects of Disorder in the Fluctuation Magnetoconductivity of Polycrystalline DyBa2Cu3O7