# Excess conductivity studies in the paracoherence region in high-temperature superconductors

TL;DR: In this article, resistivity measurements on CaREBaCu 3 O 7− y (RE=La, Sm), Ag coated YBa 2 Cu 3 O7− y, Tl 2 CaBa 2 Ca 2 O 8 and YBa2 Cu 3− x Zn x Cu 3 o 7 − y ( x = 0.05, 0.075 and 0.15) in the paracoherence region.

Abstract: Fluctuations in phase across the weak links in a granular superconductor contribute to the excess conductivity below T m , the temperature at which the plot of temperature derivative of electrical resistivity against temperature shows a maximum. In a narrow range of temperature above T ∞ (the temperature at which the resistivity at which the resistivity becomes zero), the excess conductivity, Δσ = [ σ ( T ) − σ ( T m )], is expected to vary as ( T − T ∞ ) − γ . In this work, resistivity measurements on CaREBaCu 3 O 7− y (RE=La, Sm), Ag coated YBa 2 Cu 3 O 7− y , Tl 2 CaBa 2 Cu 2 O 8 and YBa 2 Cu 3− x Zn x Cu 3 O 7− y ( x =0.05, 0.075, 0.1 and 0.15) in the paracoherence region are reported. Analysis of the data indicates the excess conductivity in this region is better described by the critical exponent y = 1.33. From an analogy between the tunneling Hamiltonian in the granular material and the Hamiltonian for an XY ferromagnet one expects the critical exponent, γ, to be the same as the critical exponent for susceptibility in the ferromangetic case.

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TL;DR: In this article, the effect of disorder on the electrical resistance near the superconducting transition temperature in the paracoherence region of high temperature YBa 2 Cu 3 O 7− δ (YBCO) thin film superconductor is reported.

Abstract: Effect of disorder on the electrical resistance near the superconducting transition temperature in the paracoherence region of high temperature YBa 2 Cu 3 O 7− δ (YBCO) thin film superconductor is reported. For this, c -axis oriented YBa 2 Cu 3 O 7− δ thin films having superconducting transition width varying between 0.27 K and 6 K were deposited using laser ablation and high pressure oxygen sputtering techniques. Disorder in these films was further created by using 100 MeV oxygen and 200 MeV silver ions with varying fluences. It is observed that the critical exponent in the paracoherence region for films with high transition temperature and small transition width is in agreement with the theoretically predicted value ( γ = 1.33) and is not affected by disorder, while for films with lower transition temperature and larger transition width the value of exponent is much larger as compared to that theoretically predicted and it varies from sample to sample and usually changes with disorder induced by radiation. This difference in the behaviour of the exponent has been explained on the basis of differences in the strength of weak links and the transition between temperatures T m and T c is interpreted as a percolation like transition with disorder.

28 citations

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TL;DR: In this paper, the temperature variations of electrical resistivity have been measured on pure and 5-wt% Ag doped (La 1−xY x)2Ba2CaCu5Oz superconductors in the absence and presence of 1-T magnetic field.

Abstract: The temperature variations of electrical resistivity have been measured on pure and 5 wt% Ag doped (La1−xY x)2Ba2CaCu5Oz superconductors in the absence and presence of 1 T magnetic field The resistivity data were analyzed in terms of excess conductivity due to phase fluctuations of the order parameter in the paracoherence regime In zero field case, we observed ordered phase fluctuations with the critical exponent mostly comparable to that of 3D XY ferromagnet and a crossover to disordered phase fluctuations with critical exponent comparable to that of diluted Heisenberg ferromagnet at higher temperatures In the presence of a magnetic field, only ordered phase fluctuations have been observed

13 citations

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TL;DR: In this article, the critical exponent of the electrical conductivity in the paracoherence region (gamma) of the high temperature superconductor YBa2Cu3O7-x (YBCO) has been estimated for high quality thin film on ZrO2 substrate prepared by high pressure oxygen sputtering.

Abstract: Critical exponent of the electrical conductivity in the paracoherence region (gamma) of the high temperature superconductor YBa2Cu3O7-x (YBCO) has been estimated for high quality thin film on ZrO2 substrate prepared by high pressure oxygen sputtering. High energy ion irradiation was carried out using 100 MeV O-16(7+) ions at liquid nitrogen to see the effects of disorder on the value of the exponent. The critical exponent from a value of about 2 to 1.62 upon irradiation. Studies were also carried film to see the effect of ageing and annealing.

6 citations

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TL;DR: In this paper, the amplitude of order parameter was found to be two dimensional in nature in the mean field region and the average phase breaking time τϕ (100 K) was estimated to be 3.9×10-16s for pure and 5 wt% Ag doped (La1.6Y0.4)Ba2Ca0.8Cu4.8Oz superconductors.

Abstract: The temperature variations of electrical resistivity have been measured on pure and 5 wt% Ag doped (La1.6Y0.4)Ba2Ca0.8Cu4.8Oz superconductors. These data were analyzed in terms of fluctuation-induced excess conductivity in the mean field regime by using the Aslamazov–Larkin (AL), Lawrence–Doniach (LD) and Maki–Thompson (MT) models. The fluctuations in the amplitude of order parameter are found to be two dimensional in nature in the mean field region. The estimated values of the average phase breaking time τϕ (100 K) are found to be 3.9×10-16s and 4.6×10-16s for pure and Ag doped samples respectively. The resistivity data were also analyzed in terms of excess conductivity due to phase fluctuations of the order parameter in the paracoherence region. The critical exponent is found to be mostly comparable to that of the 3D XY ferromagnet in the vicinity of zero resistivity temperature and the diluted Heisenberg model at a higher temperature.

2 citations

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TL;DR: In this article, the role of the Harris' criteria is discussed and the normalized Josephson coupling energy is calculated and its variation temperature is plotted both for the unirradiated and irradiated Bi-2212 samples.

Abstract: The relation between the excess conductivity and the critical exponent of conductivity is discussed in the paracoherence region. The critical exponents vary with α-particle irradiation of Bi2Sr2CaCu2O8+x (Bi-2212) superconductors. The role of the Harris' criteria is discussed. The normalized Josephson coupling energy is calculated and its variation temperature is plotted both for the unirradiated and irradiated Bi-2212 samples. Effects of the variation of normalized Josephson coupling energy are discussed in the light of variations of critical temperature and grain volumes.

1 citations

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TL;DR: In this paper, the coefficients of the renormalization functions of the ( ϕ → 2 ) 2 theory are expanded in powers of the coupling constant, and new and more precise values of critical exponents are obtained.

Abstract: We present a new calculation of the critical exponents of the n -vector model through field-theoretical methods. The coefficients of the renormalization functions of the ( ϕ → 2 ) 2 theory are expanded in powers of the coupling constant. Asymptotic estimates of large order of perturbation series are used to transform the divergent perturbation series into a convergent one. As a consequence, new and more precise values of critical exponents are obtained.

879 citations

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TL;DR: In this paper, a rectangular prism with edges in principal crystal directions is prepared with electrodes on the corners of one face, and voltage-current ratios for opposite pairs of electrodes permit calculation of components of the resistivity tensor.

Abstract: A rectangular prism with edges in principal crystal directions is prepared with electrodes on the corners of one face. Voltage‐current ratios for opposite pairs of electrodes permit calculation of components of the resistivity tensor. The method can use small samples, and is best suited to materials describable by two or three tensor components. Examples are given of measurements of V2O3–Cr and oriented amorphous graphite.

574 citations

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TL;DR: In this paper, the critical behavior of spin systems with quenched disorder is studied by renormalization-group methods, and a second-order phase transition with exponents which do not depend continuously on impurity concentration is predicted.

Abstract: The critical behavior of spin systems with quenched disorder is studied by renormalization-group methods. For the randomly dilute $m$-vector model, the $n=0$ limit is used to construct a translationally invariant effective Hamiltonian which describes the original disordered system. This Hamiltonian is analyzed in the $\ensuremath{\epsilon}$ expansion to order ${\ensuremath{\epsilon}}^{2}$. Sharp second-order phase transitions with exponents which do not depend continuously on impurity concentration are predicted. For $mg{m}_{c}\ensuremath{\equiv}4\ensuremath{-}4\ensuremath{\epsilon}+O({\ensuremath{\epsilon}}^{2})$ the isotropic $m$-component fixed point, which characterizes the critical behavior of the pure system, is stable. For $ml{m}_{c}$, a new random fixed point becomes stable. The exponents corresponding to this fixed point are $\ensuremath{\eta}=[\frac{(5{m}^{2}\ensuremath{-}8m)}{256{(m\ensuremath{-}1)}^{2}}]{\ensuremath{\epsilon}}^{2}+O({\ensuremath{\epsilon}}^{3})$, $\ensuremath{
u}=\frac{1}{2}+[\frac{3m}{32(m\ensuremath{-}1)}]\ensuremath{\epsilon}+[\frac{m(127{m}^{2}\ensuremath{-}572m\ensuremath{-}32)}{4096{(m\ensuremath{-}1)}^{3}}]{\ensuremath{\epsilon}}^{2}+O({\ensuremath{\epsilon}}^{3})$ for $m\ensuremath{
e}1$, and $\ensuremath{\eta}=\ensuremath{-}\frac{\ensuremath{\epsilon}}{106}+O({\ensuremath{\epsilon}}^{\frac{3}{2}})$, $\ensuremath{
u}=\frac{1}{2}+\frac{{(\frac{6\ensuremath{\epsilon}}{53})}^{\frac{1}{2}}}{4}+O(\ensuremath{\epsilon})$ for $m=1$. More general random systems are qualitatively discussed from the effective-Hamiltonian viewpoint.

338 citations

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TL;DR: In this paper, the authors consider a system where the local transition temperature varies in space, with a correlation function obeying a power law for large separations, and extend the Harris criterion for this case.

Abstract: As a model for a phase transition in an inhomogeneous system, we consider a system where the local transition temperature varies in space, with a correlation function obeying a power law $\ensuremath{\sim}{x}^{\ensuremath{-}a}$ for large separations $x$. We extend the Harris criterion for this case, finding that for $ald$ (where $d$ is the spatial dimension) the disorder is irrelevant if $a\ensuremath{
u}\ensuremath{-}2g0$, while if $agd$ we recover the usual Harris criterion: The disorder is irrelevant if $d\ensuremath{
u}\ensuremath{-}2=\ensuremath{-}\ensuremath{\alpha}g0$. An $m$-vector system of this type is studied with the use of a renormalization-group expansion in $\ensuremath{\epsilon}=4\ensuremath{-}d$ and $\ensuremath{\delta}=4\ensuremath{-}a$. We find a new long-range-disorder fixed point in addition to the short-range-disorder and pure fixed points found previously. The crossover between fixed points is found to follow the extended Harris criterion. The new fixed point has complex eigenvalues, leading to oscillating corrections to scaling, and has a correlation-length exponent $\ensuremath{
u}=\frac{2}{a}$. We argue that this new scaling relation is exact and applies more generally than just to the specific model. We show that the extended Harris criterion also applies to percolation with long-range-correlated site or bond-occupation probabilities, so that the scaling law should be obeyed by such systems. Results for the percolation properties of the triangular Ising model are in agreement with these predictions.

304 citations

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246 citations