Temperature dependent magnetic penetration depth of Co and Zn doped YBa2Cu3O7 obtained from the AC susceptibility of magnetically aligned powders

Porch, Adrian; Cooper, J. R.; Zheng, D. N.; Waldram, J.R.; Campbell, A.M.; Freeman, P. A.

doi:10.1016/0921-4534(93)90837-g

Cited by 78 publications

(46 citation statements)

References 39 publications

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“…where N i is the measured number of grains of radius r i [6] which are assumed to be spherical [10]. λ(T ) of our polycrystalline sample was determined from the mea-sured χ(T ) and grain size distribution by solving Eq.…”

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confidence: 99%

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Exponential Temperature Dependence of the Penetration Depth in Single CrystalMgB2

et al. 2002

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The temperature dependence of the London penetration depth, λ(T ), was measured in both single crystal and polycrystalline MgB2 samples by a high-resolution, radio frequency technique. A clear exponential temperature dependence of λ(T ) was observed at low temperature, indicating s-wave pairing. A BCS fit to the lowest temperature data gives an in-plane energy gap ∆ of 30 ± 2 K (2∆/Tc = 1.5 ± 0.1), which is significantly smaller than the standard BCS weak coupling value of 3.5. We find that the data are best described by a two-gap model.

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confidence: 99%

“…This powder has a relatively wide range of grain sizes which complicates quantitative analysis of the data. For the analysis to give a reliable estimate of the absolute values of λ(T ) grain sizes < ∼ 5λ are required [6]. To obtain such a distribution the powder was ground in an agate mortar and then sedimented in acetone for one hour [7].…”

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Exponential Temperature Dependence of the Penetration Depth in Single CrystalMgB2

et al. 2002

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“…As a next step we studied the pressure effect on the magnetic field penetration depth λ. The temperature dependence of λ was calculated from the measured FC magnetization by using the Shoenberg formula [21], modified for the known grain size distribution N (R) [22]:…”

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Anomalous electron-phonon coupling probed on the surface of superconductorZrB12

et al. 2005

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Magnetization measurements under hydrostatic pressure up to 10.5 kbar in zirconium dodecaboride ZrB12 superconductor (Tc ≃ 6.0 K at p = 0) were carried out. A negative pressure effect on Tc with dTc/dp = −0.0225(3) K/kbar was observed. The electron-phonon coupling constant λ el−ph decreases with increasing pressure with d ln λ el−ph /dp ≃ −0.20%/kbar. The magnetic field penetration depth λ was studied in the Meissner state and, therefore, probes only the surface of the sample. The absolute values of λ and the superconducting energy gap at ambient pressure and zero temperature were found to be λ(0) =140(30) nm and ∆0 =1.251(9) meV, respectively. ∆0 scales linearly with Tc as 2∆0/kBTc = 4.79(1). The studies of the pressure effect on λ reveal that λ −2 increases with pressure with d ln λ −2 (0)/dp = 0.60(23) %/kbar. This effect can not be explained within the framework of conventional adiabatic electron-phonon pairing, suggesting that close to the surface, an unconventional non-adiabatic character of the electron-phonon coupling takes place.PACS numbers: 74.70. Ad, 74.62.Fj, 74.25.Ha, 83.80.Fg The traditional concept of superconductivity is strictly associated with the electron-phonon interaction. The conventional theory is based on the Migdal-Eliashberg adiabatic approximation [1] that, in fact, leads to the prediction of many peculiar features which are a direct evidence of a phonon mediated superconductivity. The adiabatic approximation is valid if the parameter ω 0 /E f is small (ω 0 is the relevant phonon frequency and E f is the Fermi energy). Usually this parameter is regarded as a measure of nonadibaticity. However, crossover from a conventional adiabatic to an unconventional nonadiabatic regime does not depend only on the value of the ω 0 /E f ratio. Paci et al. [8] show that even in a case of small "adiabatic" ratio one would expect the nonadiabatic coupling in superconductors having high value of the electron-phonon coupling constant λ el−ph . Among BCS superconductors the zirconium dodecaboride (ZrB 12 ) is probably a candidate for the observation of such type of anomalous coupling. It stems from the rather small value of the Fermi energy ∼1 eV [3] that, together with the Debye temperature ∼20 meV [4], leads to a ratio ω 0 /E f ∼0.02. A strong coupling ratio 2∆/k B T c ≃ 4.8 was observed by surface sensitive techniques [3,6]. This suggests that the electron-phonon coupling constant, which has a bulk value λ el−ph ≃ 0.67 [3], increases at the surface. From the comparison with strong coupled metallic superconductors [7] one would expect λ surf.el−ph ≃ 1.7 − 1.9. Moreover, it was pointed out by Cappelluti et al.[2] that nonadiabatic character can be further enhanced by low charge carrier density, that is the case for ZrB 12 [4,5].One of the key feature of nonadiabatic superconductivity is the observation of unconventional isotope and pressure effects on the magnetic field penetration depth λ. Note, that in adiabatic superconductors (or in the superconductors where the nonadiabatic effects are small) the...

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“…According to Jorgenson et al [7] the decrease in orthorhombic distortion indicates the increase of oxygen vacancy in CuO chain. The structure of YBCO is basically dictated by oxygen ordering [7][8][9]. The above result on structural changes due to the presence of BaTiO 3 is due to the oxygen vacancy ordering in the system.…”

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confidence: 83%

Intergranular percolation in granular YBCO/BaTiO3 composites

et al. 2009

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: Ferroelectrics and high temperature superconductors are two promising materials for future electronic devices. Both being perovskite ceramic structures with similar crystal chemistry a set of samples were prepared from the composite of (1-x)YBa 2 Cu 3 0 7-δ -(x)BaTiO 3 (YBCO/BT). These samples were investigated with temperature dependent resistance, FTIR, X-ray diffraction and SEM-EDX analysis. It has been found that the critical exponent in the T c0 (R = 0) region is in agreement with the percolation theory. A long-range superconducting order results from thermally assisted percolation process through weak-links between the grains. The connectivity in the coherent transition region can be explained by a power law.

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Temperature dependent magnetic penetration depth of Co and Zn doped YBa2Cu3O7 obtained from the AC susceptibility of magnetically aligned powders

Cited by 78 publications

References 39 publications

Exponential Temperature Dependence of the Penetration Depth in Single CrystalMgB2

Exponential Temperature Dependence of the Penetration Depth in Single CrystalMgB2

Anomalous electron-phonon coupling probed on the surface of superconductorZrB12

Intergranular percolation in granular YBCO/BaTiO3 composites

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