Strong Pair-breaking Effect Research Articles

Experimental investigation of a local mixed state induced by a small ferroparticle in YBaCuO films

Direct local measurements of the characteristics of a mixed state induced by a magnetic field of a small ferromagnetic particle have been carried out in thin film YBaCuO samples, using a scanning Hall probe. The results provide evidence for an extremely low energy barrier for entry of vortex half-loops into high quality films and further breaking of these half-loops into vortex-antivortex pairs. The observed phenomena are consistent with the Josephson medium model. We obtained the temperature dependence of the intergrain critical current density, which points to strong pair breaking effects at the grain boundaries.

Experimental investigation of a local mixed state induced by a small ferromagnetic particle in Y-Ba-Cu-O films: Extremely low energy barrier for formation of vortex-antivortex pairs

Direct local measurements of the characteristics of a mixed state induced by a magnetic field of a small ferromagnetic particle have been carried out in thin film Y-Ba-Cu-O samples, using a scanning Hall probe. The results provide evidence for an extremely low energy barrier for entry of vortex half-loops into high quality films and further breaking of these half-loops into vortex-antivortex pairs. The observed phenomena are consistent with the Josephson medium model. We obtained the temperature dependence of the intergrain critical current density, which points to strong pair breaking effects at the grain boundaries.

Comparison of the influence of Ni and Zn impurities on the electromagnetic properties of YBa2Cu3O6.95.

A set of highly sensitive cavity perturbation techniques, quite different from the ones most commonly used to measure thin films, has been developed for measuring the surface impedance of small crystals of high-${\mathit{T}}_{\mathit{c}}$ superconductors. Since the results of these studies differ markedly from most surface-impedance measurements of thin films of high-${\mathit{T}}_{\mathit{c}}$ superconductors, we describe the techniques in detail, including recent improvements. Measurements of the surface resistance and penetration depth of a twin-free crystal of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.95}$ indicate two striking differences from the surface impedance of conventional superconductors. Both quantities exhibit linear temperature dependence at low temperatures rather than the exponentially activated behavior of an s-wave BCS superconductor. Also, a broad peak in the temperature dependence of the surface resistance indicates that the large transport scattering rate in the normal state collapses below ${\mathit{T}}_{\mathit{c}}$ as the holes condense into the superconducting condensate. Systematic studies of the influence of Zn and Ni impurities on the surface impedance indicate that these properties are very sensitive to defects and this sensitivity provides further insight into the pairing state. Although both types of impurities provide strong elastic scattering that limits the collapse of the scattering rate and suppresses the peak in the surface resistance, only Zn has a strong pair-breaking effect. As little as 0.31% Zn substitution makes ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.95}$ gapless, but Ni, which is a magnetic impurity has no such pair-breaking effect up to 0.75% substitution. This difference is the opposite of that expected for an s-wave BCS superconductor.

Superconducting properties and preferred orientations of epitaxial growth in V/Cr multilayers

We investigated the superconducting properties of V/Cr multilayers with the different preferred orientations of (100) and (110). These samples were grown epitaxially on MgO (100) and Al 2O 3 (11 2 0) single-crystal substrates. The Cr layer thickness was varied from d cr=2 to 140Å at a constant V layer thickness of 300 Å. In (100) V/Cr, the critical temperature T c abruptly drops with increasing d Cr from 5 to 40 Å and shows a plateau above 40 Å. In (110) V/Cr, T c gradually decreases up to d Cr=80 Å and keeps a higher value than in (100). For the temperature dependence of parallel upper critical fields, H c2∥( T), the 3D-to-2D dimensional crossover phenomenon takes place in d Cr=10∼25 Å for (100) V/Cr and 10∼70 Å for (110) V/Cr. 2D behavior of H c2∥( T), which implies the decoupling of superconducting V layers by the Cr layer, is achieved in the range of d Cr above 40 Å for (100) and 80 Å for (110). The ratio of H c2⊥ to H c2⊥ for (100) V/Cr is enhanced in comparison with that for (110). Our results indicate that the strong pair-breaking effect operates at the interfaces of (100) V/Cr.

Point contact tunneling studies on (Y 1− xPr x)Ba 2Cu 3O 7−δ single crystals

We present point contact tunneling studies on single crystals of (Y 1−xPr x)Ba 2Cu 3O 7−δ (x⩽0.5) with T c varying f rom 92 to 30 K. Superconducting gap features are clearly seen in undoped YBCO but the Pr-doped systems do not show any trace of such features. Based on our results we point out that gapless superconductivity in Pr-doped YBCO system arises due to strong pair breaking effects associated with magnetic scattering.

Transport processes and reduction of I/sub c/R/sub n/ product in YBa/sub 2/Cu/sub 3/O/sub x//PrBa/sub 2/Cu/sub 3/O/sub x//YBa/sub 2/Cu/sub 3/O/sub x/ ramp-type Josephson junctions

The mechanisms of current passage and the causes of I/sub c/R/sub n/ (critical-current normal-resistance) product reduction of YBCO/PBCO/YBCO ramp-type junctions are analyzed. At PBCO (PrBa/sub 2/Cu/sub 3/O/sub x/) barrier thicknesses L=8-20 nm the junction characteristics are determined by the thickness of the PBCO barrier and its nature. The boundary resistance and depression of the YBCO (YBa/sub 2/Cu/sub 3/O/sub x/) superconducting parameters near the interface do not strongly affect the junction parameters. The behaviour of the YBCO/PBCO/YBCO junctions cannot be described by simple SNS (superconductor-normal metal-superconductor) weak-link or SIS (superconductor-insulator-superconductor) tunnel models. A strong pair-breaking effect and a one-center inelastic tunneling process are taken into account to explain the Josephson and normal state characteristics of these junctions. >

ZERO FIELD EXCESS CONDUCTIVITY STUDIES IN THE Bi-Sr-Ca-Cu-O SYSTEM

The excess conductivity (∆σ) data on several samples of 85 K phase and 110 K phase of Bi-Sr-Ca-Cu-O system were fitted to a sum of direct (AL) and indirect (MT) contribution of fluctuation induced excess conductivity, in the mean field region. The samples of the 85 K phase are of compositions Bi2Sr2CaCu2Oy, Bi2Sr2CaCu2Ag0.8Oy, and Bi2Sr2.14Ca0.86Cu2Oy with Tc (zero) of 78.9 K, 86.5 K and 81.8 K respectively. The two samples of 110 K phase with composition Bi1.2Pb0.3Sr1.5Ca2Cu3Oy were prepared from nitrate and citrate routes respectively and have Tc (zero) of 105.3 K and 102.9 K. We find from our analysis relatively strong pair-breaking effects in both 85 K and 110 K phases, indicating the contribution of Maki-Thompson term to the excess conductivity to be rather insignificant. The average value estimated for the phase-breaking time for the 85 K and 110 K phases are (3.7±0.5)×10−16s and(11.3±1.6)×10−16s respectively, at 100 K.

Scattering of normal excitations by superconducting fluctuations in single-crystal and granular copper oxide superconductors.

By using a unified empirical procedure, the intrinsic paraconductivity in the ab plane, \ensuremath{\Delta}${\mathrm{\ensuremath{\sigma}}}^{\mathit{i}}$, is extracted from different available data for resistivity rounding in single-crystal and granular ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ samples, and from data in ${\mathrm{Bi}}_{1.5}$${\mathrm{Pb}}_{0.5}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{y}}$ granular samples. These results strongly suggest that both high-temperature superconductor systems will have relatively strong pair-breaking effects, and that \ensuremath{\Delta}${\mathrm{\ensuremath{\sigma}}}^{\mathit{i}}$ may be explained by direct superconducting order-parameter-fluctuation effects alone.

Superconducting properties of Fe/V/Fe sandwiches

A study has been made of the superconducting properties of Fe/V/Fe sandwiches with V thickness ranging from 153 AP (atomic planes) to 5048 AP. All 20 samples studied were prepared in a single run. At zero magnetic field, the suppression of the transition temperature follows an inverse square dependence on the thickness, implying a strong pair-breaking effect in the Fe layers. The upper critical field data cross over smoothly from the 2D regime to the 3D regime with increasing V thickness. A comparison with some thin-film theories is given.

Ion implantation a powerful technique for the production of metastable superconducting alloys

Ion implantation at liquid helium temperatures is a new method of producing metastable alloys. The special features of this alloying technique are illustrated by two selected examples of dilute and concentrated alloys, respectively. Superconductors containing small amounts of paramagnetic (3d)-impurities such asHgMn, PbMn andSnMn exhibit strong pair breaking effects. In the case ofSnMn the Kondo effect is observed. Concentrated alloys of Pd and Pd-noble metal alloys with hydrogen (deuterium) are only superconducting in a very high concentration range, being unstable at room temperature. Such concentrations can be achieved by ion implantation at liquid He-temperatures. The superconducting transition temperature can thus be raised to a maximum value of 16.6 K in aH/Pd 0.55 Cu0.45≃0.7 alloy.

Fluctuation-Induced Conductivity of Superconductors above the Transition Temperature: Regularization of the Maki Diagram

The Maki contribution to the conductivity above the superconducting transition temperature is regularized within the framework of the BCS theory. This is achieved through the renormalization of the impurity-scattering vertex by inclusion of the effects of pair fluctuations. The conductivity is evaluated for a thin film. It depends only on the reduced temperature and the normal resistance per square. Fair agreement is found with Al films over a wide temperature range. Agreement is not found with experiments on Bi, Pb, and Ga films, which apparently contain a strong additional pair-breaking effect. The temperature range in which interactions among fluctuations become important in the Maki conductivity is generally larger than that given by the Ginzburg criterion.