Ceramic samples of Tl2Ba2Ca2Cu3O10 (Tl-2223) and TlBa2Ca2Cu3O9 (Tl-1223) superconductor were resynthesized by using several preparation procedures and investigated by means of magnetic separation technique, allowing evaluation of the superconducting critical temperature of individual grains as small as 25 mu m. Although an onset of diamagnetism detected by SQUID measurements was observed around 130 K in Tl-2223 and 134 K in Tl-1223, no grains with T-c > 125 K were found for Tl-2223 and with T-c > 130 K for Tl-1223. Furthermore, the volume fraction of superconductivity of such ceramics was less than 5% and 3% in best samples of Tl-2223 and Tl-1223, respectively. Here we provide a detailed comparison of results obtained by magnetic separation technique with those by SQUID applied to individual grains and bulk samples. (C) 2011 Elsevier Masson SAS. All rights reserved.

We propose a simple scaling procedure for the normal-state magnetization Mn data collected as functions of temperature T in different magnetic fields H. As a result, the Mn(T) curves collected in different fields collapse on to a single Msc(T) line. In this representation, the onset of superconducting diamagnetism manifests itself by a sharp divergence of the Msc(T) curves for different H values. As will be demonstrated, this allows for a reliable determination of temperature Tonset, at which superconducting diamagnetism become observable.

Magneto-optical imaging and magnetization measurements were applied to investigate the local formation of a superconducting phase effected by a random neck synthesis in the Y–Ba–Cu–O system. Polished pellets of strongly inhomogeneous ceramic samples show clearly the appearance of superconducting material in the intergrain zones of binary primary particles reacted under different conditions. Susceptibility measurements allowed evaluation of the superconducting critical temperature, which turned out to be close to that of optimally doped YBa2Cu3O7−x.

By comparison of recent direct measurements of the temperature dependence of the upper critical field $H_{c2}$ in an Y-123 high temperature superconductor with the scaling analysis of magnetization data, collected in fields H << H_c2, we demonstrate that that the temperature dependence of the Ginzburg-Landau parameter kappa is negligible. Another conclusion is that the normalized temperature dependence of H_c2 is independent of the orientation of the magnetic field in respect to crystallographic axes of the sample. We also discuss that isotropy of the temperature dependence of H_c2 straightforwardly follows from the Ginzburg-Landau theory if kappa does not depend on temperature.

By comparison of recent direct measurements of the temperature dependence of the upper critical field H(c2) of an YBa(2)Cu(3)O(7-x) high-T(c) superconductor with the scaling analysis of magnetization data, collected in fields [Formula: see text], we demonstrate that the temperature dependence of the Ginzburg-Landau parameter κ is negligible. Another conclusion is that the normalized temperature dependence of H(c2) is independent of the orientation of the magnetic field with respect to the crystallographic axes of the sample. We also discuss the fact that isotropy of the temperature dependence of H(c2) straightforwardly follows from the Ginzburg-Landau theory if κ does not depend on the temperature.

We present a magnetization study of low density YBCO ceramics carried out in magnetic fields 0.5 Oe < H < 50 kOe. It was demonstrated that superconducting links between grains may be completely suppressed either by a magnetic field of the order of 100 Oe (at low temperatures) or by an increase of temperature above 70 K. This property of present samples allowed to evaluate the ratio between an average grain size and the magnetic field penetration depth lambda. Furthermore, at temperatures T > 85 K, using low-field magnetization measurements, we could evaluate the temperature dependence of lambda, which turned out to be very close to predictions of the conventional Ginzburg-Landau theory. Although present samples consisted of randomly oriented grains, specifics of magnetization measurements allowed for evaluation of lambda_ab(T). Good agreement between our estimation of the grain size with the real sample structure provides evidence for the validity of this analysis of magnetization data. Measurements of equilibrium magnetization in high magnetic fields were used for evaluation of Hc2(T). At temperatures close to T_c, the Hc2(T) dependence turned out to be linear in agreement with the Ginzburg-Landau theory. The value of temperature, at which Hc2 vanishes, coincides with the superconducting critical temperature evaluated from low-field measurements.

We reanalyze published magnetization data and demonstrate that the conclusion of the original authors, claiming enhanced two-dimensional properties of the cuprate superconductor $\mathrm{Tl}{\mathrm{Ba}}_{2}{\mathrm{Ca}}_{3}{\mathrm{Cu}}_{4}{\mathrm{O}}_{y}$, is not supported by the experimental results. Our analysis shows that the magnetic field dependence of the mixed-state magnetization for this particular sample is amazingly close to the results of numerical calculations by Brandt for an ideal vortex lattice without fluctuations. This good agreement between experiment and theory allows for the evaluation of the absolute values of the upper critical field ${H}_{c2}(T)$.

Experiments show that Aux creep in HTSC at low temperatures is very sensitive to illumination. Thus, illumination as weak as few microwatts per cm(2) drastically accelerates the flux creep. It is important to underline that the effect exists in so different HTSC materials as Bi-2212 single crystals and YBCO films, We believe that the pair breaking process is responsible for this effect. This approach can explain experimental results qualitatively. In this case the sensitivity to illumination gives an important information about relaxation processes, which has to be very slow to explain the magnitude of the effect observed experimentally.

We present an experimental study of flux creep in YBCO film at low temperatures (1.3 K<T<25 K). A ring shaped film was used to obtain directly voltage–current characteristics of the film. It has been found that even very weak illumination greatly accelerates the flux creep at liquid helium temperatures. The influence of illumination is completely different from that caused by the temperature rise. We propose a simple qualitative model to explain the high sensitivity of the flux creep to weak illumination.

We present an experimental study of cold deposited Pb films. Experimental results show that high values of the film thickness corresponding to the onset of electrical conductivity (55–65 Å) cannot be explained by the formation of the island structure. It means that metastable insulating modification of Pb is formed in this case. All the results are in good agreement with the recently proposed model of the insulating modifications of metals [I. A. Parshinet al., Phys. Rev. B 54, 1308 (1996) and A. V. Danilovet al., J. of Low Temp. Phys. 103, 35 (1996)].

It is shown that the conductivity onset in cold deposited Ga films on glass substrates corresponds to the film thickness of 60{endash}70 A in contrast to films deposited onto predeposited layers of SiO, where the conductivity onset can be observed at 10 times smaller thicknesses. A detailed investigation of superconducting properties of the films is also presented. We believe that chemical bonds between atoms are very important for the formation of the film structure, which can explain experimental results. {copyright} {ital 1996 The American Physical Society.}

It is shown that the conductivity onset in cold deposited Hg films on glass substrates covered with a SiO layer corresponds to te film thickness of 4–5 Å in a contrast to films deposited on pure glass, where the conductivity onset can be observed only at 10–15 times higher thicknesses. Study of superconducting properties of films is also presented. We believe that chemical bonds between metallic atoms themselves and their chemical interaction with atoms of the substrate are very important for formation of the film structure and responsible for a completely different behavior of films deposited onto different substrates.

We present an experimental study of the flux creep rate in YBCO film at low temperatures. It is shown that the flux creep can be described in terms of quantum tunneling of vortices up to temperatures of 10-15 K. We have investigated also the influence of a weak illumination on a flux creep rate. It is shown that a visible light with intensity as weak as 1 mW/cm(2) drastically changes the flax creep rate at liquid helium temperatures. We propose a simple model to explain this extreme sensitivity of the flux creep to a weak illumination.

Numerical calculations of superheating magnetic fields and superheating currents for superconducting slabs for a wide range of the sample thickness are presented. The calculations were made for low values of Ginzburg-Landau parameter κ, i.e., for type-1 superconductors. We propose also experimental procedures to measure superheating fields and currents in films and bulk samples.

We present experimental evidence that mercury forms an insulating phase in films condensed on glass substrates at liquid-helium temperatures. This insulating phase is metastable and it exists in films only up to some critical thickness ${\mathit{d}}_{\mathit{c}}$\ensuremath{\approxeq}60 \AA{}. Our results cannot be explained by the assumption that the films consist of isolated islands. We believe that similar insulating phases exist also in quench-condensed lead films and maybe in some other metals. As far as we know the possibility that a disordered system of metallic atoms may form an insulator has never been considered.

We present an experimental study of the current induced destruction of superconductivity in films of type-1 superconductors. The most interesting aspect of these experiments is the possibility to observe and investigate different mechanisms of the superconductivity destruction depending on the ratio of the sample thickness d to the coherence length ξ(T).If d/ξ(T) < 4.5 we observe, above a critical current value, a resistive state throughout the section of the film, but in the case of d/ξ(T) > 4.5 the destruction of superconductivity starts, as in bulk samples, with the formation of a normal shell along the outer surface of the cylindrical sample together with a layer of the two-dimensional mixed state at the inner surface. Investigations on films allowed for the first time to measure the thickness of the two-dimensional mixed state.

The superconducting transition temperature Tc of quench condensed thin films of bismuth and gallium have been observed to rise as a result of the deposition of silver on the films surface. Increase in the critical temperature δTc was depended on the thickness of the superconducting film and δTc≈1K for very rhin films. The effect stems from a suppression of the localization of electrons due to a decrease of the films resistance.

We present experimental evidences that mercury forms a metastable insulating phase in films condensed on glass substrates at low temperatures. This phase exists in films only up to some critical thickness d(c) almost-equal-to 60 angstrom. Our results cannot be explained by the assumption that films have an islands structure. We believe that similar phases exist also in quench-condensed lead films and, may be, in some other metals. The possibility that disordered system of metallic atoms can form an insulator has never been considered.

We present an experimental study of the current induced destruction of superconductivity in cylindrical indium films. The most interesting aspect of these experiments is the possibility to observe and investigate different mechanisms of the superconductivity destruction depending on the ratio of the sample thickness d to the coherence length ξ(T). If d/ξ(T)<4.5 we observe, above a critical current value, a resistive state throughout the section of the film, but in the case of d/ξ(T)>4.5 the destruction of superconductivity starts, as in bulk samples, with the formation of a normal shell along the outer surface of the cylindrical sample together with a thin layer of the two-dimensional mixed state at the inner surface.