Anomalies of the magnetic properties of granular oxide superconductor BaPb1 ?x Bi x O3

Josephson-coupled superconducting regions embedded at the interfaces of highly oriented pyrolytic graphite

“…As an example, we show the results at 100 K in figure 7. The observed changes of the hysteresis as a function of the maximum applied field, namely from a reversible (zero hysteresis) at low maximum applied fields (H 100 Oe) to a Bean-like behavior at fields of the order of 300 Oe, to the narrowing of the hysteresis at the largest fields, agrees with that expected for granular superconductors [44][45][46], as has recently been reported for water-treated graphite powders [11]. The change in the slope of the virgin first curve is also typical of a granular superconducting response.…”

Low field magnetic response of the granular superconductor La1.8Sr0.2CuO4

“…The upper one corresponds to the λ → 0 limit, where the magnetization is given simply by the Bean's formula for the cylinder: Let us now discuss the data starting from those obtained for low d.c. fields, H d.c. = 0, 0.1 G. One can see that, after the initial linear part, the absolute value of the measured susceptibility is always smaller than the calculated ones. This corresponds to the decrease of J c with increasing induction, as it is classically expected in granular materials, due to the suppression of intergrain critical currents by magnetic field penetration into the Josephson junctions [9]. This "classical" behavior for granular superconductors is usually analyzed by considering the volume-averaged Josephson medium as a kind of type II superconductor in the dirty limit, provided its macroscopic penetration depth λ J is large as compared with the grains size [11,35].…”

“…In this section, we describe successively the static magnetic response of samples A and B and present a preliminary treatment of these data, in order to distinguish between the magnetic response of individual grains and intergrain currents [9,22] (a detailed study of the latter is the subject of the next section). Firstly, we present results obtained after cooling the samples in various d.c. fields and applying small field increases.…”

“…Its relative importance is characterized by the ratio of the corresponding magnetic flux penetrating elementary loops of the ceramics Φ sf to the flux quantum Φ 0 , i.e. just by the parameter β L = 2πLI c /cΦ 0 where L is the characteristic inductance of an elementary loop [9]. Estimating the elementary inductance as L ≈ 2πa 0 µ g and using Eq.…”

“…The effect of screening on the presence and properties of the phase transition into a glassy state is not completely clear; some numerical results [8] indicate the absence of a true phase transtion in a 3D model with screening. Quantitatively, the strengh of screening is determined by the ratio β L = 2πLI c /cΦ 0 where L is the characteristic inductance of an elementary intergrain current loop [9]. In the ceramics with β L ≪ 1 , screening effects become important on a long-distance scale ∼ a 0 / √ β L only (i.e.…”