Dissipation in a weak-link-limited superconductor as a problem of percolation theory

Broadening of the resistive transition of sintered GdBa2Cu3O7-x superconductor has been studied varying the measuring current over the range of five orders of magnitude. The observed well-defined branching points in the resistive transition coincide with the quasi-Ohmic saturation of V-I curves and scale with the average grain size. The results suggest the validity of a simple percolation model for dissipation in systems whose global superconductivity is realized by Josephson coupling of the superconducting grains. A common origin of structured resistive transition, observed in many superconducting systems, has been also discussed

Homogenost i perkolacija u keramičkim visokotemperaturnim supravodičima

The main results of systematic investigation of the electric transport properties and ac susceptibility of ceramic high temperature superconductors (HTS) are presented. An effort has been made to deduce a suitable description for these properties. Due to the presence of the weak links at the grain boundaries the superconducting and the normal state properties of these systems deviate strongly from those of the corresponding homogeneous compounds. Becasue of this, ceramic HTS can be modeled as the two phase systems consisting of grains and 3D disordered array of intergrain links. The variation in the strength of the coupling between the grains determines the bulk critical currents (Ic) of these systems. A more detailed analysis shows that properties of ceramic HTS depend strongly on density and the ratio between the dimensions of the sample and the grain size. Therefore only some properties of fine-grained dense samples can probably be described by averaging the local properties over the volume of the sample. In contrast to that, the resistivity at lower densities, the diamagnetic transition below Tc and the transition to the dissipative state above Ic all exhibit the percolation effects.Prikazani su rezultati sistematskih istraživanja elektronskih transportnih svojstava i inicijalne (ac) susceptibilnosti keramičkih visokotemperaturnih supravodiča (VTS). Načinjen je pokušaj da se odredi najprikladniji opis tih svojstava. Keramički VTS mogu se opisati kao dvofazni sistemi sastavljeni od zrna dotičnog spoja i neuređene trodimenzijske (3D) mreže spojeva među zrnima. Raspon u jačini vezanja među pojedinim zrnima određuje ukupne kritične struje (Ic) u tim sistemima. Detaljnija analiza ukazuje da svojstva keramičkih VTS jako ovise o gustoći i omjeru između dimenzija zrna i dimenzija uzorka. Zbog toga se samo neka svojstva gustih, sitnozrnatih uzoraka mogu opisati postupkom homogenizacije tj. usrednjenjem lokalnih svojstava po volumenu uzorka. Nasuprot tome električna otpornost pri nižim gustoćama, dijamagnetski prijelaz i prijelaz u disipativno stanje iznad Ic su perkolativne pojave te se ne mogu opisati postupkom homogenizacije. Posljedica toga je, naprimjer, da napon na manjim gustoćama u neposrednoj blizini Ic slijedi zakon potencija, sa eksponentom praktički neovisnim o temperaturi i primjenjenom magnetskom polju

Experimental evidence of a fractal dissipative regime in high-T_c superconductors

We report on our experimental evidence of a substantial geometrical ingredient characterizing the problem of incipient dissipation in high-T_c superconductors(HTS): high-resolution studies of differential resistance-current characteristics in absence of magnetic field enabled us to identify and quantify the fractal dissipative regime inside which the actual current-carrying medium is an object of fractal geometry. The discovery of a fractal regime proves the reality and consistency of critical-phenomena scenario as a model for dissipation in inhomogeneous and disordered HTS, gives the experimentally-based value of the relevant finite-size scaling exponent and offers some interesting new guidelines to the problem of pairing mechanisms in HTS.Comment: 5 pages, 3 figures, RevTex; Accepted for publication in Physical Review B; (figures enlarged

Superconductivity in Bi 2223 compound: physics and potential applications

The main results of the systematic study of the electron transport properties and AC susceptibility in Pb-doped, almost single phase, Bi 2223 ceramic samples and Ag-clad Bi 2223 tape, are presented. Whereas the results for the ceramic samples can only be used for the study of the percolation problem in the weak-link network, those for tape can yield some intrinsic parameters of Bi 2223 compound (the upper critical fields B_C2, the coherence lengths, etc.). In addition, the results for the tape provide some technologically relevant parameters (the pinning potential U_0, volume pinning force F_p and the flux-flow viscosity h) and an insight into the nature of dissipation and pinning in these materials. Some factors limiting the critical current density J_c in the present-day tapes are briefly discussed

Supravodljivost u spoju Bi 2223: fizika i moguće primjene

The main results of the systematic study of the electron transport properties and AC susceptibility in Pb-doped, almost single phase, Bi 2223 ceramic samples and Ag-clad Bi 2223 tape, are presented. Whereas the results for the ceramic samples can only be used for the study of the percolation problem in the weak-link network, those for tape can yield some intrinsic parameters of Bi 2223 compound (the upper critical fields BC2, the coherence lengths, etc.). In addition, the results for the tape provide some technologically relevant parameters (the pinning potential U0, volume pinning force Fp and the flux-flow viscosity h) and an insight into the nature of dissipation and pinning in these materials. Some factors limiting the critical current density Jc in the present-day tapes are briefly discussed.Prikazani su glavni rezultati sustavnog istraživanja električnih svojstava i inicijalne susceptibilnosti gotovo–monofaznih Bi 2223 keramičkih uzoraka dopiranih olovom i srebrom obložene vrpce. Dok se rezultati za keramičke uzorke mogu primjenjivati isključivo za proučavanje perkolacije u sustavu slabih veza, oni za traku daju neke intrinsične parametre Bi 2223 spoja (viša kritična polja i duljine koherencije). Nadalje, rezultati za traku daju neke tehnološki važne parametre (potencijal zapinjanja U0, volumnu gustoću sile zapinjanja Fp i viskoznost tečenja magnetskih vrtloga η) i uvid u prirodu rasipanja energije i zapinjanja vrtloga u tim materijalima. Ukratko su razmotreni razlozi ograničenja kritičnih struja u vrpcama

Influence of internal disorder on the superconducting state in the organic layered superconductor [kappa]-(BEDT-TTF)[sub]2Cu[N(CN)[sub]2]Br

We report high-sensitivity AC susceptibility measurements of the penetration depth in the Meissner state of the layered organic superconductor ▫$kappa$▫-(BEDT-TTF)▫$_2$▫Cu[N(CN)▫$_2$▫]Br. We have studied nominally pure single crystals from the two different syntheses and employed controlled cooling procedures in order to minimize intrinsic remnant disorder at low temperatures associated with the glass transition, caused by ordering of the ethylene moieties in BEDT-TTF molecule at ▫$T_{text{G}}$▫ = 75°K. We find that the optimal cooling procedures (slow cooling of -0.2°K/h or annealing for 3 days in the region of ▫$T_{text{G}}$▫) needed to establish the ground state, depend critically on the sample origin indicating different relaxationtimes of terminal ethylene groups. We show that, in the ground state, the behavior observed for nominally pure single crystals from both syntheses is consistent with unconventional $d$-wave order parameter. The in-plane penetration depth ▫$lambda_{text{in}}(T)$▫ is strongly linear, whereas the out-of-plane component ▫$lambda_{text{out}}(T)$▫ varies as ▫$T^2$▫. In contrast, the behavior of single crystals with long relaxation times observed after slow (-0.2°K/h) cooling is as expected for a $d$-wave superconductor with impurities (textit{i.e.} ▫$lambda_{text{in}}(T) proptolambda_{text{out}}(T) propto T^2$▫) or might be also reasonably welldescribed by the ▫$s$▫-wave model. Our results might reconcile the contradictory findings previously reported by different authors