Two-dimensional vortex plasma in Bi_2Sr_2CaCu_2O_x single crystals in the vicinity of T_c

Quantitative analysis of results of measurements of magnetoresistance of Bi_2Sr_2CaCu_2O_x single crystals for magnetic fields 0.2-0.8 T, in the temperature interval 78.5-84 K has been made. For temperatures lower than 81 K the dissipation is predominantly determined by the suppression of the long-range superconducting order, whereas closer to T_c the flux-flow contribution becomes significant. We obtain very good agreement between the experiment and the model in which two dissipation mechanisms are additive

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

Strongly nonequilibrium flux flow in the presence of perforating submicron holes

We report on the effects of perforating submicron holes on the vortex dynamics of amorphous Nb0.7Ge0.3 microbridges in the strongly nonequilibrium mixed state, when vortex properties change substantially. In contrast to the weak nonequilibrium - when the presence of holes may result in either an increase (close to Tc) or a decrease (well below Tc) of the dissipation, in the strong nonequilibrium an enhanced dissipation is observed irrespectively of the bath temperature. Close to Tc this enhancement is similar to that in the weak nonequilibrium, but corresponds to vortices shrunk due to the Larkin-Ovchinnikov mechanism. At low temperatures the enhancement is a consequence of a weakening of the flux pinning by the holes in a regime where electron heating dominates the superconducting properties.Comment: 6 pages, 5 figure

Dvodimenzionalna plazma magnetskih vrtloga u monokristalima Bi2Sr2CaCu20x u blizini Tc

Quantitative analysis of results of measurements of magnetoresistance of Bi2Sr2CaCu2Ox single crystals for magnetic fields 0.2-0.8 T, in the temperature interval 78.5-84 K has been made. For temperatures lower than 81 K the dissipation is predominantly determined by the suppression of the long-range superconducting order, whereas closer to Tc the flux-flow contribution becomes significant. We obtain very good agreement between the experiment and the model in which two dissipation mechanisms are additive.Izvršili smo kvantitativnu analizu magnetootpora monokristala Bi2Sr2CaCu2Ox, za magnetska polja 0.2–0.8 T i u temperaturnom području 78.5–84 K. Na temperaturama ispod 81 K disipacija je uglavnom posljedica potiskivanja dugodosežnog supravodljivog uređenja, dok bliže Tc doprinos kretanja vrtloga postaje značajan. Ustanovljeno je vrlo dobro slaganje eksperimentalnih rezultata i modela u kojem su ta dva disipacijska mehanizma aditivna

Reversal of Nonlocal Vortex Motion in the Regime of Strong Nonequilibrium

We investigate nonlocal vortex motion in weakly pinning a-NbGe nanostructures, which is driven by a transport current I and remotely detected as a nonlocal voltage Vnl. At high I, the measured Vnl exhibits dramatic sign reversals that at low and high temperatures T occur for opposite polarities of I. The sign of Vnl becomes independent of that of the drive current at large abs(I). These unusual effects can be nearly quantitatively explained by a novel enhancement of magnetization, arising from a nonequilibrium distribution of quasiparticles at high T, and a Nernst-like effect resulting from local electron heating at low T

New Aspects of Variable-range-hopping Conductivity in Doped Polyaniline

Temperature, T, dependence of the electrical conductivity, σ, of own-made polyaniline pellets, highly doped either by HCl or by dodecylbenzenesulphonic acid, is discussed. For the both dopants, a variable-range-hopping exponent 2/5 is found below T* ≈ 200 − 250 K. This result can be understood on the basis of a recent theory of Fogler, Teber and Shklovskii which takes into account Coulomb correlations in a three-dimensional system of chain-like conductors, predicting a power-law density of states for charge excitations around the Fermi energy. At T > T *, the σ(T) curves are steeper, implying a tendency towards nearest-neighbour hopping by a reduction of the average hopping length.</p

Radiocezij u mahovinama u Parku prirode Kopački rit u Hrvatskoj: traganje za neprijavljenim ispuštanjem iz nuklearnih postrojenja u ratom razorenoj Ukrajini

The invasion of Ukraine and military operations around Ukrainian nuclear power plants and other nuclear facilities have prompted us to search for radiocaesium in mosses from the Kopački Rit Nature Park in Croatia, since mosses are known bioindicators of airborne radioactive pollution, and Kopački Rit is a known low radiocaesium background area. Sampling was finished in August 2023, and our analysis found no elevated radiocaesium levels. Kopački Rit therefore remains a suitable place for future detection of anthropogenic radioactive pollutants.Invazija na Ukrajinu i vojne operacije oko ukrajinskih nuklearnih elektrana i drugih nuklearnih postrojenja potaknule su nas da potražimo radiocezij u mahovinama u Parku prirode Kopački rit, jer su mahovine poznati bioindikatori radioaktivnog onečišćenja zraka, a Kopački je rit poznat po niskoj razini onečišćenja radiocezijem. Uzorkovanje je završeno u kolovozu 2023. Naša analiza nije otkrila povišene razine radiocezija. Kopački rit stoga ostaje pogodno mjesto za buduću detekciju antropogenog radioaktivnog onečišćenja