50 research outputs found
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 keramiÄkim visokotemperaturnim supravodiÄima
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 keramiÄkih visokotemperaturnih supravodiÄa (VTS). NaÄinjen je pokuÅ”aj da se odredi najprikladniji opis tih svojstava. KeramiÄki VTS mogu se opisati kao dvofazni sistemi sastavljeni od zrna dotiÄnog spoja i neureÄene trodimenzijske (3D) mreže spojeva meÄu zrnima. Raspon u jaÄini vezanja meÄu pojedinim zrnima odreÄuje ukupne kritiÄne struje (Ic) u tim sistemima. Detaljnija analiza ukazuje da svojstva keramiÄkih VTS jako ovise o gustoÄi i omjeru izmeÄu 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 elektriÄna otpornost pri nižim gustoÄama, 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 gustoÄama u neposrednoj blizini Ic slijedi zakon potencija, sa eksponentom praktiÄki 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.IzvrÅ”ili smo kvantitativnu analizu magnetootpora monokristala Bi2Sr2CaCu2Ox, za magnetska polja 0.2ā0.8 T i u temperaturnom podruÄju 78.5ā84 K. Na temperaturama ispod 81 K disipacija je uglavnom posljedica potiskivanja dugodosežnog supravodljivog ureÄenja, dok bliže Tc doprinos kretanja vrtloga postaje znaÄajan. 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, Ļ, 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* ā 200 ā 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 Ļ(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 KopaÄki rit u Hrvatskoj: traganje za neprijavljenim ispuÅ”tanjem 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 KopaÄki Rit Nature Park in Croatia, since mosses are known bioindicators of airborne radioactive pollution, and KopaÄki Rit is a known low radiocaesium background area. Sampling was finished in August 2023, and our analysis found no elevated radiocaesium levels. KopaÄki 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 KopaÄki rit, jer su mahovine poznati bioindikatori radioaktivnog oneÄiÅ”Äenja zraka, a KopaÄki je rit poznat po niskoj razini oneÄiÅ”Äenja radiocezijem. Uzorkovanje je zavrÅ”eno u kolovozu 2023. NaÅ”a analiza nije otkrila poviÅ”ene razine radiocezija. KopaÄki rit stoga ostaje pogodno mjesto za buduÄu detekciju antropogenog radioaktivnog oneÄiÅ”Äenja