Results 1 - 10 of 24
Results 1 - 10 of 24. Search took: 0.014 seconds
|Sort by: date | relevance|
[en] In this paper they present magnetoresistivity and Hall effect measurements performed on a YBCO epitaxial film as a function of the angle θ between the external magnetic field and the a-b planes. The resistivity and Hall effect measurements are analyzed in term of the general scaling approach proposed by Blatter and coworkers; the Hall conductivity data are examined to separate the contributions due to vortices and quasi particles
[en] Magnetic measurements on type-II superconducting thin films subjected to a transverse external field can be explained by a model that takes demagnetizing effects into account. They carried out measurements of magnetic flux distribution on the film surface by means of a self made apparatus, based on two Hall probes and a differential transformer and they tested the validity of a recent model presented in the literature
[en] They present paraconductivity measurements on YBCO thin films obtained by means of pulsed laser ablation in MBE condition. The films, grown in different oxygen pressure, exhibit critical temperatures ranging from 81 to 91 K. The excess conductivity was analyzed in order to study critical phenomena. They observe the Lawrence-Doniach crossover and, at higher temperatures, the crossover related to the breakdown of the Ginzburg-Landau approximation, due to the importance of short wave fluctuations; the first crossover shifts to lower temperatures as the critical temperature of the film decreases, while the second occurs for all the samples roughly at the same reduced temperature, which proves the universality of the paraconductivity behaviour
[en] The temperature-dependent fundamental ac susceptibility of a granular superconductor in the absence of dc fields has been analyzed by developing a phenomenological model for effective magnetic fields, taking into account the influence of the magnetic interaction between the intergranular and the intragranular magnetizations due to demagnetizing effects. For this purpose a policrystal Fe-based superconductor FeSe_0_._5Te_0_._5 sample has been studied. By the frequency dependence of the peaks of the temperature-dependent imaginary part of the fundamental complex susceptibility, the dependence on temperature of the characteristic times both for intergranular and intragranular relaxations of magnetic flux are derived, and the corresponding relaxation processes due to combinations of the flux creep, the flux flow and the thermally activated flux flow regimes are identified on the basis of the effective magnetic fields both at the sample surface and at the grains’ surfaces. Such characteristic times, through the Havriliak–Negami function, determine the temperature and the frequency dependences of the complex susceptibility. The comparison of the numerically obtained curves with the experimental ones confirms the relevance, for identifying the intergranular and intragranular contributions to the ac magnetic response and the corresponding flux dynamical regimes, of the interaction between the intergranular and intragranular magnetizations due to demagnetizing effects. (paper)
[en] Twisted and not twisted multifilamentary BSCCO (2223) - Ag tapes, prepared by the PIT technique, have been characterized by SQUID magnetometry with the magnetic field applied both parallel and perpendicular to the c-axis. From these measurements and estimation of the critical current densities Jab with Hext parallel to the c-axis Jab planes and Jc is obtained. These values are in fairly good agreement with those obtained by transport measurements. The tapes turned out to be very homogeneous and practically no difference was found in the hysteresis loops of twisted and not twisted tapes, which suggests that the mean grain alignment is the same in the two types of samples and the current path probably follows the 'brick wall' model
[en] In this paper we carry out a direct comparison between transport and superconducting properties-namely resistivity, magnetoresistivity, Hall effect, Seebeck effect, thermal conductivity, upper critical field-of two different families of Fe-based superconductors, which can be viewed in many respects as end members: SmFeAsO1-xFx with the largest Tc and the largest anisotropy and Fe1+yTe1-xSex, with the largest Hc2, the lowest Tc and the lowest anisotropy. In the case of the SmFeAsO1-xFx series, we find that a single-band description allows us to extract an approximate estimation of band parameters such as carrier density and mobility from experimental data, although the behaviour of the Seebeck effect as a function of doping demonstrates that a multiband description would be more appropriate. On the contrary, experimental data for the Fe1+y(Te1-x, Sex) series exhibit a strongly compensated behaviour, which can be described only within a multiband model. In the Fe1+y(Te1-, Sex) series, the role of the excess Fe, tuned by Se stoichiometry, is found to be twofold: on one hand it dopes electrons in the system and on the other hand it introduces localized magnetic moments, responsible for Kondo like scattering and likely pairbreaking of Cooper pairs. Hence, Fe excess also plays a crucial role in determining superconducting properties such as the Tc and the upper critical field Hc2. The huge Hc2 values of the Fe1+yTe1-xSex samples are described by a dirty limit law, opposed to the clean limit behaviour of the SmFeAsO1-xFx samples. Hence, magnetic scattering by excess Fe seems to drive the system in the dirty regime, but its detrimental pairbreaking role seems not to be as severe as predicted by theory. This issue has yet to be clarified, addressing the more fundamental issue of the interplay between magnetism and superconductivity.
[en] We report on measurements of the superconducting properties of FeSe0.5Te0.5 thin films grown on lanthanum aluminate. The films have high transition temperatures (above 19 K) and sharp resistive transitions in fields up to 15 T. The temperature dependence of the upper critical field and the irreversibility lines are steep and anisotropic, as recently reported for single crystals. The critical current densities, assessed by magnetization measurements in a vector VSM, were found to be well above 109 A m-2 at low temperatures. In all samples, the critical current as a function of field orientation has a maximum, when the field is oriented parallel to the film surface. The maximum indicates the presence of correlated pinning centers. A minimum occurs in three films, when the field is applied perpendicular to the film plane. In the fourth film, instead, a local maximum caused by c-axis-correlated pinning centers was found at this orientation. The irradiation of two films with fast neutrons did not change the properties drastically, where a maximum enhancement of the critical current by a factor of two was found.
[en] Superconducting epitaxial FeSe0.5Te0.5 thin films are prepared on SrTiO3(001) substrates by pulsed laser deposition. The high purity of the phase, the quality of the growth and the epitaxy are studied with different experimental techniques: x-rays diffraction, reflection high energy electron diffraction, scanning tunneling microscopy and atomic force microscopy. The substrate temperature during the deposition is found to be the main parameter governing sample morphology and superconducting critical temperature. Films obtained under optimal conditions show an epitaxial growth with the c axis perpendicular to the film surface and the a and b axes parallel to the substrate, without evidence of any other orientation. Moreover, such films exhibit a metallic behavior over the whole measured temperature range and the critical temperature is above 17 K, which is higher than the target value.
[en] The goal of this work is to study the evolution of thermoelectric transport across the members of the Ruddlesden–Popper series iridates , where a metal–insulator transition driven by bandwidth change occurs, from the strongly insulating Sr2IrO4 to the metallic non Fermi liquid behavior of SrIrO3. Sr2IrO4 (), Sr3Ir2O7 () and SrIrO3 () polycrystals are synthesized at high pressure and characterized by structural, magnetic, electric and thermoelectric transport analyses. We find a complex thermoelectric phenomenology in the three compounds. Thermal diffusion of charge carriers accounts for the Seebeck behavior of Sr2IrO4, whereas additional drag mechanisms come into play in determining the Seebeck temperature dependence of Sr3Ir2O7 and SrIrO3. These findings reveal a close relationship between magnetic, electronic and thermoelectric properties, strong coupling of charge carriers with phonons and spin fluctuations as well as the relevance of multiband description in these compounds. (paper)
[en] Full text: Magnetic nanoparticles embedded in multi walled carbon nanotubes display nowadays a continuous increasing technological interest due to their possibility to be used in magnetic storage devices, medical imaging technologies, quantum computing and other industrial applications as magnetic sensors and permanent magnets. We present here the synthesis of multi walled carbon nanotubes (MWCNT) grown from magnetic nanoparticles used as catalyst. The starting magnetic nanoparticles were synthesized according to the method developed by Hou and Gao  and the MWCNT obtained in a catalytic micro reactor through the complete decomposition of methane as CH4(g) → C (nano) + 2H2(gas) Extended physical characterisation was performed on MWCNT with nickel nanoparticles. TEM analysis indicated a wide size distribution of particles centered around two mean values of about 4 and 9 nanometers. DC magnetic measurements gave evidence of superparamagnetic behaviour of the system above the blocking temperature TB , i.e. the scaling of the magnetisation vs. H/T and the fit by a double Langevin equation confirming thus the bimodal size distribution already observed by TEM. Below TB = 68 K , obtained by the temperature dependence of the coercive field Hc, the system has the hysteretic behaviour typical of a ferromagnet, confirmed also by the field cooled (FC) and zero field cooled (ZFC) measurements at different applied magnetic fields. Preliminary results on MWCNT embedded with Sm2Co17 nanoparticles will be also presented and compared. References:  Y. Hou, S.Gao, J. Mater.Chem. 13 (2003) 1510  F.C.Fonseca, G.F.Goya, R.F.Jardim, R.Muccillo, N.L.V. Carreno, E.Longo, E.R.Leite, Phys.rev.B 66 (2002) 1044