126 research outputs found
Angular dependence of magnetoresistivity in c-oriented MgB2 thin film
The anisotropy of MgB2 is still under debate: its value, strongly dependent
on the sample and on the measuring method, ranges between 1.2 and 13. In this
work we present our results on a MgB2 c-oriented superconducting thin film. To
evaluate the anisotropy, we followed two different approaches. Firstly,
magnetoresistivity was measured as a function of temperature at selected
magnetic fields applied both parallel and perpendicular to the c-axis;
secondly, we measured magnetoresistivity at selected temperatures and magnetic
fields, varying the angle q between the magnetic field and the c-axis. The
anisotropy estimated from the ratio between the upper critical fields parallel
and perpendicular to the c-axis and the one obtained in the framework of the
scaling approach within the anisotropic Ginzburg-Landau theory are different
but show a similar trend in the temperature dependence. The obtained results
are compared and discussed in the light of the two-band nature of MgB2. A
comparison between critical fields in thin films and single crystal is also
performed.Comment: 13 pages, 4 figures, European Physical Journal B in pres
Tc=21K in epitaxial FeSe0.5Te0.5 thin films with biaxial compressive strain
High purity epitaxial FeSe0.5Te0.5 thin films with different thickness were
grown by Pulsed Laser Ablation on different substrates. By varying the film
thickness, Tc up to 21K were observed, significantly larger than the bulk
value. Structural analyses indicated that the a axis changes significantly with
the film thickness and is linearly related to the Tc. The latter result
indicates the important role of the compressive strain in enhancing Tc. Tc is
also related to both the Fe-(Se,Te) bond length and angle, suggesting the
possibility of further enhancement
Thermoelectric behavior of Ruddlesden-Popper series iridates
The goal of this work is studying the evolution of thermoelectric transport
across the members of the Ruddlesden-Popper series iridates Srn+1IrnO3n+1,
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 (n=1), Sr3Ir2O7 (n=2) and SrIrO3 (n=inf.) 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 close
relationship between magnetic, electronic and thermoelectric properties, strong
coupling of charge carriers with phonons and spin fluctuations as well as
relevance of multiband description in these compounds.Comment: main paper + supplementary informatio
Crossover between magnetism and superconductivity in low H-doped LaFeAsO
By a systematic study of the hydrogen-doped LaFeAsO system by means of dc
resistivity, dc magnetometry, and muon-spin spectroscopy we addressed the
question of universality of the phase diagram of rare-earth-1111 pnictides. In
many respects, the behaviour of LaFeAsO_(1-x)H_(x) resembles that of its widely
studied F-doped counterpart, with H^- realizing a similar (or better)
electron-doping in the LaO planes. In a x = 0.01 sample we found a long-range
SDW order with T_n = 119 K, while at x = 0.05 the SDW establishes only at 38 K
and, below T_c = 10 K, it coexists at a nanoscopic scale with bulk
superconductivity. Unlike the abrupt M-SC transition found in the parent
La-1111 compound, the presence a crossover region makes the H-doped system
qualitatively similar to other Sm-, Ce-, or Nd-1111 families.Comment: to appear in Journal of Physics: Condensed Matte
The Crossover beteween Aslamazov-Larkin and Short Wavelength Fluctuations Regimes in HTS Conductivity Experiments
We present paraconductivity (AL) measurements in three different high
temperature superconductors: a melt textured sample, a
epitaxial thin film and a highly textured
tape. The crossovers between different temperature
regimes in excess conductivity have been analysed. The Lawrence-Doniach (LD)
crossover, which separates the 2D and 3D regimes, shifts from lower to higher
temperatures as the compound anisotropy decreases. Once the LD crossover is
overcome, the fluctuation conductivity of the three compounds shows the same
universal behaviour: for all the curves bend down
according to the law. This asymptotic behaviour was
theoretically predicted previously for the high temperature region where the
short wavelength fluctuations (SWF) become important.Comment: 4 pages, Revtex, 1 PostScript figure available upon request
([email protected]); submitted to Physical Review B rapid communication
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