250 research outputs found
Role of Dirac cones in magnetotransport properties of REFeAsO (RE=rare earth) oxypnictides
In this work we study the effect of the rare earth element in iron
oxypnictides of composition REFeAsO (RE=rare earth). On one hand we carry out
Density Functional Theory calculations of the band structure, which evidence
the multiband character of these compounds and the presence of Dirac cones
along the Y-{\Gamma} and Z-R directions of the reciprocal space. On the other
hand, we explore transport behavior by means of resistivity, Hall resistance
and magnetoresistance measurements, which confirm the dominant role of Dirac
cones. By combining our theoretical and experimental approaches, we extract
information on effective masses, scattering rates and Fermi velocities for
different rare earth elements.Comment: 13 pages, 5 figures accepted for publication on European Journal of
Physics
Large phonon-drag enhancement induced by narrow quantum confinement at the LaAlO3/SrTiO3 interface
The thermoelectric power of the two-dimensional electron system (2DES) at the
LaAlO3/SrTiO3 interface is explored below room temperature, in comparison with
that of Nb-doped SrTiO3 single crystals. For the interface we find a region
below T =50 K where thermopower is dominated by phonon-drag, whose amplitude is
hugely amplified with respect to the corresponding bulk value, reaching values
~mV/K and above. The phonon-drag enhancement at the interface is traced back to
the tight carrier confinement of the 2DES, and represents a sharp signature of
strong electron-acoustic phonon coupling at the interface
Cu_{2}O as nonmagnetic semiconductor for spin transport in crystalline oxide electronics
We probe spin transport in Cu_{2}O by measuring spin valve effect in
La_{0.7}Sr_{0.3}MnO_{3}/Cu_{2}O/Co and
La_{0.7}Sr_{0.3}MnO_{3}/Cu_{2}O/La_{0.7}Sr_{0.3}MnO_{3} epitaxial
heterostructures. In La_{0.7}Sr_{0.3}MnO_{3}/Cu_{2}O/Co systems we find that a
fraction of out-of-equilibrium spin polarized carrier actually travel across
the Cu_{2}O layer up to distances of almost 100 nm at low temperature. The
corresponding spin diffusion length dspin is estimated around 40 nm.
Furthermore, we find that the insertion of a SrTiO_{3} tunneling barrier does
not improve spin injection, likely due to the matching of resistances at the
interfaces. Our result on dspin may be likely improved, both in terms of
Cu_{2}O crystalline quality and sub-micrometric morphology and in terms of
device geometry, indicating that Cu_{2}O is a potential material for efficient
spin transport in devices based on crystalline oxides.Comment: 15 pages, 10 figure
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
Critical Field of MGB2 : Crossover from Clean to Dirty Regimes
We have studied the upper critical field, Bc2, in poly-crystalline MgB2
samples in which disorder was varied in a controlled way to carry selectively p
and s bands from clean to dirty limit. We have found that the clean regime
survives when p bands are dirty and s bands are midway between clean and dirty.
In this framework we can explain the anomalous behaviour of Al doped samples,
in which Bc2 decreases as doping increases.Comment: 11 pages, 2 figure
Low-Temperature Rapid Synthesis and Superconductivity of Fe-Based Oxypnictide Superconductors
we were able to develop a novel method to synthesize Fe-based oxypnictide
superconductors. By using LnAs and FeO as the starting materials and a
ball-milling process prior to solid-state sintering, Tc as high as 50.7 K was
obtained with the sample of Sm 0.85Nd0.15FeAsO0.85F0.15 prepared by sintering
at temperatures as low as 1173 K for times as short as 20 min.Comment: 2 pages,2 figures, 1 tabl
Giant Oscillating Thermopower at Oxide Interfaces
Understanding the nature of charge carriers at the LaAlO3/SrTiO3 interface is
one of the major open issues in the full comprehension of the charge
confinement phenomenon in oxide heterostructures. Here, we investigate
thermopower to study the electronic structure in LaAlO3/SrTiO3 at low
temperature as a function of gate field. In particular, under large negative
gate voltage, corresponding to the strongly depleted charge density regime,
thermopower displays record-high negative values of the order of 10^4 - 10^5
microV/K, oscillating at regular intervals as a function of the gate voltage.
The huge thermopower magnitude can be attributed to the phonon-drag
contribution, while the oscillations map the progressive depletion and the
Fermi level descent across a dense array of localized states lying at the
bottom of the Ti 3d conduction band. This study is the first direct evidence of
a localized Anderson tail in the two-dimensional (2D) electron liquid at the
LaAlO3/SrTiO3 interface.Comment: Main text: 28 pages and 3 figures; Supplementary information: 29
pages, 5 figures and 1 tabl
Clean and Dirty Superconductivity in Pure, Al doped, and Neutron Irradiated MgB2: a Far-Infrared Study
The effects of Al substitution and neutron irradiation on the conduction
regime (clean or dirty) of the - and -band of MgB have been
investigated by means of far-infrared spectroscopy. The intensity reflected by
well characterized polycrystalline samples was measured up to 100 cm in
both normal and superconducting state. The analysis of the superconducting to
normal reflectivity ratios shows that only the effect of the opening of the
small gap in the dirty -band can be clearly observed in pure MgB,
consistently with previous results. In Al-doped samples the dirty character of
the -band is increased, while no definitive conclusion on the conduction
regime of the -band can be drawn. On the contrary, results obtained
for the irradiated sample show that the irradiation-induced disorder drives the
-band in the dirty regime, making the large gap in -band
observable for the first time in far-infrared measurements.Comment: 11 pages, 1 figur
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