9,863 research outputs found
X-ray photoelectron spectroscopy studies of non-stoichiometric superconducting NbB2+x
Polycrystalline samples of NbB2+x with nominal composition (B/Nb) = 2.0, 2.1,
2.2, 2.3, 2.4 and 2.5 were studied by X-ray photoelectron spectroscopy (XPS).
The spectra revealed Nb and B oxides on the surface of the samples, mainly B2O3
and Nb2O5. After Ar ion etching the intensity of Nb and B oxides decreased. The
Nb 3d5/2 and B 1s core levels associated with the chemical states (B/Nb) were
identified and they do not change with etching time. The Binding Energy of the
Nb 3d5/2 and B 1s core levels increase as boron content increases, suggesting a
positive chemical shift in the core levels. On the other hand, analysis of
Valence Band spectra showed that the contribution of the Nb 4d states slightly
decreased while the contribution of the B 2p(pi) states increased as the boron
content increased. As a consequence, the electronic and superconducting
properties were substantially modified, in good agreement with band-structure
calculations.Comment: 10 pages, 7 figures, 1 tabl
Cosmic magnetic fields and dark energy in extended electromagnetism
We discuss an extended version of electromagnetism in which the usual gauge
fixing term is promoted into a physical contribution that introduces a new
scalar state in the theory. This new state can be generated from vacuum quantum
fluctuations during an inflationary era and, on super-Hubble scales, gives rise
to an effective cosmological constant. The value of such a cosmological
constant coincides with the one inferred from observations as long as inflation
took place at the electroweak scale. On the other hand, the new state also
generates an effective electric charge density on sub-Hubble scales that
produces both vorticity and magnetic fields with coherent lengths as large as
the present Hubble horizon.Comment: 4 pages, 2 figures. Contribution to the proceedings of Spanish
Relativity Meeting 2010, Granada, Spain, 6-10 September 201
The Peierls substitution in an engineered lattice potential
Artificial gauge fields open new possibilities to realize quantum many-body
systems with ultracold atoms, by engineering Hamiltonians usually associated
with electronic systems. In the presence of a periodic potential, artificial
gauge fields may bring ultracold atoms closer to the quantum Hall regime. Here,
we describe a one-dimensional lattice derived purely from effective
Zeeman-shifts resulting from a combination of Raman coupling and radiofrequency
magnetic fields. In this lattice, the tunneling matrix element is generally
complex. We control both the amplitude and the phase of this tunneling
parameter, experimentally realizing the Peierls substitution for ultracold
neutral atoms.Comment: 6 pages, 5 figure
The onset of solar cycle 24: What global acoustic modes are telling us
We study the response of the low-degree, solar p-mode frequencies to the
unusually extended minimum of solar surface activity since 2007. A total of
4768 days of observations collected by the space-based, Sun-as-a-star
helioseismic GOLF instrument are analyzed. A multi-step iterative
maximum-likelihood fitting method is applied to subseries of 365 days and 91.25
days to extract the p-mode parameters. Temporal variations of the l=0, 1, and 2
p-mode frequencies are then obtained from April 1996 to May 2009. While the
p-mode frequency shifts are closely correlated with solar surface activity
proxies during the past solar cycles, the frequency shifts of the l=0 and l=2
modes show an increase from the second half of 2007, when no significant
surface activity is observable. On the other hand, the l=1 modes follow the
general decreasing trend of the solar surface activity. The different
behaviours between the l=0 and l=2 modes and the l=1 modes can be interpreted
as different geometrical responses to the spatial distribution of the solar
magnetic field beneath the surface of the Sun. The analysis of the low-degree,
solar p-mode frequency shifts indicates that the solar activity cycle 24
started late 2007, despite the absence of activity on the solar surface.Comment: To be accepted by A&A (with minor revisions), 4 pages, 3 figures, 1
tabl
Tailoring magnetic anisotropy in epitaxial half metallic La0.7Sr0.3MnO3 thin films
We present a detailed study on the magnetic properties, including anisotropy,
reversal fields, and magnetization reversal processes, of well characterized
half-metallic epitaxial La0.7Sr0.3MnO3 (LSMO) thin films grown onto SrTiO3
(STO) substrates with three different surface orientations, i.e. (001), (110)
and (1-18). The latter shows step edges oriented parallel to the [110]
(in-plane) crystallographic direction. Room temperature high resolution
vectorial Kerr magnetometry measurements have been performed at different
applied magnetic field directions in the whole angular range. In general, the
magnetic properties of the LSMO films can be interpreted with just the uniaxial
term with the anisotropy axis given by the film morphology, whereas the
strength of this anisotropy depends on both structure and film thickness. In
particular, LSMO films grown on nominally flat (110)-oriented STO substrates
presents a well defined uniaxial anisotropy originated from the existence of
elongated in-plane [001]-oriented structures, whereas LSMO films grown on
nominally flat (001)-oriented STO substrates show a weak uniaxial magnetic
anisotropy with the easy axis direction aligned parallel to residual substrate
step edges. Elongated structures are also found for LSMO films grown on vicinal
STO(001) substrates. These films present a well-defined uniaxial magnetic
anisotropy with the easy axis lying along the step edges and its strength
increases with the LSMO thickness. It is remarkable that this step-induced
uniaxial anisotropy has been found for LSMO films up to 120 nm thickness. Our
results are promising for engineering novel half-metallic magnetic devices that
exploit tailored magnetic anisotropy.Comment: 10 pages, 10 figures, 1 tabl
A 3D geological model of Campo de Cartagena, SE Spain : Hydrogeological implications
Knowledge and understanding of geologic basins for hydrogeologic purposes requires an accurate 3D geological architecture representation. For model building, surface and subsurface data integration with the interpretation of geophysical survey and lithologic logs is needed. A methodology to reconstruct the geometric architecture of the sedimentary basin and relationships among stratigraphic formations, as well as to define hydrostratigraphic units, has been applied to the Campo de Cartagena Neogene formations. Data analysis included seismic reflection profiles and gravimetric data from oil exploration, electric resistivity surveys and 491 lithologic logs. The 3D model obtained from a close integration of stratigraphic and geophysical data was generated through a computerbased tool. It presents a common framework and a good starting point for hydrogeologic applications
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