134 research outputs found
Hydrogen site occupancy and strength of forces in nano-sized metal hydrides
The dipole force components in nano-sized metal hydrides are quantitatively
determined with curvature and x-ray diffraction measurements. Ab-initio density
functional theory is used to calculate the dipole components and the symmetry
of the strain field. The hydrogen occupancy in a 100 nm thick V film is shown
to be tetrahedral with a slight asymmetry at low concentration and a transition
to octahedral occupancy is shown to take place at around 0.07 [H/V] at 360 K.
When the thickness of the V layer is reduced to 3 nm and biaxially strained, in
a Fe_0.5V_0.5/V superlattice, the hydrogen unequivocally occupies octahedral
z-like sites, even at and below concentrations of 0.02 [H/V]
Mg/Ti multilayers: structural, optical and hydrogen absorption properties
Mg-Ti alloys have uncommon optical and hydrogen absorbing properties,
originating from a "spinodal-like" microstructure with a small degree of
chemical short-range order in the atoms distribution. In the present study we
artificially engineer short-range order by depositing Pd-capped Mg/Ti
multilayers with different periodicities and characterize them both
structurally and optically. Notwithstanding the large lattice parameter
mismatch between Mg and Ti, the as-deposited metallic multilayers show good
structural coherence. Upon exposure to H2 gas a two-step hydrogenation process
occurs, with the Ti layers forming the hydride before Mg. From in-situ
measurements of the bilayer thickness L at different hydrogen pressures, we
observe large out-of-plane expansions of the Mg and Ti layers upon
hydrogenation, indicating strong plastic deformations in the films and a
consequent shortening of the coherence length. Upon unloading at room
temperature in air, hydrogen atoms remain trapped in the Ti layers due to
kinetic constraints. Such loading/unloading sequence can be explained in terms
of the different thermodynamic properties of hydrogen in Mg and Ti, as shown by
diffusion calculations on a model multilayered systems. Absorption isotherms
measured by hydrogenography can be interpreted as a result of the elastic
clamping arising from strongly bonded Mg/Pd and broken Mg/Ti interfaces
Observation by resonant angle-resolved photoemission of a critical thickness for 2-dimensional electron gas formation in SrTiO embedded in GdTiO
For certain conditions of layer thickness, the interface between GdTiO
(GTO) and SrTiO (STO) in multilayer samples has been found to form a
two-dimensional electron gas (2DEG) with very interesting properties including
high mobilities and ferromagnetism. We have here studied two trilayer samples
of the form [2 nm GTO/1.0 or 1.5 unit cells STO/10 nm GTO] as grown on (001)
(LaAlO)(SrAlTaO) (LSAT), with the STO layer
thicknesses being at what has been suggested is the critical thickness for 2DEG
formation. We have studied these with Ti-resonant angle-resolved (ARPES) and
angle-integrated photoemission and find that the spectral feature in the
spectra associated with the 2DEG is present in the 1.5 unit cell sample, but
not in the 1.0 unit cell sample. We also observe through core-level spectra
additional states in Ti and Sr, with the strength of a low-binding-energy state
for Sr being associated with the appearance of the 2DEG, and we suggest it to
have an origin in final-state core-hole screening.Comment: 12 pages, 4 figure
The hyperfine properties of a hydrogenated Fe/V superlattice
: We study the effect of hydrogen on the electronic, magnetic and hyperfine
structures of an iron-vanadium superlattice consisting of three Fe monolayers
and nine V monolayers. The contact charge density ({\rho}), the contact
hyperfine field (Bhf) and the electronic field gradient (EFG) at the Fe sites
for different H locations and H fillings are calculated using the first
principle full-potential linear-augmented-plane-wave (FP-LAPW) method . It is
found that sizeable changes in the hyperfine properties are obtained only when
H is in the interface region.Comment: 6 pages, 2 figures, 3 tables, ICAME 2011 conference (Kobe, Japan
Depth-Resolved Composition and Electronic Structure of Buried Layers and Interfaces in a LaNiO/SrTiO Superlattice from Soft- and Hard- X-ray Standing-Wave Angle-Resolved Photoemission
LaNiO (LNO) is an intriguing member of the rare-earth nickelates in
exhibiting a metal-insulator transition for a critical film thickness of about
4 unit cells [Son et al., Appl. Phys. Lett. 96, 062114 (2010)]; however, such
thin films also show a transition to a metallic state in superlattices with
SrTiO (STO) [Son et al., Appl. Phys. Lett. 97, 202109 (2010)]. In order to
better understand this transition, we have studied a strained LNO/STO
superlattice with 10 repeats of [4 unit-cell LNO/3 unit-cell STO] grown on an
(LaAlO)(SrAlTaO) substrate using soft x-ray
standing-wave-excited angle-resolved photoemission (SWARPES), together with
soft- and hard- x-ray photoemission measurements of core levels and
densities-of-states valence spectra. The experimental results are compared with
state-of-the-art density functional theory (DFT) calculations of band
structures and densities of states. Using core-level rocking curves and x-ray
optical modeling to assess the position of the standing wave, SWARPES
measurements are carried out for various incidence angles and used to determine
interface-specific changes in momentum-resolved electronic structure. We
further show that the momentum-resolved behavior of the Ni 3d eg and t2g states
near the Fermi level, as well as those at the bottom of the valence bands, is
very similar to recently published SWARPES results for a related
LaSrMnO/SrTiO superlattice that was studied using the
same technique (Gray et al., Europhysics Letters 104, 17004 (2013)), which
further validates this experimental approach and our conclusions. Our
conclusions are also supported in several ways by comparison to DFT
calculations for the parent materials and the superlattice, including
layer-resolved density-of-states results
General Formula for the Thermoelectric Transport Phenomena based on the Fermi Liquid Theory: Thermopower, Nernst Coefficient, and Thermal Conductivity
On the basis of the linear response transport theory, the general expressions
for the thermoelectric transport coefficients, such as thermoelectric power
(S), Nernst coefficient (\nu), and thermal conductivity (\kappa), are derived
by using the Fermi liquid theory. The obtained expression is exact as for the
most singular term in terms of 1/\gamma_k^* (\gamma_k^* being the quasiparticle
damping rate). We utilize the Ward identities for the heat current which is
derived by the local energy conservation law. Based on the derived expressions,
we can calculate various thermoelectric transport coefficients within the
framework of the Baym-Kadanoff type conserving approximation. Thus, the present
expressions are very useful for studying the strongly correlated electrons such
as high-Tc superconductors, organic metals, and heavy Fermion systems, where
the current vertex corrections are expected to play important roles. By using
the derived expression, we calculate the thermal conductivity \kappa in a
free-dispersion model up to the second-order with respect to U. We find that it
is slightly enhanced due to the vertex correction for the heat current,
although the vertex correction for electron current makes the conductivity
(\sigma) of this system diverge, reflecting the absence of the Umklapp process.Comment: 22 pages, 11 figures; accepted for publication in PR
Radiographic closure time of appendicular growth plates in the Icelandic horse
<p>Abstract</p> <p>Background</p> <p>The Icelandic horse is a pristine breed of horse which has a pure gene pool established more than a thousand years ago, and is approximately the same size as living and extinct wild breeds of horses. This study was performed to compare the length of the skeletal growth period of the "primitive" Icelandic horse relative to that reported for large horse breeds developed over the recent centuries. This information would provide practical guidance to owners and veterinarians as to when the skeleton is mature enough to commence training, and would be potentially interesting to those scientists investigating the pathogenesis of osteochondrosis. Interestingly, osteochondrosis has not been documented in the Icelandic horse.</p> <p>Methods</p> <p>The radiographic closure time of the appendicular growth plates was studied in 64 young Icelandic horses. The results were compared with previously published closure times reported for other, larger horse breeds. The radiographs were also examined for any signs of developmental orthopaedic diseases. In order to describe further the growth pattern of the Icelandic horse, the total serum alkaline phosphatase (ALP) activity was determined and the height at the withers was measured.</p> <p>Results</p> <p>Most of the examined growth plates were fully closed at the age of approximately three years. The horses reached adult height at this age; however ALP activity was still mildly increased over baseline values. The growth plates in the digits were the first to close at 8.1 to 8.5 months of age, and those in the regions of the distal radius (27.4 to 32.0 months), tuber olecrani (31.5 to 32.2 months), and the stifle (27.0 to 40.1 months) were the last to close. No horse was found to have osteochondrosis type lesions in the neighbouring joints of the evaluated growth plates.</p> <p>Conclusion</p> <p>The Icelandic horse appears to have similar radiographic closure times for most of the growth plates of its limbs as reported for large new breeds of horses developed during the past few centuries. It thus appears that different breeding goals and the intensity of breeding have not altered the length of the growth period in horses. Instead, it can be assumed that the pristine and relatively small Icelandic horse has a slower rate of growth. The appendicular skeleton of Icelandic horses has completed its bone growth in length at approximately 3 years of age, and therefore may be able to enter training at this time.</p
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