1,779 research outputs found
Anisotropic Dirac cones in monatomic hexagonal lattices
In the last few years, the fascinating properties of graphene have been
thoroughly investigated. The existence of Dirac cones is the most important
characteristic of the electronic band-structure of graphene. In this
theoretical paper, hexagonal monolayers of silicon (h-Si) and germanium (h-Ge)
are examined using density functional theory, within the generalized gradient
approximation. Our numerical results indicate that both h-Si and h-Ge are
chemically stable. The lattice parameters, electronic dispersion relations and
densities of states for these systems are reported. The electronic dispersion
relations display Dirac cones with the symmetry of an equilateral triangle (the
group D) in the vicinity of the K points. Hence, the Fermi velocity depends
on the wave vector direction around points. Fermi velocities for holes and
electrons are significantly different. The maximum and minimum Fermi velocities
are also reported.Comment: 7 pages, 9 figures. Accepted for publication in The European Physical
Journal B (EPJB
Twin disc assessment of wear regime transitions and rolling contact fatigue in R400HT – E8 pairs
Twin disc tests were carried out to evaluate the wear resistance and Rolling Contact Fatigue (RCF) of premium R400HT rail samples in contact with E8 wheel samples. The wear rate and friction coefficient were correlated with the frictional work expended at the contact interface (the Tgamma approach). Accelerated RCF tests were also carried out on the premium R400HT rail and the results were compared to those obtained for standard R260 rail. The wear rates of rail samples were consistently lower than those reported in the literature for other contacting pairs in which the rail material studied is softer than R400HT. Also, the energy needed for the transition from the moderate to severe wear regime significantly increased for the hardened rail. Fatigue cracks were shallower for R400HT when compared with standard rail material. Hardened rails also showed lower mean spacing between fatigue cracks. This new information can be used to improve wear simulations of wheels and rails by using more realistic wear equations
Metallic spin-glasses beyond mean-field: An approach to the impurity-concentration dependence of the freezing temperature
A relation between the freezing temperature () and the exchange
couplings () in metallic spin-glasses is derived, taking the
spin-correlations () into account. This approach does not involve a
disorder-average. The expansion of the correlations to first order in
leads to the molecular-field result from
Thouless-Anderson-Palmer. Employing the current theory of the spin-interaction
in disordered metals, an equation for as a function of the
concentration of impurities is obtained, which reproduces the available data
from {\sl Au}Fe, {\sl Ag}Mn, and {\sl Cu}Mn alloys well.Comment: 4 figures. This is a strongly revised version, where several aspects
have been improved, and the equation for the freezing temperature has been
refined. It is equivalent to the published version in J. Phys.: Condens.
Matter 25 (2013) 13600
On the existence of supersolid helium-4 monolayer films
Extensive Monte Carlo simulations of helium-4 monolayer films adsorbed on
weak substrates have been carried out, aimed at ascertaining the possible
occurrence of a quasi-two-dimensional supersolid phase. Only crystalline films
not registered with underlying substrates are considered. Numerical results
yield strong evidence that helium-4 will not form a supersolid film on {any}
substrate strong enough to stabilize a crystalline layer. On weaker substrates,
continuous growth of a liquid film takes place
Regulation of lamp2a levels in the lysosomal membrane
The selective degradation of cytosolic proteins in lysosomes by chaperone-mediated autophagy depends, at least in part, on the levels of a substrate receptor at the lysosomal membrane. We have previously identified this receptor as the lysosome-associated membrane protein type 2a (lamp2a) and showed that levels of lamp2a at the lysosomal membrane directly correlate with the activity of the proteolytic pathway. Here we show that levels of lamp2a at the lysosomal membrane are mainly controlled by changes in its half-life and its distribution between the lysosomal membrane and the matrix. The lysosomal degradation of lamp2a requires the combined action of at least two different proteolytic activities at the lysosomal membrane. Lamp2a is released from the membrane by the action of these proteases, and then the truncated lamp2a is rapidly degraded within the lysosomal matrix. Membrane degradation of lamp2a is a regulated process that is inhibited in the presence of substrates for chaperone-mediated autophagy and under conditions that activate that type of autophagy. Uptake of substrate proteins also results in transport of some intact lamp2a from the lysosomal membrane into the matrix. This fraction of lamp2a can be reinserted back into the lysosomal membrane. The traffic of lamp2a through the lysosomal matrix is not mediated by vesicles, and lamp2a reinsertion requires the lysosomal membrane potential and protein components of the lysosomal membrane. The distribution of lamp2a between the lysosomal membrane and matrix is a dynamic process that contributes to the regulation of lysosomal membrane levels of lamp2a and consequently to the activity of the chaperone-mediated autophagic pathway
Biobased polymers derived from itaconic acid bearing clickable groups with potent antibacterial activity and negligible hemolytic activity.
Herein, we report, for the first time, the synthesis of clickable polymers derived from biobased itaconic
acid, which was then used for the preparation of novel cationic polymers with antibacterial properties and
low hemotoxicity via click chemistry. Itaconic acid (IA) was subjected to chemical modification by incorporating
clickable alkyne groups on the carboxylic acids. The resulting monomer with pendant alkyne
groups was easily polymerized and copolymerized with dimethyl itaconate (DMI) by radical polymerization.
The feed molar ratio of comonomers was varied to precisely tune the content of alkyne groups in
the copolymers and the amphiphilic balance. Subsequently, an azide with a thiazole group, which is a
component of the vitamin thiamine (B1), was attached onto the polymers by copper-catalyzed azidealkyne
cycloaddition (CuAAC) click chemistry leading to triazole linkages. N-Alkylation reactions of the
thiazole and triazole groups with methyl and butyl iodides provide the corresponding itaconate derivatives
with pendant azolium groups. The copolymers with variable cationic charge densities and hydrophobic/
hydrophilic balances, depending on the comonomer feed ratio, display potent antibacterial activity
against Gram-positive bacteria, whereas the activity was almost null against Gram-negative bacteria.
Hemotoxicity assays demonstrated that the copolymers exhibited negligible hemolysis and excellent
selectivity, more than 1000-fold, for Gram-positive bacteria over human red blood cells.post-print1945 K
Influencia de la inclinación acetabular en la luxación de la artroplastia parcial de cadera
La luxación de la prótesis parcial de cadera, representa en la actualidad un grave problema de incapacidad para el paciente, debido no solo, a la complicación de la marcha en sÃ, sino a todas aquellas derivadas de la estancia hospitalaria de estos pacientes en nuestros centros. Respecto a la vÃa de abordaje, se ha sentado una mayor incidencia de luxaciones en la vÃa posterolateral, frente a las anteriores o laterales transtrocantéreas en decúbito supino. Objetivo: evaluar la influencia del ángulo de inclinación anatómico del acetábulo, techo y cobertura en la luxación de la artroplastia parcial de cadera. Material y método: el estudio incluyó un total de 718 pacientes intervenidos por fractura cervical de fémur, durante los años de 1992 a 2005. Las prótesis parciales implantadas fueron del modelo Thompson; los tipos de vÃas de abordaje quirúrgico utilizados fueron; la anterior de Smith Petersen, y la posterior de Gibson Moore. Se realizaron las mediciones radiográficas del ángulo formado entre las lÃnea trazada en el borde externo del acetábulo y la lÃnea biisquiática.
Resultados: Del total de pacientes incluidos en el estudio el número de luxaciones fue de 24 prótesis, lo que representa el 3,3 %. La edad media fue de 84 años, y un intervalo de 76 a 92 años. El sexo predominante fue el femenino con un 83%. La vÃa más utilizada fue la anterior con un 67 %. El mayor porcentaje de luxación fue para aquellas pacientes con un ángulo de inclinación anatómico del acetábulo de > 55º. El tiempo medio seguimiento hasta producirse la luxación fue de 35 dÃas y un intervalo entre 1 a 89 dÃas. El tratamiento más frecuente fue el conservador 88%.
Conclusión: El presente estudio demuestra que la luxación temprana de una prótesis parcial de cadera está directamente relacionado con un ángulo de inclinación anatómico del acetábulo mayor de 55º.The dislocation of partial hip replacements
represents an important postoperative complication. Apart from
the most important problem which is patient mobility, another
aspect is the prolonged hospital stay detrimental to patient health
and the evonawy of hospital. With respect to the incidence of pos-
toperative dislocations and the type of surgical approach, it seated
that there is a greater incidence using the lateroposterior hip sur-
gical approach, in comparision to the anterior or lateral access.
Objective: To evidence the influence of acetabular inclination in
postoperative dislocations of partial hip arthroplasty. Methods: A
study involuing 718 cases of intervened femoral neck fractures
using Thompson endoprosthesis during the period including 1992 to
2005.With respect to type of surgical approaches, these included
the Smith-Petersen anterior and the Gibson-Moore posterior
access. The acetabular inclination was measurements radiologi-
cally, obtaining the angle between the bi-ischial line and the exter-
nal border of the acetabulum. Results: A total of twentyfour (24)
prosthesis suffered dislocation, with respect to the total of seven
hundred and eighteen (718) patients intervened for partial hip arth-
roplasty, representing 3,3% of the total.
The average age for dislocated arthroplasties was 84 years, with a
range between 76-92 years, and the predominant sex was female.
Concerning surgical access, the anterior was the most frecuently
used (67%). There was found to be a greater percentage of dislo-
cated arthroplasties when the angle of acetabular inclination exce-
eded fifty five (55º) degrees. The average time elapsed before dis-
location was diagnosed included a 35 days time period, with a
range between to 89 days. The most frequent management of pos-
toperative dislocations was conservative treatment. Conclusion:
This study demonstrates that when the acetabular inclination is
greater than 55º, there is a greater incidence of early dislocation of
partial hip replacements
Food consumption analysis in Spanish elderly based upon the Mini Nutritional Assessment Test
Background/Aims: Aged people are an increasing population group worldwide, and nutritional impairments may contribute to additional health problems. The characterization of food consumption of elderly people is a good approach to implementing adequate nutritional policies in order to improve their nutritional status. The aim of this study was to describe and analyze specific aspects of food intake based upon the Mini Nutritional Assessment (MNA) test in a representative country sample.
Methods: The survey consisted of a cross-sectional study assessing the nutritional status of 22,007 Spanish people aged 65 years or older using the MNA test. These data, including dietary information, were obtained by health professionals specifically trained to carry out the questionnaire. A multiple regression analysis was used to evaluate the contributing impact of the diet-related MNA questions on the total MNA score.
Results: Differences in the food choices between men and women were found, but not in the mode of feeding. Obese and non-obese individuals, categorized by a body mass index (BMI) more or less than 30, showed similar intake patterns of protein-rich foods, but differences in fruits, vegetables and fluids. On the other hand, undernourished and non-undernourished subjects, according to the MNA criteria, revealed different food consumption. Loss of appetite and mode of feeding were the items with the highest influence the MNA total score.
Conclusions: While age, gender and BMI considered together are responsible for 11.3% of the total MNA score, dietary-related items can predict the 62.4% of the total MNA classification in the overall elderly Spanish population
On the possible "supersolid" character of parahydrogen clusters
We present results of a theoretical study of structural and superfluid
properties of parahydrogen clusters comprising 25, 26 and 27 molecules at low
temperature. The microscopic model utilized here is based on the
Silvera-Goldman pair potential. Numerical results are obtained by means of
Quantum Monte Carlo simulations, making use of the continuous-space Worm
Algorithm. The clusters are superfluid in the low temperature limit, but
display markedly different physical behaviours. For N=25 and 27, superfluidity
at low temperature arises as clusters melt, i.e., become progressively
liquid-like as a result of quantum effects. On the other hand, for N = 26 the
cluster remains rigid and solid-like. We argue that this cluster can be
regarded as a mesoscopic "supersolid". This physical picture is supported by
results of simulations in which a single parahydrogen molecule in the cluster
is isotopically substituted.Comment: 18 pages, 7 figure
Solidification of small para-H2 clusters at zero temperature
We have determined the ground-state energies of para-H clusters at zero
temperature using the diffusion Monte Carlo method. The liquid or solid
character of each cluster is investigated by restricting the phase through the
use of proper importance sampling. Our results show inhomogeneous
crystallization of clusters, with alternating behavior between liquid and solid
phases up to N=55. From there on, all clusters are solid. The ground-state
energies in the range N=13--75 are established and the stable phase of each
cluster is determined. In spite of the small differences observed between the
energy of liquid and solid clusters, the corresponding density profiles are
significantly different, feature that can help to solve ambiguities in the
determination of the specific phase of H clusters.Comment: 17 pages, accepted for publication in J. Phys. Chem.
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