474 research outputs found
Topological phonon analysis of the 2D buckled honeycomb lattice: an application to real materials
By means of group theory, topological quantum chemistry, first-principles and
Monte Carlo calculations, we analyze the topology of the 2D buckled honeycomb
lattice phonon spectra. Taking the pure crystal structure as an input, we show
that eleven distinct phases are possible, five of which necessarily have
non-trivial topology according to topological quantum chemistry. Another four
of them are also identified as topological using Wilson loops in an analytical
model that includes all the symmetry allowed force constants up to third
nearest neighbors, making a total of nine topological phases. We then compute
the ab initio phonon spectra for the two-dimensional crystals of Si, Ge, P, As
and Sb in this structure and construct its phase diagram. Despite the large
proportion of topological phases found in the analytical model, all of the
crystals lie in a trivial phase. By analyzing the force constants space using
Monte Carlo calculations, we elucidate why topological phonon phases are
physically difficult to realize in real materials with this crystal structure
Microstructural control of the transport properties of β-FeSe films grown by sputtering
We have investigated the correlation between structural and transport properties in sputtered β-FeSe films grown onto SrTiO3 (100). The growth parameters, such as substrate temperature and thickness, have been varied in order to explore different regimes. In the limit of textured thick films, we found promising features like an enhanced Tc ∼ 12K, a relatively high Hc2 and a low anisotropy. By performing magnetoresistance and Hall coefficient measurements, we investigate the influence of the disorder associated with the textured morphology on some features attributed to subtle details of the multi-band electronic structure of β-FeSe. Regarding the superconductor-insulator transition (SIT) induced by reducing the thickness, we found a non-trivial evolution of the structural properties and morphology associated with a strained initial growth and the coalescence of grains. Our results reveal the key effects on the macroscopic electronic behaviour played by the lattice distortion in thin insulating samples and by the grain morphology in thicker superconducting films.Fil: Ale Crivillero, María Victoria. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche); Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Amigo, M.. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche); Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Haberkorn, Nestor Fabian. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche); Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche | Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche; ArgentinaFil: Nieva, G.. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche); ArgentinaFil: Guimpel, Julio Juan. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche); Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche | Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche; Argentin
Dirac gaugino as leptophilic dark matter
We investigate the leptophilic properties of Dirac gauginos in an
R--symmetric N=2 supersymmetric model with extended gauge and Higgs sectors.
The annihilation of Dirac gauginos to leptons requires no chirality flip in the
final states so that it is not suppressed as in the Majorana case. This implies
that it can be sizable enough to explain the positron excess observed by the
PAMELA experiment with moderate or no boost factors. When squark masses are
heavy, the annihilation of Dirac gauginos to hadrons is controlled by their
Higgsino fraction and is driven by the and final states.
Moreover, at variance with the Majorana case, Dirac gauginos with a
non-vanishing higgsino fraction can also have a vector coupling with the
gauge boson leading to a sizable spin--independent scattering cross section off
nuclei. Saturating the current antiproton limit, we show that Dirac gauginos
can leave a signal in direct detection experiments at the level of the
sensitivity of dark matter searches at present and in the near future.Comment: 24 pages, 10 figures, typos corrected, final version published on
JCA
R-symmetric Gauge Mediation and the MRSSM
This is an invited summary of a seminar talk given at various institutions in
the United States and Canada. After a brief introduction, a review of the
minimal R-symmetric supersymmetric standard model is given, and the benefits to
the flavor sector are discussed. R-symmetric gauge mediation is an attempt to
realize this model using metastable supersymmetry breaking techniques. Sample
low energy spectra are presented and tuning is discussed. Various other
phenomenological results are summarized.Comment: 14 pages, invited Brief Review, submitted to Modern Physics Letters
A; v2: replaced Figure 1, updated acknowledgments, fixed typo
Dirac Gauginos, Negative Supertraces and Gauge Mediation
In an attempt to maximize General Gauge Mediated parameter space, I propose
simple models in which gauginos and scalars are generated from disconnected
mechanisms. In my models Dirac gauginos are generated through the supersoft
mechanism, while independent R-symmetric scalar masses are generated through
operators involving non-zero messenger supertrace. I propose several new
methods for generating negative messenger supertraces which result in viable
positive mass squareds for MSSM scalars. The resultant spectra are novel,
compressed and may contain light fermionic SM adjoint fields.Comment: 16 pages 3 figure
Experiencia y resultados preliminares en el uso de cilindros intersomáticos para la artrodesis lumbar
El dolor lumbar crónico es una patología que puede llegar a ser incapacitante y en la que a menudo fracasan los tratamientos conservadores. Como alternativa entre las cirugías que pretenden atajar este problema mediante una artrodesis está la fusión intersomática con cilindros, que presenta algunas ventajas respecto a otras técnicas de fijación. Esta técnica ha sido introducida recientemente en nuestro país. Se presentan los primeros resultados obtenidos tras esta cirugía en los primeros 16 pacientes operados. Desde 1997 a diciembre de 2000 se han implantado 11 cilindros BAK y 5 LIFEC. Diez pacientes tenían un seguimiento mínimo de 1 año y éstos constituyen la serie evaluada. Siete pacientes (70%) no presentaban dolor. Cuatro pacientes (40%) se habían incorporado a sus trabajos respectivos. Seis pacientes (60%) no precisaban tratamiento médico. Los buenos resultados obtenidos hasta el momento nos invitan a pensar que esta técnica, de menor agresividad y que no cierra la puerta a otros tipos de instrumentación, es una opción válida para el tratamiento del dolor lumbar crónico.Chronic low back pain is a medical cause of disability in which conservative management is often not effective. As an alternative, interbody fusion with cylinders offers several advantages as compared to other surgical methods of fusion. This technique has been recently introduced in our Country. The preliminary results of 16 patients managed in our Department are shown. From 1997 to December 2000, eleven BAK devices and 5 LIFEC have been implanted. Ten patients had a minimum follow-up of 1 year and these constitute the series evaluated. Seven patients (70%) were free of pain. Four patients (40%) returned to their previous job. Six patients (60%) did not need medical treatment. The good results with this technique encourage us and suggest that these devices are a valid option for the management of chronic low back pain
On the fraction of dark matter in charged massive particles (CHAMPs)
From various cosmological, astrophysical and terrestrial requirements, we
derive conservative upper bounds on the present-day fraction of the mass of the
Galactic dark matter (DM) halo in charged massive particles (CHAMPs). If dark
matter particles are neutral but decay lately into CHAMPs, the lack of
detection of heavy hydrogen in sea water and the vertical pressure equilibrium
in the Galactic disc turn out to put the most stringent bounds. Adopting very
conservative assumptions about the recoiling velocity of CHAMPs in the decay
and on the decay energy deposited in baryonic gas, we find that the lifetime
for decaying neutral DM must be > (0.9-3.4)x 10^3 Gyr. Even assuming the
gyroradii of CHAMPs in the Galactic magnetic field are too small for halo
CHAMPs to reach Earth, the present-day fraction of the mass of the Galactic
halo in CHAMPs should be < (0.4-1.4)x 10^{-2}. We show that redistributing the
DM through the coupling between CHAMPs and the ubiquitous magnetic fields
cannot be a solution to the cuspy halo problem in dwarf galaxies.Comment: 21 pages, 2 figures. To appear in JCA
Absence of disparities in anthropometric measures among Chilean indigenous and non-indigenous newborns
<p>Abstract</p> <p>Background</p> <p>Studies throughout North America and Europe have documented adverse perinatal outcomes for racial/ethnic minorities. Nonetheless, the contrast in newborn characteristics between indigenous and non-indigenous populations in Latin America has been poorly characterized. This is due to many challenges, including a lack of vital registration information on ethnicity. The objective of this study was to analyze trends in anthropometric measures at birth in Chilean indigenous (Mapuche) and non-indigenous children over a 5-year period.</p> <p>Methods</p> <p>We examined weight and length at birth using information available through a national data base of all birth records for the years 2000 through 2004 (n = 1,166.513). Newborns were classified ethnically according to the origins of the parents' last names.</p> <p>Result</p> <p>The average birthweight was stable over the 5 year period with variations of less than 20 g in each group, and with mean values trivially higher in indigenous newborns. The proportion weighing less than 2500 g at birth increased modestly from 5.2% to 5.6% in non-indigenous newborns whereas the indigenous births remained constant at 5.2%. In multiple regression analyses, adjusting flexibly for gestational age and maternal characteristics, the occurrence of an indigenous surname added only 14 g to an average infant's birthweight while holding other factors constant. Results for length at birth were similar, and adjusted time trend variation in both outcomes was trivially small after adjustment. Anthropometric indexes at birth in Chile are quite favorable by international standards.</p> <p>Conclusion</p> <p>There is only a trivial degree of ethnic disparity in these values, in contrast to conditions for ethnic minorities in other countries. Moreover, these values remained roughly constant over the 5 years of observation in this study.</p
- …