867 research outputs found
Wannier interpolation of the electron-phonon matrix elements in polar semiconductors: Polar-optical coupling in GaAs
We generalize the Wannier interpolation of the electron-phonon matrix
elements to the case of polar-optical coupling in polar semiconductors. We
verify our methodological developments against experiments, by calculating the
widths of the electronic bands due to electron-phonon scattering in GaAs, the
prototype polar semiconductor. The calculated widths are then used to estimate
the broadenings of excitons at critical points in GaAs and the electron-phonon
relaxation times of hot electrons. Our findings are in good agreement with
available experimental data. Finally, we demonstrate that while the Fr\"ohlich
interaction is the dominant scattering process for electrons/holes close to the
valley minima, in agreement with low-field transport results, at higher
energies, the intervalley scattering dominates the relaxation dynamics of hot
electrons or holes. The capability of interpolating the polar-optical coupling
opens new perspectives in the calculation of optical absorption and transport
properties in semiconductors and thermoelectrics.Comment: To appear on Phys. Rev.
Angular dependence of core hole screening in LiCoO2: A DFT+U calculation of the oxygen and cobalt K-edge x-ray absorption spectra
Angular dependent core-hole screening effects have been found in the cobalt
K-edge x-ray absorption spectrum of LiCoO2, using high-resolution data and
parameter-free GGA+U calculations. The Co 1s core-hole on the absorber causes
strong local attraction. The core-hole screening on the nearest neighbours
cobalt induces a 2 eV shift in the density of states with respect to the
on-site 1s-3d transitions, as detected in the Co K pre-edge spectrum. Our DFT+U
calculations reveal that the off-site screening is different in the
out-of-plane direction, where a 3 eV shift is visible in both calculations and
experiment. The detailed analysis of the inclusion of the core-hole potential
and the Hubbard parameter U shows that the core-hole is essential for the
off-site screening, while U improves the description of the angular dependent
screening effects. In the case of oxygen K-edge, both the core-hole potential
and the Hubbard parameter improve the relative positions of the spectral
features
Effects of magnetism and doping on the electron-phonon coupling in BaFeAs
We calculate the effect of local magnetic moments on the electron-phonon
coupling in BaFeAs using the density functional perturbation
theory. We show that the magnetism enhances the total electron-phonon coupling
by , up to , still not enough to explain the
high critical temperature, but strong enough to have a non-negligible effect on
superconductivity, for instance, by frustrating the coupling with spin
fluctuations and inducing order parameter nodes. The enhancement comes mostly
from a renormalization of the electron-phonon matrix elements. We also
investigate, in the rigid band approximation, the effect of doping, and find
that versus doping does not mirror the behavior of the density of
states; while the latter decreases upon electron doping, the former does not,
and even increases slightly.Comment: 4 pages, 3 figure
The stability in the most external region of the Oort Cloud: The evolution of the ejected comets
In this paper, we present a study about the dynamical effects of the Galaxy
on the external region of the Oort Cloud. The aims of this paper are: i) to
determine an outer limit for the Oort Cloud; and ii) to analyse the dynamical
behaviour of the most external objects of the Cloud and how they are ejected
from the Solar System. This is undertaken by following the temporal evolution
of massless test particles in the Galactic environment of the solar
neighbourhood. Here we show that the effect of the perturbations from the
Galactic tide in the particles is similar to that find for the evolution of
wide binary stars population. Moreover, in the Oort Cloud we found a dynamical
structure around 10 5 au conformed by objects unbound of the Sun. This
structure allows us to define a transition region of stability and an outer
boundary for the Oort Cloud, and it is also in agreement with previous results
about the disruption of wide binary stars.Comment: Accepted for publishing in MNRAS. 13 pages, 9 figures, 2 table
Violation of Ioffe-Regel condition but saturation of resistivity of the high Tc cuprates
We demonstrate that the resistivity data of a number of high Tc cuprates, in
particular La(2-x)SrxCuO4, are consistent with resistivity saturation, although
the Ioffe-Regel condition is strongly violated. By using the f-sum rule
together with calculations of the kinetic energy in the t-J model, we show that
the saturation resistivity is unusually large. This is related to the strong
reduction of the kinetic energy due to strong correlation effects. The
fulfilment of the Ioffe-Regel condition for conventional transition metal
compounds is found to be somewhat accidental.Comment: 4 pages, RevTeX, 2 eps figures, additional material available at
http://www.mpi-stuttgart.mpg.de/andersen/saturation
Autonomic dysfunction in progressive supranuclear palsy
Background: The degree of involvement of the autonomic nervous system in progressive supranuclear palsy (PSP) has been investigated in several studies, often providing conflicting results. There is a need for a better characterization of autonomic dysfunction in PSP, to enhance our understanding of this highly disabling neurodegenerative disease including patients’ needs and possibly be of value for clinicians in the differential diagnosis among Parkinsonian syndromes. Methods: We applied a systematic methodology to review existing literature on Pubmed regarding autonomic nervous system involvement in PSP. Results: PSP reported quite frequently symptoms suggestive of autonomic dysfunction in all domains. Cardiovascular autonomic testing showed in some cases a certain degree of impairment (never severe). There was some evidence suggesting bladder dysfunction particularly in the storage phase. Dysphagia and constipation were the most common gastrointestinal symptoms. Instrumental tests seemed to confirm sudomotor and pupillomotor disturbances. Conclusions: PSP patients frequently reported visceral symptoms, however objective testing showed that not always these reflected actual autonomic impairment. Further studies are needed to better delineate autonomic profile and its prognostic role in PSP
Cefepime monotherapy for the empirical treatment of fever in granulocytopenic cancer patients
In a pilot study, we evaluated the efficacy and the safety of cefepime, a new cephalosporin with extended-spectrum activity against both Gram-positive and Gram-negative bacteria, as empirical monotherapy for 108 febrile episodes in 84 granulocytopenic cancer patients. Cefepime (2 g tds) was given for a minimum of 7 days or until resolution of infection. Of the 108 episodes, 91 were evaluable. Microbiologically documented infections occurred in 25 patients (27%) (18 Gram-positive, 7 Gram-negative), of whom 18 had bacteraemia. Infection was clinically documented in 47 patients (52%) and fever was unexplained in 19 (21%). Overall, 71% (65/91) of the infections resolved. Response rates were 86% (6/7) for Gram-negative infections, 44% (8/18) for Gram-positive infections (57%%for cefepime-susceptible Gram-positive bacteria), 77% (36/47) for clinically documented infections and 79% (15/19) for unexplained fevers. Of the 26 patients (29%) whose primary infections did not improve with cefepime monotherapy, 23 responded after the addition of other antibiotics. Sixteen patients (18%) developed secondary infections of which 13 were microbiologically documented; Gram-positive bacteria were isolated from seven patients, Gram-negative bacteria from two, fungi from three and a virus from one. Adverse effects were mild and did not require premature discontinuation of therapy except for one patient who developed an immediate allergic reaction after the first dose of cefepime from which he recovered fully. The survival rate after resolution of granulocytopenia was 96%; three patients died of primary bacterial infection and one from secondary disseminated candidiasis. In this pilot study, cefepime monotherapy appeared safe and effective as empirical therapy for fever in cancer patients with granulocytopenia. Whether cefepime is superior to other advanced-generation cephalosporins for the treatment of Gram-positive infections will require evaluation in a larger comparative stud
Exact bounds on the ground-state energy of the infinite-U Hubbard model
We give upper and lower bounds for the ground-state energy of the infinite-U
Hubbard model. In two dimensions, using these bounds we are able to rule out
the possibility of phase separation between the undoped-insulating state and an
hole-rich state.Comment: 2 pages, 1 figure, to appear in Phys. Rev.
AI and blockchain as new triggers in the education arena
Several scholars have examined the potential use of AI and Blockchain in education, primarily focusing on the contributions of such technologies with a goal to improve learning possibilities and outcomes for students. Such technologies may be utilized to ensure that all students have equal access to education, including people with disabilities, refugees, and those living in isolated communities or rural areas. AI-empowered applications such as holograms and robotics may enable children with special needs to attend school from home or from hospital, as well as ensure learning continuity in emergencies or crises, such as during the recent COVID pandemic. Moreover, teachers and lecturers can utilize such new systems to monitor asynchronous discussion groups, boosting the involvement of the participants through Intelligent Tutoring Systems, making group discussions more engaging even at a distance, coupled with e-learning. In larger cohorts, AI can support the fast grading of tests and homework and can provide tools to prevent cheating.info:eu-repo/semantics/publishedVersio
A new insight into the Galactic potential: a simple secular model for the evolution of binary systems in the solar neighbourhood
Context. Among the main effects that the Milky Way exerts in binary systems, the Galactic tide is the only one that is not probabilistic and can be deduced from a potential. Therefore, it is possible to perform an analysis of the global structure of the phase space of binary systems in the solar neighbourhood using the Galactic potential. Aims. The aim of this work is to obtain a simple model to study the collisionless dynamical evolution of generic wide binaries systems in the solar neighbourhood. Methods. Through an averaging process, we reduced the three-dimensional potential of the Galaxy to a secular one-degree of freedom model. The accuracy of this model was tested by comparing its predictions with numerical simulations of the exact equations of motion of a two-body problem disturbed by the Galaxy. Results. Using the one-degree of freedom model, we developed a detailed dynamical study, finding that the secular Galactic tide period changes as a function of the separation of the pair, which also gives a dynamical explanation for the arbitrary classification between wide and tight binaries. Moreover, the secular phase space for a generic gravitationally bound pair is similar to the dynamical structure of a Lidov-Kozai resonance, but surprisingly this structure is independent of the masses and semimajor axis of the binary system. Thus, the Galactic potential is able to excite the initially circular orbit of binary systems to high values of eccentricity, which has important implications for studies of binary star systems (with and without exoplanets), comets, and Oort cloud objects.Facultad de Ciencias Astronómicas y GeofÃsica
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