2,087 research outputs found
Atomic Effective Pseudopotentials for Semiconductors
We derive an analytic connection between the screened self-consistent
effective potential from density functional theory (DFT) and atomic effective
pseudopotentials (AEPs). The motivation to derive AEPs is to address structures
with thousands to hundred thousand atoms, as given in most nanostructures. The
use of AEPs allows to bypass a self-consistent procedure and to address
eigenstates around a certain region of the spectrum (e.g., around the band
gap). The bulk AEP construction requires two simple DFT calculations of
slightly deformed elongated cells. The ensuing AEPs are given on a fine
reciprocal space grid, including the small reciprocal vector components, are
free of parameters, and involve no fitting procedure. We further show how to
connect the AEPs of different bulk materials, which is necessary to obtain
accurate band offsets. We derive a total of 20 AEPs for III-V, II-VI and group
IV semiconductors and demonstrate their accuracy and transferability by
comparison to DFT calculations of strained bulk structures, quantum wells with
varying thickness, and semiconductor alloys.Comment: 10 pages, 5 figures, submitted to PR
Degree of Quantumness in Quantum Synchronization
We introduce the concept of degree of quantumness in quantum synchronization,
a measure of the quantum nature of synchronization in quantum systems.
Following techniques from quantum information, we propose the number of
non-commuting observables that synchronize as a measure of quantumness. This
figure of merit is compatible with already existing synchronization
measurements, and it captures different physical properties. We illustrate it
in a quantum system consisting of two weakly interacting cavity-qubit systems,
which are coupled via the exchange of bosonic excitations between the cavities.
Moreover, we study the synchronization of the expectation values of the Pauli
operators and we propose a feasible superconducting circuit setup. Finally, we
discuss the degree of quantumness in the synchronization between two quantum
van der Pol oscillators
Could people with stereo-deficiencies have a rich 3D experience using HMDs?
People with stereo-deficiencies usually have problems for the perception of depth using stereo devices. This paper presents a study that involves participants who did not have stereopsis and participants who had stereopsis. The two groups of participants were exposed to a maze navigation task in a 3D environment in two conditions, using a HMD and a large stereo screen. Fifty-nine adults participated in our study. From the results, there were no statistically significant differences for the performance on the task between the participants with stereopsis and those without stereopsis. We found statistically significant differences between the two conditions in favor of the HMD for the two groups of participants. The participants who did not have stereopsis and could not perceive 3D when looking at the Lang 1 Stereotest did have the illusion of depth perception using the HMD. The study suggests that for the people who did not have stereopsis, the head tracking largely influences the 3D experience
Some Remarks on Oscillating Inflation
In a recent paper Damour and Mukhanov describe a scenario where inflation may
continue during the oscillatory phase. This effect is possible because the
scalar field spends a significant fraction of each period of oscillation on the
upper part of the potential. Such additional period of inflation could push
perturbations after the slow roll regime to observable scales. Although in this
work we show that the small region of the Damour-Mukhanov parameter q gives the
main contribution to oscillating inflation, it was not satisfactory understood
until now. Furthermore, it gives an expression for the energy density spectrum
of perturbations, which is well behaved in the whole physical range of q .Comment: 4 pages including figures caption, 3 ps-figures. To appear in Phys.
Rev.
Bilateral Internuclear Ophthalmoplegia in a Young Woman with Vertebral Artery Dissection
Background: Internuclear ophthalmoplegia (INO) is an eye movement disorder caused by a lesion in the medial longitudinal fasciculus (MLF)
located in the midbrain. Adduction paralysis of both eyes and bilateral abduction nystagmus are the main features of INO[1].
Case presentation: A 29-year-old Hispanic woman was admitted to the emergency department complaining of an intense holocranial
headache lasting 9 days, associated with nausea and vomiting. She was discharged home with resolution of the headache but persistence
of symptoms. However, she subsequently developed horizontal diplopia and gait abnormalities. She was readmitted to hospital because of
anomalous eye movements and conjugate gaze palsy, manifested as bilateral INO. Magnetic resonance angiography (MRA) findings were
consistent with dissection of the left V4 vertebral artery with multiple brain infarcts in the superior cerebellar artery territory, comprising
both MLF tracts.
Conclusions: In young adults, bilateral INO is normally caused by demyelinating disease. In other patients, common causes include trauma,
infections and autoimmune diseases with neurological symptoms. Vascular disease is implicated in over a third of cases
Space-time calibration of wind speed forecasts from regional climate models
Numerical weather predictions (NWP) are systematically subject to errors due
to the deterministic solutions used by numerical models to simulate the
atmosphere. Statistical postprocessing techniques are widely used nowadays for
NWP calibration. However, time-varying bias is usually not accommodated by such
models. Its calibration performance is also sensitive to the temporal window
used for training. This paper proposes space-time models that extend the main
statistical postprocessing approaches to calibrate NWP model outputs.
Trans-Gaussian random fields are considered to account for meteorological
variables with asymmetric behavior. Data augmentation is used to account for
censuring in the response variable. The benefits of the proposed extensions are
illustrated through the calibration of hourly 10 m wind speed forecasts in
Southeastern Brazil coming from the Eta model.Comment: 43 pages, 13 figure
Climatic elements variability affecting maize yield in Northern Minas Gerais, Brazil.
Conference
PERFORMANCE CORRECTION FACTORS FOR VAPOR COMPRESSION REFRIGERATION AND HEAT PUMP SYSTEMS TESTED WITH UNCONTROLLED CONDENSER CONDITIONS
A method for experimental data adjustment consisting of correction equations for the performance parameters of the refrigeration/heat pump vapor compression cycle, when operation conditions depart from those established in testing standards, is here presented. A basic thermodynamic model allowed for a methodology to be developed so as to correct vapor compression cycle performance to a desirable operating condition. Correction factor equations are proposed for refrigerant mass flow rate, compressor specific enthalpy gain and evaporator refrigeration effect, for situations when condensing pressure has not followed standards conditions or has not been properly controlled during experiments. The method was verified against experimental data from a vapor compression water-to-water heat pump with controlled condensing temperatures of 30oC, 40oC and 50oC. In spite of the purposely excessive correction, ±10oC, a relatively good smoothness, as well as a good agreement among all conversions, was obtained with the standardized points. The model was also applied to a refrigeration system running with water-SWCNT nanofluid (single walled carbon nanotube with water as the base fluid) as the secondary fluid. It contributed to a better discernment of the actual influence of the nanofluid in the system performance
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