115 research outputs found
Fermionic current densities induced by magnetic flux in a conical space with a circular boundary
We investigate the vacuum expectation value of the fermionic current induced
by a magnetic flux in a (2+1)-dimensional conical spacetime in the presence of
a circular boundary. On the boundary the fermionic field obeys MIT bag boundary
condition. For irregular modes, a special case of boundary conditions at the
cone apex is considered, when the MIT bag boundary condition is imposed at a
finite radius, which is then taken to zero. We observe that the vacuum
expectation values for both charge density and azimuthal current are periodic
functions of the magnetic flux with the period equal to the flux quantum
whereas the expectation value of the radial component vanishes. For both
exterior and interior regions, the expectation values of the current are
decomposed into boundary-free and boundary-induced parts. For a massless field
the boundary-free part in the vacuum expectation value of the charge density
vanishes, whereas the presence of the boundary induces nonzero charge density.
Two integral representations are given for the boundary-free part in the case
of a massive fermionic field for arbitrary values of the opening angle of the
cone and magnetic flux. The behavior of the induced fermionic current is
investigated in various asymptotic regions of the parameters. At distances from
the boundary larger than the Compton wavelength of the fermion particle, the
vacuum expectation values decay exponentially with the decay rate depending on
the opening angle of the cone. We make a comparison with the results already
known from the literature for some particular cases.Comment: 34 pages, 6 figure
Vibrational properties of single-wall nanotubes and monolayers of hexagonal BN
We report a detailed study of the vibrational properties of BN single-walled
nanotubes and of the BN monolayer. Our results have been obtained from a
well-established Tight-Binding model complemented with an electrostatic model
to account for the long-range interactions arising from the polar nature of the
material, and which are not included in the Tight-Binding model. Our study
provides a wealth of data for the BN monolayer and nanotubes, such as phonon
band structure, vibrational density of states, elastic constants, etc. For the
nanotubes we obtain the behavior of the optically active modes as a function of
the structural parameters, and we compare their frequencies with those derived
from a zone-folding treatment applied to the phonon frequencies of the BN
monolayer, finding general good agreement between the two.Comment: 14 pages with 10 postscript figures, to appear in PRB, January 15th
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Communication and visiting policies in Italian intensive care units during the first COVID-19 pandemic wave and lockdown: a nationwide survey
Background: During the first coronavirus disease 2019 (COVID-19) pandemic wave, an unprecedented number of patients with respiratory failure due to a new, highly contagious virus needed hospitalization and intensive care unit (ICU) admission. The aim of the present study was to describe the communication and visiting policies of Italian intensive care units (ICUs) during the first COVID-19 pandemic wave and national lockdown and compare these data with prepandemic conditions. Methods: A national web-based survey was conducted among 290 Italian hospitals. Each ICU (active between February 24 and May 31, 2020) was encouraged to complete an individual questionnaire inquiring the hospital/ICU structure/organization, communication/visiting habits and the role of clinical psychology prior to, and during the first COVID-19 pandemic wave. Results: Two hundred and nine ICUs from 154 hospitals (53% of the contacted hospitals) completed the survey (202 adult and 7 pediatric ICUs). Among adult ICUs, 60% were dedicated to COVID-19 patients, 21% were dedicated to patients without COVID-19 and 19% were dedicated to both categories (Mixed). A total of 11,102 adult patients were admitted to the participating ICUs during the study period and only approximately 6% of patients received at least one visit. Communication with family members was guaranteed daily through an increased use of electronic devices and was preferentially addressed to the same family member. Compared to the prepandemic period, clinical psychologists supported physicians more often regarding communication with family members. Fewer patients received at least one visit from family members in COVID and mixed-ICUs than in non-COVID ICUs, l (0 [0–6]%, 0 [0–4]% and 11 [2–25]%, respectively, p < 0.001). Habits of pediatric ICUs were less affected by the pandemic. Conclusions: Visiting policies of Italian ICUs dedicated to adult patients were markedly altered during the first COVID-19 wave. Remote communication was widely adopted as a surrogate for family meetings. New strategies to favor a family-centered approach during the current and future pandemics are warranted
Large-Diameter Single-Wall Carbon Nanotubes Formed Alongside Small-Diameter Double-Walled Carbon Nanotubes
Samples containing a majority of either single-wall carbon nanotubes (SWCNTs) or double-walled carbon nanotubes (DWCNTs) are prepared in the same catalytic chemical vapor deposition conditions but using slightly different catalytic materials, based on alumina impregnated with iron and molybdenum salts. There is a sharp SWCNTs-to-DWCNTs transition. By contrast to the usual findings, the selectivity is not correlated to the size of the iron-based catalyst nanoparticles, nor does the transition occur upon a decreasing carbon/catalyst ratio. The result is attributed to the increasing MoO3 concentration inducing modifications of the gas atmosphere, such as the formation of more reactive C2 species through C2H4 dissociation, which thus favors the nucleation and growth of a DWCNT. In the DWCNT sample, the average diameter of the SWCNTs is higher than the average outer diameter of the DWCNTs, which is uncommon, as many authors stress that SWCNTs show a lower diameter than DWCNTs. The study could provide guidelines for the synthesis of very small diameter DWCNTs
Electronic Durability of Flexible Transparent Films from Type-Specific Single-Wall Carbon Nanotubes
Influence of Acoustic Cavitation on the Controlled Ultrasonic Dispersion of Carbon Nanotubes
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