144 research outputs found
Superconductivity in carbon nanotube ropes
We investigate the conditions in which superconductivity may develop in ropes
of carbon nanotubes. It is shown that the interaction among a large number of
metallic nanotubes favors the appearance of a metallic phase in the ropes,
intermediate between respective phases with spin-density-wave and
superconducting correlations. These arise in samples with about 100 metallic
nanotubes or more, where the long-range Coulomb interaction is very effectively
reduced and it may be overcome by the attractive interaction from the exchange
of optical phonons within each nanotube. We estimate that the probability for
the tunneling of Cooper pairs between neighboring nanotubes is much higher than
that for single electrons in a disordered rope. The effect of pair hopping is
therefore what establishes the intertube coherence, and the tunneling amplitude
of the Cooper pairs dictates the scale of the transition to the superconducting
state.Comment: 12 page
Closed Strings with Low Harmonics and Kinks
Low-harmonic formulas for closed relativistic strings are given. General
parametrizations are presented for the addition of second- and third-harmonic
waves to the fundamental wave. The method of determination of the
parametrizations is based upon a product representation found for the finite
Fourier series of string motion in which the constraints are automatically
satisfied. The construction of strings with kinks is discussed, including
examples. A procedure is laid out for the representation of kinks that arise
from self-intersection, and subsequent intercommutation, for harmonically
parametrized cosmic strings.Comment: 39, CWRUTH-93-
Magnetic fields in supernova remnants and pulsar-wind nebulae
We review the observations of supernova remnants (SNRs) and pulsar-wind
nebulae (PWNe) that give information on the strength and orientation of
magnetic fields. Radio polarimetry gives the degree of order of magnetic
fields, and the orientation of the ordered component. Many young shell
supernova remnants show evidence for synchrotron X-ray emission. The spatial
analysis of this emission suggests that magnetic fields are amplified by one to
two orders of magnitude in strong shocks. Detection of several remnants in TeV
gamma rays implies a lower limit on the magnetic-field strength (or a
measurement, if the emission process is inverse-Compton upscattering of cosmic
microwave background photons). Upper limits to GeV emission similarly provide
lower limits on magnetic-field strengths. In the historical shell remnants,
lower limits on B range from 25 to 1000 microGauss. Two remnants show
variability of synchrotron X-ray emission with a timescale of years. If this
timescale is the electron-acceleration or radiative loss timescale, magnetic
fields of order 1 mG are also implied. In pulsar-wind nebulae, equipartition
arguments and dynamical modeling can be used to infer magnetic-field strengths
anywhere from about 5 microGauss to 1 mG. Polarized fractions are considerably
higher than in SNRs, ranging to 50 or 60% in some cases; magnetic-field
geometries often suggest a toroidal structure around the pulsar, but this is
not universal. Viewing-angle effects undoubtedly play a role. MHD models of
radio emission in shell SNRs show that different orientations of upstream
magnetic field, and different assumptions about electron acceleration, predict
different radio morphology. In the remnant of SN 1006, such comparisons imply a
magnetic-field orientation connecting the bright limbs, with a non-negligible
gradient of its strength across the remnant.Comment: 20 pages, 24 figures; to be published in SpSciRev. Minor wording
change in Abstrac
Factors Associated with Severity of COVID-19 Disease in a Multicenter Cohort of People with HIV in the United States, March-December 2020
Background: Understanding the spectrum of COVID-19 in people with HIV (PWH) is critical to provide clinical guidance and risk reduction strategies.Setting:Centers for AIDS Research Network of Integrated Clinic System, a US multisite clinical cohort of PWH in care.Methods:We identified COVID-19 cases and severity (hospitalization, intensive care, and death) in a large, diverse HIV cohort during March 1, 2020-December 31, 2020. We determined predictors and relative risks of hospitalization among PWH with COVID-19, adjusted for disease risk scores. Results: Of 16,056 PWH in care, 649 were diagnosed with COVID-19 between March and December 2020. Case fatality was 2%; 106 (16.3%) were hospitalized, and 12 died. PWH with current CD4 count <350 cells/mm3[aRR 2.68; 95% confidence interval (CI): 1.93 to 3.71; P < 0.001] or lowest recorded CD4 count <200 cells/mm3(aRR 1.67; 95% CI: 1.18 to 2.36; P < 0.005) had greater risks of hospitalization. HIV viral load and antiretroviral therapy status were not associated with hospitalization, although most of the PWH were suppressed (86%). Black PWH were 51% more likely to be hospitalized with COVID-19 compared with other racial/ethnic groups (aRR 1.51; 95% CI: 1.04 to 2.19; P = 0.03). Chronic kidney disease, chronic obstructive pulmonary disease, diabetes, hypertension, obesity, and increased cardiovascular and hepatic fibrosis risk scores were associated with higher hospitalization risk. PWH who were older, not on antiretroviral therapy, and with current CD4 count <350 cells/mm3, diabetes, and chronic kidney disease were overrepresented among PWH who required intubation or died. Conclusions: PWH with CD4 count <350 cells/mm3, and a history of CD4 count <200 cells/mm3, have a clear excess risk of severe COVID-19, accounting for comorbidities associated with severe outcomes. PWH with these risk factors should be prioritized for COVID-19 vaccination and early treatment and monitored closely for worsening illness
Human Resource Flexibility as a Mediating Variable Between High Performance Work Systems and Performance
Much of the human resource management literature has demonstrated the impact of high performance
work systems (HPWS) on organizational performance. A new generation of studies is
emerging in this literature that recommends the inclusion of mediating variables between HPWS
and organizational performance. The increasing rate of dynamism in competitive environments
suggests that measures of employee adaptability should be included as a mechanism that may
explain the relevance of HPWS to firm competitiveness. On a sample of 226 Spanish firms, the
studyâs results confirm that HPWS influences performance through its impact on the firmâs
human resource (HR) flexibility
Racial and ethnic disparities in coronavirus disease 2019 disease incidence independent of comorbidities, among people with HIV in the United States
Objectives: To define the incidence of clinically detected coronavirus disease 2019 (COVID-19) in people with HIV (PWH) in the United States and evaluate how racial and ethnic disparities, comorbidities, and HIV-related factors contribute to risk of COVID-19. Design: Observational study within the CFAR Network of Integrated Clinical Systems cohort in seven cities during 2020. Methods: We calculated cumulative incidence rates of COVID-19 diagnosis among PWH in routine care by key characteristics including race/ethnicity, current and lowest CD4ĂŸ cell count, and geographic area. We evaluated risk factors for COVID-19 among PWH using relative risk regression models adjusted with disease risk scores. Results: Among 16 056 PWH in care, of whom 44.5% were black, 12.5% were Hispanic, with a median age of 52 years (IQR 40 - 59), 18% had a current CD4ĂŸ cell count less than 350 cells/ml, including 7% less than 200; 95.5% were on antiretroviral therapy (ART), and 85.6% were virologically suppressed. Overall in 2020, 649 PWH were diagnosed with COVID-19 for a rate of 4.94 cases per 100 person-years. The cumulative incidence of COVID-19 was 2.4-fold and 1.7-fold higher in Hispanic and black PWH respectively, than non-Hispanic white PWH. In adjusted analyses, factors associated with COVID-19 included female sex, Hispanic or black identity, lowest historical CD4ĂŸ cell count less than 350 cells/ml (proxy for CD4ĂŸ nadir), current low CD4ĂŸ : CD8ĂŸ ratio, diabetes, and obesity. Conclusion: Our results suggest that the presence of structural racial inequities above and beyond medical comorbidities increased the risk of COVID-19 among PWH. PWH with immune exhaustion as evidenced by lowest historical CD4ĂŸ cell count or current low CD4ĂŸ : CD8ĂŸ ratio had greater risk of COVID-19
Measurement of forward charged hadron flow harmonics in peripheral PbPb collisions at âsNN = 5.02 TeV with the LHCb detector
Flow harmonic coefficients,
v
n
, which are the key to studying the hydrodynamics of the quark-gluon plasma (QGP) created in heavy-ion collisions, have been measured in various collision systems and kinematic regions and using various particle species. The study of flow harmonics in a wide pseudorapidity range is particularly valuable to understand the temperature dependence of the shear viscosity to entropy density ratio of the QGP. This paper presents the first LHCb results of the second- and the third-order flow harmonic coefficients of charged hadrons as a function of transverse momentum in the forward region, corresponding to pseudorapidities between 2.0 and 4.9, using the data collected from PbPb collisions in 2018 at a center-of-mass energy of 5.02
TeV
. The coefficients measured using the two-particle angular correlation analysis method are smaller than the central-pseudorapidity measurements at ALICE and ATLAS from the same collision system but share similar features
Helium identification with LHCb
The identification of helium nuclei at LHCb is achieved using a method based on measurements of ionisation losses in the silicon sensors and timing measurements in the Outer Tracker drift tubes. The background from photon conversions is reduced using the RICH detectors and an isolation requirement. The method is developed using pp collision data at â(s) = 13 TeV recorded by the LHCb experiment in the years 2016 to 2018, corresponding to an integrated luminosity of 5.5 fb-1. A total of around 105 helium and antihelium candidates are identified with negligible background contamination. The helium identification efficiency is estimated to be approximately 50% with a corresponding background rejection rate of up to O(10^12). These results demonstrate the feasibility of a rich programme of measurements of QCD and astrophysics interest involving light nuclei
- âŠ