613 research outputs found
Absence of room temperature ferromagnetism in bulk Mn-doped ZnO
Structural and magnetic properties have been studied for polycrystalline
Zn_1-xMn_xO (x=0.02, 0.03, 0.05). Low-temperature (~500 oC) synthesis leaves
unreacted starting ZnO and manganese oxides. Contrary to a recent report, no
bulk ferromagnetism was observed for single-phase materials synthesized in air
at temperatures above 900 oC. Single-phase samples show paramagnetic
Curie-Weiss behavior.Comment: Accepted for publication in J. Appl. Phys., RevTeX, 3 pages, 4
figure
Loop Corrections in the Spectrum of 2D Hawking Radiation
We determine the one-loop and the two-loop back-reaction corrections in the
spectrum of the Hawking radiation for the CGHS model of 2d dilaton gravity by
evaluating the Bogoliubov coefficients for a massless scalar field propagating
on the corresponding backgrounds. Since the back-reaction can induce a small
shift in the position of the classical horizon, we find that a positive shift
leads to a non-Planckian late-time spectrum, while a null or a negative shift
leads to a Planckian late-time spectrum in the leading-order stationary-point
approximation. In the one-loop case there are no corrections to the classical
Hawking temperature, while in the two-loop case the temperature is three times
greater than the classical value. We argue that these results are consistent
with the behaviour of the Hawking flux obtained from the operator quantization
only for the times which are not too late, in accordance with the limits of
validity of the semiclassical approximation.Comment: 20 pages, latex, no figure
One-Loop Effective Action for Spherical Scalar Field Collapse
We calculate the complete one-loop effective action for a spherical scalar
field collapse in the large radius approximation. This action gives the
complete trace anomaly, which beside the matter loop contributions, receives a
contribution from the graviton loops. Our result opens a possibility for a
systematic study of the back-reaction effects for a real black hole.Comment: 14 pages, slightly expanded version to appear in Class. Quant. Gra
Zitterbewegung of Klein-Gordon particles and its simulation by classical systems
The Klein-Gordon equation is used to calculate the Zitterbewegung (ZB,
trembling motion) of spin-zero particles in absence of fields and in the
presence of an external magnetic field. Both Hamiltonian and wave formalisms
are employed to describe ZB and their results are compared. It is demonstrated
that, if one uses wave packets to represent particles, the ZB motion has a
decaying behavior. It is also shown that the trembling motion is caused by an
interference of two sub-packets composed of positive and negative energy states
which propagate with different velocities. In the presence of a magnetic field
the quantization of energy spectrum results in many interband frequencies
contributing to ZB oscillations and the motion follows a collapse-revival
pattern. In the limit of non-relativistic velocities the interband ZB
components vanish and the motion is reduced to cyclotron oscillations. The
exact dynamics of a charged Klein-Gordon particle in the presence of a magnetic
field is described on an operator level. The trembling motion of a KG particle
in absence of fields is simulated using a classical model proposed by Morse and
Feshbach -- it is shown that a variance of a Gaussian wave packet exhibits ZB
oscillations.Comment: 16 pages and 7 figure
Helmet CPAP to Treat Acute Hypoxemic Respiratory Failure in Patients with COVID-19 : a Management Strategy Proposal
Since the beginning of March 2020, the coronavirus disease 2019 (COVID-19) pandemic has caused more than 13,000 deaths in Europe, almost 54% of which has occurred in Italy. The Italian healthcare system is experiencing a stressful burden, especially in terms of intensive care assistance. In fact, the main clinical manifestation of COVID-19 patients is represented by an acute hypoxic respiratory failure secondary to bilateral pulmonary infiltrates, that in many cases, results in an acute respiratory distress syndrome and requires an invasive ventilator support. A precocious respiratory support with non-invasive ventilation or high flow oxygen should be avoided to limit the droplets' air-dispersion and the healthcare workers' contamination. The application of a continuous positive airway pressure (CPAP) by means of a helmet can represent an effective alternative to recruit diseased alveolar units and improve hypoxemia. It can also limit the room contamination, improve comfort for the patients, and allow for better clinical assistance with long-term tolerability. However, the initiation of a CPAP is not free from pitfalls. It requires a careful titration and monitoring to avoid a delayed intubation. Here, we discuss the rationale and some important considerations about timing, criteria, and monitoring requirements for patients with COVID-19 respiratory failure requiring a CPAP treatment
The effect of a multidisciplinary approach for smoking cessation in patients with Crohn's disease : results from an observational cohort study
Introduction: Cigarette smoking is the most important risk factor for Crohn's disease (CD). The effectiveness of smoking cessation programs (SCPs) in patients with CD is still poorly understood.
Methods: This was a retrospective, observational, single-centre, cohort study of 136 active smokers with mean age 55 years (SD=11), 58% males, including 27 (19.8%) patients with CD who entered the multidisciplinary SCP of the Luigi Sacco University Hospital of Milan from January 2017 through January 2019. A pulmonologist was responsible for the clinical and pharmacological management, while a psychiatrist and a psychologist conducted the counselling and assessed the motivation to quit, anxiety and depression using the Brief Psychiatric Rating Scale (BPRS) and the nicotine dependence with the Fagerstr\uf6m test. Patients were defined as quitters after 12 months.
Results: Demographic and clinical characteristics, and Fagerstr\uf6m score, did not differ in patients with and without CD. At baseline, patients with CD had a higher BPRS (median: 27, IQR: 22-32; vs 25 and 22-28.5; p=0.03), and a lower motivation to quit score (median: 10, IQR: 9-13; vs 14 and 12-15; p<0.001). After 12 months, the quitting rate of smokers with CD was significantly lower (14.8% vs 36.7%; p<0.022) and the chance of quitting was negatively associated with the baseline BPRS (r=-0.256; p<0.003). Varenicline and nicotine replacement therapy tended to be less effective in patients with CD.
Conclusions: The lower efficacy of SCPs in patients with CD might be secondary to a higher prevalence of anxiety and depression. Psychological issue recognition and support should be enhanced to increase SCP effectiveness in CD
Renormalizability of noncommutative SU(N) gauge theory
We analyze the renormalizability properties of pure gauge noncommutative
SU(N) theory in the -expanded approach. We find that the theory is
one-loop renormalizable to first order in .Comment: 11 pages, minor changes, accepted for publication in JHE
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