379 research outputs found
Field-induced suppression of the pi-band superconductivity and magnetic hysteresis in the microwave surface resistance of MgB_2 at temperatures near T_c
We report on the magnetic-field-induced variations of the microwave surface
resistance, R_s, in a polycrystalline MgB_2 sample, at different values of
temperature. We have detected a magnetic hysteresis in R_s, which exhibits an
unexpected plateau on decreasing the DC magnetic field below a certain value.
In particular, at temperatures near T_c the hysteresis manifests itself only
through the presence of the plateau. Although we do not quantitatively justify
the anomalous shape of the magnetic hysteresis, we show that the results
obtained in the reversible region of the R_s(H) curve can be quite well
accounted for by supposing that, in this range of magnetic field, the pi-gap is
almost suppressed by the applied field and, consequently, all the pi-band
charge carriers are quasiparticles. On this hypothesis, we have calculated
R_s(H) supposing that fluxons assume a conventional (single core) structure and
the flux dynamics can be described in the framework of conventional models.
From the fitting of the experimental results, we determine the values of
H_{c2}^pi(T) at temperatures near T_c. In our opinion, the most important
result of our investigation is that, at least at temperatures near T_c, the
value of the applied field that separates the reversible and irreversible
regions of the R_s(H) curves is just H_{c2}^pi(T); a qualitative discussion of
the possible reason of this finding is given.Comment: 20 pages, 8 embedded figures, 2 Appendices, accepted for publication
in Supercond. Sci. Techno
Critical-state effects on microwave losses in type-II superconductors
We discuss the microwave energy losses in superconductors in the critical
state. The field-induced variations of the surface resistance are determined,
in the framework of the Coffey and Clem model, by taking into account the
distribution of the vortex magnetic field inside the sample. It is shown that
the effects of the critical state cannot generally be disregarded to account
for the experimental data. Results obtained in bulk niobium at low temperatures
are quantitatively justified.Comment: 4 pages, 4 embedded figures, to be published on Eur. Phys. J.
Depinning frequency in a heavily neutron-irradiated MgB2 sample
The magnetic-field-induced variations of the microwave surface resistance
have been investigated in a heavily neutron-irradiated MgB2 sample, in which
the irradiation has caused the merging of the two gaps into a single value. The
experimental results have been analyzed in the framework of the Coffey and Clem
model. By fitting the experimental data, we have determined the field
dependence of the depinning frequency, omega_0, at different values of the
temperature. Although the pinning is not particularly effective, the value of
omega_0 obtained at low temperatures is considerably higher than that observed
in conventional low-temperature superconductors.Comment: 15 pages, 6 figures; to be published in Physica
Extended-spectrum ß-lactamase, AmpC-producing, and fluoroquinolone-resistant Escherichia coli in retail broiler chicken meat, Italy
Background: Globally, antimicrobial drug-resistant Escherichia coli is among the most common etiological agents of invasive disease in humans. In Europe, increasing proportions of infections due to third-generation cephalosporins and/or fluoroquinolone-resistant extraintestinal pathogenic E. coli (ExPEC) strains are reported. E. coli from poultry are those more closely linked to human E. coli, but lack of reliable data makes it difficult to assess the attributable risk of different food sources. In the present study, our objective was to investigate the antimicrobial resistance profile, phylogenetic background, and virulence factors of E. coli isolates from broiler chicken meat sold at retail in Palermo, Italy. Materials and Methods: Isolation of multidrug resistant (MDR) E. coli was performed during April-December 2013 on a total of 163 chicken meat samples. Susceptibility to a panel of nine antimicrobial agents was determined. PCR assays were carried out to detect extended-spectrum β-lactamase (ESBL), plasmid-mediated AmpC β-lactamase, and plasmid-mediated quinolone resistance (PMQR) genes, phylogenetic group, and ExPEC-associated traits. A single nucleotide polymorphism (SNP) PCR was done to detect E. coli sequence type (ST)131. Results: One hundred thirty-four isolates from 109 meat samples were MDR. B1 was the most prevalent phylogenetic group (47.8%), followed by groups D (25.4%), A (22.3%), and B2 (4.5%). ESBLs and AmpC β-lactamases were detected by PCR in 132 (98.5%) and 15 (11.2%) isolates. PMQR determinants were detected in 122 (91%) isolates. Twenty-two MDR isolates met the molecular definition of ExPEC. SNP-PCR results confirmed that four B2 isolates were ST131. Enterobacterial Repetitive Intergenic Consensus sequence-PCR analysis showed a large heterogeneity with 55 unique profiles and 31 clusters including 2-4 isolates. Conclusions: An alarmingly high prevalence of MDR E. coli from broiler chicken meat is evident in our geographic area. The ongoing use of antimicrobial drugs in livestock should be urgently restricted, particularly in the poultry sector
Microwave surface resistance of pristine and neutron-irradiated MgB2 samples in magnetic field
We report on the microwave surface resistance of two polycrystalline Mg11B2
samples; one consists of pristine material, the other has been irradiated at
very high neutron fluence. It has already been reported that in the strongly
irradiated sample the two gaps merge into a single value. The mw surface
resistance has been measured in the linear regime as a function of the
temperature and the DC magnetic field, at increasing and decreasing fields. The
results obtained in the strongly irradiated sample are quite well justified in
the framework of a generalized Coffey and Clem model, in which we take into
account the field distribution inside the sample due to the critical state. The
results obtained in the pristine sample show several anomalies, especially at
low temperatures, which cannot be justified in the framework of standard models
for the fluxon dynamics. Only at temperatures near Tc and for magnetic fields
greater than 0.5Hc2(T) the experimental data can quantitatively be accounted
for by the Coffey and Clem model, provided that the upper-critical-field
anisotropy is taken into due account.Comment: RevTeX, 13 pages with 10 eps figures, in press on EPJ
Electromagnetic response of LaO_0.94F_0.06FeAs: AC susceptibility and microwave surface resistance
We discuss on the electromagnetic response of a polycrystalline sample of
LaO_0.94F_0.06FeAs exposed to DC magnetic fields up to 10 kOe. The low- and
high-frequency responses have been investigated by measuring the AC
susceptibility at 100 kHz and the microwave surface resistance at 9.6 GHz. At
low as well as high DC magnetic fields, the susceptibility strongly depends on
the amplitude of the AC driving field, highlighting enhanced nonlinear effects.
The field dependence of the AC susceptibility exhibits a magnetic hysteresis
that can be justified considering the intragrain-field-penetration effects on
the intergrain critical current density. The microwave surface resistance
exhibits a clockwise magnetic hysteresis, which cannot be justified in the
framework of the critical-state models of the Abrikosov-fluxon lattice; it may
have the same origin as that detected in the susceptibility.Comment: 8 pages, 4 embedded eps figures; Proceedings of the 9th EUCAS
Conference (Dresden, Germany, September 13-17, 2009
La sorveglianza della tubercolosi in Sicilia
Obiettivi. Descrivere l’andamento dei casi di tubercolosi (TB) in Sicilia negli anni 2010-2013.
Disegno e partecipanti. Indagine osservazionale delle notifiche di TB relative agli anni 2010-2013 mediante l’analisi dei dati del
Sistema informativo delle malattie infettive (SIMI) e indagine epidemiologico-molecolare mediante tipizzazione spoligo- e 24-
loci mycobacterial interspersed repetitive units-variable number of tandem repeats (MIRU-VNTR) dei ceppi di Mycobacterium tuberculosis
complex (MTBC) isolati a Palermo negli anni 2012-2013.
Risultati. Negli anni 2010-2013 sono stati notificati in Sicilia 876 casi di TB con un tasso medio annuale di notifica pari a 4,4
casi per 100.000 abitanti. La proporzione di casi di TB in soggetti nati all’estero è crescente e nel 2012 e 2013 supera quella
nella popolazione autoctona. L’età mediana dei pazienti stranieri è risultata significativamente inferiore agli italiani (30,5 vs. 51,2
anni, p <0,001). La distribuzione per classi di età è stata significativamente differente nelle due subpopolazioni, con la proporzione
più elevata di casi nelle fascia di età 15-44 anni tra gli stranieri e, al contrario, una distribuzione uniforme dei pazienti italiani
tra le fasce di età 15-44, 45-64 e >64 anni. Nell’82,5% dei casi è stata diagnosticata una localizzazione polmonare.
La tipizzazione molecolare è stata effettuata su 151 ceppi di MTBC. Mediante spoligotyping essi sono stati attribuiti a 14 differenti
lineage e 33 sublineage rappresentati con frequenza diversa tra le due subpopolazioni di pazienti. Soltanto nove ceppi (6,2%)
sono stati inclusi in quattro cluster. Sono stati identificati, infine, due ceppi multiresistenti.
Conclusioni. L’epidemiologia della TB in Sicilia è complessa e in rapida evoluzione, con una crescente proporzione di casi in soggetti
non italiani e una grande eterogeneità dei ceppi di MTBC circolanti. L’integrazione di strumenti convenzionali e molecolari
è necessaria per una sorveglianza efficace e un’interpretazione accurata dell’epidemiologia della TB in Sicilia
Isolation and expression of a novel MBL-like collectin cDNA enhanced by LPS injection in the body wall of the ascidian Ciona intestinalis
Collectins are a family of calcium-dependent lectins that are characterized by their collagen-like domains. Considerable interest has been focused on this class of proteins because of their ability to interact with components of the complement system activating a cascade of events responsible for the activation of the innate immune system. A differential screening between LPS-challenged and na\uefve Ciona intestinalis has been performed allowing the isolation of a full length cDNA encoding for a 221 AA protein. In silico analysis has shown that this polypeptide displays protein domains with similarities to mannose-binding lectins. A phylogenetic analysis suggested that C. intestinalis MBL has evolved early as a prototype of vertebrate MBL. Real-time PCR assay demonstrated that this gene is strongly activated after LPS injection in the tunica. In situ hybridization performed in LPS-induced animals has shown that this gene is expressed in granular amoebocytes and large granules hemocytes in the inflamed body wall tissue. Finally, an antimicrobial activity of the C. intestinalis MBL has been demonstrate
Staphylococcal cassette chromosome mec typing and meca sequencing in methicillin-resistant staphylococci from Algeria: A highly diversified element with new mutations in mecA
Genetic mechanisms of methicillin resistance are still relevant in staphylococci. The aims of this study are to assess the possible exchanges of staphylococcal cassette chromosome mec (SCCmec) among isolates of methicillin-resistant staphylococci (MRS) and to check for known or new mutations in mecA DNA. A total of 35 MRS non-repetitive isolates were recovered, including 20 Staphylococcus haemolyticus, 7 Staphylococcus aureus, 4 Staphylococcus sciuri, 2 Staphylococcus saprophyticus and 1 isolate each of Staphylococcus xylosus and Staphylococcus lentus. Only 16 of the 35 strains were assigned to known SCCmec types: 7 SCCmec VII, 6 SCCmec IV and 3 SCCmec III, with possible horizontal transfer of the SCCmec VII from methicillin-resistant S. haemolyticus to methicillin-susceptible S. aureus. mecA gene sequencing in ten selected isolates allowed description of nine punctual mutations, seven of which were reported for the first time. The most frequent mutation was G246E, identified in isolates of methicillin-resistant S. aureus, S. sciuri, S. saprophyticus and S. lentus. These results emphasized the high degree of genetic diversity of SCCmec element in MRS and describe new missense mutations in mecA, which might be important in understanding the evolution of methicillin and new b-lactam resistance
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