4,607 research outputs found
Taming the magnetoresistance anomaly in graphite
At low temperatures, graphite presents a magnetoresistance anomaly which
manifests as a transition to a high-resistance state (HRS) above a certain
critical magnetic field . Such HRS is currently attributed
to a c-axis charge-density-wave taking place only when the lowest Landau level
is populated. By controlling the charge carrier concentration of a gated sample
through its charge neutrality level (CNL), we were able to experimentally
modulate the HRS in graphite for the first time. We demonstrate that the HRS is
triggered both when electrons and holes are the majority carriers but is
attenuated near the CNL. Taking screening into account, our results indicate
that the HRS possess a strong in-plane component and can occur below the
quantum limit, being at odds with the current understanding of the phenomenon.
We also report the effect of sample thickness on the HRS
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The properties and genesis environments of South Atlantic cyclones
A new climatology of South Atlantic cyclones is produced to provide new insights into the conditions leading to genesis in different regions of the domain. Cyclones are identifed and tracked based on the relative vorticity at 850 hPa computed from the winds. The characteristics of the cyclones are obtained by diagnostic variables sampled within a radial distance from the center to produce a spatial distribution of cyclone properties at the time of genesis. Also, cyclone centered composites are used to analyze the cyclone structure and evolution of cyclones during their genesis. There are four main cyclogenesis region in the South Atlantic Ocean: the Southern Brazilian coast (SE-BR, 30â—¦S), over the continent near the La Plata river discharge region (LA PLATA, 35â—¦S), the southeastern coast of Argentina (ARG, 40â—¦S-55â—¦S) and the Southeastern Atlantic (SE-SAO, centered at 45â—¦S and 10â—¦W). We found that cyclogenesis northward of 35â—¦S occurs mainly due to low-level forcing associated with moisture transport in the summer, and is associated with upper-level forcing in the winter due to a strong baroclinic environment and potential vorticity intrusions. Southward of 35â—¦S, cyclones develop in a high baroclinic environment throughout the year with only a small influence from moist processes. The cyclone composites reveal that SE-BR and SE-SAO cyclones are associated with secondary development, the LA PLATA cyclones development is influenced by an orographic low in their early stages, and ARG cyclones are influenced by thermal advection as an essential mechanism in the reduction of static stability
Lifetimes of ultralong-range strontium Rydberg molecules in a dense BEC
The lifetimes and decay channels of ultralong-range Rydberg molecules created
in a dense BEC are examined by monitoring the time evolution of the Rydberg
population using field ionization. Studies of molecules with values of
principal quantum number, , in the range to that contain tens
to hundreds of ground state atoms within the Rydberg electron orbit show that
their presence leads to marked changes in the field ionization characteristics.
The Rydberg molecules have lifetimes of s, their destruction
being attributed to two main processes: formation of Sr ions through
associative ionization, and dissociation induced through -changing
collisions. The observed loss rates are consistent with a reaction model that
emphasizes the interaction between the Rydberg core ion and its nearest
neighbor ground-state atom. The measured lifetimes place strict limits on the
time scales over which studies involving Rydberg species in cold, dense atomic
gases can be undertaken and limit the coherence times for such measurements.Comment: 9 pages, 8 figure
Effect of structural disorder on quantum oscillations in graphite
PublishedWe have studied the effect of structural disorder on the de Haas van Alphen and Shubnikov de Haas quantum oscillations measured in natural, Kish, and highly oriented pyrolytic graphite samples at temperatures down to 30 mK and at magnetic fields up to 14 T. The measurements were performed on different samples characterized by means of x-ray diffractometry, transmission electron microscopy, and atomic-force microscopy techniques. Our results reveal a correlation between the amplitude of quantum oscillations and the sample surface roughness.This work was carried out with the support of CNPq (Conselho Nacional de Desenvolvimento CientÃfico e Tecnológico—Brasil) and FAPESP (Fundação de Amparo a Pesquisa do Estado de São Paulo)
Extraordinary magnetoresistance in graphite: experimental evidence for the time-reversal symmetry breaking
The ordinary magnetoresistance (MR) of doped semiconductors is positive and
quadratic in a low magnetic field, B, as it should be in the framework of the
Boltzmann kinetic theory or in the conventional hopping regime. We observe an
unusual highly-anisotropic in-plane MR in graphite, which is neither quadratic
nor always positive. In a certain current direction MR is negative and linear
in B in fields below a few tens of mT with a crossover to a positive MR at
higher fields, while in a perpendicular current direction we observe a giant
super-linear and positive MR. These extraordinary MRs are respectively
explained by a hopping magneto-conductance via non-zero angular momentum
orbitals, and by the magneto-conductance of inhomogeneous media. The linear
orbital NMR is a unique signature of the broken time-reversal symmetry (TRS) in
graphite. While some local paramagnetic centers could be responsible for the
broken TRS, the observed large diamagnetism suggests a more intriguing
mechanism of this breaking, involving superconducting clusters with
unconventional (chiral) order parameters and spontaneously generated
normal-state current loops in graphite.Comment: 4 pages, 5 figure
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Virulence factors in vancomycin-resistant and vancomycin- susceptible Enterococcus faecalis from Brazil
Enterococci are members of commensal flora of animals and insects, but are also important opportunistic pathogens. Our objective was to observe if there was any difference of virulence in several groups of E. faecalis, mainly between vancomycin-resistant E. faecalis (VREFS) of colonization and infection. VREFS and vancomycin-sensitive E. faecalis from Brazil were screened for the presence of virulence factor genes. Phenotypic assays were used to assess in vitro expression, to understand the pathogenic potential of these isolates and to determine whether a correlation exists between virulence and antibiotic resistance. Different virulence profiles were found suggesting that the disseminating clone may have generated several variations. However, our study showed that one constellation of traits appeared most commonly: gelatinase, aggregation substance and esp (GEA). These factors are important because they have been implicated in cell aggregation and biofilm formation. Biofilm formation may promote the conjugation of plasmids harboring resistance and virulence genes, enhancing the probability of entry of new resistance genes into species. Curiously, the profile GEA was not exclusive to VREFS, it was the second most observed in VSEFS isolates from colonization and infection in hospitalized patients and also from rectal swabs of healthy volunteers. Such strains appear to represent the entry gateway to new resistance genes into E. faecalis and may contribute to the spreading of E. faecalis mainly in hospitals
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