9,047 research outputs found
Topology-Induced Inverse Phase Transitions
Inverse phase transitions are striking phenomena in which an apparently more
ordered state disorders under cooling. This behavior can naturally emerge in
tricritical systems on heterogeneous networks and it is strongly enhanced by
the presence of disassortative degree correlations. We show it both
analytically and numerically, providing also a microscopic interpretation of
inverse transitions in terms of freezing of sparse subgraphs and coupling
renormalization.Comment: 4 pages, 4 figure
Experimental evidence of laser power oscillations induced by the relative Fresnel (Goos-Haenchen) phase
The amplification of the relative Fresnel (Goos-Haenchen) phase by an
appropriate number of total internal reflections and the choice of favorable
incidence angles allow to observe full oscillations in the power of a DPSS
laser transmitted through sequential BK7 blocks. The experimental results
confirm the theoretical predictions. The optical apparatus used in this letter
can be seen as a new type of two-phase ellipsometric system where the phase of
the complex refractive index is replaced by the relative Fresnel
(Goos-Haenchen) phase.Comment: 7 pages, 3 figures, 1 tabl
Jeans analysis of self-gravitating systems in f(R)-gravity
Dynamics and collapse of collisionless self-gravitating systems is described
by the coupled collisionless Boltzmann and Poisson equations derived from
-gravity in the weak field approximation. Specifically, we describe a
system at equilibrium by a time-independent distribution function
and two potentials and solutions of the modified
Poisson and collisionless Boltzmann equations. Considering a small perturbation
from the equilibrium and linearizing the field equations, it can be obtained a
dispersion relation. A dispersion equation is achieved for neutral
dust-particle systems where a generalized Jeans wave-number is obtained. This
analysis gives rise to unstable modes not present in the standard Jeans
analysis (derived assuming Newtonian gravity as weak filed limit of ).
In this perspective, we discuss several self-gravitating astrophysical systems
whose dynamics could be fully addressed in the framework of -gravity.Comment: 8 pages, 2 figures, Accepted for publication in PR
The Cerebellar Fastigial Nucleus Contributes to CO\u3csub\u3e2\u3c/sub\u3e-H\u3csup\u3e+\u3c/sup\u3e Ventilatory Sensitivity in Awake Goats
The purpose of this study was to test the hypothesis that an intact cerebellar fastigial nucleus (CFN) is an important determinant of CO2-H+ sensitivity during wakefulness. Bilateral, stainless steel microtubules were implanted into the CFN (N = 9) for injection (0.5â10 ÎŒl) of the neurotoxin ibotenic acid. Two or more weeks after implantation of the microtubules, eupneic breathing and CO2-H+ sensitivity did not differ significantly (P \u3e 0.10) from pre-implantation conditions. Injection of ibotenic acid (50 mM) did not significantly alter eupneic PaCO2 (P \u3e 0.10). The coefficient of variation of eupneic PaCO2 was 4.0 ± 0.6 and 3.7 ± 0.4% over the 2 weeks before and after the lesion, respectively. CO2-H+ sensitivity expressed as inspired ventilation/PaCO2 decreased from 2.15 ± 0.17 pre-lesion to 1.58 ± 0.26 l/(min mmHg) 3â6 days post-lesion (P \u3c 0.02, â27%). There was no significant (P \u3e 0.10) recovery of sensitivity between 7 and 10 days post-lesion. The lesion also increased (P \u3c 0.05) the day-to-day variability of this index by nearly 100%. When CO2 sensitivity was expressed as elevated inspired CO2/room air VI, values at 7%, but not 3 and 5% inspired CO2, were reduced and more variable (P \u3c 0.05) after the ibotenic acid injections. We conclude that during wakefulness, the CFN contributes relatively more to overall ventilatory drive at high relative to low levels of hypercapnia
Magnetic field and unstable accretion during AM Herculis low states
A study of AM Her low states in September 1990 and 1991 and June-July 1997 is
reported from a coordinated campaign with observations obtained at the
Haute-Provence observatory, at the 6-m telescope of the Special Astrophysical
Observatory and at the 2.6m and 1.25m telescopes of the Crimean observatory.
Spectra obtained at different dates when the source was in low states at a
comparable V magnitude, show the presence of strong Zeeman absorption features
and marked changes in emission lines with a day-to-day reappearance of the HeII
(4686\AA) emission lines in 1991. Despite this variability, the magnetic field
inferred from the fitting of the absorption spectrum with Zeeman hydrogen
splitting, is remarkably constant with a best value of (12.50.5)MG.
Detailed analysis of the UBVRI light curves shows the presence of repetitive
moderate amplitude ( 0.3-0.5 mag) flares predominantly red in colour.
These flares are attributed to small accretion events and are compared to the
large ( 2 mag.) blue flare reported by Shakhovskoy et al. (1993). We
suggest that the general flaring activity observed during the low states is
generated by accretion events. The different characteristics of the flares
(colour and polarization) are the results of different shock geometries
depending on the net mass accretion flux.Comment: accepted in Astronomy & Astrophysics (Main Journal), 10 pages, 6
Figures, Late
A2163: Merger events in the hottest Abell galaxy cluster II. Subcluster accretion with galaxy-gas separation
Located at z = 0.203, A2163 is a rich galaxy cluster with an intra-cluster
medium (ICM) that exhibits extraordinary properties, including an exceptionally
high X-ray luminosity, average temperature, and a powerful and extended radio
halo. The irregular and complex morphology of its gas and galaxy structure
suggests that this cluster has recently undergone major merger events that
involve two or more cluster components. In this paper, we study the gas
structure and dynamics by means of spectral-imaging analysis of X-ray data
obtained from XMM-Newton and Chandra observations. From the evidence of a cold
front, we infer the westward motion of a cool core across the E-W elongated
atmosphere of the main cluster A2163-A. Located close to a galaxy over-density,
this gas 'bullet' appears to have been spatially separated from its galaxy (and
presumably dark matter component) as a result of high-velocity accretion.
From gas brightness and temperature profile analysis performed in two
opposite regions of the main cluster, we show that the ICM has been
adiabatically compressed behind the crossing 'bullet' possibly because of shock
heating, leading to a strong departure of the ICM from hydrostatic equilibrium
in this region. Assuming that the mass estimated from the Yx proxy best
indicates the overall mass of the system and that the western cluster sector is
in approximate hydrostatic equilibrium before subcluster accretion, we infer a
merger scenario between two subunits of mass ratio 1:4, leading to a present
total system mass of M500 . The exceptional
properties of A2163 present various similarities with those of 1E0657-56, the
so-called 'bullet-cluster'. These similarities are likely to be related to a
comparable merger scenario.Comment: A&A, in pres
Electron-electron interaction effects in quantum point contacts
We consider electron-electron interaction effects in quantum point contacts on the first quantization plateau, taking into account all scattering processes. We compute the low-temperature linear and nonlinear conductance, shot noise, and thermopower, by perturbation theory and a self-consistent nonperturbative method. On the conductance plateau, the low-temperature corrections are solely due to momentum-nonconserving processes that change the relative number of left- and right-moving electrons. This leads to a suppression of the conductance for increasing temperature or voltage. The size of the suppression is estimated for a realistic saddle-point potential, and is largest in the beginning of the conductance plateau. For large magnetic field, interaction effects are strongly suppressed by the Pauli principle, and hence the first spin-split conductance plateau has a much weaker interaction correction. For the nonperturbative calculations, we use a self-consistent nonequilibrium Green's function approach, which suggests that the conductance saturates at elevated temperatures. These results are consistent with many experimental observations related to the so-called 0.7 anomaly
EUVE J0425.6-5714: A Newly Discovered AM Herculis Star
We detected a new AM Her star serendipitously in a 25 day observation with
the EUVE satellite. A coherent period of 85.82 min is present in the EUVE Deep
Survey imager light curve of this source. A spectroscopic identification is
made with a 19th magnitude blue star that has H and He emission lines, and
broad cyclotron humps typical of a magnetic cataclysmic variable. A lower limit
to the polar magnetic field of 46 MG is estimated from the spacing of the
cyclotron harmonics. EUVE J0425.6-5714 is also detected in archival ROSAT HRI
observations spanning two months, and its stable and highly structured light
curve permits us to fit a coherent ephemeris linking the ROSAT and EUVE data
over a 1.3 yr gap. The derived period is 85.82107 +/- 0.00020 min, and the
ephemeris should be accurate to 0.1 cycles until the year 2005. A narrow but
partial X-ray eclipse suggests that this object belongs to the group of Am Her
stars whose viewing geometry is such that the accretion stream periodically
occults the soft X-ray emitting accretion spot on the surface of the white
dwarf. A non-detection of hard X-rays from ASCA observations that are
contemporaneous with the ROSAT HRI shows that the soft X-rays must dominate by
at least an order of magnitude, which is consistent with a known trend among AM
Her stars with large magnetic field. This object should not be confused with
the Seyfert galaxy 1H 0419-577 (= LB 1727), another X-ray/EUV source which lies
only 4' away, and was the principal target of these monitoring observations.Comment: 13 pages, 5 figures, to appear in PASP, Dec. 1998 issu
Qualitative analysis of small (â€2 cm) regenerative nodules, dysplastic nodules and well-differentiated HCCs with gadoxetic acid MRI
: The characterization of small lesions in cirrhotic patients is extremely difficult due to the overlap of imaging features among different entities in the step-way of the hepatocarcinogenesis. The aim of our study was to evaluate the role of gadoxetic-acid MRI in the differentiation of small (â€2 cm) well-differentiated hepatocellular carcinomas from regenerative and dysplastic nodules.
: Seventy-three cirrhotic patients, with 118 focal liver lesions (â€2 cm) were prospectively recruited. MRI examination was performed with a 3T magnet and the study protocol included T1 - and T2-weighted pre-contrast sequences and T1 -weighted gadoxetic-acid enhanced post-contrast sequences obtained during the arterial, venous, late dynamic and hepatobiliary phases. All lesions were pathologically confirmed. Two radiologists blinded to clinical and pathological information evaluated two imaging datasets; another radiologist analysed the signal intensity characteristics of each lesion. Sensitivity, specificity and diagnostic accuracy were considered for statistical analysis.
: Good agreement was reported between the two readers (Îș 0.70). Both readers reported a significantly improved sensitivity (57.7 and 66.2 vs 74.6 and 83.1) and diagnostic accuracy (0.717 and 0.778 vs 0.843 and 0.901) with the adjunction of the hepatobiliary phase 57.7 vs 74.6 and 66.2 vs 83.1 (pââ€â0.04).
: Gadoxetic-acid MRI is a reliable tool for the characterization of HCC and lesions at high risk to further develop
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