Equation of state of hard oblate ellipsoids by replica exchange Monte Carlo

We implemented the replica exchange Monte Carlo technique to produce the equation of state of hard 1:5 aspect-ratio oblate ellipsoids for a wide density range. For this purpose, we considered the analytical approximation of the overlap distance given by Bern and Pechukas and the exact numerical solution given by Perram and Wertheim. For both cases we capture the expected isotropic-nematic transition at low densities and a nematic-crystal transition at larger densities. For the exact case, these transitions occur at the volume fraction 0.341, and in the interval $0.584-0.605$, respectively.Comment: 4 pages, 2 figure

Considerations on bubble fragmentation models

n this paper we describe the restrictions that the probability density function (p.d.f.) of the size of particles resulting from the rupture of a drop or bubble must satisfy. Using conservation of volume, we show that when a particle of diameter, D0, breaks into exactly two fragments of sizes D and D2 = (D30−D3)1/3 respectively, the resulting p.d.f., f(D; D0), must satisfy a symmetry relation given by D22 f(D; D0) = D2 f(D2; D0), which does not depend on the nature of the underlying fragmentation process. In general, for an arbitrary number of resulting particles, m(D0), we determine that the daughter p.d.f. should satisfy the conservation of volume condition given by m(D0) ∫0D0 (D/D0)3 f(D; D0) dD = 1. A detailed analysis of some contemporary fragmentation models shows that they may not exhibit the required conservation of volume condition if they are not adequately formulated. Furthermore, we also analyse several models proposed in the literature for the breakup frequency of drops or bubbles based on different principles, g(ϵ, D0). Although, most of the models are formulated in terms of the particle size D0 and the dissipation rate of turbulent kinetic energy, ϵ, and apparently provide different results, we show here that they are nearly identical when expressed in dimensionless form in terms of the Weber number, g*(Wet) = g(ϵ, D0) D2/30 ϵ−1/3, with Wet ~ ρ ϵ2/3 D05/3/σ, where ρ is the density of the continuous phase and σ the surface tension

A deep and wide-field view at the IC 2944 / 2948 complex in Centaurus

We employed the ESO MPI wide-field camera and obtained deep images in the VIc pass-bands in the region of the IC 2944/2948 complex (l ~ 294; b ~ -1), and complemented them with literature and archival data. We used this material to derive the photometric, spectroscopic and kinematic properties of the brightest (V < 16) stars in the region. The VI deep photometry on the other end, helped us to unravel the lower main sequence of a few, possibly physical, star groups in the area. Our analysis confirmed previous suggestions that the extinction toward this line of sight follows the normal law (Rv = 3.1). We could recognize B-type stars spread in distance from a few hundred pc to at least 2 kpc. We found two young groups (age ~ 3 Myr) located respectively at about 2.3 and 3.2 kpc from the Sun. They are characterized by a significant variable extinction (E(B-V) ranging from 0.28 to 0.45 mag), and host a significant pre-main sequence population. We computed the initial mass functions for these groups and obtained slopes Gamma from -0.94 to -1.02 (e_Gamma = 0.3), in a scale where the classical Salpeter law is -1.35. We estimated the total mass of both main stellar groups in ~ 1100$and ~ 500$ Mo, respectively. Our kinematic analysis indicated that both groups of stars deviate from the standard rotation curve of the Milky Way, in line with literature results for this specific Galactic direction. Finally, along the same line of sight we identified a third group of early-type stars located at ~ 8 kpc from the Sun. This group might be located in the far side of the Sagittarius-Carina spiral arm.Comment: 13 pages, 11 figures, accepted for publication in MNRA

High temperature behavior of Sr-doped layered cobaltites Y(Ba1-xSrx)Co2O5.5: phase stability and structural properties

In this article we present a neutron diffraction in-situ study of the thermal evolution and high-temperature structure of layered cobaltites Y(Ba, Sr)Co2 O5+{\delta}. Neutron thermodiffractograms and magnetic susceptibility measurements are reported in the temperature range 20 K <= T <= 570 K, as well as high resolution neutron diffraction experiments at selected temperatures. Starting from the as-synthesized samples with {\delta} ~ 0.5, we show that the room temperature phases remain stable up to 550 K, where they start loosing oxygen and transform to a vacancy-disordered "112" structure with tetragonal symmetry. Our results also show how the so-called "122" structure can be stabilized at high temperature (around 450 K) in a sample in which the addition of Sr at the Ba site had suppressed its formation. In addition, we present the structural and magnetic properties of the resulting samples with a new oxygen content {\delta} ~ 0.25 in the temperature range 20 K <= T <= 300 K

Signatures of superfluidity for Feshbach-resonant Fermi gases

We consider atomic Fermi gases where Feshbach resonances can be used to continuously tune the system from weak to strong interaction regime, allowing to scan the whole BCS-BEC crossover. We show how a probing field transferring atoms out of the superfluid can be used to detect the onset of the superfluid transition in the high-$T_c$ and BCS regimes. The number of transferred atoms, as a function of the energy given by the probing field, peaks at the gap energy. The shape of the peak is asymmetric due to the single particle excitation gap. Since the excitation gap includes also a pseudogap contribution, the asymmetry alone is not a signature of superfluidity. Incoherent nature of the non-condensed pairs leads to broadening of the peak. The pseudogap and therefore the broadening decay below the critical temperature, causing a drastic increase in the asymmetry. This provides a signature of the transition.Comment: Revised version, accepted to Phys. Rev. Letters. Figures changed, explanations adde