22,571 research outputs found
Studies of hadronic B decays to final states containing open charm mesons at LHCb
International audienceThe LHCb experiment is a general purpose forward spectrometer operating at the Large Hadron Collider, optimized for the study of B and D hadrons. LHCb collected 1.0 fb-1 of integrated luminosity during 2011 data taking, collecting unprecedented large samples of B hadron decays to final states involving charmed hadrons. These decays offer many complementary measurements of CP violation and CKM matrix parameters, and serve as a laboratory for testing effective theories of hadron decays. We present a selection of new world leading results in these types of decays, including first observations of new decay modes, world best branching ratio measurements and studies of resonant structure
Nanowires: A route to efficient thermoelectric devices
Miniaturization of electronic devices aims at manufacturing ever smaller
products, from mesoscopic to nanoscopic sizes. This trend is challenging
because the increased levels of dissipated power demands a better understanding
of heat transport in small volumes. A significant amount of the consumed energy
is transformed into heat and dissipated to the environment. Thermoelectric
materials offer the possibility to harness dissipated energy and make devices
less energy-demanding. Heat-to-electricity conversion requires materials with a
strongly suppressed thermal conductivity but still high electronic conduction.
Nanowires can meet nicely these two requirements because enhanced phonon
scattering at the surface and defects reduces the lattice thermal conductivity
while electric conductivity is not deteriorated, leading to an overall
remarkable thermoelectric efficiency. Therefore, nanowires are regarded as a
promising route to achieving valuable thermoelectric materials at the
nanoscale. In this paper, we present an overview of key experimental and
theoretical results concerning the thermoelectric properties of nanowires. The
focus of this review is put on the physical mechanisms by which the efficiency
of nanowires can be improved. Phonon scattering at surfaces and interfaces,
enhancement of the power factor by quantum effects and topological protection
of electron states to prevent the degradation of electrical conductivity in
nanowires are thoroughly discussed
A pilot search for mm-wavelength recombination lines from emerging ionized winds in pre-planetary nebulae candidates
We report the results from a pilot search for radio recombination line (RRL)
emission at millimeter wavelengths in a small sample of pre-planetary nebulae
(pPNe) and young PNe (yPNe) with emerging central ionized regions. Observations
of the H30\alpha, H31a, H39a, H41a, H48b, H49b, H51b, and H55g lines at 1 and
3mm have been performed with the IRAM 30 m radio telescope. These lines are
excellent probes of the dense inner (<~150 au) and heavily obscured regions of
these objects, where the yet unknown agents for PN-shaping originate. We
detected mm-RRLs in three objects: CRL 618, MWC 922, and M 2-9. For CRL 618,
the only pPN with previous published detections of H41a, H35a, and H30a
emission, we find significant changes in the line profiles indicating that
current observations are probing regions of the ionized wind with larger
expansion velocities and mass-loss rate than ~29 years ago. In the case of MWC
922, we observe a drastic transition from single-peaked profiles at 3mm to
double-peaked profiles at 1mm, which is consistent with maser amplification of
the highest frequency lines; the observed line profiles are compatible with
rotation and expansion of the ionized gas, probably arranged in a disk+wind
system around a ~5-10 Msun central mass. In M 2-9, the mm-RRL emission appears
to be tracing a recent mass outburst by one of the stars of the central binary
system. We present the results from non-LTE line and continuum radiative
transfer models, which enables us to constrain the structure, kinematics, and
physical conditions (electron temperature and density) of the ionized cores of
our sample. (abridged). We deduce mass-loss rates of ~1e-6-1e-7 Msun/yr, which
are significantly higher than the values adopted by stellar evolution models
currently in use and would result in a transition from the asymptotic giant
branch to the PN phase faster than hitherto assumed.Comment: Accepted by Astronomy and Astrophysics. 28 pages, including figure
The diffuse Nitsche method: Dirichlet constraints on phase-field boundaries
We explore diffuse formulations of Nitsche's method for consistently imposing Dirichlet boundary conditions on phase-field approximations of sharp domains. Leveraging the properties of the phase-field gradient, we derive the variational formulation of the diffuse Nitsche method by transferring all integrals associated with the Dirichlet boundary from a geometrically sharp surface format in the standard Nitsche method to a geometrically diffuse volumetric format. We also derive conditions for the stability of the discrete system and formulate a diffuse local eigenvalue problem, from which the stabilization parameter can be estimated automatically in each element. We advertise metastable phase-field solutions of the Allen-Cahn problem for transferring complex imaging data into diffuse geometric models. In particular, we discuss the use of mixed meshes, that is, an adaptively refined mesh for the phase-field in the diffuse boundary region and a uniform mesh for the representation of the physics-based solution fields. We illustrate accuracy and convergence properties of the diffuse Nitsche method and demonstrate its advantages over diffuse penalty-type methods. In the context of imaging based analysis, we show that the diffuse Nitsche method achieves the same accuracy as the standard Nitsche method with sharp surfaces, if the inherent length scales, i.e., the interface width of the phase-field, the voxel spacing and the mesh size, are properly related. We demonstrate the flexibility of the new method by analyzing stresses in a human vertebral body
Interplay between the magnetic anisotropy contributions of Cobalt nanowires
We report on the magnetic properties and the crystallographic structure of
the cobalt nanowire arrays as a function of their nanoscale dimensions. X-ray
diffraction measurements show the appearance of an in-plane HCP-Co phase for
nanowires with 50 nm diameter, suggesting a partial reorientation of the
magnetocrystalline anisotropy axis along the membrane plane with increasing
pore diameter. No significant changes in the magnetic behavior of the nanowire
system are observed with decreasing temperature, indicating that the effective
magnetoelastic anisotropy does not play a dominant role in the remagnetization
processes of individual nanowires. An enhancement of the total magnetic
anisotropy is found at room temperature with a decreasing nanowire
diameter-to-length ratio (d/L), a result that is quantitatively analyzed on the
basis of a simplified shape anisotropy model.Comment: 8 pages, 4 figure
VIMOS-VLT spectroscopy of the giant Ly-alpha nebulae associated with three z~2.5 radio galaxies
The morphological and spectroscopic properties of the giant (>60 kpc)
Ly-alpha nebulae associated with three radio galaxies at z~2.5 (MRC 1558-003,
MRC 2025-218 and MRC 0140-257) have been investigated using integral field
spectroscopic data obtained with VIMOS on VLT.
The morphologies are varied. The nebula of one source has a centrally peaked,
rounded appearance. In the other two objects, it consists of two spatial
components. The three nebulae are aligned with the radio axis within <30 deg.
The total Ly-alpha luminosities are in the range (0.3-3.4) x 1e44 erg s-1. The
Ly-alpha spectral profile shows strong variation through the nebulae, with FWHM
values in the range ~400-1500 km s-1 and velocity shifts V~120-600 km s-1.
We present an infall model which can explain successfully most Ly-alpha
morphological and spectroscopic properties of the nebula associated with MRC
1558-003. This adds further support to our previous conclusion that the
_quiescent_ giant nebulae associated with this and other high redshift powerful
radio galaxies are in infall. A problem for this model is the difficulty to
reproduce the large Ly-alpha FWHM values.
We have discovered a giant (~85 kpc) Ly-alpha nebula associated with the
radio galaxy MRC 0140-257 at z=2.64. It shows strikingly relaxed kinematics
(FWHM2) radio galaxies.Comment: 14 pages, 13 figures. Accepted for publication in MNRA
Discovery of a wide companion near the deuterium burning mass limit in the Upper Scorpius association
We present the discovery of a companion near the deuterium burning mass limit
located at a very wide distance, at an angular separation of 4.6+/-0.1 arcsec
(projected distance of ~ 670 AU) from UScoCTIO108, a brown dwarf of the very
young Upper Scorpius association. Optical and near-infrared photometry and
spectroscopy confirm the cool nature of both objects, with spectral types of M7
and M9.5, respectively, and that they are bona fide members of the association,
showing low gravity and features of youth. Their masses, estimated from the
comparison of their bolometric luminosities and theoretical models for the age
range of the association, are 60+/-20 and 14^{+2}_{-8} MJup, respectively. The
existence of this object around a brown dwarf at this wide orbit suggests that
the companion is unlikely to have formed in a disk based on current planet
formation models. Because this system is rather weakly bound, they did not
probably form through dynamical ejection of stellar embryos.Comment: 10 pages, including 4 figures and 2 table
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