42,165 research outputs found
Room temperature ferromagnetic-like behavior in Mn-implanted and post-annealed InAs layers deposited by Molecular Beam Epitaxy
We report on the magnetic and structural properties of Ar and Mn implanted
InAs epitaxial films grown on GaAs (100) by Molecular Beam Epitaxy (MBE) and
the effect of Rapid Thermal Annealing (RTA) for 30 seconds at 750C. Channeling
Particle Induced X- ray Emission (PIXE) experiments reveal that after Mn
implantation almost all Mn atoms are subsbtitutional in the In-site of the InAs
lattice, like in a diluted magnetic semiconductor (DMS). All of these samples
show diamagnetic behavior. But, after RTA treatment the Mn-InAs films exhibit
room-temperature magnetism. According to PIXE measurements the Mn atoms are no
longer substitutional. When the same set of experiments were performed with As
as implantation ion all of the layers present diamagnetism without exception.
This indicates that the appearance of room-temperature ferromagnetic-like
behavior in the Mn-InAs-RTA layer is not related to lattice disorder produce
during implantation, but to a Mn reaction produced after a short thermal
treatment. X-ray diffraction patterns (XRD) and Rutherford Back Scattering
(RBS) measurements evidence the segregation of an oxygen deficient-MnO2 phase
(nominally MnO1.94) in the Mn-InAs-RTA epitaxial layers which might be on the
origin of room temperature ferromagnetic-like response observed.Comment: 16 pages, 5 figures. Acepted in J. Appl. Phy
Radiation Magnetohydrodynamics In Global Simulations Of Protoplanetary Disks
Our aim is to study the thermal and dynamical evolution of protoplanetary
disks in global simulations, including the physics of radiation transfer and
magneto-hydrodynamic (MHD) turbulence caused by the magneto-rotational
instability. We develop a radiative transfer method based on the flux-limited
diffusion approximation that includes frequency dependent irradiation by the
central star. This hybrid scheme is implemented in the PLUTO code. The focus of
our implementation is on the performance of the radiative transfer method.
Using an optimized Jacobi preconditioned BiCGSTAB solver, the radiative module
is three times faster than the MHD step for the disk setup we consider. We
obtain weak scaling efficiencies of 70% up to 1024 cores. We present the first
global 3D radiation MHD simulations of a stratified protoplanetary disk. The
disk model parameters are chosen to approximate those of the system AS 209 in
the star-forming region Ophiuchus. Starting the simulation from a disk in
radiative and hydrostatic equilibrium, the magnetorotational instability
quickly causes MHD turbulence and heating in the disk. For the disk parameters
we use, turbulent dissipation heats the disk midplane and raises the
temperature by about 15% compared to passive disk models. A roughly flat
vertical temperature profile establishes in the disk optically thick region
close to the midplane. We reproduce the vertical temperature profile with a
viscous disk models for which the stress tensor vertical profile is flat in the
bulk of the disk and vanishes in the disk corona. The present paper
demonstrates for the first time that global radiation MHD simulations of
turbulent protoplanetary disks are feasible with current computational
facilities. This opens up the windows to a wide range of studies of the
dynamics of protoplanetary disks inner parts, for which there are significant
observational constraints.Comment: Accepted to A&
Unconventional quasiparticle lifetime in undoped graphene
We address the question of how small can the quasiparticle decay rate be at
low energies in undoped graphene, where kinematical constraints are known to
prevent the decay into particle-hole excitations. For this purpose, we study
the renormalization of the phonon dispersion by many-body effects, which turns
out to be very strong in the case of the out-of-plane phonons at the K point of
the spectrum. We show that these evolve into a branch of very soft modes that
provide the relevant channel for quasiparticle decay, at energies below the
scale of the optical phonon modes. In this regime, we find that the decay rate
is proportional to the cube of the quasiparticle energy. This implies that a
crossover should be observed in transport properties from the linear dependence
characteristic of the high-energy regime to the much slower decay rate due to
the soft phonon modes.Comment: 5 pages, 1 figur
Combined analysis of the decays and
In a combined study of the decay spectra of and
decays within a dispersive representation of the
required form factors, we illustrate how the resonance parameters,
defined through the pole position in the complex plane, can be extracted with
improved precision as compared to previous studies. While we obtain a
substantial improvement in the mass, the uncertainty in the width is only
slightly reduced, with the findings MeV and
MeV. Further constraints on the width
could result from updated analyses of the and/or spectra using
the full Belle-I data sample. Prospects for Belle-II are also discussed. As the
vector form factor enters the description of the decay , we are in a position to investigate isospin violations in its
parameters like the form factor slopes. In this respect also making available
the spectrum of the transition would be extremely
useful, as it would allow to study those isospin violations with much higher
precision.Comment: 20 pages, 1figur
Suspensions Thermal Noise in the LIGO Gravitational Wave Detector
We present a calculation of the maximum sensitivity achievable by the LIGO
Gravitational wave detector in construction, due to limiting thermal noise of
its suspensions. We present a method to calculate thermal noise that allows the
prediction of the suspension thermal noise in all its 6 degrees of freedom,
from the energy dissipation due to the elasticity of the suspension wires. We
show how this approach encompasses and explains previous ways to approximate
the thermal noise limit in gravitational waver detectors. We show how this
approach can be extended to more complicated suspensions to be used in future
LIGO detectors.Comment: 28 pages, 13 figure
Isospin breaking in the nucleon mass and the sensitivity of β decays to new physics
We discuss the consequences of the approximate conservation of the vector and axial currents for the hadronic matrix elements appearing in β decay if nonstandard interactions are present. In particular, the isovector (pseudo)scalar charge gS(P) of the nucleon can be related to the difference (sum) of the nucleon masses in the absence of electromagnetic effects. Using recent determinations of these quantities from phenomenological and lattice QCD studies we obtain the accurate values gS=1.02(11) and gP=349(9) in the modified minimal subtraction scheme at μ=2 GeV. The consequences for searches of nonstandard scalar interactions in nuclear β decays are studied, finding for the corresponding Wilson coefficient εS=0.0012(24) at 90% C.L., which is significantly more stringent than current LHC bounds and previous low-energy bounds using less precise gS values. We argue that our results could be rapidly improved with updated computations and the direct calculation of certain ratios in lattice QCD. Finally, we discuss the pion-pole enhancement of gP, which makes β decays much more sensitive to nonstandard pseudoscalar interactions than previously thought
Numerical Implementation of a Critical State Model for Soft Rocks
This paper details the basic tasks for the numerical implementation of a simple elasto-plastic critical state model for bonded materials (i.e. soft rocks-hard soils) into the finite element program SNAC developed at the University of Newcastle in Australia. The first task described focusses on the derivation of the incremental constitutive relationships used to represent the mechanical response of a bonded/cemented material under saturated conditions. The second task presents how these stress-strain relations can be numerically integrated using an explicit substepping scheme with automatic error control. The third task concentrates on the verification of the substepping algorithm proposed. The model used to represent the saturated mechanical response of a bonded material combines the modified Cam clay with the constitutive relationships for cemented materials proposed in Gens & Nova (1993), but incorporates some flexibility on the degradation law adopted. The role of suction and other relevant aspects of unsaturated behaviour are also discussed at the end of the paper
Searching for a link between the presence of chemical spots on the surface of HgMn stars and their weak magnetic fields
We present the results of mapping the HgMn star AR Aur using the Doppler
Imaging technique for several elements and discuss the obtained distributions
in the framework of a magnetic field topology.Comment: 2 pages, 1 figure, to appear in Proceedings of IAU Symposium 259
"Cosmic Magnetic Fields: from Planets, to Stars and Galaxies", Tenerife,
Spain, November 3-7, 200
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