2,672 research outputs found
Synchrotron emission in molecular cloud cores: the SKA view
Understanding the role of magnetic fields in star-forming regions is of
fundamental importance. In the near future, the exceptional sensitivity of SKA
will offer a unique opportunity to evaluate the magnetic field strength in
molecular clouds and cloud cores through synchrotron emission observations. The
most recent Voyager 1 data, together with Galactic synchrotron emission and
Alpha Magnetic Spectrometer data, constrain the flux of interstellar cosmic-ray
electrons between MeV and GeV, in particular in the
energy range relevant for synchrotron emission in molecular cloud cores at SKA
frequencies. Synchrotron radiation is entirely due to primary cosmic-ray
electrons, the relativistic flux of secondary leptons being completely
negligible. We explore the capability of SKA in detecting synchrotron emission
in two starless molecular cloud cores in the southern hemisphere, B68 and FeSt
1-457, and we find that it will be possible to reach signal-to-noise ratios of
the order of at the lowest frequencies observable by SKA ( MHz)
with one hour of integration.Comment: 5 pages, 4 figures, accepted by Astronomy & Astrophysic
Large scale GW calculations
We present GW calculations of molecules, ordered and disordered solids and
interfaces, which employ an efficient contour deformation technique for
frequency integration, and do not require the explicit evaluation of virtual
electronic states, nor the inversion of dielectric matrices. We also present a
parallel implementation of the algorithm which takes advantage of separable
expressions of both the single particle Green's function and the screened
Coulomb interaction. The method can be used starting from density functional
theory calculations performed with semi-local or hybrid functionals. We applied
the newly developed technique to GW calculations of systems of unprecedented
size, including water/semiconductor interfaces with thousands of electrons
Nonempirical Range-separated Hybrid Functionals for Solids and Molecules
Dielectric-dependent hybrid (DDH) functionals were recently shown to yield
accurate energy gaps and dielectric constants for a wide variety of solids, at
a computational cost considerably less than that of GW calculations. The
fraction of exact exchange included in the definition of DDH functionals
depends (self-consistently) on the dielectric constant of the material. Here we
introduce a range-separated (RS) version of DDH functionals where short and
long-range components are matched using system dependent, non-empirical
parameters. We show that RS DDHs yield accurate electronic properties of
inorganic and organic solids, including energy gaps and absolute ionization
potentials. Furthermore we show that these functionals may be generalized to
finite systems.Comment: In press. 13 pages, 7 figures, 8 tables, Physical Review B 201
A Finite-field Approach for Calculations Beyond the Random Phase Approximation
We describe a finite-field approach to compute density response functions,
which allows for efficient and calculations beyond
the random phase approximation. The method is easily applicable to density
functional calculations performed with hybrid functionals. We present results
for the electronic properties of molecules and solids and we discuss a general
scheme to overcome slow convergence of quasiparticle energies obtained from
calculations, as a function of the basis set used to represent
the dielectric matrix
Interstellar dust charging in dense molecular clouds: cosmic ray effects
The local cosmic-ray (CR) spectra are calculated for typical characteristic
regions of a cold dense molecular cloud, to investigate two so far neglected
mechanisms of dust charging: collection of suprathermal CR electrons and
protons by grains, and photoelectric emission from grains due to the UV
radiation generated by CRs. The two mechanisms add to the conventional charging
by ambient plasma, produced in the cloud by CRs. We show that the CR-induced
photoemission can dramatically modify the charge distribution function for
submicron grains. We demonstrate the importance of the obtained results for
dust coagulation: While the charging by ambient plasma alone leads to a strong
Coulomb repulsion between grains and inhibits their further coagulation, the
combination with the photoemission provides optimum conditions for the growth
of large dust aggregates in a certain region of the cloud, corresponding to the
densities between cm and
cm. The charging effect of CR is of generic nature, and therefore is
expected to operate not only in dense molecular clouds but also in the upper
layers and the outer parts of protoplanetary discs.Comment: accepted by Ap
Designing defect-based qubit candidates in wide-gap binary semiconductors for solid-state quantum technologies
The development of novel quantum bits is key to extend the scope of
solid-state quantum information science and technology. Using first-principles
calculations, we propose that large metal ion - vacancy complexes are promising
qubit candidates in two binary crystals: 4H-SiC and w-AlN. In particular, we
found that the formation of neutral Hf- and Zr-vacancy complexes is
energetically favorable in both solids; these defects have spin-triplet ground
states, with electronic structures similar to those of the diamond NV center
and the SiC di-vacancy. Interestingly, they exhibit different spin-strain
coupling characteristics, and the nature of heavy metal ions may allow for easy
defect implantation in desired lattice locations and ensure stability against
defect diffusion. In order to support future experimental identification of the
proposed defects, we report predictions of their optical zero-phonon line,
zero-field splitting and hyperfine parameters. The defect design concept
identified here may be generalized to other binary semiconductors to facilitate
the exploration of new solid-state qubits.Comment: 23 pages, 5 figures, 6 tables, Supplementary Information is added at
the en
Cosmic-ray ionisation in circumstellar discs
Galactic cosmic rays are a ubiquitous source of ionisation of the
interstellar gas, competing with UV and X-ray photons as well as natural
radioactivity in determining the fractional abundance of electrons, ions and
charged dust grains in molecular clouds and circumstellar discs. We model the
propagation of different components of Galactic cosmic rays versus the column
density of the gas. Our study is focussed on the propagation at high densities,
above a few g cm, especially relevant for the inner regions of
collapsing clouds and circumstellar discs. The propagation of primary and
secondary CR particles (protons and heavier nuclei, electrons, positrons, and
photons) is computed in the continuous slowing down approximation, diffusion
approximation, or catastrophic approximation, by adopting a matching procedure
for the different transport regimes. A choice of the proper regime depends on
the nature of the dominant loss process, modelled as continuous or
catastrophic. The CR ionisation rate is determined by CR protons and their
secondary electrons below g cm and by electron/positron
pairs created by photon decay above g cm. We show that a
proper description of the particle transport is essential to compute the
ionisation rate in the latter case, since the electron/positron differential
fluxes depend sensitively on the fluxes of both protons and photons. Our
results show that the CR ionisation rate in high-density environments, like,
e.g., the inner parts of collapsing molecular clouds or the mid-plane of
circumstellar discs, is larger than previously assumed. It does not decline
exponentially with increasing column density, but follows a more complex
behaviour due to the interplay of different processes governing the generation
and propagation of secondary particles.Comment: 19 pages, 11 figures, accepted by A&
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