9,165 research outputs found
Energetic stability and magnetic properties of Mn dimers in silicon
We present an accurate first-principles study of magnetism and energetics of single Mn impurities and Mn dimers in Si. Our results, in general agreement with available experiments, show that (i) Mn atoms tend to aggregate, the formation energy of dimers being lower than the sum of the separate constituents, (ii) ferromagnetic coupling is favored between the Mn atoms constituting the dimers in p-type Si, switching to an antiferromagnetic coupling in n-type Si, (iii) Mn atoms show donors (acceptor) properties in p-type (n-type) Si, therefore they tend to compensate doping, while dimers being neutral or acceptors allow for Si to be doped p-type. (C) 2004 American Institute of Physics
Gap opening in ultrathin Si layers: Role of confined and interface states
We present first principle calculations of ultrathin silicon (111) layers embedded in CaF2, a lattice matched insulator. Our all electron calculation allows a check of the quantum confinement hypothesis for the Si band gap opening as a function of thickness. We find that the gap opening is mostly due to the valence band while the lowest conduction band states shift very modestly due to their pronounced interface character. The latter states are very sensitive to the sample design. We suggest that a quasidirect band gap can be achieved by stacking Si layers of different thickness
The phase diagrams of iron-based superconductors: theory and experiments
Phase diagrams play a primary role in the understanding of materials
properties. For iron-based superconductors (Fe-SC), the correct definition of
their phase diagrams is crucial because of the close interplay between their
crystallo-chemical and magnetic properties, on one side, and the possible
coexistence of magnetism and superconductivity, on the other. The two most
difficult issues for understanding the Fe-SC phase diagrams are: 1) the origin
of the structural transformation taking place during cooling and its
relationship with magnetism; 2) the correct description of the region where a
crossover between the magnetic and superconducting electronic ground states
takes place. Hence a proper and accurate definition of the structural, magnetic
and electronic phase boundaries provides an extremely powerful tool for
material scientists. For this reason, an exact definition of the thermodynamic
phase fields characterizing the different structural and physical properties
involved is needed, although it is not easy to obtain in many cases. Moreover,
physical properties can often be strongly dependent on the occurrence of
micro-structural and other local-scale features (lattice micro-strain, chemical
fluctuations, domain walls, grain boundaries, defects), which, as a rule, are
not described in a structural phase diagram. In this review, we critically
summarize the results for the most studied 11-, 122- and 1111-type compound
systems, providing a correlation between experimental evidence and theory
Searching for tau neutrinos with Cherenkov telescopes
Cherenkov telescopes have the capability of detecting high energy tau
neutrinos in the energy range of 1--1000 PeV by searching for very inclined
showers. If a tau lepton, produced by a tau neutrino, escapes from the Earth or
a mountain, it will decay and initiate a shower in the air which can be
detected by an air shower fluorescence or Cherenkov telescope. In this paper,
we present detailed Monte Carlo simulations of corresponding event rates for
the VERITAS and two proposed Cherenkov Telescope Array sites: Meteor Crater and
Yavapai Ranch, which use representative AGN neutrino flux models and take into
account topographic conditions of the detector sites. The calculated neutrino
sensitivities depend on the observation time and the shape of the energy
spectrum, but in some cases are comparable or even better than corresponding
neutrino sensitivities of the IceCube detector. For VERITAS and the considered
Cherenkov Telescope Array sites the expected neutrino sensitivities are up to
factor 3 higher than for the MAGIC site because of the presence of surrounding
mountains.Comment: arXiv admin note: text overlap with arXiv:1308.019
Relation between phase and dwell times for quantum tunneling of a relativistically propagating particle
The general and explicit relation between the phase time and the dwell time
for quantum tunneling of a relativistically propagating particle is
investigated and quantified. In analogy with previously obtained
non-relativistic results, it is shown that the group delay can be described in
terms of the dwell time and a self-interference delay. Lessons concerning the
phenomenology of the relativistic tunneling are drawn
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