3,737 research outputs found
Fourier Decomposition of RR Lyrae light curves and the SX Phe population in the central region of NGC 3201
CCD time-series observations of the central region of the globular cluster
NGC~3201 were obtained with the aim of performing the Fourier decomposition of
the light curves of the RR~Lyrae stars present in that field. This procedure
gave the mean values, for the metallicity, of [Fe/H]
(statistical) (systematical), and for the distance, ~kpc (statistical) (systematical). The values found from two
RRc stars are consistent with those derived previously. The differential
reddening of the cluster was investigated and individual reddenings for the RR
Lyrae stars were estimated from their curves. We found an average value
of . An investigation of the light curves of stars in
the {\it blue stragglers} region led to the discovery of three new SX~Phe
stars. The period-luminosity relation of the SX~Phe stars was used for an
independent determination of the distance to the cluster and of the individual
reddenings. We found a distance of 5.0 kpcComment: To appear in Revista Mexicana de Astronom\'ia y Astrof\'isica,
Octuber 2014 issue, Vol 50. 17 pages, 10 figure
Theory of one and two donors in Silicon
We provide here a roadmap for modeling silicon nano-devices with one or two
group V donors (D). We discuss systems containing one or two electrons, that
is, D^0, D^-, D_2^+ and D_2^0 centers. The impact of different levels of
approximation is discussed. The most accurate instances -- for which we provide
quantitative results -- are within multivalley effective mass including the
central cell correction and a configuration interaction account of the
electron-electron correlations. We also derive insightful, yet less accurate,
analytical approximations and discuss their validity and limitations -- in
particular, for a donor pair, we discuss the single orbital LCAO method, the
Huckel approximation and the Hubbard model. Finally we discuss the connection
between these results and recent experiments on few dopant devices.Comment: 13 pages, 6 figure
Re-entrant ferromagnetism in a generic class of diluted magnetic semiconductors
Considering a general situation where a semiconductor is doped by magnetic
impurities leading to a carrier-induced ferromagnetic exchange coupling between
the impurity moments, we show theoretically the possible generic existence of
three ferromagnetic transition temperatures, T_1 > T_2 > T_3, with two distinct
ferromagnetic regimes existing for T_1 > T > T_2 and T < T_3. Such an
intriguing re-entrant ferromagnetism, with a paramagnetic phase (T_2 > T > T_3)
between two ferromagnetic phases, arises from a subtle competition between
indirect exchange induced by thermally activated carriers in an otherwise empty
conduction band versus the exchange coupling existing in the impurity band due
to the bound carriers themselves. We comment on the possibility of observing
such a re-entrance phenomenon in diluted magnetic semiconductors and magnetic
oxides.Comment: 4 pages, 3 figure
Impact of the valley degree of freedom on the control of donor electrons near a Si/SiO_2 interface
We analyze the valley composition of one electron bound to a shallow donor
close to a Si/barrier interface as a function of an applied electric field. A
full six-valley effective mass model Hamiltonian is adopted. For low fields,
the electron ground state is essentially confined at the donor. At high fields
the ground state is such that the electron is drawn to the interface, leaving
the donor practically ionized. Valley splitting at the interface occurs due to
the valley-orbit coupling, V_vo^I = |V_vo^I| e^{i theta}. At intermediate
electric fields, close to a characteristic shuttling field, the electron states
may constitute hybridized states with valley compositions different from the
donor and the interface ground states. The full spectrum of energy levels shows
crossings and anti-crossings as the field varies. The degree of level
repulsion, thus the width of the anti-crossing gap, depends on the relative
valley compositions, which vary with |V_vo^I|, theta and the interface-donor
distance. We focus on the valley configurations of the states involved in the
donor-interface tunneling process, given by the anti-crossing of the three
lowest eigenstates. A sequence of two anti-crossings takes place and the
complex phase theta affects the symmetries of the eigenstates and level
anti-crossing gaps. We discuss the implications of our results on the practical
manipulation of donor electrons in Si nanostructures.Comment: 8 pages, including 5 figures. v2: Minor clarifying changes in the
text and figures. Change of title. As published in PR
Systematic challenges for future gravitational wave measurements of precessing binary black holes
The properties of precessing, coalescing binary black holes are presently
inferred through comparison with two approximate models of compact binary
coalescence. In this work we show these two models often disagree substantially
when binaries have modestly large spins () and modest mass ratios
(). We demonstrate these disagreements using standard figures of
merit and the parameters inferred for recent detections of binary black holes.
By comparing to numerical relativity, we confirm these disagreements reflect
systematic errors. We provide concrete examples to demonstrate that these
systematic errors can significantly impact inferences about astrophysically
significant binary parameters. For the immediate future, parameter inference
for binary black holes should be performed with multiple models (including
numerical relativity), and carefully validated by performing inference under
controlled circumstances with similar synthetic events.Comment: 12 pages, 9 figure
Effect of strain on the orbital and magnetic ordering of manganite thin films and their interface with an insulator
We study the effect of uniform uniaxial strain on the ground state electronic
configuration of a thin film manganite. Our model Hamiltonian includes the
double-exchange, the Jahn-Teller electron-lattice coupling, and the
antiferromagnetic superexchange. The strain arises due to the lattice mismatch
between an insulating substrate and a manganite which produces a tetragonal
distortion. This is included in the model via a modification of the hopping
amplitude and the introduction of an energy splitting between the Mn e_g
levels. We analyze the bulk properties of half-doped manganites and the
electronic reconstruction at the interface between a ferromagnetic and metallic
manganite and the insulating substrate. The strain drives an orbital selection
modifying the electronic properties and the magnetic ordering of manganites and
their interfaces.Comment: 8 pages, 8 figure
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