2,045 research outputs found
Long-range excitations in time-dependent density functional theory
Adiabatic time-dependent density functional theory fails for excitations of a
heteroatomic molecule composed of two open-shell fragments at large separation.
Strong frequency-dependence of the exchange-correlation kernel is necessary for
both local and charge-transfer excitations. The root of this is static
correlation created by the step in the exact Kohn-Sham ground-state potential
between the two fragments. An approximate non-empirical kernel is derived for
excited molecular dissociation curves at large separation. Our result is also
relevant for the usual local and semi-local approximations for the ground-state
potential, as static correlation there arises from the coalescence of the
highest occupied and lowest unoccupied orbital energies as the molecule
dissociates.Comment: 7 pages, 2 figure
Magnetic and orbital order in overdoped bilayer manganites
The magnetic and orbital orders for the bilayer manganites in the doping
region have been investigated from a model that incorporates the
two orbitals at each Mn site, the inter-orbital Coulomb interaction and
lattice distortions. The usual double exchange operates via the orbitals.
It is shown that such a model reproduces much of the phase diagram recently
obtained for the bilayer systems in this range of doping. The C-type phase with
() spin order seen by Ling et al. appears as a natural consequence
of the layered geometry and is stabilised by the static distortions of the
system. The orbital order is shown to drive the magnetic order while the
anisotropic hopping across the orbitals, layered nature of the underlying
structure and associated static distortions largely determine the orbital
arrangements.Comment: 8 pages, 5 figure
Identification of two new HMXBs in the LMC: a 2013 s pulsar and a probable SFXT
We report on the X-ray and optical properties of two high-mass X-ray binary
systems located in the Large Magellanic Cloud (LMC). Based on the obtained
optical spectra, we classify the massive companion as a supergiant star in both
systems. Timing analysis of the X-ray events collected by XMM-Newton revealed
the presence of coherent pulsations (spin period 2013 s) for XMMU
J053108.3-690923 and fast flaring behaviour for XMMU J053320.8-684122. The
X-ray spectra of both systems can be modelled sufficiently well by an absorbed
power-law, yielding hard spectra and high intrinsic absorption from the
environment of the systems. Due to their combined X-ray and optical properties
we classify both systems as SgXRBs: the 19 confirmed X-ray pulsar
and a probable supergiant fast X-ray transient in the LMC, the second such
candidate outside our Galaxy.Comment: 12 pages, 10 figures, accepted for publication in MNRA
Adaptive Seeding for Gaussian Mixture Models
We present new initialization methods for the expectation-maximization
algorithm for multivariate Gaussian mixture models. Our methods are adaptions
of the well-known -means++ initialization and the Gonzalez algorithm.
Thereby we aim to close the gap between simple random, e.g. uniform, and
complex methods, that crucially depend on the right choice of hyperparameters.
Our extensive experiments indicate the usefulness of our methods compared to
common techniques and methods, which e.g. apply the original -means++ and
Gonzalez directly, with respect to artificial as well as real-world data sets.Comment: This is a preprint of a paper that has been accepted for publication
in the Proceedings of the 20th Pacific Asia Conference on Knowledge Discovery
and Data Mining (PAKDD) 2016. The final publication is available at
link.springer.com (http://link.springer.com/chapter/10.1007/978-3-319-31750-2
24
Undoing static correlation: Long-range charge transfer in time-dependent density functional theory
Long-range charge transfer excited states are notoriously badly
underestimated in time-dependent density functional theory (TDDFT). We resolve
how {\it exact} TDDFT captures charge transfer between open-shell species: in
particular the role of the step in the ground-state potential, and the severe
frequency-dependence of the exchange-correlation kernel. An expression for the
latter is derived, that becomes exact in the limit that the charge-transfer
excitations are well-separated from other excitations. The exchange-correlation
kernel has the task of undoing the static correlation in the ground state
introduced by the step, in order to accurately recover the physical
charge-transfer states.Comment: 2 figure
Nature of the spiral state, electric polarisation and magnetic transitions in Sr-doped YBaCuFeO: A first-principles study
Contradictory results on the ferroelectric response of type II multiferroic
YBaCuFeO, in its incommensurate phase, has of late, opened up a lively
debate. There are ambiguous reports on the nature of the spiral magnetic state.
Using first-principles DFT calculations for the parent compound within
LSDA+U+SO approximation, the multiferroic response and the nature of spiral
state is revealed. The helical spiral is found to be more stable below the
transition temperature as spins prefer to lie in ab plane. The
Dzyaloshinskii-Moriya (DM) interaction and the spin current mechanism were
earlier invoked to account for the electric polarisation in this system.
However, the DM interaction is found to be absent, spin current mechanism is
not valid in the helical spiral state and there is no electric polarisation
thereof. These results are in good agreement with the recent single-crystal
data. We also investigate the magnetic transitions in
YBaSrCuFeO for the entire range of doping. The
exchange interactions are estimated as a function of doping and a quantum Monte
Carlo (QMC) calculation on an effective spin Hamiltonian shows that the
paramagnetic to commensurate phase transition temperature increases with doping
till and decreases beyond. Our observations are consistent with
experimental findings.Comment: 8 pages, 7 figure
Modeling Willingness-to-Pay Values for Rural Bus Attributes Under Different Trip Purposes
Modeling Willingness-to-Pay Values for Rural Bus Attributes Under Different Trip Purpose
The Jet in the Galactic Center: An Ideal Laboratory for Magnetohydrodynamics and General Relativity
In this paper we review and discuss some of the intriguing properties of the
Galactic Center supermassive black hole candidate Sgr A*. Of all possible black
hole sources, the event horizon of Sgr A*, subtends the largest angular scale
on the sky. It is therefore a prime candidate to study and image plasma
processes in strong gravity and it even allows imaging of the shadow cast by
the event horizon. Recent mm-wave VLBI and radio timing observations as well as
numerical GRMHD simulations now have provided several breakthroughs that put
Sgr A* back into the focus. Firstly, VLBI observations have now measured the
intrinsic size of Sgr A* at multiple frequencies, where the highest frequency
measurements have approached the scale of the black hole shadow. Moreover,
measurements of the radio variability show a clear time lag between 22 GHz and
43 GHz. The combination of size and timing measurements, allows one to actually
measure the flow speed and direction of magnetized plasma at some tens of
Schwarzschild radii. This data strongly support a moderately relativistic
outflow, consistent with an accelerating jet model. This is compared to recent
GRMHD simulation that show the presence of a moderately relativistic outflow
coupled to an accretion flow Sgr A*. Further VLBI and timing observations
coupled to simulations have the potential to map out the velocity profile from
5-40 Schwarzschild radii and to provide a first glimpse at the appearance of a
jet-disk system near the event horizon. Future submm-VLBI experiments would
even be able to directly image those processes in strong gravity and directly
confirm the presence of an event horizon.Comment: invited talk to appear in "Jets on All Scales", IAU Symposium 275,
G.E. Romero, R.A. Sunyaev & T. Belloni, eds., Cambridge University Press, 9
pages, LaTex, 4 figure
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