981 research outputs found
Mechanism of structural phase transitions in KCrF3
We study the origin of the cubic to tetragonal and tetragonal to monoclinic
structural transitions in KCrF3, and the associated change in orbital order,
paying particular attention to the relevance of super-exchange in both phases.
We show that super-exchange is not the main mechanism driving these
transitions. Specifically, it is not strong enough to be responsible for the
high-temperature cubic to tetragonal transition and does not yield the type of
orbital order observed in the monoclinic phase. The energy difference between
the tetragonal and the monoclinic structure is tiny, and most likely results
from the interplay between volume, covalency, and localization effects. The
transition is rather driven by Slater exchange than super-exchange.
Nevertheless, once the monoclinic distortions are present, super-exchange helps
in stabilizing the low symmetry structure. The orbital order we obtain for this
monoclinic phase is consistent with the magnetic transition at 80 K.Comment: 8 pages, 6 figure
Thermally assisted ordering in Mott insulators
Ginzburg-Landau theory describes phase transitions as the competition between
energy and entropy: The ordered phase has lower energy, while the disordered
phase has larger entropy. When heating the system, ordering is reduced
entropically until it vanishes at the critical temperature. This established
picture implicitly assumes that the energy difference between ordered and
disordered phase does not change with temperature. We show that for the Mott
insulator KCuF3 this assumption is strongly violated: thermal expansion
energetically stabilizes the orbitally-ordered phase to such and extent that no
phase transition is observed. This new mechanism explains not only the absence
of a phase transition in KCuF3 but even suggests the possibility of an inverted
transition in closed-shell systems, where the ordered phase emerges only at
high temperatures.Comment: 5 pages, 5 figure
Multiplet effects in orbital and spin ordering phenomena: A hybridization-expansion quantum impurity solver study
Orbital and spin ordering phenomena in strongly correlated systems are
commonly studied using the local-density approximation + dynamical mean-field
theory approach. Typically, however, such simulations are restricted to
simplified models (density-density Coulomb interactions, high symmetry
couplings and few-band models). In this work we implement an efficient general
hybridization-expansion continuous-time quantum Monte Carlo impurity solver
(Krylov approach) which allows us to investigate orbital and spin ordering in a
more realistic setting, including interactions that are often neglected (e.g.,
spin-flip and pair-hopping terms), enlarged basis sets (full d versus eg),
low-symmetry distortions, and reaching the very low-temperature (experimental)
regime. We use this solver to study ordering phenomena in a selection of
exemplary low-symmetry transition-metal oxides: LaMnO3 and rare-earth
manganites as well as the perovskites CaVO3 and YTiO3. We show that spin-flip
and pair hopping terms do not affect the Kugel-Khomskii orbital-order melting
transition in rare-earth manganites, or the suppression of orbital fluctuations
driven by crystal field and Coulomb repulsion. For the Mott insulator YTiO3 we
find a ferromagnetic transition temperature 50 K, in remarkably good agreement
with experiments. For LaMnO3 we show that the classical t2g-spin approximation,
commonly adopted for studying manganites, yields indeed an occupied eg orbital
in very good agreement with that obtained for the full d 5-orbital Hubbard
model, while the spin-spin e_g-t_{2g} correlation function calculated from the
full d model is 0.74, very close to the value expected for aligned eg and t2g
spins; the eg spectral function matrix is also well reproduced. Finally, we
show that the t2g screening reduces the eg-eg Coulomb repulsion by about 10%Comment: 9 pages, 5 figure
Purveyors of fine halos: Re-assessing globular cluster contributions to the Milky Way halo build-up with SDSS-IV
There is ample evidence in the Milky Way for globular cluster (GC)
disruption. Hence one may expect that also part of the Galactic halo field
stars may once have formed in GCs. We quantify the fraction of halo stars
donated by GCs by searching for stars that bear the unique chemical
fingerprints typical for a subset of GC stars often dubbed `second-generation
stars'. These are stars showing light element abundance anomalies such as a
pronounced CN-band strength accompanied by weak CH-bands. Based on this
indicator, past studies have placed the fraction of halo stars with a GC origin
between a few to up to 50%. Using low-resolution spectra from the most recent
data release of the latest extension of the Sloan Digital Sky Survey (SDSS-IV),
we were able to identify 118 metal-poor ([Fe/H]) CN-strong
stars in a sample of 4470 halo giant stars out to 50 kpc. This results in an
observed fraction of these stars of 2.60.2%. Using an updated formalism to
account for the fraction of stars lost early on in the GCs' evolution we
estimate the fraction of the halo that stems from disrupted clusters to be
111%. This number represents the case that stars lost from GCs were
entirely from the first generation and is thus merely an upper limit. Our
conclusions are sensitive to our assumptions of the mass lost early on from the
first generation formed in the GCs, the ratio of first-to-second generation
stars, and other GC parameters. We carefully test the influence of varying
these parameters on the final result and find that, under realistic scenarios,
the above fraction depends on the main assumptions at less than 10%. We further
recover a flat trend in this fraction with Galactocentric radius, with a
marginal indication of a rise beyond 30 kpc that could reflect the ex-situ
origin of the outer halo. (abridged)Comment: 13 pages, 11 figures, accepted for publication in Astronomy &
Astrophysic
A Wide-Field View of Leo II -- A Structural Analysis Using the SDSS
Using SDSS I data, we have analysed the stellar distribution of the Leo II
dwarf spheroidal galaxy (distance of 233 kpc) to search for evidence of tidal
deformation. The existing SDSS photometric catalogue contains gaps in regions
of high stellar crowding, hence we filled the area at the centre of Leo II
using the DAOPHOT algorithm applied to the SDSS images. The combined
DAOPHOT-SDSS dataset contains three-filter photometry over a 4x4 square degree
region centred on Leo II. By defining a mask in three-filter colour-magnitude
space, we removed the majority of foreground field stars. We have measured the
following Leo II structural parameters: a core radius of r_c = 2.64 +/- 0.19
arcmin (178 +/- 13 pc), a tidal radius of r_t = 9.33 +/- 0.47 arcmin (632 +/-
32 pc) and a total V-band luminosity of L_V = (7.4 +/- 2.0) times 10^5 L_sun
(M_V = -9.9 +/- 0.3). Our comprehensive analysis of the Leo II structure did
not reveal any significant signs of tidal distortion. The internal structure of
this object contains only mild isophotal twisting. A small overdensity was
discovered appoximately 4.5 tidal radii from the Leo II centre, however we
conclude it is unlikely to be material tidally stripped from Leo II based on
its stellar population, and is most likely a foreground overdensity of stars.
Our results indicate that the influence of the Galactic graviational field on
the structure of Leo II has been relatively mild. We rederived the
mass-to-light ratio of this system using existing kinematic data combined with
our improved structural measurements, and favour the scenario in which Leo II
is strongly dominated by dark matter with (M/L)_V ~ 100 in solar units.Comment: 41 pages, 15 figures, accepted for publication in the Astronomical
Journa
Complexity in small-scale dwarf spheroidal galaxies
Our knowledge about the dynamics, the chemical abundances and the
evolutionary histories of the more luminous dwarf spheroidal (dSph) galaxies is
constantly growing. However, very little is known about the enrichment of the
ultra-faint systems recently discovered in large numbers in large sky surveys.
Current low-resolution spectroscopy and photometric data indicate that these
galaxies are highly dark matter dominated and predominantly metal poor. On the
other hand, recent high-resolution abundance analyses indicate that some dwarf
galaxies experienced highly inhomogenous chemical enrichment, where star
formation proceeds locally on small scales. In this article, I will review the
kinematic and chemical abundance information of the Milky Way satellite dSphs
that is presently available from low- and high resolution spectroscopy.
Moreover, some of the most peculiar element and inhomogeneous enrichment
patterns will be discussed and related to the question of to what extent the
faintest dSph candidates could have contributed to the Galactic halo, compared
to more luminous systems.Comment: 34 pages, 8 figures; Ludwig-Biermann Award Lecture 2008, to appear in
Reviews in Modern Astronomy, 21,
Effects of fact‐checking warning labels and social endorsement cues on climate change fake news credibility and engagement on social media
Online fake news can have noxious consequences. Social media platforms are experimenting with different interventions to curb fake news' spread, often employing them simultaneously. However, research investigating the interaction of these interventions is limited. Here, we use the heuristic-systematic model of information processing (HSM) as a theoretical framework to jointly test two interventions against fake news that are implemented at scale by social media platforms: (1) adding warning labels from fact checkers to initiate systematic processing and (2) removing social endorsement cues (e.g., engagement counts) to reduce the influence of this heuristic cue. Moreover, we accounted for dispositions previously found to affect a person's response to fake news through motivated reasoning or cognitive style. An online experiment in Germany (N = 571) confirmed that warning labels reduced the perceived credibility of a fake news post exaggerating the consequences of climate change. Warning labels also lowered the (self-reported) likelihood to amplify fake news. Removing social endorsement cues did not have an effect. In line with research on motivated reasoning, left-leaning individuals perceived the climate fake news to be more credible and reported a higher likelihood to amplify it. Supporting research on cognitive style, participants with lower educational levels and a less analytic thinking style also reported a higher likelihood of amplification. Elaboration likelihood was associated only with age, involvement, and political leaning, but not affected by warning labels. Our findings contribute to the mounting evidence for the effectiveness of warning labels while questioning their relevance for systematic processing
Regional migration governance: contributions to a sustainable international migration architecture
Technical and financial capacities of regional organizations ought to be improved so that they can tackle regional migratory challenges effectively and in line with global regulatory requirements. This is not only true for security-relevant areas such as the protection of borders or migration control but also for the implementation of regional standards pertaining to the freedom of movement and, in particular, for the protection of refugees and vulnerable migrants. It also applies to promoting the positive effect migration has on human development.
Regional and inter-regional dialogue forums help to mediate conflicts between different national interests and can strengthen the overall coherence of international migration policy. Building up these forums ought to be promoted, the exchange between national, regional and global actors intensified, and the inclusion of civil society actors in regional cooperation formats improved.
Regional organizations have developed groundbreaking norms and standards in very different areas of refugee and migration policy, for instance ECOWAS in West Africa or IGAD in East Africa. In the preparation of the global compacts for migration and on refugees, they have so far primarily been able to provide their input in the consultation phase. To give their positions more weight, they should be granted the right to contribute their own texts in the negotiation phase. It is also important to grant them an active role in the review and follow-up process of the two global compacts
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