62 research outputs found
Sensitivity of Greenland ice sheet projections to spatial resolution in higher-order simulations: the Alfred Wegener Institute (AWI) contribution to ISMIP6 Greenland using the Ice-sheet and Sea-level System Model (ISSM)
Projections of the contribution of the Greenland ice sheet to future sea-level rise include uncertainties primarily due to the imposed climate forcing and the initial state of the ice sheet model. Several state-of-the-art ice flow models are currently being employed on various grid resolutions to estimate future mass changes in the framework of the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6). Here we investigate the sensitivity to grid resolution of centennial sea-level contributions from the Greenland ice sheet and study the mechanism at play. We employ the finite-element higher-order Ice-sheet and Sea-level System Model (ISSM) and conduct experiments with four different horizontal resolutions, namely 4, 2, 1 and 0.75 km. We run the simulation based on the ISMIP6 core climate forcing from the MIROC5 global circulation model (GCM) under the high-emission Representative Concentration Pathway (RCP) 8.5 scenario and consider both atmospheric and oceanic forcing in full and separate scenarios. Under the full scenarios, finer simulations unveil up to approximately 5 % more sea-level rise compared to the coarser resolution. The sensitivity depends on the magnitude of outlet glacier retreat, which is implemented as a series of retreat masks following the ISMIP6 protocol. Without imposed retreat under atmosphere-only forcing, the resolution dependency exhibits an opposite behaviour with approximately 5 % more sea-level contribution in the coarser resolution. The sea-level contribution indicates a converging behaviour below a 1 km horizontal resolution. A driving mechanism for differences is the ability to resolve the bedrock topography, which highly controls ice discharge to the ocean. Additionally, thinning and acceleration emerge to propagate further inland in high resolution for many glaciers. A major response mechanism is sliding, with an enhanced feedback on the effective normal pressure at higher resolution leading to a larger increase in sliding speeds under scenarios with outlet glacier retreat
Zero-field incommensurate spin-Peierls phase with interchain frustration in TiOCl
We report on the magnetic, thermodynamic and optical properties of the
quasi-one-dimensional quantum antiferromagnets TiOCl and TiOBr, which have been
discussed as spin-Peierls compounds. The observed deviations from canonical
spin-Peierls behavior, e.g. the existence of two distinct phase transitions,
have been attributed previously to strong orbital fluctuations. This can be
ruled out by our optical data of the orbital excitations. We show that the
frustration of the interchain interactions in the bilayer structure gives rise
to incommensurate order with a subsequent lock-in transition to a commensurate
dimerized state. In this way, a single driving force, the spin-Peierls
mechanism, induces two separate transitions.Comment: 4 pages, 4 figure
Anisotropic Susceptibility of La_2-xSr_xCoO_4 related to the Spin States of Cobalt
We present a study of the magnetic susceptibility of La_2-xSr_xCoO_4 single
crystals in a doping range 0.3<=x<=0.8. Our data shows a pronounced magnetic
anisotropy for all compounds. This anisotropy is in agreement with a low-spin
ground state (S=0) of Co^3+ for x>=0.4 and a high-spin ground state (S=3/2) of
Co^2+. We compare our data with a crystal-field model calculation assuming
local moments and find a good description of the magnetic behavior for x>=0.5.
This includes the pronounced kinks observed in the inverse magnetic
susceptibility, which result from the anisotropy and low-energy excited states
of Co^2+ and are not related to magnetic ordering or temperature-dependent
spin-state transitions
МЕТОДИКА ПРОВЕДЕННЯ ІСТОРИКО-КРАЄЗНАВЧИХ ЕКСПЕДИЦІЙ НА ПІВДНІ УКРАЇНИ ІНСТИТУТОМ ЕТНОГРАФІЇ АН СРСР у 50-60-ті рр. ХХ ст.
Український народ у своїй історії має великий досвід духовного життя, бо з покоління в покоління передає набуті навички, що стали невід’ємною части-ною нашої національності. Але з кожним роком наше покоління, на жаль, втра-чає зв’язок з попередніми поколіннями. Тому, щоб зберегти традиції, націона-льні особливості, в різні часи різноманітні інститути, товариства, окремі дослі-дники намагались створити найоптимальнішу методику організації та прове-дення історико-краєзнавчих експедицій
Intrinsic and extrinsic corrugation of monolayer graphene deposited on SiO2
Using scanning tunneling microscopy (STM) in ultra high vacuum and atomic
force microscopy, we investigate the corrugation of graphene flakes deposited
by exfoliation on a Si/SiO2 (300 nm) surface. While the corrugation on SiO2 is
long-range with a correlation length of about 25 nm, some of the graphene
monolayers exhibit an additional corrugation with a preferential wave length of
about 15 nm. A detailed analysis shows that the long range corrugation of the
substrate is also visible on graphene, but with a reduced amplitude, leading to
the conclusion that the graphene is partly freely suspended between hills of
the substrate. Thus, the intrinsic rippling observed previously on artificially
suspended graphene can exist as well, if graphene is deposited on SiO2.Comment: 10 pages, 11 figures, including supplementary materia
Mobilität von CeO2-Nanomaterialien in Böden
Der zunehmende Einsatz von Cerdioxid-Nanomaterialien (CeO2 NM) führt zu ihrer Freisetzung in die aquatische und terrestrische Umwelt. Aufgrund der ungeklärten human- und umwelttoxischen Wirkung von CeO2 NM muss deren Verbleib in der Umwelt untersucht werden. Es wurden zwei verschiedene CeO2 NM (OECD-Standard CeO2 NM-212, Europium-gedopte CeO2 NM (5 % Eu)) zu elf luftgetrockneten Ackerböden pulverförmig zugegeben und mit künstlichem Regenwasser dispergiert (Batchversuche), um Nachweismethoden für Bodenfestphase und Bodenlösung zu entwickeln. Darüber hinaus sollen diese Batchversuche erste Informationen über die Mobilität der NM in Böden liefern. Zusätzlich wurden Säulenversuche mit einer schluffigen Parabraunerde durchgeführt, um die Wiederfindung und mögliche Remobilisierung des Eu-gedopten CeO2 NM in Böden bzw. Bodenlösungen mit hohen Ce-Hintergrundgehalten zu untersuchen. Nach der pulverförmigen Applikation des gedopten NM wurde die Parabraunerde jeweils für 3 und 92 Tage inkubiert, der Wassergehalt wurde wöchentlich auf 55 % der maximalen Wasserhaltekapazität eingestellt. Anschließend erfolgte die Elution unter gesättigten Bedingungen mit künstlichem Regenwasser unter Austausch von maximal zwölf Porenvolumina.
Die Bodenlösungen der Batchversuche und die Säuleneluate wurden nach der Fraktionierung (Filtration, Zentrifugation und Ultrazentrifugation) mit HNO3 und H2O2 aufgeschlossen. Die Messung der Ce- und Eu-Gehalte erfolgte mit ICP-OES und ICP-MS.
Durch das Eu-Doping wird weder das Kristallgitter noch die Größe der Primärpartikel verändert. Die Bestimmungsgrenzen der applizierten NM in Böden und Bodenlösungen werden im Vergleich zum ungedopten Material, insbesondere bei hohen Ce-Hintergrundgehalten, deutlich erniedrigt. Dadurch können umweltrelevante Konzentrationen des NM getestet werden. Batchversuche sind für die Untersuchung der Mobilität von NM stark limitiert, dennoch deutet die variierende Freisetzung der NM aus den verschiedenen Bodenproben darauf hin, dass die unterschiedlichen Bodencharakteristika die NM Mobilität steuern. Die CeO2 NM Freisetzung aus den Säulenversuchen lag auf einem sehr niedrigen Niveau, war nach der Langzeitinkubation jedoch minimal erhöht. Aus den Ergebnissen lässt sich folgern, dass Böden wahrscheinlich eine deutliche Senke für die getesteten NM darstellen
Collective orbital excitations in orbitally ordered YVO3 and HoVO3
We study orbital excitations in the optical absorption spectra of YVO3 and
HoVO3. We focus on an orbital absorption band observed at 0.4 eV for
polarization E parallel c. This feature is only observed in the intermediate,
monoclinic phase. By comparison with the local crystal-field excitations in
VOCl and with recent theoretical predictions for the crystal-field levels we
show that this absorption band cannot be interpreted in terms of a local
crystal-field excitation. We discuss a microscopic model which attributes this
absorption band to the exchange of two orbitals on adjacent sites, i.e., to the
direct excitation of two orbitons. This model is strongly supported by the
observed dependence on polarization and temperature. Moreover, the calculated
spectral weight is in good agreement with the experimental result.Comment: 12 pages, 9 figure
How chemistry controls electron localization in 3d1 perovskites: A Wannier-function study
In the series of 3d1 t2g perovskites, SrVO3--CaVO3--LaTiO3--YTiO3 the
transition-metal d electron becomes increasingly localized and undergoes a Mott
transition between CaVO3 and LaTiO3. By defining a low-energy Hubbard
Hamiltonian in the basis of Wannier functions for the t2g LDA band and solving
it in the single-site DMFT approximation, it was recently shown[1] that
simultaneously with the Mott transition there occurs a strong suppression of
orbital fluctuations due to splitting of the t2g levels. The present paper
reviews and expands this work, in particular in the direction of exposing the
underlying chemical mechanisms by means of ab initio LDA Wannier functions
generated with the NMTO method. The Wannier functions for the t2g band exhibit
covalency between the transition-metal t2g, the large cation-d, and the
oxygen-p states; this covalency, which increases along the series, turns out to
be responsible not only for the splittings of the t2g levels, but also for
non-cubic perturbations of the hopping integrals, both of which are decisive
for the Mott transition. We find good agreement with the optical and
photoemission spectra, with the crystal-field splittings and orbital
polarizations recently measured for the titanates, and with the metallization
volume for LaTiO3. The metallization volume for YTiO3 is predicted. Using
super-exchange theory, we reproduce the observed magnetic orders in LaTiO3 and
YTiO3, but the results are sensitive to detail, in particular for YTiO3 which,
without the Jahn-Teller distortion, would be AFM C- or A-type, rather than FM.
Finally, we show that it possible to unfold the orthorhombic t2g LDA
bandstructure to a pseudocubic zone. In this zone, the lowest band is separated
from the two others by a direct gap and has a width, W_I, which is
significantly smaller than that, W, of the entire t2g band. The progressive
GdFeO3-type distortion favours electron localization by decreasing W, by
increasing the splitting of the t2g levels and by decreasing W_I. Our
conclusions concerning the roles of GdFeO3-type and JT distortions agree with
those of Mochizuki and Imada [2].Comment: Published version, final. For high resolution figures see
http://www.fkf.mpg.de/andersen/docs/pub/abstract2004+/pavarini_02.pd
Cluster Dynamical Mean-field calculations for TiOCl
Based on a combination of cluster dynamical mean field theory (DMFT) and
density functional calculations, we calculated the angle-integrated spectral
density in the layered quantum magnet TiOCl. The agreement with recent
photoemission and oxygen K-edge X-ray absorption spectroscopy experiments is
found to be good. Th e improvement achieved with this calculation with respect
to previous single-site DMFT calculations is an indication of the correlated
nature and low-dimensionality of TiOCl.Comment: 9 pages, 3 figures, improved version as publishe
A confined–unconfined aquifer model for subglacial hydrology and its application to the Northeast Greenland Ice Stream
Subglacial hydrology plays an important role in ice sheet dynamics as it determines
the sliding velocity. It also drives freshwater into the ocean, leading to undercutting of calving fronts by plumes.
Modeling subglacial water has been a challenge for decades. Only
recently have new approaches been developed such as representing subglacial channels
and thin water sheets by separate layers of variable hydraulic conductivity.
We extend this concept by modeling a confined–unconfined
aquifer system (CUAS) in a single layer of an equivalent porous medium (EPM). The advantage of this formulation is
that it prevents unphysical values of pressure at reasonable computational
cost. We performed sensitivity tests to investigate the effect of
different model parameters. The strongest influence of model parameters was
detected in terms of governing the opening and closure of the system. Furthermore,
we applied the model to the Northeast Greenland Ice Stream, where
an efficient system independent of seasonal input was identified about
500 km downstream from the ice divide.
Using the effective pressure from the hydrology model, the Ice
Sheet System Model (ISSM) showed considerable improvements in
modeled velocities in the coastal region.</p
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