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 $s=1/2$ 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&thinsp;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