227 research outputs found
EuO and Eu on metal crystals and graphene: interface effects and epitaxial films
Growth of the ferromagnetic semiconductor EuO was studied on the metal crystals Ni(100) and Ir(111) and on graphene. Primarily, characterisation was done by means of in-situ scanning tunnelling microscopy (STM) and low energy electron diffraction. The epitaxy on the metal crystals is strongly influenced by interface effects which lead to a complicated growth behaviour in the sub-monolayer regime, especially on Ni(100). Therefore, also films of sub-monolayer thickness were analysed in detail for these substrates.
Eu oxide on Ni(100) shows a variety of different surface phases in the sub-monolayer regime, depending on the growth temperature and the ratio of the Eu and O fluxes. Hence, a careful selection of the initial growth parameters is decisive to obtain a surface oxide suitable for subsequent epitaxy of single phase EuO(100). After creation of a 3 layer thick coalesced oxide film, for subsequent growth a distillation technique can be applied. Ex-situ X-ray adsorption spectroscopy and magneto-optical Kerr effect microscopy measurements of thicker films on Ni(100) are consistent with stoichiometric single phase EuO with bulk properties.
On Ir(111) initially only islands of polar EuO(111) grow, but formation of EuO(100) sets in before the first oxide layer is completed. The ratio of EuO(100) to EuO(111) is thereby influenced by the ratio of the Eu and O fluxes. Thus, the EuO films on Ir(111) consist of a phase mixture of EuO(111) and three rotational domains of EuO(100). The thinnest structure of the EuO(111) is a bilayer. Field emission resonances revealed a work function increase of 6 eV for this structure compared to EuO(100). Despite the polarity, the bilayer shows no obvious reconstruction which could reduce the high electric field. Triangular reconstruction motifs were found for the third EuO(111) layer.
On graphene EuO can be grown as thin film of distinct, {100}-faceted grains which are oriented to the substrate at a sufficiently high growth temperature. As the EuO on graphene is not affected by interface effects, the initial growth stage is not crucial. Thus, the growth of these grains is far less sensitive to the ratio of Eu and O fluxes than the EuO growth on Ni(100).
Appropriate annealing of EuO(100) films generates sufficient conductivity for STM and electron spectroscopies, even for films of 100 nm thickness. Oxygen vacancies were directly imaged by STM. They are of decisive importance for the metal-to-insulator transition of EuO around the temperature of the ferromagnetic-to-paramagnetic transition. Tunnelling spectra of EuO were recorded for the first time. For EuO(100) with 1% O vacancies in the topmost layer they exhibit states about 500 meV above the Fermi level which are most probably related to O vacancies.
On all substrates, monolayer high EuO(100) films have a contracted lattice which expands with increasing film thickness. Even if the substrate applies compressive biaxial stress, the EuO bulk lattice constant is almost reached for 5 nm film thickness. This leaves little hope for an increase of the Curie temperature through epitaxial compression.
During the investigation of the EuO on graphene, intercalation of Eu between the graphene and its Ir(111) substrate was observed and analysed further. For Eu deposition at 720 K a variety of equilibrium intercalate structures occur, dependent on the deposited Eu amount, all of which have a height of one monolayer. The dimensions and orientations of these structures are determined by binding energy differences within the unit cell of the graphene moiré on Ir(111). The energetically preferred lattice of the intercalated Eu is a p(2x2) structure, but intercalation continues until a denser (1.73x1.73)R30° structure is saturated. Angular resolved photoemission spectroscopy finds a shift of the graphene's Dirac cone by -1.5 eV for both of these structures.
For closed graphene films, intercalation is hindered by a penetration barrier for temperatures below 400 K. The adsorption and equilibrium surface phases of Eu on graphene were investigated in the temperature range from 35 K to 400 K and for coverages ranging from a small fraction of a saturated monolayer to the second layer. Using density functional theory, including the 4f-shell Coulomb interactions and modelling of the electronic interactions, excellent agreement with the experimental results for the equilibrium adsorbate phase, adsorbate diffusion, and work function was obtained. Most remarkable, at 300 K in an intermediate coverage range a phase of uniformly distributed Eu clusters coexists in two dimensional equilibrium with large Eu-islands in a (1.73x1.73)R30° structure. The formation of the cluster phase is driven by the interplay of three effects: First, the metallic Eu-Eu binding leads to the local stability of (1.73x1.73)R30° structures. Second, electrons lower their kinetic energy by leaving the Eu clusters, thereby doping graphene. Third, the Coulomb energy penalty associated with the charge transfer from Eu to graphene is strongly reduced for smaller clusters
Europium Underneath Graphene on Ir(111): Intercalation Mechanism, Magnetism, and Band Structure
The intercalation of Eu underneath Gr on Ir(111) is comprehensively
investigated by microscopic, magnetic, and spectroscopic measurements, as well
as by density functional theory. Depending on the coverage, the intercalated Eu
atoms form either a or a R superstructure with respect to Gr. We investigate the
mechanisms of Eu penetration through a nominally closed Gr sheet and measure
the electronic structures and magnetic properties of the two intercalation
systems. Their electronic structures are rather similar. Compared to Gr on
Ir(111), the Gr bands in both systems are essentially rigidly shifted to larger
binding energies resulting in n-doping. The hybridization of the Ir surface
state with Gr states is lifted, and the moire superperiodic potential is
strongly reduced. In contrast, the magnetic behavior of the two intercalation
systems differs substantially as found by X-ray magnetic circular dichroism.
The Eu structure displays plain paramagnetic behavior, whereas
for the R structure the large
zero-field susceptibility indicates ferromagnetic coupling, despite the absence
of hysteresis at 10 K. For the latter structure, a considerable easy-plane
magnetic anisotropy is observed and interpreted as shape anisotropy.Comment: 18 pages with 14 figures, including Supplemental Materia
Is keV ion induced pattern formation on Si(001) caused by metal impurities?
We present ion beam erosion experiments performed in ultra high vacuum using
a differentially pumped ion source and taking care that the ion beam hits the
Si(001) sample only. Under these conditions no ion beam patterns form on Si for
angles below 45 degrees with respect to the global surface normal using 2 keV
Kr ions and fluences of 2 x 10^22 ions/m^2. In fact, the ion beam induces a
smoothening of preformed patterns. Simultaneous sputter deposition of stainless
steel in this angular range creates a variety of patterns, similar to those
previously ascribed to clean ion beam induced destabilization of the surface
profile. Only for grazing incidence with incident angles between 60 degrees and
83 degrees pronounced ion beam patterns form. It appears that the angular
dependent stability of Si(001) against pattern formation under clean ion beam
erosion conditions is related to the angular dependence of the sputtering
yield, and not primarily to a curvature dependent yield as invoked frequently
in continuum theory models.Comment: 15 pages, 7 figures. This is an author-created, un-copyedited version
of an article published in Nanotechnology. IOP Publishing Ltd is not
responsible for any errors or omissions in this version of the manuscript or
any version derived from i
The Majority of Compact Massive Galaxies at z~2 are Disk Dominated
We investigate the stellar structure of massive, quiescent galaxies at z~2,
based on Hubble Space Telescope/WFC3 imaging from the Early Release Science
program. Our sample of 14 galaxies has stellar masses of M* > 10^{10.8} Msol
and photometric redshifts of 1.5 < z < 2.5. In agreement with previous work,
their half-light radii are <2 kpc, much smaller than equally massive galaxies
in the present-day universe. A significant subset of the sample appears highly
flattened in projection, which implies, considering viewing angle statistics,
that a significant fraction of the galaxies in our sample have pronounced
disks. This is corroborated by two-dimensional surface brightness profile fits.
We estimate that 65% +/- 15% of the population of massive, quiescent z~2
galaxies are disk-dominated. The median disk scale length is 1.5 kpc,
substantially smaller than the disks of equally massive galaxies in the
present-day universe. Our results provide strong observational evidence that
the much-discussed ultra-dense high-redshift galaxies should generally be
thought of as disk-like stellar systems with the majority of stars formed from
gas that had time to settle into a disk.Comment: published versio
Priming by Chemokines Restricts Lateral Mobility of the Adhesion Receptor LFA-1 and Restores Adhesion to ICAM-1 Nano-Aggregates on Human Mature Dendritic Cells
LFA-1 is a leukocyte specific β2 integrin that plays a major role in regulating adhesion and migration of different immune cells. Recent data suggest that LFA-1 on mature dendritic cells (mDCs) may function as a chemokine-inducible anchor during homing of DCs through the afferent lymphatics into the lymph nodes, by transiently switching its molecular conformational state. However, the role of LFA-1 mobility in this process is not yet known, despite that the importance of lateral organization and dynamics for LFA-1-mediated adhesion regulation is broadly recognized. Using single particle tracking approaches we here show that LFA-1 exhibits higher mobility on resting mDCs compared to monocytes. Lymphoid chemokine CCL21 stimulation of the LFA-1 high affinity state on mDCs, led to a significant reduction of mobility and an increase on the fraction of stationary receptors, consistent with re-activation of the receptor. Addition of soluble monomeric ICAM-1 in the presence of CCL21 did not alter the diffusion profile of LFA-1 while soluble ICAM-1 nano-aggregates in the presence of CCL21 further reduced LFA-1 mobility and readily bound to the receptor. Overall, our results emphasize the importance of LFA-1 lateral mobility across the membrane on the regulation of integrin activation and its function as adhesion receptor. Importantly, our data show that chemokines alone are not sufficient to trigger the high affinity state of the integrin based on the strict definition that affinity refers to the adhesion capacity of a single receptor to its ligand in solution. Instead our data indicate that nanoclustering of the receptor, induced by multi-ligand binding, is required to maintain stable cell adhesion once LFA-1 high affinity state is transiently triggered by inside-out signals.Peer ReviewedPostprint (published version
Galaxy Zoo: CANDELS barred discs and bar fractions
The formation of bars in disc galaxies is a tracer of the dynamical maturity of the population. Previous studies have found that the incidence of bars in discs decreases from the local Universe to z ~ 1, and by z > 1 simulations predict that bar features in dynamically mature discs should be extremely rare. Here, we report the discovery of strong barred structures in massive disc galaxies at z ~ 1.5 in deep rest-frame optical images from the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey. From within a sample of 876 disc galaxies identified by visual classification in Galaxy Zoo, we identify 123 barred galaxies. Selecting a subsample within the same region of the evolving galaxy luminosity function (brighter than L*), we find that the bar fraction across the redshift range 0.5 ≤ z ≤ 2 (fbar = 10.7+6.3 -3.5 per cent after correcting for incompleteness) does not significantly evolve.We discuss the implications of this discovery in the context of existing simulations and our current understanding of the way disc galaxies have evolved over the last 11 billion yearsPeer reviewedFinal Accepted Versio
Exciton bimolecular annihilation dynamics in supramolecular nanostructures of conjugated oligomers
We present femtosecond transient absorption measurements on -conjugated
supramolecular assemblies in a high pump fluence regime.
Oligo(\emph{p}-phenylenevinylene) monofunctionalized with
ureido-\emph{s}-triazine (MOPV) self-assembles into chiral stacks in dodecane
solution below 75C at a concentration of M. We
observe exciton bimolecular annihilation in MOPV stacks at high excitation
fluence, indicated by the fluence-dependent decay of B-exciton
spectral signatures, and by the sub-linear fluence dependence of time- and
wavelength-integrated photoluminescence (PL) intensity. These two
characteristics are much less pronounced in MOPV solution where the phase
equilibrium is shifted significantly away from supramolecular assembly,
slightly below the transition temperature. A mesoscopic rate-equation model is
applied to extract the bimolecular annihilation rate constant from the
excitation fluence dependence of transient absorption and PL signals. The
results demonstrate that the bimolecular annihilation rate is very high with a
square-root dependence in time. The exciton annihilation results from a
combination of fast exciton diffusion and resonance energy transfer. The
supramolecular nanostructures studied here have electronic properties that are
intermediate between molecular aggregates and polymeric semiconductors
CANDELS: The progenitors of compact quiescent galaxies at z~2
We combine high-resolution HST/WFC3 images with multi-wavelength photometry
to track the evolution of structure and activity of massive (log(M*) > 10)
galaxies at redshifts z = 1.4 - 3 in two fields of the Cosmic Assembly
Near-infrared Deep Extragalactic Legacy Survey (CANDELS). We detect compact,
star-forming galaxies (cSFGs) whose number densities, masses, sizes, and star
formation rates qualify them as likely progenitors of compact, quiescent,
massive galaxies (cQGs) at z = 1.5 - 3. At z > 2 most cSFGs have specific
star-formation rates (sSFR = 10^-9 yr^-1) half that of typical, massive SFGs at
the same epoch, and host X-ray luminous AGN 30 times (~30%) more frequently.
These properties suggest that cSFGs are formed by gas-rich processes (mergers
or disk-instabilities) that induce a compact starburst and feed an AGN, which,
in turn, quench the star formation on dynamical timescales (few 10^8 yr). The
cSFGs are continuously being formed at z = 2 - 3 and fade to cQGs by z = 1.5.
After this epoch, cSFGs are rare, thereby truncating the formation of new cQGs.
Meanwhile, down to z = 1, existing cQGs continue to enlarge to match local QGs
in size, while less-gas-rich mergers and other secular mechanisms shepherd
(larger) SFGs as later arrivals to the red sequence. In summary, we propose two
evolutionary scenarios of QG formation: an early (z > 2), fast-formation path
of rapidly-quenched cSFGs that evolve into cQGs that later enlarge within the
quiescent phase, and a slow, late-arrival (z < 2) path for SFGs to form QGs
without passing through a compact state.Comment: Submitted to the Astrophysical Journal Letters, 6 pages, 4 figure
Phylogeography of red muntjacs reveals three distinct mitochondrial lineages
Background: The members of the genus Muntiacus are of particular interest to evolutionary biologists due to their
extreme chromosomal rearrangements and the ongoing discussions about the number of living species. Red
muntjacs have the largest distribution of all muntjacs and were formerly considered as one species. Karyotype
differences led to the provisional split between the Southern Red Muntjac (Muntiacus muntjak) and the Northern
Red Muntjac (M. vaginalis), but uncertainties remain as, so far, no phylogenetic study has been conducted. Here, we
analysed whole mitochondrial genomes of 59 archival and 16 contemporaneous samples to resolve uncertainties
about their taxonomy and used red muntjacs as model for understanding the evolutionary history of other species
in Southeast Asia.
Results: We found three distinct matrilineal groups of red muntjacs: Sri Lankan red muntjacs (including the
Western Ghats) diverged first from other muntjacs about 1.5 Mya; later northern red muntjacs (including North
India and Indochina) and southern red muntjacs (Sundaland) split around 1.12 Mya. The diversification of red
muntjacs into these three main lineages was likely promoted by two Pleistocene barriers: one through the Indian
subcontinent and one separating the Indochinese and Sundaic red muntjacs. Interestingly, we found a high level of
gene flow within the populations of northern and southern red muntjacs, indicating gene flow between
populations in Indochina and dispersal of red muntjacs over the exposed Sunda Shelf during the Last Glacial
Maximum.
Conclusions: Our results provide new insights into the evolution of species in South and Southeast Asia as we
found clear genetic differentiation in a widespread and generalist species, corresponding to two known
biogeographical barriers: The Isthmus of Kra and the central Indian dry zone. In addition, our molecular data
support either the delineation of three monotypic species or three subspecies, but more importantly these data
highlight the conservation importance of the Sri Lankan/South Indian red muntjac
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