585 research outputs found
Magnetism and half-metallicity at the O surfaces of ceramic oxides
The occurence of spin-polarization at ZrO, AlO and MgO
surfaces is proved by means of \textit{ab-initio} calculations within the
density functional theory. Large spin moments, as high as 1.56 , develop
at O-ended polar terminations, transforming the non-magnetic insulator into a
half-metal. The magnetic moments mainly reside in the surface oxygen atoms and
their origin is related to the existence of holes of well-defined spin
polarization at the valence band of the ionic oxide. The direct relation
between magnetization and local loss of donor charge makes possible to extend
the magnetization mechanism beyond surface properties
A comparative DFT study of electronic properties of 2H-, 4H- and 6H-SiC(0001) and SiC(000-1) clean surfaces: Significance of the surface Stark effect
Electric field, uniform within the slab, emerging due to Fermi level pinning
at its both sides is analyzed using DFT simulations of the SiC surface slabs of
different thickness. It is shown that for thicker slab the field is nonuniform
and this fact is related to the surface state charge. Using the electron
density and potential profiles it is proved that for high precision simulations
it is necessary to take into account enough number of the Si-C layers. We show
that using 12 diatomic layers leads to satisfactory results. It is also
demonstrated that the change of the opposite side slab termination, both by
different type of atoms or by their location, can be used to adjust electric
field within the slab, creating a tool for simulation of surface properties,
depending on the doping in the bulk of semiconductor. Using these simulations
it was found that, depending on the electric field, the energy of the surface
states changes in a different way than energy of the bulk states. This
criterion can be used to distinguish Shockley and Tamm surface states. The
electronic properties, i.e. energy and type of surface states of the three
clean surfaces: 2H-, 4H-, 6H-SiC(0001), and SiC() are analyzed and
compared using field dependent DFT simulations.Comment: 18 pages, 10 figures, 4 table
Contrast Mechanisms for the Detection of Ferroelectric Domains with Scanning Force Microscopy
We present a full analysis of the contrast mechanisms for the detection of
ferroelectric domains on all faces of bulk single crystals using scanning force
microscopy exemplified on hexagonally poled lithium niobate. The domain
contrast can be attributed to three different mechanisms: i) the thickness
change of the sample due to an out-of-plane piezoelectric response (standard
piezoresponse force microscopy), ii) the lateral displacement of the sample
surface due to an in-plane piezoresponse, and iii) the electrostatic tip-sample
interaction at the domain boundaries caused by surface charges on the
crystallographic y- and z-faces. A careful analysis of the movement of the
cantilever with respect to its orientation relative to the crystallographic
axes of the sample allows a clear attribution of the observed domain contrast
to the driving forces respectively.Comment: 8 pages, 8 figure
Investigating Atomic Details of the CaF(111) Surface with a qPlus Sensor
The (111) surface of CaF has been intensively studied with
large-amplitude frequency-modulation atomic force microscopy and atomic
contrast formation is now well understood. It has been shown that the apparent
contrast patterns obtained with a polar tip strongly depend on the tip
terminating ion and three sub-lattices of anions and cations can be imaged.
Here, we study the details of atomic contrast formation on CaF(111) with
small-amplitude force microscopy utilizing the qPlus sensor that has been shown
to provide utmost resolution at high scanning stability. Step edges resulting
from cleaving crystals in-situ in the ultra-high vacuum appear as very sharp
structures and on flat terraces, the atomic corrugation is seen in high clarity
even for large area scans. The atomic structure is also not lost when scanning
across triple layer step edges. High resolution scans of small surface areas
yield contrast features of anion- and cation sub-lattices with unprecedented
resolution. These contrast patterns are related to previously reported
theoretical results.Comment: 18 pages, 9 Figures, presented at 7th Int Conf Noncontact AFM
Seattle, USA Sep 12-15 2004, accepted for publication in Nanotechnology,
http://www.iop.or
Composition, structure and stability of RuO_2(110) as a function of oxygen pressure
Using density-functional theory (DFT) we calculate the Gibbs free energy to
determine the lowest-energy structure of a RuO_2(110) surface in thermodynamic
equilibrium with an oxygen-rich environment. The traditionally assumed
stoichiometric termination is only found to be favorable at low oxygen chemical
potentials, i.e. low pressures and/or high temperatures. At realistic O
pressure, the surface is predicted to contain additional terminal O atoms.
Although this O excess defines a so-called polar surface, we show that the
prevalent ionic model, that dismisses such terminations on electrostatic
grounds, is of little validity for RuO_2(110). Together with analogous results
obtained previously at the (0001) surface of corundum-structured oxides, these
findings on (110) rutile indicate that the stability of non-stoichiometric
terminations is a more general phenomenon on transition metal oxide surfaces.Comment: 12 pages including 5 figures. Submitted to Phys. Rev. B. Related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
Surface electronic structure of the Fe3O4(100): Evidence of a half-metal to metal transition
In situ prepared Fe3O4(100) thin films were studied by means of scanning tunneling microscopy (STM) and spin-polarized photoelectron spectroscopy (SP-PES). The atomically resolved (2×2)R45°wavelike surface atomic structure observed by STM is explained based on density functional theory (DFT) and ab initio atomistic thermodynamics calculations as a laterally distorted surface layer containing octahedral iron and oxygen, referred to as a modified B layer. The work-function value of the Fe3O4(100) surface extracted from the cutoff of the photoelectron spectra is in good agreement with that predicted from DFT. On the Fe3O4(100) surface both the SP-PES measurements and the DFT results show a strong reduction of the spin polarization at the Fermi level (EF) compared to the bulk density of states. The nature of the states in the majority band gap of the Fe3O4 surface layer is analyzed
First-principles study of the polar O-terminated ZnO surface in thermodynamic equilibrium with oxygen and hydrogen
Using density-functional theory in combination with a thermodynamic formalism
we calculate the relative stability of various structural models of the polar
O-terminated (000-1)-O surface of ZnO. Model surfaces with different
concentrations of oxygen vacancies and hydrogen adatoms are considered.
Assuming that the surfaces are in thermodynamic equilibrium with an O2 and H2
gas phase we determine a phase diagram of the lowest-energy surface structures.
For a wide range of temperatures and pressures we find that hydrogen will be
adsorbed at the surface, preferentially with a coverage of 1/2 monolayer. At
high temperatures and low pressures the hydrogen can be removed and a structure
with 1/4 of the surface oxygen atoms missing becomes the most stable one. The
clean, defect-free surface can only exist in an oxygen-rich environment with a
very low hydrogen partial pressure. However, since we find that the
dissociative adsorption of molecular hydrogen and water (if also the
Zn-terminated surface is present) is energetically very preferable, it is very
unlikely that a clean, defect-free (000-1)-O surface can be observed in
experiment.Comment: 10 pages, 4 postscript figures. Uses REVTEX and epsf macro
Perspective of the Surviving Sepsis Campaign on the Management of Pediatric Sepsis in the Era of Coronavirus Disease 2019
Severe acute respiratory syndrome coronavirus 2 is a novel cause of organ dysfunction in children, presenting as either coronavirus disease 2019 with sepsis and/or respiratory failure or a hyperinflammatory shock syndrome. Clinicians must now consider these diagnoses when evaluating children for septic shock and sepsis-associated organ dysfunction. The Surviving Sepsis Campaign International Guidelines for the Management of Septic Shock and Sepsis-associated Organ Dysfunction in Children provide an appropriate framework for the early recognition and initial resuscitation of children with sepsis or septic shock caused by all pathogens, including severe acute respiratory syndrome coronavirus 2. However, the potential benefits of select adjunctive therapies may differ from non-coronavirus disease 2019 sepsis
Cell-free (RNA) and cell-associated (DNA) HIV-1 and postnatal transmission through breastfeeding
<p>Introduction - Transmission through breastfeeding remains important for mother-to-child transmission (MTCT) in resource-limited settings. We quantify the relationship between cell-free (RNA) and cell-associated (DNA) shedding of HIV-1 virus in breastmilk and the risk of postnatal HIV-1 transmission in the first 6 months postpartum.</p>
<p>Materials and Methods - Thirty-six HIV-positive mothers who transmitted HIV-1 by breastfeeding were matched to 36 non-transmitting HIV-1 infected mothers in a case-control study nested in a cohort of HIV-infected women. RNA and DNA were quantified in the same breastmilk sample taken at 6 weeks and 6 months. Cox regression analysis assessed the association between cell-free and cell-associated virus levels and risk of postnatal HIV-1 transmission.</p>
<p>Results - There were higher median levels of cell-free than cell-associated HIV-1 virus (per ml) in breastmilk at 6 weeks and 6 months. Multivariably, adjusting for antenatal CD4 count and maternal plasma viral load, at 6 weeks, each 10-fold increase in cell-free or cell-associated levels (per ml) was significantly associated with HIV-1 transmission but stronger for cell-associated than cell-free levels [2.47 (95% CI 1.33–4.59) vs. aHR 1.52 (95% CI, 1.17–1.96), respectively]. At 6 months, cell-free and cell-associated levels (per ml) in breastmilk remained significantly associated with HIV-1 transmission but was stronger for cell-free than cell-associated levels [aHR 2.53 (95% CI 1.64–3.92) vs. 1.73 (95% CI 0.94–3.19), respectively].</p>
<p>Conclusions - The findings suggest that cell-associated virus level (per ml) is more important for early postpartum HIV-1 transmission (at 6 weeks) than cell-free virus. As cell-associated virus levels have been consistently detected in breastmilk despite antiretroviral therapy, this highlights a potential challenge for resource-limited settings to achieve the UNAIDS goal for 2015 of eliminating vertical transmission. More studies would further knowledge on mechanisms of HIV-1 transmission and help develop more effective drugs during lactation.</p>
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