40 research outputs found
Ultrathin cobalt films on ruthenium (0001): growth, structure, and magnetism
Tesis doctoral inédita. Universidad Autónoma de Madrid. Facultad de Ciencias, Departamento de FÃsica Teórica de la Materia Condensada. Fecha de lectura: 24-11-2006Texto en inglés. Conclusiones en españo
In-plane orientation effects on the electronic structure, stability and Raman scattering of monolayer graphene on Ir(111)
We employ angle-resolved photoemission spectroscopy (ARPES) to investigate
the electronic structures of two rotational variants of epitaxial, single-layer
graphene on Ir(111). As grown, the more-abundant R0 variant is nearly
charge-neutral, with strong hybridization between graphene and Ir bands near
the Fermi level. The graphene Fermi surface and its replicas exactly coincide
with Van Hove singularities in the Ir Fermi surface. Sublattice symmetry
breaking introduces a small gap-inducing potential at the Dirac crossing, which
is revealed by n-doping the graphene using K atoms. The energy gaps between
main and replica bands (originating from the moir\'e interference pattern
between graphene and Ir lattices) is shown to be non-uniform along the mini-
zone boundary due to hybridization with Ir bands. An electronically mediated
interaction is proposed to account for the stability of the R0 variant. The
variant rotated 30{\deg} in-plane, R30, is p-doped as grown and K doping
reveals no band gap at the Dirac crossing. No replica bands are found in ARPES
measurements. Raman spectra from the R30 variant exhibit the characteristic
phonon modes of graphene, while R0 spectra are featureless. These results show
that the film/substrate interaction changes from chemisorption (R0) to
physisorption (R30) with in-plane orientation. Finally, graphene-covered Ir has
a work function lower than the clean substrate but higher than graphite.Comment: Manuscript plus 7 figure
Imaging Spin Reorientation Transitions in Consecutive Atomic Co layers
By means of spin-polarized low-energy electron microscopy (SPLEEM) we show
that the magnetic easy-axis of one to three atomic-layer thick cobalt films on
ruthenium crystals changes its orientation twice during deposition:
one-monolayer and three-monolayer thick films are magnetized in-plane, while
two-monolayer films are magnetized out-of-plane, with a Curie temperature well
above room temperature. Fully-relativistic calculations based on the Screened
Korringa-Kohn-Rostoker (SKKR) method demonstrate that only for two-monolayer
cobalt films the interplay between strain, surface and interface effects leads
to perpendicular magnetization.Comment: 5 pages, 4 figures. Presented at the 2005 ECOSS conference in Berlin,
and at the 2005 Fall meeting of the MRS. Accepted for publication at Phys.
Rev. Lett., after minor change
Mechanisms for charge-transfer processes at electrode/solid-electrolyte interfaces.
This report summarizes the accomplishments of a Laboratory-Directed Research and Development (LDRD) project focused on developing and applying new x-ray spectroscopies to understand and improve electric charge transfer in electrochemical devices. Our approach studies the device materials as they function at elevated temperature and in the presence of sufficient gas to generate meaningful currents through the device. We developed hardware and methods to allow x-ray photoelectron spectroscopy to be applied under these conditions. We then showed that the approach can measure the local electric potentials of the materials, identify the chemical nature of the electrochemical intermediate reaction species and determine the chemical state of the active materials. When performed simultaneous to traditional impedance-based analysis, the approach provides an unprecedented characterization of an operating electrochemical system
Coinage-metal capping effects on the spin-reorientations of Co/Ru(0001)
Thin films of Co/Ru(0001) are known to exhibit an unusual spin reorientation
transition (SRT) coupled to the completion of Co atomic layers for Co
thicknesses under 4 layers. By means of spin-polarized low-energy electron
microscopy, we follow in real space the magnetization orientation during the
growth of atomically thick capping layers on Co/Ru(0001). Capping with
coinage-metal (Cu, Ag, Au) elements modifies the SRT depending on the Co and
overlayer thickness and on the overlayer material, resulting in an expanded
range of structures with high perpendicular magnetic anisotropy. The origin of
the SRT can be explained in terms of ab-initio calculations of the
layer-resolved contributions to the magnetic anisotropy energy. Besides the
changes in the SRT introduced by the capping, a quantitative enhancement of the
magnetic anisotropy is identified. A detailed analysis of the interplay between
strain and purely electronic effects allows us to identify the conditions that
lead to a high perpendicular magnetic anisotropy in thin hcp Co films.Comment: 25 pages, 10 figures. Corrected several typos, added a reference. The
experimental and theory discussion has been rewritten in places for improved
readability. The experimental observations are summarized in a table instead
of a figur
Current-induced transition from particle-by-particle to concurrent intercalation in phase-separating battery electrodes
Many battery electrodes contain ensembles of nanoparticles that phase-separate on (de)intercalation. In such electrodes, the fraction of actively intercalating particles directly impacts cycle life: a vanishing population concentrates the current in a small number of particles, leading to current hotspots. Reports of the active particle population in the phase-separating electrode ​lithium iron phosphate (​LiFePO4; ​LFP) vary widely, ranging from near 0% (particle-by-particle) to 100% (concurrent intercalation). Using synchrotron-based X-ray microscopy, we probed the individual state-of-charge for over 3,000 ​LFP particles. We observed that the active population depends strongly on the cycling current, exhibiting particle-by-particle-like behaviour at low rates and increasingly concurrent behaviour at high rates, consistent with our phase-field porous electrode simulations. Contrary to intuition, the current density, or current per active internal surface area, is nearly invariant with the global electrode cycling rate. Rather, the electrode accommodates higher current by increasing the active particle population. This behaviour results from thermodynamic transformation barriers in ​LFP, and such a phenomenon probably extends to other phase-separating battery materials. We propose that modifying the transformation barrier and exchange current density can increase the active population and thus the current homogeneity. This could introduce new paradigms to enhance the cycle life of phase-separating battery electrodes
Measuring individual overpotentials in an operating solid-oxide electrochemical cell
We use photo-electrons as a non-contact probe to measure local electrical
potentials in a solid-oxide electrochemical cell. We characterize the cell in
operando at near-ambient pressure using spatially-resolved X-ray photoemission
spectroscopy. The overpotentials at the interfaces between the Ni and Pt
electrodes and the yttria-stabilized zirconia (YSZ) electrolyte are directly
measured. The method is validated using electrochemical impedance spectroscopy.
Using the overpotentials, which characterize the cell's inefficiencies, we
compare without ambiguity the electro-catalytic efficiencies of Ni and Pt,
finding that on Ni H_2O splitting proceeds more rapidly than H2 oxidation,
while on Pt, H2 oxidation proceeds more rapidly than H2O splitting.Comment: corrected; Phys. Chem. Chem. Phys., 201