147 research outputs found
Photoemission Electron Microscopy as a tool for the investigation of optical near fields
Photoemission electron microscopy was used to image the electrons
photoemitted from specially tailored Ag nanoparticles deposited on a Si
substrate (with its native oxide SiO). Photoemission was induced by
illumination with a Hg UV-lamp (photon energy cutoff eV,
wavelength nm) and with a Ti:Sapphire femtosecond laser
( eV, nm, pulse width below 200 fs),
respectively. While homogeneous photoelectron emission from the metal is
observed upon illumination at energies above the silver plasmon frequency, at
lower photon energies the emission is localized at tips of the structure. This
is interpreted as a signature of the local electrical field therefore providing
a tool to map the optical near field with the resolution of emission electron
microscopy.Comment: 10 pages, 4 figures; submitted to Physical Review Letter
Interface Engineering to Create a Strong Spin Filter Contact to Silicon
Integrating epitaxial and ferromagnetic Europium Oxide (EuO) directly on
silicon is a perfect route to enrich silicon nanotechnology with spin filter
functionality.
To date, the inherent chemical reactivity between EuO and Si has prevented a
heteroepitaxial integration without significant contaminations of the interface
with Eu silicides and Si oxides.
We present a solution to this long-standing problem by applying two
complementary passivation techniques for the reactive EuO/Si interface:
() an hydrogen-Si passivation and () the
application of oxygen-protective Eu monolayers --- without using any additional
buffer layers.
By careful chemical depth profiling of the oxide-semiconductor interface via
hard x-ray photoemission spectroscopy, we show how to systematically minimize
both Eu silicide and Si oxide formation to the sub-monolayer regime --- and how
to ultimately interface-engineer chemically clean, heteroepitaxial and
ferromagnetic EuO/Si in order to create a strong spin filter contact to
silicon.Comment: 11 pages of scientific paper, 10 high-resolution color figures.
Supplemental information on the thermodynamic problem available (PDF).
High-resolution abstract graphic available (PNG). Original research (2016
Raman and fluorescence contributions to resonant inelastic soft x-ray scattering on LaAlO/SrTiO heterostructures
We present a detailed study of the Ti 3 carriers at the interface of
LaAlO/SrTiO heterostructures by high-resolution resonant inelastic soft
x-ray scattering (RIXS), with special focus on the roles of overlayer thickness
and oxygen vacancies. Our measurements show the existence of interfacial Ti
3 electrons already below the critical thickness for conductivity and an
increase of the total interface charge up to a LaAlO overlayer thickness of
6 unit cells before it levels out. By comparing stoichiometric and oxygen
deficient samples we observe strong Ti 3 charge carrier doping by oxygen
vacancies. The RIXS data combined with photoelectron spectroscopy and transport
measurements indicate the simultaneous presence of localized and itinerant
charge carriers. However, it is demonstrated that the relative amount of
localized and itinerant Ti electrons in the ground state cannot be deduced
from the relative intensities of the Raman and fluorescence peaks in excitation
energy dependent RIXS measurements, in contrast to previous interpretations.
Rather, we attribute the observation of either the Raman or the fluorescence
signal to the spatial extension of the intermediate state reached in the RIXS
excitation process.Comment: 9 pages, 6 figure
Monitoring surface resonances on Co2MnSi(100) by spin-resolved photoelectron spectroscopy
The magnitude of the spin polarization at the Fermi level of ferromagnetic
materials at room temperature is a key property for spintronics. Investigating
the Heusler compound CoMnSi a value of 93 for the spin polarization has
been observed at room temperature, where the high spin polarization is related
to a stable surface resonance in the majority band extending deep into the
bulk. In particular, we identified in our spectroscopical analysis that this
surface resonance is embedded in the bulk continuum with a strong coupling to
the majority bulk states. The resonance behaves very bulk-like, as it extends
over the first six atomic layers of the corresponding (001)-surface. Our study
includes experimental investigations, where the bulk electronic structure as
well as surface-related features have been investigated using spin-resolved
photoelectron spectroscopy (SR-UPS) and for a larger probing depth
spin-integrated high energy x-ray photoemission spectroscopy (HAXPES). The
results are interpreted in comparison with first-principles band structure and
photoemission calculations which consider all relativistic, surface and
high-energy effects properly.Comment: 9 pages, 8 figures, Heusler alloy, electronic structure and
photoemissio
Layer-resolved electronic behavior in a Kondo lattice system, CeAgAs2
We investigate the electronic structure of an antiferromagnetic Kondo lattice
system CeAgAs2 employing hard x-ray photoemission spectroscopy. CeAgAs2, an
orthorhombic variant of HfCuSi2 structure, exhibits antiferromagnetic ground
state, Kondo like resistivity upturn and compensation of magnetic moments at
low temperatures. The photoemission spectra obtained at different photon
energies suggest termination of the cleaved surface at cis-trans-As layers. The
depth-resolved data show significant surface-bulk differences in the As and Ce
core level spectra. The As 2p bulk spectrum shows distinct two peaks
corresponding to two different As layers. The peak at higher binding energy
correspond to cis-trans-As layers and is weakly hybridized with the adjacent Ce
layers. The As layers between Ce and Ag-layers possess close to trivalent
configuration due to strong hybridization with the neighboring atoms and the
corresponding feature appear at lower binding energy. Ce 3d core level spectra
show multiple features reflecting strong Ce-As hybridization and strong
correlation. Intense f0 peak is observed in the surface spectrum while it is
insignificant in the bulk. In addition, we observe a features at binding energy
lower than the well-screened feature indicating the presence of additional
interactions. This feature becomes more intense in the bulk spectra suggesting
it to be a bulk property. Increase in temperature leads to a spectral weight
transfer to higher binding energies in the core level spectra and a depletion
of spectral intensity at the Fermi level as expected in a Kondo material. These
results reveal interesting surface-bulk differences, complex interplay of
intra- and inter-layer covalency, and electron correlation in the electronic
structure of this novel Kondo lattice system
Complexity in the hybridization physics revealed by depth-resolved photoemission spectroscopy of single crystalline novel Kondo lattice systems, CeCuX (X = As/Sb)
We investigate the electronic structure of a novel Kondo lattice system
CeCuX2 (X = As/Sb) employing high resolution depth-resolved photoemission
spectroscopy of high quality single crystalline materials. CeCuSb2 and CeCuAs2
represent different regimes of the Doniach phase diagram exhibiting Kondo-like
transport properties and CeCuSb2 is antiferromagnetic (TN ~ 6.9 K) while
CeCuAs does not show long-range magnetic order down to the lowest
temperature studied. In this study, samples were cleaved in ultrahigh vacuum
before the photoemission measurements and the spectra at different surface
sensitivity establish the pnictogen layer having squarenet structure as the
terminated surface which is weakly bound to the other layers. Cu 2p and As 2p
spectra show spin-orbit split sharp peaks along with features due to plasmon
excitations. Ce 3d spectra exhibit multiple features due to the hybridization
of the Ce 4f/5d states with the valence states. While overall lineshape of the
bulk spectral functions look similar in both the cases, the surface spectra are
very different; the surface-bulk difference is significantly weaker in CeCuAs2
compared to that observed in CeCuSb2. A distinct low binding energy peak is
observed in the Ce 3d spectra akin to the scenario observed in cuprates and
manganites due to the Zhang-Rice singlets and/or high degree of itineracy of
the conduction holes. The valence band spectra of CeCuSb manifest highly
metallic phase. In CeCuAs2, intensity at the Fermi level is significantly small
suggesting a pseudogap-type behavior. These results bring out an interesting
scenario emphasizing the importance and subtlety of hybridization physics
underlying the exoticity of this novel Kondo system
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