411 research outputs found
X-ray absorption spectroscopy on layered cobaltates Na_xCoO_2
Measurements of polarization and temperature dependent soft x-ray absorption
have been performed on Na_xCoO_2 single crystals with x=0.4 and x=0.6. They
show a deviation of the local trigonal symmetry of the CoO_6 octahedra, which
is temperature independent in a temperature range between 25 K and 372 K. This
deviation was found to be different for Co^{3+} and Co^{4+} sites. With the
help of a cluster calculation we are able to interpret the Co L_{23}-edge
absorption spectrum and find a doping dependent energy splitting between the
t_{2g} and the e_g levels (10Dq) in Na_xCoO_2.Comment: 7 pages, 8 figure
Bare electron dispersion from photoemission experiments
Performing an in-depth analysis of the photoemission spectra along the nodal
direction of the high temperature superconductor Bi-2212 we have developed a
procedure to determine the underlying electronic structure and established a
precise relation of the measured quantities to the real and imaginary parts of
the self-energy of electronic excitations. The self-consistency of the
procedure with respect to the Kramers-Kronig transformation allows us to draw
conclusions on the applicability of the spectral function analysis and on the
existence of well defined quasiparticles along the nodal direction even for the
underdoped Bi-2212 in the pseudogap state.Comment: 4 pages 3 figures revtex, corrected misprint
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Spectromicroscopic measurements of electronic structure variations in atomically thin WSe2
Atomically thin transition metal dichalcogenides (TMDCs) are promising candidates for implementation in next generation semiconducting devices, for which laterally homogeneous behavior is needed. Here, we study the electronic structure of atomically thin exfoliated WSe2, a prototypical TMDC with large spin–orbit coupling, by photoemission electron microscopy, electron energy-loss spectroscopy, and density functional theory. We resolve the inhomogeneities of the doping level by the varying energy positions of the valence band. There appear to be different types of inhomogeneities that respond differently to electron doping, introduced by potassium intercalation. In addition, we find that the doping process itself is more complex than previously anticipated and entails a distinct orbital and thickness dependence that needs to be considered for effective band engineering. In particular, the density of selenium vs tungsten states depends on the doping level, which leads to changes in the optical response beyond increased dielectric screening. Our work gives insight into the inhomogeneity of the electron structure of WSe2 and the effects of electron doping, provides microscopic understanding thereof, and improves the basis for property engineering of 2D materials
Fine-tuning the functional properties of carbon nanotubes via the interconversion of encapsulated molecules
Tweaking the properties of carbon nanotubes is a prerequisite for their
practical applications. Here we demonstrate fine-tuning the electronic
properties of single-wall carbon nanotubes via filling with ferrocene
molecules. The evolution of the bonding and charge transfer within the tube is
demonstrated via chemical reaction of the ferrocene filler ending up as
secondary inner tube. The charge transfer nature is interpreted well within
density functional theory. This work gives the first direct observation of a
fine-tuned continuous amphoteric doping of single-wall carbon nanotubes
A photoemission study of interfaces between organic semiconductors and Co as well as Al<sub>2</sub>O<sub>3</sub>/Co contacts
We have studied the energy-level alignment of ex situ, acetone cleaned Co and Al2O3/Co contacts to the organic semiconductors pentacene and rubrene by combined X-ray and ultraviolet photoemission spectroscopy. Our results demonstrate that the work function under these conditions is smaller than in the in situ cleaned, atomically clean case. Moreover, the studied interfaces are characterized by very small, short range interfaces dipoles and substantial injection barriers for holes. This represents essential information in view of their use in organic spintronic devices. Our core-level photoemission spectroscopy measurements rule out chemical reactions
Linear plasmon dispersion in single-wall carbon nanotubes and the collective excitation spectrum of graphene
We have measured a strictly linear pi-plasmon dispersion along the axis of
individualized single wall carbon nanotubes, which is completely different from
plasmon dispersions of graphite or bundled single wall carbon nanotubes.
Comparative ab initio studies on graphene based systems allow us to reproduce
the different dispersions. This suggests that individualized nanotubes provide
viable experimental access to collective electronic excitations of graphene,
and it validates the use of graphene to understand electronic excitations of
carbon nanotubes. In particular, the calculations reveal that local field
effects (LFE) cause a mixing of electronic transitions, including the 'Dirac
cone', resulting in the observed linear dispersion
Circular dichroism and bilayer splitting in the normal state of underdoped (Pb,Bi)Sr(CaY)CuO and overdoped (Pb,Bi)SrCaCuO
We report an ARPES investigation of the circular dichroism in the first
Brillouin zone (BZ) of under- and overdoped Pb-Bi2212 samples. We show that the
dichroism has opposite signs for bonding and antibonding components of the
bilayer-split CuO-band and is antisymmetric with respect to reflections in both
mirror planes parallel to the c-axis. Using this property of the energy and
momentum intensity distributions we prove the existence of the bilayer
splitting in the normal state of the underdoped compound and compare its value
with the splitting in overdoped sample. In agreement with previous studies the
magnitude of the interlayer coupling does not depend significantly on doping.
We also discuss possible origins of the observed dichroism.Comment: 4 RevTex pages, 4 EPS figure
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