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Polarization control at the microscopic and electronic structure observatory
The new Microscopic and Electronic Structure Observatory (MAESTRO) at the Advanced Light Source (ALS) in Berkeley provides X-rays of variable polarization, produced by an elliptically polarized undulator (EPU), for angle resolved photoemission (ARPES) and photoemission electron microscopy (PEEM) experiments. The interpretation of photoemission data, in particular of dichroism effects in ARPES, requires the precise knowledge of the exact polarization state. Numerical simulations show that the first harmonics of the EPU at MAESTRO provides soft X-rays of almost 100% on axis polarization. However, the higher harmonics as well as the downstream optical elements of the beamline, have a considerable impact on the polarization of the light delivered to the experimental end-station. Employing a simple reflective polarimeter, the polarization is characterized for variable EPU and beamline settings and the overall degree of polarization in the MAESTRO end-stations is estimated to be on the order of 83%
Cluster sum rules for three-body systems with angular-momentum dependent interactions
We derive general expressions for non-energy weighted and energy-weighted
cluster sum rules for systems of three charged particles. The interferences
between pairs of particles are found to play a substantial role. The
energy-weighted sum rule is usually determined by the kinetic energy operator,
but we demonstrate that it has similar additional contributions from the
angular momentum and parity dependence of two- and three-body potentials
frequently used in three-body calculations. The importance of the different
contributions is illustrated with the dipole excitations in He. The results
are compared with the available experimental data.Comment: 11 pages, 3 figures, 2 table
Strictly One-Dimensional Electron System in Au Chains on Ge(001) Revealed By Photoelectron K-Space Mapping
Atomic nanowires formed by Au on Ge(001) are scrutinized for the band
topology of the conduction electron system by k-resolved photoemission. Two
metallic electron pockets are observed. Their Fermi surface sheets form
straight lines without undulations perpendicular to the chains within
experimental uncertainty. The electrons hence emerge as strictly confined to
one dimension. Moreover, the system is stable against a Peierls distortion down
to 10 K, lending itself for studies of the spectral function. Indications for
unusually low spectral weight at the chemical potential are discussed.Comment: 4 pages, 4 figures - revised version with added Fig. 2e) and
additional reference
Anderson Transition in Disordered Graphene
We use the regularized kernel polynomial method (RKPM) to numerically study
the effect disorder on a single layer of graphene. This accurate numerical
method enables us to study very large lattices with millions of sites, and
hence is almost free of finite size errors. Within this approach, both weak and
strong disorder regimes are handled on the same footing. We study the
tight-binding model with on-site disorder, on the honeycomb lattice. We find
that in the weak disorder regime, the Dirac fermions remain extended and their
velocities decrease as the disorder strength is increased. However, if the
disorder is strong enough, there will be a {\em mobility edge} separating {\em
localized states around the Fermi point}, from the remaining extended states.
This is in contrast to the scaling theory of localization which predicts that
all states are localized in two-dimensions (2D).Comment: 4 page
Tunable Polaronic Conduction in Anatase TiO2
Oxygen vacancies created in anatase TiO2 by UV photons (80–130 eV) provide an effective electron-doping mechanism and induce a hitherto unobserved dispersive metallic state. Angle resolved photoemission reveals that the quasiparticles are large polarons. These results indicate that anatase can be tuned from an insulator to a polaron gas to a weakly correlated metal as a function of doping and clarify the nature of conductivity in this material.open1192sciescopu
Electronic Instability in a Zero-Gap Semiconductor: The Charge-DensityWave in (TaSe4)(2)I
We report a comprehensive study of the paradigmatic quasi-1D compound (TaSe4)(2)I performed by means of angle-resolved photoemission spectroscopy (ARPES) and first-principles electronic structure calculations. We find it to be a zero-gap semiconductor in the nondistorted structure, with non-negligible interchain coupling. Theory and experiment support a Peierls-like scenario for the charge-density wave formation below T-CDW = 263 K, where the incommensurability is a direct consequence of the finite interchain coupling. The formation of small polarons, strongly suggested by the ARPES data, explains the puzzling semiconductor-to-semiconductor transition observed in transport at T-CDW.open114sciescopu
The electronic structure of the high-symmetry perovskite iridate Ba2IrO4
We report angle-resolved photoemission (ARPES) measurements, density
functional and model tight-binding calculations on BaIrO (Ba-214), an
antiferromagnetic ( K) insulator. Ba-214 does not exhibit the
rotational distortion of the IrO octahedra that is present in its sister
compound SrIrO (Sr-214), and is therefore an attractive reference
material to study the electronic structure of layered iridates. We find that
the band structures of Ba-214 and Sr-214 are qualitatively similar, hinting at
the predominant role of the spin-orbit interaction in these materials.
Temperature-dependent ARPES data show that the energy gap persists well above
, and favour a Mott over a Slater scenario for this compound.Comment: 13 pages, 9 figure
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