127 research outputs found
Concept of a multichannel spin-resolving electron analyzer based on Mott scattering
The concept of a multichannel electron spin detector based on optical imaging principles and Mott scattering (iMott) is presented. A multichannel electron image produced by a standard angle-resolving (photo) electron analyzer or microscope is re-imaged by an electrostatic lens at an accelerating voltage of 40 kV onto the Au target. Quasi-elastic electrons bearing spin asymmetry of the Mott scattering are imaged by magnetic lenses onto position-sensitive electron CCDs whose differential signals yield the multichannel spin asymmetry image. Fundamental advantages of this concept include acceptance of inherently divergent electron sources from the electron analyzer or microscope focal plane as well as small aberrations achieved by virtue of high accelerating voltages, as demonstrated by extensive ray-tracing analysis. The efficiency gain compared with the single-channel Mott detector can be a factor of more than 10 4 which opens new prospects of spin-resolved spectroscopies in application not only to standard bulk and surface systems (Rashba effect, topological insulators, etc.) but also to buried heterostructures. The simultaneous spin detection combined with fast CCD readout enables efficient use of the iMott detectors at X-ray free-electron laser facilities
Self-doping processes between planes and chains in the metal-to-superconductor transition of YBa2Cu3O6.9
The interplay between the quasi 1-dimensional CuO-chains and the
2-dimensional CuO2 planes of YBa2Cu3O6+x (YBCO) has been in focus for a long
time. Although the CuO-chains are known to be important as charge reservoirs
that enable superconductivity for a range of oxygen doping levels in YBCO, the
understanding of the dynamics of its temperature-driven metal-superconductor
transition (MST) remains a challenge. We present a combined study using x-ray
absorption spectroscopy and resonant inelastic x-ray scattering (RIXS)
revealing how a reconstruction of the apical O(4)-derived interplanar orbitals
during the MST of optimally doped YBCO leads to substantial hole-transfer from
the chains into the planes, i.e. self-doping. Our ionic model calculations show
that localized divalent charge-transfer configurations are expected to be
abundant in the chains of YBCO. While these indeed appear in the RIXS spectra
from YBCO in the normal, metallic, state, they are largely suppressed in the
superconducting state and, instead, signatures of Cu trivalent charge-transfer
configurations in the planes become enhanced. In the quest for understanding
the fundamental mechanism for high-Tc-superconductivity (HTSC) in perovskite
cuprate materials, the observation of such an interplanar self-doping process
in YBCO opens a unique novel channel for studying the dynamics of HTSC.Comment: 9 pages, 4 Figure
Critical Diagnosis of Electronic Dimensionality Reduction in Semiconductor Quantum Well Structures
Two-dimensional (2D) systems, such as high-temperature superconductors,
surface states of topological insulators, and layered materials, have been
intensively studied using vacuum-ultraviolet (VUV) angle-resolved photoemission
spectroscopy (ARPES). In semiconductor films (heterostructures), quantum well
(QW) states arise due to electron/hole accumulations at the surface
(interface). The quantized states due to quantum confinement can be observed by
VUV-ARPES, while the periodic intensity modulations along the surface normal
(kz) direction of these quantized states are also observable by varying
incident photon energy, resembling three-dimensional (3D) band dispersion. We
have conducted soft X-ray (SX) ARPES measurements on thick and ultrathin III-V
semiconductor InSb(001) films to investigate the electronic dimensionality
reduction in semiconductor QWs. In addition to the dissipation of the kz
dispersion, the SX-ARPES observations demonstrate the changes of the symmetry
and periodicity of the Brillouin zone in the ultrathin film as 2D QW compared
with these of the 3D bulk one, indicating the electronic dimensionality
reduction of the 3D bulk band dispersion caused by the quantum confinement. The
results provide a critical diagnosis using SX-ARPES for the dimensionality
reduction in semiconductor QW structures
Intra-layer doping effects on the high-energy magnetic correlations in NaFeAs
We have used Resonant Inelastic X-ray Scattering (RIXS) and dynamical
susceptibility calculations to study the magnetic excitations in
NaFeCoAs (x = 0, 0.03, and 0.08). Despite a relatively low ordered
magnetic moment, collective magnetic modes are observed in parent compounds (x
= 0) and persist in optimally (x = 0.03) and overdoped (x = 0.08) samples.
Their magnetic bandwidths are unaffected by doping within the range
investigated. High energy magnetic excitations in iron pnictides are robust
against doping, and present irrespectively of the ordered magnetic moment.
Nevertheless, Co doping slightly reduces the overall magnetic spectral weight,
differently from previous studies on hole-doped BaFeAs, where it
was observed constant. Finally, we demonstrate that the doping evolution of
magnetic modes is different for the dopants being inside or outside the Fe-As
layer.Comment: 19 pages, 7 figure
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