75 research outputs found
On the origin of circular dichroism in angular resolved photoemission from graphene, graphite, and WSe family of materials
Circular dichroism in angle-resolved photoemission (CD-ARPES) is one of the
promising techniques for obtaining experimental insight into topological
properties of novel materials, in particular to the orbital angular momentum
(OAM) in dispersive bands, which might be related, albeit certainly in a
non-trivial way, to the momentum resolved Berry curvature of the bands.
Therefore, it is important to understand how non-vanishing CD-ARPES signal
arises in graphene, a material where Dirac bands are made from C
orbitals that carry zero OAM, spin-orbit-coupling (SOC) can be neglected, and
Berry curvature effectively vanishes. Dubs et al., Phys. Rev. B 32, 8389 (1985)
have demonstrated non-vanishing cricular dichroism in angular distribution
(CDAD) from an oriented orbital, and this process can be responsible for
the experimentally observed CD-ARPES in graphene. In this paper, we derive the
CD-ARPES from orbitals by elementary means, using only simple algebraic
formulas and tabulated numerical values, and show that it leads to significant
CD-ARPES signal over the entire vacuum ultraviolet and soft x-ray energy range,
with an exception of the photon energy region near eV. We
also demonstrate that another process, emerging from the finite electron
inelastic mean free path, also leads to CD-ARPES of the potentially similar
order of magnitude, as previously discussed by Moser, J. Electron Spectrosc.
Relat. Phenom. 214, 29 (2017). We present calculated CDAD maps for selected
orbitals and briefly discuss the consequences of the findings for CD-ARPES,
focusing on graphene, graphite and WSe.Comment: 12 pages, 15 figure
Mixed topological semimetals driven by orbital complexity in two-dimensional ferromagnets
The concepts of Weyl fermions and topological semimetals emerging in
three-dimensional momentum space are extensively explored owing to the vast
variety of exotic properties that they give rise to. On the other hand, very
little is known about semimetallic states emerging in two-dimensional magnetic
materials, which present the foundation for both present and future information
technology. Here, we demonstrate that including the magnetization direction
into the topological analysis allows for a natural classification of
topological semimetallic states that manifest in two-dimensional ferromagnets
as a result of the interplay between spin-orbit and exchange interactions. We
explore the emergence and stability of such mixed topological semimetals in
realistic materials, and point out the perspectives of mixed topological states
for current-induced orbital magnetism and current-induced domain wall motion.
Our findings pave the way to understanding, engineering and utilizing
topological semimetallic states in two-dimensional spin-orbit ferromagnets
Quasi 2D electronic states with high spin-polarization in centrosymmetric MoS bulk crystals
Time reversal dictates that nonmagnetic, centrosymmetric crystals cannot be
spin-polarized as a whole. However, it has been recently shown that the
electronic structure in these crystals can in fact show regions of high
spin-polarization, as long as it is probed locally in real and in reciprocal
space. In this article we present the first observation of this type of
compensated polarization in MoS bulk crystals. Using spin- and
angle-resolved photoemission spectroscopy (ARPES) we directly observed a
spin-polarization of more than 65% for distinct valleys in the electronic band
structure. By additionally evaluating the probing depth of our method we find
that these valence band states at the point in the
Brillouin zone are close to fully polarized for the individual atomic trilayers
of MoS, which is confirmed by our density functional theory calculations.
Furthermore, we show that this spin-layer locking leads to the observation of
highly spin-polarized bands in ARPES since these states are almost completely
confined within two dimensions. Our findings prove that these highly desired
properties of MoS can be accessed without thinning it down to the monolayer
limit
Geometry-induced spin-filtering in photoemission maps from WTe surface states
We demonstrate that an important quantum material WTe exhibits a new type
of geometry-induced spin-filtering effect in photoemission, stemming from low
symmetry that is responsible for its exotic transport properties. Through the
laser-driven spin-polarized angle-resolved photoemission Fermi surface mapping,
we showcase highly asymmetric spin textures of electrons photoemitted from the
surface states of WTe. Such asymmetries are not present in the initial
state spin textures, which are bound by the time-reversal and crystal lattice
mirror plane symmetries. The findings are reproduced qualitatively by
theoretical modeling within the one-step model photoemission formalism. The
effect could be understood within the free-electron final state model as an
interference due to emission from different atomic sites. The observed effect
is a manifestation of time-reversal symmetry breaking of the initial state in
the photoemission process, and as such it cannot be eliminated, but only its
magnitude influenced, by special experimental geometries.Comment: 5 pages, 3 figure
Electrical resistance of individual defects at a topological insulator surface
Three-dimensional topological insulators host surface states with linear
dispersion, which manifest as a Dirac cone. Nanoscale transport measurements
provide direct access to the transport properties of the Dirac cone in real
space and allow the detailed investigation of charge carrier scattering. Here,
we use scanning tunnelling potentiometry to analyse the resistance of different
kinds of defects at the surface of a (Bi0.53Sb0.47)2Te3 topological insulator
thin film. The largest localized voltage drop we find to be located at domain
boundaries in the topological insulator film, with a resistivity about four
times higher than that of a step edge. Furthermore, we resolve resistivity
dipoles located around nanoscale voids in the sample surface. The influence of
such defects on the resistance of the topological surface state is analysed by
means of a resistor network model. The effect resulting from the voids is found
to be small compared to the other defects
Direct observation of the band gap transition in atomically thin ReS
ReS is considered as a promising candidate for novel electronic and
sensor applications. The low crystal symmetry of the van der Waals compound
ReS leads to a highly anisotropic optical, vibrational, and transport
behavior. However, the details of the electronic band structure of this
fascinating material are still largely unexplored. We present a
momentum-resolved study of the electronic structure of monolayer, bilayer, and
bulk ReS using k-space photoemission microscopy in combination with
first-principles calculations. We demonstrate that the valence electrons in
bulk ReS are - contrary to assumptions in recent literature - significantly
delocalized across the van der Waals gap. Furthermore, we directly observe the
evolution of the valence band dispersion as a function of the number of layers,
revealing a significantly increased effective electron mass in single-layer
crystals. We also find that only bilayer ReS has a direct band gap. Our
results establish bilayer ReS as a advantageous building block for
two-dimensional devices and van der Waals heterostructures
Does Exchange Splitting persist above ? A spin-resolved photoemission study of EuO
The electronic structure of the ferromagnetic semiconductor EuO is
investigated by means of spin- and angle-resolved photoemission spectroscopy
and density functional theory (GGA+). Our spin-resolved data reveals that,
while the macroscopic magnetization of the sample vanishes at the Curie
temperature, the exchange splitting of the O 2 band persists up to .
Thus, we provide evidence for short-range magnetic order being present at the
Curie temperature
Sub-nm wide electron channels protected by topology
Helical locking of spin and momentum and prohibited backscattering are the
key properties of topologically protected states. They are expected to enable
novel types of information processing such as spintronics by providing pure
spin currents, or fault tolerant quantum computation by using the Majorana
fermions at interfaces of topological states with superconductors. So far, the
required helical conduction channels used to realize Majorana fermions are
generated through application of an axial magnetic field to conventional
semiconductor nanowires. Avoiding the magnetic field enhances the possibilities
for circuit design significantly. Here, we show that sub-nanometer wide
electron channels with natural helicity are present at surface step-edges of
the recently discovered topological insulator Bi14Rh3I9. Scanning tunneling
spectroscopy reveals the electron channels to be continuous in both energy and
space within a large band gap of 200 meV, thereby, evidencing its non-trivial
topology. The absence of these channels in the closely related, but
topologically trivial insulator Bi13Pt3I7 corroborates the channels'
topological nature. The backscatter-free electron channels are a direct
consequence of Bi14Rh3I9's structure, a stack of 2D topologically insulating,
graphene-like planes separated by trivial insulators. We demonstrate that the
surface of Bi14Rh3I9 can be engraved using an atomic force microscope, allowing
networks of protected channels to be patterned with nm precision.Comment: 17 pages, 4 figures, and supplementary material, Nature Physics in
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