268 research outputs found
Suppression of electron scattering resonances in graphene by quantum dots
Transmission of low-energetic electrons through two-dimensional materials
leads to unique scattering resonances. These resonances contribute to
photoemission from occupied bands where they appear as strongly dispersive
features of suppressed photoelectron intensity. Using angle-resolved
photoemission we have systematically studied scattering resonances in epitaxial
graphene grown on the chemically differing substrates Ir(111), Bi/Ir, Ni(111)
as well as in graphene/Ir(111) nanopatterned with a superlattice of uniform Ir
quantum dots. While the strength of the chemical interaction with the substrate
has almost no effect on the dispersion of the scattering resonances, their
energy can be controlled by the magnitude of charge transfer from/to graphene.
At the same time, a superlattice of small quantum dots deposited on graphene
eliminates the resonances completely. We ascribe this effect to a
nanodot-induced buckling of graphene and its local rehybridization from
sp to sp towards a three-dimensional structure. Our results suggest
nanopatterning as a prospective tool for tuning optoelectronic properties of
two-dimensional materials with graphene-like structure.Comment: The following article has been submitted to Applied Physics Letters.
If it is published, it will be found online at http://apl.aip.or
Rashba splitting of 100 meV in Au-intercalated graphene on SiC
Intercalation of Au can produce giant Rashba-type spin-orbit splittings in
graphene but this has not yet been achieved on a semiconductor substrate. For
graphene/SiC(0001), Au intercalation yields two phases with different doping.
Here, we report the preparation of an almost pure p-type graphene phase after
Au intercalation. We observe a 100 meV Rashba-type spin-orbit splitting at 0.9
eV binding energy. We show that this giant splitting is due to hybridization
and much more limited in energy and momentum space than for Au-intercalated
graphene on Ni
Laser-induced persistent photovoltage on the surface of a ternary topological insulator at room temperature
Using time- and angle-resolved photoemission, we investigate the ultrafast
response of excited electrons in the ternary topological insulator (BiSb)Te to fs-infrared pulses. We demonstrate that at the
critical concentration =0.55, where the system becomes bulk insulating, a
surface voltage can be driven at room temperature through the topological
surface state solely by optical means. We further show that such a photovoltage
persists over a time scale that exceeds 6 s, i.e, much longer than
the characteristic relaxation times of bulk states. We attribute the origin of
the photovoltage to a laser-induced band-bending effect which emerges near the
surface region on ultrafast time scales. The photovoltage is also accompanied
by a remarkable increase in the relaxation times of excited states as compared
to undoped topological insulators. Our findings are relevant in the context of
applications of topological surface states in future optical devices.Comment: 5 pages, 4 figure
Ultrafast spin polarization control of Dirac fermions in topological insulators
Three-dimensional topological insulators (TIs) are characterized by
spin-polarized Dirac-cone surface states that are protected from backscattering
by time-reversal symmetry. Control of the spin polarization of topological
surface states (TSSs) using femtosecond light pulses opens novel perspectives
for the generation and manipulation of dissipationless surface spin currents on
ultrafast timescales. Using time-, spin-, and angle-resolved spectroscopy, we
directly monitor for the first time the ultrafast response of the spin
polarization of photoexcited TSSs to circularly-polarized femtosecond pulses of
infrared light. We achieve all-optical switching of the transient out-of-plane
spin polarization, which relaxes in about 1.2 ps. Our observations establish
the feasibility of ultrafast optical control of spin-polarized Dirac fermions
in TIs and pave the way for novel optospintronic applications at ultimate
speeds.Comment: 9 pages, 4 figure
Design issues for the VLSI implementation of universal approximator fuzzy systems
Comunicación presentada al "CSCC'99" celebrado en Atenas en Julio de 1999.Several VLSI realizations of fuzzy systems have been proposed in the literature in the recent years. They employ analog or digital circuitry, offering more or less programmability, implementing different inference methods, with different types of membership functions as well as different antecedents’ connectives. This paper centers this wide design space by fixing several parameters that allow efficient VLSI implementations of programmable fuzzy systems featuring first, second and third order accurate approximation. Hardware requirements are discussed and compared from the point of view of approximation capability or precision, thus attempting to a formalization that has never been applied before to the field of fuzzy hardware.This work has been partially supported by the Spanish CICYT Project TIC98-0869.Peer reviewe
Hidden spin-orbital texture at the -located valence band maximum of a transition metal dichalcogenide semiconductor
Finding stimuli capable of driving an imbalance of spin-polarised electrons
within a solid is the central challenge in the development of spintronic
devices. However, without the aid of magnetism, routes towards this goal are
highly constrained with only a few suitable pairings of compounds and driving
mechanisms found to date. Here, through spin- and angle-resolved photoemission
along with density functional theory, we establish how the -derived bulk
valence bands of semiconducting 1T-HfSe possess a local, ground-state spin
texture spatially confined within each Se-sublayer due to strong
sublayer-localised electric dipoles orientated along the -axis. This hidden
spin-polarisation manifests in a `coupled spin-orbital texture' with
in-equivalent contributions from the constituent -orbitals. While the
overall spin-orbital texture for each Se sublayer is in strict adherence to
time-reversal symmetry (TRS), spin-orbital mixing terms with net polarisations
at time-reversal invariant momenta are locally maintained. These apparent
TRS-breaking contributions dominate, and can be selectively tuned between with
a choice of linear light polarisation, facilitating the observation of
pronounced spin-polarisations at the Brillouin zone centre for all . We
discuss the implications for the generation of spin-polarised populations from
1T-structured transition metal dichalcogenides using a fixed energy, linearly
polarised light source.Comment: 11 pages, 6 figure
Angle-resolved and core-level photoemission study of interfacing the topological insulator Bi1.5Sb0.5Te1.7Se1.3 with Ag, Nb and Fe
Interfaces between a bulk-insulating topological insulator (TI) and metallic
adatoms have been studied using high-resolution, angle-resolved and core-level
photoemission. Fe, Nb and Ag were evaporated onto Bi1.5Sb0.5Te1.7Se1.3 (BSTS)
surfaces both at room temperature and 38K. The coverage- and
temperature-dependence of the adsorption and interfacial formation process have
been investigated, highlighting the effects of the overlayer growth on the
occupied electronic structure of the TI. For all coverages at room temperature
and for those equivalent to less than 0.1 monolayer at low temperature all
three metals lead to a downward shift of the TI's bands with respect to the
Fermi level. At room temperature Ag appears to intercalate efficiently into the
van der Waals gap of BSTS, accompanied by low-level substitution of the Te/Se
atoms of the termination layer of the crystal. This Te/Se substitution with
silver increases significantly for low temperature adsorption, and can even
dominate the electrostatic environment of the Bi/Sb atoms in the BSTS
near-surface region. On the other hand, Fe and Nb evaporants remain close to
the termination layer of the crystal. On room temperature deposition, they
initially substitute isoelectronically for Bi as a function of coverage, before
substituting for Te/Se atoms. For low temperature deposition, Fe and Nb are too
immobile for substitution processes and show a behaviour consistent with
clustering on the surface. For both Ag and Fe/Nb, these differing adsorption
pathways leads to the qualitatively similar and remarkable behavior for low
temperature deposition that the chemical potential first moves upward (n-type
dopant behavior) and then downward (p-type behavior) on increasing coverage.Comment: 10 pages, 4 figures. In our Phys. Rev. B manuscript an error was made
in formulating the last sentence of the abstract that, unfortunately, was
missed in the page proofs. Version 2 on arxiv has the correct formulation of
this sentenc
Magnetostatic coupling of 90° domain walls in Fe19Ni81/Cu/Co trilayers
The magnetic interlayer coupling of Fe19Ni81/Cu/Co trilayered microstructures
has been studied by means of x-ray magnetic circular dichroism in combination
with photoelectron emission microscopy (XMCD-PEEM). We find that a parallel
coupling between magnetic domains coexists with a non-parallel coupling
between magnetic domain walls (DWs) of each ferromagnetic layer. We attribute
the non-parallel coupling of the two magnetic layers to local magnetic stray
fields arising at DWs in the magnetically harder Co layer. In the magnetically
softer FeNi layer, non-ordinary DWs, such as 270° and 90° DWs with overshoot
of the magnetization either inwards or outwards relative to the turning
direction of the Co magnetization, are identified. Micromagnetic simulations
reveal that in the absence of magnetic anisotropy, both types of overshooting
DWs are energetically equivalent. However, if a uniaxial in-plane anisotropy
is present, the relative orientation of the DWs with respect to the anisotropy
axis determines which of these DWs is energetically favorable
Ferrimagnetic nanostructures for magnetic memory bits
Increasing the magnetic data recording density requires reducing the size of
the individual memory elements of a recording layer as well as employing
magnetic materials with temperature-dependent functionalities. Therefore, it is
predicted that the near future of magnetic data storage technology involves a
combination of energy-assisted recording on nanometer-scale magnetic media. We
present the potential of heat-assisted magnetic recording on a patterned
sample; a ferrimagnetic alloy composed of a rare earth and a transition metal,
DyCo, which is grown on a hexagonal-ordered nanohole array membrane. The
magnetization of the antidot array sample is out-of-plane oriented at room
temperature and rotates towards in-plane upon heating above its
spin-reorientation temperature (T) of ~350 K, just above room temperature.
Upon cooling back to room temperature (below T), we observe a well-defined
and unexpected in-plane magnetic domain configuration modulating with ~45 nm.
We discuss the underlying mechanisms giving rise to this behavior by comparing
the magnetic properties of the patterned sample with the ones of its extended
thin film counterpart. Our results pave the way for novel applications of
ferrimagnetic antidot arrays of superior functionality in magnetic nano-devices
near room temperature.Comment: 19 pages, 4 figure
Probing two topological surface bands of Sb2Te3 by spin-polarized photoemission spectroscopy
Using high resolution spin- and angle-resolved photoemission spectroscopy, we
map the electronic structure and spin texture of the surface states of the
topological insulator Sb2Te3. In combination with density functional
calculations (DFT), we directly show that Sb2Te3 exhibits a partially occupied,
single spin-Dirac cone around the Fermi energy, which is topologically
protected. DFT obtains a spin polarization of the occupied Dirac cone states of
80-90%, which is in reasonable agreement with the experimental data after
careful background subtraction. Furthermore, we observe a strongly spin-orbit
split surface band at lower energy. This state is found at 0.8eV below the
Fermi level at the gamma-point, disperses upwards, and disappears at about
0.4eV below the Fermi level into two different bulk bands. Along the gamma-K
direction, the band is located within a spin-orbit gap. According to an
argument given by Pendry and Gurman in 1975, such a gap must contain a surface
state, if it is located away from the high symmetry points of the Brillouin
zone. Thus, the novel spin-split state is protected by symmetry, too.Comment: 8 pages, 10 figure
- …