30 research outputs found
Step and kink correlations on vicinal Ge(100) surfaces investigated by electron diffraction
Using spot profile analysis in low-energy electron diffraction, we have investigated vicinal Ge(100) surfaces, which were miscut by 2.7° and 5.4°, respectively, in [011] direction with respect to the surface normal. Within the kinematic approximation the morphology was evaluated quantitatively both perpendicular and parallel to the step edge direction. In contrast to vicinal Si(100) surfaces with similar miscut angles, the Ge(100) surfaces still show an alternating configuration of (2×1) and (1×2) reconstructed (100) terraces, which are separated by steps of single atomic height. From the spot profiles and their energy dependence we extracted the morphological parameters such as the average terrace width, the variance of the terrace size distribution, and the average kink separation. Furthermore, step energies on the vicinal Ge(100) surfaces were estimated. These turn out to be significantly lower than for Si(100) and lead to the formation of the observed double domain structure. © 2002 The American Physical SocietyDFGK+S Grupp
Momentum space separation of quantum path interferences between photons and surface plasmon polaritons in nonlinear photoemission microscopy
Quantum path interferences occur whenever multiple equivalent and coherent
transitions result in a common final state. Such interferences strongly modify
the probability of a particle to be found in that final state, a key concept of
quantum coherent control. When multiple nonlinear and energy-degenerate
transitions occur in a system, the multitude of possible quantum path
interferences is hard to disentangle experimentally. Here, we analyze quantum
path interferences during the nonlinear emission of electrons from hybrid
plasmonic and photonic fields using time-resolved photoemission electron
microscopy. We experimentally distinguish quantum path interferences by
exploiting the momentum difference between photons and plasmons and through
balancing the relative contributions of their respective fields. Our work
provides a fundamental understanding of the nonlinear photon-plasmon-electron
interaction. Distinguishing emission processes in momentum space, as introduced
here, will ultimately allow nano-optical quantum-correlations to be studied
without destroying the quantum path interferences
In situ observation of stress relaxation in epitaxial graphene
Upon cooling, branched line defects develop in epitaxial graphene grown at
high temperature on Pt(111) and Ir(111). Using atomically resolved scanning
tunneling microscopy we demonstrate that these defects are wrinkles in the
graphene layer, i.e. stripes of partially delaminated graphene. With low energy
electron microscopy (LEEM) we investigate the wrinkling phenomenon in situ.
Upon temperature cycling we observe hysteresis in the appearance and
disappearance of the wrinkles. Simultaneously with wrinkle formation a change
in bright field imaging intensity of adjacent areas and a shift in the moire
spot positions for micro diffraction of such areas takes place. The stress
relieved by wrinkle formation results from the mismatch in thermal expansion
coefficients of graphene and the substrate. A simple one-dimensional model
taking into account the energies related to strain, delamination and bending of
graphene is in qualitative agreement with our observations.Comment: Supplementary information: S1: Photo electron emission microscopy and
LEEM measurements of rotational domains, STM data of a delaminated bulge
around a dislocation. S2: Movie with increasing brightness upon wrinkle
formation as in figure 4. v2: Major revision including new experimental dat
Europium Underneath Graphene on Ir(111): Intercalation Mechanism, Magnetism, and Band Structure
The intercalation of Eu underneath Gr on Ir(111) is comprehensively
investigated by microscopic, magnetic, and spectroscopic measurements, as well
as by density functional theory. Depending on the coverage, the intercalated Eu
atoms form either a or a R superstructure with respect to Gr. We investigate the
mechanisms of Eu penetration through a nominally closed Gr sheet and measure
the electronic structures and magnetic properties of the two intercalation
systems. Their electronic structures are rather similar. Compared to Gr on
Ir(111), the Gr bands in both systems are essentially rigidly shifted to larger
binding energies resulting in n-doping. The hybridization of the Ir surface
state with Gr states is lifted, and the moire superperiodic potential is
strongly reduced. In contrast, the magnetic behavior of the two intercalation
systems differs substantially as found by X-ray magnetic circular dichroism.
The Eu structure displays plain paramagnetic behavior, whereas
for the R structure the large
zero-field susceptibility indicates ferromagnetic coupling, despite the absence
of hysteresis at 10 K. For the latter structure, a considerable easy-plane
magnetic anisotropy is observed and interpreted as shape anisotropy.Comment: 18 pages with 14 figures, including Supplemental Materia
Short-range surface plasmonics: localized electron emission dynamics from a 60-nm spot on an atomically flat single-crystalline gold surface
We experimentally and theoretically visualize the propagation of short-range surface plasmon polaritons using atomically flat single-crystalline gold platelets on silicon substrates. We study their excitation and subfemtosecond dynamics via normal-incidence two-photon photoemission electron microscopy. By milling a plasmonic disk and grating structure into a single-crystalline gold platelet, we observe nanofocusing of the short-range surface plasmon polariton. Localized two-photon ultrafast electron emission from a spot with a smallest dimension of 60 nm is observed. Our novel approach opens the door toward reproducible plasmonic nanofocusing devices, which do not degrade upon high light intensity or heating due to the atomically flat surface without any tips, protrusions, or holes. Our nanofoci could also be used as local emitters for ultrafast electron bunches in time-resolved electron microscopes
Focused surface plasmon polaritons coherently couple to electronic states in above-threshold electron emission
Floquet theory describes transient states driven by a light-matter interaction and could potentially be used to engineer the band structure and the topology of solid-state systems. Here, the authors investigate coherent photoemission from a gold surface caused by a strong surface plasmon polariton excitation, which could be used to realize surface plasmon polariton driven Floquet effects in nanostructures
Topology of Surface Plasmon Polaritons with Integer and Fractional Orbital Angular Momentum
The
topology of surface plasmon polariton fields (SPPs) with orbital
angular momentum (OAM) is characterized by the winding numbers of
the phase singularities in the field, also known as topological charges.
Using theoretical expressions for the surface plasmon fields, we identify
the phase singularities as points where the field is zero and investigate
their properties for both integer and noninteger, or fractional, orbital
angular momentum. The phase singularities act as vortex centers for
the rotating fields. We analyze the behavior of the vortex–antivortex
pairs and the breakup of the central vortex and discuss their influence
on the measured topology as the orbital angular momentum changes from
one integer value l to the next l +1 via the fractional states. Our work highlights the fact that
measures of the topological charges do not always equate with the
orbital angular momentum and shows how the topology can change discontinuously,
even though all of the parameters controlling the fields change smoothly