38 research outputs found
Ultrafast Electron Diffraction at Surfaces:From Nanoscale Heat Transport to Driven Phase Transitions
Many fundamental processes of structural changes at surfaces occur on a pico-
or femtosecond time scale. In order to study such ultra-fast processes, we have
combined modern surface science techniques with fs-laser pulses in a pump-probe
scheme. Reflection high energy electron diffraction (RHEED) with grazing
incident electrons ensures surface sensitivity for the probing electron pulses.
Utilizing the Debye-Waller effect, we studied the nanoscale heat transport from
an ultrathin film through a hetero-interface or the damping of vibrational
excitations in monolayer adsorbate systems on the lower ps-time scale. By means
of spot profile analysis the different cooling rates of epitaxial Ge
nanostructures of different size and strain state were determined. The
excitation and relaxation dynamics of a driven phase transition far away from
thermal equilibrium is demonstrated using the In-induced (8x2) reconstruction
on Si(111). This Peierls-distorted surface charge density wave system exhibits
a discontinuous phase transition at 130 K from a (8x2) insulating ground state
to (4x1) metallic excited state. Upon excitation by a fs-laser pulse, this
structural phase transition is non-thermally driven in only 700 fs into the
excited state. A small barrier of 40 meV hinders the immediate recovery of the
groundstate and the system is found in a metastable supercooled state for up to
few nanoseconds
Cool Cooling Collar for Bake-Out of Temperature-Sensitive Devices
A combination of a pumpable gate valve and a self-built cooling collar
permits bake-out of an ultra-high vacuum chamber without having to dismount
sensitive equipment. A small pump port on the closed gate valve maintains
ultra-high vacuum conditions for a TVIPS TemCam-XF416 imaging electron detector
in the case of venting the main chamber. The water-cooled collar mounted to the
detector housing prevents heating of the detector upon bake-out of the
ultra-high vacuum chamber.Comment: 3 pages, 2 figure
Disentangling the Electronic and Lattice Contributions to the Dielectric Response of Photoexcited Bismuth
Elucidating the interplay between nuclear and electronic degrees of freedom
that govern the complex dielectric behavior of materials under intense
photoexcitation is essential for tailoring optical properties on demand.
However, conventional transient reflectivity experiments have been unable to
differentiate between real and imaginary components of the dielectric response,
omitting crucial electron-lattice interactions. Utilizing thin film
interference we unambiguously determined the photoinduced change in complex
dielectric function in the Peierls semimetal bismuth and examined its
dependence on the excitation density and nuclear motion of the A phonon.
Our modeled transient reflectivity data reveals a progressive broadening and
redshift of Lorentz oscillators with increasing excitation density and
underscores the importance of both, electronic and nuclear coordinates in the
renormalization of interband transitions.Comment: Manuscript (6 pages) plus supplemental material (6 pages
Scattering at magnetic and nonmagnetic impurities on surfaces with strong spin-orbit coupling
Adsorption-induced reduction of surface-state conductivity in epitaxial Bi(111) films, a prototype system with large Rashba-induced surface-state splitting, by adsorbed atoms of Bi, Fe, and Co has been investigated by macroscopic surface magnetotransport measurements at a temperature of 10 K. A detailed analysis of magnetotransport, dc transport, and Hall data reveals that the scattering efficiencies for Co and Fe are larger by a factor of 2 than that for Bi. While for the latter charge transfer and change of band filling near the Fermi level are negligible, we find an increase of hole concentration upon Co and Fe adsorption. These atoms act as acceptors and immobilize on average about 0.5 electrons per adsorbed atom. Besides the dominant classical magnetoconductance signal the films show signatures of weak antilocalization, reflecting the strong spin-orbit coupling in Bi(111) surface states. This behavior can be changed to weak localization by the adsorption of high concentrations (0.1 monolayers) of magnetic impurities (Fe,Co), similarly to results found on the topological insulator Bi2Se3. Our results demonstrate that details of chemical bond formation for impurities are crucial for local spin moments and electronic scattering properties. © 2012 American Physical Society.DFGDAA
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
Heat Transport in Nanoscale Heterosystems: A Numerical and Analytical Study
The numerical integration of the heat diffusion equation applied to the Bi/Si-heterosystem is presented for times larger than the characteristic time of electron-phonon coupling. By comparing the numerical results to experimental data, it is shown that the thermal boundary resistance of the interface can be directly determined from the characteristic decay time of the observed surface cooling, and an elaborate simulation of the temporal surface temperature evolution can be omitted. Additionally, the numerical solution shows that the substrate temperature only negligibly varies with time and can be considered constant. In this case, an analytical solution can be found. A thorough examination of the analytical solution shows that the surface cooling behavior strongly depends on the initial temperature distribution which can be used to study energy transport properties at short delays after the excitation
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