36 research outputs found
Probing transitions and phase-ordering of charge-density waves
Due to their reduced dimensionality, surfaces and quasi two-dimensional materials exhibit
numerous intriguing physical phenomena that drastically differ from the bulk. To resolve
these effects and the associated dynamics at their intrinsic timescales requires experimental
methodologies combining a high surface sensitivity with the essential temporal
resolution. However, to date, there are still very few methods that facilitate investigation
of the structural degrees of freedom of surfaces on the atomic scale along with a temporal
resolution of femtoseconds or picoseconds.
Addressing these challenges, this thesis covers the development and application of ultrafast
low-energy electron diffraction in a backscattering geometry to study structural dynamics
at surfaces.
In this context, a central aspect is the development of a miniaturized and laser-driven
electron source based on a nanometric needle photocathode. Using such a sharp metal tip,
the photoemitted electron bunches offer a particularly high coherence and remarkably
short pulse durations, which were also successfully implemented recently in ultrafast
transmission electron microscopy, as well as in time-resolved transmission low-energy
electron diffraction.
Employing the capabilities of this novel technique, so-called transition metal dichalcogenides
constitute an ideal prototype system. Specifically, in the present work, the
transient structural disorder of charge-density waves at the surface of 1T-TaS2 has been
examined. Following the optically induced transition between two temperature-dependent
charge-density wave phases, this method enables the observation of a highly disordered
transient state and the subsequent phase-ordering kinetics. More precisely, the temporal
evolution of the growing charge-density correlation length is traced over several hundreds
of picoseconds and found to obey a power-law scaling behavior. Due to the particular
properties of the charge-density wave system at hand, the observed transient disorder
can be explained by the ultrafast formation of topological defects and their subsequent
annihilation. These results are complemented by a numerical modeling using a timedependent
Ginzburg-Landau approach.
Finally, two different excitation schemes demonstrating the possibility to study the
relaxation of the investigated sample on the nanosecond and microsecond timescale
are presented, as well as future prospects of ultrafast low-energy electron diffraction,
including other promising surface sample systems
Structural Dynamics of incommensurate Charge-Density Waves tracked by Ultrafast Low-Energy Electron Diffraction
We study the non-equilibrium structural dynamics of the incommensurate and
nearly-commensurate charge-density wave phases in 1T-TaS. Employing
ultrafast low-energy electron diffraction (ULEED) with 1 ps temporal
resolution, we investigate the ultrafast quench and recovery of the CDW-coupled
periodic lattice distortion. Sequential structural relaxation processes are
observed by tracking the intensities of main lattice as well as satellite
diffraction peaks as well as the diffuse scattering background. Comparing
distinct groups of diffraction peaks, we disentangle the ultrafast quench of
the PLD amplitude from phonon-related reductions of the diffraction intensity.
Fluence-dependent relaxation cycles reveal a long-lived partial suppression of
the order parameter for up to 60 picoseconds, far outlasting the initial
amplitude recovery and electron-phonon scattering times. This delayed return to
a quasi-thermal level is controlled by lattice thermalization and coincides
with the population of zone-center acoustic modes, as evidenced by a structured
diffuse background. The long-lived non-equilibrium order parameter suppression
suggests hot populations of CDW-coupled lattice modes. Finally, a broadening of
the superlattice peaks is observed at high fluences, pointing to a nonlinear
generation of phase fluctuations.Comment: Main text and Appendice
k-space Imaging of the Eigenmodes of Sharp Gold Tapers for Scanning Near-Field Optical Microscopy
We investigate the radiation patterns of sharp conical gold tapers, designed
as adiabatic nanofocusing probes for scanning near-field optical microscopy
(SNOM). Field calculations show that only the lowest order eigenmode of such a
taper can reach the very apex and thus induce the generation of strongly
enhanced near-field signals. Higher order modes are coupled into the far field
at finite distances from the apex. Here, we demonstrate experimentally how to
distinguish and separate between the lowest and higher order eigenmodes of such
a metallic taper by filtering in the spatial frequency domain. Our approach has
the potential to considerably improve the signal-to-background ratio in
spectroscopic experiments on the nanoscale
Discovery of the inhibitory effect of a phosphatidylinositol derivative on P-glycoprotein by virtual screening followed by <i>in vitro</i> cellular studies
P-glycoprotein is capable of effluxing a broad range of cytosolic and membrane penetrating xenobiotic substrates, thus leading to multi-drug resistance and posing a threat for the therapeutic treatment of several diseases, including cancer and central nervous disorders. Herein, a virtual screening campaign followed by experimental validation in Caco-2, MDKCII, and MDKCII mdr1 transfected cell lines has been conducted for the identification of novel phospholipids with P-gp transportation inhibitory activity. Phosphatidylinositol-(1,2-dioctanoyl)-sodium salt (8∶0 PI) was found to significantly inhibit transmembrane P-gp transportation in vitro in a reproducible-, cell line-, and substrate-independent manner. Further tests are needed to determine whether this and other phosphatidylinositols could be co-administered with oral drugs to successfully increase their bioavailability. Moreover, as phosphatidylinositols and phosphoinositides are present in the human diet and are known to play an important role in signal transduction and cell motility, our finding could be of substantial interest for nutrition science as well
Nanotip-based photoelectron microgun for ultrafast LEED
We present the design and fabrication of a micrometer-scale electron gun for the
implementation of ultrafast low-energy electron diffraction from surfaces. A multi-step
process involving photolithography and focused-ion-beam nanostructuring is used to assemble and
electrically contact the photoelectron gun, which consists of a nanotip photocathode in a
Schottky geometry and an einzel lens for beam collimation. We characterize the low-energy
electron pulses by a transient electric field effect and achieve pulse durations of 1.3 ps
at an electron energy of 80 eV. First diffraction images in a backscattering geometry (at 50
eV electron energy) are shown
Structural phase transitions and phase ordering at surfaces probed by ultrafast LEED
We demonstrate the capability of ultrafast low-energy electron diffraction to resolve phase-ordering kinetics and structural phase transitions on their intrinsic time scales with ultimate surface sensitivity
Structural phase transitions and phase ordering at surfaces probed by ultrafast LEED
We demonstrate the capability of ultrafast low-energy electron diffraction to resolve phase-ordering kinetics and structural phase transitions on their intrinsic time scales with ultimate surface sensitivity