arXiv subjects

Katrin Meier

Publications and source records attributed to Katrin Meier.

5 recordsLinked to original sources

arXiv2026

Spatially-resolved multiphoton photoemission from a lateral transition metal dichalcogenide heterostructure

Transition metal dichalcogenides (TMDs) in their monolayer form offer a premier platform for next-generation optoelectronics, particularly through the local manipulation of their robust excitonic states using nanoscale electric fields. These localized states can be dynamically controlled through spatial structuring as well as through ultrafast field modulations driven by tailored optical pulses. Characterizing the resulting rapid, nanoscale charge carrier dynamics requires a technique with exceptional spatial and temporal resolution. Here, we report on the spatio-temporally resolved investigation of ground state and excited state photoemission from a lateral heterostructure built of monolayers of WSe$_2$ and MoSe$_2$ using few-cycle light pulses with a photon energy of 0.62 eV. We utilize photoemission electron microscopy to spatially resolve the highly nonlinear photoemission from the monolayer structure with few tens of nanometer resolution. By varying the laser pulse energy, we extract the nonlinearity of the photoemission process and thus the dynamic binding energy of the photoelectrons before and after optical excitation with high spatial and temporal resolution.

physics.optics↗

arXiv2024

Multiscale carrier-envelope phase characterization of 2-\mu m pulses delivered by a 200-kHz optical parametric amplifier

Light fields with a central wavelength of 2 um are very well suited for strong-field-driven charge carrier control: Their photon energy lies far below the band gap of many materials, while their oscillation period remains significantly shorter than the coherence time of charge carrier oscillations. The resulting potential for field-driven charge carrier control is contingent on the reproducibility of the field structure of such ultrashort laser pulses. Here, we present a compact 200-kHz laser system that delivers ultrashort pulses with a duration of less than 20 fs in the spectral range around 2 um and with a pulse energy of 25 uJ. The electric field structure of the 2-um pulses is characterized in detail. In particular, the carrier-envelope phase (CEP) is measured over a wide range of timescales, from microseconds to hours. Passive stabilization due to difference frequency generation results in a root mean square value of carrier-envelope phase noise of less than 70 mrad over all measured time scales. The applicability of the pulses is demonstrated by measuring CEP-dependent high-order harmonic spectra with energies of up to 160 eV.

physics.optics↗

arXiv2011

Nanostructuring of Ba8Ga16Ge30 clathrates

First thermoelectric properties measurements on bulk nanostructured Ba8Ga16Ge30 clathrate-I are presented. A sol-gel-calcination route was developed for preparing amorphous nanosized precursor oxides. The further reduction of the oxides led to quantitative yield of crystalline nanosized Ba8Ga16Ge30 clathrate-I. TEM investigations show the clathrate nanoparticles retain the size and morphology of the precursor oxides. The clathrate nanoparticles contain mainly thin plates (approx. 300 nm x 300 nm x 50 nm) and a small amount of nanospheres (diameter ~ 10 nm). SAED patterns confirm the clathrate-I structure type for both morphologies. The powders were compacted via Spark Plasma Sintering (SPS) to obtain a bulk nano-structured material. The Seebeck coefficient S, measured on low-density samples (53% of δx-ray), reaches -145 μV/k at 375 °C. The ZT values are quite low (0.02) due to the high resistivity of the sample (two orders of magnitude larger than bulk materials) and the low sample density. The trend of the temperature dependence of S is in agreement with the values obtained from electronic structure calculations and semi-classical Boltzmann transport theory within the constant scattering approximation. The total thermal conductivity (1.61 W/mK), measured on high density samples (93% of δx-ray), shows a reduction of 20-25% in relation to the bulk materials (2.1 W/mK). A further shaping of the sample for the Seebeck coefficient and electrical conductivity measurements was not possible due to the presence of cracks. An improvement on the design of the pressing tools, loading of the sample and profile of the applied pressure will enhance the mechanical stability of the samples. These investigations are now in progress.

cond-mat.mtrl-sci↗

arXiv2011

Symmetry preserving lattice collapse in tetragonal SrFe_(2-x)Ru_xAs_2 (x = 0, 0.2) -- a combined experimental and theoretical study

In a joint experimental and theoretical study, we investigate the isostructural collapse from the ambient pressure tetragonal phase to a collapsed tetragonal phase for non-superconducting metallic SrFe2As2 and SrFe_1.8Ru_0.2As_2. The crystallographic details have been studied using X-ray powder diffraction up to 20 GPa pressure in a diamond anvil cell. The structural phase transition occurs at 10 GPa and 9 GPa for SrFe2As2 and SrFe_1.8Ru_0.2As_2, respectively. The changes in the unit cell dimensions are highly anisotropic with a continuous decrease of the c lattice parameter with pressure, while the a-axis length increases until the transition to a collapsed tetragonal phase and then continues to decrease. Across the phase transition, we observe a volume reduction of 5% and 4% for SrFe2As2 and SrFe_1.8Ru_0.2As_2, respectively. We are able to discern that Ru substitution on the Fe-site acts like `chemical pressure' to the system. Density-functional theory-based calculations of the electronic structure and electron localizability indicator are consistent with the experimental observations. Detailed analysis of the electronic structure in k-space and real space reveals As 4pz interlayer bond formation as the driving force of the c/a collapse with a change in the As-As bond length of about 0.35ang.

cond-mat.mtrl-sci↗

arXiv2010

Synthesis and thermoelectric characterization of Bi2Te3 nanoparticles

We report a novel synthesis for near monodisperse, sub-10-nm Bi2Te3 nanoparticles. At first, a new reduction route to bismuth nanoparticles is described which are applied as starting materials in the formation of rhombohedral Bi2Te3 nanoparticles. After ligand removal by a novel hydrazine hydrate etching procedure, the nanoparticle powder is spark plasma sintered to a pellet with preserved crystal grain sizes. Unlike previous works on the properties of Bi2Te3 nanoparticles, the full thermoelectric characterization of such sintered pellets shows a highly reduced thermal conductivity and the same electric conductivity as bulk n-type Bi2Te3.

cond-mat.mtrl-sci↗