arXiv subjects

Wolf Gero Schmidt

Publications and source records attributed to Wolf Gero Schmidt.

15 recordsLinked to original sources

arXiv2026

Second harmonic study of thermally oxidized mono- and few-layer 2H-MoS2

A comprehensive study of second harmonic generation on thermally oxidized MoS2 flakes with thickness ranging from monolayer up to seven layers is presented. Observing the fundamental nonlinear behavior for non-treated and oxidized MoS2 reveals that oxidation causes significant changes in the second harmonic (SH) response for all investigated structures. Excitation power dependent measurements to analyze the nonlinear behavior with respect to the oxidation time show progressive oxidation within the maximum oxidation time of six hours, under the considered oxidation conditions. Here, polarization dependent measurements reveal the structural changes due to oxidation. Additionally, it is found that the oxidation depth is restricted to the top most layer and the oxidation behavior exhibits a layer dependency. These findings are supported by theoretical band structure calculations. The results demonstrate that the thermal oxidation progress of two dimensional MoS2 can be monitored with non-resonant and non-invasive SH microscopy, by following distinct fingerprints of structural modification in the nonlinear response.

cond-mat.mtrl-sci↗

arXiv2026

Role of Defects in the Paramagnetism of Fe-doped Cs$_{2}$AgBiBr$_{6}$ Double Perovskite

Transition-metal doping enables the introduction of spin functionality into halide double perovskites, while simultaneously modifying optical properties. Here, we combine controlled single-crystal growth, optical characterization, comprehensive electron paramagnetic resonance (EPR) spectroscopy, and first-principles modeling to identify the microscopic nature of Fe-related centers in Fe-doped Cs$_{2}$AgBiBr$_{6}$. Single crystals with nominal Fe$^{3+}$ concentrations up to 15% in the precursor stage were grown using a controlled-cooling method, yielding reproducible Fe incorporation up to 0.1% w.r.t. Bi, without secondary phases. Despite this low concentration, Fe doping introduces electronic states that influence optical absorption and photoluminescence. EPR measurements reveal an S = 5/2 Fe$^{3+}$-related center whose anisotropy follows the cubic-to-tetragonal phase transition below 120 K. Angular-dependent EPR resolves two configurations of this nearly axial spin center, with principal axes rotated by 90$^\circ$ and aligned with the $a/b$ plane of the tetragonal lattice. Density-functional calculations attribute these centers to impurity-vacancy complexes, most likely Fe$_{\rm Bi}$-V$_{\rm Br}$, that stabilise in a basal configuration of the low-temperature phase. This approach resolves vacancy-coupled defect orientations, narrowing possible models to Fe$^{3+}$-vacancy complexes and establishing them as stable, orientation-sensitive spin probes of structural symmetry in halide double perovskites, while providing a microscopic basis for tuning their magnetic and optical responses.

cond-mat.mtrl-sci↗

arXiv2025

Mechanistic Origin of Charge Separation and Enhanced Photocatalytic Activity in D-$π$-A-Functionalized UiO-66-NH$_2$ MOFs

Donor-$π$-acceptor (D-$π$-A) functionalization of MOF linkers can enhance visible-light photocatalytic activity, yet the mechanisms responsible for these effects remain unclear. Here we combine EPR spectroscopy, transient photoluminescence, and first-principles calculations to examine how diazo-coupled anisole, diphenylamine (DPA), and N,N-dimethylaniline (NNDMA) groups modify the photophysics of UiO-66-NH$_2$. All donor units introduce new occupied states near the valence-band edge, enabling charge separation through dye-to-framework electron transfer. Among them, the anisole-modified material stands out for facilitating efficient intersystem crossing into a triplet charge-transfer configuration that suppresses fast recombination and yields long-lived charge carriers detectable by photo-EPR. Meanwhile, bulkier donors such as DPA and NNDMA - despite their stronger electron-donating character - also tend to introduce defect-associated trap states. These results underscore the interplay between donor-induced electronic-structure changes, triplet pathways, and defect-mediated recombination, offering a mechanistic basis for tuning photocatalytic response in D-$π$-A-modified MOFs.

cond-mat.mtrl-sci↗

arXiv2025

Unveiling Linker-Born Electron Spin Centers in UiO-66-NH2 MOF

Metal-organic frameworks (MOFs), with their high porosity and large internal surface area, provide versatile platforms for integrating spin centers with potential applications in catalysis and quantum sensing. Here, we identify a stable NH radical spin center in UiO-66-NH2, a zirconium-based MOF with aminoterephthalic acid (TPA-NH2) linkers. Using electron paramagnetic resonance spectroscopy and density functional theory calculations, we determine the nature and spin Hamiltonian parameters of this radical. We also demonstrate that the NH spin center exhibits a relatively long coherence time, making it a strong candidate for quantum sensing. Since this spin center is intrinsic to TPA-NH2 linkers, our findings open new directions for leveraging organic radicals in MOFs for quantum technologies beyond the UiO-66-NH2 family.

cond-mat.mtrl-sci↗

arXiv2025

Intrinsic defects as a source of $\textit{n}$-type conductivity in CrSBr

Understanding and controlling native defects is essential for unlocking the full potential of two-dimensional magnetic semiconductors. Here, angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations are used to explore the electronic properties of bulk CrSBr. ARPES measurements reveal clear signatures of conduction band filling in as-grown crystals, indicative of unintentional doping. An analysis of intrinsic defects based on density functional theory (DFT) identifies chromium interstitials ($Cr_i$) stabilized between CrSBr layers as the most favorable shallow donors. Bromine-on-sulfur antisites ($Br_S$) and bromine vacancies ($V_{Br}$) are also found to act as potential donors, albeit with deeper ionization energies. Our results shed light on the origin of unintentional $\textit{n}$-type doping of CrSBr and pave the way for new strategies for defect control and electronic property tuning in this van der Waals magnet.

cond-mat.mtrl-sci↗

arXiv2025

Quantifying Spin Defect Density in hBN via Raman and Photoluminescence Analysis

Negatively charged boron vacancies ($\mathrm{V_B^-}$) in hexagonal boron nitride (hBN) are emerging as promising solid-state spin qubits due to their optical accessibility, structural simplicity, and compatibility with photonic platforms. However, quantifying the density of such defects in thin hBN flakes has remained elusive, limiting progress in device integration and reproducibility. Here, we present an all-optical method to quantify $\mathrm{V_B^-}$ defect density in hBN by correlating Raman and photoluminescence (PL) signatures with irradiation fluence. We identify two defect-induced Raman modes, D1 and D2, and assign them to vibrational modes of $\mathrm{V_B^-}$ using polarization-resolved Raman measurements and density functional theory (DFT) calculations. By adapting a numerical model originally developed for graphene, we establish an empirical relationship linking Raman (D1, $E_\mathrm{2g}$) and PL intensities to absolute defect densities. This method is universally applicable across various irradiation types and uniquely suited for thin flakes, where conventional techniques fail. Our approach enables accurate, direct, and non-destructive quantification of spin defect densities down to $10^{15}$ defects/ cm${}^3$, offering a powerful tool for optimizing and benchmarking hBN for quantum optical applications.

quant-ph↗

arXiv2025

On the origin of bulk-related anisotropies in surface optical spectra

Reflection anisotropy spectroscopy (RAS) is a powerful method for probing the optical properties of surfaces, used routinely in research and industrial applications, yet the origin of 'bulk-related' features that appear in the spectra of various surfaces has been debated for nearly 40 years. It is often argued that these features are related to surface-induced bulk anisotropy (SIBA) because they coincide with critical energies of the bulk dielectric function. In general, any quantitative RAS theory must include excitonic effects as they significantly influence the spectra and are believed to be the key to determining the origin of SIBA features. Here, we introduce a layer-resolved exciton localization (LREL) measure within the framework of many-body perturbation theory, which enables a quantitative analysis of the origins of 'bulk-related' RAS features. Applying LREL to arsenic-modified silicon reconstructions reveals that, depending on the surface reconstruction, the 'apparent' SIBA features arise primarily from states localized at the surface, with only a small contribution from the underlying layers. Our findings, further supported by the fact that the calculated spectra agree well with low-temperature RAS measurements, challenge the conventional explanation of 'bulk-related' RAS features. They indicate that in many instances bulk-enhanced surface anisotropies (BESA)-the opposite of SIBA-contribute to, or are even responsible for, 'bulk-related' RAS features. Therefore, we suggest that previously studied semiconductor surfaces, which exhibit 'bulk-related' features in their spectra, should be reanalyzed using the presented method.

cond-mat.mtrl-sci↗

arXiv2025

Topological Defects in Semiconducting Carbon Nanotubes as Triplet Exciton Traps and Single-Photon Emitters

We investigate the role of topological defects in exciton behavior in (6,5) semiconducting single-walled carbon nanotubes using density functional theory. Our study identifies the helical Stone-Wales defect as a prominent trap for triplet excitons, characterized by a large zero-field splitting consistent with experimental data and a small singlet-triplet gap. The weak electron-phonon coupling, as evidenced by a Huang-Rhys factor of 0.74, renders it a promising single-photon emitter, with the zero-phonon line predicted at 1.6 $μ$m, within the telecom range. These insights into defect-engineered electronic structure and exciton dynamics offer promising opportunities for improving the performance of carbon nanotube-based quantum light sources and optoelectronic devices.

cond-mat.mes-hall↗

arXiv2024

DFT-Assisted Investigation of the Electric Field and Charge Density Distribution of Pristine and Defective 2D WSe$_2$ by Differential Phase Contrast Imaging

Most properties of solid materials are defined by their internal electric field and charge density distributions which so far are difficult to measure with high spatial resolution. Especially for 2D materials, the atomic electric fields influence the optoelectronic properties. In this study, the atomic-scale electric field and charge density distribution of WSe$_2$ bi- and trilayers are revealed by using an emerging microscopy technique, differential phase contrast (DPC) imaging in the scanning transmission electron microscope (STEM). For pristine material, a higher positive charge density located at the selenium atomic columns compared to the tungsten atomic columns is obtained and tentatively explained by a coherent scattering effect. Furthermore, the change in the electric field distribution induced by a missing selenium atomic column is investigated. A characteristic electric field distribution in the vicinity of the defect with locally reduced magnitudes compared to the pristine lattice is observed. This effect is accompanied by a considerable inward relaxation of the surrounding lattice, which according to first principles DFT calculation is fully compatible with a missing column of Se atoms. This shows that DPC imaging, as an electric field sensitive technique, provides additional and remarkable information to the otherwise only structural analysis obtained with conventional STEM imaging.

cond-mat.mtrl-sci↗

arXiv2021

Polaronic Enhancement of Second-Harmonic Generation in Lithium Niobate

Density-functional theory within a Berry-phase formulation of the dynamical polarization is used to determine the second-order susceptibility $χ^{(2)}$ of lithium niobate (LiNbO$_3$). Defect trapped polarons and bipolarons are found to strongly enhance the nonlinear susceptibility of the material, in particular if localized at Nb$_\mathrm{V}$-V$_{\mathrm{Li}}$ defect pairs. This is essentially a consequence of the polaronic excitation resulting in relaxation-induced gap states. The occupation of these levels leads to strongly enhanced $χ^{(2)}$ coefficients and allows for the spatial and transient modification of the second-harmonic generation of macroscopic samples.

physics.optics↗

arXiv2020

Spin decontamination for magnetic dipolar coupling calculations: Application to high-spin molecules and solid-state spin qubits

An accurate description of the two-electron density, crucial for magnetic coupling in spin systems, provides in general a major challenge for density functional theory calculations. It affects, e.g., the calculated zero-field splitting (ZFS) energies of spin qubits in semiconductors that frequently deviate significantly from experiment. In the present work (i) we propose an efficient and robust strategy to correct for spin contamination in both extended periodic and finite-size systems, (ii) verify its accuracy using model high-spin molecules, and finally (iii) apply the methodology to calculate accurate ZFS of spin qubits (NV$^-$ centers, divacancies) in diamond and silicon carbide. The approach is shown to reduce the dependence on the used exchange-correlation functional to a minimum.

quant-ph↗

arXiv2020

Tetracene ultrathin film growth on silicon

Inorganic-organic interfaces are important for enhancing the power conversion efficiency of silicon-based solar cells through singlet exciton fission (SF). We elucidated the structure of the first monolayers of tetracene (Tc), a SF molecule, on hydrogen-passivated Si(111) [H-Si(111)] and hydrogenated amorphous Si (a-Si:H) by combining near-edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) experiments with density functional theory (DFT) calculations. For samples grown at or below substrate temperatures of 265 K, the resulting ultrathin Tc films are dominated by almost upright-standing molecules. The molecular arrangement is very similar to the Tc bulk phase, with only slightly higher average angle between the conjugated molecular plane normal and the surface normal ($α$) around 77°. Judging from carbon K-edge X-ray absorption spectra, the orientation of the Tc molecules are almost identical when grown on H-Si(111) and a-Si:H substrates as well as for (sub)mono- to several-monolayer coverages. Annealing to room temperature, however, changes the film structure towards a smaller $α$ of about 63°. A detailed DFT-assisted analysis suggests that this structural transition is correlated with a lower packing density and requires a well-chosen amount of thermal energy. Therefore, we attribute the resulting structure to a distinct monolayer configuration that features less inclined, but still well-ordered molecules. The larger overlap with the substrate wavefunctions makes this arrangement attractive for an optimized interfacial electron transfer in SF-assisted silicon solar cells.

cond-mat.mtrl-sci↗

arXiv2018

Beyond the molecular movie: dynamics of bands and bonds during a photo-induced phase transition

Ultrafast non-equilibrium dynamics offer a route to study the microscopic interactions that govern macroscopic behavior. In particular, photo-induced phase transitions (PIPTs) in solids provide a test case for how forces, and the resulting atomic motion along a reaction coordinate, originate from a non-equilibrium population of excited electronic states. Utilizing femtosecond photoemission we obtain access to the transient electronic structure during an ultrafast PIPT in a model system: indium nanowires on a silicon(111) surface. We uncover a detailed reaction pathway, allowing a direct comparison with the dynamics predicted by ab initio simulations. This further reveals the crucial role played by localized photo-holes in shaping the potential energy landscape, and enables a combined momentum and real space description of PIPTs, including the ultrafast formation of chemical bonds.

cond-mat.str-el↗

arXiv2016

Grand canonical Peierls transition in In/Si(111)

Starting from a Su-Schrieffer-Heeger-like model inferred from first-principles simulations, we show that the metal-insulator transition in In/Si(111) is a first-order grand canonical Peierls transition in which the substrate acts as an electron reservoir for the wires. This model explains naturally the existence of a metastable metallic phase over a wide temperature range below the critical temperature and the sensitivity of the transition to doping. Raman scattering experiments corroborate the softening of the two Peierls deformation modes close to the transition.

cond-mat.mes-hall↗

arXiv2015

Mechanism for nuclear and electron spin excitation by radio frequency current

Recent radio frequency scanning tunneling spectroscopy (rf-STS) experiments have demonstrated nuclear and electron spin excitations up to $\pm12\hbar$ in a single molecular spin quantum dot (qudot). Despite the profound experimental evidence, the observed independence of the well-established dipole selection rules is not described by existing theory of magnetic resonance -- pointing to a new excitation mechanism. Here we solve the puzzle of the underlying mechanism by presenting all relevant mechanistic steps. At the heart of the mechanism, periodic transient charging and electric polarization due to the rf-modulated tunneling process cause a periodic asymmetric deformation of the qudot, enabling spin transitions via spin-phonon-like coupling. The mechanism has general relevance for a broad variety of different spin qudots exhibiting internal mechanical degrees of freedom (organic molecules, doped semiconductor qudots, nanocrystals, etc.).

cond-mat.mes-hall↗