arXiv subjects

Friedrich Schöppler

Publications and source records attributed to Friedrich Schöppler.

6 recordsLinked to original sources

arXiv2026

A Visible-Frequency Excitonic Reststrahlen Band in (PEA)$_2$PbI$_4$ Slabs

Layered halide perovskites host exceptionally strong excitons, whose optical signatures are usually interpreted as absorptive resonances on a smooth dielectric background. Strong excitons, however, can also reshape the dielectric response itself and drive the real permittivity negative, opening a reflective band: the visible, excitonic analogue of an infrared Reststrahlen band. Whether bare (PEA)$_2$PbI$_4$ slabs reach this regime has remained unclear. Here we show that low-temperature transmission of (PEA)$_2$PbI$_4$ slabs, driven by the intralayer-exciton manifold, evolves with increasing thickness from an excitonic dip into a broad near-zero-transmission interval with compressed Fabry-Pérot-like fringes. Transfer-matrix analysis with an effective Lorentz-oscillator dielectric response reproduces this crossover, reconstructs a finite negative-Re($\varepsilon$) window, and implies near-ultrastrong exciton-photon coupling. Calculated field maps show suppressed in-plane field penetration within this interval and a driven longitudinal response near the high-energy ($\varepsilon=0$) edge. These results identify (PEA)$_2$PbI$_4$ slabs as a cavity-free visible-frequency excitonic Reststrahlen material.

cond-mat.mtrl-sci↗

arXiv2026

Liquid-phase encapsulation of $π$-conjugated dyes in boron nitride nanotubes: Ensemble and single-nanotube optical characterization

Boron nitride nanotubes (BNNTs) provide wide-bandgap, optically transparent one-dimensional hosts for molecular dyes, limiting direct electronic participation of the host. Whether dye@BNNT systems produce bright, well-defined J- or H-aggregates or instead heterogeneous emissive ensembles whose character depends on chain length and local packing remains only partly resolved. We address this question using ensemble extinction, photoluminescence, quantum-yield measurements, and TCSPC-derived radiative and non-radiative rates, together with polarization-resolved single-nanotube microscopy on encapsulated quaterthiophene, sexithiophene, octithiophene, and Nile Red, selected from a ten-dye screening. In the oligothiophene series, confinement modifies spectra and excited-state dynamics in a length-dependent manner, with all three oligothiophenes forming weakly emissive ensembles with suppressed effective radiative rates and 6T showing the strongest redistribution between effective radiative and non-radiative decay. The absence of radiative-rate enhancement or fluorescence-lifetime shortening across the series disfavors bright J-aggregate assignments. Polarization-resolved single-nanotube microscopy reveals strongly polarized emission, but with tube-to-tube and intratube variations, identifying oligothiophene@BNNTs as ordered yet structurally heterogeneous confined ensembles. Nile Red provides a complementary case in which the dominant response is dielectric tuning of a solvatochromic charge-transfer state rather than oligothiophene-like aggregate formation. These findings establish dye-filled BNNTs as optically quiet nanoconfined systems in which molecular ordering, dielectric confinement, and guest-guest coupling can be distinguished through combined ensemble and single-nanotube spectroscopy.

cond-mat.mtrl-sci↗

arXiv2025

From Canonical to Complex: Benchmarking LLM Capabilities in Undergraduate Thermodynamics

Large language models (LLMs) are increasingly considered as tutoring aids in science education. Yet their readiness for unsupervised use in undergraduate instruction remains uncertain, as reliable teaching requires more than fluent recall: it demands consistent, principle-grounded reasoning. Thermodynamics, with its compact laws and subtle distinctions between state and path functions, reversibility, and entropy, provides an ideal testbed for evaluating such capabilities. Here we present UTQA, a 50-item undergraduate thermodynamics question answering benchmark, covering ideal-gas processes, reversibility, and diagram interpretation. No leading 2025-era model exceeded our 95\% competence threshold: the best LLMs achieved 82\% accuracy, with text-only items performing better than image reasoning tasks, which often fell to chance levels. Prompt phrasing and syntactic complexity showed modest to little correlation with performance. The gap concentrates in finite-rate/irreversible scenarios and in binding visual features to thermodynamic meaning, indicating that current LLMs are not yet suitable for unsupervised tutoring in this domain.

physics.ed-ph↗

arXiv2023

Diffusive and Unimolecular Nonradiative Decay of Excited States in Doped Carbon Nanotubes

Doping can profoundly affect the electronic- and optical-structure of semiconductors. Here we address the effect of surplus charges on non-radiative (NR) exciton and trion decay in doped semiconducting single-wall carbon nanotubes. The dependence of exciton photoluminescence quantum yields and exciton decay on the doping level, with its characteristically stretched-exponential kinetics, is attributed to diffusion-limited NR decay at charged impurity sites. By contrast, trion decay is unimolecular with a rate constant of $2.0\,\rm ps^{-1}$. Our experiments thus show that charged impurities not only trap trions and scavenge mobile excitons but that they also facilitate efficient NR energy dissipation for both.

cond-mat.mes-hall↗

arXiv2023

Trions, Exciton Dynamics and Spectral Modifications in Doped Carbon Nanotubes: A Singular Defect-Driven Mechanism

Doping substantially influences the electronic and photophysical properties of semiconducting single-wall carbon nanotubes (s-SWNTs). Although prior studies have noted that surplus charge carriers modify optical spectra and accelerate non-radiative exciton decay in doped s-SWNTs, a direct mechanistic correlation of trion formation, exciton dynamics and energetics remains elusive. This work examines the influence of doping-induced non-radiative decay and exciton confinement on s-SWNT photophysics. Using photoluminescence, continuous-wave absorption, and pump-probe spectroscopy, we show that localization of and barrier formation by trapped charges can be jointly quantified using diffusive exciton transport- and particle-in-the-box models, yielding a one-to-one correlation between charge carrier concentrations derived from these models. The study highlights the multifaceted role of exohedral counterions, which trap charges to create quenching sites, form barriers to exciton movement, and host trion states. This contributes significantly to understanding and optimizing the photophysical properties of doped SWNTs.

cond-mat.mtrl-sci↗

arXiv2019

Quantifying Doping Levels in Carbon Nanotubes by Optical Spectroscopy

Controlling doping is essential for a successful integration of semiconductor materials into device technologies. However, the assessment of doping levels and the distribution of charge carriers in carbon nanotubes or other nanoscale semiconductor materials is often either limited to a qualitative attribution of being 'high' or 'low' or it is entirely absent. Here, we describe efforts toward a quantitative characterization of doping in redox- or electrochemically doped semiconducting carbon nanotubes (s-SWNTs) using VIS-NIR absorption spectroscopy. We discuss how carrier densities up to about 0.5 $\rm nm^{-1}$ can be quantified with a sensitivity of roughly one charge per $10^4$ carbon atoms assuming in-homogeneous or homogeneous carrier distributions.

cond-mat.mtrl-sci↗