arXiv subjects

Michael Müller

Publications and source records attributed to Michael Müller.

At least 19 recordsLinked to original sources

arXiv2026

Hidden excimer formation in the gas-phase photodynamics of a BN-doped phenanthrene

Replacing CC units by isoelectronic BN motifs provides a powerful strategy to tune the electronic structure and excited-state chemistry of polycyclic aromatic hydrocarbons (PAHs). Here, we combine multiphoton ionization spectroscopy, time-resolved photoelectron imaging, ion velocity-map imaging, and quantum-chemical calculations to disentangle the monomer and dimer photophysics of 4a,4b-azaboraphenanthrene. The monomer exhibits a structured S$_1 \leftarrow$ S$_0$ spectrum with an origin at $22880 \pm 15\,\mathrm{cm}^{-1}$, corresponding to $2.837 \,\mathrm{eV}$, and pronounced activity in low-wavenumber deformation modes. Photoelectron spectroscopy yields an adiabatic ionization energy of $7.18 \pm 0.02\,\mathrm{eV}$. While the structured spectrum, high fluorescence quantum yield, small computed geometry changes, and weak spin-orbit couplings all point to a long-lived monomer S$_1$ state, time-resolved photoelectron images reveal an additional picosecond component. Ion imaging shows that this component originates from dissociative ionization of the molecular dimer, which projects dimer excited-state dynamics into the monomer mass channel. Computations identify the initially excited dimer state as a bright H-aggregate-like exciton, followed by ultrafast S$_2 \rightarrow$ S$_1$ internal conversion and subsequent structural relaxation toward an excimeric S$_1$ minimum. The experimentally observed $\approx 15\,\mathrm{ps}$ time constant is therefore assigned to excimer formation in the neutral dimer.

physics.chem-ph↗

arXiv2026

On the Reliability of User-Centric Evaluation of Conversational Recommender Systems

User-centric evaluation has become a key paradigm for assessing Conversational Recommender Systems (CRS), aiming to capture subjective qualities such as satisfaction, trust, and rapport. To enable scalable evaluation, recent work increasingly relies on third-party annotations of static dialogue logs by crowd workers or large language models. However, the reliability of this practice remains largely unexamined. In this paper, we present a large-scale empirical study investigating the reliability and structure of user-centric CRS evaluation on static dialogue transcripts. We collected 1,053 annotations from 124 crowd workers on 200 ReDial dialogues using the 18-dimensional CRS-Que framework. Using random-effects reliability models and correlation analysis, we quantify the stability of individual dimensions and their interdependencies. Our results show that utilitarian and outcome-oriented dimensions such as accuracy, usefulness, and satisfaction achieve moderate reliability under aggregation, whereas socially grounded constructs such as humanness and rapport are substantially less reliable. Furthermore, many dimensions collapse into a single global quality signal, revealing a strong halo effect in third-party judgments. These findings challenge the validity of single-annotator and LLM-based evaluation protocols and motivate the need for multi-rater aggregation and dimension reduction in offline CRS evaluation.

cs.IR↗

arXiv2026

Measurements of absolute gamma-ray energies using an ultra-high energy resolution magnetic microcalorimeter

We present new measurements of 27 gamma ray energies ranging from 14 keV to 136 keV, obtained using high-energy-resolution magnetic microcalorimeters for energy-dispersive spectrometry. The spectrometer has eight pixels and achieves an energy resolution between 15 and 30 eV across the entire energy range. It faces a cryogenic source sampler with four movable sources. Each source contains a mixture of radionuclides, including Yb-169 and Co-57 used to calibrate the spectrometer and correct its non-linearity. The gamma-ray energies, emitted through the decay of Co-57, Cd-109, Ba-133, Gd-153, Eu-154, Eu-155, Yb-169, Tm-170, Pb-210, Np-239, Am-241 and Am-243, have been reassessed. The lowest absolute uncertainty achieved is 0.13 eV at 105.3 keV, which corresponds to a relative uncertainty of 1.3 ppm. Of the 27 measured gamma rays, the uncertainties of 19 energies were improved with respect to the available literature: 10 by a factor of 5 and 4 by more than one order of magnitude. Good agreement is observed with energies obtained elsewhere by wavelength-dispersive spectrometry. This work significantly improves upon previously reported gamma energies obtained by energy-dispersive spectrometry using semiconductor detectors, thanks to the ultra-high energy resolution of magnetic microcalorimeters.

physics.ins-det↗

arXiv2026

Scaling of broadband Ho:CALGO regenerative amplifier to multi-mJ pulse energy

We report on energy scaling of a 2.08-{\mu}m wavelength regenerative amplifier (RA) system based on the broadband gain material Ho:CaAlGdO4 (CALGO) to multi-mJ pulse energy at kHz repetition rates. Compared to previous reports, energy scaling was enabled thanks to an upgraded seed laser with a higher fluence and better spectral overlap to the gain spectrum of Ho:CALGO, which increased amplification efficiency. Bifurcation-free energy extraction was investigated experimentally and numerically for various repetition rates. A stable output was obtained at 10 W average power for repetition rates of 30 kHz and above. In addition, stable 3.4-mJ energy extraction was achieved at a 1-kHz repetition rate. We discuss the further scaling potential of pulse energy and pulse duration.

physics.optics↗

arXiv2025

Distinguishing between Black Holes and Neutron Stars within a Population of Weak Tidal Measurements

We study the ability of tidal signatures within the inspiral of compact binaries observed through gravitational waves (GWs) to distinguish between neutron stars (NSs) and black holes (BHs). After quantifying how hard this measurement is on a single-event basis, we investigate the ability of a large catalog of GW detections to constrain the fraction of NS in the population as a function of mass: $f_{\mathrm{NS}}(m)$. Using simulated catalogs with realistic measurement uncertainty, we find that $> O(200)$ events will be needed before we can precisely measure $f_{\mathrm{NS}}$, and catalogs of $> O(100)$ events will be needed before we can even rule out the possibility that all low-mass objects are BHs with GW data alone (i.e., without electromagnetic counterparts). Therefore, this is unlikely to occur with advanced detectors, even at design sensitivity. Nevertheless, it could be feasible with next-generation facilities like Cosmic Explorer and Einstein Telescope.

gr-qc↗

arXiv2025

Advanced fabrication process for particle absorbers of highly pure electroplated gold for microcalorimeter applications

Magnetic microcalorimeters (MMCs) have become a key technology for applications requiring outstanding energy resolution, fast signal rise time and excellent linearity. MMCs measure the temperature rise upon absorption of a single particle within a particle absorber by using a paramagnetic temperature sensor that is thermally coupled to the absorber. The design and fabrication of the particle absorber is key for excellent detector performance. Here, we present a microfabrication process for free-standing particle absorbers made of two stacked and independently electroplated high-purity Au layers. This enables, for example, embedding of radioactive sources within the absorber for realizing a $4π$ detection geometry in radionuclide metrology or preparing detector arrays with variable quantum efficiency and energy resolution as requested for future applications in high energy physics. Due to careful optimization of photoresist processing and electroplating parameters, the Au films are of very high purity and very high residual resistivity ratio values above 40, allowing for fast internal absorber thermalization.

physics.ins-det↗

arXiv2025

Comparison of the detector response and calibration function of metallic microcalorimeters for X-ray photons and external electrons

Metallic microcalorimeters (MMCs) are cryogenic single-particle detectors that rely on a calorimetric detection principle. Due to their excellent energy resolution, close-to-ideal linear detector response, fast signal rise time and the potential for \SI{100}{\%} quantum efficiency, MMCs outperform conventional detectors by several orders of magnitude in resolution. These attributes make them particularly interesting for a broad spectrum of applications, including a next-generation neutrino mass experiment based on the measurement of the tritium beta-decay spectrum, with an objective of achieving a sensitivity surpassing that of the pioneering KATRIN experiment. However, although MMCs have been used in measurements of photons and heavy ions with great success, no information is currently available on the interaction between MMCs and external light charged particles such as electrons. This work aims to provide such missing information and to demonstrate that MMC-based detectors are suitable for high-resolution spectroscopy of external electron sources. Particularly, we present the first-ever measurements of external electrons using a metallic microcalorimeter, comprehensively discuss the characteristics of the signal shape and the calibration function and give a direct comparison between well-defined conversion electron and X-ray photon signals from the same $^{83}$Rb/$^{83m}$Kr source.

physics.ins-det↗

arXiv2025

DFingerNet: Noise-Adaptive Speech Enhancement for Hearing Aids

The DeepFilterNet (DFN) architecture was recently proposed as a deep learning model suited for hearing aid devices. Despite its competitive performance on numerous benchmarks, it still follows a `one-size-fits-all' approach, which aims to train a single, monolithic architecture that generalises across different noises and environments. However, its limited size and computation budget can hamper its generalisability. Recent work has shown that in-context adaptation can improve performance by conditioning the denoising process on additional information extracted from background recordings to mitigate this. These recordings can be offloaded outside the hearing aid, thus improving performance while adding minimal computational overhead. We introduce these principles to the DFN model, thus proposing the DFingerNet (DFiN) model, which shows superior performance on various benchmarks inspired by the DNS Challenge.

cs.SD↗

arXiv2024

Be careful in multi-messenger inference of the Hubble constant: A path forward for robust inference

Multi-messenger observations of coalescing binary neutron stars (BNSs) are a direct probe of the expansion history of the universe and carry the potential to shed light on the disparity between low- and high-redshift measurements of the Hubble constant $H_0$. To measure the value of $H_0$ with such observations requires pristine inference of the luminosity distance and the true source redshift with minimal impact from systematics. In this analysis, we carry out joint inference on mock gravitational wave (GW) signals and their electromagnetic (EM) afterglows from BNS coalescences and find that the inclination angle inferred from the afterglow light curve and apparent superluminal motion can be precise, but need not be accurate and is subject to systematic uncertainty that could be as large as $1.5σ$. This produces a disparity between the EM and GW inferred inclination angles, which if not carefully treated when combining observations can bias the inferred value of $H_0$. We also find that already small misalignments of $3^{\circ}-6^{\circ}$ between the inherent system inclinations for the GW and EM emission can bias the inference by $\mathcal{O}(1-2σ)$ if not taken into account. As multi-messenger BNS observations are rare, we must make the most out of a small number of events and harness the increased precision, while avoiding reduced accuracy. We demonstrate how to mitigate these potential sources of bias by jointly inferring the mismatch between the GW- and EM-based inclination angles and $H_0$.

astro-ph.CO↗

arXiv2024

Achieving Different Stoichiometries and Morphologies in Vapor Phase Deposition of Inorganic Halide Perovskites: Single or Dual Precursor Sources?

Inorganic halide perovskites have become attractive for many optoelectronic applications due to their outstanding properties. While chemical synthesis techniques have been successful in producing high-quality perovskite crystals, scaling up to wafer-scale thin films remains challenging. Vapor deposition methods, particularly physical vapor deposition and chemical vapor deposition, have emerged as potential solutions for large-scale thin film fabrication. However, the control of phase purity during deposition remains problematic. Here, we investigate single-source (CsPbBr3) and dual-source (CsBr and PbBr2) vapor deposition techniques to achieve phase-pure CsPbBr3 thin films. Utilizing Knudsen Effusion Mass Spectrometry, we demonstrate that while the single-source CsPbBr3 evaporation is partially congruent, it leads to compositional changes in the evaporant over time. The dual-source evaporation, with a precise control of the PbBr2/CsBr flux ratio, can improve phase purity, particularly at elevated substrate temperatures at excess PbBr2 conditions. Our results give direct evidence that the growth is CsBr-limited. Overall, our findings provide critical insights into the vapor phase deposition processes, highlighting the importance of evaporation conditions in achieving the desired inorganic perovskite stoichiometry and morphology.

cond-mat.mtrl-sci↗

arXiv2024

Uncertainty Analysis of Limit Cycle Oscillations in Nonlinear Dynamical Systems with the Fourier Generalized Polynomial Chaos Expansion

In engineering, simulations play a vital role in predicting the behavior of a nonlinear dynamical system. In order to enhance the reliability of predictions, it is essential to incorporate the inherent uncertainties that are present in all real-world systems. Consequently, stochastic predictions are of significant importance, particularly during design or reliability analysis. In this work, we concentrate on the stochastic prediction of limit cycle oscillations, which typically occur in nonlinear dynamical systems and are of great technical importance. To address uncertainties in the limit cycle oscillations, we rely on the recently proposed Fourier generalized Polynomial Chaos expansion (FgPC), which combines Fourier analysis with spectral stochastic methods. In this paper, we demonstrate that valuable insights into the dynamics and their variability can be gained with a FgPC analysis, considering different benchmarks. These are the well-known forced Duffing oscillator and a more complex model from cell biology in which highly non-linear electrophysiological processes are closely linked to diffusive processes. With our spectral method, we are able to predict complicated marginal distributions of the limit cycle oscillations and, additionally, for self-excited systems, the uncertainty in the base frequency. Finally we study the sparsity of the FgPC coefficients as a basis for adaptive approximation.

cs.CE↗

arXiv2023

Magnetic microcalorimeters for primary activity standardization within the EMPIR project PrimA-LTD

The precision of existing decay data of radionuclides for activity determination is often a limitation for actual applications in science, society, and industry. For this reason, the EMPIR project PrimA-LTD aims to introduce an advanced primary activity standardization technique that is based on magnetic microcalorimeters (MMCs) and that will offer very low energy threshold of few eV and a decay scheme independent detection efficiency close to 100%. As a proof of concept, we developed two MMC-based detector types in order to standardize an $α$-decaying, a $β$-decaying and an electron capture decaying isotope. One detector type aims to introduce a reusable detector setup, while the other aims to provide highly accurate decay spectra by high resolution measurements with high statistics. We present the designs, fabrication status and first characterization measurements of both detectors types and outline next steps.

physics.ins-det↗

arXiv2023

Exploratory applications of the Fröhlich-Morchio-Strocchi mechanism in quantum gravity

A manifestly diffeomorphism-invariant approach to canonical quantum gravity requires to use composite operators. These can be considered to be bound states of matter and/or gravitons, intrinsically non-perturbative objects. An analytical approach to determine the properties of such bound states could be the Fröhlich-Morchio-Strocchi mechanism. We explore the necessary technology by applying it to various $n$-point functions, including geon propagators and black-hole-particle vertices.

hep-th↗

arXiv2021

1 kW, 10 mJ, 120 fs coherently combined fiber CPA laser system

An ultrafast fiber chirped-pulse amplification laser system based on coherent combination of 16 ytterbium-doped rod-type amplifiers is presented. It generates 10 mJ pulse energy at 1 kW average power and 120 fs pulse duration. A partially helium-protected, two-staged chirped-pulse amplification grating compressor is implemented to maintain the close to diffraction-limited beam quality by avoiding nonlinear absorption in air.

physics.optics↗

arXiv2021

Kilowatt-average-power compression of millijoule pulses in a gas-filled multi-pass cell

We demonstrate the reliable generation of 1-mJ, 31-fs pulses with an average power of 1 kW by post-compression of 200-fs pulses from a coherently combined Yb:fiber laser system in an argon-filled Herriott-type multi-pass cell with an overall compression efficiency of 96%. We also analyze the output beam, revealing essentially no spatio-spectral couplings or beam quality loss.

physics.optics↗

arXiv2021

Fiber laser-driven gas plasma-based generation of THz radiation with 50-mW average power

We present on THz generation in the two-color gas plasma scheme driven by a high-power, ultrafast fiber laser system. The applied scheme is a promising approach for scaling the THz average power but it has been limited so far by the driving lasers to repetition rates up to 1 kHz. Here, we demonstrate recent results of THz generation operating at a two orders of magnitude higher repetition rate. This results in a unprecedented THz average power of 50 mW. The development of compact, table-top THz sources with high repetition rate and high field strength is crucial for studying nonlinear responses of materials, particle acceleration or faster data acquisition in imaging and spectroscopy.

physics.optics↗

arXiv2021

23 mJ high-power fiber CPA system using electro-optically controlled divided-pulse amplification

The pulse-energy scaling technique electro-optically controlled divided-pulse amplification is implemented in a high-power ultrafast fiber laser system based on coherent beam combination. A fiber-integrated front end and a multi-pass cell based back end allow for a small footprint and a modular implementation. Bursts of 8 pulses are amplified parallel in up to 12 ytterbium-doped large-pitch fiber amplifiers. Subsequent spatio-temporal coherent combination of the 96 total amplified pulse replicas to a single pulse results in a pulse energy of 23 mJ at an average power of 674 W, compressible to a pulse duration of 235 fs. To the best of our knowledge, this is the highest pulse energy ever accomplished with a fiber CPA system.

physics.optics↗

arXiv2021

Electro-optically controlled divided-pulse amplification

A novel technique for divided-pulse amplification is presented in a proof-of-principle experiment. A pulse burst, cut out of the pulse train of a mode-locked oscillator, is amplified and temporally combined into a single pulse. High combination efficiency and excellent pulse contrast are demonstrated. The system is mostly fiber-coupled, enabling a high interferometric stability. This approach provides access to the amplitude and phase of the individual pulses in the burst to be amplified, potentially allowing the compensation of gain saturation and nonlinear phase mismatches within the burst. Therefore, this technique enables the scaling of the peak power and pulse energy of pulsed laser systems beyond currently prevailing limitations.

physics.optics↗