arXiv subjects

Andreas Wagner

Publications and source records attributed to Andreas Wagner.

At least 19 recordsLinked to original sources

arXiv2026

From phase transformation to amorphization: damage accumulation in Yb-implanted $\beta-Ga_2O_3$

This study provides a comprehensive analysis of the radiation response and structural evolution of differently oriented$\beta-Ga_2O_3$ single crystals subjected to Yb ion implantation over a wide fluence range from $5 \times 10^{12}$ to $1 \times 10^{16}$~cm$^{-2}$ ($0.04$--$74$~dpa). A multi-technique approach (RBS/c, PAS, HRTEM, and HRXRD) was employed to investigate the mechanisms of damage accumulation. The results reveal a multi-stage process of defect evolution. At a critical threshold of around $0.4$~dpa, the accumulation of lattice strain triggers a phase transformation from monoclinic $\beta$-Ga$_{2}$O$_{3}$ to a defective spinel structure of $\gamma$-Ga$_{2}$O$_{3}$. Notably, the formation of this new phase is accompanied by strain relaxation. With further irradiation, defects develop within the crystal structure of $\gamma$-Ga$_{2}$O$_{3}$. The associated atomic reorganization at this stage is reflected by a distinct dip in the damage accumulation curve and the appearance of stacking faults in the subsurface region of the implanted layer. In contrast to previous reports suggesting high radiation stability of this phase, the present study clearly demonstrates that continuous defect accumulation results in a significant increase in both displaced atoms and vacancy-type defects, with a strong depth dependence in their type and density. Ultimately, at an irradiation level of approximately $7$~dpa, the surface layer amorphizes. With further irradiation, the amorphous layer expands, gradually replacing the transient $\gamma$-Ga$_{2}$O$_{3}$ phase. These findings reveal that the radiation tolerance of gallium oxide is highly sensitive to ion-specific interactions and strain-induced instabilities, thereby challenging the previously assumed robustness of this material under high-fluence ion irradiation.

cond-mat.mtrl-sci↗

arXiv2026

The Gentle Collapse: Distributional Metrics for Continual Learning

Accuracy degradation is the standard metric for Catastrophic Forgetting (CF), however, it records only whether forgetting occurred or not. It saturates at the extremes and collapses discretely at task boundaries, hiding the internal structure of what is being forgotten. We introduce six softmax-derived metrics spanning true-label rank (TLR), predictive confidence, and distributional divergence that characterize forgetting continuously, each normalized to [0, 1] with no modification to training. On CIFAR-100, these metrics carry information where accuracy does not: at 0% accuracy, the Confusion Margin spans an IQR of [0.32, 0.50] across classes that accuracy treats identically. We demonstrate that this richer signal is actionable in mitigating catastrophic forgetting. Per-sample metric scores used as loss weights reduce forgetting by 1.3 percentage points over uniform experience replay (ER) on CIFAR-100. Furthermore, the slope of a metric over a small window provides a stable sampling criterion: at a small-window size (e.g. 3 epochs), accuracy-trend degrades to 34.79% (std. = 2.32) while log-TLR achieves 41.07% (std. = 0.57). This gap is structural since reliable small-window trend estimation requires a continuous signal. On TinyImageNet, log-TLR trend sampling reduces forgetting by 7.7 percentage points over the ER baseline.

cs.LG↗

arXiv2026

Carrier Localization in Pnictogen-Based Chalcohalides from Defect-Bound Hot Polarons

Pnictogen-based solar absorbers have gained prominence as promising nontoxic and stable alternatives to lead-halide perovskites (LHPs), but are severely limited by carrier localization, preventing their performance from approaching those of LHPs. Recent efforts have uncovered routes to overcome carrier localization, but these early efforts only considered intrinsic factors. Herein, we push beyond these limited early efforts, examining the role of defects, not only on cold carriers but also hot carriers. Focusing on the structurally one-dimensional pnictogen chalcohalide BiSBr, we find that whilst this material intrinsically does not exhibit carrier localization, vacancies introduced during synthesis or post-treatment lead to pronounced extrinsic self-trapping via the formation of defect-bound hot polarons-excited charge-carriers strongly coupled to local defect-induced vibrational modes. These above-gap defect states divert hot carriers from cooling to the band edge, thus depleting the mobile carrier population. Our findings establish the key role of defect-bound hot polarons in mediating extrinsic localization and offer new mechanistic insights into the interplay between defects, lattice coupling, and excited-state charge-carrier transport, which are critical to designing efficient perovskite-inspired solar absorbers.

cond-mat.mtrl-sci↗

arXiv2026

Self-Assembled Telecom Color Centers in Silicon and Their Growth Environment

Artificial atoms based on color centers in silicon (SiCCs) have recently emerged as promising candidates for highly integrable and scalable key components in photonic quantum technology, including telecom single-photon sources and spin memory devices. A novel all-epitaxial fabrication technique for SiCCs, based on ultra-low-temperature (ULT) molecular beam epitaxy (MBE), addresses limitations of conventional fabrication via ion implantation, such as vertical ion straggle and collateral crystal lattice damage. This method solely relies on self-assembly of SiCCs during kinetically-limited growth of (carbon-doped) Si(:C) at ULTs <~350{\deg}C. The latter requires an extraordinary pristine growth environment to prevent unintended defect formation caused by the incorporation of impurities from the background vapor; however, so far, no study has specifically addressed how exactly the vacuum conditions during epitaxy influence SiCC formation, their optical properties, and the quality of the surrounding crystal matrix. Here, we investigate the impact of the growth pressure and the substrate temperature on the self-assembly and photoluminescence (PL) properties of important SiCCs, such as W, G, G', and T centers. Further, we use PL and Doppler broadening variable energy positron annihilation spectroscopy to emphasize the role of the growth pressure in suppressing the luminescence background, which is crucial for advancing quantum photonics applications.

cond-mat.mes-hall↗

arXiv2026

Structural and Optical Characteristics of $\beta-Ga_2O_3$ Implanted with Rare Earth Ions

We investigated the structural evolution and optical properties of $\beta-Ga_2O_3$ crystals implanted with different rare-earth (RE) ions using channeling Rutherford Backscattering Spectrometry, Positron Annihilation, Photoluminescence, and Photoluminescence Excitation spectroscopies. The studies reveal that implantation-induced disorder, accompanying phase transitions, and post-annealing structural recovery are largely insensitive to the implanted RE species. The defect microstructure is also found to be similar for all implanted RE ions. Thermal annealing does not completely remove radiation-induced defects but instead drives their rearrangement into larger defect complexes. Unimplanted (virgin) $\beta-Ga_2O_3$ exhibits strong UV-visible emission attributed to oxygen vacancies, whereas the introduction of RE ions produces additional emission lines originating from electronic transitions within RE3+ ions. The results indicate that RE3+ ions are excited through the host conduction band, followed by non-radiative relaxation to the 4f excited states and radiative decay to the respective ground states. Fluence-dependent studies of Yb3+ reveal the onset of concentration quenching, while RE-related emission remains efficient even in the presence of substantial lattice disorder. These findings provide new insight into defect evolution in ion-implanted beta-Ga2O3 and clarify the excitation mechanisms of RE3+ ions, offering guidance for optimizing the optical performance of $\beta-Ga_2O_3$:RE materials.

cond-mat.mtrl-sci↗

arXiv2026

Defect Landscape Engineering Suppresses Helium Damage in Ceramics

Helium accumulation in structural ceramics used in nuclear, fusion, and aerospace systems causes swelling, cracking, and early failure, yet controlling this damage has remained elusive. Here, we introduce defect landscape engineering, the deliberate creation of vacancy clusters prior to helium exposure, as a general strategy to suppress helium-induced degradation. Using {\alpha}-SiC as a model, we combine advanced microscopy, strain mapping, helium depth profiling, positron annihilation spectroscopy, and atomistic simulations to demonstrate that tailored pre-damage transforms helium defect evolution. Instead of forming extended platelets and nanocracks, helium is trapped in stable, uniformly dispersed nanobubbles. Simulations reveal that small vacancy clusters act as dual-function sinks for irradiation-induced interstitials and preferential helium traps, fundamentally altering cascade recombination dynamics. This mechanism is composition-independent and scalable, offering a new design principle for radiation-tolerant ceramics across carbides, nitrides, and oxides. By viewing defect control as a tunable parameter instead of a fixed material property, this work outlines a possible design route toward enhanced radiation tolerance in ceramics used in extreme environments.

physics.app-ph↗

arXiv2025

Defect analysis of the $\beta$- to $\gamma$-Ga$_{2}$O$_{3}$ phase transition

In this study, we investigate the ion-irradiation-induced phase transition in gallium oxide (Ga2O3) from the $\beta$ to the $\gamma$ phase, the role of defects during the transformation, and the quality of the resulting crystal structure. Using a multi-method analysis approach including X-ray diffraction (XRD), transmission electron microscopy (TEM), Rutherford backscattering spectrometry in channeling mode (RBS/c), Doppler broadening variable energy positron annihilation spectroscopy (DB-VEPAS) and variable energy positron annihilation lifetime spectroscopy (VEPALS) supported by density functional theory (DFT) calculations, we have characterized defects at all the relevant stages before, during, and after the phase transition. Reduction in backscattering yield was observed in RBS/c spectra after the transition to the $\gamma$ phase. This is corroborated by a significant decrease in the positron trapping center density due to generation of embedded vacancies intrinsic for the $\gamma$-Ga2O3 but too shallow in order to trap positrons. A comparison of the observed positron lifetime of $\gamma$-Ga2O3 with different theoretical models shows good agreement with the three-site $\gamma$ phase approach. A characteristic increase in the effective positron diffusion length and the positron lifetime at the transition point from $\beta$-Ga2O3 to $\gamma$-Ga2O3 enables visualization of the phase transition with positrons for the first time. Moreover, a subsequent reduction of these quantities with increasing irradiation fluence was observed, which we attribute to further evolution of the $\gamma$-Ga2O3 and changes in the gallium vacancy density as well as relative occupation in the crystal lattice.

cond-mat.mtrl-sci↗

arXiv2025

The generalized scalar auxiliary variable applied to the incompressible Boussinesq Equation

This paper introduces a second-order time discretization for solving the incompressible Boussinesq equation. It uses the generalized scalar auxiliary variable (GSAV) and a backward differentiation formula (BDF), based on a Taylor expansion around $t^{n+k}$ for $k\geq3$. An exponential time integrator is used for the auxiliary variable to ensure stability independent of the time step size. We give rigorous asymptotic error estimates of the time-stepping scheme, thereby justifying its accuracy and stability. The scheme is reformulated into one amenable to a $H^1$-conforming finite element discretization. Finally, we validate our theoretical results with numerical experiments using a Taylor--Hood-based finite element discretization and show its applicability to large-scale 3-dimensional problems.

math.NA↗

arXiv2025

A 3D-1D-0D Multiscale Model of the Neuro-Glial-Vascular Unit for Synaptic and Vascular Dynamics in the Dorsal Vagal Complex

Cerebral blood flow regulation is critical for brain function, and its disruption is implicated in various neurological disorders. Many existing models do not fully capture the complex, multiscale interactions among neuronal activity, astrocytic signaling, and vascular dynamics--especially in key brainstem regions. In this work, we present a 3D-1D-0D multiscale computational framework for modeling the neuro-glial-vascular unit (NGVU) in the dorsal vagal complex (DVC). Our approach integrates a quadripartite synapse model--which represents the interplay among excitatory and inhibitory neurons, astrocytes, and vascular smooth muscle cells--with a hierarchical description of vascular dynamics that couples a three-dimensional microcirculatory network with a one-dimensional macrocirculatory representation and a zero-dimensional synaptic component. By linking neuronal spiking, astrocytic calcium and gliotransmitter signaling, and vascular tone regulation, our model reproduces key features of functional hyperemia and elucidates the feedback loops that help maintain cerebral blood flow. Simulation results demonstrate that neurotransmitter release triggers astrocytic responses that modulate vessel radius to optimize oxygen and nutrient delivery. This integrated framework, to our knowledge the first model to combine these elements for the NGVU in the DVC, provides a robust and modular platform for future investigations into the pathophysiology of cerebral blood flow regulation and its role in autonomic control, including the regulation of stomach function.

q-bio.TO↗

arXiv2025

Carbon magneto-ionics: Control of magnetism through voltage-driven carbon transport

Control of magnetism through voltage-driven ionic processes (i.e., magneto-ionics) holds potential for next-generation memories and computing. This stems from its non-volatility, flexibility in adjusting the magnitude and speed of magnetic modulation, and energy efficiency. Since magneto-ionics depends on factors like ionic radius and electronegativity, identifying alternative mobile ions is crucial to embrace new phenomena and applications. Here, the feasibility of C as a prospective magneto-ionic ion is investigated in a Fe-C system by electrolyte gating. In contrast to most magneto-ionic systems, Fe-C presents a dual-ion mechanism: Fe and C act as cation and anion, respectively, moving uniformly in opposite directions under an applied electric field. This leads to a 7-fold increase in saturation magnetization with magneto-ionic rates larger than 1 emu cm-3 s-1, and a 25-fold increase in coercivity. Since carbides exhibit minimal cytotoxicity, this introduces a biocompatible dimension to magneto-ionics, paving the way for the convergence of spintronics and biotechnology.

cond-mat.mtrl-sci↗

arXiv2025

Charge-transfer-mediated boron magneto-ionics: Towards voltage-driven multi-ion transport

Voltage control of magnetism via magneto-ionics, where ion transport and/or redox processes drive magnetic modulation, holds great promise for next-generation memories and computing. This stems from its non-volatility and ability to precisely tune both the magnitude and speed of magnetic properties in an energy-efficient manner. However, expanding magneto-ionics to incorporate novel mobile ions or even multiple ion species is crucial for unlocking new phenomena and enabling multifunctional capabilities. Here, we demonstrate voltage-driven multi-ion transport in a FeBO system with increasing oxygen content, progressively transitioning from an electrostatic-like response to a more pronounced electrochemical (magneto-ionic) behavior. The voltage-driven transport of both B and Fe is activated by oxidation state tuning, owing to the larger electronegativity of oxygen. Such charge-transfer effects allow multi-ion magneto-ionics, where O ions move oppositely to Fe and B ions. These results pave the way for programmable functionalities by leveraging elements with different electron affinities through charge-transfer engineering.

cond-mat.mtrl-sci↗

arXiv2024

The effect of data-driving and relaxation model on magnetic flux rope evolution and stability

We investigate the effect of data-driving on flux rope eruptivity in magnetic field simulations by analysing fully data-driven modelling results of active region (AR) 12473 and AR11176, as well as preforming relaxation runs for AR12473 (found to be eruptive). Here, the driving is switched off systematically at different time steps. We analyse the behaviour of fundamental quantities, essential for understanding the eruptivity of magnetic flux ropes (MFRs). The data-driven simulations are carried out with the time-dependent magnetofrictional model (TMFM) for AR12473 and AR11176. For the relaxation runs, we employ the magnetofrictional method (MFM) and a zero-beta magnetohydrodynamic (MHD) model to investigate how significant the differences between the two relaxation procedures are when started from the same initial conditions. To determine the eruptivity of the MFRs, we calculate characteristic geometric properties, such as the cross-section, MFR height along with stability parameters, such as MFR twist and the decay index. For eruptive cases, we investigate the effect of sustained driving beyond the point of eruptivity on the MFR properties. We find that the fully-driven AR12473 MFR is eruptive while the AR11176 MFR is not. For the relaxation runs, we find that the MFM MFRs are eruptive when the driving is stopped around the flare time or later, while the MHD MFRs show eruptive behaviour even if the driving is switched off one and a half days before the flare occurs. We find that characteristic MFR properties can vary greatly even for the eruptive cases of different relaxation simulations. The results suggest that data driving can significantly influence the evolution of the eruption, with differences appearing even when the relaxation time is set to later stages of the simulation when the MFRs have already entered an eruptive phase.

astro-ph.SR↗

arXiv2024

Ge epitaxy at ultra-low growth temperatures enabled by a pristine growth environment

Germanium (Ge), the next-in-line group-IV material, bears great potential to add functionality and performance to next-generation nanoelectronics and solid-state quantum transport based on silicon (Si) technology. Here, we investigate the direct epitaxial growth of two-dimensional high-quality crystalline Ge layers on Si deposited at ultra-low growth temperatures ($T_{Ge} = 100^{\circ}\mathrm{C}-350^{\circ}\mathrm{C}$) and pristine growth pressures ($\lesssim 10^{-10}\,\mathrm{mbar}$). First, we show that $T_{Ge}$ does not degrade the crystal quality of homoepitaxial Ge/Ge(001) by comparing the point defect density using positron annihilation lifetime spectroscopy. Subsequently, we present a systematic investigation of the Ge/Si(001) heteroepitaxy, varying the Ge coverage (${\theta}_{Ge}$, 1, 2, 4, 8, 12, and 16 nm) and $T_{Ge}$ ($100^{\circ}\mathrm{C}$ to $300^{\circ}\mathrm{C}$, in increments of $50^{\circ}\mathrm{C}$) to assess the influence of these parameters on the layer's structural quality. Atomic force microscopy revealed a rippled surface topography with superimposed grainy features and the absence of three-dimensional structures, such as quantum dots. Transmission electron microscopy unveiled pseudomorphic, grains of highly crystalline growth separated by defective domains. Thanks to nanobeam scanning x-ray diffraction measurements, we were able to evidence the lattice strain fluctuations due to the ripple-like structure of the layers. We conclude that the heteroepitaxial strain contributes to the formation of the ripples, which originate from the kinetic limitations of the ultra-low temperatures.

cond-mat.mtrl-sci↗

arXiv2024

Physics-Regularized Multi-Modal Image Assimilation for Brain Tumor Localization

Physical models in the form of partial differential equations serve as important priors for many under-constrained problems. One such application is tumor treatment planning, which relies on accurately estimating the spatial distribution of tumor cells within a patient's anatomy. While medical imaging can detect the bulk of a tumor, it cannot capture the full extent of its spread, as low-concentration tumor cells often remain undetectable, particularly in glioblastoma, the most common primary brain tumor. Machine learning approaches struggle to estimate the complete tumor cell distribution due to a lack of appropriate training data. Consequently, most existing methods rely on physics-based simulations to generate anatomically and physiologically plausible estimations. However, these approaches face challenges with complex and unknown initial conditions and are constrained by overly rigid physical models. In this work, we introduce a novel method that integrates data-driven and physics-based cost functions, akin to Physics-Informed Neural Networks (PINNs). However, our approach parametrizes the solution directly on a dynamic discrete mesh, allowing for the effective modeling of complex biomechanical behaviors. Specifically, we propose a unique discretization scheme that quantifies how well the learned spatiotemporal distributions of tumor and brain tissues adhere to their respective growth and elasticity equations. This quantification acts as a regularization term, offering greater flexibility and improved integration of patient data compared to existing models. We demonstrate enhanced coverage of tumor recurrence areas using real-world data from a patient cohort, highlighting the potential of our method to improve model-driven treatment planning for glioblastoma in clinical practice.

cs.CV↗

arXiv2024

Spatially-resolved charge detectors for particle beam optimization with femtoampere resolution achieved by in-vacuum signal preamplification

We present the design of a Faraday cup-like charged particle detector in a four quadrant configuration aimed at facilitating the alignment of low-intensity beams of exotic particles. The device is capable of assessing the current on the electrodes with a resolution of 33fA within 15ms or a maximal resolution of 1.8fA with a measurement time of 12.4s. This performance is achieved by minimizing the noise through a preamplification circuit installed in vacuum, as close as possible to the electrodes. We tested the detector with the positron beam of ELBE, achieving the nominal maximum resolution with high reproducibility. We then exploited the capabilities of the detector to resolve the two-dimensional shape of the beam, and revealed the presence of a weak electron beam being transported alongside the positrons. Characterization of the detector performance showed that in a variety of scenarios it can be used to quickly center positron beams thus allowing for the prompt optimization of beam transport.

physics.ins-det↗

arXiv2023

Insights into the LiMn2O4 Cathode Stability in Aqueous Electrolyte

LiMn2O4 (LMO), cathodes present large stability when cycled in aqueous electrolytes, contrasting its behavior in conventional organic electrolytes in Lithium-ion batteries (LIBs). To elucidate the mechanisms underlying this distinctive behavior, we employ unconventional characterization techniques, including Variable Energy Positron Annihilation Lifetime Spectroscopy (VEPALS), Tip-Enhanced Raman Spectroscopy (TERS) and macro-Raman Spectroscopy (with mm-size laser spot). These still rather unexplored techniques in the battery field provide complementary information across different length scales, revealing previously hidden features. VEPALS offers atomic-scale insights, uncovering cationic defects and sub-nanometer pores that tend to collapse with cycling. TERS, operating at the nanometric range at the surface, captured the presence of Mn3O4 and its dissolution with cycling, elucidating dynamic changes during operation. Additionally, TERS highlights SO42- accumulation at grain boundaries. Macro-Raman Spectroscopy focuses on the micrometer scale, depicting small changes in the cathode's long-range order, suggesting a slow but progressive loss of crystalline quality under operation. Integrating these techniques provides a comprehensive assessment of LMO cathode stability in aqueous electrolytes, offering multifaceted insights into phase and defect evolution that can help to rationalize the origin of such stability when compared to conventional organic electrolytes. Our findings advance the understanding of LMO behavior in aqueous environments and provide guidelines for its development for next-generation LIBs.

cond-mat.mtrl-sci↗

arXiv2023

The Automatic Identification and Tracking of Coronal Flux Ropes -- Part II: New Mathematical Morphology-based Flux Rope Extraction Method and Deflection Analysis

We present a magnetic flux rope (FR) extraction tool for solar coronal magnetic field modelling data, which builds upon the methodology from Wagner et al. (2023). We apply the scheme to magnetic field simulations of active regions AR12473 and AR11176. We compare the method to its predecessor and study the 3D movement of the newly extracted FRs up to heights of 200 and 300 Mm, respectively. The extraction method is based on the twist parameter and a variety of mathematical morphology algorithms, including the opening transform and the morphological gradient. We highlight the differences between the methods by investigating the circularity of the FRs in the plane we extract from. The simulations for the active regions are carried out with a time-dependent data-driven magnetofrictional model (TMFM; Pomoell et al. (2019)). We investigate the FR trajectories by tracking their apex throughout the full simulation time span. We demonstrate that this upgraded methodology provides the user with more tools and less a-priori assumptions about the FR shape that, in turn, leads to a more accurate set of field lines. The propagation analysis yields that the erupting FR from AR12473 showcases stronger dynamics than the AR11176 FR and a significant deflection during its ascent through the domain. The AR11176 FR appears more stable, though there still is a notable deflection. This confirms that at these low coronal heights, FRs do undergo significant changes in the direction of their propagation even for less dynamic cases. The modelling results are also verified with observations, with AR12473 being indeed dynamic and eruptive, while AR11176 only features an eruption outside of our simulation time window.

astro-ph.SR↗

arXiv2023

Photometry of Type II Supernova SN 2023ixf with a Worldwide Citizen Science Network

We present highly sampled photometry of the supernova (SN) 2023ixf, a Type II SN in M101, beginning 2 days before its first known detection. To gather these data, we enlisted the global Unistellar Network of citizen scientists. These 252 observations from 115 telescopes show the SN's rising brightness associated with shock emergence followed by gradual decay. We measure a peak $M_{V}$ = -18.18 $\pm$ 0.09 mag at 2023-05-25 21:37 UTC in agreement with previously published analyses.

astro-ph.HE↗