arXiv subjects

Alexander Stangl

Publications and source records attributed to Alexander Stangl.

12 recordsLinked to original sources

arXiv2026

The frameset of the second-order B-Spline along $ab=1/q$

We give a complete characterization of the zero set of the Zak transform of the second-order B-spline $Q_2$. As a consequence, we obtain a new proof of the Lemvig--Nielsen obstructions to the Gabor frame property of $Q_2$, and, along the hyperbola $ab=1/q$, we show these obstructions are sharp: every point they do not exclude belongs to the frame set. This gives the first complete characterization of $\mathcal F(Q_2)$ along an entire curve in this family, resolving a question left open by Lemvig and Nielsen.

math.CA↗

arXiv2026

On the structure of the Gram matrix for Gabor systems generated by B-splines

We consider the Gabor system $\mathcal{G}(g,a\mathbb{Z}\times b\mathbb{Z})$ generated by a continuous, compactly supported function $g$ over the time-frequency lattice generated by the parameters $a$ and $b$. We show that, under an appropriate ordering of the Gabor elements, certain submatrices of the Gram matrix of $\mathcal{G}(g,a\mathbb{Z}\times b\mathbb{Z})$ exhibit a block-Toeplitz structure. This structural property enables us to derive spectral results for finite sub-blocks of the Gram matrix by appealing to the spectral theory of Toeplitz matrices. In particular, we apply our results to the Gram matrix of Gabor systems generated by the $N$th-order B-spline.

math.GM↗

arXiv2026

Bias-triggered conductivity relaxation (BCR): a unique tool to simultaneously investigate thermodynamics, kinetics and electrostatic effects of oxygen reactions in MIEC thin films

Mixed ionic electronic transfer (MIET) reactions, such as the oxygen reduction reaction (ORR) at oxide surfaces, are of paramount importance to manifold technologically highly relevant processes and fundamental understanding must be developed to improve performance and tailor highly efficient electrodes and catalysts. Understanding such complex multi-step reactions, requires the study of kinetic processes, underlying thermodynamic properties, i.e. ionic and electronic defect concentrations and electrostatic surface effects. However conventional techniques struggle to uncover the complete picture within the same sample/measurement. Here, we overcome this limitation by introducing bias-triggered conductivity relaxation (BCR) as a novel tool to investigate MIET reactions on oxides. It is based on alternating out-of-plane coulometric titration/polarization and in-plane electrical conductivity relaxation measurements, providing simultaneous electronic, ionic and extraordinarily rich surface kinetics information. This innovative combination of electrical and chemical driving forces synergizes information depth, with enhanced time resolution, versatility and speed, yet it lifts the weaknesses of the individual approaches, while remaining cost-effective and surprisingly simple. Furthermore, BCR allows to disentangle overpotential induced electrostatic modifications of the surface kinetics in a unique manner. We showcase the advantages of BCR in this work by studying the ORR in model (La,Sr)FeO$_{3-{\delta}}$ thin film electrodes and reporting on their thermodynamic and kinetic properties.

cond-mat.mtrl-sci↗

arXiv2025

Fast track to the overdoped regime of superconducting YBa2Cu3O7-{\delta} thin films via electrochemical oxidation

High temperature superconductors, especially YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO), are considered a key enabling technology towards a clean energy future. Hole doping in YBCO is a prerequisite for the emergence of its unchallenged superconducting properties. Up to now, research was focused on the under- and optimally doped region, due to practical limitations in reaching the overdoped state, despite being highly interesting from fundamental and applied aspects as competing orders vanish and critical current densities are expected to peak. Here, we deploy for the first time an electrochemical method to access the mostly uncharted overdoped region. We demonstrate precise control over the bulk oxygen concentration in YBCO thin films across the full off-stoichiometry window (0$\le{\delta}\le$1) using electrochemical oxidation combined with in situ XRD and electrical measurements. Resulting high doping states and critical current densities are confirmed using a multi modal approach, including x-ray diffraction, electrical, Hall and magnetic characterization. Thus, this work opens a promising pathway based on electrochemical oxidation towards electronically clean, oxygen overdoped cuprate superconductors and therefore will assist to further push the critical current density to its intrinsic limit.

cond-mat.supr-con↗

arXiv2025

Investigating oxides by electrochemical projection of the oxygen off-stoichiometry diagram onto a single sample

The oxygen stoichiometry is an essential key to tune functional properties of advanced oxide materials and thus has motivated numerous studies of the oxygen off-stoichiometry diagram, with the aim to determine and control structural, electronic, ionic, electrochemical and optical properties, as well as thermodynamic quantities such as the oxygen storage capacity, among others. Here, a novel approach is developed, which allows to project a broad range of oxygen chemical potentials onto a single thin film sample with unprecedented control via electrochemical polarization. Therefore, a specifically designed electrochemical cell geometry is deployed, resulting in a well-defined, linear, 1D in-plane oxygen concentration gradient, independent of variations in the materials electrical resistivity, whose endpoints can be flexibly controlled via the external pO2 and applied overpotential. This allows for an unparalleled study of materials properties as a continuous function of the oxygen content using spatially resolved tools (spectroscopic, diffraction, microscopy, local electrical probes, etc.) and thereby greatly reduces experimental efforts while also avoiding sample-to-sample variability, multi-step treatments, sample evolution effects, etc. This work presents the proof-of-concept of in-plane oxygen gradients, based on spatially resolved ex/in situ and novel fixed-energy X-ray absorption near edge spectroscopy (XANES), X-ray diffraction, ellipsometry and electrical resistivity measurements in hyper-stoichiometric La2NiO4+{\delta} and sub-stoichiometric (La,Sr)FeO3-{\delta} thin films. It thereby demonstrates the readiness and wide applicability of this innovative approach, which can be highly relevant for fundamental as well as applied research.

cond-mat.mtrl-sci↗

arXiv2025

Oxygen incorporation in YBa$_2$Cu$_3$O$_{7-\delta}$ thin films: surface activation and degradation

The oxygen off-stoichiometry plays a pivotal role for the physical properties of superconducting oxides. Yet, there is a lack of knowledge on the fundamental processes of oxygen incorporation and correlated phenomena during oxygen post annealing treatments. Here, we deployed electrical probes ($\rho(T)$ and electrical conductivity relaxation (ECR) measurements) to gain a better understanding of the kinetics of the oxygen reduction reaction (ORR) in epitaxial YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO) thin films. We identified a new indicator for the onset temperature of oxygen incorporation and report for the first time drastic kinetic deactivation in bare YBCO. We demonstrate that surface decoration using silver micro islands both catalytically activates oxygen incorporation and bypasses surface degradation processes. Our results suggest that the ORR in the studied YBCO samples is limited by a surface reaction. Additionally, weak XRD signatures of the formation of extended bulk defects were identified, which have to be considered in the designing of an optimal oxygenation treatment to obtain best performing superconducting YBCO.

cond-mat.supr-con↗

arXiv2025

In situ Study of Phase Transitions in La$_2$NiO$_{4+\delta}$ using Raman Spectroscopy

La$_2$NiO$_{4+\delta}$ has attracted increasing interest in recent years, both as oxygen electrode in solid oxide fuel cells and electrolysers due to its high electrochemical activity at intermediate to high temperatures, and as key component of memristive devices for neuromorphic computing, owing to its variable oxygen stoichiometry. The integration of La$_2$NiO$_{4+\delta}$ into devices operating at different temperatures and oxygen partial pressures requires knowledge of the effects of hyper-stoichiometry ($\delta$) on its crystalline structure. La$_2$NiO$_{4+\delta}$ is known to accommodate oxygen at interstitial sites allowing for large delta values, up to ~ 0.16. In addition, the O-doping - temperature phase diagram is known to be complex, exhibiting several phase transitions with increasing delta. Herein, we use Raman spectroscopy to monitor the effects of O-doping in the phase diagram and the various structures it contains. Throughout this work, we studied this material in its usual ceramic form, as well as in the form of thin films. Results are discussed in terms of phase transitions, chemical expansion, and some of the possible consequences of the low mean grain size inherent to such thin films.

cond-mat.mtrl-sci↗

arXiv2024

Exploring the potential of combining over- and under-stoichiometric MIEC materials for Oxygen-Ion Batteries

The increasing demand for energy storage solutions has spurred intensive research into next-generation battery technologies. Oxygen-ion batteries (OIBs), which leverage mixed ionic-electronic conducting (MIEC) oxides, have emerged as promising candidates due to their solid, non-flammable nature and potential for high power densities. This study investigates the use of over-stoichiometric La2NiO4+delta (L2NO4) as a cathode material for OIBs, exploring its capacity for electrochemical energy storage. Half-cell measurements reveal that L2NO4 with a closed-pore microstructure can store oxygen, achieving a volumetric charge of 63 mA.h.cm-3 at 400 {\deg}C with a current density of 3.6 uA.cm-2 and potentials up to 0.75 V vs. 1 bar O2. Additionally, a functional full cell combining over-stoichiometric L2NO4 and under-stoichiometric La0.5Sr0.5Cr0.2Mn0.8O3-delta (LSCrMn) has been successfully developed, demonstrating excellent cyclability and coulomb efficiency. The full cell reaches a maximum volumetric charge of 90 mA.h.cm-3 at 400 {\deg}C, 17.8 uA.cm-2, and a cut-off voltage of 1.8 V. This proof of concept underscores the viability of combining over- and under-stoichiometric MIEC materials in OIBs and provides critical insights into optimizing electrode materials and tuning oxygen content for improved performance. This research lays the groundwork for future advancements in OIB technology, aiming to develop materials with lower resistance and higher efficiency.

cond-mat.mtrl-sci↗

arXiv2024

Real time observation of oxygen diffusion in CGO thin films using spatially resolved Isotope Exchange Raman Spectroscopy

The exploitation of advanced materials for novel energy, health and computing applications requires fundamental understanding of enabling physicochemical mechanisms, including ionic and electronic conductivity, defect formation processes and reaction kinetics. Therefore, access to underlying constants of functional materials via advanced but straightforward experimental techniques is key. We present a novel, cheap and widely applicable approach to analyze oxygen-tracer-diffusion in thin films with unprecedented time resolution based on the novel in situ isotope-exchange Raman spectroscopy (IERS) methodology. Raman spectroscopy is sensitive to changes in the local isotopic composition, manifested by a frequency shift of the oxygen Raman modes. Employing a Raman transparent capping layer allows to establish an in-plane tracer gradient and follow the isotope exchange and diffusion processes via consecutive spatial and time resolved in situ Raman line scans. Mass-transport coefficients are calculated from these isotopic gradients, similar to conventional techniques, but with an additional time-component, not accessible by other techniques. Here, we study gadolinium doped ceria (CGO) thin films, capped with Si3N4 or Al2O3. We report diffusion coefficients within the temperature range of interest for intermediate temperature emerging applications (300-500{\deg}C) and confirm the validity of the measurement procedure and extracted parameters by comparison with FEM simulations and literature results.

cond-mat.mtrl-sci↗

arXiv2023

Isotope Exchange Raman Spectroscopy (IERS): a novel technique to probe physicochemical processes $in$ $situ$

We have developed a novel in situ methodology for the direct study of mass transport properties in oxides with spatial and unprecedented time resolution, based on Raman spectroscopy coupled to isothermal isotope exchanges. Changes in the isotope concentration, resulting in a Raman frequency shift, can be followed in real time, not accessible by conventional methods, enabling complementary insights for the study of ion transport properties of electrode and electrolyte materials for advanced solid-state electrochemical devices. The proof of concept and strengths of isotope exchange Raman spectroscopy (IERS) are demonstrated by studying the oxygen isotope back-exchange in gadolinium-doped ceria (CGO) thin films. Resulting oxygen self-diffusion and surface exchange coefficients are compared to conventional time-of-flight secondary ion mass spectrometry (ToF-SIMS) characterisation and literature values, showing good agreement, while at the same time providing additional insight, challenging established assumptions. IERS captivates through its rapidity, simple setup, non-destructive nature, cost effectiveness and versatile fields of application and thus can readily be integrated as new standard tool for in situ and operando characterization in many laboratories worldwide. The applicability of this method is expected to consolidate our understanding of elementary physicochemical processes and impact various emerging fields including solid oxide cells, battery research and beyond.

cond-mat.mtrl-sci↗

arXiv2021

Tailored nano-columnar La$_2$NiO$_4$ cathodes for improved electrode performance

La$_2$NiO$_4$ is a very promising cathode material for intermediate and low temperature solid oxide cell applications, due to its good electronic and ionic conductivity, together with its high oxygen exchange activity with a low activation energy. Oxygen incorporation and transport in La$_2$NiO$_4$ (L2NO4) thin films is limited by surface reactions. Hence, tailoring the morphology is expected to lead to an overall improvement of the electrode performance. We report on the growth of nano-architectured La$_2$NiO$_4$ thin film electrodes by Pulsed Injection Metal Organic Vapour Deposition (PI-MOCVD), achieving vertically gapped columns with multi-fold active surface area, leading to much faster oxygen exchange. This nano-columnar structure is rooted in a dense bottom layer serving as good electronic and ionic conduction pathway. The microstructure is tuned by modification of the growth temperature and characterised by SEM, TEM and XRD. We studied the effect of surface activity by electrical conductivity relaxation measurements in fully dense and nano-columnar La$_2$NiO$_4$ thin films of various thicknesses grown on several different single crystal substrates. Our results demonstrate that the increased surface area, in combination with the opening of different surface terminations, leads to a significant enhancment of the total exchange activity in our films with optimized nano-architectured microstructure.

cond-mat.mtrl-sci↗

arXiv2020

$In$ $situ$ and $operando$ characterisation techniques for solid oxide electrochemical cells: Recent advances

Oxygen activity and surface stability are two key parameters in the search for advanced materials for intermediate temperature solid oxide electrochemical cells, as overall device performance depends critically on them. In particular $in$ $situ$ and $operando$ characterisation techniques have accelerated the understanding of degradation processes and the identification of active sites, motivating the design and synthesis of improved, nanoengineered materials. In this short topical review we report on the latest developments of various sophisticated $in$ $situ$ and $operando$ characterization techniques, including Transmission and Scanning Electron Microscopy (TEM and SEM), surface-enhanced Raman spectroscopy (SERS), Electrochemical Impedance Spectroscopy (EIS), X-ray Diffraction (XRD) and synchrotron based X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS), among others. We focus on their use in three emerging topics, namely: (i) the analysis of general electrochemical reactions and the surface defect chemistry of electrode materials; (ii) the evolution of electrode surfaces achieved by nanoparticle exsolution for enhanced oxygen activity and (iii) the study of surface degradation caused by Sr segregation, leading to reduced durability. For each of these topics we highlight the most remarkable examples recently published. We anticipate that ongoing improvements in the characterisation techniques and especially a complementary use of them by multimodal approaches will lead to improved knowledge of $operando$ processes, hence allowing a significant advancement in cell performance in the near future.

cond-mat.mtrl-sci↗