arXiv subjects

András Kovács

Publications and source records attributed to András Kovács.

At least 19 recordsLinked to original sources

arXiv2026

Compositional and Magnetic Characterisation of Oblique Co and Fe Nanowire Structures Fabricated Using Focused Electron Beam Induced Deposition

Focused electron beam induced deposition (FEBID) is an additive manufacturing technique uniquely suited for fabricating functional 3D nanostructures for a range of applications, including spintronic devices. However, the variation of growth dynamics associated with lateral movement of the electron beam results in structures with non-uniform composition when fabricating intricate 3D geometries. Herein, we measure changes in atomic composition and corresponding changes in magnetic induction in 3D ferromagnetic nanostructures with overhanging elements, e.g. bridges or arches. To investigate the effects of electron beam lateral movement, we fabricated Co and Fe nanowire (NW) structures with growth angle relative to the optic axis varying from 0° to 90°. The (scanning) transmission electron microscopy techniques of electron energy loss spectroscopy and off-axis electron holography were performed to map the NW elemental composition and magnetic induction as a function of NW growth angle. Comparison of the results reveals a reduction in metal content with increased growth angle in FEBID NWs. The magnitude of metal content reduction can be tuned by controlling electron beam parameters, and ferromagnetic NWs with approximately equal metal content at growth angles from 0° to 60° were fabricated by using the lowest viable electron beam voltage and the highest viable beam current to reduce the interaction volume and increase the metal content, respectively.

cond-mat.mes-hall↗

arXiv2026

Realization of ZnSe-based Field-Effect Transistors operating at Cryogenic Temperatures as a Platform for Future Spin-Qubit Applications

The wide-bandgap compound semiconductor ZnSe is a promising host material for the realization of electron spin-qubits. Its non-degenerate conduction band and the potential for isotopic nuclear spin purification promise long spin coherence times. Key requirements for such devices include reliable electrostatic control of electrons in ZnSe and low-resistance ohmic contacts that remain functional at cryogenic temperatures. In this work, we utilize a novel Shadow Wall technique for molecular-beam epitaxy combined with in-situ deposition of Al ohmic contacts to realize normally-off ZnSe-based field-effect transistors. The devices exhibit linear output characteristics and effective gate control of the channel from room temperature down to 5 K, confirming low-resistance ohmic contacts to the undoped ZnSe channel. The drain current can be modulated by several orders of magnitude through electrostatic gating, with threshold voltages of approximately 3 V and field-effect mobilities exceeding 100 cm2/Vs over the investigated temperature range. Self-consistent Schrödinger-Poisson and drift-diffusion simulations reproduce the measured transfer characteristics and provide insight into the role of interface electrostatics in determining the channel formation and threshold voltage. These results demonstrate the potential of gated ZnSe heterostructures for future spin-qubit applications.

cond-mat.mtrl-sci↗

arXiv2026

Retrieving Biblical Intertextual References in Karen Blixen's Seven Gothic Tales

Identifying intertextual references is central to literary scholarship, but computationally difficult when source material is transformed through paraphrase, allusion, historical language, and translation. We investigate this problem through biblical intertextuality in Karen Blixen's Seven Gothic Tales. Drawing on the commentary to a critical edition, we construct a benchmark of 189 annotated references and evaluate retrieval against all 31,170 verses of historically plausible Danish Old and New Testament translations. We compare TF-IDF and BM25 with multilingual and Danish sentence encoders, examine the effect of linguistic normalization, and fine-tune a Danish encoder using hard negatives and five-fold cross-validation. We analyze performance across automatically derived lexical-overlap strata representing quotations, paraphrases, and allusions. Linguistically normalized BM25 provides a strong zero-shot baseline, attaining an overall R@10 of 0.365 and retrieving every quotation within its ten highest-ranked verses. The best zero-shot dense model achieves a comparable overall score of 0.360 while performing better on allusions. Fine-tuning DFM-large raises its overall R@10 from 0.265 to 0.508 and more than doubles its performance on allusions, from 0.138 to 0.339. However, evaluation against editorial annotations alone understates the model's scholarly usefulness: a literary scholar judged seven of 30 selected rank-one predictions counted as false positives to be meaningful additional references. These findings show both the potential and the epistemic limits of computational intertextual retrieval. Rather than treating scholarly annotations as exhaustive or model outputs as discoveries, we propose retrieval models as heuristic co-readers that recover documented references and generate candidates for expert-led close reading.

cs.CL↗

arXiv2026

Towards precision cosmology with Voids x CMB correlations (I): Roman-Agora mock catalogs and pipeline validation

We construct and validate a set of multi-purpose mock galaxy catalogs designed to capture, to different degrees of accuracy, the main characteristics of the Nancy Grace Roman Space Telescope survey. These catalogs provide a foundation for void statistics and various CMB cross-correlation analyses. Our approach differs from traditional halo occupation or abundance matching methods by directly translating a reference mock catalog -- containing basic properties of the host halos -- into a new simulation (in our case Agora). This technique, which we call analog matching, assigns a halo counterpart in the new simulation to each reference galaxy through a nearest-neighbor search in a multi-dimensional parameter space. This space can include halo mass, environmental measures and other galaxy-specific attributes. By varying the composition of this parameter vector, we can generate catalogs of differing complexity and test how galaxy-halo prescriptions influence LSS statistics and CMB-related observables. We find that analog matching based on halo mass alone, or halo mass and galaxy-type indicators, successfully reproduces the expected Roman emission-line galaxy statistics. We also show that reproducing two-dimensional galaxy clustering does not guarantee consistent void properties. Our results highlight the importance of matching void statistics for improved mock accuracy, and demonstrate that measuring voids provides independent and sensitive constraints on galaxy-halo connections beyond the matter power spectrum. An important by-product of our setup is that it is fully general and can be applied to any combination of simulation and reference catalog, provided that the desired parameter space for both is specified. The resulting Roman-Agora mock catalogs offer a versatile resource for LSS x CMB studies and a benchmark for assessing the impact of mock accuracy on cosmological observables.

astro-ph.CO↗

arXiv2026

Towards precision cosmology with Void x CMB correlations (II): Impact of mock catalogs on the Void x CMB lensing signal

Gravitational lensing by large-scale structure imprints secondary anisotropies on the Cosmic Microwave Background (CMB) that can be exploited to probe cosmology. In particular, cosmic voids produce a characteristic lensing signature detectable through Void x CMB cross-correlations. This signal has been robustly measured in the past but its cosmological constraining power remains limited by the incomplete knowledge of how methodological choices affect its measurement and by its uncertain dependence on cosmological parameters. Using a set of validated Roman mock catalogs, we first quantify how mock construction impacts the measured signal and then forecast the capabilities of Roman, in combination with current and upcoming CMB surveys such as Planck, SO and CMB-S4-like experiments. We analyze the signal-to-noise ratio (S/N) for different void definitions (2D and 3D), stacking approaches (rescaled versus non-rescaled profiles), CMB map filtering schemes and noise levels. In contrast to galaxy and void statistics, we find that the Void x CMB lensing signal is less sensitive to the choice of mock catalog, indicating that future tensions with data are unlikely to stem from mock inaccuracies alone. The highest S/N is achieved for 2D voids with rescaled profiles. We forecast S/N ~13$σ$ (8$σ$) for 2D (3D) Roman voids combined with Planck, increasing to 22$σ$ (13$σ$) for SO and 31$σ$ (18$σ$) for CMB-S4-like surveys. While the cosmological dependence of this observable remains to be quantified, Roman together with next-generation of LSS and CMB surveys opens a path toward the first direct cosmological constraints from Void x CMB lensing.

astro-ph.CO↗

arXiv2026

Revisiting the angular size-redshift cosmological test with milliarcsecond radio structures in active galactic nuclei

VLBI measurements of the sizes of compact extragalactic radio sources, jetted active galactic nuclei, provide data for probing the angular size--redshift relation, offering a complementary test to other distance--redshift methods. We analyse a significantly expanded dataset to reassess the angular size--redshift relation and its potential for constraining cosmological model parameters, focusing on the matter density parameter $Ω_{\mathrm{m}}$ in a flat $Λ$ Cold Dark Matter Universe. This is the first major update of the compact-source angular size test in the past quarter of a century, using a dataset an order of magnitude larger than in previous studies. MCMC analysis on real data and on multiple mock catalogues to evaluate parameter constraints in the presence of observational scatter. In addition, we conducted a test with 100 randomized catalogues created by shuffling redshifts while preserving other observables. We also explored how astrophysical parameters depend on fixed cosmological models with different $Ω_{\mathrm{m}}$ values. The randomization test showed that the posterior distributions from randomized data do not overlap with those from real observations, confirming that the measured angular size--redshift relation is physically meaningful. The astrophysical model parameter that describes the redshift dependence of the source angular size exhibits degeneracy with $Ω_{\mathrm{m}}$. Simulated mock catalogues indicate that the method is able to constrain $Ω_{\mathrm{m}}$ if the data scatter is below $\sim20\%$. Scaling estimates suggest that high-quality data of samples of several thousands to $\sim 100\,000$ sources, a standardisation calibration approach, and/or refining sample selection criteria are needed to fully exploit the potential of the angular size--redshift test with this type of objects (abridged).

astro-ph.CO↗

arXiv2026

A Path to an All-Sky Survey with Roman

A deep, space-based, all-sky near-infrared survey carried out with the Nancy Grace Roman Space Telescope would constitute a foundational astronomical infrastructure for decades to come. In this white paper, we present a concrete and feasible path to imaging the entire sky at $\sim0.1''$ resolution, beginning with high-impact fields in Cycle 1 and scaling to ultra-wide coverage within the nominal mission. This first-epoch survey will reach $\mathrm{H}\sim25.5$ AB mag (5$σ$) and maximize synergies with contemporaneous observatories, while preserving substantial time for other ambitious Roman programs. We outline representative scheduling scenarios and an example Cycle 1 program that triples early Roman-LSST overlap and delivers high-value community data products such as LSST forced photometry, joint \textit{Gaia}-Roman astrometry, and catalogs of Galactic substructure, stong lenses, and other rare systems. The Cycle 1 program will lay the foundation for an eventual all-sky survey, while also delivering high-impact early science. We invite broad community participation in shaping and carrying out both the initial program and the long-term vision of an all-sky Roman survey.

astro-ph.IM↗

arXiv2025

Integrated Sachs-Wolfe maps from the Gower Street $w$CDM simulations

The late-time linear Integrated Sachs-Wolfe (ISW) effect directly probes the dynamics of cosmic acceleration and the nature of dark energy. Detecting these weak, secondary temperature anisotropy signals of the CMB requires accurate theoretical predictions of their amplitude across cosmological models. By extending the pyGenISW package, previously limited to $Λ$CDM, we aim to generate full-sky ISW maps for a suite of 791 $w$CDM cosmologies using the Gower Street N-body simulations, thereby enabling ISW analyses across a broader dark-energy parameter space. We make our code and ISW data publicly available. We compute the ISW signals by tracing the time evolution of the gravitational potential across large-volume simulations that span dark energy equation of state parameters from phantom to quintessence, $-1.79 \lesssim w \lesssim -0.34$. These data are projected onto the sphere using HEALPix to obtain full-sky temperature maps. We validate our pipeline by comparing the measured ISW angular power spectra and ISW-density cross-correlations against linear theory expectations ($2 \leq \ell \leq 200$) computed with benchmarks from the pyCCL library. The agreement is excellent across the multipole range where the ISW contribution is expected to dominate, confirming the reliability of our modelling of gravitational-potential evolution. With additional tests of the ISW signal's strength in density extrema, as well as comparing all models to a reference $Λ$CDM cosmology, we found that quintessence-like models ($w > -1$) show higher ISW amplitudes than phantom models ($w < -1$), consistent with enhanced late-time decay of gravitational potentials. The consistency of our $w$CDM ISW maps and their agreement with theory predictions confirm the robustness of our methodology, establishing it as a reliable tool for theoretical and observational ISW-LSS analyses.

astro-ph.CO↗

arXiv2025

Identifying grain boundary and intragranular pinning centres in Sm2(Co,Fe,Cu,Zr)17 permanent magnets to guide performance optimisation

Permanent magnets draw their properties from a complex interplay of chemical composition and phase, each with their associated intrinsic magnetic properties. Gaining an understanding of these interactions is the key to deciphering the origins of a permanent magnets' magnetic performance and facilitate the engineering of much improved-performing magnets. Here, we use advanced multiscale microscopy and microanalysis on a bulk Sm2(Co,Fe,Cu,Zr)17 pinning-type high-performance magnet with outstanding thermal and chemical stability. Comparison of the microstructure in regions of different composition, we demonstrate that the pinning of magnetic domains, imaged by nanoscale magnetic induction mapping, is controlled by the composition and atomic arrangement of copper. This is confirmed by micromagnetic simulations. Contrary to the belief that grain boundaries are "weak links" in magnetic materials, we demonstrate grain boundaries undergo magnetization reversal at relatively low fields (0.1-0.3 T), but this remains confined to these regions and does not significantly impact the magnet's coercivity. Our results showcase that it is the optimal microstructure within the grain itself that is crucial for achieving the desired magnetic properties.

cond-mat.mtrl-sci↗

arXiv2025

Direct observation of nanoscale pinning centers in Ce(Co0.8Cu0.2)5.4 permanent magnets

Permanent magnets containing rare earth elements are essential components for the electrification of society. Ce(Co1-xCux)5 permanent magnets are a model system known for their substantial coercivity, yet the underlying mechanism remains unclear. Here, we investigate Ce(Co0.8Cu0.2)5.4 magnets with a coercivity of ~1 T. Using transmission electron microscopy (TEM) and atom probe tomography (APT), we identify a nanoscale cellular structure formed by spinodal decomposition. Cu-poor cylindrical cells (~5-10 nm in diameter, ~20 nm long) have a disordered CeCo5-type structure and a composition Ce(Co0.9Cu0.1)5.3. Cu-rich cell boundaries are ~ 5 nm thick and exhibit a modified CeCo5 structure, with Cu ordered on the Co sites and a composition Ce(Co0.7Cu0.3)5.0. Micromagnetic simulations demonstrate that the intrinsic Cu concentration gradients up to 12 at.% Cu/nm lead to a spatial variation in magnetocrystalline anisotropy and domain wall energy, resulting in effective pinning and high coercivity. Compared to Sm2Co17-type magnets, Ce(Co0.8Cu0.2)5.4 displays a finer-scale variation of conventional pinning with lower structural and chemical contrast in its underlying nanostructure. The identification of nanoscale chemical segregation in nearly single-phase Ce(Co0.8Cu0.2)5.4 magnets provides a microstructural basis for the long-standing phenomenon of "giant intrinsic magnetic hardness" in systems such as SmCo5-xMx, highlighting avenues for designing rare-earth-lean permanent magnets via controlled nanoscale segregation.

cond-mat.mtrl-sci↗

arXiv2025

Radio-loudness statistics of quasars from Quaia-VLASS

Quasars are objects of high interest in extragalactic astrophysics, cosmology, and astrometry. One of their useful qualities is their potential radio loudness. However, the fraction of radio-loud vs. radio-quiet quasars is subject to ongoing investigations, where the statistical power is limited by the low number of known quasars with radio counterparts. In this analysis, we revisited the radio loudness statistics of quasars by significantly expanding the pool of known sources. Our main goal was to create a new, value-added quasar catalog with information about their extinction-corrected magnitudes, radio flux density, possible contamination levels, and other flags, besides their sky coordinates and photometric redshifts. We cross-matched the optical Quaia catalog of about 1.3 million quasars (from the Gaia data) with 1.9 million sources from the Very Large Array Sky Survey (VLASS) radio catalog. We explored different thresholds for the matching radius, balancing completeness and purity of the resulting Quaia-VLASS catalog, and found 1.5 arcseconds a sufficient choice. Our main finding is that the quasar radio-loud fraction is in good agreement with previous works (< 10%), and there is no significant large-scale sky pattern in radio loudness. The exact estimate depends on the G-band magnitude limit, and we observed weak trends with redshift and absolute optical magnitude, possibly indicating remnant systematic effects in our data. The cross-matched Quaia-VLASS catalog with 43,650 sources and accompanying code are freely available at doi:10.5281/zenodo.16035690. This latest census of QSOs with radio counterparts will facilitate further investigations of the dichotomy of radio-loud and radio-quiet quasars, and it may also support other lines of research.

astro-ph.GA↗

arXiv2025

The CosmoVerse White Paper: Addressing observational tensions in cosmology with systematics and fundamental physics

The standard model of cosmology has provided a good phenomenological description of a wide range of observations both at astrophysical and cosmological scales for several decades. This concordance model is constructed by a universal cosmological constant and supported by a matter sector described by the standard model of particle physics and a cold dark matter contribution, as well as very early-time inflationary physics, and underpinned by gravitation through general relativity. There have always been open questions about the soundness of the foundations of the standard model. However, recent years have shown that there may also be questions from the observational sector with the emergence of differences between certain cosmological probes. In this White Paper, we identify the key objectives that need to be addressed over the coming decade together with the core science projects that aim to meet these challenges. These discordances primarily rest on the divergence in the measurement of core cosmological parameters with varying levels of statistical confidence. These possible statistical tensions may be partially accounted for by systematics in various measurements or cosmological probes but there is also a growing indication of potential new physics beyond the standard model. After reviewing the principal probes used in the measurement of cosmological parameters, as well as potential systematics, we discuss the most promising array of potential new physics that may be observable in upcoming surveys. We also discuss the growing set of novel data analysis approaches that go beyond traditional methods to test physical models. [Abridged]

astro-ph.CO↗

arXiv2025

Model-Based Iterative Reconstruction of Three-Dimensional Magnetisation in a Nanowire Structure Using Electron Holographic Vector Field Tomography

Methods for characterisation of 3D magnetic spin structures are necessary to advance the performance of 3D magnetic nanoscale technologies. However, as the component dimensions approach the nanometre range, it becomes more challenging to analyse 3D magnetic configurations with the appropriate spatial resolution. In this paper, we present a method based on Lorentz transmission electron microscopy in which model-based iterative reconstruction (MBIR) is used to reconstruct the most probable magnetisation in an exemplar nanostructure. This method is based on relating electron phase measurements to the magnetic configuration of the nanostructure, and therefore, the method is subject to certain limitations. In this proof-of-concept experiment, MBIR was tested on an L-shaped ferromagnetic cobalt nanowire, fabricated using focused electron beam induced deposition. Off-axis electron holography was used to acquire a tomographic tilt series of electron holograms, which were analysed to measure magnetic electron phase shift over two tilt arcs with up to $ \pm 60$ degree tilt range. Then, a 3D magnetisation vector field consistent with the tomographic phase measurements was reconstructed, revealing multiple magnetic domains within the nanowire. The reconstructed magnetisation is accurate for magnetic domains larger than 50 nm, and higher resolution can be achieved by the continued development of tomographic reconstruction algorithms.

cond-mat.mes-hall↗

arXiv2025

All-optical stochastic switching of magnetization textures in Fe$_3$Sn$_2$

The all-optical control of magnetization at room temperature broadens the scope of applications of spin degrees-of-freedom in data storage, spintronics, and quantum computing. Topological magnetic spin structures, such as skyrmions, are of particular interest due to their particle-like properties, small size and inherent stability. Controlling skyrmion states without strong magnetic fields or large current densities would create new possibilities for their application. In this work, we utilize femtosecond optical pulses to alter the helicity of the spin configuration in dipolar skyrmions formed in the kagome magnet Fe$_3$Sn$_2$ in the absence of an external magnetic field and at room temperature. In situ Lorentz transmission electron microscopy is used to visualize the stochastic, light-induced switching process of chiral Néel caps, while the internal Bloch component of the dipolar skyrmions remain unchanged. In addition to this switching process, we observe the interconversion between type I skyrmionic and type II bubble configurations depending on the external magnetic field and illumination conditions. To corroborate the spin states and the light-induced magnetization dynamics, micromagnetic modelling and simulations of the resulting electron phase shift maps are conducted to elucidate the spin rearrangement induced by individual femtosecond optical pulses.

physics.optics↗

arXiv2023

Type-Theoretic Signatures for Algebraic Theories and Inductive Types

We develop the usage of certain type theories as specification languages for algebraic theories and inductive types. We observe that the expressive power of dependent type theories proves useful in the specification of more complicated algebraic theories. We describe syntax and semantics for three classes of algebraic theories: finitary quotient inductive-inductive theories, their infinitary generalization, and finally higher inductive-inductive theories. In each case, an algebraic signature is a typing context or a closed type in a specific type theory.

cs.LO↗

arXiv2023

Magnetic Phase Imaging using Lorentz Near-field Electron Ptychography

Over the past few years, the combination of diffuser and near-field electron ptychography has drawn more attention by its ability to recover large field of view with few diffraction patterns. In this paper, we purpose a novel design and implementation of amplitude diffuser. The amplitude diffuser introduces structures to the illumination while reducing the inelastic scattering. And the amplitude diffuser is implemented at the condenser lens aperture, allowing us to vary the illumination size under the same microscope setup. We demonstrate the reconstruction results under both conventional Transmission Electron Microscopy (TEM) mode as well as Lorentz mode.

cond-mat.mtrl-sci↗

arXiv2023

Element-Specific Study of Magnetic Anisotropy and Hardening in SmCo$_{5-x}$Cu$_{x}$ Thin Films

This work investigates the effect of copper substitution on the magnetic properties of SmCo$_{5}$ thin films synthesized by molecular beam epitaxy. A series of thin films with varying concentrations of Cu were grown under otherwise identical conditions to disentangle structural and compositional effects on the magnetic behavior. The combined experimental and theoretical studies show that Cu substitution at the Co$_{3g}$ sites not only stabilizes the formation of the SmCo$_{5}$ structure but enhances magnetic anisotropy and coercivity. Density functional theory calculations indicate that Sm(Co$_4$Cu$_{3g}$)$_5$ possesses a higher single-ion anisotropy as compared to pure SmCo$_{5}$. In addition, X-ray magnetic circular dichroism reveals that Cu substitution causes an increasing decoupling of the Sm 4\textit{f} and Co 3\textit{d} moments. Scanning transmission electron microscopy confirms predominantly SmCo$_{5}$ phase formation and reveals nanoscale inhomogeneities in the Cu and Co distribution. Our study based on thin film model systems and advanced characterization as well as modeling reveals novel aspects of the complex interplay of intrinsic and extrinsic contributions to magnetic hysteresis in rare earth-based magnets, \textit{i.e.} the combination of increased intrinsic anisotropy due to Cu substitution and the extrinsic effect of inhomogeneous elemental distribution of Cu and Co.

cond-mat.mtrl-sci↗

arXiv2022

Staged Compilation with Two-Level Type Theory

The aim of staged compilation is to enable metaprogramming in a way such that we have guarantees about the well-formedness of code output, and we can also mix together object-level and meta-level code in a concise and convenient manner. In this work, we observe that two-level type theory (2LTT), a system originally devised for the purpose of developing synthetic homotopy theory, also serves as a system for staged compilation with dependent types. 2LTT has numerous good properties for this use case: it has a concise specification, well-behaved model theory, and it supports a wide range of language features both at the object and the meta level. First, we give an overview of 2LTT's features and applications in staging. Then, we present a staging algorithm and prove its correctness. Our algorithm is "staging-by-evaluation", analogously to the technique of normalization-by-evaluation, in that staging is given by the evaluation of 2LTT syntax in a semantic domain. The staging algorithm together with its correctness constitutes a proof of strong conservativity of 2LLT over the object theory. To our knowledge, this is the first description of staged compilation which supports full dependent types and unrestricted staging for types.

cs.PL↗