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A. Paul

Publications and source records attributed to A. Paul.

18 recordsLinked to original sources
arXiv2026

Dynamic nanoscale structural correlations in strontium ruthenate

Strontium ruthenate (Sr$_2$RuO$_4$, SRO) has been the subject of intense research as a model quasi-two-dimensional metal with strong electronic correlations and potential exotic multi-component superconductor. Yet the nature of the superconducting state and its emergence remain debated, despite highly detailed knowledge of the normal-state electronic properties. Here we use diffuse neutron and x-ray scattering to uncover self-organized structural fluctuations on the picosecond timescale in SRO. We show that these nanoscale correlations do not originate from extrinsic disorder but rather involve cooperative displacements of oxygen atoms in the quintessential RuO$_2$ planes. Moreover, the observed displacement pattern is consistent with distortions due to incipient orbital order that we obtain in first-principles calculations, which suggests that orbital effects could play a pivotal role in the physics of SRO. Similar dynamic correlations may play a role in the physical properties of a wide range of prominent oxides with closely-related lamellar structures, such as the cuprates and nickelates.

cond-mat.str-el↗
arXiv2026

Sungrazer comets as analogs of star-planet magnetic interactions

Star-planet magnetic interactions (SPMIs) can transfer energy from an exoplanet to its host star via Alfv\'en waves when the planet orbits within a sub-Alfv\'enic stellar wind. Similar conditions were encountered by the sungrazing comet Lovejoy as it passed through the solar corona in December 2011. The possibility that comets could trigger solar activity via magnetic interactions has never been investigated. The aim of this paper is to quantify the energetics of such interaction and to assess if enough energy could be deposited to either form a hotspot or trigger eruptions on the Sun. We used the magnetohydrodynamic WindPredict-AW model to reconstruct the coronal magnetic field and solar wind conditions along the comet's orbit, and determined the magnetic footpoints that connected the comet to the solar surface. By estimating the travel time of hypothetical Alfv\'en waves, we identified a brightening event observed by EUVI aboard STEREO-A that is spatially and temporally consistent with comet Lovejoy's passage. We then computed the SPMI power for all magnetic field lines anchored within 5 degrees of the brightening using SPMI power scaling laws. The resulting power distribution spans approximately 10$^{14}$-10$^{16}$ W. In comparison, the estimated radiative power of the brightening event in the STEREO-A/EUVI 195$\r{A}$ channel is approximately 10$^{17}$ W. We find that comet Lovejoy does not generate sufficient SPMI power to be the energy source of the observed brightening intensity. However, it may still act as a perturbation to existing magnetic structures, and lead to the triggering of solar flares. Confirming this hypothesis would require additional observations that do not exist for this event, which make future passages of sungrazing comets valuable opportunities to study SPMI processes within our solar system.

astro-ph.SR↗
arXiv2025

Detecting and characterising the magnetic field of exoplanets

Magnetic fields play a crucial role in planetary evolution and habitability. While the intrinsic magnetic fields of solar system planets are relatively well understood, the magnetic properties of exoplanets remain largely unconstrained, despite their potential ubiquity. Detecting exoplanetary magnetic fields is essential to advancing our understanding of planetary habitability beyond the solar system. This paper focuses on two promising spectropolarimetric techniques for detecting magnetic fields in hot exoplanets: direct detection through polarization signatures in the He I 1083 nm triplet and indirect detection via star-planet magnetic interactions manifesting as stellar hot spots. The direct method is particularly suited to close-in gas giants, leveraging the Hanle and Zeeman effects to detect low-amplitude magnetic fields. The indirect method can apply to both giant and low-mass planets by identifying magnetic connectivity-induced features in the stellar atmosphere. Although the interpretation of current detections remain tentative, upcoming high-resolution spectropolarimetric capabilities in the UV and near-infrared, particularly with future missions like HWO, promise to enable definitive measurements of exoplanetary magnetic fields. These advancements will open new avenues for probing the magnetic environments of exoplanets and their implications for atmospheric retention and habitability.

astro-ph.IM↗
arXiv2025

Further exploration of binding energy residuals using machine learning and the development of a composite ensemble model

This paper describes the development of the Four Model Tree Ensemble (FMTE). The FMTE is a composite of machine learning models trained on experimental binding energies from the Atomic Mass Evaluation (AME) 2012. The FMTE predicts binding energy values for all nuclei with N > 7 and Z > 7 from AME 2020 with a standard deviation of 76 keV and a mean average deviation of 34 keV. The FMTE model was developed by combining three new models with one prior model. The new models presented here have been trained on binding energy residuals from mass models using four machine learning approaches. The models presented in this work leverage shape parameters along with other physical features. We have determined the preferred machine learning approach for binding energy residuals is the least-squares boosted ensemble of trees. This approach appears to have a superior ability to both interpolate and extrapolate binding energy residuals. A comparison with the masses of isotopes that were not measured previously and a discussion of extrapolations approaching the neutron drip line have been included.

nucl-th↗
arXiv2024

Ohmic heating in the upper atmosphere of hot exoplanets The influence of a time-varying magnetic field

Exoplanets on close-in orbit are subject to intense X-ray and ultraviolet (XUV) irradiation from their star. Their atmosphere therefore heats up, sometimes to the point where it thermally escapes from the gravitational potential of the planet. Nonetheless, XUV is not the only source of heating in such atmospheres. Indeed, close-in exoplanets are embedded in a medium (the stellar wind) with strong magnetic fields that can significantly vary along the orbit. The variations of this magnetic field can induce currents in the upper atmosphere, which dissipate and locally heat it up through Ohmic heating. The aim of this work is to quantify Ohmic heating in the upper atmosphere of hot exoplanets due to an external time-varying magnetic field, and to compare it to the XUV heating. Ohmic heating depends strongly on the conductivity properties of the upper atmosphere. A 1D formalism is developed to assess the level and the localization of Ohmic heating depending on the conductivity profile, and applied to the specific cases of Trappist-1 b and $\pi$ Men c. Ohmic heating can reach values up to 10$^{-3}$ erg s$^{-1}$ cm$^{-3}$ in the upper atmospheres of hot exoplanets. It is expected to be stronger the closer the planet is and the lower the central star mass is, as these conditions maximize the strength of the ambient magnetic field around the planet. We confirm that Ohmic heating can play an important role in setting the thermal budget of the upper atmosphere of hot exoplanets, and can even surpass the XUV heating in the most favorable cases. When it is strong, a corollary is that the upper atmosphere screens efficiently time-varying external magnetic fields, preventing them to penetrate deeper in the atmosphere or inside the planet itself. We find that both Trappist-1b and $\pi$ Men c are likely subject to intense Ohmic heating.

astro-ph.EP↗
arXiv2021

CutLang V2: towards a unified Analysis Description Language

We will present the latest developments in CutLang, the runtime interpreter of a recently-developed analysis description language (ADL) for collider data analysis. ADL is a domain-specific, declarative language that describes the contents of an analysis in a standard and unambiguous way, independent of any computing framework. In ADL, analyses are written in human-readable plain text files, separating object, variable and event selection definitions in blocks, with a syntax that includes mathematical and logical operations, comparison and optimisation operators, reducers, four-vector algebra and commonly used functions. Adopting ADLs would bring numerous benefits to the LHC experimental and phenomenological communities, ranging from analysis preservation beyond the lifetimes of experiments or analysis software to facilitating the abstraction, design, visualization, validation, combination, reproduction, interpretation and overall communication of the analysis contents. Since their initial release, ADL and CutLang have been used for implementing and running numerous LHC analyses. In this process, the original syntax from CutLang v1 has been modified for better ADL compatibility, and the interpreter has been adapted to work with that syntax, resulting in the current release v2. Furthermore, CutLang has been enhanced to handle object combinatorics, to include tables and weights, to save events at any analysis stage, to benefit from multi-core/multi-CPU hardware among other improvements. In this contribution, these and other enhancements are discussed in details. In addition, real life examples from LHC analyses are presented together with a user manual.

hep-ph↗
arXiv2020

Pore morphology evolution and atom distribution of doped Fe2O3 foams developed by freeze-casting after redox cycling

Chemical looping water splitting systems operate at relatively high temperatures (450-800 degree C) to produce, purify, or store hydrogen by the cyclic reduction and oxidation (redox) of a solid oxygen carrier. Therefore, to improve long-term operation, it is necessary to develop highly stable oxygen carriers with large specific surface areas. In this work, highly interconnected doped Fe2O3 foams are fabricated through the freeze-casting technique, and the aid of a submicrometric camphene-based suspension to prevent Fe sintering and pore clogging during redox operation. The influence of the dopant elements (Al and Ce) over the pore morphology evolution, and redox performances are examined. The use of an Fe2O3 porous structure with initial pore size above 100 microns shows a significant reduction of the sample densification, and the addition of Al2O3 by the co-precipitation process proves to be beneficial in preventing the generation of a core-shell structure following redox processing.

cond-mat.mtrl-sci↗
arXiv2020

Effect of processing parameters on the properties of freeze-cast Ni wick with gradient porosity

Wicks are the main component of Loop Heat Pipe systems, whereby coolant liquid flow through their porous structure. They are usually formed by a primary wick to produce liquid transportation by capillary force, and a secondary wick that is continuously wetted by the liquid coolant. Traditionally, the two wicks are manufactured separately and subsequently joined, thereby creating an interface that reduces the liquid transfer efficiency. In order to overcome this situation, a gradient porous wick is proposed and successfully manufactured through the freeze-casting method in a single operation. The influence of two different dispersant agents, KD4 and stearic acid was studied on the processing parameters, final pore size and morphology, and capillarity performances. A variate of gradient porosity was obtained by applying a diverse thermal gradient and solidification front velocity during directional solidification. The rheological characterisation of the camphene-based NiO suspensions was performed using a rotational viscometer. The final pore size and morphology were characterised by Optical Microscopy, Field Emission Scanning Electron Microscopy, and X-ray computed tomography. The use of stearic acid improves the particle stabilisation and generates pore enlargement with an equiaxed pore structure, while commercial dispersant KD4 shows a dendritic pore morphology at lower thermal gradient.

cond-mat.mtrl-sci↗
arXiv2020

Composition dependence of tracer diffusion coefficients in Fe-Ga alloys: a case study by a tracer-interdiffusion couple method

The problem of estimation of the tracer diffusion coefficients is solved by utilizing a novel tracer-interdiffusion couple technique even in the absence suitable radioisotope of one of the components and absence of reliable thermodynamic parameters. This is demonstrated by generating reliable and reproducible mobility data in the alloys of the Fe-Ga system with a strong composition dependence of the diffusion coefficients. Tracer- (59Fe) and inter-diffusion are simultaneously measured in three couples Fe/Fe-16Ga, Fe/Fe-24Ga and Fe-16Ga/Fe-24Ga at 1143 K. The results obtained for the couples with different end-members are in an excellent agreement with each other for the overlapping composition intervals. The influence of the molar volume on the measured tracer- (59Fe) and inter-diffusion coefficients is evaluated. Using thermodynamic calculations, the Ga tracer diffusion coefficient and the vacancy wind factor are determined via the Darken-Manning relation for the composition range of 0-24 at.% Ga. The results confirm the reliability of the tracer-interdiffusion couple technique for producing highly accurate diffusion data, in the present case for optimizing the mobility description of the bcc phase of the Fe-Ga system. The Ga tracer diffusion coefficients are further estimated via experimental determination of the ratio of the Fe and Ga tracer diffusivities at the Kirkendall marker planes and utilizing the Fe tracer diffusion coefficients measured directly by the radiotracer method.

cond-mat.mtrl-sci↗
arXiv2020

Study of charging up effect in a triple GEM detector

The advancement of Micro Pattern Gaseous Detector technology offers us different kinds of detectors with good spatial resolution and high rate capability and the Gas Electron Multiplier (GEM) detector is one of them. Typically GEM is made up of a thin polyimide foil having a thickness of 50 micrometers with 5 micrometers copper cladding on top and bottom sides. The presence of polyimide changes the gain of the detector under the influence of external radiation and the phenomenon is referred to as the charging up effect. The charging up effect is investigated with a double mask triple GEM detector prototype with Ar/CO$_2$ gas mixture in 70/30 ratio under continuous irradiation from a strong Fe$^{55}$ X-ray source. The detailed method of measurements and the test results are presented in this article.

physics.ins-det↗
arXiv2019

The Belle II Physics Book

We present the physics program of the Belle II experiment, located on the intensity frontier SuperKEKB $e^+e^-$ collider. Belle II collected its first collisions in 2018, and is expected to operate for the next decade. It is anticipated to collect 50/ab of collision data over its lifetime. This book is the outcome of a joint effort of Belle II collaborators and theorists through the Belle II theory interface platform (B2TiP), an effort that commenced in 2014. The aim of B2TiP was to elucidate the potential impacts of the Belle II program, which includes a wide scope of physics topics: B physics, charm, tau, quarkonium, electroweak precision measurements and dark sector searches. It is composed of nine working groups (WGs), which are coordinated by teams of theorist and experimentalists conveners: Semileptonic and leptonic B decays, Radiative and Electroweak penguins, phi_1 and phi_2 (time-dependent CP violation) measurements, phi_3 measurements, Charmless hadronic B decay, Charm, Quarkonium(like), tau and low-multiplicity processes, new physics and global fit analyses. This book highlights "golden- and silver-channels", i.e. those that would have the highest potential impact in the field. Theorists scrutinised the role of those measurements and estimated the respective theoretical uncertainties, achievable now as well as prospects for the future. Experimentalists investigated the expected improvements with the large dataset expected from Belle II, taking into account improved performance from the upgraded detector.

hep-ex↗
arXiv2019

Rich diversity of crystallographic phase formation in 2D Rex:Mo1-xS2 (x< 0.5) alloy

We report on the observation of rich variety of crystallographic phase formation in RexMo1-xS2 alloy for x < 0.5. For x < 0.23, no low dimensional super-structural modulation is observed and inter-cation hybridization remains discrete forming dimers to tetramers with increasing Re concentration. For x > 0.23, super-strutural modulaton is observed. Depending on the Re concentrations (x = 0.23, 0.32, 0.38 and 0.45) and its distributions, various types of cation hybridization results in rich variety of low dimensional super-structural modulation as directly revealed by high resolution transmission electron microscopy. These layered alloy system may be useful for various energy and novel device applications.

cond-mat.mtrl-sci↗
arXiv2019

The CLIC Potential for New Physics

The Compact Linear Collider (CLIC) is a mature option for the future of high energy physics. It combines the benefits of the clean environment of $e^+e^-$ colliders with operation at high centre-of-mass energies, allowing to probe scales beyond the reach of the Large Hadron Collider (LHC) for many scenarios of new physics. This places the CLIC project at a privileged spot in between the precision and energy frontiers, with capabilities that will significantly extend knowledge on both fronts at the end of the LHC era. In this report we review and revisit the potential of CLIC to search, directly and indirectly, for physics beyond the Standard Model.

hep-ph↗
arXiv2018

Growth of epitaxial ReS2 thin film by pulsed laser deposition

We present results on growth of large area epitaxial ReS2 thin film both on c plane sapphire substrate and MoS2 template by pulsed laser deposition (PLD). Films tend to grow with (0001) ReS2 perpendicular to (0001) Al2O3 and (0001) ReS2 perpendicular to (0001) MoS2 parallel to (0001) Al2O3 at deposition temperature below 300 deg C. Films are polycrystalline grown at temperature above 300 deg C. The smoothness and quality of the films are significantly improved when grown on MoS2 template compared to sapphire substrate. The results show that PLD is suitable to grow ReS2 epitaxial thin film over large area for practical device application.

cond-mat.mtrl-sci↗
arXiv2017

Observing the metal-poor solar neighbourhood: a comparison of galactic chemical evolution predictions

Atmospheric parameters and chemical compositions for ten stars with metallicities in the region of -2.2< [Fe/H] <-0.6 were precisely determined using high resolution, high signal to noise, spectra. For each star the abundances, for 14 to 27 elements, were derived using both LTE and NLTE approaches. In particular, differences by assuming LTE or NLTE are about 0.10 dex; depending on [Fe/H], Teff, gravity and element lines used in the analysis. We find that the O abundance has the largest error, ranging from 0.10 and 0.2 dex. The best measured elements are Cr, Fe, and Mn; with errors etween 0.03 and 0.11 dex. The stars in our sample were included in previous different observational work. We provide a consistent data analysis. The data dispersion introduced in the literature by different techniques and assumptions used by the different authors is within the observational errors, excepting for HD103095. We compare these results with stellar observations from different data sets and a number of theoretical galactic chemical evolution (GCE) simulations. We find a large scatter in the GCE results, used to study the origin of the elements. Within this scatter as found in previous GCE simulations, we cannot reproduce the evolution of the elemental ratios [Sc/Fe], [Ti/Fe], and [V/Fe] at different metallicities. The stellar yields from core collapse supernovae (CCSN) are likely primarily responsible for this discrepancy. Possible solutions and open problems are discussed.

astro-ph.SR↗
arXiv2012

Measurement and simulation of anisotropic magnetoresistance in single GaAs/MnAs core/shell nanowires

We report four probe measurements of the low field magnetoresistance in single core/shell GaAs/MnAs nanowires synthesized by molecular beam epitaxy, demonstrating clear signatures of anisotropic magnetoresistance that track the field-dependent magnetization. A comparison with micromagnetic simulations reveals that the principal characteristics of the magnetoresistance data can be unambiguously attributed to the nanowire segments with a zinc blende GaAs core. The direct correlation between magnetoresistance, magnetization and crystal structure provides a powerful means of characterizing individual hybrid ferromagnet/semiconductor nanostructures.

cond-mat.mes-hall↗
arXiv2011

New tools for the direct characterisation of FinFETs

This paper discusses how classical transport theories such as the thermionic emission, can be used as a powerful tool for the study and the understanding of the most complex mechanisms of transport in Fin Field Effect Transistors (FinFETs). By means of simple current and differential conductance measurements, taken at different temperatures and different gate voltages ($V_G$'s), it is possible to extrapolate the evolution of important parameters such as the spatial region of transport and the height of thermionic barrier at the centre of the channel. Furthermore, if the measurements are used in conjunction with simulated data, it becomes possible to also extract the interface trap density of these objects. These are important results, also because these parameters are extracted directly on state-of-the-art devices and not in specially-designed test structures. The possible characterisation of the different regimes of transport that can arise in these ultra-scaled devices having a doped or an undoped channel are also discussed. Examples of these regimes are, full body inversion and weak body inversion. Specific cases demonstrating the strength of the thermionic tool are discussed in sections \ref{sec:II}, \ref{sec:III} and \ref{sec:IV}. This text has been designed as a comprehensive overview of 4 related publications (see Ref. [2-5]) and has been submitted as a book chapter in Ref. [6]).

cond-mat.mes-hall↗