Research Articles (Physics)

Permanent URI for this collectionhttp://hdl.handle.net/2263/1725

A collection containing some of the full text peer-reviewed/ refereed articles published by researchers from
the Department of Physics

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Now showing 1 - 20 of 837

First-principles investigation of structural, mechanical, electronic, and thermal properties of half-Heusler ZrPtSn
Allan, Lynet; Mwabora, Julius M.; Mulwa, Winfred M.; Mapasha, Refilwe Edwin; Musembi, Robinson J. (Springer, 2025-12)
This study explores the structural, mechanical, electronic, lattice dynamical, and thermal properties of the half-Heusler ZrPtSn using first-principles density functional theory. The goal is to assess its suitability for electronic and thermoelectric applications. Structural optimization confirmed stability under ambient conditions. Mechanical properties, including bulk, shear, and Young’s moduli, were evaluated for stiffness and ductility. Electronic structure analysis determined its semiconducting nature, with band gaps of 1.10 eV (without SOC) and 0.95 eV (with SOC). Phonon dispersion was analyzed to assess dynamical stability. ZrPtSn was dynamically stable, with no imaginary phonon modes. Its band gap suggests potential for optoelectronic applications. These findings provide a comprehensive understanding of ZrPtSn’s properties, supporting its potential use in electronic and thermoelectric devices and paving the way for further experimental and theoretical studies.
Synoptic wide-field EVN-e-MERLIN public survey (SWEEPS) - I. First steps towards commensal surveys with VLBI
Herbe-George, Celestin; McKean, John P.; Morganti, Raffaella; Radcliffe, Jack Frederick (Oxford University Press, 2025-02)
The high angular resolution and sensitivity of Very Long Baseline Interferometry (VLBI) offer a unique tool to identify and study active galactic nuclei and star-formation activity o v er cosmic time. However, despite recent technical advances, such as multiple phase centre correlation, VLBI surveys have thus far been limited to either a few well-studied deep-fields or wide-areas to a relatively shallow depth. To enter the era of extensive statistical studies at high angular resolution, a significantly larger area of the sky must be observed to much better sensitivity with VLBI. The Synoptic Wide-field EVN–e -MERLIN Public Survey (SWEEPS) is a proposed commensal observing mode for the EVN and e -MERLIN, where single-target principle investigator- led observations are re-correlated at the position of known radio sources within 12 arcmin of the pointing centre. Here, we demonstrate a proof-of-concept of this methodology by detecting a 5.6 mJy core-jet object at 1.7 GHz that would have otherwise been lost from the parent data set. This is the first object to be recovered as part of the SWEEPS pilot programme, which highlights the potential for increasing sample sizes of VLBI-detected radio sources with commensal observing modes in the near future.
Statistical selection of high-redshift, neutral-hydrogen-rich, lensed galaxies with the Square Kilometre Array
Button, Charissa Bronwyn; Deane, Roger (Oxford University Press, 2025-02)
Please read abstract in the article.
SHARP–IX. The dense, low-mass perturbers in B1938 + 666 and J0946 + 1006 : implications for cold and self-interacting dark matter
Tajalli, M.; Vegetti, S.; O’Riordan, C.M.; White, S.D.M.; Fassnacht, C.D.; Powell, D.M.; McKean, John P.; Despali, G. (Oxford University Press, 2025-10)
Please read abstract in the article.
Reducing false positives in strong lens detection through effective augmentation and ensemble learning
Rezaei, Samira; Chegeni, Amirmohammad; Nagam, Bharath Chowdhary; McKean, John P.; Baratchi, Mitra; Kuijken, Koen; Koopmans, Leon V.E. (Oxford University Press, 2025-03)
This research studies the impact of high-quality training data sets on the performance of Convolutional Neural Networks (CNNs) in detecting strong gravitational lenses. We stress the importance of data diversity and representativeness, demonstrating how variations in sample populations influence CNN performance. In addition to the quality of training data, our results highlight the effectiveness of various techniques, such as data augmentation and ensemble learning, in reducing false positives while maintaining model completeness at an acceptable level. This enhances the robustness of gravitational lens detection models and advancing capabilities in this field. Our experiments, employing variations of DenseNet and EfficientNet, achieved a best false positive rate (FP rate) of 10 −4 , while successfully identifying o v er 88 per cent of genuine gravitational lenses in the test data set. This represents an 11-fold reduction in the FP rate compared to the original training data set. Notably, this substantial enhancement in the FP rate is accompanied by only a 2.3 per cent decrease in the number of true positive samples. Validated on the Kilo Degree Survey data set, our findings offer insights applicable to ongoing missions, like Euclid .
Radio emission from a nearby M dwarf binary
Wandia, Kelvin; Garrett, Michael A.; Beswick, Robert J.; Radcliffe, Jack Frederick; Gajjar, Vishal; Williams-Baldwin, David; Tremblay, Chenoa; McDonald, Iain; Andersson, Alex; Siemion, Andrew (Oxford University Press, 2025-11)
We present the detection of the binary system 2MASS J02132062 + 3648506 AB using the Karl G. Jansky Very Large Array archive data observed at 4–8 GHz. The system is a triple consisting of a tight binary (∼0 . 2arcsec ) of two M dwarfs of spectral types M4.5 and M6.5 and a wide T3 brown dwarf companion ( ∼16.4 arcsec). The binary displays coronal and chromospheric activity as traced by previously measured X-ray flux and H αemission. We detect the unresolved binary at a peak flux density of∼356μJybeam −1 at a signal-to-noise ratio (SNR) of∼36 and determine a radio luminosity oflog LR /log Lbol ∼−7 . 76. The radio emission is quiescent, polarized at a mean circular polarization fraction fc = 45 . 20 ±1 . 58% and exhibits a spectral index α= −0 . 44 ±0 . 07. We probe the binary using the Enhanced Multi-Element Remotely Linked Interferometer Network (e-MERLIN) with an angular resolution of∼40 mas at 5 GHz and detect a component at a peak flux density of∼90μJybeam −1 at a SNR ∼5. We propose a gyrosynchrotron origin for the radio emission and estimate a magnetic field strength B < 174 . 86 G, an emitting region of size L < 1 . 54 times the radius of the M4.5 primary and a plasma number density ne < 2 . 91 ×105 cm −3 . The brown dwarf companion is not detected. Additionally, we have analysed observations of 2MASS J04183483 + 213127, a chromospherically active L5 brown dwarf which is also not detected and can only place 3 σflux density upper limits at 36 . 9μJybeam −1 and 42 . 3μJybeam −1 for Stokes I and V, respectively.
MeerKAT-derived H I kinematics and the baryonic Tully–Fisher relation in the X-ray luminous cluster Abell 3408
Ndaliso, Xola; Blecher, Tariq; Deane, Roger; Elson, Ed (Oxford University Press, 2025-09)
Please read abstract in the article.
A million-solar-mass object detected at a cosmological distance using gravitational imaging
Powell, D.M.; McKean, John P.; Vegetti, S.; Spingola, C.; White, S.D.M.; Fassnacht, C.D. (Nature Research, 2025-11)
Structure on subgalactic scales provides important tests of galaxy formation models and the nature of dark matter. However, such objects are typically too faint to provide robust mass constraints. Here we report the discovery of an extremely low-mass object detected by means of its gravitational perturbation to a thin lensed arc observed with milli-arcsecond-resolution very long baseline interferometry. The object was identified using a non-parametric gravitational imaging technique and confirmed using independent parametric modelling. It contains a mass of m80 = (1.13 ± 0.04) × 106 M⊙ within a projected radius of 80 pc at an assumed redshift of 0.881. This detection is extremely robust and precise, with a statistical significance of 26σ, a 3.3% fractional uncertainty on m80 and an astrometric uncertainty of 194 μas. This is the lowest-mass object known to us, by two orders of magnitude, to be detected at a cosmological distance by its gravitational effect. This work demonstrates the observational feasibility of using gravitational imaging to probe the million-solar-mass regime far beyond our local Universe.
A self-consistent framework to study magnetic fields with strong gravitational lensing and polarized radio sources
Ndiritu, S.W.; Vegetti, S.; Powell, D.M.; McKean, John P. (Oxford University Press, 2025-04)
We introduce a unified approach that, given a strong gravitationally lensed polarized source, self-consistently infers its complex surface brightness distribution and the lens galaxy mass–density profile, magnetic field and electron density from interferometric data. The method is fully Bayesian, pixellated, and three-dimensional: the source light is reconstructed in each frequency channel on a Delaunay tessellation with a magnification-adaptive resolution. We tested this technique using simulated interferometric observations with a realistic model of the lens, for two different levels of source polarization and two different lensing configurations. For all data sets, the presence of a Faraday rotating screen in the lens is supported by the data with strong statistical significance. In the region probed by the lensed images, we can recover the rotation measure and the parallel component of the magnetic field with an average error between 0.6 and 11 rad m −2 and 0.3 and 3nG, respectively. Given our choice of model, we find the electron density is the least well-constrained component due to a degeneracy with the magnetic field and disc inclination. The background source total intensity, polarization fraction, and polarization angle are inferred with an error between 4 and 10 per cent, 15 and 50 per cent, and 1–12 de g, respectively. Our analysis shows that both the lensing configuration and the intrinsic model degeneracies play a role in the quality of the constraints that can be obtained.
A grand-design spiral galaxy with an ordered magnetic field at redshift 2.6 as resolved with ALMA and gravitational lensing
De Roo, W.; Vegetti , S.; Powell , D.M.; Ndiritu, S.W.; Pakmor, R.; McKean, John P. (Oxford University Press, 2025-06)
Magnetic fields play an important role in the evolution of galaxies and in shaping the dynamics of their interstellar medium. However, the formation history of magnetic fields from initial seed-fields to well-ordered systems is not clear. Favoured scenarios include a turbulent dynamo that amplifies the field, and a mean-field dynamo that organizes it. Such a model can be tested through observing the magnetic-field structure of galaxies in the early Universe given the relative formation time-scales involved. Here, we combine the high angular resolution of the Atacama Large Millimetre Array (ALMA) and gravitational lensing to resolve the magnetic field structure of a 4 kpc in extent grand-design spiral when the Universe was just 2.6 Gyr old. We find that the spiral arm structure, as traced by the heated dust emission, is coincident with the linearly polarized emission, which is consistent with a highly ordered magnetic field. The time-scale needed to produce such an ordered field is likely within at least several rotations of the disc. Our study highlights the importance of combining the long baselines of ALMA and gravitational lensing to resolve the structure of galaxies at cosmologically interesting epochs.
A first glimpse at the MeerKAT DEEP2 field at S-band
Ranchod, S.; Wagenveld, J.D.; Klockner, H.-R.; Wucknitz, O.; Deane, Roger; Sridhar, S.S.; Barr, E.; Buchner, S.; Camilo, F.; Damas-Segovia, A.; Kasemann, C.; Kramer , M.; Legodi, L.S.; Mao, S.A.; Menten, K.; Rammala, I.; Rugel, M.R.; Wieching, G. (Oxford University Press, 2025-02)
Please read abstract in the article.
Hi gas in the rejuvenated radio galaxy PKS 2014-55
Mtshweni, Leon; Thorat, Kshitij; Deane, Roger; Frank, Bradley S.; Maccagni, Filippo M.; Jozsa, Gyula I.; Cotton, William D.; Giri, Gourab; White, Sarah V.; Atemkeng, Marcellin; Bester, Hertzog L.; Fanaroff, Bernie L.; Heywood, Ian; Lawrie, Graham; Manamela, Thato Eugine; Magolego, Isaac; Mauch, Tom; Oozeer, Nadeem; Smirnov, Oleg; Kupa, Masacheba S. (Oxford University Press, 2025-10)
Please read abstract in the article.
MeerKAT discovery of an infalling cold gas tail on to the nearby barred spiral galaxy, NGC 5643
Santana, Karina C.; Maccagni, Filippo M.; Deane, Roger; Healy, Julia (Oxford University Press, 2025-07)
Please read abstract in the article.
An extended and extremely thin gravitational arc from a lensed compact symmetric object at redshift of 2.059
McKean, John P.; Spingola, C.; Powell, D.M.; Vegetti, S. (Oxford University Press, 2025-11)
Compact symmetric objects (CSOs) are thought to be short-lived radio sources with two lobes of emission that are separated by less than a kpc in projection. However, studies of such systems at high redshift are challenging due to the limited resolution of present-day telescopes, and can be biased to the most luminous objects. Here, we report imaging of a gravitationally lensed CSO at a redshift of 2.059 using very long baseline interferometry at 1.7 GHz. The data are imaged using Bayesian forward modelling deconvolution, which reveals a spectacularly extended and thin gravitational arc, and several resolved features within the lensed images. The surface brightness of the lensing-corrected source shows two mini-lobes separated by 642 pc in projection, with evidence of multiple hotspots that have brightness temperatures of 108 . 6 to 109 . 2 K, and a total luminosity density of 1026 . 3 W Hz−1 . By combining the well-resolved radio source morphology with previous multiwavelength studies, we conclude that this object is likely a CSO of type 2, and that the properties are consistent with the bow-shock model for compact radio sources. Our analysis highlights the importance of combining high-quality data sets with sophisticated imaging and modelling algorithms for studying the high-redshift Universe.
A spatially resolved spectral analysis of giant radio galaxies with MeerKAT
Charlton , K.K.L.; Delhaize, J.; Thorat, Kshitij; Heywood, I.; Jarvis , M.J.; Hardcastle, M.J.; An, F.; Delvecchio, I.; Hale, C.L.; Whittam, I.H.; Bruggen, M.; Marchetti, L.; Morabito, L.; Randriamanakoto, Z.; White, S.V.; Taylor, A.R. (Oxford University Press, 2025)
In this study we report the spatially resolved, wideband spectral properties of three giant radio galaxies (GRGs) in the COSMOS field: MGTC J095959.63 + 024608.6, MGTC J100016.84 + 015133.0, and MGTC J100022.85 + 031520.4. One of these galaxies, MGTC J100022.85 + 031520.4, is reported here for the first time, with a projected linear size of 1.29 Mpc at a redshift of 0.1034. Unlike the other two, it is associated with a brightest cluster galaxy (BCG), making it one of the few GRGs known to inhabit cluster environments. We examine the spectral age distributions of the three GRGs using new MeerKAT UHF-band (544–1088 MHz) observations, and L -band (900–1670 MHz) data from the MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE) survey. We test two models of spectral ageing, the Jaffe–Perola and Tribble models, using the Broadband Radio Astronomy Tools ( BRATS ) software, and find that they agree well with each other. We estimate the Tribble spectral age for MGTC J095959.63 + 024608.6 as 68 Myr, for MGTC J100016.84 + 015133.0 as 47 Myr, and for MGTC J100022.85 + 031520.4 as 67 Myr. We find significant disagreements between these spectral age estimates and the estimates of the dynamical ages of these GRGs, modelled in cluster and group environments. Our results highlight the need for additional processes that are not accounted for in either the dynamic age or the spectral age estimations.
Guanidinium pseudo-halide salt vapor post-treatment of 3D/2D Sn-based perovskite heterostructure thin films prepared by sequential physical vapor deposition for solar cell applications
Sembito, Alex; Mwabora, Julius M.; Nyongesa, Francis W.; Diale, M. (Mmantsae Moche) (Elsevier, 2026-01)
Please read abstract in the article.
Upper limits on radio emission from the K2-18 system
Wandia, Kelvin; Tremblay, Chenoa; Garrett, Michael A.; Andersson, Alex; Li, Megan G.; Gajjar, Vishal; Beswick, Robert J.; Radcliffe, Jack Frederick; Czech, Daniel; Deboer, David R.; Demorest, P.B.; Farah, Wael; Heywood, Ian; Siemion, Andrew (Oxford University Press, 2026-03)
Please read abstract in the article.
The effects of in‑plane strains on the electrochemical properties of Li adatoms on the ZrS2 monolayer : a first-principles study
Mahapane, B.D.; Fwalo, Chewe; Bekeur, Craig Arthur; Mapasha, Refilwe Edwin (Springer, 2025-12)
In this study, we use density functional theory (DFT) with a Hubbard (U) parameter, implemented in the Quantum Espresso code, to investigate the interactions between Li-ions and the ZrS2 monolayer under the influence of in-plane uniaxial and biaxial strains, specifically within the context of lithium-ion batteries. This is to ensure the ZrS2 monolayer is more robust against the Coulomb forces arising from interactions between multiple lithium ions. This study objectively examines the impact of tensile and compressive strains ranging from − 5% to 5% on the energetic stability and electrochemical properties of the lithiated ZrS2 electrode monolayer. For a single Li adatom on a 3 × 3 ZrS2 monolayer, the compressed structure (at − 5% strain) becomes more energetically favorable, exhibiting a low adsorption energy of − 1.41 eV. In contrast, the stretched structure (at + 5% strain) has a higher adsorption energy of − 0.95 eV compared to the unstrained structure (− 1.16 eV), although exothermic interaction is maintained. The ZrS2 electrode monolayer has a shallow energy barrier of 0.23 eV for Li-ion diffusion, indicating greater mobility, which is slightly enhanced by compressive strain. The application of − 5% (compressive strain) resulted in an average OCV of 0.93 V, and 0.78 V for unstrained, while + 5% (tensile strain) yielded an OCV of 0.69 V, which is in the range of commercial anode materials. The tensile strain on a ZrS2 electrode monolayer would be more effective in mitigating the dendrite formation. The introduction of a Li adatom rearranged the conduction band minimum, leading to the hybridized Zr d orbital states crossing the Fermi level and becoming more populated as the number of Li adatoms increases, leading to a more conductive electrode. Additionally, the strain reduced the band gap, causing the induced electronic states to be continuous from the VBM to the CBM edges, which enhances the electronic conductivity of the material, ensuring the excellent LIBs operation during the charge and discharge processes.
Optimization FTO/c-TiO2 capped with m-TiO2 electron transport layer for efficient interfacial charge carriers' extraction and transport prepared by spray pyrolysis
Sibiya, Sizwe Bhekithemba; Diale, M. (Mmantsae Moche) (Wiley, 2026-01)
Please read abstract in the article.
Defect engineering with group III dopants in 2D monolayer SiC for improved electronic devices
Igumbor, Emmanuel; Ajeh, Patience; Mapasha, Refilwe Edwin; Omotoso, Ezekiel; Raji, Abdulrafiu (Elsevier, 2026-06)
Please read abstract in the article. HIGHLIGHTS • Hybrid DFT was used to study the effects of group III dopants on 2D SiC.. • Group III dopant in 2D monolayer SiC distorts its lattice, inducing varying strain. • Group III dopants at C sites induce magnetism, enabling spintronic 2D SiC devices. • This study reveals how group III dopants tune electronic and magnetic properties of 2D SiC.

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