arXiv subjects

Zabeada Aslam

Publications and source records attributed to Zabeada Aslam.

6 recordsLinked to original sources

arXiv2026

A critical consideration of X-ray detectors based on Ga2O3: excitation, carrier transport mechanisms and performance standardization

X-ray detection underpins a wide range of applications in medicine, security, industrial inspection, scientific research for non-destructive imaging and material analysis. The rapid development of Ga2O3-based X-ray detectors offers a promising pathway toward next-generation detectors with high sensitivity, low noise, and harsh environment applications, benefiting from its intrinsic material properties such as high density, wide band gap energy, and high thermal-chemical stability. However, the underlying device operating mechanisms, including both carrier excitation and transport processes, have not yet been adequately studied, largely due to the misuse of X-ray sources in previous studies. Besides, benchmarking of device characteristics has been problematic due to experimental or data analysis issues, as well as misunderstandings of the applied equations associated with parameter definitions. In this work, we have designed and performed an instructive research work based on epitaxial beta-Ga2O3:Si and its planar Schottky detectors, measured with energy-tuneable monochromatic X-ray beams on a synchrotron beamline, clarifying the device excitation and carrier transport mechanisms with properly benchmarked device performance. In the end, we propose a set of protocols for correctly measuring and analysing the device performance. The proposed protocols are broadly applicable and can be readily extended to other semiconductor X-ray detectors.

physics.ins-det↗

arXiv2026

In Situ Dynamics of the Microscopic Crystallographic Dehydration Pathway in a Model Channel Hydrate, Theophylline

Solid-state phase transformations in molecular crystal hydrates govern stability and functional performance across a range of applications, including pharmaceutical, agrochemical and coordination framework materials. During dehydration, these hydrates can undergo substantial structural reorganisation involving changes in molecular orientation, intermolecular interactions, and lattice symmetry. Despite extensive study, the microscopic crystallographic pathways by which such transformations proceed remain poorly understood. Here, we investigate the dynamics of solid-state dehydration of theophylline monohydrate as a model molecular hydrate using in situ low-dose scanning electron diffraction (SED). Simultaneous observations of changes in morphology and crystallographic phase and orientation mapped across single particles reveal how complete dehydration proceeds via a two-step, reconstructive topotactic solid-state transformation: anisotropic, surface-specific mass loss of material near water channel sides (suggesting the monohydrate adopts a non-centrosymmetric crystal structure) is followed by surface-localised nucleation and growth of anhydrous form II on the parent monohydrate while preserving similar molecular orientations at a common plane. By providing direct, local crystallographic insight into hydrate dehydration, this work demonstrates how surface-controlled mass loss, morphological changes, and lattice orientation jointly govern solid-state transformations in molecular hydrates. More broadly, it establishes low-dose SED as an effective approach for probing dynamic phase transformations in beam-sensitive molecular crystals.

cond-mat.mtrl-sci↗

arXiv2026

Structural and magnetic properties of co-sputtered epitaxial Fe-Sn kagome thin films

In recent years the intermetallic alloys of Fe and Sn have gained significant interest due to a rich variety of magnetic properties present in these materials. The crystal Fe3Sn2 is a frustrated ferromagnet, while the crystallographically similar FeSn, which differs only by the stacking sequence of its Fe-containining kagome and stanene layers, is an antiferromagnet. Thin-film growth techniques such as magnetron sputtering allow for these different stoichiometric compositions to be grown through adjustments of the rate of deposition of the individual Fe and Sn sources, while all other conditions remain constant. Here, we report the production of high quality epitaxial thin films of Fe$_3$Sn$_2$ and FeSn on sapphire with a Pt seed layer, as well as a mixed phase containing intergrowths of both crystals, all of which we have characterized using both X-ray and four-dimensional scanning transmission electron microscopy (4D-STEM) methods. The resulting crystallographic phase content is compared to the results of magnetization measurements, with correspondence between the predicted ferromagnetic phase content and the resulting magnetization. Further magnetic properties of these films can then also be compared, leading to the discovery of a unique behavior in the temperature dependent coercivity within highly mixed phase alloys, a feature that is absent in either pure Fe3Sn2 and FeSn.

cond-mat.mtrl-sci↗

arXiv2025

The Microscopic Structure of Stacking Faults in Sr$_2$NaNb$_5$O$_{15}$

Stacking faults and other topological defects in ferroics can have a significant influence on the electronic and mechanical properties of the material. Here, regular stacking faults in the tetragonal tungsten bronze material Sr$_2$NaNb$_5$O$_{15}$ are investigated through transmission electron microscopy, symmetry mode analysis and machine-learned force-field calculations. It is shown that the faults, with a fault vector of $\frac{1}{4}[\bar{2}12]_o$, annihilate in sets of four in the material, owing to the $\frac{1}{4}$ unit cell displacement along the b-axis. The four resulting domains emerge as four possible directions of the S$_3$ order parameter, related to NbO$_6$ octahedral tilts in the material. Force-field calculations reveal that the stacking faults are likely placed at positions where the octahedra in neighbouring domains have similar magnitudes of rotation, and that the estimated stacking fault energy is 46 mJ/m$^2$. The investigation shows that the stacking faults have a significant local effect on the polar modes present in the structure, and therefore could affect the ferroelectric properties.

cond-mat.mtrl-sci↗

arXiv2020

A nano-carbon route to rare earth free permanent magnetism

High coercivity magnets are an important resource for renewable energy, electric vehicles and memory technologies. Most hard magnetic materials incorporate rare-earths such as neodymium and samarium, but the concerns about the environmental impact and supply stability of these materials is prompting research into alternatives. Here, we present a hybrid bilayer of cobalt and the nano-carbon molecule C60 which exhibits significantly enhanced coercivity with minimal reduction in magnetisation. We demonstrate how this anisotropy enhancing effect cannot be described by existing models of molecule-metal magnetic interfaces. We outline a new form of magnetic anisotropy, arising from asymmetric magneto-electric coupling in the metal-molecule interface. Because this phenomenon arises from pi-d hybrid orbitals, we propose calling this effect pi-anisotropy. While the critical temperature of this effect is currently limited by the rotational degree of freedom of the chosen molecule, C60, we describe how surface functionalisation would allow for the design of room-temperature, carbon based hard magnetic films.

physics.app-ph↗

arXiv2018

Helical magnetic structure and the anomalous and topological Hall effects in epitaxial B20 Fe$_{1-y}$Co$_y$Ge films

Epitaxial films of the B20-structure alloy Fe$_{1-y}$Co$_y$Ge were grown by molecular beam epitaxy on Si (111) substrates. The magnetization varied smoothly from the bulk-like values of one Bohr magneton per Fe atom for FeGe to zero for non-magnetic CoGe. The chiral lattice structure leads to a Dzyaloshinskii-Moriya interaction (DMI), and the films' helical magnetic ground state was confirmed using polarized neutron reflectometry measurements. The pitch of the spin helix, measured by this method, varies with Co content $y$ and diverges at $y \sim 0.45$. This indicates a zero-crossing of the DMI, which we reproduced in calculations using first principle methods. We also measured the longitudinal and Hall resistivity of our films as a function of magnetic field, temperature, and Co content $y$. The Hall resistivity is expected to contain contributions from the ordinary, anomalous, and topological Hall effects. Both the anomalous and topological Hall resistivities show peaks around $y \sim 0.5$. Our first principles calculations show a peak in the topological Hall constant at this value of $y$, related to the strong spin-polarisation predicted for intermediate values of $y$. Half-metallicity is predicted for $y = 0.6$, consistent with the experimentally observed linear magnetoresistance at this composition. Whilst it is possible to reconcile theory with experiment for the various Hall effects for FeGe, the large topological Hall resistivities for $y \sim 0.5$ are much larger then expected when the very small emergent fields associated with the divergence in the DMI are taken into account.

cond-mat.mtrl-sci↗