arXiv subjects

Samuel J. Magorrian

Publications and source records attributed to Samuel J. Magorrian.

8 recordsLinked to original sources

arXiv2026

Microscopic origin of polytype-dependent melting in SiC revealed by machine-learning molecular dynamics

Predicting how crystal structure influences high-temperature stability remains a key challenge in materials modelling and design. Silicon carbide (SiC), one of the most thermally and chemically stable materials known, provides an ideal system for studying this problem because its many polytypes preserve similar local tetrahedral bonding while differing in long-range stacking geometry. Here, we combine phase-coexistence machine-learning molecular dynamics with finite-temperature phonon analysis, enabled by a fine-tuned MACE interatomic potential that accurately describes crystalline, high-temperature, and disordered configurations across multiple SiC polytypes. We identify a clear relative stability ordering, 3C > 2H > 9R, reflected consistently in structural disordering, interlayer sliding, and finite-temperature phonon spectra. Across all polytypes, melting initiates through the formation of short C-C contacts and carbon-rich local regions, followed by a progressive loss of tetrahedral Si-C connectivity. The reduced stability of the long-period 9R polytype is traced to low-frequency transverse-acoustic shear modes associated with relative bilayer sliding, which are already present in the 0 K phonon spectra and soften further at high temperature. These modes generate larger lateral bilayer displacements, linking enhanced interlayer sliding to local chemical disordering and ultimately melting. More broadly, our results show that high-temperature stability in polytypic covalent materials is governed not only by local bond strength, but also by stacking-dependent transverse dynamics.

cond-mat.mtrl-sci↗

arXiv2025

Computational tuning of the elastic properties of low- and high-entropy ultra-high temperature ceramics

Ultra-high temperature ceramics (UHTCs) represent a class of crystalline materials for extreme environments. They can withstand extremely high temperatures but are mechanically difficult to work with due to their inherent brittleness. Mixture compounds, in particular high-entropy mixtures, offer a pathway to tune the physical properties of UHTCs such as their elastic constants. Here we fine-tune the MACE-MPA-0 universal machine-learning potential on rocksalt carbide UHTCs containing group IV-V metals and demonstrate that not only do the elastic constants deviate from the rule of mixtures approximation in the high-entropy limit, but also in the low-entropy limit of binary and ternary mixtures. We find that this is caused by distortion imposed by the lattice mismatch, enabling the tuning of the physical properties of UHTC mixtures in both low- and high-entropy compounds. We identify a three-component mixture compound, HfCVCZrC, as the best balance between synthesizability and toughness, and apply our developed MACE-UHTC model to identify a range of non-equimolar candidate compositions of this compound which may enable the synthesis of a mixture UHTC with a Young's modulus up to 40 GPa below that of ZrC.

cond-mat.mtrl-sci↗

arXiv2025

Electronic structure of the interface between Au and WSe2

Understanding the interface between metals and two-dimensional materials is critical for their application in electronics and for the development of metal-mediated exfoliation of large area monolayers. Studying the intricate interactions at the interface requires model systems that enable control of the roughness, purity, and crystallinity of the metal surface. Here, we investigate the layer-dependent electronic structure of WSe_2 on template-stripped gold substrates fabricated using both silicon and mica templates, giving crystallographically disordered and Au(111) ordered surfaces, respectively, and contrast these findings with ab initio predictions. We observe strong hybridization around the Brillouin zone centre at $\overlineΓ$, indicating a covalent admixture in the gold-\WSe~interaction, and band shifts that suggest charge rearrangement at the Au(111) / WSe_2 interface. Core-level spectroscopy shows a single chemical environment for the interfacial WSe_2 layer on the template-stripped gold, distinct from the subsequent layers. These results reveal a mixture of van der Waals and covalent interactions, best described as a covalent-like quasi-bonding with intermediate interaction strength.

cond-mat.mtrl-sci↗

arXiv2024

Strong atomic reconstruction in twisted bilayers of highly flexible InSe: Machine-Learned Interatomic Potential and continuum model approaches

The relaxation of atomic positions to their optimal structural arrangement is crucial for understanding the emergence of new physical behavior in long scale superstructures in twisted bilayers of two-dimensional materials. The amount of deviation from a rigid moiré structure will depend on the elastic properties of the constituent monolayers which for the twisted bilayer - the more flexible the monolayers are, the lower the energy required to deform the layers to maximize the areas with an energetically optimal interlayer arrangement of atoms. We investigate this atomic reconstruction for twisted bilayers of highly flexible InSe. Results using two methods are demonstrated - first we train a machine-learned interatomic potential (MLIP) to enable fully atomistic relaxations of small-twist-angle large-length-scale moiré supercells while retaining density functional theory (DFT) level accuracy. We find substantial out-of-plane corrugation and in-plane domain formation for a wide range of twist angles and moiré length scales. We then adapt an existing continuum approach and show that it can reproduce some, but not all, features of the fully atomistic calculations.

cond-mat.mtrl-sci↗

arXiv2024

Strain-dependent one-dimensional confinement channels in twisted bilayer 1T$'$-WTe$_2$

The low symmetry and anistropic lattice of 1T$'$ WTe$_2$ is responsible for the existence of parallel one-dimensional channels in the moiré patterns of twisted bilayers. This gives the opportunity to explore moiré physics of a different nature to that widely observed in twisted bilayers of materials with hexagonal symmetries. Here, we combine plane-wave and linear-scaling density functional theory calculations to describe the electronic properties of twisted bilayer 1T$'$ WTe$_2$. For a small change in the lattice parameters of the constituent 1T$'$ WTe$_2$ monolayers, we find a substantial moiré-induced striped electrostatic potential landscape in the twisted bilayer, with a peak-to-trough magnitude $>$200~meV.

cond-mat.mtrl-sci↗

arXiv2020

Crossover from weakly indirect to direct excitons in atomically thin films of InSe

We perform a $\mathbf{k \cdot p}$ theory analysis of the spectra of the lowest energy and excited states of the excitons in few-layer atomically thin films of InSe taking into account in-plane electric polarizability of the film and the influence of the encapsulation environment. For the thinner films, the lowest-energy state of the exciton is weakly indirect in momentum space, with its dispersion showing minima at a layer-number-dependent wave number, due to an inverted edge of a relatively flat topmost valence band branch of the InSe film spectrum and we compute the activation energy from the momentum dark exciton ground state into the bright state. For the films with more than seven In$_2$Se$_2$ layers, the exciton dispersion minimum shifts to $Γ$-point.

cond-mat.mes-hall↗

arXiv2018

Infrared-to-violet tunable optical activity in atomic films of GaSe, InSe, and their heterostructures

Two-dimensional semiconductors - atomic layers of materials with covalent intra-layer bonding and weak (van der Waals or quadrupole) coupling between the layers - are a new class of materials with great potential for optoelectronic applications. Among those, a special position is now being taken by post-transition metal chalcogenides (PTMC), InSe and GaSe. It has recently been found that the band gap in 2D crystals of InSe more than doubles in the monolayer compared to thick multilayer crystals, while the high mobility of conduction band electrons is promoted by their light in-plane mass. Here, we use Raman and PL measurements of encapsulated few layer samples, coupled with accurate atomic force and transmission electron microscope structural characterisation to reveal new optical properties of atomically thin GaSe preserved by hBN encapsulation. The band gaps we observe complement the spectral range provided by InSe films, so that optical activity of these two almost lattice-matched PTMC films and their heterostructures densely cover the spectrum of photons from violet to infrared. We demonstrate the realisation of the latter by the first observation of interlayer excitonic photoluminescence in few-layer InSe-GaSe heterostructures. The spatially indirect transition is direct in k-space and therefore is bright, while its energy can be tuned in a broad range by the number of layers.

cond-mat.mes-hall↗

arXiv2018

Hybrid $\mathbf{k\cdot p}$-tight-binding model for intersubband optics in atomically thin InSe films

We propose atomic films of n-doped $γ$-InSe as a platform for intersubband optics in the infrared (IR) and far infrared (FIR) range, coupled to out-of-plane polarized light. Depending on the film thickness (number of layers) of the InSe film these transitions span from $\sim 0.7$ eV for bilayer to $\sim 0.05$ eV for 15-layer InSe. We use a hybrid $\mathbf{k} \cdot \mathbf{p}$ theory and tight-binding model, fully parametrized using density functional theory, to predict their oscillator strengths and thermal linewidths at room temperature.

cond-mat.mes-hall↗