arXiv subjects

Graeme J. Ackland

Publications and source records attributed to Graeme J. Ackland.

At least 19 recordsLinked to original sources

arXiv2026

Stacking-Dependent Magnetism and Tunable Half-Metallicity in Bilayer Janus 1T-MnSSe

We investigate the structural, electronic, and magnetic properties of bilayer Janus 1T-MnSSe using first-principles calculations. Various AA- and AB-type stacking configurations are considered to examine the influence of interlayer registry on magnetic ordering and exchange interactions. The nonmagnetic state is unstable for all stackings, confirming intrinsic magnetism. The AA2 stacking is identified as the ground state and exhibits A-type antiferromagnetic ordering, indicating antiferromagnetic interlayer coupling. Monte Carlo simulations based on an effective Ising model reveal enhanced magnetic transition temperatures in the bilayer relative to the monolayer, with Néel temperatures above 300~K for antiferromagnetic stackings and Curie temperatures up to 250~K for ferromagnetic phases. Several stacking configurations exhibit robust half-metallic ferromagnetism with nearly 100\% spin polarization at the Fermi level. Moreover, the half-metallic state can be tuned and ultimately transformed into a metallic ferromagnetic phase through carrier doping and biaxial strain. These findings establish bilayer MnSSe as a promising platform for controllable interlayer magnetism and spintronic applications in two-dimensional materials.

cond-mat.mtrl-sci↗

arXiv2026

Triangular Charge-Density Waves (T-CDW) Stabilize Janus Group-VI Chalcogenide Hydrides

Hydrogenation is an effective strategy for enhancing electron--phonon coupling (EPC) and superconductivity in two-dimensional materials. However, excessively strong EPC can also induce lattice instabilities, leading to charge-density-wave (CDW) formation and structural phase transitions. Here, using first-principles calculations, we investigate CDW order in the Janus transition-metal chalcogenide hydrides 1T-WSH and 1T-WSeH. We find that the high-symmetry phases exhibit pronounced phonon softening at the M point, driving a transition to a commensurate $2\times2$ distorted structure characterized by an emergent triangular charge-density-wave (T-CDW) pattern. Analysis of the electronic structure, susceptibility, and phonon spectrum reveals that the instability is not driven by conventional Fermi-surface nesting but originates from strong momentum-dependent EPC. The T-CDW transition reconstructs the electronic structure and reduces the density of states at the Fermi level, leading to a substantial renormalization of the EPC strength. Consequently, the electron--phonon coupling constants decrease from $λ=2.04$ to $1.50$ in 1T-WSH and from $λ=3.94$ to $1.06$ in 1T-WSeH, while superconductivity remains robust in CDW phase with predicted transition temperatures of $T_c=12.28$ K and $7.75$ K, respectively. Together with previous results for MoSH and MoSeH, our findings establish a universal mechanism in the 1T-$MCH$ family ($M=\mathrm{Mo},\mathrm{W}$ and $C=\mathrm{S},\mathrm{Se}$), where the primary role of the T-CDW phase is not to eliminate superconductivity but to stabilize the lattice through EPC renormalization. The T-CDW phase therefore acts as an intrinsic self-stabilizing response that relieves excessively strong EPC while preserving phonon-mediated superconductivity.

cond-mat.supr-con↗

arXiv2026

A computational model of spatial politics: Hotelling-Downs model as statistical physics

The Hotelling-Downs model considers parties changing policy to maximise their vote-share. Where policy position lies on a left-right axis, it describes a tendency for political parties to move towards centrist platforms. This is in contrast with widely observed political polarisation. We extend the model to two dimensions, with many parties and with single and multiple-peaked voter distribution. We find that a two party system reduces polarisation, even if voters are polarised with a bimodal distribution. By contrast, multiparty systems induce polarisation, even when most voters favour moderate position. We model the effect of turnout and activists as influences on the parties, showing that this results in more polarisation, even in a two-party system. This suggests that polarisation of parties can be driven by abstention, intra-party politics and turnout on the extremes. In the two-party case, the winning party's positions are more moderate than the views of their supporters but better representative of the electorate as a whole. With polarisation, individual voters are better able to find a party which represents their views, but the government (winning part or coalition) is less representative of the population, even when the population has a clear consensus on all issues.

physics.soc-ph↗

arXiv2026

Understanding the lifetime of water with dynamic network analysis: the case of CsOH.H2O

We describe the atomic-level motions in caesium hydroxide monohydrate (CsOH$\cdot$H$_2$O), which is a chemical compound containing layers of water and hydroxide ions. At this composition, each oxygen is involved in three hydrogen bonds which, in the hexagonal structure, form a quasi-2D honeycomb lattice. While oxygen and caesium atoms form a typical crystal lattice, the dynamics of the hydrogen atoms are more complex. Here we show that the covalent and hydrogen bonds are continually interconverting, meaning that the water and hydroxyl are interconverting by proton exchange. The order-disorder transition of the water and hydroxyl proceeds by chemical reaction rather than rotation or diffusion of the molecules. A hydrogen can rotate out of the layer, leaving a vacant site in the 2D layer. Such a hydrogen vacancy can diffuse rapidly by single molecule rotation, leading to fast-ionic conduction. The proton exchange leads to a novel type of Raman activity combining stretch and exchange processes, for which we develop a theoretical model. This would manifest in a broad single peak associated with both H$_2$O and OH stretches and a low frequency peak appearing at elevated temperature.

cond-mat.mtrl-sci↗

arXiv2026

Observation of ΔJ=0 Rotational Excitation in Dense Hydrogens

Raman measurements performed on dense H2, D2 and H2+D2 in a wide pressure-temperature range reveal the presence of the ΔJ=0 rotational excitation. In the gas/fluid state this excitation has zero Raman shift, but in the solid, the crystal field drive s it away from the zero value e.g. 75 cm-1 at around 50 GPa and 10 K for both isotopes and their mixture. In the case of deuterium, the ΔJ=0 mode splits upon entering phase II suggesting a very complex molecular environment of the broken symmetry phase (BSP). In the fluid state and phases I and II the frequencies (energies) of the ΔJ=0 transition for H2 and D2 do not scale either as rotational (by factor of 2) nor vibrational (by square 2) modes and appear to be completely isotope independent. This independence on mass marks this transition as unique and a fundamentally different type of excitation from the commonly considered harmonic oscillator and quantum rotor.

quant-ph↗

arXiv2025

Stability, electronic disruption, and anisotropic superconductivity of hydrogenated trilayer metal tetraborides (MB$_{4}$H; M=Be, Mg, Ca, Al)

The discovery of superconductivity in MgB$_2$ ($T_c = 39$ K) \cite{nagamatsu2001superconductivity} established metal diborides (MB$_2$) as a promising class of conventional superconductors. Recent advances in fabrication techniques have enabled the synthesis of 2D MgB$_2$ with a $T_c$ of 36 K \cite{cheng2018fabrication}, reigniting interest in layered metal borides. This has led to predictions of superconductivity in various 2D metal borides, including MB$_4$ (M = Be, Mg, Ca, Al), with CaB$_4$ exhibiting the highest estimated $T_c$ of 36.1 K. To explore the impact of hydrogenation on superconductivity, we systematically investigate two-dimensional hydrogenated trilayer metal borides (MB$_4$H; M = Be, Mg, Ca, Al). Our results reveal that these materials retain a metallic nature dominated by boron $p$-orbitals, while hydrogenation significantly alters their band dispersion and Fermi surface topology. Phonon calculations confirm their dynamical stability and reveal strong electron-phonon interactions, leading to multi-gap superconductivity. Among the studied compounds, MgB$_4$H, AlB$_4$H, and CaB$_4$H exhibit possible two superconducting gaps, with CaB$_4$H showing the strongest electron-phonon coupling, resulting in an intrinsic superconducting transition temperature of 64 K. In contrast, AlB$_4$H shows the weakest coupling, with $T_c = 22$ K. The calculated electron-phonon coupling constants ($λ$) range from 0.62 to 0.99, demonstrating the tunability of superconducting properties through elemental substitution. These findings provide valuable insights into superconductivity in hydrogenated metal borides and highlight their potential for high-$T_c$ applications.

cond-mat.supr-con↗

arXiv2024

Understanding solid nitrogen through machine learning simulation

We construct a fast, transferable, general purpose, machine-learning interatomic potential suitable for large-scale simulations of $N_2$. The potential is trained only on high quality quantum chemical molecule-molecule interactions, no condensed phase information is used. The potential reproduces the experimental phase diagram including the melt curve and the molecular solid phases of nitrogen up to 10 GPa. This demonstrates that many-molecule interactions are unnecessary to explain the condensed phases of $N_2$. With increased pressure, transitions are observed from cubic ($α-N_2$), which optimises quadrupole-quadrupole interactions, through tetragonal ($γ-N_2$) which allows more efficient packing, through to monoclinic ($λ-N_2$) which packs still more efficiently. On heating, we obtain the hcp 3D rotor phase ($β-N_2$) and, at pressure, the cubic $δ-N_2$ phase which contains both 3D and 2D rotors, tetragonal $δ^\star-N_2$ phase with 2D rotors and the rhombohedral $ε-N_2$. Molecular dynamics demonstrates where these phases are indeed rotors, rather than frustrated order. The model does not support the existence of the wide range of bondlengths reported for the complex $ι-N_2$ phase. The thermodynamic transitions involve both shifts of molecular centres and rotations of molecules. We simulate these phase transitions between finding that the onset of rotation is rapid whereas motion of molecular centres is inhibited and the cause of the observed sluggishness of transitions. Routine density functional theory calculations give a similar picture to the potential.

physics.comp-ph↗

arXiv2023

A comparison of different Fourier transform procedures for analysis of diffraction data from noble gas fluids

A comparison is made between the three principal methods for analysis of neutron and X-ray diffraction data from noble gas fluids by direct Fourier transform. All three methods (standard Fourier transform, Lorch modification and Soper-Barney modification) are used to analyse four different sets of diffraction data from noble gas fluids. The results are compared to the findings of a full-scale real space structure determination, namely Empirical Potential Structure Refinement. Conclusions are drawn on the relative merits of the three Fourier transform methods, what information can be reliably obtained using each method, and which method is most suitable for analysis of different kinds of diffraction data. The mathematical validity of the Lorch method is critically analysed.

physics.comp-ph↗

arXiv2023

Competition between superconductivity and molecularization in quantum nuclear behavior of Lanthanum Hydride

Lanthanum hydride is the superconductor with the highest known critical temperature. It is believed that the superconductivity is of standard BCS-type, with electrons forming Cooper pairs and opening the superconducting band gap. Here we show that the BCS electron pairing is in competition with an alternative pairing: covalent bonding. We show that the covalent pairing is favored at lower pressures, the superconducting cubic phase becomes unstable as pressure is reduced. Previous calculations based on static relaxation neglect three factors, all of which are important in stabilizing the cubic phase. Finite temperature plays a role, and two quantum effects are also important - the nuclear wavefunction contributes to destablizing the H$_2$ molecules, and the zero-point pressure means that calculated pressures are significantly overestimated by standard methods. We demonstrate these phenomena using Born-Oppenheimer and path-integral molecular dynamics: these give the same qualitative picture, with nuclear quantum effects (NQE) increasing the transition pressure significantly. This competition between molecularization and superconducting gap formation is the fundamental reason why hydride superconductors have so far been found only at high pressure.

cond-mat.supr-con↗

arXiv2022

Detectability of core level crossing and electronic topological transformations: the case of Osmium

Osmium, the least compressible metal, has recently been observed to undergo abrupt changes in the c/a ratio at extreme pressures. These are claimed to provide evidence for two unusual electronic behaviors: a crossing of the semicore 4f and 5p levels, and an electronic topological transition. We demonstrate that these two electronic phenomena are readily reproduced and understood in density functional theory, but that neither perturbs the trend in c/a ratio against pressure. Hence the observed anomalies in c/a must have another cause. Osmium is also notable for its high yield stress: the c/a anomalies lie well within the differential strains which osmium can support. We propose that observed c/a changes can arise from mechanical yield of crystallites with strong preferred orientation under high deviatoric stress in the experimental data. We discuss what evidence remains for the more general hypothesis that core-level overlap under pressure can have measurable effects on the crystal structure in any material.

cond-mat.mtrl-sci↗

arXiv2022

Calculating the Raman Signal Beyond Perturbation Theory for a Diatomic Molecular Crystal

We calculate the eigenstates of a diatomic molecule in a range of model mean-field potentials, and evaluate the evolution of their associated Raman spectra with field strength. We demonstrate that dramatic changes in the appearance of the Raman spectrum for a diatomic molecule occur without any associated change in the symmetry of the surrounding potential. The limiting cases of the quantum eigenstates correspond, in the classical sense, to free rotation, and libration of well-oriented molecules. However, there are also many mixed modes which are neither rotons nor librons. The consequence for Raman spectroscopy is a series of complications - the non-harmonic potential splits the Raman active modes, and breaks the selection rules on forbidden modes. The mass-dependence of the various states is different - rotors, oscillators and reorientations have $1/m$, $1/\sqrt{m}$ and weaker mass dependence respectively. This may allow one to identify the character of the mode with isotope spectroscopy. However it is complicated by mixed modes and transitions between two different eigenstates with different character. We conclude that significant changes in the Raman spectrum of molecular systems are insufficient to demonstrate a phase transition since such changes can also occur in a fixed symmetry potential upon increasing field strength.

cond-mat.mtrl-sci↗

arXiv2021

Non-existence of the s-f transition in structures of solid gadolinium at pressure

Gadolinium has long been believed to undergo a high pressure phase transition with a volume collapse around 5%. Theoretical explanations have focused on the idea of electrons transferring from the extended s-orbital to the compact f-orbital. However, experimental measurement has been unable to detect any associated change in the magnetic properties of the f-electrons. Here we resolve this discrepancy by showing that there is no significant volume collapse, beyond what is typical in high pressure phase transformations. We present density functional theory calculations of solid gadolinium under high pressure using a range of methods, and revisit the experimental situation using X-ray diffraction (XRD). The standard lanthanide pressure-transformation sequence involving different stackings of close-packed planes: hcp to 9R to dhcp to fcc to d-fcc is reproduced. The so-called "volume collapsed" high-pressure phase is shown to be an unusual stacking of close-packed planes, with Fddd symmetry and a density change less than 2%. The distorted fcc (d-fcc) structure is revealed to arise as a consequence of antiferromagnetism. The theoretical results are shown to be remarkably robust to various treatments of the f-electrons. The key result is that there is no XRD evidence for volume collapse in Gadolinium. The sequence of phase transitions is well described by standard DFT. There is no need for special treatment of the f-electrons or evidence of f-electron bonding. Noting previous spectroscopic evidence is that there is no change in the f-electrons we conclude that high pressure Gadolinium has no complicated f-electron physics such as Mott-Hubbard, Kondo or valence transition.

cond-mat.mtrl-sci↗

arXiv2021

Equation of state and strength of diamond in high pressure ramp loading

Diamond is used extensively as a component in high energy density experiments, but existing equation of state (EOS) models do not capture its observed response to dynamic loading. In particular, in contrast with first principles theoretical EOS models, no solid-solid phase changes have been detected, and no general-purpose EOS models match the measured ambient isotherm. We have performed density functional theory (DFT) calculations of the diamond phase to ~10TPa, well beyond its predicted range of thermodynamic stability, and used these results as the basis of a Mie-Greuneisen EOS. We also performed DFT calculations of the elastic moduli, and calibrated an algebraic elasticity model for use in simulations. We then estimated the flow stress of diamond by comparison with the stress-density relation measured experimentally in ramp-loading experiments. The resulting constitutive model allows us to place a constraint on the Taylor-Quinney factor (the fraction of plastic work converted to heat) from the observation that diamond does not melt on ramp compression.

cond-mat.mtrl-sci↗

arXiv2021

Isotope quantum effects in the metallization transition in liquid hydrogen

Quantum effects in condensed matter normally only occur at low temperatures. Here we show a large quantum effect in high-pressure liquid hydrogen at thousands of Kelvins. We show that the metallization transition in hydrogen is subject to a very large isotope effect, occurring hundreds of degrees lower than the equivalent transition in deuterium. We examined this using path integral molecular dynamics simulations which identify a liquid-liquid transition involving atomization, metallization, and changes in viscosity, specific heat and compressibility. The difference between H$_2$ and D$_2$ is a quantum mechanical effect which can be associated with the larger zero-point energy in H$_2$ weakening the covalent bond. Our results mean that experimental results on deuterium must be corrected before they are relevant to understanding hydrogen at planetary conditions.

cond-mat.mtrl-sci↗

arXiv2021

Two state model for critical points and the negative slope of the melting-curve

We present a thermodynamic model which explains the presence of a negative slope in the melt curve, as observed in systems as diverse as the alkali metals and molecular hydrogen at high pressure. We assume that components of the system can be in one of two well defined states - one associated with low energy, the other with low volume. The model exhibits a number of measurable features which are also observed in these systems and are therefore expected to be associated with all negative Clapeyron-slope systems: first order phase transitions, thermodynamic anomalies along Widom lines. The melt curve maximum is a feature of the model, but appears well below the pressures where the change in state occurs in the solid: the solid-solid transition is related to the melt line minimum. An example of the model fitted to the electride transition in potassium is discussed.

cond-mat.stat-mech↗

arXiv2020

Phase behaviour of the quantum Lennard-Jones solid

The Lennard-Jones potential is perhaps one of the most widely-used models for the interaction of uncharged particles, such as noble gas solids. The phase diagram of the classical LJ solid is known to exhibit transitions between hcp and fcc phases. However, the phase behaviour of the quantum Lennard-Jones solid remains unknown. Thermodynamic integration based on path integral molecular dynamics and lattice dynamics calculations are used to study the phase stability of the hcp and fcc Lennard-Jones solids. The hcp phase is shown to be stabilized by quantum effects in PIMD while fcc is shown to be favoured by lattice dynamics, which suggests a possible re-entrant low pressure hcp phase for highly quantum systems. Implications for the phase stability of noble gas solids are discussed. For parameters equating to Helium, the expansion due to zero-point vibrations is associated with quantum melting: neither crystal structure is stable at zero pressure.

cond-mat.mtrl-sci↗

arXiv2020

Understanding high pressure hydrogen with a hierarchical machine-learned potential

The hydrogen phase diagram has a number of unusual features which are generally well reproduced by density functional calculations. Unfortunately, these calculations fail to provide good physical insights into why those features occur. In this paper, we parameterize a model potential for molecular hydrogen which permits long and large simulations. The model shows excellent reproduction of the phase diagram, including the broken-symmetry Phase II, an efficiently-packed phase III and the maximum in the melt curve. It also gives an excellent reproduction of the vibrational frequencies, including the maximum in the vibrational frequency $ν(P)$ and negative thermal expansion. By detailed study of lengthy molecular dynamics, we give intuitive explanations for observed and calculated properties. All solid structures approximate to hexagonal close packed, with symmetry broken by molecular orientation. At high pressure, Phase I shows significant short-ranged correlations between molecular orientations. The turnover in Raman frequency is due to increased coupling between neighboring molecules, rather than weakening of the bond. The liquid is denser than the close-packed solid because, at molecular separations below 2.3Å, the favoured relative orientation switches from quadrupole-energy-minimising to steric-repulsion-minimising. The latter allows molecules to get closer together, without atoms getting closer but this cannot be achieved within the constraints of a close-packed layer.

cond-mat.mtrl-sci↗

arXiv2020

The raman signal of a hindered rotor

We present a method for calculation of Raman modes of the quantum solid phase I solid hydrogen and deuterium. We use the mean-field assumption that the quantised excitations are localized on one molecule. This is done by explicit solution of the time-dependent Schroedinger equation in an angle-dependent potential, and direct calculation of the polarisation. We show that in the free-rotor limit, the H$_2$ and D$_2$ frequencies differ by a factor of 2, which evolves toward $\sqrt{2}$ as the modes acquire librational character due to stronger interactions. The ratio overshoots $\sqrt{2}$ if anharmonic terms weaken the harmonic potential. We also use density functional theory and molecular dynamics to calculate the E$_{2_g}$ optical phonon frequency and the Raman linewidths. The molecular dynamics shows that the molecules are not free rotors except at very low pressure and high temperature, and become like oscillators as phase II is approached. We fit the interaction strengths to experimental frequencies, but good agreement for intensities requires us to also include strong preferred-orientation and stimulated Raman effects between S$_0$(1) and S$_0$(0) contributions. The experimental Raman spectrum for phase II cannot be reproduced, showing that the mean-field assumption is invalid in that case.

cond-mat.mtrl-sci↗