arXiv subjects

R. E. Cohen

Publications and source records attributed to R. E. Cohen.

At least 19 recordsLinked to original sources

arXiv2026

Novel Transformations of PbTiO3 with Pressure and Temperature

We investigated the behavior of lead titanate (PbTiO3) up to 100 GPa, both at room temperature and upon laser heating, using synchrotron X ray diffraction combined with density functional theory (DFT) computations. At the high pressure temperature (PT) conditions produced in laser heated diamond anvil cells, PbTiO3 dissociates into PbO and TiO2, consistent with our DFT computations showing that decomposition becomes enthalpically favored above 65 GPa. In contrast, on room temperature compression, PbTiO3 persists in the tetragonal I4mcm phase up to at least 100 GPa. Laser heating produces distinct PbO phases: a compressed form of alpha PbO and a previously unreported delta PbO polymorph, both of which transform to beta PbO on decompression. The calculations predict that alpha PbO undergoes pressure-induced band gap closure, metallizing above 70 GPa, whereas the delta and beta phases remain semiconducting with a band gap above 1 eV even at megabar pressures. The experimental and confirming theoretical results reveal an unanticipated dimension of the behavior of PbTiO3, showing that distinct equilibrium and metastable phases can be stabilized along different PT synthesis paths.

cond-mat.mtrl-sci↗

arXiv2025

Order-Disorder in Fe-Si Alloys: Implications for Seismic Anisotropy and Thermal Evolution of Earth's Inner Core

Understanding the structure and dynamics of Earth's inner core is essential for constraining its composition, thermal evolution, and seismic properties. Silicon is a probable major component of Earth's core. Using first-principles molecular dynamics and thermodynamic modeling, we investigate the structural, elastic, and transport properties of Fe-Si alloys at high pressures and temperatures. By computing the Gibbs free energies of B2, hcp, fcc, and bcc solid solutions, we construct the Fe-Si phase diagram applicable to the Earth's inner core. Our results reveal a pronounced miscibility gap between hcp and B2 Fe-Si, with the two phases coexisting over the compositional range of 6-11 wt% Si at 6000 K. The B2 Fe-Si alloy exhibits strong single-crystal shear anisotropy (22.9% at 6000 K) compared to the nearly isotropic hcp phase (0.6%), and yields a shear wave velocity (3.73 km/s) and Poisson's ratio consistent with seismological observations. Moreover, the computed transport properties reveal substantially lower thermal conductivity of B2 Fe-Si relative to pure iron or hcp Fe-Si under inner-core conditions. These results imply that Earth's inner core likely comprises multiple phases, whose distribution and crystallographic texture critically influence its seismic and thermal properties.

cond-mat.mtrl-sci↗

arXiv2025

Infrared photometry and CaT spectroscopy of the most metal-poor in-situ globular cluster VVV-CL001

Globular clusters in the Galactic bulge are difficult to study due to high extinction and severe crowding. VVV-CL001 is an old, metal-poor, and fast cluster in the inner bulge, whose extreme properties make it a key probe of the early chemical and dynamical evolution of the Milky Way. We derive its fundamental parameters by combining spectroscopy, astrometry, and near-infrared photometry. Metallicity and radial velocity were measured from medium-resolution FORS2/VLT spectra; proper motions from Gaia DR3; and FourStar/Magellan photometry was used to refine the cluster centre, derive its structure, and estimate age, distance, and reddening. VVV-CL001 is confirmed to be an old ($12.1^{+1.0}_{-1.2}$ Gyr), metal-poor ($[\text{Fe}/\text{H}] = -2.25 \pm 0.05$) cluster at a heliocentric distance of $7.1^{+1.3}_{-1.1}$ kpc, with reddening $E(J-K_s) = 1.40^{+0.01}_{-0.02}$. Its mean proper motions are $μ_α^* = -3.68 \pm 0.09$ and $μ_δ= -1.76 \pm 0.10$ mas yr$^{-1}$, and its radial velocity is $-334 \pm 4$ km s$^{-1}$. The orbit is eccentric ($e = 0.76^{+0.10}_{-0.14}$), confined to the inner Galaxy ($|Z|_{\max} \approx 1$ kpc) and within the bar's influence ($R < 5$ kpc), with pericentre $0.6^{+0.3}_{-0.2}$ kpc and apocentre $4.5^{+2.5}_{-1.2}$ kpc. Its old age, low metallicity, and orbital properties support an in-situ origin, identifying VVV-CL001 as one of the most metal-poor inner-Galaxy clusters formed in the early Milky Way. It likely belongs to the primordial disk cluster population later trapped by the bar, making it a fossil remnant of the earliest phases of Galactic assembly.

astro-ph.GA↗

arXiv2025

Stabilization of Ferroelectric Hafnia and Zirconia through Y2O3 doping

We investigate the possible stabilization of ferroelectricity in bulk Y2O3-doped hafnia and zirconia. We use density functional theory (DFT) with large random supercells of hafnia and zirconia and study the relative phase stability of the centrosymmetric cubic and monoclinic phases compared with the polar orthorhombic phase. We find that Y2O3-doping stabilizes the polar ferroelectric phase over the monoclinic baddeleyite phase in both hafnia and zirconia.

cond-mat.mtrl-sci↗

arXiv2025

Pressure dependence of liquid iron viscosity from machine-learning molecular dynamics

We have developed a machine-learning potential that accurately models the behavior of iron under the conditions of Earth's core. By performing numerous nanosecond scale equilibrium molecular dynamics simulations, the viscosities of liquid iron for the whole outer core conditions are obtained with much less uncertainty. We find that the Einstein-Stokes relation is not accurate for outer core conditions. The viscosity is on the order of 10s \si{mPa.s}, in agreement with previous first-principles results. We present a viscosity map as a function of pressure and temperature for liquid iron useful for geophysical modeling.

physics.geo-ph↗

arXiv2024

Absence of high pressure ground state re-entrant ferroelectricity in PbTiO$_3$

We study ferroelectricity in the classic perovskite ferroelectric PbTiO$_3$ to high pressures with density functional theory (DFT) and experimental diamond-anvil techniques. We use second harmonic generation (SHG) spectroscopy to detect lack of inversion symmetry. Consistent with early understanding and experiments, we find that ferroelectricity disappears at moderate pressures. However, DFT computations show that the disappearance arises from the overtaking of zone boundary instabilities, and not from the squeezing out of the off-centering ferroelectric displacements with pressure, as previously thought. Moreover, at high pressures the distorted perovskite phases are metastable with respect to a new dense centrosymmetric post-perovskite phase with P$2_1/m$ symmetry and 8-coordinated Ti, which becomes stable at about 70 GPa.

cond-mat.mtrl-sci↗

arXiv2024

The JWST Resolved Stellar Populations Early Release Science Program VII. Stress Testing the NIRCam Exposure Time Calculator

We empirically assess estimates from v3.0 of the JWST NIRCam Exposure Time Calculator (ETC) using observations of resolved stars in Local Group targets taken as part of the Resolved Stellar Populations Early Release Science (ERS) Program. For bright stars, we find that: (i) purely Poissonian estimates of the signal-to-noise ratio (SNR) are in good agreement between the ETC and observations, but non-ideal effects (e.g., flat field uncertainties) are the current limiting factor in the photometric precision that can be achieved; (ii) source position offsets, relative to the detector pixels, have a large impact on the ETC saturation predictions and introducing sub-pixel dithers in the observation design can improve the saturation limits by up to ~1 mag. For faint stars, for which the sky dominates the error budget, we find that the choice in ETC extraction strategy (e.g., aperture size relative to point spread function size) can affect the exposure time estimates by up to a factor of 5. We provide guidelines for configuring the ETC aperture photometry to produce SNR predictions in line with the ERS data. Finally, we quantify the effects of crowding on the SNRs over a large dynamic range in stellar density and provide guidelines for approximating the effects of crowding on SNRs predicted by the ETC.

astro-ph.GA↗

arXiv2024

High-pressure Phase Transition of Olivine-type Mg$_2$GeO$_4$ to a Metastable Forsterite-III type Structure and their Equation of States

Germanates are often used as structural analogs of planetary silicates. We have explored the high-pressure phase relations in Mg$_2$GeO$_4$ using diamond anvil cell experiments combined with synchrotron x-ray diffraction and computations based on density functional theory. Upon room temperature compression, forsterite-type Mg$_2$GeO$_4$ remains stable up to 30 GPa. At higher pressures, a phase transition to a forsterite-III type (Cmc21) structure was observed, which remained stable to the peak pressure of 105 GPa. Using a 3rd order Birch Murnaghan fit to the experimental data, we obtained V0 = 305.1 (3) Å3, K0 = 124.6 (14) GPa and K0' = 3.86 (fixed) for forsterite- and V0 = 263.5 (15) Å3, K0 = 175 (7) GPa and K0' = 4.2 (fixed) for the forsterite-III type phase. The forsterite-III type structure was found to be metastable when compared to the stable assemblage of perovskite/post-perovskite + MgO, as observed during laser-heating experiments. Understanding the phase relations and physical properties of metastable phases is crucial for studying the mineralogy of impact sites, understanding metastable wedges in subducting slabs and interpreting the results of shock compression experiments.

physics.geo-ph↗

arXiv2023

Hafnia HfO$_2$ is a Proper Ferroelectric

We clarify the nature of hafnia as a proper ferroelectric and show that there is a shallow double well involving a single soft polar mode as in well-known classic ferroelectrics. Using symmetry analysis, density-functional theory (DFT) structural optimizations with and without epitaxial strain, and density functional perturbation theory (DFPT), we examine several important possible hafnia structures derived ultimately from the cubic fluorite structure, including baddeleyite ($P2_{1}/c$) tetragonal antiferroelectric $P4_{2}nmc$, $Pbca$ (nonpolar and brookite), ferroelectric rhombohedral ($R3m$ and $R3$), $Pmn2_{1}$, and $Pca2_{1}$ structures. The latter is considered to be the most likely ferroelectric phase seen experimentally, and has an antiferroelectric parent with space group $Pbcn$, with a single unstable polar mode and a shallow double well with a well depth of 24 meV/atom. Strain is not required for switching or other ferroelectric properties, nor is coupling of the soft-mode with any other modes within the ferroelectric $Pca2_{1}$, $Pmn2_{1}$, $R3m$ or $R3$ phases.

cond-mat.mtrl-sci↗

arXiv2023

High-pressure order-disorder transition in Mg$_2$SiO$_4$: Implications for super-Earth mineralogy

(Mg, Fe)SiO$_3$ post-perovskite is the highest pressure silicate mineral phase in the Earth's interior. The extreme pressure and temperature conditions inside large extrasolar planets will likely lead to phase transitions beyond pPv. In this work we have explored the high-pressure phase relations in Mg$_2$SiO$_4$ using computations based on density functional theory. We find that a partially disordered I-42d type structure would be stable in the interiors of these super-Earth planets. The discovery of a structure where two very dissimilar cations, Mg$^{2+}$ and Si$^{4+}$ occupy the same crystallographic site opens up a domain of interesting crystal chemistry and provides a foundation for other silicates and oxides with mixed occupancy. We have explored the mechanism of the phase transition from the ordered ground state and the effect of the disordering on electronic properties of the silicate phase.

physics.geo-ph↗

arXiv2023

Origin of Wake-Up Effect in Hafnia

We performed density functional theory (DFT) calculations on epitaxially strained hafnia. We demonstrate the stabilization of the ferroelectric ($Pca2_{1}$) phase from the antiferroelectric ($Pbcn$) in bulk hafnia in the presence of electric field. We found that the polar ($Pca2_{1}$) phase can be efficiently stabilized with an adequate choice of film orientation. We show that for a (010)-oriented Pbcn, the ferroelectric $Pca2_{1}$ phase can be reached with a relatively small electric field ($\mid\varepsilon\mid\geq 150$ KV/m). We thus provide a simple explanation to the experimental observation of polarization enhancement through electric field cycling, or wake-up effect, as a ferroelectric phase transition driven by electric field. We find, in contrast, that stress free pure hafnia does not become ferroelectric for any reasonable electric field. So we explain the wake up effect and stabilization of ferroelectric pure hafnia as coming from a combination of epitaxial strain under applied electric field perpendicular to the film. We find that strain (or doping) primarily destabilizes the baddeleyite structure, so that the antiferroelectric Pbcn and ferroelectric phases can form.

cond-mat.mtrl-sci↗

arXiv2023

First-principles thermal equation of state of fcc iridium

The thermal equation of states for fcc iridium (Ir) is obtained from first-principles molecular dynamics up to 3000 K and 540 GPa. The equation of state (EoS) is globally fitted to a simplified free energy model and various parameters are derived. The theoretical principal Hugoniot is compared with shockwave experiments, where discrepancy suggests formation of new Ir phases. A few representative EoS parameters, such as bulk modulus $K_T$, thermal expansivity $α$, Grüneisen parameter $γ$, and constant pressure capacity $C_P$, Debye temperature, $Θ_D$ are computed to compare with experimental data

cond-mat.mtrl-sci↗

arXiv2022

Ca Triplet Metallicities and Velocities for twelve Globular Clusters towards the Galactic Bulge

Globular clusters (GCs) are excellent tracers of the formation and early evolution of the Milky Way. The bulge GCs (BGCs) are particularly important because they can reveal vital information about the oldest, in-situ component of the Milky Way. We aim at deriving mean metallicities and radial velocities for 13 GCs that lie towards the bulge and are generally associated with this component. We use near infrared low resolution spectroscopy with the FORS2 instrument on the VLT to measure the wavelengths and equivalent widths of the CaII triplet (CaT) lines for a number of stars per cluster. We derive radial velocities, ascertain membership and apply known calibrations to determine metallicities for cluster members, for a mean of 11 members per cluster. We derive mean cluster RV values to 3 km/s, and mean metallicities to 0.05 dex. Our sample has metallicities lying between -0.21 and -1.64 and is distributed between the traditional metal-rich BGC peak near [Fe/H] aprox. -0.5 and a more metal-poor peak around [Fe/H] aprox. -1.1, which has recently been identified. These latter are candidates for the oldest GCs in the Galaxy, if blue horizontal branches are present, and include BH 261, NGC 6401, NGC 6540, NGC 6642, and Terzan 9. Finally, Terzan 10 is even more metal-poor. However, dynamically, Terzan 10 is likely an intruder from the halo, possibly associated with the Gaia-Enceladus or Kraken accretion events. Terzan 10 is also confirmed as an Oosterhotype II GC based on our results. The lone halo intruder in our sample, Terzan 10, is conspicuous for also having by far the lowest metallicity, and casts doubt on the possibility of any bonafide BGCs at metallicities below about aprox. -1.5.

astro-ph.GA↗

arXiv2022

First-principles Calculations of Raman and Infrared Spectroscopy For Phase Identification and Strain Calibration of Hafnia

Using density functional perturbation theory (DFPT) we computed the phonon frequencies, Raman and IR activities of hafnia polymorphs (P4$_{2}$nmc, Pca2$_{1}$, Pmn2$_{1}$, Pbca OI, brookite, and baddeleyite) for phase identification. We investigated the evolution of Raman and IR activities with respect to epitaxial strain and provide plots of frequency differences as a function of strain for experimental calibration and identification of the strain state of the sample. We found Raman signatures of different hafnia polymorphs: $ω(A_{1g})=300$ cm$^{-1}$ for P4$_{2}$nmc, $ω(A_{1})=343$ cm$^{-1}$ for Pca2$_{1}$, $ω(B_{2})=693$ cm$^{-1}$ for Pmn2$_{1}$, $ω(A_{g})=513$ cm$^{-1}$ for Pbca (OI), $ω(A_{g})=384$ cm$^{-1}$ for brookite, and $ω(A_{g}) = 496$ cm$^{-1}$ for baddeleyite. We also identified the Raman $B_{1g}$ mode, an anti-phase vibration of dipole moments, ( $ω(B_{1g}) = 758$ cm$^{-1}$ for OI, $ω(B_{1g})= 784$ cm$^{-1}$ for brookite) as the Raman signature of antipolar Pbca structures. We calculated a large splitting between longitudinal optical (LO) and transverse optical (TO) modes ($Δ{ω_{\text{LO-TO}}(A^{z}_{1})}=255$ cm$^{-1}$ in Pca2$_{1}$, and $Δ{ω_{\text{LO-TO}}(A_{1})}=263$ cm$^{-1}$ in Pmn2$_{1}$) to the same order as those observed in perovskite ferroelectrics, and related them to the anomalously large Born effective charges of Hf atoms ($Z^{*}(\text{Hf}) = 5.54$).

cond-mat.mtrl-sci↗

arXiv2022

Origin of Ferroelectricity in Hafnia from Epitaxial Strain

Ferroelectric hafnia is being explored for next generation electronics due to its robust ferroelectricity in nanoscale samples and its compatibility with silicon. However, its ferroelectricity is not understood. Other ferroelectrics usually lose their ferroelectricity for nanoscopic samples and thin films, and the hafnia ground state is non-polar baddeleyite. Here we study hafnia with density functional theory (DFT) under epitaxial strain, and find that strain not only stabilizes the ferroelectric phases, but also leads to unstable modes and a downhill path in energy from the high temperature tetragonal structure. We find that for tensile epitaxial strains corresponding to a square substrate of lattice constant $a_{epi} \geqslant 5.38$ Å~the ferroelectric \oI is most stable, even more stable than baddeleyite. Furthermore, we find that under tensile epitaxial strain $η$ the tetragonal phase will distort to one of the two ferroelectric phases: for $η> 1.5$\%, the $Γ^{-}_{5}$ mode is unstable and leads to \oII , and at $η> 3.75$\% coupling between this mode and the zone boundary M1 mode leads to \oI.

cond-mat.mtrl-sci↗

arXiv2021

Superconductivity in SrB3C3 clathrate

We predict superconductivity for the carbon-boron clathrate SrB3C3 at 27-43 K for Coulomb pseudopotential (mu*) values between 0.17 and 0.10 using first-principles calculations with conventional electron-phonon coupling. Electrical transport measurements, facilitated by a novel in situ experimental design compatible with extreme synthesis conditions (>3000 K at 50 GPa), show non-hysteretic resistivity drops that track the calculated magnitude and pressure dependence of superconductivity for mu*=0.15, and transport measurements collected under applied magnetic fields confirm superconductivity with an onset Tc of approximately 20 K at 40 GPa. Carbon-based clathrates thus represent a new class of superconductors similar to other covalent metals like MgB2 and doped fullerenes. Carbon clathrates share structures similar to superconducting superhydrides, but covalent C-B bonds allow metastable persistence at ambient conditions.

cond-mat.mtrl-sci↗

arXiv2021

The Milky Way bar and bulge revealed by APOGEE DR16 and Gaia EDR3

We investigate the inner regions of the Milky Way with a sample of unprecedented size and coverage thanks to APOGEE DR16 and Gaia EDR3 data. Our inner Galactic sample has more than 26,000 stars within $|X_{\rm Gal}| <5$ kpc, $|Y_{\rm Gal}| <3.5$ kpc, $|Z_{\rm Gal}| <1$ kpc, and we also make the analysis for a foreground-cleaned sub-sample of 8,000 stars more representative of the bulge-bar populations. The inner Galaxy shows a clear chemical discontinuity in key abundance ratios [$α$/Fe], [C/N], and [Mn/O], probing different enrichment timescales, which suggests a star formation gap (quenching) between the high- and low-$α$ populations. For the first time, we are able to fully characterize the different populations co-existing in the innermost regions of the Galaxy via joint analysis of the distributions of rotational velocities, metallicities, orbital parameters and chemical abundances. The chemo-kinematic analysis reveals the presence of the bar; of an inner thin disk; of a thick disk, and of a broad metallicity population, with a large velocity dispersion, indicative of a pressure supported component. We find and characterize chemically and kinematically a group of counter-rotating stars, which could be the result of a gas-rich merger event or just the result of clumpy star formation during the earliest phases of the early disk, which migrated into the bulge. Finally, based on the 6D information we assign stars a probability value of being on a bar orbit and find that most of the stars with large bar orbit probabilities come from the innermost 3 kpcs. Even stars with a high probability of belonging to the bar show the chemical bimodality in the [$α$/Fe] vs. [Fe/H] diagram. This suggests bar trapping to be an efficient mechanism, explaining why stars on bar orbits do not show a significant distinct chemical abundance ratio signature.

astro-ph.GA↗

arXiv2021

Stable Polar Oxynitrides through Epitaxial Strain

We investigate energetically favorable structures of ABO$_2$N oxynitrides as functions of pressure and strain via swarm-intelligence-based structure prediction methods, DFT lattice dynamics and first-principles molecular dynamics. We predict several thermodynamically stable polar oxynitride perovskites under high pressures. In addition, we find that ferroelectric polar phases of perovskite-structured oxynitrides can be thermodynamically stable and synthesized at high pressure on appropriate substrates. The dynamical stability of the ferroelectric oxynitrides under epitaxial strain at ambient pressure also imply the possibility to synthesize them using pulsed laser deposition or other atomic layer deposition methods. Our results have broad implications for further exploration of other oxynitride materials as well. We performed first-principles molecular dynamics and find that the polar perovskite of YSiO$_2$N is metastable up to at least 600 K under compressive epitaxial strain before converting to the stable wollastonite-like structures. YSiO$_2$N is stabilized under pressure with extensional epitaxial strain. We predict that LaSi$_2$N, LaGeO$_2$N, BiSiO$_2$N, and BiGeO$_2$N are metastable as ferroelectric perovskites at zero pressure even without epitaxial strain.

cond-mat.mtrl-sci↗