arXiv subjects

R. E. Cohen

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

At least 55 records · Page 3Linked to original sources

arXiv2017

Effect of aging and Mn substitution on anisotropy of third generation piezoelectrics

We predict a new candidate high-temperature high-pressure structure of FeSiO$_3$ with space-group symmetry Cmmm by applying an evolutionary algorithm within DFT+U that we call post-perovskite II (PPv-II). An exhaustive search found no other competitive candidate structures with ABO$_3$ composition. We compared the X-ray diffraction (XRD) pattern of FeSiO$_3$ PPv-II with experimental results of the recently reported "H-phase" of (Fe,Mg)SiO$_3$. The intensities and positions of two main X-ray diffraction peaks of PPv-II FeSiO$_3$ compare well with those of the H-phase. We also calculated the static equation of state, the enthalpy and the bulk modulus of the PPv-II phase and compared it with those of perovskite (Pv) and post-perovskite (PPv) phases of FeSiO$_3$. According to the static DFT+U computations the PPv-II phase of FeSiO$_3$ is less stable than Pv and PPv phases under lower mantle pressure conditions at T=0 K and has a higher volume. PPv-II may be entropically stabilized, and may be a stable phase in Earth's lower mantle, coexisting with {α-PbO$_2$ (Columbite-structure) silica and perovskite, or with magnesiowustite and/or ferropericlase, depending on bulk composition.

cond-mat.mtrl-sci↗

arXiv2017

Origin of Negative Longitudinal Piezoelectric Effect

Piezoelectrics with negative longitudinal piezoelectric coefficients will contract in the direction of an applied electric field. Such piezoelectrics are thought to be rare but there is no fundamental physics preventing the realization of negative longitudinal piezoelectric effect in a single-phase material. Using first-principles calculations, we demonstrate that several hexagonal $ABC$ ferroelectrics possess significant negative longitudinal piezoelectric effect. The data mining of a first-principles-based database of piezoelectrics reveals that this effect is a general phenomenon. The origin of this unusual piezoelectric response relies on the strong ionic bonds associated with small effective charges and rigid potential energy surfaces. Moreover, ferroelectrics with negative longitudinal piezoelectric coefficients show anomalous pressure-enhanced ferroelectricity. Our results offer design principles to aid the search of new piezoelectrics for novel electromechanical device applications.

cond-mat.mtrl-sci↗

arXiv2017

Electric field induced phase transition and electrocaloric effect in PMN-PT

Ferroelectric perovskite oxides possess a large electrocaloric (EC) effect, but usually at high temperatures near the ferroelectric/paraelectric phase transition temperature, which limits their potential application as next-generation solid-state cooling devices. We use classical molecular dynamics to study the electric field induced phase transitions and EC effect in PMN-PT (PbMg1/3Nb2/3O3-PbTiO3). We find that the maximum EC strength of PMN-PT occurs within the morphotropic phase boundary (MPB) region at 300 K. The large adiabatic temperature change is caused by easy rotation of polarization within the MPB region.

cond-mat.mtrl-sci↗

arXiv2017

Multiscale Simulations of Defect Dipole-Enhanced Electromechanical Coupling at Dilute Defect Concentrations

The role of defects in solids of mixed ionic-covalent bonds such as ferroelectric oxides is complex. Current understanding of defects on ferroelectric properties at the single-defect level remains mostly at the empirical level, and the detailed atomistic mechanisms for many defect-mediated polarization-switching processes have not been convincingly revealed quantum mechanically. We simulate the polarization-electric field (P-E) and strain-electric field ({$ε$}-E) hysteresis loops for BaTiO3 in the presence of generic defect dipoles with large-scale molecular dynamics and provide a detailed atomistic picture of the defect dipole-enhanced electromechanical coupling. We develop a general first-principles-based atomistic model, enabling a quantitative understanding of the relationship between macroscopic ferroelectric properties and dipolar impurities of different orientations, concentrations, and dipole moments. We find that the collective orientation of dipolar defects relative to the external field is the key microscopic structure feature that strongly affects materials hardening/softening and electromechanical coupling. We show that a small concentration (0.1 at.%) of defect dipoles dramatically improves electromechanical response. This offers the opportunity to improve the performance of inexpensive polycrystalline ferroelectric ceramics through defect dipole engineering for a range of applications including piezoelectric sensors, actuators, and transducers.

cond-mat.mtrl-sci↗

arXiv2017

Synthesis of Polar Ordered Oxynitride Perovskite

For decades, numerous attempts have been made to produce polar oxynitride perovskites, where some of the oxygen are replaced by nitrogen, but a polar ordered oxynitride has never been demonstrated. Caracas and Cohen studied possible ordered polar oxynitrides within density functional theory (DFT) and found a few candidates that were predicted to be insulating and at least metastable. YSiO2N stood out with huge predicted polarization and nonlinear optic coefficients. In this study, we demonstrate the synthesis of perovskite-structured YSiO2N by using a combination of a diamond anvil cell and in-situ laser heating technique. Subsequent in-situ X-ray diffraction, second harmonic generation, and Raman scattering measurements confirm that it is polar and a strong nonlinear optical material, with structure and properties similar to those predicted by DFT.

cond-mat.mtrl-sci↗

arXiv2017

Improving the Functional Control of Aged Ferroelectrics using Insights from Atomistic Modelling

We provide a fundamental insight into the microscopic mechanisms of the ageing processes. Using large scale molecular dynamics simulations of the prototypical ferroelectric material PbTiO3, we demonstrate that the experimentally observed ageing phenomena can be reproduced from intrinsic interactions of defect-dipoles related to dopant-vacancy associates, even in the absence of extrinsic effects. We show that variation of the dopant concentration modifies the material's hysteretic response. We identify a universal method to reduce loss and tune the electromechanical properties of inexpensive ceramics for efficient technologies.

cond-mat.mtrl-sci↗

arXiv2017

The peculiar Na-O anticorrelation of the bulge globular cluster NGC 6440

Context. Galactic Globular Clusters (GCs) are essential tools to understand the earliest epoch of the Milky Way, since they are among the oldest objects in the Universe and can be used to trace its formation and evolution. Current studies using high resolution spectroscopy for many stars in each of a large sample of GCs allow us to develop a detailed observational picture about their formation and their relation with the Galaxy. However, it is necessary to complete this picture by including GCs that belong to all major Galactic components, including the Bulge. Aims. Our aim is to perform a detailed chemical analyses of the bulge GC NGC 6440 in order to determine if this object has Multiple Populations (MPs) and investigate its relation with the Bulge of the Milky Way and with the other Galactic GCs, especially those associated with the Bulge, which are largely poorly studied. Methods. We determined the stellar parameters and the chemical abundances of light elements (Na, Al), iron-peak elements (Fe, Sc, Mn, Co, Ni), $α$-elements (O, Mg, Si, Ca, Ti) and heavy elements (Ba, Eu) in seven red giant members of NGC 6440 using high resolution spectroscopy from FLAMES@UVES. Results. We found a mean iron content of [Fe/H]=-0.50$\pm$0.03 dex in agreement with other studies. We found no internal iron spread. On the other hand, Na and Al show a significant intrinsic spread, but the cluster has no significant O-Na anticorrelation nor exhibits a Mg-Al anticorrelation. The $α$-elements show good agreement with the Bulge field star trend, although they are at the high alpha end and are also higher than those of other GCs of comparable metallicity. The heavy elements are dominated by the r-process, indicating a strong contribution by SNeII. The chemical analysis suggests an origin similar to that of the Bulge field stars.

astro-ph.SR↗

arXiv2017

Interpenetrating Graphene Networks: Three-dimensional Node-line Semimetals with Massive Negative Linear Compressibilities

We investigated the stability and mechanical and electronic properties of fifteen metastable mixed $sp^2$-$sp^3$ carbon allotropes in the family of interpenetrating graphene networks (IGNs) using density functional theory (DFT) within the generalized gradient approximation (GGA). IGN allotropes exhibit non-monotonic bulk and linear compressibilities before their structures irreversibly transform into new configurations under large hydrostatic compression. The maximum bulk compressibilities vary widely between structures and range from 3.6 to 306 TPa$^{-1}$. We find all the IGN allotropes have negative linear compressibilities with maximum values varying from -0.74 to -133 TPa$^{-1}$. The maximal negative linear compressibility of Z33 (-133 TPa$^{-1}$ at 3.4 GPa) exceeds previously reported values at pressures higher than 1.0 GPa. IGN allotropes can be classified as either armchair- or zigzag-type, and these two types of IGNs exhibit different electronic properties. Zigzag-type IGNs are node-line semimetals, while armchair-type IGNs are either semiconductors or node-loop or node-line semimetals. Experimental synthesis of these IGN allotropes might be realized since their formation enthalpies relative to graphite are only 0.1 - 0.5 eV/atom (that of C$_{60}$ fullerene is about 0.4 eV/atom), and energetically feasible binary compound pathways are possible.

cond-mat.mtrl-sci↗

arXiv2017

Stable Charged Antiparallel Domain Walls in Hyperferroelectrics

Charge-neutral 180$^\circ$ domain walls that separate domains of antiparallel polarization directions are common structural topological defects in ferroelectrics. In normal ferroelectrics, charged 180$^\circ$ domain walls running perpendicular to the polarization directions are highly energetically unfavorable because of the depolarization field and are difficult to stabilize. We explore both neutral and charged 180$^\circ$ domain walls in hyperferroelectrics, a class of proper ferroelectrics with persistent polarization in the presence of a depolarization field, using density functional theory. We obtain zero temperature equilibrium structures of head-to-head and tail-to-tail walls in recently discovered $ABC$-type hexagonal hyperferroelectrics. Charged domain walls can also be stabilized in canonical ferroelectrics represented by LiNbO$_3$ without any dopants, defects or mechanical clamping. First-principles electronic structure calculations show that charged domain walls can reduce and even close the band gap of host materials and support quasi-two-dimensional electron(hole) gas with enhanced electrical conductivity.

cond-mat.mtrl-sci↗

arXiv2017

The Correlation Between Mixing Length and Metallicity on the Giant Branch: Implications for Ages in the Gaia Era

In the updated APOGEE-Kepler catalog, we have asteroseismic and spectroscopic data for over 3000 first ascent red giants. Given the size and accuracy of this sample, these data offer an unprecedented test of the accuracy of stellar models on the post-main-sequence. When we compare these data to theoretical predictions, we find a metallicity dependent temperature offset with a slope of around 100 K per dex in metallicity. We find that this effect is present in all model grids tested and that theoretical uncertainties in the models, correlated spectroscopic errors, and shifts in the asteroseismic mass scale are insufficient to explain this effect. Stellar models can be brought into agreement with the data if a metallicity dependent convective mixing length is used, with $ Δα_{\rm ML, YREC} \sim 0.2$ per dex in metallicity, a trend inconsistent with the predictions of three dimensional stellar convection simulations. If this effect is not taken into account, isochrone ages for red giants from the Gaia data will be off by as much as a factor of 2 even at modest deviations from solar metallicity ([Fe/H]=$-$0.5).

astro-ph.SR↗

arXiv2017

Origin of Stationary Domain Wall Enhanced Ferroelectric Susceptibility

Ferroelectrics usually adopt a multi-domain state with domain walls separating domains with polarization axes oriented differently. It has long been recognized that domain walls can dramatically impact the properties of ferroelectric materials. The enhancement of low-field susceptibility/permittivity under subswitching conditions is usually attributed to the reversible domain wall vibration. Recent experiments highlight the stationary domain wall contribution to the dielectric susceptibility irrespective of any lateral displacements or deformations of the wall. We study the effects of domain walls on low-field permittivity of PbTiO$_3$ with density functional theory and molecular dynamics simulations. The static dielectric constant is calculated as a function of increasing domain wall density and temperature. We find an increase of dielectric permittivity with increasing domain wall density, which is expected to occur at low driving field where the lateral motion of domain walls is forbidden. Real-space decomposition of dielectric response reveals that frustrated dipoles within the finite width of the domain walls are responsible for the enhanced low-field permittivity.

cond-mat.mtrl-sci↗

arXiv2017

Baade's window with APOGEE: Metallicities, ages and chemical abundances

Baade's window (BW) is one of the most observed Galactic bulge fields in terms of chemical abundances. Due to its low and homogeneous interstellar absorption it is considered as a calibration field for Galactic bulge studies. In the era of large spectroscopic surveys, calibration fields such as BW are necessary to cross calibrate the stellar parameters and individual abundances of the APOGEE survey. We use the APOGEE BW stars to derive their metallicity distribution function (MDF) and individual abundances, for $α$- and iron-peak elements of the APOGEE ASPCAP pipeline (DR13), as well as the age distribution for stars in BW. We determine the MDF of APOGEE stars in BW and find a remarkable agreement with that of the Gaia-ESO survey (GES). Both exhibit a clear bimodal distribution. We also find that the Mg-metallicity planes of both surveys agree well, except for the metal-rich part ([Fe/H] >0.1), where APOGEE finds systematically higher Mg abundances with respect to the GES. The ages based on the [C/N] ratio reveal a bimodal age distribution, with a major old population at 10 Gyr, with a decreasing tail towards younger stars. A comparison between APOGEE estimates and stellar parameters, and those determined by other sources reveals detectable systematic offsets, in particular for spectroscopic surface gravity estimates. In general, we find a good agreement between individual abundances of O, Na, Mg, Al, Si, K, Ca, Cr, Mn, Co, and Ni from APOGEE with that of literature values. We have shown that in general APOGEE data show a good agreement in terms of MDF and individual chemical abundances with respect to literature works. Using the [C/N] ration we found a significant fraction of young stars in BW which is in agreement with the model of Haywood et al. (2016).

astro-ph.GA↗

arXiv2017

The $Gaia$-ESO Survey: the inner disk intermediate-age open cluster NGC 6802

Milky Way open clusters are very diverse in terms of age, chemical composition, and kinematic properties. Intermediate-age and old open clusters are less common, and it is even harder to find them inside the solar Galactocentric radius, due to the high mortality rate and strong extinction inside this region. NGC 6802 is one of the inner disk open clusters (IOCs) observed by the $Gaia$-ESO survey (GES). This cluster is an important target for calibrating the abundances derived in the survey due to the kinematic and chemical homogeneity of the members in open clusters. Using the measurements from $Gaia$-ESO internal data release 4 (iDR4), we identify 95 main-sequence dwarfs as cluster members from the GIRAFFE target list, and eight giants as cluster members from the UVES target list. The dwarf cluster members have a median radial velocity of $13.6\pm1.9$ km s$^{-1}$, while the giant cluster members have a median radial velocity of $12.0\pm0.9$ km s$^{-1}$ and a median [Fe/H] of $0.10\pm0.02$ dex. The color-magnitude diagram of these cluster members suggests an age of $0.9\pm0.1$ Gyr, with $(m-M)_0=11.4$ and $E(B-V)=0.86$. We perform the first detailed chemical abundance analysis of NGC 6802, including 27 elemental species. To gain a more general picture about IOCs, the measurements of NGC 6802 are compared with those of other IOCs previously studied by GES, that is, NGC 4815, Trumpler 20, NGC 6705, and Berkeley 81. NGC 6802 shows similar C, N, Na, and Al abundances as other IOCs. These elements are compared with nucleosynthetic models as a function of cluster turn-off mass. The $α$, iron-peak, and neutron-capture elements are also explored in a self-consistent way.

astro-ph.GA↗

arXiv2016

Ultra-deep GEMINI near-infrared observations of the bulge globular cluster NGC 6624

We used ultra-deep $J$ and $K_s$ images secured with the near-infrared GSAOI camera assisted by the multi-conjugate adaptive optics system GeMS at the GEMINI South Telescope in Chile, to obtain a ($K_s$, $J-K_s$) color-magnitude diagram (CMD) for the bulge globular cluster NGC 6624. We obtained the deepest and most accurate near-infrared CMD from the ground for this cluster, by reaching $K_s$ $\sim$ 21.5, approximately 8 magnitudes below the horizontal branch level. The entire extension of the Main Sequence (MS) is nicely sampled and at $K_s$ $\sim$ 20 we detected the so-called MS "knee" in a purely near-infrared CMD. By taking advantage of the exquisite quality of the data, we estimated the absolute age of NGC 6624 ($t_{age}$ = 12.0 $\pm$ 0.5 Gyr), which turns out to be in good agreement with previous studies in the literature. We also analyzed the luminosity and mass functions of MS stars down to M $\sim$ 0.45 M$_{\odot}$ finding evidence of a significant increase of low-mass stars at increasing distances from the cluster center. This is a clear signature of mass segregation, confirming that NGC 6624 is in an advanced stage of dynamical evolution.

astro-ph.SR↗

arXiv2016

Structural Diversity in Lithium Carbides

The lithium-carbon binary system possesses a broad range of chemical compounds, which exhibit fascinating chemical bonding characteristics that give rise diverse and technologically important properties. While lithium carbides with various compositions have been studied or suggested previously, the crystal structures of these compounds are far from well understood. In this work we present the first comprehensive survey of all ground state (GS) structures of lithium carbides over a broad range of thermodynamic conditions, using \textit{ab initio} density functional theory (DFT) crystal structure searching methods. Thorough searches were performed for 29 stoichiometries ranging from Li$_{12}$C to LiC$_{12}$ at 0 GPa as well as 40 GPa. Based on formation enthalpies from optimized van der Waals density functional calculations, three thermodynamically stable phases (Li$_4$C$_3$, Li$_2$C$_2$ and LiC$_{12}$) were identified at 0 GPa, and seven thermodynamically stable phases (Li$_8$C, Li$_6$C, Li$_4$C, Li$_8$C$_3$, Li$_2$C, Li$_3$C$_4$, and Li$_2$C$_3$) were predicted at 40 GPa. A rich diversity of carbon bonding, including monomers, dimers, trimers, nanoribbons, sheets and frameworks, was found within these structures, and the dimensionality of carbon connectivity existing within each phase was observed to increase with increasing carbon concentration. Of particular interest, we find that the well-known composition LiC$_6$ is actually a metastable one. We also find a unique coexistence of carbon monomers and dimers within the predicted thermodynamically stable phase Li$_8$C$_3$, and different widths of carbon nanoribbons coexist in a metastable phase of Li$_2$C$_2$ (\textit{Imm}2). Interesting mixed \textit{sp}$^2$-\textit{sp}$^3$ carbon frameworks are predicted in metastable phases with composition LiC$_6$.

cond-mat.mtrl-sci↗

arXiv2016

Low-pressure phase diagram of crystalline benzene from quantum Monte Carlo

We study the low-pressure (0 to 10 GPa) phase diagram of crystalline benzene using quantum Monte Carlo (QMC) and density functional theory (DFT) methods. We consider the $Pbca$, $P4_32_12$, and $P2_1/c$ structures as the best candidates for phase I and phase II. We perform diffusion quantum Monte Carlo (DMC) calculations to obtain accurate static phase diagrams as benchmarks for modern van der Waals density functionals. We use density functional perturbation theory to compute phonon contribution in the free-energy calculations. Our DFT enthalpy-pressure phase diagram indicates that the $Pbca$ and $P2_1/c$ structures are the most stable phases within the studied pressure range. The DMC Gibbs free-energy calculations predict that the room temperature $Pbca$ to $P2_1/c$ phase transition occurs at 2.1(1) GPa. This prediction is consistent with available experimental results at room temperature. Our DMC calculations show an estimate of 50.6$\pm$0.5 kJ/mol for crystalline benzene lattice energy.

physics.chem-ph↗

arXiv2015

Chemical accuracy from quantum Monte Carlo for the Benzene Dimer

We report an accurate study of interactions between Benzene molecules using variational quantum Monte Carlo (VMC) and diffusion quantum Monte Carlo (DMC) methods. We compare these results with density functional theory (DFT) using different van der Waals (vdW) functionals. In our QMC calculations, we use accurate correlated trial wave functions including three-body Jastrow factors, and backflow transformations. We consider two benzene molecules in the parallel displaced (PD) geometry, and find that by highly optimizing the wave function and introducing more dynamical correlation into the wave function, we compute the weak chemical binding energy between aromatic rings accurately. We find optimal VMC and DMC binding energies of -2.3(4) and -2.7(3) kcal/mol, respectively. The best estimate of the CCSD(T)/CBS limit is -2.65(2) kcal/mol [E. Miliordos et al, J. Phys. Chem. A 118, 7568 (2014)]. Our results indicate that QMC methods give chemical accuracy for weakly bound van der Waals molecular interactions, comparable to results from the best quantum chemistry methods.

physics.chem-ph↗

arXiv2014

Effects of Manganese Addition on the Electronic Structure of BaTiO$_3$

Mn is used as a dopant to improve the electromechanical properties of perovskite oxides. We elucidate the role of Mn defects and associated vacancies on the electronic, atomic and ferroelectric properties of BaTiO$_3$. Using density functional theory (DFT) and DFT+U we investigate the equilibrium geometry and electronic properties of Mn ion on A or B sites and with compensating oxygen vacancies. We study the change in the oxidation state of Mn in response to local environment changes, such as the presence of oxygen vacancies.

cond-mat.mtrl-sci↗