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R. E. Cohen

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

106 records · Page 6Linked to original sources
arXiv2003

Non-collinear magnetism in iron at high pressures

Using a first principles based, magnetic tight-binding total energy model, the magnetization energy and moments are computed for various ordered spin configurations in the high pressure polymorphs of iron (fcc, or $γ$-Fe, and hcp, or $ε$-Fe), as well ferromagnetic bcc iron ($α$-Fe). For hcp, a non-collinear, antiferromagnetic, spin configuration that minimizes unfavorable ferromagnetic nearest neighbor ordering is the lowest energy state and is more stable than non-magnetic $ε$ iron up to about 75 GPa. Accounting for non-collinear magnetism yields better agreement with the experimental equation of state, in contrast to the non-magnetic equation of state, which is in poor agreement with experiment below 50 GPa.

cond-mat.mtrl-sci↗
arXiv2002

Importance of Magnetism in Phase Stability, Equations of State, and Elasticity

The effects of magnetism on high pressure properties of transition metals and transition metal compounds can be quite important. In the case of Fe, magnetism is responsible for stability of the body-centered cubic (bcc) phase at ambient conditions, and the large thermal expansivity in face-centered cubic (fcc) iron, and also has large effects on the equation of state and elasticity of hexagonal close-packed (hcp) iron. In transition metal oxides, local magnetic moments are responsible for their insulating behavior. LDA+U results are presented for CoO and FeO, and predictions are made for high pressure metallization. The inclusion of a local Coulomb repulsion, U, greatly inhibits the high-spin low-spin transitions found with conventional exchange-correlation functionals (i.e. generalized gradient corrections, GGA). We discuss theory and computations for the effects of magnetism on high pressure cohesive properties.

cond-mat.mtrl-sci↗
arXiv2002

Physical Properties of Iron in the Inner Core

The Earth's inner core plays a vital role in the dynamics of our planet and is itself strongly exposed to dynamic processes as evidenced by a complex pattern of elastic structure. To gain deeper insight into the nature of these processes we rely on a characterization of the physical properties of the inner core which are governed by the material physics of its main constituent, iron. Here we review recent research on structure and dynamics of the inner core, focusing on advances in mineral physics. We will discuss results on core composition, crystalline structure, temperature,and various aspects of elasticity. Based on recent computational results, we will show that aggregate seismic properties of the inner core can be explained by temperature and compression effects on the elasticity of pure iron, and use single crystal anisotropy to develop a speculative textural model of the inner core that can explain major aspects of inner core anisotropy.

physics.geo-ph↗
arXiv2002

Metal-insulator and magnetic collapse transitions in FeO at high pressure: predictions of the LDA+U method

The accuracy of LDA + U total energies is tested by predicting the high pressure behavior of rocksalt-structured FeO, including metal-insulator and magnetic transitions, equation of state, and lattice strain, as functions of the Coulomb repulsion U. For U = 4.6 eV, 6.0 eV and 8.0 eV, two self-consistent solutions, one with rhombohedral and one with monoclinic electronic symmetry, are found, and for U = 4.6 eV the monoclinic solution becomes more stable than the rhombohedral solution at 100 GPa, leading to an insulator-metal transition. With increasing U, metallization occurs at higher pressures. Despite the insulator-metal transition, the high-spin magnetic state persists to pressures greater than 300 GPa for U > 0. The LDA + U method also gives improved agreement with experiments for ground state properties as compared to LDA and GGA methods.

cond-mat.str-el↗
arXiv2001

Evidence of local magnetic order in hcp iron from Raman mode splitting

Experimental measurements of Raman spectra for hcp iron at high pressure show two modes over a considerable pressure range in contrast to the prediction of one doubly degenerate mode for the hcp lattice. We use density functional theory to investigate the influence of magnetic order on the Raman active modes of hcp iron. We find an antiferromagnetic state that lifts the degeneracy of the transverse optical mode, and yields stable antiferromagnetic moments up to approximately 60 GPa (55 Bohr$^3$). The resulting frequencies of the two transverse optical modes are in good agreement with the experimental Raman shifts, lending support to the existence of local antiferromagnetic order in hcp iron.

cond-mat.mtrl-sci↗
arXiv2001

High Pressure Thermoelasticity of Body-centered Cubic Tantalum

We have investigated the thermoelasticity of body-centered cubic (bcc) tantalum from first principles by using the linearized augmented plane wave (LAPW) and mixed--basis pseudopotential methods for pressures up to 400 GPa and temperatures up to 10000 K. Electronic excitation contributions to the free energy were included from the band structures, and phonon contributions were included using the particle-in-a-cell (PIC) model. The computed elastic constants agree well with available ultrasonic and diamond anvil cell data at low pressures, and shock data at high pressures. The shear modulus $c_{44}$ and the anisotropy change behavior with increasing pressure around 150 GPa because of an electronic topological transition. We find that the main contribution of temperature to the elastic constants is from the thermal expansivity. The PIC model in conjunction with fast self-consistent techniques is shown to be a tractable approach to studying thermoelasticity.

cond-mat.mtrl-sci↗
arXiv2001

First-principles Calculation of the Formation Energy in MgO-CaO Solid Solutions

The electronic structure and total energy were calculated for ordered and disordered MgO-CaO solid solutions within the multiple scattering theory in real space and the local density approximation. Based on the dependence of the total energy on the unit cell volume the equilibrium lattice parameter and formation energy were determined for different solution compositions. The formation energy of the solid solutions is found to be positive that is in agreement with the experimental phase diagram, which shows a miscibility gap.

cond-mat.mtrl-sci↗
arXiv2001

Sulfur, Chlorine, and Argon in Planetary Nebulae. IIA: Observations of a Southern Sample

In this paper we present fully reduced and dereddened emission line strengths for a sample of 45 southern Type II planetary nebulae (PNe). The spectrophotometry for these PNe covers an extended optical/near-IR range from 3600 - 9600 Angstroms. This PN study and subsequent analysis (presented in a companion paper), together with a similar treatment for a northern PN sample, is aimed at addressing the lack of homogeneous, consistently observed, reduced, and analyzed data sets that include the near-IR [S III] lines at 9069 and 9532 Angstroms. The use of Type II objects only is intended to select disk nebulae that are uncontaminated by nucleosynthetic products of the progenitor star. Extending spectra redward to include the strong [S III] lines enables us to look for consistency between S++ abundances inferred from these lines and from the more accessible, albeit weaker, [S III] line at 6312 Angstroms.

astro-ph↗
arXiv2000

Vacancy Formation Energy at High Pressures in Tantalum

We have computed the variation of the vacancy formation energy with pressure for Ta. Total energy calculations were performed for 16 and 54 atom supercells using a mixed basis pseudopotential method which uses pseudo-atomic orbitals and low energy plane waves as the basis set, within density functional theory (LDA and GGA). The vacancy formation energy is found to increase from 2.95 eV at ambient pressures to 6.96 eV at 300 GPa, and the vacancy formation volume decreases from being 53.4% of the bulk volume per atom at ambient pressure to 19.6% at 300 GPa, for a 54 atom supercell. The structural relaxation energy is found to increase with pressure from 14% of the vacancy formation energy at ambient pressure to 16% at 300 GPa.

cond-mat.mtrl-sci↗
arXiv1999

First-Principles Elastic Constants for the hcp Transition Metals Fe, Co, and Re at High Pressure

The elastic constant tensors for the hcp phases of three transition metals (Co, Re, and Fe) are computed as functions of pressure using the Linearized Augmented Plane Wave method with both the local density and generalized gradient approximations. Spin-polarized states are found to be stable for Co (ferromagnetic) and Fe (antiferromagnetic at low pressure). The elastic constants of Co and Re are compared to experimental measurements near ambient conditions and excellent agreement is found. Recent measurements of the lattice strain in high pressure experiments when interpreted in terms of elastic constants for Re and Fe are inconsistent with the calculated moduli.

cond-mat.mtrl-sci↗
arXiv1996

The Melting Curve and Premelting of MgO

The melting curve for MgO was obtained using molecular dynamics and a non-empirical, many-body potential. We also studied premelting effects by computing the dynamical structure factor in the crystal on approach to melting. The melting curve simulations were performed with periodic boundary conditions with cells up to 512 atoms using the ab-initio Variational Induced Breathing (VIB) model. The melting curve was obtained by computing $% ΔH_m$ and $ΔV_m$ and integrating the Clapeyron equation. Our $% ΔH_m$ is in agreement with previous estimates and we obtain a reasonable $ΔV_m$, but our melting slope dT/dP (114 K/GPa) is three times greater than that of Zerr and Boehler [1994] (35 K/GPa), suggesting a problem with the experimental melting curve, or an indication of exotic, non-ionic behavior of MgO liquid. We computed $S(q,ω)$ from simulations of 1000 atom clusters using the Potential Induced Breathing (PIB) model. A low frequency peak in the dynamical structure factor $% S(q,ω)$ arises below the melting point which appears to be related to the onset of bulk many-atom diffusive exchanges. These exchanges may help destabilize the crystalline state and be related to intrinsic crystalline instability suggested in earlier simulations.

mtrl-th↗
arXiv1996

Notes on the static dielectric response function in the density functional theory

We discuss several aspects of the dielectric response theory application to the density functional theory. This field has been an unceasing source of confusion during several decades. The most frequent reasons for this confusion are (a) uncritical tranfer of the results, especially regarding so-called local field corrections, obtained in many-body perturbation theory onto density functional theory, and (b) mixing up the statements true for the exact density functional theory with those applicable to the local density approximation only. In these notes we try to draw an appropriate lines between those theories. We also discuss a newly introduced (X. Gonze, Ph. Ghosez, and R.W. Godby, Phys. Rev. Lett., 74, 4035, 1995) "polarization+density functional" and show that within a given (e.g., local density) approximation to the exchange-correlation energy the Gonze et al approach is exactly equivalent to the conventional one.

cond-mat↗
arXiv1995

Hohenberg-Kohn theorem is valid

In a recent PRL, Gonze et al claim that the density functional theory in Hohenberg-Kohn formulation is not valid for crystalline solids in a homogenious electric field. They introduce another theory, density-and-polarization functional theory, which has to substitute the DFT in the calculations of the long-wave dielectric response. We show that the theorem proven by Gonzo et al holds in addition to, and not instead of, the Hohenberg-Kohn theorem.

cond-mat↗
arXiv1995

Periodic Slab LAPW Computations for Ferroelectric BaTiO$_3$

Linearized augmented plane wave (LAPW) calculations are performed for periodic (001) and (111) slabs of BaTiO$_3$ to understand the effects of surfaces on ferroelectric BaTiO$_3$. The (111) slab is found to be much less stable than the (001) slab. The average surface energies are respectively 3700 erg/cm$^2$ and 1600 erg/cm$^2$. The depolarization field is sufficiently large in the ideal unrelaxated slab to completely inhibit the ferroelectric instability. No mid-gap surface states are evident, but there are surface states in the upper gap in the unrelaxed slab and at the top of the valence band. The dangling surface Ti bonds self-heal making the Ti-O surface very reactive and an excellent epitaxial substrate. The charge density on atoms only one unit cell away from the surface are almost identical to the bulk. Keywords: ferroelectric, surface, slab, electronic structure, depolarization, BaTiO$_3$, thin film

mtrl-th↗
arXiv1995

High Pressure Effects on Thermal Properties of MgO

Using the non-empirical Variational Induced Breathing (VIB) model, the thermal properties of periclase (MgO) under high pressures and temperatures are investigated using molecular dynamics, which includes all anharmonic effects. Equations of state for temperatures up to 3000K and pressures up to 310 GPa were calculated. Bulk modulus, thermal expansivity, Anderson-Gruneisen parameter, thermal pressure, Gruneisen parameter and their pressure and temperature dependencies are studied in order to better understand high pressure effects on thermal properties. The results agree very well with experiments and show that the thermal expansivity decreases with pressure up to about 100 GPa ($η$=0.73), and is almost pressure and temperature independent above this compression. It is also effected by anharmonicity at zero pressure and temperatures above 2500K. The thermal pressure changes very little with increasing pressures and temperatures, and the Gruneisen parameter is temperature independent and decreases slightly with pressure.

mtrl-th↗