192 research outputs found
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 -Fe,
and hcp, or -Fe), as well ferromagnetic bcc iron (-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 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
Thermodynamic stability of Fe/O solid solution at inner-core conditions
We present a new technique which allows the fully {\em ab initio} calculation
of the chemical potential of a substitutional impurity in a high-temperature
crystal, including harmonic and anharmonic lattice vibrations. The technique
uses the combination of thermodynamic integration and reference models
developed recently for the {\em ab initio} calculation of the free energy of
liquids and anharmonic solids. We apply the technique to the case of the
substitutional oxygen impurity in h.c.p. iron under Earth's core conditions,
which earlier static {\em ab initio} calculations indicated to be
thermodynamically very unstable. Our results show that entropic effects arising
from the large vibrational amplitude of the oxygen impurity give a major
reduction of the oxygen chemical potential, so that oxygen dissolved in h.c.p.
iron may be stabilised at concentrations up a few mol % under core conditions
Compressibility and Electronic Structure of MgB2 up to 8 GPa
The lattice parameters of MgB2 up to pressures of 8 GPa were determined using
high-resolution x-ray powder diffraction in a diamond anvil cell. The bulk
modulus, B0, was determined to be 151 +-5 GPa. Both experimental and
first-principles calculations indicate nearly isotropic mechanical behavior
under pressure. This small anisotropy is in contrast to the 2 dimensional
nature of the boron pi states. The pressure dependence of the density of states
at the Fermi level and a reasonable value for the average phonon frequency
account within the context of BCS theory for the reduction of Tc under
pressure.Comment: REVTeX file. 4 pages, 4 figure
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.Comment: 23 pages, 16 figures. To appear in AGU Geodynamics Series book on
"Core structure, dynamics, and rotation", V. Dehant et al. (eds.
Pressure-dependence of electron-phonon coupling and the superconducting phase in hcp Fe - a linear response study
A recent experiment by Shimizu et al. has provided evidence of a
superconducting phase in hcp Fe under pressure. To study the
pressure-dependence of this superconducting phase we have calculated the phonon
frequencies and the electron-phonon coupling in hcp Fe as a function of the
lattice parameter, using the linear response (LR) scheme and the full potential
linear muffin-tin orbital (FP-LMTO) method. Calculated phonon spectra and the
Eliashberg functions indicate that conventional s-wave
electron-phonon coupling can definitely account for the appearance of the
superconducting phase in hcp Fe. However, the observed change in the transition
temperature with increasing pressure is far too rapid compared with the
calculated results. For comparison with the linear response results, we have
computed the electron-phonon coupling also by using the rigid muffin-tin (RMT)
approximation. From both the LR and the RMT results it appears that
electron-phonon interaction alone cannot explain the small range of volume over
which superconductivity is observed. It is shown that
ferromagnetic/antiferromagnetic spin fluctuations as well as scattering from
magnetic impurities (spin-ordered clusters) can account for the observed values
of the transition temperatures but cannot substantially improve the agreeemnt
between the calculated and observed presure/volume range of the superconducting
phase. A simplified treatment of p-wave pairing leads to extremely small ( K) transition temperatures. Thus our calculations seem to rule out
both - and - wave superconductivity in hcp Fe.Comment: 12 pages, submitted to PR
Elasticity of (Mg,Fe)SiO 3 ‐perovskite at high pressures
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95173/1/grl15683.pd
Structure and Dynamics of Liquid Iron under Earth's Core Conditions
First-principles molecular dynamics simulations based on density-functional
theory and the projector augmented wave (PAW) technique have been used to study
the structural and dynamical properties of liquid iron under Earth's core
conditions. As evidence for the accuracy of the techniques, we present PAW
results for a range of solid-state properties of low- and high-pressure iron,
and compare them with experimental values and the results of other
first-principles calculations. In the liquid-state simulations, we address
particular effort to the study of finite-size effects, Brillouin-zone sampling
and other sources of technical error. Results for the radial distribution
function, the diffusion coefficient and the shear viscosity are presented for a
wide range of thermodynamic states relevant to the Earth's core. Throughout
this range, liquid iron is a close-packed simple liquid with a diffusion
coefficient and viscosity similar to those of typical simple liquids under
ambient conditions.Comment: 13 pages, 8 figure
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