493 research outputs found
Order parameters in the Verwey phase transition
The Verwey phase transition in magnetite is analyzed on the basis of the
Landau theory. The free energy functional is expanded in a series of components
belonging to the primary and secondary order parameters. A low-temperature
phase with the monoclinic P2/c symmetry is a result of condensation of two
order parameters X_3 and \Delta_5 . The temperature dependence of the shear
elastic constant C_44 is derived and the mechanism of its softening is
discussed.Comment: 4 pages, 1 figur
Mechanism of the Verwey transition in magnetite
By combining {\it ab initio} results for the electronic structure and phonon
spectrum with the group theory, we establish the origin of the Verwey
transition in FeO. Two primary order parameters with and
symmetries are identified. They induce the phase transformation from
the high-temperature cubic to the low-temperature monoclinic structure. The
on-site Coulomb interaction between 3d electrons at Fe ions plays a crucial
role in this transition -- it amplifies the coupling of phonons to conduction
electrons and thus opens a gap at the Fermi energy. {\it Published in Phys.
Rev. Lett. {\bf 97}, 156402 (2006).}Comment: 5 pages, 3 figure
Comparative study of the electronic structures of Fe3O4 and Fe2SiO4
The electronic properties of two spinels FeO and FeSiO are
studied by the density functional theory. The local Coulomb repulsion and
the Hund's exchange between the electrons on iron are included. For
, both spinels are half-metals, with the minority states at the
Fermi level. Magnetite remains a metal in a cubic phase even at large values of
. The metal-insulator transition is induced by the phonon, which
lowers the total energy and stabilizes the charge-orbital ordering.
FeSiO transforms to a Mott insulating state for eV with a gap
. The antiferromagnetic interactions induce the tetragonal
distortion, which releases the geometrical frustration and stabilizes the
long-range order. The differences of electronic structures in the high-symmetry
cubic phases and the distorted low-symmetry phases of both spinels are
discussed.Comment: 6 pages, 6 figure
Vibrational properties of alpha- and sigma-phase Fe-Cr alloy
Experimental investigation as well as theoretical calculations, of the
Fe-partial phonon density-of-states (DOS) for nominally Fe_52.5Cr_47.5 alloy
having (a) alpha- and (b) sigma-phase structure were carried out. The former at
sector 3-ID of the Advanced Photon Source, using the method of nuclear resonant
inelastic X-ray scattering, and the latter with the direct method [K. Parlinski
et al., Phys. Rev. Lett. {78, 4063 (1997)]. The characteristic features of
phonon DOS, which differentiate one phase from the other, were revealed and
successfully reproduced by the theory. Various data pertinent to the dynamics
such as Lamb-Mossbauer factor, f, kinetic energy per atom, E_k, and the mean
force constant, D, were directly derived from the experiment and the
theoretical calculations, while vibrational specific heat at constant volume,
C_V, and vibrational entropy, S were calculated using the Fe-partial DOS. Using
the values of f and C_V, we determined values for Debye temperatures, T_D. An
excellent agreement for some quantities derived from experiment and
first-principles theory, like C_V and quite good one for others like D and S
was obtained.Comment: 4 pages, 3 figure
Ab initio and nuclear inelastic scattering studies of FeSi/GaAs heterostructures
The structure and dynamical properties of the FeSi/GaAs(001) interface
are investigated by density functional theory and nuclear inelastic scattering
measurements. The stability of four different atomic configurations of the
FeSi/GaAs multilayers is analyzed by calculating the formation energies and
phonon dispersion curves. The differences in charge density, magnetization, and
electronic density of states between the configurations are examined. Our
calculations unveil that magnetic moments of the Fe atoms tend to align in a
plane parallel to the interface, along the [110] direction of the FeSi
crystallographic unit cell. In some configurations, the spin polarization of
interface layers is larger than that of bulk FeSi. The effect of the
interface on element-specific and layer-resolved phonon density of states is
discussed. The Fe-partial phonon density of states measured for the FeSi
layer thickness of three monolayers is compared with theoretical results
obtained for each interface atomic configuration. The best agreement is found
for one of the configurations with a mixed Fe-Si interface layer, which
reproduces the anomalous enhancement of the phonon density of states below 10
meVComment: 14 pages, 9 figures, 4 table
Origin of the Verwey transition in magnetite: Group theory, electronic structure, and lattice dynamics study
The Verwey phase transition in magnetite has been analyzed using the group
theory methods. It is found that two order parameters with the symmetries
and induce the structural transformation from the high-temperature
cubic to the low-temperature monoclinic phase. The coupling between the order
parameters is described by the Landau free energy functional. The electronic
and crystal structure for the cubic and monoclinic phases were optimized using
the {\it ab initio} density functional method. The electronic structure
calculations were performed within the generalized gradient approximation
including the on-site interactions between 3d electrons at iron ions -- the
Coulomb element and Hund's exchange . Only when these local interactions
are taken into account, the phonon dispersion curves, obtained by the direct
method for the cubic phase, reproduce the experimental data. It is shown that
the interplay of local electron interations and the coupling to the lattice
drives the phonon order parameters and is responsible for the opening of the
gap at the Fermi energy. Thus, it is found that the metal-insulator transition
in magnetite is promoted by local electron interactions, which significantly
amplify the electron-phonon interaction and stabilize weak charge order
coexisting with orbital order of the occupied states at Fe ions. This
provides a scenario to understand the fundamental problem of the origin of the
Verwey transition in magnetite.Comment: 17 pages, 5 figures, 8 tables. Accepted version to be published in
Phys. Rev.
Spin-Phonon Coupling in Iron Pnictide Superconductors
The magnetic moment in the parent phase of the iron-pnictide superconductors
varies with composition even when the nominal charge of iron is unchanged. We
propose the spin-lattice coupling due to the magneto-volume effect as the
primary origin of this effect, and formulate a Landau theory to describe the
dependence of the moment to the Fe-As layer separation. We then compare the
superconductive critical temperature of doped iron pnictides to the local
moment predicted by the theory, and suggest that the spin-phonon coupling may
play a role in the superconductivity of this compound
prototype conductor for the transmission line magnet
The Very Large Hadron Collider (VLHC), under consideration for construction at Fermilab in the next 1-2 decades, is a 100 TeV cm pp collider. A major cost driver is the magnet. R&D is underway on several possible magnet designs. A low-field (2T) superferric magnet, sometimes called a transmission line magnet, may be the most cost- effective route to the VLHC. Although NbTi is now the cheapest superconductor measured in cost/kA-meter, Nb/sub 3/Al has the potential advantage that it remains superconducting at higher temperature. It may be particularly suited to the single "turn" and long straight lengths of the transmission line design. The combination of the simple magnet design and the higher strain tolerance than e.g. Nb/sub 3/Sn allows a simple process of cable fabrication, reaction, and magnet assembly. This higher strain tolerance is an advantage for splicing in the field. Sumitomo Electric Industries is producing an Nb/sub 3/Al conductor for the Fermilab low-field magnet program. (9 refs)
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