332 research outputs found
Domain structure of bulk ferromagnetic crystals in applied fields near saturation
We investigate the ground state of a uniaxial ferromagnetic plate with
perpendicular easy axis and subject to an applied magnetic field normal to the
plate. Our interest is the asymptotic behavior of the energy in macroscopically
large samples near the saturation field. We establish the scaling of the
critical value of the applied field strength below saturation at which the
ground state changes from the uniform to a branched domain magnetization
pattern and the leading order scaling behavior of the minimal energy.
Furthermore, we derive a reduced sharp-interface energy giving the precise
asymptotic behavior of the minimal energy in macroscopically large plates under
a physically reasonable assumption of small deviations of the magnetization
from the easy axis away from domain walls. On the basis of the reduced energy,
and by a formal asymptotic analysis near the transition, we derive the precise
asymptotic values of the critical field strength at which non-trivial
minimizers (either local or global) emerge. The non-trivial minimal energy
scaling is achieved by magnetization patterns consisting of long slender
needle-like domains of magnetization opposing the applied fieldComment: 38 pages, 7 figures, submitted to J. Nonlin. Sci
Systematic generation of finite-range atomic basis sets for linear-scaling calculations
Basis sets of atomic orbitals are very efficient for density functional
calculations but lack a systematic variational convergence.
We present a variational method to optimize numerical atomic orbitals using a
single parameter to control their range.
The efficiency of the basis generation scheme is tested and compared with
other schemes for multiple zeta basis sets.
The scheme shows to be comparable in quality to other widely used schemes
albeit offering better performance for linear-scaling computations
Electron localization : band-by-band decomposition, and application to oxides
Using a plane wave pseudopotential approach to density functional theory we
investigate the electron localization length in various oxides. For this
purpose, we first set up a theory of the band-by-band decomposition of this
quantity, more complex than the decomposition of the spontaneous polarization
(a related concept), because of the interband coupling. We show its
interpretation in terms of Wannier functions and clarify the effect of the
pseudopotential approximation. We treat the case of different oxides: BaO,
-PbO, BaTiO and PbTiO. We also investigate the variation of the
localization tensor during the ferroelectric phase transitions of BaTiO as
well as its relationship with the Born effective charges
Uniaxial Phase Transition in Si : Ab initio Calculations
Based on a previously proposed thermodynamic analysis, we study the relative
stabilities of five Si phases under uniaxial compression using ab initio
methods. The five phases are diamond, beta-tin, sh, sc, and hcp structures. The
possible phase-transition patterns were investigated by considering the phase
transitions between any two chosen phases of the five phases. By analyzing the
different conributions to the relative pahse stability, we identified the most
important factors in reducing the phase-transition pressures at uniaxial
compression. We also show that it is possible to have phase transitions occur
only when the phases are under uniaxial compression, in spite of no phase
transition when under hydrostatic commpression. Taking all five phases into
consideration, the phase diagram at uniaxial compression was constructed for
pressures under 20 GPa. The stable phases were found to be diamond, beta-tin
and sh structures, i.e. the same as those when under hydrostatic condition.
According to the phase diagram, direct phase transition from the diamond to the
sh phase is possible if the applied uniaxial pressures, on increasing, satisfy
the condition of Px>Pz. Simiilarly, the sh-to-beta-tin transition on
increeasing pressures is also possible if the applied uniaxial pressures are
varied from the condition of Px>Pz, on which the phase of sh is stable, to that
of Px<Pz, on which the beta-tin is stable
Impact ionization in GaAs: a screened exchange density functional approach
Results are presented of a fully ab-initio calculation of impact ionization
rates in GaAs within the density functional theory framework, using a
screened-exchange formalism and the highly precise all-electron full-potential
linearized augmented plane wave (FLAPW) method. The calculated impact
ionization rates show a marked orientation dependence in {\bf k} space,
indicating the strong restrictions imposed by the conservation of energy and
momentum. This anisotropy diminishes as the impacting electron energy
increases. A Keldysh type fit performed on the energy-dependent rate shows a
rather soft edge and a threshold energy greater than the direct band gap. The
consistency with available Monte Carlo and empirical pseudopotential
calculations shows the reliability of our approach and paves the way to
ab-initio calculations of pair production rates in new and more complex
materials.Comment: 11 pages, 4 figures, Submitted to Phys. Rev.
Superconducting and pseudogap phases from scaling near a Van Hove singularity
We study the quantum corrections to the Fermi energy of a two-dimensional
electron system, showing that it is attracted towards the Van Hove singularity
for a certain range of doping levels. The scaling of the Fermi level allows to
cure the infrared singularities left in the BCS channel after renormalization
of the leading logarithm near the divergent density of states. A phase of
d-wave superconductivity arises beyond the point of optimal doping
corresponding to the peak of the superconducting instability. For lower doping
levels, the condensation of particle-hole pairs due to the nesting of the
saddle points takes over, leading to the opening of a gap for quasiparticles in
the neighborhood of the singular points.Comment: 4 pages, 6 Postscript figures, the physical discussion of the results
has been clarifie
First-principles study of the polar O-terminated ZnO surface in thermodynamic equilibrium with oxygen and hydrogen
Using density-functional theory in combination with a thermodynamic formalism
we calculate the relative stability of various structural models of the polar
O-terminated (000-1)-O surface of ZnO. Model surfaces with different
concentrations of oxygen vacancies and hydrogen adatoms are considered.
Assuming that the surfaces are in thermodynamic equilibrium with an O2 and H2
gas phase we determine a phase diagram of the lowest-energy surface structures.
For a wide range of temperatures and pressures we find that hydrogen will be
adsorbed at the surface, preferentially with a coverage of 1/2 monolayer. At
high temperatures and low pressures the hydrogen can be removed and a structure
with 1/4 of the surface oxygen atoms missing becomes the most stable one. The
clean, defect-free surface can only exist in an oxygen-rich environment with a
very low hydrogen partial pressure. However, since we find that the
dissociative adsorption of molecular hydrogen and water (if also the
Zn-terminated surface is present) is energetically very preferable, it is very
unlikely that a clean, defect-free (000-1)-O surface can be observed in
experiment.Comment: 10 pages, 4 postscript figures. Uses REVTEX and epsf macro
Electronic and Structural Properties of a 4d-Perovskite: Cubic Phase of SrZrO
First-principles density functional calculations are performed within the
local density approximation to study the electronic properties of SrZrO, an
insulating 4d-perovskite, in its high-temperature cubic phase, above 1400 K, as
well as the generic 3d-perovskite SrTiO, which is also a d^0-insulator and
cubic above 105 K, for comparison reasons. The energy bands, density of states
and charge density distributions are obtained and a detailed comparison between
their band structures is presented. The results are discussed also in terms of
the existing data in the literature for both oxides.Comment: 5 pages, 2 figure
Spin Stiffness of Mesoscopic Quantum Antiferromagnets
We study the spin stiffness of a one-dimensional quantum antiferromagnet in
the whole range of system sizes and temperatures . We show that for
integer and half-odd integer spin case the stiffness differs fundamentally in
its and dependence, and that in the latter case the stiffness exhibits
a striking dependence on the parity of the number of sites. Integer spin chains
are treated in terms of the non-linear sigma model, while half-odd integer spin
chains are discussed in a renormalization group approach leading to a Luttinger
liquid with Aharonov-Bohm type boundary conditions.Comment: 12 pages, LaTe
Structural, Electronic, and Magnetic Properties of MnO
We calculate the structural, electronic, and magnetic properties of MnO from
first principles, using the full-potential linearized augmented planewave
method, with both local-density and generalized-gradient approximations to
exchange and correlation. We find the ground state to be of rhombohedrally
distorted B1 structure with compression along the [111] direction,
antiferromagnetic with type-II ordering, and insulating, consistent with
experiment. We show that the distortion can be understood in terms of a
Heisenberg model with distance dependent nearest-neighbor and
next-nearest-neighbor couplings determined from first principles. Finally, we
show that magnetic ordering can induce significant charge anisotropy, and give
predictions for electric field gradients in the ground-state rhombohedrally
distorted structure.Comment: Submitted to Physical Review B. Replaced: regenerated figures to
resolve font problems in automatically generated pd
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