37,545 research outputs found
Spin Dependent Tunneling in FM|semiconductor|FM structures
Here we show that ordinary band structure codes can be used to understand the
mechanisms of coherent spin-injection at interfaces between ferromagnets and
semiconductors. This approach allows the screening of different material
combinations for properties useful for obtaining high tunneling
magnetoresistance (TMR). We used the Vienna Ab-initio Simulation Code (VASP) to
calculate the wave function character of each band in periodic epitaxial
Fe(100)|GaAs(100) and Fe(100)|ZnSe(100) structures. It is shown that Fe wave
functions of different symmetry near Fermi energy decay differently in the GaAs
and ZnSe.Comment: Accepted for publication in MMM'05 Proceedings. 7 pages, 5 figure
Effects of 3-d and 4-d-transition metal substitutional impurities on the electronic properties of CrO2
We present first-principles based density functional theory calculations of
the electronic and magnetic structure of CrO2 with 3d (Ti through Cu) and 4d
(Zr through Ag) substitutional impurities. We find that the half-metallicity of
CrO2 remains intact for all of the calculated substitutions. We also observe
two periodic trends as a function of the number of valence electrons: if the
substituted atom has six or fewer valence electrons (Ti-Cr or Zr-Mo), the
number of down spin electrons associated with the impurity ion is zero,
resulting in ferromagnetic (FM) alignment of the impurity magnetic moment with
the magnetization of the CrO2 host. For substituent atoms with eight to ten
(Fe-Ni or Ru-Pd with the exception of Ni), the number of down spin electrons
contributed by the impurity ion remains fixed at three as the number
contributed to the majority increases from one to three resulting in
antiferromagnetic (AFM) alignment between impurity moment and host
magnetization. The origin of this variation is the grouping of the impurity
states into 3 states with approximate "t2g" symmetry and 2 states with
approximate "eg" symmetry. Ni is an exception to the rule because a
Jahn-Teller-like distortion causes a splitting of the Ni eg states. For Mn and
Tc, which have 8 valence electrons, the zero down spin and 3 down spin
configurations are very close in energy. For Cu and Ag atoms, which have 11
valence electrons, the energy is minimized when the substituent ion contributes
5 Abstract down-spin electrons. We find that the interatomic exchange
interactions are reduced for all substitutions except for the case of Fe for
which a modest enhancement is calculated for interactions along certain
crystallographic directions.Comment: 26 pages, 10 figures, 2 table
Theoretical investigation into the possibility of very large moments in Fe16N2
We examine the mystery of the disputed high-magnetization \alpha"-Fe16N2
phase, employing the Heyd-Scuseria-Ernzerhof screened hybrid functional method,
perturbative many-body corrections through the GW approximation, and onsite
Coulomb correlations through the GGA+U method. We present a first-principles
computation of the effective on-site Coulomb interaction (Hubbard U) between
localized 3d electrons employing the constrained random-phase approximation
(cRPA), finding only somewhat stronger on-site correlations than in bcc Fe. We
find that the hybrid functional method, the GW approximation, and the GGA+U
method (using parameters computed from cRPA) yield an average spin moment of
2.9, 2.6 - 2.7, and 2.7 \mu_B per Fe, respectively.Comment: 8 pages, 3 figure
Determining the Anisotropic Exchange Coupling of CrO_2 via First-Principles Density Functional Theory Calculations
We report a study of the anisotropic exchange interactions in bulk CrO_2
calculated from first principles within density functional theory. We determine
the exchange coupling energies, using both the experimental lattice parameters
and those obtained within DFT, within a modified Heisenberg model Hamiltonian
in two ways. We employ a supercell method in which certain spins within a cell
are rotated and the energy dependence is calculated and a spin-spiral method
that modifies the periodic boundary conditions of the problem to allow for an
overall rotation of the spins between unit cells. Using the results from each
of these methods, we calculate the spin-wave stiffness constant D from the
exchange energies using the magnon dispersion relation. We employ a Monte Carlo
method to determine the DFT-predicted Curie temperature from these calculated
energies and compare with accepted values. Finally, we offer an evaluation of
the accuracy of the DFT-based methods and suggest implications of the competing
ferro- and antiferromagnetic interactions.Comment: 10 pages, 13 figure
The "quasi-stable" lipid shelled microbubble in response to consecutive ultrasound pulses
Controlled microbubble stability upon exposure to consecutive ultrasound exposures is important for increased sensitivity in contrast enhanced ultrasound diagnostics and manipulation for localised drug release. An ultra high-speed camera operating at 13 × 10 6 frames per second is used to show that a physical instability in the encapsulating lipid shell can be promoted by ultrasound, causing loss of shell material that depends on the characteristics of the microbubble motion. This leads to well characterized disruption, and microbubbles follow an irreversible trajectory through the resonance peak, causing the evolution of specific microbubble spectral signatures. © 2012 American Institute of Physics
Neutrino Physics and Nuclear Axial Two-Body Interactions
We consider the counter-term describing isoscalar axial two-body currents in
the nucleon-nucleon interaction, L1A, in the effective field theory approach.
We determine this quantity using the solar neutrino data. We investigate the
variation of L1A when different sets of data are used.Comment: 8 pages with 4 figures. To be published in the Proceedings of the
Conference "Blueprints For The Nucleus: From First Principles to Collective
Motion" held at Feza Gursey Institute, Istanbul, Turkey; May 17 -22, 200
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