10,198 research outputs found
Local structure of In_(0.5)Ga_(0.5)As from joint high-resolution and differential pair distribution function analysis
High resolution total and indium differential atomic pair distribution
functions (PDFs) for In_(0.5)Ga_(0.5)As alloys have been obtained by high
energy and anomalous x-ray diffraction experiments, respectively. The first
peak in the total PDF is resolved as a doublet due to the presence of two
distinct bond lengths, In-As and Ga-As. The In differential PDF, which involves
only atomic pairs containing In, yields chemical specific information and helps
ease the structure data interpretation. Both PDFs have been fit with structure
models and the way in that the underlying cubic zinc-blende lattice of
In_(0.5)Ga_(0.5)As semiconductor alloy distorts locally to accommodate the
distinct In-As and Ga-As bond lengths present has been quantified.Comment: 9 pages, 7 figur
Lattice dynamics and correlated atomic motion from the atomic pair distribution function
The mean-square relative displacements (MSRD) of atomic pair motions in
crystals are studied as a function of pair distance and temperature using the
atomic pair distribution function (PDF). The effects of the lattice vibrations
on the PDF peak widths are modelled using both a multi-parameter Born
von-Karman (BvK) force model and a single-parameter Debye model. These results
are compared to experimentally determined PDFs. We find that the near-neighbor
atomic motions are strongly correlated, and that the extent of this correlation
depends both on the interatomic interactions and crystal structure. These
results suggest that proper account of the lattice vibrational effects on the
PDF peak width is important in extracting information on static disorder in a
disordered system such as an alloy. Good agreement is obtained between the BvK
model calculations of PDF peak widths and the experimentally determined peak
widths. The Debye model successfully explains the average, though not detailed,
natures of the MSRD of atomic pair motion with just one parameter. Also the
temperature dependence of the Debye model largely agrees with the BvK model
predictions. Therefore, the Debye model provides a simple description of the
effects of lattice vibrations on the PDF peak widths.Comment: 9 pages, 11 figure
Implications of the B20 Crystal Structure for the Magneto-electronic Structure of MnSi
Due to increased interest in the unusual magnetic and transport behavior of
MnSi and its possible relation to its crystal structure (B20) which has unusual
coordination and lacks inversion symmetry, we provide a detailed analysis of
the electronic and magnetic structure of MnSi. The non-symmorphic P2_13
spacegroup leads to unusual fourfold degenerate states at the zone corner R
point, as well as ``sticking'' of pairs of bands throughout the entire
Brillouin zone surface. The resulting Fermi surface acquires unusual features
as a result of the band sticking. For the ferromagnetic system (neglecting the
long wavelength spin spiral) with the observed moment of 0.4 \mu_B/Mn, one of
the fourfold levels at R in the minority bands falls at the Fermi energy (E_F),
and a threefold majority level at k=0 also falls at E_F. The band sticking and
presence of bands with vanishing velocity at E_F imply an unusually large phase
space for long wavelength, low energy interband transitions that will be
important for understanding the unusual resistivity and far infrared optical
behavior.Comment: Nine two-column pages with eight figures include
Direct observation of the formation of polar nanoregions in Pb(MgNb)O using neutron pair distribution function analysis
Using neutron pair distribution function (PDF) analysis over the temperature
range from 1000 K to 15 K, we demonstrate the existence of local polarization
and the formation of medium-range, polar nanoregions (PNRs) with local
rhombohedral order in a prototypical relaxor ferroelectric
Pb(MgNb)O. We estimate the volume fraction of the PNRs as a
function of temperature and show that this fraction steadily increases from 0 %
to a maximum of 30% as the temperature decreases from 650 K to 15 K.
Below T200 K the PNRs start to overlap as their volume fraction reaches
the percolation threshold. We propose that percolating PNRs and their
concomitant overlap play a significant role in the relaxor behavior of
Pb(MgNb)O.Comment: 4 pages, 3 figure
Giant Magnetoelectric Effect in a Multiferroic Material with a High Ferroelectric Transition Temperature
We present a unique example of giant magnetoelectric effect in a conventional
multiferroic HoMnO3, where polarization is very large (~56 mC/m2) and the
ferroelectric transition temperature is higher than the magnetic ordering
temperature by an order. We attribute the uniqueness of the giant
magnetoelectric effect to the ferroelectricity induced entirely by the
off-center displacement of rare earth ions with large magnetic moments. This
finding suggests a new avenue to design multiferroics with large polarization
and higher ferroelectric transition temperature as well as large
magnetoelectric effects
The MHD Kelvin-Helmholtz Instability III: The Role of Sheared Magnetic Field in Planar Flows
We have carried out simulations of the nonlinear evolution of the
magnetohydrodynamic (MHD) Kelvin-Helmholtz (KH) instability for compressible
fluids in -dimensions, extending our previous work by Frank et al
(1996) and Jones \etal (1997). In the present work we have simulated flows in
the x-y plane in which a ``sheared'' magnetic field of uniform strength
``smoothly'' rotates across a thin velocity shear layer from the z direction to
the x direction, aligned with the flow field. We focus on dynamical evolution
of fluid features, kinetic energy dissipation, and mixing of the fluid between
the two layers, considering their dependence on magnetic field strength for
this geometry. The introduction of magnetic shear can allow a Cat's Eye-like
vortex to form, even when the field is stronger than the nominal linear
instability limit given above. For strong fields that vortex is asymmetric with
respect to the preliminary shear layer, however, so the subsequent dissipation
is enhanced over the uniform field cases of comparable field strength. In fact,
so long as the magnetic field achieves some level of dynamical importance
during an eddy turnover time, the asymmetries introduced through the magnetic
shear will increase flow complexity, and, with that, dissipation and mixing.
The degree of the fluid mixing between the two layers is strongly influenced by
the magnetic field strength. Mixing of the fluid is most effective when the
vortex is disrupted by magnetic tension during transient reconnection, through
local chaotic behavior that follows.Comment: 14 pages including 9 figures (4 figures in degraded jpg format), full
paper with original quality figures available via anonymous ftp at
ftp://canopus.chungnam.ac.kr/ryu/mhdkh2d.uu, to appear in The Astrophysical
Journa
Preheating and Affleck-Dine leptogenesis after thermal inflation
Previously, we proposed a model of low energy Affleck-Dine leptogenesis in
the context of thermal inflation. The lepton asymmetry is generated at the end
of thermal inflation, which occurs at a relatively low energy scale with the
Hubble parameter somewhere in the range 1 \keV \lesssim H \lesssim 1 \MeV.
Thus Hubble damping will be ineffective in bringing the Affleck-Dine field into
the lepton conserving region near the origin, leaving the possibility that the
lepton number could be washed out. Previously, we suggested that preheating
could damp the amplitude of the Affleck-Dine field allowing conservation of the
lepton number. In this paper, we demonstrate numerically that preheating does
efficiently damp the amplitude of the Affleck-Dine field and that the lepton
number is conserved as the result. In addition to demonstrating a crucial
aspect of our model, it also opens the more general possibility of low energy
Affleck-Dine baryogenesis.Comment: 38 pages, 17 figure
Giant negative magnetoresistance in semiconductors doped by multiply charged deep impurities
A giant negative magnetoresistance has been observed in bulk germanium doped
with multiply charged deep impurities. Applying a magnetic field the resistance
may decrease exponentially at any orientation of the field. A drop of the
resistance as much as about 10000% has been measured at 6 T. The effect is
attributed to the spin splitting of impurity ground state with a very large
g-factor in the order of several tens depending on impurity.Comment: 4 pages, 4 figure
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