857 research outputs found
X-ray diffraction peak profiles from threading dislocations in GaN epitaxial films
We analyze the lineshape of x-ray diffraction profiles of GaN epitaxial
layers with large densities of randomly distributed threading dislocations. The
peaks are Gaussian only in the central, most intense part of the peak, while
the tails obey a power law. The decay typical for random dislocations
is observed in double-crystal rocking curves. The entire profile is well fitted
by a restricted random dislocation distribution. The densities of both edge and
screw threading dislocations and the ranges of dislocation correlations are
obtained
Pressure dependence of two-level systems in disordered atomic chain
The dependence of two-level systems in disordered atomic chain on pressure,
both positive and negative was studied numerically. The disorder was produced
through the use of interatomic pair potentials having more than one energy
minimum. It was found that there exists a correlation between the energy
separation of the minima of two-level systems Delta and the variation of this
separation with pressure. The correlation may have either positive or negative
sign, implying that the asymmetry of two-level systems may in average increase
or decrease with pressure depending on the interplay of different interactions
between atoms in disordered state. The values of Delta depend on the sign of
pressure.Comment: 5 pages, 5 figure
Geometric approach to the phenomenological theory of phase transitions of the second kind
Geometrical approach to the phenomenological theory of phase transitions of
the second kind at constant pressure and variable temperature is
proposed. Equilibrium states of a system at zero external field and fixed
and are described by points in three-dimensional space with coordinates
, the order parameter, , the temperature and , the thermodynamic
potential. These points form the so-called zero field curve in the space. Its branch point coincides with the critical point of the system.
The small parameter of the theory is the distance from the critical point along
the zero-field curve. It is emphasized that no explicit functional dependency
of on and is imposed. It is shown that using space one cannot overcome well-known difficulties of the Landau theory
of phase transitions and describe non-analytical behavior of real systems in
the vicinity of the critical point. This becomes possible only if one increases
the dimensionality of the space, taking into account the dependency of the
thermodynamic potential not only on and , but also on near (local)
order parameters . In this case under certain conditions it is
possible to describe anomalous increase of the specific heat when the
temperature of the system approaches the critical point from above as well as
from below the critical temperature .Comment: 20 pages. 2 figures. Requires elsart package available at
ftp://ftp.shsu.edu/tex-archive/macros/latex209/contrib/elsevier
A new neutron study of the short range order inversion in FeCr
We have performed new neutron diffuse scattering measurements in
FeCr solid solutions, in a concentration range 0x0.15, where
the atomic distribution shows an inversion of the short range order. By
optimizing the signal-background ratio, we obtain an accurate determination of
the concentration of inversion x =0.110(5). We determine the near neighbor
atomic short range order parameters and pair potentials, which change sign at
x. The experimental results are compared with previous first principle
calculations and atomistic simulations.Comment: 6 pages; 6 figure
Synthesis, crystal structure and chemical stability of the superconductor FeSe_{1-x}
We report on a comparative study of the crystal structure and the magnetic
properties of FeSe1-x (x= 0.00 - 0.15) superconducting samples by neutron
powder diffraction and magnetization measurements. The samples were synthesized
by two different methods: a 'low-temperature' one using powders as a starting
material at T =700 C and a "high-temperature' method using solid pieces of Fe
and Se at T= 1070 C. The effect of a starting (nominal) stoichiometry on the
phase purity of the obtained samples, the superconducting transition
temperature Tc, as well as the chemical instability of FeSe1-x at ambient
conditions were investigated. It was found that in the Fe-Se system a stable
phase exhibiting superconductivity at Tc~8K exists in a narrow range of
selenium concentration (FeSe0.974(2)).Comment: 7 pages, 7 figures, 1 tabl
Theory of temperature dependence of the Fermi surface-induced splitting of the alloy diffuse-scattering intensity peak
The explanation is presented for the temperature dependence of the fourfold
intensity peak splitting found recently in diffuse scattering from the
disordered Cu3Au alloy. The wavevector and temperature dependence of the
self-energy is identified as the origin of the observed behaviour. Two
approaches for the calculation of the self-energy, the high-temperature
expansion and the alpha-expansion, are proposed. Applied to the Cu3Au alloy,
both methods predict the increase of the splitting with temperature, in
agreement with the experimental results.Comment: 4 pages, 3 EPS figures, RevTeX, submitted to J. Phys. Condens. Matter
(Letter to the Editor
Depth-dependent critical behavior in V2H
Using X-ray diffuse scattering, we investigate the critical behavior of an
order-disorder phase transition in a defective "skin-layer" of V2H. In the
skin-layer, there exist walls of dislocation lines oriented normal to the
surface. The density of dislocation lines within a wall decreases continuously
with depth. We find that, because of this inhomogeneous distribution of
defects, the transition effectively occurs at a depth-dependent local critical
temperature. A depth-dependent scaling law is proposed to describe the
corresponding critical ordering behavior.Comment: 5 pages, 4 figure
Electron self-trapping at quantum and classical critical points
Using Feynman path integral technique estimations of the ground state energy
have been found for a conduction electron interacting with order parameter
fluctuations near quantum critical points. In some cases only \textit{singular}
perturbation theory in the coupling constant emerges for the electron ground
state energy. It is shown that an autolocalized state (quantum fluctuon) can be
formed and its characteristics have been calculated depending on critical
exponents for both weak and strong coupling regimes. The concept of fluctuon is
considered also for the classical critical point (at finite temperatures) and
the difference between quantum and classical cases has been investigated. It is
shown that, whereas the quantum fluctuon energy is connected with a true
boundary of the energy spectrum, for classical fluctuon it is just a
saddle-point solution for the chemical potential in the exponential density of
states fluctuation tail.Comment: 45 pages, 1 eps figure, elsart, submitted to Annals of Physic
Origin of the anomaly in diffuse scattering from disordered Pt-V alloys
An explanation of the anomalous concentration dependence of diffuse
scattering from the Pt-V alloy system (splitting of the (100) short-range order
intensity peak with increasing Pt content) is proposed. The effect is
attributed to the competition between the interaction and self-energy
curvatures. A similar temperature behaviour is predicted.Comment: 5 pages, 5 EPS figures, RevTeX; minor editorial corrections, text as
publishe
Temperature independent diffuse scattering and elastic lattice deformations in relaxor PbMg1/3Nb2/3O3
The results of diffuse neutron scattering experiment on PbMg1/3Nb2/3O3 single
crystal above the Burns temperature are reported. It is shown that the high
temperature elastic diffuse component is highly anisotropic in low-symmetry
Brillouin zones and this anisotropy can be described using Huang scattering
formalism assuming that the scattering originates from mesoscopic lattice
deformations due to elastic defects. The qualitative agreement between this
model and the experimental data is achieved with simple isotropic defects. It
is demonstrated that weak satellite maxima near the Bragg reflections can be
interpreted as the finite resolution effect.Comment: 7 pages, 7 figure
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