453 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
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
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
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
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
Fermi-surface induced modulation in an optimally doped YBCO superconductor
We have observed a Fermi-surface (FS) induced lattice modulation in a YBCO
superconductor with a wavevector along CuO chains, {\it i.e.} =(0,,0). The value of is twice the Fermi
wavevector () along {\bf b*} connecting nearly nested FS `ridges'.
The modulation exists only within O-vacancy-ordered islands
(characterized by = and persists well above and
below . Our results are consistent with the presence of a FS-induced
charge-density wave
Concentration phase diagram of Ba(x)Sr(1-x)TiO3 solid solutions
Method of derivation of phenomenological thermodynamic potential of solid
solutions is proposed in which the interaction of the order parameters of
constituents is introduced through the account of elastic strain due to misfit
of the lattice parameters of the end-members. The validity of the method is
demonstrated for Ba(x)Sr(1-x)TiO3 system being a typical example of
ferroelectric solid solution. Its phase diagram is determined using
experimental data for the coefficients in the phenomenological potentials of
SrTiO3 and BaTiO3. In the phase diagram of the Ba(x)Sr(1-x)TiO3 system for
small Ba concentration, there are a tricritical point and two multiphase points
one of which is associated with up to 6 possible phases.Comment: 8 pages, 3 figure
Phenomenological theory of phase transitions in epitaxial BaxSr(1-x)TiO3 thin films
A phenomenological thermodynamic theory of BaxSr(1-x)TiO3 (BST-x) thin films
epitaxially grown on cubic substrates is developed using the Landau-Devonshire
approach. The eighth-order thermodynamic potential for BT single crystal and
modified fourth-order potential for ST single crystal were used as starting
potentials for the end-members of the solid solution with the aim to develop
potential of BST- solid solution valid at high temperatures. Several
coefficients of these potentials for BT were changed to obtain reasonable
agreement between theory and experimental phase diagram for BST-x (x > 0.2)
solid solutions. For low Ba content we constructed the specific phase diagram
where five phases converge at the multiphase point (T_N2 = 47 K, x = 0.028) and
all transitions are of the second order. The "concentration-misfit strain"
phase diagrams for BST-x thin films at room temperature and "temperature-misfit
strain" phase diagrams for particular concentrations are constructed and
discussed. Near T_N2 coupling between polarization and structural order
parameter in the epitaxial film is modified considerably and large number of
new phases not present in the bulk materials appear on the phase diagram.Comment: 8 pages 5 figure
The evolution with temperature of magnetic polaron state in an antiferromagnetic chain with impurities
The thermal behavior of a one-dimensional antiferromagnetic chain doped by
donor impurities was analyzed. The ground state of such a chain corresponds to
the formation of a set of ferromagnetically correlated regions localized near
impurities (bound magnetic polarons). At finite temperatures, the magnetic
structure of the chain was calculated simultaneously with the wave function of
a conduction electron bound by an impurity. The calculations were performed
using an approximate variational method and a Monte Carlo simulation. Both
these methods give similar results. The analysis of the temperature dependence
of correlation functions for neighboring local spins demonstrated that the
ferromagnetic correlations inside a magnetic polaron remain significant even
above the N\'eel temperature implying rather high stability of the
magnetic polaron state. In the case when the electron-impurity coupling energy
is not too high (for lower that the electron hopping integral ), the
magnetic polaron could be depinned from impurity retaining its magnetic
structure. Such a depinning occurs at temperatures of the order of . At
even higher temperatures () magnetic polarons disappear and the chain
becomes completely disordered.Comment: 17 pages, 5 figures, RevTe
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