10,714 research outputs found
First-order nature of the ferromagnetic phase transition in (La-Ca)MnO_3 near optimal doping
Neutron scattering has been used to study the nature of the ferromagnetic
transition in single crystals of La_0.7Ca_0.3MnO_3 and La_0.8Ca_0.2MnO_3, and
polycrystalline samples of La_0.67Ca_0.33MnO_3 and La_5/8Ca_3/8MnO_3 where the
naturally occurring O-16 can be replaced with the O-18 isotope. Small angle
neutron scattering on the x=0.3 single crystal reveals a discontinuous change
in the scattering at the Curie temperature for wave vectors below ~0.065 A^-1.
Strong relaxation effects are observed for this domain scattering, for the
magnetic order parameter, and for the quasielastic scattering, demonstrating
that the transition is not continuous in nature. There is a large oxygen
isotope effect observed for the T_C in the polycrystalline samples. For the
optimally doped x=3/8 sample we observed T_C(O-16)=266.5 K and T_C(O-18)=261.5
K at 90% O-18 substitution. The temperature dependence of the spin-wave
stiffness is found to be identical for the two samples despite changes in T_C.
Hence, T_C is not solely determined by the magnetic subsystem, but instead the
ferromagnetic phase is truncated by the formation of polarons which cause an
abrupt transition to the paramagnetic, insulating state. Application of
uniaxial stress in the x=0.3 single crystal sharply enhances the polaron
scattering at room temperature. Measurements of the phonon density-of-states
show only modest differences above and below T_C and between the two different
isotopic samples.Comment: 13 pages, 16 figures, submitted to Phys. Rev.
Magnetic Order and Spin Dynamics in Ferroelectric HoMnO
Hexagonal HoMnO is a frustrated antiferromagnet (T=72 K)
ferroelectric (T=875 K) in which these two order parameters are coupled.
Our neutron measurements of the spin wave dispersion for the S=2 Mn on
the layered triangular lattice are well described by a two-dimensional
nearest-neighbor Heisenberg exchange J=2.44 meV, and an anisotropy that is
0.093 meV above the spin reorientation transition at 40 K, and 0.126 meV below.
For the magnetic structures and phase diagram have been
determined, and reveal additional transitions below 8 K where the
ferroelectrically displaced Ho ions are ordered magnetically.Comment: To be published in Physical Review Letter
I-V curves and intergranular flux creep activation energy in the magnetic superconductor RuSr2GdCu2O8
A systematic study of I-V characteristic curves for RuSr2GdCu2O8 [Ru-(1212)]
is presented, with magnetic fields up to 3 T and 5 K<T<30 K, in the region of
the superconducting transition. The activation energy E_{a}(H,T) for flux line
depinning was determined by fitting the nonlinear region of the curves using
the flux creep model. E_{a}(H,T) was found to vary linearly with temperature,
while a power-law dependence on the magnetic field was observed up to H=0.1 T,
where an abrupt reduction in its decreasing rate occurs. The extrapolated
value, E_{a}(0,0)=50 meV, is twice the reported value for YBa2Cu3O7, but the
critical current density J_{C}(0,0)=70 A/cm2 is about one order of magnitude
lower. These results are explained as a consequence of the contribution of the
magnetization in the grains to the effective field at the intergranular links
and to a spin-flop transition of the Ru-sub-lattice.Comment: 4 pages, 3 figure
Small business lending in low- and Moderate-income areas: The effects of credit scoring
Credit scoring systems ; Lender liability ; Small business
Uncorrelated and correlated nanoscale lattice distortions in the paramagnetic phase of magnetoresistive manganites
Neutron scattering measurements on a magnetoresistive manganite
La(CaSr)MnO show that uncorrelated
dynamic polaronic lattice distortions are present in both the orthorhombic (O)
and rhombohedral (R) paramagnetic phases. The uncorrelated distortions do not
exhibit any significant anomaly at the O-to-R transition. Thus, both the
paramagnetic phases are inhomogeneous on the nanometer scale, as confirmed
further by strong damping of the acoustic phonons and by the anomalous
Debye-Waller factors in these phases. In contrast, recent x-ray measurements
and our neutron data show that polaronic correlations are present only in the O
phase. In optimally doped manganites, the R phase is metallic, while the O
paramagnetic state is insulating (or semiconducting). These measurements
therefore strongly suggest that the {\it correlated} lattice distortions are
primarily responsible for the insulating character of the paramagnetic state in
magnetoresistive manganites.Comment: 10 pages, 8 figures embedde
Institutional development work in the World Bank : a review of 84 bank projects
Institutional weakness is a critical constraint to economic development. The goal of this paper is to review the design of recent Bank projects to assess the quality of their institutional development (ID) components and the factors that may affect that quality. A major focus is Bank staffing and organization, and the following issues are addressed: (a) the quality of institutional analysis and ID components in the design of current Bank project; (b) the ID work that is being done in Bank projects; (c) qualifications needed for effective ID work; (d) the impact the Bank's organizational structure has on ID work; and (e) suggestions that can be made to broaden and strengthen the ID work in Bank projects.Banks&Banking Reform,Health Monitoring&Evaluation,Health Economics&Finance,Municipal Financial Management,Rural Portfolio Improvement
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