300 research outputs found
Strain control of superlattice implies weak charge-lattice coupling in LaCaMnO
We have recently argued that manganites do not possess stripes of charge
order, implying that the electron-lattice coupling is weak [Phys Rev Lett
\textbf{94} (2005) 097202]. Here we independently argue the same conclusion
based on transmission electron microscopy measurements of a nanopatterned
epitaxial film of LaCaMnO. In strain relaxed regions, the
superlattice period is modified by 2-3% with respect to the parent lattice,
suggesting that the two are not strongly tied.Comment: 4 pages, 4 figures It is now explained why the work provides evidence
to support weak-coupling, and rule out charge orde
Volume contraction at the Jahn-Teller transition of LaMnO
We have studied the volume collapse of LaMnO at the Jahn- Teller (JT)
transition temperature T=750 K which has recently been found in high
temperature powder x- ray and neutron diffraction experiments. We construct a
model Hamiltonian involving the pseudospin of Mn e states, the
staggered JT distortion and the volume strain coordinate. We show that the
anharmonic coupling between these primary and secondary order parameters leads
to the first order JT phase transition associated with a comparatively large
reduction of the unit cell volume of V/V 10. We explain
the temperature dependence of JT distortions and volume strain and discuss the
volume change as function of the anharmonic coupling constant. A continuous
change to a second order transition as function of model parameters is
obtained. This behaviour is also observed under Ba doping.Comment: 5 pages, 4 figure
Cooperative Jahn-Teller Effect and Electron-Phonon Coupling in
A classical model for the lattice distortions of \lax is derived and, in a
mean field approximation, solved. The model is based on previous work by
Kanamori and involves localized Mn d-electrons (which induce tetragonal
distortions of the oxygen octahedra surrounding the Mn) and localized holes
(which induce breathing distortions). Parameters are determined by fitting to
the room temperature structure of . The energy gained by formation of
a local lattice distortion is found to be large, most likely eV
per site, implying a strong electorn-phonon coupling and supporting polaronic
models of transport in the doped materials. The structural transition is shown
to be of the order-disorder type; the rapid x-dependence of the transition
temperature is argued to occur because added holes produce a "random" field
which misaligns the nearby sites.Comment: 24 pages. No figures. One Table. Late
Structural and magnetic properties of Mn3-xCdxTeO6 (x = 0, 1, 1.5 and 2)
Mn3TeO6 exhibits a corundum-related A3TeO6 structure and a complex magnetic
structure involving two magnetic orbits for the Mn atoms [*]. Mn3-xCdxTeO6
(x=0, 1, 1.5 and 2) ceramics were synthesized by solid state reaction and
investigated using X-ray powder diffraction, electron microscopy, calorimetric
and magnetic measurements. Cd2+ replaces Mn2+ cations without greatly affecting
the structure of the compound. The Mn and Cd cations were found to be randomly
distributed over the A-site. Magnetization measurements indicated that the
samples order antiferromagnetically at low temperature with a transition
temperature that decreases with increasing Cd doping. The nuclear and magnetic
structure of one specially prepared 114Cd containing sample:
Mn1.5(114Cd)1.5TeO6, was studied using neutron powder diffraction over the
temperature range 2 to 295 K. Mn1.5(114Cd)1.5TeO6 was found to order in an
incommensurate helical magnetic structure, very similar to that of Mn3TeO6 [*].
However, with a lower transition temperature and the extension of the ordered
structure confined to order 240(10) {\AA}. [*] S. A. Ivanov et al. Mater. Res.
Bull. 46 (2011) 1870.Comment: 20 pages, 8 figure
Interplay of the CE-type charge ordering and the A-type spin ordering in a half-doped bilayer manganite La{1}Sr{2}Mn{2}O{7}
We demonstrate that the half-doped bilayer manganite La_{1}Sr_{2}Mn_{2}O_{7}
exhibits CE-type charge-ordered and spin-ordered states below K and below K, respectively. However, the volume
fraction of the CE-type ordering is relatively small, and the system is
dominated by the A-type spin ordering. The coexistence of the two types of
ordering is essential to understand its transport properties, and we argue that
it can be viewed as an effective phase separation between the metallic
orbital ordering and the charge-localized
orbital ordering.Comment: 5 pages, 4 figures, submitted to Phys. Rev.
Spin Excitation Spectrum of LaMnO
As an effective model to describe perovskite-type manganates (La,)MnO,
the double-exchange model on a cubic lattice is investigated. Spin excitation
spectrum of the model in the ground state is studied using the spin wave
approximation. Spin wave dispersion relation observed in the inelastic neutron
scattering experiment of LaPbMnO is reproduced. Effective
values for the electron bandwidth as well as Hund's coupling is estimated from
the data.Comment: 10 pages LaTeX including 4 PS figure
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