38 research outputs found
A Resonant X-ray Scattering Study of Octahedral Tilt Ordering in LaMnO and PrCaMnO
We report an x-ray scattering study of octahedral tilt ordering in the
manganite series PrCaMnO with x=0.4 and 0.25 and in LaMnO.
The sensitivity to tilt ordering is achieved by tuning the incident x-ray
energy to the L, L and L absorption edges of Pr and La,
respectively. The resulting energy-dependent profiles are characterized by a
dipole-resonant peak and higher energy fine structure. The polarization
dependence is predominantly -to- and the azimuthal dependence
follows a sin-squared behavior. These results are similar to those obtained in
recent x-ray scattering studies of orbital ordering carried out in these same
materials at the Mn K edge. They lead to a description of the cross-section in
terms of Templeton scattering in which the tilt ordering breaks the symmetry at
the rare earth site. The most interesting result of the present work is our
observation that octahedral tilt ordering persists above the orbital ordering
transition temperatures in all three samples. Indeed, we identify separate
structural transitions which may be associated with the onset of orbital and
tilt ordering, respectively, and characterize the loss of tilt ordering versus
temperature in LaMnO.Comment: 24 pages, 8 figure
Coherent X-ray Scattering from Manganite Charge and Orbital Domains
We report coherent x-ray scattering studies of charge and orbital domains in
manganite systems. The experiments were carried out on LaMnO_3 and
Pr_{0.6}Ca_{0.4}MnO_3, with the incident photon energy tuned near the Mn K
edge. At room temperature, the orbital speckle pattern of LaMnO_3 was observed
to be constant over a timescale of at least minutes, which is indicative of
static orbital domains on this timescale. For Pr_{0.6}Ca_{0.4}MnO_3, both
charge and orbital speckle patterns were observed. The observation of the
latter rules out the presence of fast orbital fluctuations, while long time
series data-- on the order of several minutes-- were suggestive of slow dynamic
behavior. In contrast, the charge order speckle patterns were static.Comment: 6 pages, 4 figure
Electronic Structure and Phase Transition in V2O3: Importance of 3d Spin-Orbit Interaction and Lattice Distortion
The 3d electronic structure and phase transition in pure and Cr doped V2O3
are theoretically investigated in relation to the 3d spin-orbit interaction and
lattice distortion. A model consisting of the nearest-neighbor V ion pair with
full degeneracy of the 3d orbitals is studied within the many-body point of
view. It is shown that each V ion with S=1 spin state has a large orbital
magnetic moment and no orbital ordering occurs in the
antiferromagnetic insulating (AFI) phase. The anomalous resonant Bragg
reflection found in the AFI phase is attributed to the magnetic ordering. In
the AFI and paramagnetic insulating (PI) phases, Jahn-Teller like lattice
instability leads to tilting of the V ion pairs from the corundum c-axis and
this causes large difference in the orbital occupation between the paramagnetic
metal and the insulating phases, which is consistent with linear dichroic V 2p
XAS measurements.
To understand the AFI to PI transition, a model spin Hamiltonian is also
proposed. The transition is found to be simultaneous order-disorder transition
of the magnetic moments and tilting directions of the V ion pairs. Softening of
elastic constant C44 and abrupt change in short range spin correlations
observed at the transition are also explained.Comment: 18 pages, 16 figure
X-ray Resonant Scattering Studies of Orbital and Charge Ordering in PrCaMnO
We present the results of a systematic x-ray scattering study of the charge
and orbital ordering in the manganite series PrCaMnO with
=0.25, 0.4 and 0.5. The temperature dependence of the scattering at the
charge and orbital wavevectors, and of the lattice constants, was characterized
throughout the ordered phase of each sample. It was found that the charge and
orbital order wavevectors are commensurate with the lattice, in striking
contrast to the results of earlier electron diffraction studies of samples with
=0.5. High momentum-transfer resolution studies of the x=0.4 and 0.5 samples
further revealed that while long-range charge order is present, long-range
orbital order is never established. Above the charge/orbital ordering
temperature T, the charge order fluctuations are more highly correlated
than the orbital fluctuations. This suggests that charge order drives orbital
order in these samples. In addition, a longitudinal modulation of the lattice
with the same periodicity as the charge and orbital ordering was discovered in
the x=0.4 and 0.5 samples. For x=0.25, only long-range orbital order was
observed with no indication of charge ordering, nor of an additional lattice
modulation. We also report the results of a preliminary investigation of the
loss of charge and orbital ordering in the x=0.4 sample by application of a
magnetic field. Finally, the polarization and azimuthal dependence of the
charge and orbital ordering in these compounds is characterized both in the
resonant and nonresonant limits, and compared with the predictions of current
theories. The results are qualitatively consistent with both cluster and LDA+U
calculations of the electronic structure.Comment: 37 pages, 22 figure
Correlated Polarons in Dissimilar Perovskite Manganites
We report x-ray scattering studies of broad peaks located at a (0.5 0 0)/(0
0.5 0)-type wavevector in the paramagnetic insulating phases of
La_{0.7}Ca_{0.3}MnO_{3} and Pr_{0.7}Ca_{0.3}MnO_{3}. We interpret the
scattering in terms of correlated polarons and measure isotropic correlation
lengths of 1-2 lattice constants in both samples. Based on the wavevector and
correlation lengths, the correlated polarons are found to be consistent with
CE-type bipolarons. Differences in behavior between the samples arise as they
are cooled through their respective transition temperatures and become
ferromagnetic metallic (La_{0.7}Ca_{0.3}MnO_{3}) or charge and orbitally
ordered insulating (Pr_{0.7}Ca_{0.3}MnO_{3}). Since the primary difference
between the two samples is the trivalent cation size, these results illustrate
the robust nature of the correlated polarons to variations in the relative
strength of the electron-phonon coupling, and the sensitivity of the
low-temperature ground state to such variations.Comment: 13 pages, 6 figure
Resonant x-ray diffraction study of the magnetoresistant perovskite Pr0.6Ca0.4MnO3
We report a x-ray resonant diffraction study of the perovskite
Pr0.6Ca0.4MnO3. At the Mn K-edge, this technique is sensitive to details of the
electronic structure of the Mn atoms. We discuss the resonant x-ray spectra
measured above and below the charge and orbital ordering phase transition
temperature (TCOO = 232 K), and present a detailed analysis of the energy and
polarization dependence of the resonant scattering. The analysis confirms that
the structural transition is a transition to an orbitally ordered phase in
which inequivalent Mn atoms are ordered in a CE-type pattern. The Mn atoms
differ mostly by their 3d orbital occupation. We find that the charge
disproportionation is incomplete, 3d^{3.5-\delta} and 3d^{3.5+\delta} with
\delta\ll0.5 . A revised CE-type model is considered in which there are two Mn
sublattices, each with partial e_{g} occupancy. One sublattice consists of Mn
atoms with the 3x^{2}-r^{2} or 3y^{2}-r^{2} orbitals partially occupied, the
other sublattice with the x^{2}-y^{2} orbital partially occupied.Comment: 15 pages, 15 figure
Substrate-induced strain effects on Pr_{0.6}Ca_{0.4}MnO_{3} films
We report the characterization of the crystal structure, low-temperature
charge and orbital ordering, transport, and magnetization of
Pr_{0.6}Ca_{0.4}MnO_{3} films grown on LaAlO_{3}, NdGaO_{3}, and SrTiO_{3}
substrates, which provide compressive (LaAlO_{3}) and tensile (NdGaO_{3} and
SrTiO_{3}) strain. The films are observed to exhibit different crystallographic
symmetries than the bulk material, and the low-temperature ordering is found to
be more robust under compressive-- as opposed to tensile-- strain. In fact,
bulk-like charge and orbital ordering is not observed in the film grown on
NdGaO_{3}, which is the substrate that provides the least amount of nominal and
measured, but tensile, strain. This result suggests the importance of the role
played by the Mn--O--Mn bond angles in the formation of charge and orbital
ordering at low temperatures. Finally, in the film grown on LaAlO_{3}, a
connection between the lattice distortion associated with orbital ordering and
the onset of antiferromagnetism is reported.Comment: 12 pages, 7 figure
Local optical excitations in metals
U of I OnlyThesi
Recent Highlights of X-Ray Magnetic Scattering Studies from Surfaces.
Abstract not availableJRC.E-Institute for Transuranium Elements (Karlsruhe
Recent Highlights of X-Ray Magnetic Scattering Studies from Surfaces
In this work, recent studies of surface magnetism, as observed by x-ray scattering techniques, are described. The experiments were concerned with uranium dioxide crystals for which x-ray resonance effects enhance the small magnetic signal from the surface. The main result is that, in contrast to the bulk which exhibits a discontinuous magnetic ordering transition, both the (001) and (111) surface layers order continuously. This is reminiscent of the general phenomenon of surface wetting, but had not been previously observed for magnetic materials. Magnetic reflectivity studies show further that the near-surface magnetic layers are more disordered than layers deep in the bulk, even at low temperatures