144 research outputs found
An X-Ray Induced Structural Transition in La_0.875Sr_0.125MnO_3
We report a synchrotron x-ray scattering study of the magnetoresistive
manganite La_0.875Sr_0.125MnO_3. At low temperatures, this material undergoes
an x-ray induced structural transition at which charge ordering of Mn^3+ and
Mn^4+ ions characteristic to the low-temperature state of this compound is
destroyed. The transition is persistent but the charge-ordered state can be
restored by heating above the charge-ordering transition temperature and
subsequently cooling. The charge-ordering diffraction peaks, which are
broadened at all temperatures, broaden more upon x-ray irradiation, indicating
the finite correlation length of the charge-ordered state. Together with the
recent reports on x-ray induced transitions in Pr_(1-x)Ca_xMnO_3, our results
demonstrate that the photoinduced structural change is a common property of the
charge-ordered perovskite manganites.Comment: 5 pages, 4 embedded EPS figures; significant changes in the data
analysis mad
Persistent X-Ray Photoconductivity and Percolation of Metallic Clusters in Charge-Ordered Manganites
Charge-ordered manganites of composition exhibit persistent photoconductivity upon
exposure to x-rays. This is not always accompanied by a significant increase in
the {\it number} of conduction electrons as predicted by conventional models of
persistent photoconductivity. An analysis of the x-ray diffraction patterns and
current-voltage characteristics shows that x-ray illumination results in a
microscopically phase separated state in which charge-ordered insulating
regions provide barriers against charge transport between metallic clusters.
The dominant effect of x-ray illumination is to enhance the electron {\it
mobility} by lowering or removing these barriers. A mechanism based on magnetic
degrees of freedom is proposed.Comment: 8 pages, 4 figure
Multiphase segregation and metal-insulator transition in single crystal La(5/8-y)Pr(y)Ca(3/8)MnO3
The insulator-metal transition in single crystal La(5/8-y)Pr(y)Ca(3/8)MnO3
with y=0.35 was studied using synchrotron x-ray diffraction, electric
resistivity, magnetic susceptibility, and specific heat measurements. Despite
the dramatic drop in the resistivity at the insulator-metal transition
temperature Tmi, the charge-ordering (CO) peaks exhibit no anomaly at this
temperature and continue to grow below Tmi. Our data suggest then, that in
addition to the CO phase, another insulating phase is present below Tco. In
this picture, the insulator-metal transition is due to the changes within this
latter phase. The CO phase does not appear to play a major role in this
transition. We propose that a percolation-like insulator-metal transition
occurs via the growth of ferromagnetic metallic domains within the parts of the
sample that do not exhibit charge ordering. Finally, we find that the
low-temperature phase-separated state is unstable against x-ray irradiation,
which destroys the CO phase at low temperatures.Comment: 9 pages, 9 encapsulated eps figure
Martensitic accommodation strain and the metal-insulator transition in manganites
In this paper, we report polarized optical microscopy and electrical
transport studies of manganese oxides that reveal that the charge ordering
transition in these compounds exhibits typical signatures of a martensitic
transformation. We demonstrate that specific electronic properties of
charge-ordered manganites stem from a combination of martensitic accommodation
strain and effects of strong electron correlations. This intrinsic strain is
strongly affected by the grain boundaries in ceramic samples. Consistently, our
studies show a remarkable enhancement of low field magnetoresistance and the
grain size effect on the resistivity in polycrystalline samples and suggest
that the transport properties of this class of manganites are governed by the
charge-disordered insulating phase stabilized at low temperature by virtue of
martensitic accommodation strain. High sensitivity of this phase to strains and
magnetic field leads to a variety of striking phenomena, such as unusually high
magnetoresistance (10^10 %) in low magnetic fields.Comment: Short paper, 4 figures, to appear in Rapid Communicatio
Temperature dependent d-d excitations in manganites probed by resonant inelastic x-ray scattering
We report the observation of temperature dependent electronic excitations in
various manganites utilizing resonant inelastic x-ray scattering (RIXS) at the
Mn K-edge. Excitations were observed between 1.5 and 16 eV with temperature
dependence found as high as 10 eV. The change in spectral weight between 1.5
and 5 eV was found to be related to the magnetic order and independent of the
conductivity. On the basis of LDA+U and Wannier function calculations, this
dependence is associated with intersite d-d excitations. Finally, the
connection between the RIXS cross-section and the loss function is addressed.Comment: 5 pages, 5 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
Direct Observation of Field-Induced Incommensurate Fluctuations in a One-Dimensional S=1/2 Antiferromagnet
Neutron scattering from copper benzoate, Cu(C6D5COO)2 3D2O, provides the
first direct experimental evidence for field-dependent incommensurate low
energy modes in a one-dimensional spin S = 1/2 antiferromagnet. Soft modes
occur for wavevectors q=\pi +- dq(H) where dq(H) ~ 2 \pi M(H)/g\mu_B as
predicted by Bethe ansatz and spinon descriptions of the S = 1/2 chain.
Unexpected was a field-induced energy gap , where
as determined from specific heat measurements. At H = 7 T
(g\mu_B H/J = 0.52), the magnitude of the gap varies from 0.06 - 0.3 J
depending on the orientation of the applied field.Comment: 11 pages, 5 postscript figures, LaTeX, Submitted to PRL 3/31/97,
e-mail comments to [email protected]
Orbital Correlations in Doped Manganites
We review our recent x-ray scattering studies of charge and orbital order in
doped manganites, with specific emphasis on the role of orbital correlations in
Pr_1-xCa_xMnO_3. For x=0.25, we find an orbital structure indistinguishable
from the undoped structure with long range orbital order at low temperatures.
For dopings 0.3<x<0.5, we find scattering consistent with a charge and
orbitally ordered CE-type structure. While in each case the charge order peaks
are resolution limited, the orbital order exhibits only short range
correlations. We report the doping dependence of the correlation length and
discuss the connection between the orbital correlations and the finite magnetic
correlation length observed on the Mn^3+ sublattice with neutron scattering
techniques. The physical origin of these domains, which appear to be isotropic,
remains unclear. We find that weak orbital correlations persist well above the
phase transitions, with a correlation length of 1-2 lattice constants at high
temperatures. Significantly, we observe similar correlations at high
temperatures in La_0.7Ca_0.3MnO_3, which does not have an orbitally ordered
ground state, and we conclude that such correlations are robust to variations
in the relative strength of the electron-phonon coupling.Comment: 22 pagegs, 7 figure
Thermal Conductivity of the Spin Peierls Compound CuGeO_3
The thermal conductivity of the Spin-Peierls (SP) compound CuGeO_3 was
measured in magnetic fields up to 16 T. Above the SP transition, the heat
transport due to spin excitations causes a peak at around 22 K, while below the
transition the spin excitations rapidly diminish and the heat transport is
dominated by phonons; however, the main scattering process of the phonons is
with spin excitations, which demonstrates itself in an unusual peak in the
thermal conductivity at about 5.5 K. This low-temperature peak is strongly
suppressed with magnetic fields in excess of 12.5 T.Comment: 6 pages, including 2 postscript figure
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