626 research outputs found
Suppressed Coherence due to Orbital Correlations in the Ferromagnetically Ordered Metallic Phase of Mn Compounds
Small Drude weight together with small specific heat coefficient
observed in the ferromagnetic phase of RAMnO (R=La, Pr, Nd, Sm;
A=Ca, Sr, Ba) are analyzed in terms of a proximity effect of the Mott
insulator. The scaling theory for the metal-insulator transition with the
critical enhancement of orbital correlations toward the staggered ordering of
two orbitals such as and symmetries may lead to the
critical exponents of and with
and . The result agrees with the experimental indications.Comment: 4 pages LaTeX using jpsj.sty. To appear in J. Phys. Soc. Jpn.
67(1998)No.
One-Center Charge Transfer Transitions in Manganites
In frames of a rather conventional cluster approach, which combines the
crystal field and the ligand field models we have considered different charge
transfer (CT) states and O 2p-Mn 3d CT transitions in MnO octahedra.
The many-electron dipole transition matrix elements were calculated using the
Racah algebra for the cubic point group. Simple "local" approximation allowed
to calculate the relative intensity for all dipole-allowed and
CT transitions. We present a self-consistent description of
the CT bands in insulating stoichiometric LaMnO compound with the
only Mn valent state and idealized octahedral MnO centers
which allows to substantially correct the current interpretation of the optical
spectra. Our analysis shows the multi-band structure of the CT optical response
with the weak low-energy edge at 1.7 eV, associated with forbidden
transition and a series of the weak and strong
dipole-allowed high-energy transitions starting from 2.5 and 4.5 eV,
respectively, and extending up to nearly 11 eV. The most intensive features are
associated with two strong composite bands near eV and
eV, respectively, resulting from the superposition of the dipole-allowed
and CT transitions. These predictions are in good
agreement with experimental spectra. The experimental data point to a strong
overscreening of the crystal field parameter in the CT states of
MnO centers.Comment: 10 pages, 3 figure
Optical conductivity in doped manganites with planar x-y orbital order
We investigate a planar model for the ferromagnetic (FM) phase of manganites,
which develops orbital order of electrons with x-y-symmetry at
low temperature. The dynamic structure factor of orbital excitations and the
optical conductivity are studied with help of a
finite-temperature diagonalization method. Our calculations provide a
theoretical prediction for for the 2D FM state and are of
possible relevance for the recently found A-type phase of manganites at high
doping which consists of FM layers coupled antiferromagnetically. In the
x-y ordered regime shows both a Drude peak and a
gapped incoherent absorption due to a gap in the orbital excitations.Comment: 5 pages, 5 figures, to appear in Phys. Rev. Let
Dimensional Crossover driven by Magnetic Ordering in Optical Conductivity of Pr_{1/2}Sr_{1/2}MnO_3
We investigated optical properties of Pr_{0.5}Sr_{0.5}MnO_3, which has the
A-type antiferromagnetic ordering at a low temperature. We found that T-
dependence of spectral weight transfer shows a clear correlation with the
magnetic phase transition. In comparison with the optical conductivity results
of Nd_{0.5}Sr_{0.5}MnO_3, which has the CE-type antiferromagnetic charge
ordering, we showed that optical properties of Pr_{0.5}Sr_{0.5}MnO_3 near the
Neel temperature could be explained by a crossover from 3D to 2D metals.
Details of spectral weight changes are consistent with the polaron picture.Comment: 11 pages, 4 figures, submitted to PRL at June
Ultrafast Photoinduced Formation of Metallic State in a Perovskite-type Manganite with Short Range Charge and Orbital Order
Femtosecond reflection spectroscopy was performed on a perovskite-type
manganite, Gd0.55Sr0.45MnO3, with the short-range charge and orbital order
(CO/OO). Immediately after the photoirradiation, a large increase of the
reflectivity was detected in the mid-infrared region. The optical conductivity
spectrum under photoirradiation obtained from the Kramers-Kronig analyses of
the reflectivity changes demonstrates a formation of a metallic state. This
suggests that ferromagnetic spin arrangements occur within the time resolution
(ca. 200 fs) through the double exchange interaction, resulting in an ultrafast
CO/OO to FM switching.Comment: 4 figure
Spin/Orbital Pattern-Dependent Polaron Absorption in Nd(1-x)Sr(x)MnO3
We investigated optical properties of Nd(1-x)Sr(x)MnO3 (x= 0.40, 0.50, 0.55,
and 0.65) single crystals. In the spin/orbital disordered state, their
conductivity spectra look quite similar, and the strength of the mid-infrared
absorption peak is proportional to x(1-x) consistent with the polaron picture.
As temperature lowers, the Nd(1-x)Sr(x)MnO3 samples enter into various
spin/orbital ordered states, whose optical responses are quite different. These
optical responses can be explained by the spin/orbital ordering
pattern-dependent polaron hopping.Comment: 3 figures (gzipped
Incoherent Charge Dynamics in Perovskite Manganese Oxides
A minimal model is proposed for the perovskite manganese oxides showing the
strongly incoherent charge dynamics with a suppressed Drude weight in the
ferromagnetic and metallic phase near the insulator. We investigate a
generalized double-exchange model including three elements; the orbital
degeneracy of conduction bands, the Coulomb interaction and fluctuating
Jahn-Teller distortions. We demonstrate that Lanczs
diagonalization calculations combined with Monte Carlo sampling of the largely
fluctuating lattice distortions result in the optical conductivity which
quantitatively accounts for the experimental indications. It is found that all
the three elements are indispensable to understand the charge dynamics in these
compounds.Comment: 4 pages with 1 page of figures. To appear in J. Phys. Soc. Jp
Dynamic conductivity of semiconducting manganites approaching the metal-insulator transition
We report the frequency-dependent conductivity of the manganite system
La1-xSrxMnO3 (x <= 0.2) when approaching the metal-insulator transition from
the insulating side. Results from low-frequency dielectric measurements are
combined with spectra in the infrared region. For low doping levels the
behavior is dominated by hopping transport of localized charge carriers at low
frequencies and by phononic and electronic excitations in the infrared region.
For the higher Sr contents the approach of the metallic state is accompanied by
the successive suppression of the hopping contribution at low frequencies and
by the development of polaronic excitations in the infrared region, which
finally become superimposed by a strong Drude contribution in the fully
metallic state.Comment: 10 pages, 7 figure
Effects of Orbital Degeneracy and Electron Correlation on Charge Dynamics in Perovskite Manganese Oxides
Taking the orbital degeneracy of conduction bands and the Coulomb
interaction into account in a double-exchange model, we investigate charge
dynamics of perovskite Mn oxides by the Lanczs diagonalization
method. In the metallic phase near the Mott insulator, it is found that the
optical conductivity for a spin-polarized two-dimensional system exhibits a
weight transfer to a broad and incoherent structure within the lower-Hubbard
band together with a suppressed Drude weight. It reproduces qualitative feature
of the experimental results. As an orbital effect, we find that an anomalous
charge correlation at quarter filling suppresses the coherent charge dynamics
and signals precursor to the charge ordering.Comment: 4 pages LaTeX including 3 PS figures, to appear in J. Phys. Soc. Jp
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