574 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.
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
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
Dynamic Kerr Effect and Spectral Weight Transfer in the Manganites
We perform pump-probe Kerr spectroscopy in the colossally magnetoresistive
manganite Pr0.67Ca0.33MnO3. Kerr effects uncover surface magnetic dynamics
undetected by established methods based on reflectivity and optical spectral
weight transfer. Our findings indicate the connection between spin and charge
dynamics in the manganites may be weaker than previously thought. Additionally,
important differences between this system and conventional ferromagnetic metals
manifest as long-lived, magneto-optical coupling transients, which may be
generic to all manganites.Comment: 12 text pages, 4 figure
Dominant role of charge ordering on high harmonic generation in Pr_{0.6}Ca_{0.4}MnO_{3}
High-harmonic generation (HHG) is a typical high-order nonlinear optical
phenomenon and can be used to probe electronic structures of solids. Here, we
investigate the temperature dependence of HHG from Pr_{0.6}Ca_{0.4}MnO_{3} in
the range of 7 K to 294 K including the charge ordering (CO) transition and
magnetic transition temperatures. The high-harmonic intensity remains almost
constant in the high-temperature charge-disordered phase. However, as the
temperature is lowered, it starts to gradually increase near the CO transition
temperature where an optical gap related to the CO phase appears. The anomalous
gap energy dependence resembles the one recently reported in a Mott insulator.
We attribute the HHG suppression at high temperatures to the destructive
interference among high-harmonic emissions from thermally activated multiple CO
configurations. Our results suggest that HHG is a promising tool for probing
the fluctuation of local order in strongly correlated systems.Comment: 16 pages, 8 figure
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
Review of the initial validation and characterization of a chicken 3K SNP array.
In 2004 the chicken genome sequence and more than 2.8 million single nucleotide polymorphisms (SNPs) were reported. This information greatly enhanced the ability of poultry scientists to understand chicken biology, especially with respect to identification of quantitative trait loci (QTL) and genes that control simple and complex traits. To validate and address the quality of the reported SNPs, assays for 3072 SNPS were developed and used to genotype 2576 DNAs isolated from commercial and experimental birds. Over 90% of the SNPs were valid based on the criterion used for segregating, and over 88% had a minor allele frequency of 2% or greater. As the East Lansing (EL) and Wageningen University (WAU) reference panels were genotyped, 1933 SNPs were added to the chicken genetic map, which was used in the second chicken genome sequence assembly. It was also discovered that linkage disequilibrium varied considerably between commercial layers and broilers; with the latter having haplotype blocks averaging 10 to 50 kb in size. Finally, it was estimated that commercial lines have lost 70% or more of their genetic diversity, with the majority of allele loss attributable to the limited number of chicken breeds used
Charge and orbital order in half-doped manganites
An explanation is given for the charge order, orbital order and insulating
state observed in half-doped manganese oxides, such as
NdSrMnO. The competition between the kinetic energy of
the electrons and the magnetic exchange energy drives the formation of
effectively one-dimensional ferromagnetic zig-zag chains. Due to a topological
phase factor in the hopping, the chains are intrinsically insulating and
orbital-ordered. Most surprisingly, the strong Coulomb interaction between
electrons on the same Mn-ion leads to the experimentally observed charge
ordering. For doping less than 1/2 the system is unstable towards phase
separation into a ferromagnetic metallic and charge-ordered insulating phase.Comment: To appear in Phys. Rev. Lett., 4 pages, 4 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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