491 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.
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
Magnetic Field resulting from non-linear electrical transport in single crystals of charge-ordered Pr Ca MnO}
In this letter we report that the current induced destabilization of the
charge ordered (CO) state in a rare-earth manganite gives rise to regions with
ferromagnetic correlation. We did this experiment by measurement of the I-V
curves in single crystal of the CO system
PrCaMnO and simultanously measuring the magnetization
of the current carrying conductor using a high T SQUID working at T = 77K.
We have found that the current induced destabilization of the CO state leads to
a regime of negative differential resistance which leads to a small enhancement
of the magnetization of the sample, indicating ferromagnetically aligned
moments.Comment: 4 pages LateX, 4 eps figure
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
Static and Dynamical Properties of the Ferromagnetic Kondo Model with Direct Antiferromagnetic Coupling Between the localized Electrons
The phase diagram of the Kondo lattice Hamiltonian with ferromagnetic Hund's
coupling in the limit where the spin of the localized electrons is
classical is analyzed in one dimension as a function of temperature, electronic
density, and a direct antiferromagnetic coupling between the localized
spins. Studying static and dynamical properties, a behavior that qualitatively
resembles experimental results for manganites occurs for smaller than 0.11
in units of the hopping amplitude. In particular a coexistence of
ferromagnetic and antiferromagnetic excitations is observed at low-hole density
in agreement with neutron scattering experiments on
with. This effect is caused by the
recently reported tendency to phase separation between hole-rich ferromagnetic
and hole-undoped antiferromagnetic domains in electronic models for manganites.
As increases metal-insulator transitions are detected by monitoring the
optical conductivity and the density of states. The magnetic correlations
reveal the existence of spiral phases without long-range order but with fairly
large correlation lengths. Indications of charge ordering effects appear in the
analysis of charge correlations.Comment: 14 pages with 25 eps figures embeded in the tex
Oxygen isotope effect and phase separation in the optical conductivity of (LaPr)CaMnO thin films
The optical conductivities of films of
(LaPr)CaMnO with different oxygen isotopes
(O and O) have been determined in the spectral range from 0.3 to
4.3 eV using a combination of transmission in the mid-infrared and ellipsometry
from the near-infrared to ultra-violet regions. We have found that the isotope
exchange strongly affects the optical response in the ferromagnetic phase in a
broad frequency range, in contrast to the almost isotope-independent optical
conductivity above . The substitution by O strongly suppresses the
Drude response and a mid-infrared peak while enhancing the conductivity peak at
1.5 eV. A qualitative explanation can be given in terms of the phase separation
present in these materials. Moreover, the optical response is similar to the
one extracted from measurements in polished samples and other thin films, which
signals to the importance of internal strain.Comment: 11 pages, 11 figures, to appear in PR
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
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
Spin-polarized tunneling of La0.67Sr0.33MnO3/YBa2Cu3O7-d junctions
The transport properties between ferromagnets and high-Tc superconductors are
investigated in La0.67Sr0.33MnO3/YBa2Cu3O7-d (LSMO/YBCO)junctions in the
geometry of cross-strip lines. The conductance spectra show zero-bias
conductance peaks (ZBCP), reflecting the charge transport in the ab-plane. When
an external magnetic field is applied to the junctions, the conductance spectra
show two notable features, i.e., an increase of background conductance and an
asymmetric ZBCP splitting whose amplitude responds nonlinearly to the applied
field. It is shown that the magnetic field response are consistent with a
theoretical prediction of tunneling spectroscopy when the presence of a
ferromagnetic barrier between a spin-polarized ferromagnet and a d-wave
superconductor is assumed.Comment: 10 Pages, 7 EPS figures, Submitted to Phys. Rev.
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
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