142 research outputs found
Jahn-Teller effect and Electron correlation in manganites
Jahn-Teller (JT) effect both in the absence and presence of the strong
Coulomb correlation is studied theoretically focusing on the reduction of the kinetic energy gain which is directly related to the spin wave
stiffness. Without the Coulomb interaction, the perturbative analysis gives
depending on the electron number
[: electron-phonon(el-ph) coupling constant, : mass of the oxygen atom,
: frequency of the phonon]. Although there occurs many channels of
the JT el-ph interaction in the multiband system, the final results of roughly scales with the density of states at the Fermi energy. In the limit
of strong electron correlation, the magnitude of the orbital polarization
saturate and the relevant degrees of freedom are the direction (phase) of it.
An effective action is derived for the phase variable including the effect of
the JT interaction. In this limit, JT interaction is {\it{enhanced}} compared
with the non-interacting case, and is given by the lattice
relaxation energy for the localized electrons, although the electrons
remains itinerant. Discussion on experiments are given based on these
theoretical results.Comment: 24 pages, 7 figure
Microwave properties of Nd_0.5Sr_0.5MnO_3: a key role of the (x^2-y^2)-orbital effects
Transmittance of the colossal magnetoresistive compound Nd_0.5Sr_0.5MnO_3
showing metal-insulator phase transition has been studied by means of the
submm- and mm-wavelength band spectroscopy. An unusually high transparency of
the material provided direct evidence for the significant suppression of the
coherent Drude weight in the ferromagnetic metallic state. Melting of the
A-type antiferromagnetic states has been found to be responsible for a
considerable increase in the microwave transmission, which was observed at the
transition from the insulating to the metallic phase induced by magnetic field
or temperature. This investigation confirmed a dominant role of the
(x^2-y^2)-orbital degree of freedom in the low-energy optical properties of
Nd_0.5Sr_0.5MnO_3 and other doped manganites with planar (x^2-y^2)-orbital
order, as predicted theoretically. The results are discussed in terms of the
orbital-liquid concept.Comment: 8 pages, 3 figure
Magnetic and orbital order in overdoped bilayer manganites
The magnetic and orbital orders for the bilayer manganites in the doping
region have been investigated from a model that incorporates the
two orbitals at each Mn site, the inter-orbital Coulomb interaction and
lattice distortions. The usual double exchange operates via the orbitals.
It is shown that such a model reproduces much of the phase diagram recently
obtained for the bilayer systems in this range of doping. The C-type phase with
() spin order seen by Ling et al. appears as a natural consequence
of the layered geometry and is stabilised by the static distortions of the
system. The orbital order is shown to drive the magnetic order while the
anisotropic hopping across the orbitals, layered nature of the underlying
structure and associated static distortions largely determine the orbital
arrangements.Comment: 8 pages, 5 figure
Complex orbital state in manganites
The -orbital states with complex coefficients of the linear combination
of and are studied for the ferromagnetic state in doped
manganites. Especially the focus is put on the competition among uniform
complex, staggered complex, and real orbital states. As the hole-doping
increases, the real, the canted complex, and the staggered complex orbital
states appears successively. Uniform complex state analoguous to Nagaoka
ferromagnet does not appear. These complex states can be expressed as a
resonating state among the planer orbitals as the orbital liquid, accompanied
by no Jahn-Teller distortion.Comment: 14 pages, 6 figure
Charge order and phase segregation in overdoped bilayer manganites
There have been recent reports of charge ordering around in the
bilayer manganites. At , there appears to be a coexistence region of
layered A-type antiferromagnetc and charge order. There are also reports of
orbital order in this region without any Jahn-Teller effect. Based on physical
grounds, this region is investigated from a model that incorporates the two
orbitals at each Mn site and a near-neighbour Coulomb repulsion. It is
shown that there indeed is both charge and orbital order close to the
half-doped region coincident with a layered magnetic structure. Although the
orbital order is known to drive the magnetic order, the layered magnetic
structure is also favoured in this system by the lack of coherent transport
across the planes and the reduced dimensionality of the lattice. The
anisotropic hopping across the orbitals and the underlying layered
structure largely determine the orbital arrangements in this region, while the
charge order is primarily due to the long range interactions.Comment: 6 pages, 6 figure
Orbital polarons versus itinerant e_g electrons in doped manganites
We study an effective one-dimensional (1D) orbital t-J model derived for
strongly correlated e_g electrons in doped manganites. The ferromagnetic spin
order at half filling is supported by orbital superexchange prop. to J which
stabilizes orbital order with alternating x^2-y^2 and 3z^2-r^2 orbitals. In a
doped system it competes with the kinetic energy prop. to t. When a single hole
is doped to a half-filled chain, its motion is hindered and a localized orbital
polaron is formed. An increasing doping generates either separated polarons or
phase separation into hole-rich and hole-poor regions, and eventually polarizes
the orbitals and gives a it metallic phase with occupied 3z^2-r^2 orbitals.
This crossover, investigated by exact diagonalization at zero temperature, is
demonstrated both by the behavior of correlation functions and by spectral
properties, showing that the orbital chain with Ising superexchange is more
classical and thus radically different from the 1D spin t-J model. At finite
temperature we derive and investigate an effective 1D orbital model using a
combination of exact diagonalization with classical Monte-Carlo for spin
correlations. A competition between the antiferromagnetic and ferromagnetic
spin order was established at half filling, and localized polarons were found
for antiferromagnetic interactions at low hole doping. Finally, we clarify that
the Jahn-Teller alternating potential stabilizes the orbital order with
staggered orbitals, inducing the ferromagnetic spin order and enhancing the
localized features in the excitation spectra. Implications of these findings
for colossal magnetoresistance manganites are discussed.Comment: 19 pages, 20 figure
Spin and orbital ordering in double-layered manganites
We study theoretically the phase diagram of the double-layered perovskite
manganites taking into account the orbital degeneracy, the strong Coulombic
repulsion, and the coupling with the lattice deformation. Observed spin
structural changes as the increased doping are explained in terms of the
orbital ordering and the bond-length dependence of the hopping integral along
-axis. Temperature dependence of the neutron diffraction peak corresponding
to the canting structure is also explained. Comparison with the 3D cubic system
is made.Comment: 7 figure
Magnetic Phases of Electron-Doped Manganites
We study the anisotropic magnetic structures exhibited by electron-doped
manganites using a model which incorporates the double-exchange between orbital
ly degenerate electrons and the super-exchange between
electrons with realistic values of the Hund's coupling(), the
super-exchange coupling(), and the bandwidth(). We look at the
relative stabilities of the G, C and A type antiferromagnetic ph ases. In
particular we find that the G-phase is stable for low electron doping as seen
in experiments. We find good agreement with the experimentally observed
magnetic phase diagrams of electron-doped manganites
() such as NdSrMnO, PrSrMnO,
and SmCaMnO. We can also explain the experimentally
observed orbital structures of the C a nd A phases.
We also extend our calculation for electron-doped bilayer manganites of the
form RAMnO and predict that the C-phase will be
absent in t hese systems due to their reduced dimensionality.Comment: 7 .ps files included. To appear in Phys. Rev. B (Feb 2001
Phase diagram of a generalized Hubbard model applied to orbital order in manganites
The magnetic phase diagram of a two-dimensional generalized Hubbard model
proposed for manganites is studied within Hartree-Fock approximation. In this
model the hopping matrix includes anisotropic diagonal hopping matrix elements
as well as off-diagonal elements. The antiferromagnetic (AF), ferromagnetic
(F), canted (C) and paramagnetic (P) states are included in the analysis as
possible phases. It is found that away from half-filling only the canted and F
states may exist and AF and P states which are possible for the usual Hubbard
model do not appear. This is because the F order has already developed for
on-site repulsion U=0 due to the hopping matrix of the generalized model. When
applied for manganites the orbital degree is described by a pseudospin. Thus
our ``magnetic'' phase diagram obtained physically describes how orbital order
changes with and with doping for manganites. Part of our results are
consistent with other numerical calculations and some experiments.Comment: 5 eps figures; a note added, to appear in Phys. Rev.
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