3,192 research outputs found
Orbitally-driven Peierls state in spinels
We consider the superstructures, which can be formed in spinels containing on
B-sites the transition-metal ions with partially filled t2g levels. We show
that, when such systems are close to itinerant state (e.g. have an
insulator-metal transition), there may appear in them an orbitally-driven
Peierls state. We explain by this mechanism the very unusual superstructures
observed in CuIr2S4 (octamers) and MgTi2O4 (chiral superstructures) and suggest
that similar phenomenon should be observed in NaTiO2 and possibly in some other
systems.Comment: 4 pages, 3 figure
Oxygen-stripes in La0.5Ca0.5MnO3 from ab initio calculations
We investigate the electronic, magnetic and orbital properties of
La0.5Ca0.5MnO3 perovskite by means of an ab initio electronic structure
calculation within the Hartree-Fock approximation. Using the experimental
crystal structure reported by Radaelli et al. [Phys. Rev B 55, 3015 (1997)], we
find a charge-ordering stripe-like ground state. The periodicity of the
stripes, and the insulating CE-type magnetic structure are in agreement with
neutron x-ray and electron diffraction experiments. However, the detailed
structure is more complex than that envisaged by simple models of charge and
orbital order on Mn d-levels alone, and is better described as a charge-density
wave of oxygen holes, coupled to the Mn spin/orbital order.Comment: 4 pages, 3 figures. Version accepted for publication in PR
Spin-orbital gap of multiorbital antiferromagnet
In order to discuss the spin-gap formation in a multiorbital system, we
analyze an e_g-orbital Hubbard model on a geometrically frustrated zigzag chain
by using a density-matrix renormalization group method. Due to the appearance
of a ferro-orbital arrangement, the system is regarded as a one-orbital system,
while the degree of spin frustration is controlled by the spatial anisotropy of
the orbital. In the region of strong spin frustration, we observe a finite
energy gap between ground and first-excited states, which should be called a
spin-orbital gap. The physical meaning is clarified by an effective Heisenberg
spin model including correctly the effect of the orbital arrangement influenced
by the spin excitation.Comment: 8 pages, 6 figures, extended versio
Phase separation in systems with charge ordering
A simple model of charge ordering is considered. It is shown explicitly that
at any deviation from half-filling () the system is unstable with
respect to phase separation into charge ordered regions with and
metallic regions with smaller electron or hole density. Possible structure of
this phase-separated state (metallic droplets in a charge-ordered matrix)is
discussed. The model is extended to account for the strong Hund-rule onsite
coupling and the weaker intersite antiferromagnetic exchange. An analysis of
this extended model allows us to determine the magnetic structure of the
phase-separated state and to reveal the characteristic features of manganites
and other substances with charge ordering.Comment: 9 pages, revte
Orbital ordering in charge transfer insulators
We discuss a new mechanism of orbital ordering, which in charge transfer
insulators is more important than the usual exchange interactions and which can
make the very type of the ground state of a charge transfer insulator, i.e. its
orbital and magnetic ordering, different from that of a Mott-Hubbard insulator.
This purely electronic mechanism allows us to explain why orbitals in
Jahn-Teller materials typically order at higher temperatures than spins, and to
understand the type of orbital ordering in a number of materials, e.g.
K_2CuF_4, without invoking the electron-lattice interaction.Comment: 4 pages, 2 figure
Jahn-Teller distortions and phase separation in doped manganites
A "minimal model" of the Kondo-lattice type is used to describe a competition
between the localization and metallicity in doped manganites and related
magnetic oxides with Jahn-Teller ions. It is shown that the number of itinerant
charge carriers can be significantly lower than that implied by the doping
level x. A strong tendency to the phase separation is demonstrated for a wide
range of intermediate doping concentrations vanishing at low and high doping.
The phase diagram of the model in the x-T plane is constructed. At low
temperatures, the system is in a state with a long-range magnetic order:
antiferromagnetic (AF), ferromagnetic (FM), or AF-FM phase separated (PS)
state. At high temperatures, there can exist two types of the paramagnetic (PM)
state with zero and nonzero density of the itinerant electrons. In the
intermediate temperature range, the phase diagram includes different kinds of
the PS states: AF-FM, FM-PM, and PM with different content of itinerant
electrons. The applied magnetic field changes the phase diagram favoring the FM
ordering. It is shown that the variation of temperature or magnetic field can
induce the metal-insulator transition in a certain range of doping levels.Comment: 14 pages, 7 figures, submitted to Phys. Rev. B.; v.2 contains the
changes introduced according to comments of the PRB Referees; in v. 3, some
misprints are correcte
Dilution Effects in Two-dimensional Quantum Orbital System
We study dilution effects in a Mott insulating state with quantum orbital
degree of freedom, termed the two-dimensional orbital compass model. This is a
quantum and two-dimensional version of the orbital model where the interactions
along different bond directions cause frustration between different orbital
configurations. A long-range correlation of a kind of orbital at each row or
column, termed the directional order, is studied by means of the quantum
Monte-Carlo method. It is shown that decrease of the ordering temperature due
to dilution is much stronger than that in spin models. Quantum effect enhances
the effective dimensionality in the system and makes the directional order
robust against dilution. We discuss an essential mechanism of the dilute
orbital systems.Comment: 5pages, 4 figure
Integer filling metal insulator transitions in the degenerate Hubbard model
We obtain exact numerical solutions of the degenerate Hubbard model in the
limit of large dimensions (or large lattice connectivity). Successive
Mott-Hubbard metal insulator transitions at integer fillings occur at
intermediate values of the interaction and low enough temperature in the
paramagnetic phase. The results are relevant for transition metal oxides with
partially filled narrow degenerate bands.Comment: 4 pages + 4 figures (in 5 ps-files), revte
Clear Experimental Signature of Charge-Orbital density wave in NdCaMnO
Single Crystals of NdCaMnO have been prepared by the
travelling floating-zone method, and possible evidence of a charge -orbital
density wave in this material presented earlier [PRB68,092405 (2003)] using
High Resolution Electron Microscopy [HRTEM] and Electron Diffraction [ED]. In
the current note we present direct evidence of charge-orbital ordering in this
material using heat capacity measurements. Our heat capacity measurements
indicate a clear transition consistent with prior observation. We find two main
transitions, one at temperature K, and other at
K. In addition, we may also conclude that there is a strong electron-phonon
coupling in this material.Comment: 7 pages, 8 figure
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