41 research outputs found
t-J model of coupled CuO ladders in SrCaCuO
Starting from the proper charge transfer model for CuO coupled
ladders in SrCaCuO we derive the low energy
Hamiltonian for this system. It occurs that the widely used ladder t-J model is
not sufficient and has to be supplemented by the Coulomb repulsion term between
holes in the neighboring ladders. Furthermore, we show how a simple mean-field
solution of the derived t-J model may explain the onset of the charge density
wave with the odd period in SrCaCuO.Comment: 8 pages, 4 figures, 2 table
Long-range disassortative correlations in generic random trees
We explicitly calculate the distance dependent correlation functions in a
maximal entropy ensemble of random trees. We show that correlations remain
disassortative at all distances and vanish only as a second inverse power of
the distance. We discuss in detail the example of scale-free trees where the
diverging second moment of the degree distribution leads to some interesting
phenomena.Comment: Revised according to referee comments; 7 pages and 7 figures
Orbital liquid in ferromagnetic manganites: The orbital Hubbard model for electrons
We have analyzed the symmetry properties and the ground state of an orbital
Hubbard model with two orbital flavors, describing a partly filled
spin-polarized band on a cubic lattice, as in ferromagnetic manganites.
We demonstrate that the off-diagonal hopping responsible for transitions
between and orbitals, and the absence of SU(2) invariance
in orbital space, have important implications. One finds that superexchange
contributes in all orbital ordered states, the Nagaoka theorem does not apply,
and the kinetic energy is much enhanced as compared with the spin case.
Therefore, orbital ordered states are harder to stabilize in the Hartree-Fock
approximation (HFA), and the onset of a uniform ferro-orbital polarization and
antiferro-orbital instability are similar to each other, unlike in spin case.
Next we formulate a cubic (gauge) invariant slave boson approach using the
orbitals with complex coefficients. In the mean-field approximation it leads to
the renormalization of the kinetic energy, and provides a reliable estimate for
the ground state energy of the disordered state. Using this approach one finds
that the HFA fails qualitatively in the regime of large Coulomb repulsion
-- the orbital order is unstable, and instead a strongly
correlated orbital liquid with disordered orbitals is realized at any electron
filling.Comment: 25 pages, 9 figure
Dimerization versus Orbital Moment Ordering in the Mott insulator YVO
We use exact diagonalization combined with mean-field theory to investigate
the phase diagram of the spin-orbital model for cubic vanadates. The spin-orbit
coupling competes with Hund's exchange and triggers a novel phase, with the
ordering of orbital magnetic moments stabilized by the tilting of
VO octahedra. It explains qualitatively spin canting and reduction of
magnetization observed in YVO. At finite temperature an orbital Peierls
instability in the -type antiferromagnetic phase induces modulation of
magnetic exchange constants even in absence of lattice distortions. The
calculated spin structure factor shows a magnon splitting due to the orbital
Peierls dimerization.Comment: 4 pages, 5 figures, Revte
Influence of anharmonicity on the negative thermal expansion of
The lattice vibrational properties of (gray tin) were investigated experimentally by temperature-dependent x-ray diffraction and theoretically by density functional theory calculations. Similar to the other elements of group IV, exhibits a lattice anomaly at low temperatures and negative thermal expansion, with a minimum at and a magnitude three times larger than in Si. The influence of anharmonic effects up to fourth-order potential terms on the phonon dispersion relations, the lattice parameters, and the thermal expansion coefficient have been tested. The performed analysis gives an excellent agreement with experiment when quartic potential terms are included in the theory. We point out that negative thermal expansion in is not driven by the anharmonicity of the interatomic potential. This resolves the long-standing puzzle in the thermal behavior of
One-dimensional orbital fluctuations and the exotic magnetic properties of YVO
Starting from the Mott insulator picture for cubic vanadates, we derive and
investigate the model of superexchange interactions between V ions, with
nearly degenerate orbitals occupied by two electrons each. The
superexchange interactions are strongly frustrated and demonstrate a strong
interrelation between possible types of magnetic and orbital order. We
elucidate the prominent role played by fluctuations of and orbitals
which generate ferromagnetic superexchange interactions even in the absence of
Hund's exchange. In this limit we find orbital valence bond state which is
replaced either by -type antiferromagnetic order with weak -type orbital
order at increasing Hund's exchange, or instead by -type antiferromagnetic
order when the lattice distortions stabilize -type orbital order. Both
phases are observed in YVO and we argue that a dimerized -type
antiferromagnetic phase with stronger and weaker FM bonds alternating along the
c axis may be stabilized by large spin-orbital entropy at finite temperature.
This suggests a scenario which explains the origin of the exotic -AF order
observed in YVO in the regime of intermediate temperatures and allows one
to specify the necessary ingredients of a more complete future theory.Comment: 23 pages, 15 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
Low-Temperature Phase of the CdReO Superconductor: Ab initio Phonon Calculations and Raman Scattering
Using an {\it ab initio} approach, we report a phonon soft mode in the
tetragonal structure described by the space group of the K
superconductor CdReO. It induces an orthorhombic distortion to a
crystal structure described by the space group which hosts the
superconducting state. This new phase has a lower total energy than the other
known crystal structures of CdReO. Comprehensive temperature
dependent Raman scattering experiments on isotope enriched samples,
CdReO, not only confirm the already known structural
phase transitions but also allow us to identify a new characteristic
temperature regime around K, below which the Raman spectra undergo
remarkable changes with the development of several sharp modes and mode
splitting. Together with the results of the \textit{ab initio} phonon
calculations we take these observations as strong evidence for another phase
transition to a novel low-temperature crystal structure of CdReO.Comment: 7 pages, 8 figure
Fingerprints of spin-orbital physics in cubic Mott insulators: Magnetic exchange interactions and optical spectral weights
The temperature dependence and anisotropy of optical spectral weights
associated with different multiplet transitions is determined by the spin and
orbital correlations. To provide a systematic basis to exploit this close
relationship between magnetism and optical spectra, we present and analyze the
spin-orbital superexchange models for a series of representative
orbital-degenerate transition metal oxides with different multiplet structure.
For each case we derive the magnetic exchange constants, which determine the
spin wave dispersions, as well as the partial optical sum rules. The magnetic
and optical properties of early transition metal oxides with degenerate
orbitals (titanates and vanadates with perovskite structure) are shown
to depend only on two parameters, viz. the superexchange energy and the
ratio of Hund's exchange to the intraorbital Coulomb interaction, and on
the actual orbital state. In systems important corrections follow from
charge transfer excitations, and we show that KCuF can be classified as a
charge transfer insulator, while LaMnO is a Mott insulator with moderate
charge transfer contributions. In some cases orbital fluctuations are quenched
and decoupling of spin and orbital degrees of freedom with static orbital order
gives satisfactory results for the optical weights. On the example of cubic
vanadates we describe a case where the full quantum spin-orbital physics must
be considered. Thus information on optical excitations, their energies,
temperature dependence and anisotropy, combined with the results of magnetic
neutron scattering experiments, provides an important consistency test of the
spin-orbital models, and indicates whether orbital and/or spin fluctuations are
important in a given compound.Comment: 34 pages, 16 figure