106 research outputs found
Damped orbital excitations in the titanates
A possible mechanism for the removal of the orbital degeneracy in RTiO3
(where R=La, Y, ...) is considered. The calculation is based on the
Kugel-Khomskii Hamiltonian for electrons residing in the t2g orbitals of the Ti
ions, and uses a self-consistent pe rturbation expansion in the interaction
between the orbital and the spin degrees of freedom. The latter are assumed to
be ordered in a Neel state, brought about by delicate interactions that are not
included in the Kugel-Khomskii Hamiltonian. Within our model calculations, each
of the t2g bands is found to acquire a finite, temperature-dependent
dispersion, that lifts the orbital degeneracy. The orbital excitations are
found to be heavily damped over a rather wide band. Consequently, they do not
participate as a separate branch of excitations in the low-temperature
thermodynamics.eComment: 6 pages, 3 figure
Hidden Symmetries and their Consequences in Cubic Perovskites
The five-band Hubbard model for a band with one electron per site is a
model which has very interesting properties when the relevant ions are located
at sites with high (e. g. cubic) symmetry. In that case, if the crystal field
splitting is large one may consider excitations confined to the lowest
threefold degenerate orbital states. When the electron hopping matrix
element () is much smaller than the on-site Coulomb interaction energy
(), the Hubbard model can be mapped onto the well-known effective
Hamiltonian (at order ) derived by Kugel and Khomskii (KK). Recently
we have shown that the KK Hamiltonian does not support long range spin order at
any nonzero temperature due to several novel hidden symmetries that it
possesses. Here we extend our theory to show that these symmetries also apply
to the underlying three-band Hubbard model. Using these symmetries we develop a
rigorous Mermin-Wagner construction, which shows that the three-band Hubbard
model does not support spontaneous long-range spin order at any nonzero
temperature and at any order in -- despite the three-dimensional lattice
structure. Introduction of spin-orbit coupling does allow spin ordering, but
even then the excitation spectrum is gapless due to a subtle continuous
symmetry. Finally we showed that these hidden symmetries dramatically simplify
the numerical exact diagonalization studies of finite clusters.Comment: 26 pages, 3 figures, 520 KB, submitted Phys. Rev.
Manifestation of quantum rotor orbital excitations in Raman spectra of Jahn-Teller crystal LaMnO3
© Published under licence by IOP Publishing Ltd.Materials, consisting of Jahn-Teller (JT) ions, such as cuprates and manganites, display many outstanding properties, including high temperature superconductivity and colossal magnetoresistance. There, the role of JT effect, although widely recognized, is still elusive. Here we show that these materials have vibronic excitations, related to local deformations rotating around JT ions in the dynamic limit, arising from linear electron-vibrational coupling in the "Mexican hat" potential profile. Their energy depends on total angular momentum, which is quantized, as in quantum rotors. We found them in the representative JT compound of orthorhombic manganites, LaMnO3. Since the "Mexican hat" potential energy surface is double-valued, they show up near the ground and excited states of JT ions. Recently, by using spectroscopic ellipsometry technique, we showed that they appear in the excited state in the form of sidebands, accompanying the electron transition between the JT split orbitals at neighboring Mn3+ ions. Here, by using Raman scattering technique, we show that they also exist near the ground state. The found quantum rotor excitations may play an important role in many unusual properties observed in these materials
Orbital Ordering in Paramagnetic LaMnO3 and KCuF3
{\it Ab-initio} studies of the stability of orbital ordering, its coupling to
magnetic structure and its possible origins (electron-phonon and/or
electron-electron interactions) are reported for two perovskite systems,
LaMnO and KCuF. We present a new Average Spin State (ASS) calculational
scheme that allowed us to treat a paramagnetic state. Using this scheme, we
succesfully described the experimental magnetic/orbital phase diagram of both
LaMnO and KCuF in crystal structures when the Jahn-Teller distortions
are neglected. Hence, we conclude that the orbital ordering in both compounds
is purely electronic in origin.Comment: 10 pages, 5 figure
Spin-orbital gapped phase with least symmetry breaking in the one-dimensional symmetrically coupled spin-orbital model
To describe the spin-orbital energy gap formation in the one-dimensional
symmetrically coupled spin-orbital model, we propose a simple mean field theory
based on an SU(4) constraint fermion representation of spins and orbitals. A
spin-orbital gapped phase is formed due to a marginally relevant spin-orbital
valence bond pairing interaction. The energy gap of the spin and orbital
excitations grows extremely slowly from the SU(4) symmetric point up to a
maximum value and then decreases rapidly. By calculating the spin, orbital, and
spin-orbital tensor static susceptibilities at zero temperature, we find a
crossover from coherent to incoherent magnetic excitations as the spin-orbital
coupling decreasing from large to small values.Comment: 10 pages, Revtex file, 5 figure
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Formation of metallic magnetic clusters in a Kondo-lattice metal: Evidence from an optical study
Magnetic materials are usually divided into two classes: those with localised magnetic moments, and those with itinerant charge carriers. We present a comprehensive experimental (spectroscopic ellipsomerty) and theoretical study to demonstrate that these two types of magnetism do not only coexist but complement each other in the Kondo-lattice metal, Tb2PdSi3. In this material the itinerant charge carriers interact with large localised magnetic moments of Tb(4f) states, forming complex magnetic lattices at low temperatures, which we associate with self-organisation of magnetic clusters. The formation of magnetic clusters results in low-energy optical spectral weight shifts, which correspond to opening of the pseudogap in the conduction band of the itinerant charge carriers and development of the low- and high-spin intersite electronic transitions. This phenomenon, driven by self-trapping of electrons by magnetic fluctuations, could be common in correlated metals, including besides Kondo-lattice metals, Fe-based and cuprate superconductors
Metal insulator transition in TlSr2CoO5 from orbital degeneracy and spin disproportionation
To describe the metal insulator transition in the new oxide TlSr2CoO5 we
investigate its electronic structure by LDA and model Hartree-Fock
calculations. Within LDA we find a homogeneous metallic and ferromagnetic
ground state, but when including the Coulomb interaction more explicitly within
the Hartree-Fock approximation, we find an insulating state of lower energy
with both spin and orbital order. We also interpret our results in terms of a
simple model.Comment: 8 pages, 9 figure
First integrals of Ginzburg-Landau equations and stability criteria for vortex-free state in unconventional superconductors
The first integrals of the Ginzburg-Landau equations for a vortex-free state
of superconductors with different mixed symmetries of the order parameter are
found. The general boundary conditions for the order parameter at the ideal
interface between the superconductor and vacuum are derived. Based on these
integrals and boundary conditions, we analyze the stability criteria for
vortex-free state in unconventional superconductors. The threshold field above
which the Abrikosov vortices can enter the superconductor is found to be higher
or equal to the thermodynamic critical field for all states under study.Comment: 8 pages, pdf file, no figure
Theory of orbital state and spin interactions in ferromagnetic titanates
A spin-orbital superexchange Hamiltonian in a Mott insulator with
orbital degeneracy is investigated. More specifically, we focus on a spin
ferromagnetic state of the model and study a collective behavior of orbital
angular momentum. Orbital order in the model occurs in a nontrivial way -- it
is stabilized exclusively by quantum effects through the order-from-disorder
mechanism. Several energetically equivalent orbital orderings are identified.
Some of them are specified by a quadrupole ordering and have no unquenched
angular momentum at low energy. Other states correspond to a noncollinear
ordering of the orbital angular momentum and show the magnetic Bragg peaks at
specific positions. Order parameters are unusually small because of strong
quantum fluctuations. Orbital contribution to the resonant x-ray scattering is
discussed. The dynamical magnetic structure factor in different ordered states
is calculated. Predictions made should help to observe elementary excitations
of orbitals and also to identify the type of the orbital order in ferromagnetic
titanates. Including further a relativistic spin-orbital coupling, we derive an
effective low-energy spin Hamiltonian and calculate a spin-wave spectrum, which
is in good agreement with recent experimental observations in YTiO.Comment: 25 pages, 17 figure
Volume contraction at the Jahn-Teller transition of LaMnO
We have studied the volume collapse of LaMnO at the Jahn- Teller (JT)
transition temperature T=750 K which has recently been found in high
temperature powder x- ray and neutron diffraction experiments. We construct a
model Hamiltonian involving the pseudospin of Mn e states, the
staggered JT distortion and the volume strain coordinate. We show that the
anharmonic coupling between these primary and secondary order parameters leads
to the first order JT phase transition associated with a comparatively large
reduction of the unit cell volume of V/V 10. We explain
the temperature dependence of JT distortions and volume strain and discuss the
volume change as function of the anharmonic coupling constant. A continuous
change to a second order transition as function of model parameters is
obtained. This behaviour is also observed under Ba doping.Comment: 5 pages, 4 figure
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