636 research outputs found
Classical frustration and quantum disorder in spin-orbital models
The most elementary of all physical spin-orbital models is the Kugel-Khomskii
model describing a S=1/2, degenerate Mott-insulator. Recent theoretical
work is reviewed revealing that the classical limit is characterized by a point
of perfect dynamical frustration. It is suggested that this might give rise to
a quantum disordered ground state.Comment: 7 pages Revtex, 3 ps figures, proceedings 1998 NEC symposium, Nasu,
Japa
Quantum disorder versus order-out-of-disorder in the Kugel-Khomskii model
The Kugel-Khomskii model, the simplest model for orbital degenerate magnetic
insulators, exhibits a zero temperature degeneracy in the classical limit which
could cause genuine quantum disorder. Khaliullin and Oudovenko [Phys. Rev. B
56, R14 243 (1997)] suggested recently that instead a particular classical
state could be stabilized by quantum fluctuations. Here we compare their
approach with standard random phase approximation and show that it strongly
underestimates the strength of the quantum fluctuations, shedding doubts on the
survival of any classical state.Comment: 4 pages, ReVTeX, 4 figure
Single-electron tunneling in InP nanowires
We report on the fabrication and electrical characterization of field-effect
devices based on wire-shaped InP crystals grown from Au catalyst particles by a
vapor-liquid-solid process. Our InP wires are n-type doped with diameters in
the 40-55 nm range and lengths of several microns. After being deposited on an
oxidized Si substrate, wires are contacted individually via e-beam fabricated
Ti/Al electrodes. We obtain contact resistances as low as ~10 kOhm, with minor
temperature dependence. The distance between the electrodes varies between 0.2
and 2 micron. The electron density in the wires is changed with a back gate.
Low-temperature transport measurements show Coulomb-blockade behavior with
single-electron charging energies of ~1 meV. We also demonstrate energy
quantization resulting from the confinement in the wire.Comment: 4 pages, 3 figure
Non-perturbative corrections to mean-field behavior: spherical model on spider-web graph
We consider the spherical model on a spider-web graph. This graph is
effectively infinite-dimensional, similar to the Bethe lattice, but has loops.
We show that these lead to non-trivial corrections to the simple mean-field
behavior. We first determine all normal modes of the coupled springs problem on
this graph, using its large symmetry group. In the thermodynamic limit, the
spectrum is a set of -functions, and all the modes are localized. The
fractional number of modes with frequency less than varies as for tending to zero, where is a constant. For an
unbiased random walk on the vertices of this graph, this implies that the
probability of return to the origin at time varies as ,
for large , where is a constant. For the spherical model, we show that
while the critical exponents take the values expected from the mean-field
theory, the free-energy per site at temperature , near and above the
critical temperature , also has an essential singularity of the type
.Comment: substantially revised, a section adde
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
Orbital dynamics in ferromagnetic transition metal oxides
We consider a model of strongly correlated electrons interacting by
superexchange orbital interactions in the ferromagnetic phase of LaMnO. It
is found that the classical orbital order with alternating occupied
orbitals has a full rotational symmetry at orbital degeneracy, and the
excitation spectrum derived using the linear spin-wave theory is gapless. The
quantum (fluctuation) corrections to the order parameter and to the ground
state energy restore the cubic symmetry of the model. By applying a uniaxial
pressure orbital degeneracy is lifted in a tetragonal field and one finds an
orbital-flop phase with a gap in the excitation spectrum. In two dimensions the
classical order is more robust near the orbital degeneracy point and quantum
effects are suppressed. The orbital excitation spectra obtained using finite
temperature diagonalization of two-dimensional clusters consist of a
quasiparticle accompanied by satellite structures. The orbital waves found
within the linear spin-wave theory provide an excellent description of the
dominant pole of these spectra.Comment: 13 pages, 12 figures, to appear in Phys. Rev.
Orbital and spin physics in LiNiO2 and NaNiO2
We derive a spin-orbital Hamiltonian for a triangular lattice of e_g orbital
degenerate (Ni^{3+}) transition metal ions interacting via 90 degree
superexchange involving (O^{2-}) anions, taking into account the on-site
Coulomb interactions on both the anions and the transition metal ions. The
derived interactions in the spin-orbital model are strongly frustrated, with
the strongest orbital interactions selecting different orbitals for pairs of Ni
ions along the three different lattice directions. In the orbital ordered
phase, favoured in mean field theory, the spin-orbital interaction can play an
important role by breaking the U(1) symmetry generated by the much stronger
orbital interaction and restoring the threefold symmetry of the lattice. As a
result the effective magnetic exchange is non-uniform and includes both
ferromagnetic and antiferromagnetic spin interactions. Since ferromagnetic
interactions still dominate, this offers yet insufficient explanation for the
absence of magnetic order and the low-temperature behaviour of the magnetic
susceptibility of stoichiometric LiNiO_2. The scenario proposed to explain the
observed difference in the physical properties of LiNiO_2 and NaNiO_2 includes
small covalency of Ni-O-Li-O-Ni bonds inducing weaker interplane superexchange
in LiNiO_2, insufficient to stabilize orbital long-range order in the presence
of stronger intraplane competition between superexchange and Jahn-Teller
coupling.Comment: 33 pages, 12 postscript figures, uses iopams.sty . This article
features in New Journal of Physics as part of a Focus Issue on Orbital
Physics - all contributions may be freely accessed at
(http://stacks.iop.org/1367-2630/6/i=1/a=E05). The published version of this
article may be found at http://stacks.iop.org/1367-2630/7/12
Suppression of static stripe formation by next-neighbor hopping
We show from real-space Hartree-Fock calculations within the extended Hubbard
model that next-nearest neighbor (t') hopping processes act to suppress the
formation of static charge stripes. This result is confirmed by investigating
the evolution of charge-inhomogeneous corral and stripe phases with increasing
t' of both signs. We propose that large t' values in YBCO prevent static stripe
formation, while anomalously small t' in LSCO provides an additional reason for
the appearance of static stripes only in these systems.Comment: 4 pages, 5 figure
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
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