4,670 research outputs found
Spin and orbital excitation spectrum in the Kugel-Khomskii model
We discuss spin and orbital ordering in the twofold orbital degenerate
superexchange model in three dimensions relevant to perovskite transition metal
oxides. We focus on the particular point on the classical phase diagram where
orbital degeneracy is lifted by quantum effects exclusively. Dispersion and
damping of the spin and orbital excitations are calculated at this point taking
into account their mutual interaction. Interaction corrections to the
mean-field order parameters are found to be small. We conclude that
quasi-one-dimensional Neel spin order accompanied by the uniform
d_{3z^2-r^2}-type orbital ordering is stable against quantum fluctuations.Comment: 4 pages with 3 PS figures, 1 table, RevTeX, accepted to Phys. Rev. B.
Rapid Communicatio
Orbital effects in manganites
In this paper I give a short review of some properties of the colossal
magnetoresistance manganites, connected with the orbital degrees of freedom.
Ions Mn{3+}, present in most of these compounds, have double orbital degeneracy
and are strong Jahn-Teller ions, causing structural distortions and orbital
ordering. Mechanisms leading to such ordering are shortly discussed, and the
role of orbital degrees of freedom in different parts of the phase diagram of
manganites is described. Special attention is paid to the properties of
low-doped systems (doping 0.1 - 0.25), to overdoped systems (x > 0.5), and to
the possibility of a novel type of orbital ordering in optimally doped
ferromagnetic metallic manganites.Comment: 28 pages, 7 figures, to be published in J. Mod. Phys.
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
Dynamical Jahn-Teller Effect in Spin-Orbital Coupled System
Dynamical Jahn-Teller (DJT) effect in a spin-orbital coupled system on a
honeycomb lattice is examined, motivated from recently observed spin-liquid
behavior in BaCuSbO. An effective vibronic Hamiltonian, where the
superexchange interaction and the DJT effect are taken into account, is
derived. We find that the DJT effect induces a spin-orbital resonant state
where local spin-singlet states and parallel orbital configurations are
entangled with each other. This spin-orbital resonant state is realized in
between an orbital ordered state, where spin-singlet pairs are localized, and
an antiferromagnetic ordered state. Based on the theoretical results, a
possible scenario for BaCuSbO is proposed.Comment: 13 pages, 8 figures, to be published in PR
Parametric resonance at the critical temperature in high energy heavy ion collisions
Parametric resonance in soft modes at the critical temperature () in
high energy heavy ion collisions is studied in the case when the temperature
() of the system is almost constant for a long time. By deviding the fields
into three parts, zero mode (condensate), soft modes and hard modes and
assuming that the hard modes are in thermal equilibrium, we derive the equation
of motion for soft modes at . Enhanced modes are extracted by
comparing with the Mathieu equation for the condensate oscillating along the
sigma axis at . It is found that the soft mode of fields at
about 174 MeV is enhanced.Comment: 8 pages, 1 figure Some statements and equations are modified to
clarif
Topology Change of Coalescing Black Holes on Eguchi-Hanson Space
We construct multi-black hole solutions in the five-dimensional
Einstein-Maxwell theory with a positive cosmological constant on the
Eguchi-Hanson space, which is an asymptotically locally Euclidean space. The
solutions describe the physical process such that two black holes with the
topology of S^3 coalesce into a single black hole with the topology of the lens
space L(2;1)=S^3/Z_2. We discuss how the area of the single black hole after
the coalescence depends on the topology of the horizon.Comment: 10 pages, Some comments are added. to be published as a letter in
Classical and Quantum Gravit
Strings in five-dimensional anti-de Sitter space with a symmetry
The equation of motion of an extended object in spacetime reduces to an
ordinary differential equation in the presence of symmetry. By properly
defining of the symmetry with notion of cohomogeneity, we discuss the method
for classifying all these extended objects. We carry out the classification for
the strings in the five-dimensional anti-de Sitter space by the effective use
of the local isomorphism between \SO(4,2) and \SU(2,2). We present a
general method for solving the trajectory of the Nambu-Goto string and apply to
a case obtained by the classification, thereby find a new solution which has
properties unique to odd-dimensional anti-de Sitter spaces. The geometry of the
solution is analized and found to be a timelike helicoid-like surface
Orbital Compass Model as an Itinerant Electron System
Two-dimensional orbital compass model is studied as an interacting itinerant
electron model. A Hubbard-type tight-binding model, from which the orbital
compass model is derived in the strong coupling limit, is identified. This
model is analyzed by the random-phase approximation (RPA) and the
self-consistent RPA methods from the weak coupling. Anisotropy for the orbital
fluctuation in the momentum space is qualitatively changed by the on-site
Coulomb interaction. This result is explained by the fact that the dominant
fluctuation is changed from the intra-band nesting to the inter-band one by
increasing the interaction.Comment: 7 pages, 8 figure
How Do Nonlinear Voids Affect Light Propagation ?
Propagation of light in a clumpy universe is examined. As an inhomogeneous
matter distribution, we take a spherical void surrounded by a dust shell where
the ``lost mass'' in the void is compensated by the shell. We study how the
angular-diameter distance behaves when such a structure exists. The
angular-diameter distance is calculated by integrating the Raychaudhuri
equation including the shear. An explicit expression for the junction condition
for the massive thin shell is calculated. We apply these results to a dust
shell embedded in a Friedmann universe and determine how the distance-redshift
relation is modified compared with that in the purely Friedmann universe. We
also study the distribution of distances in a universe filled with voids. We
show that the void-filled universe gives a larger distance than the FRW
universe by at if the size of the void is of the
Horizon radius.Comment: To appear in Prog. Theor. Phys. 10
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