555 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
Trimer Formation and Metal-Insulator Transition in Orbital Degenerate Systems on a Triangular Lattice
As a prototypical self-organization in the system with orbital degeneracy, we
theoretically investigate trimer formation on a triangular lattice, as observed
in LiVO2. From the analysis of an effective spin-orbital coupled model in the
strong correlation limit, we show that the previously-proposed orbital-ordered
trimer state is not the lowest-energy state for a finite Hund's-rule coupling.
Instead, exploring the ground state in a wide range of parameters for a
multiorbital Hubbard model, we find an instability toward a different
orbital-ordered trimer state in the intermediately correlated regime in the
presence of trigonal crystal field. The trimer phase appears in the competing
region among a paramagnetic metal, band insulator, and Mott insulator. The
underlying mechanism is nesting instability of the Fermi surface by a
synergetic effect of Coulomb interactions and trigonal-field splitting. The
results are compared with experiments in triangularlattice compounds, LiVX2
(X=O, S, Se) and NaVO2.Comment: 4 pages, 4 figures, accepted for publication in J. Phys. Soc. Jp
Superconductivity Induced by Bond Breaking in the Triangular Lattice of IrTe2
IrTe2, a layered compound with a triangular iridium lattice, exhibits a
structural phase transition at approximately 250 K. This transition is
characterized by the formation of Ir-Ir bonds along the b-axis. We found that
the breaking of Ir-Ir bonds that occurs in Ir1-xPtxTe2 results in the
appearance of a structural critical point in the T = 0 limit at xc = 0.035.
Although both IrTe2 and PtTe2 are paramagnetic metals, superconductivity at Tc
= 3.1 K is induced by the bond breaking in a narrow range of x > xc in
Ir1-xPtxTe2. This result indicates that structural fluctuations can be involved
in the emergence of superconductivity.Comment: 10 pages, 4 figure
Constraining q_0 with Cluster Gas Mass Fractions: A Feasibility Study
As the largest gravitationally bound objects in the universe, clusters of
galaxies may contain a fair sample of the baryonic mass fraction of the
universe. Since the gas mass fraction from the hot ICM is believed to be
constant in time, the value of the cosmological deceleration parameter
can be determined by comparing the calculated gas mass fraction in nearby and
distant clusters (Pen 1997). To test the potential of this method, we compare
the gas fractions derived for a sample of luminous (erg
s), nearby clusters with those calculated for eight luminous, distant
() clusters using ASCA and ROSAT observations. For consistency,
we evaluate the gas mass fraction at a fixed physical radius of 1
Mpc (assuming ). We find a best fit value of with -0.47 <
q_0 < 0.67 at 95% confidence. We also determine the gas fraction using the
method of Evrard, Metzler, & Navarro (1997) to find the total mass within
, the radius where the mean overdensity of matter is 500 times the
critical density. In simulations, this method reduces the scatter in the
determination of gravitational mass without biasing the mean. We find that it
also reduces the scatter in actual observations for nearby clusters, but not as
much as simulations suggest. Using this method, the best fit value is with -0.50 < q_0 < 0.64. The excellent agreement between these two
methods suggests that this may be a useful technique for determining . The
constraints on should improve as more distant clusters are studied and
precise temperature profiles are measured to large radii.Comment: 8 pages, 4 figures, uses emulateapj.sty, onecolfloat.st
Three-Point Correlations in Weak Lensing Surveys: Model Predictions and Applications
We use the halo model of clustering to compute two- and three-point
correlation functions for weak lensing, and apply them in a new statistical
technique to measure properties of massive halos. We present analytical results
on the eight shear three-point correlation functions constructed using
combination of the two shear components at each vertex of a triangle. We
compare the amplitude and configuration dependence of the functions with
ray-tracing simulations and find excellent agreement for different scales and
models. These results are promising, since shear statistics are easier to
measure than the convergence. In addition, the symmetry properties of the shear
three-point functions provide a new and precise way of disentangling the
lensing E-mode from the B-mode due to possible systematic errors.
We develop an approach based on correlation functions to measure the
properties of galaxy-group and cluster halos from lensing surveys. Shear
correlations on small scales arise from the lensing matter within halos of mass
M > 10^13 solar masses. Thus the measurement of two- and three-point
correlations can be used to extract information on halo density profiles,
primarily the inner slope and halo concentration. We demonstrate the
feasibility of such an analysis for forthcoming surveys. We include covariances
in the correlation functions due to sample variance and intrinsic ellipticity
noise to show that 10% accuracy on profile parameters is achievable with
surveys like the CFHT Legacy survey, and significantly better with future
surveys. Our statistical approach is complementary to the standard approach of
identifying individual objects in survey data and measuring their properties.Comment: 30 pages, 21 figures. Corrected typos in equations (23) and (28).
Matches version for publication in MNRA
Spin Order due to Orbital Fluctuations: Cubic Vanadates
We investigate the highly frustrated spin and orbital superexchange
interactions in cubic vanadates. The fluctuations of orbitals trigger
a {\it novel mechanism of ferromagnetic interactions} between spins S=1 of
V ions along one of the cubic directions which operates already in the
absence of Hund's rule exchange , and leads to the C-type
antiferromagnetic phase in LaVO. The Jahn-Teller effect can stabilize the
orbital ordering and the G-type antiferromagnetic phase at low temperatures,
but large entropy due to orbital fluctuations favors again the C-phase at
higher temperatures, as observed in YVO.Comment: 4 pages, 3 figures, to appear in Phys. Rev. Let
Ground State and Excitations of Spin Chain with Orbital Degeneracy
The one dimensional Heisenberg model in the presence of orbital degeneracy is
studied at the SU(4) symmetric viewpoint by means of Bethe ansatz. Following
Sutherland's previous work on an equivalent model, we discuss the ground state
and the low-lying excitations more extensively in connection to the spin
systems with orbital degeneracy. We show explicitly that the ground state is a
SU(4) singlet. We study the degeneracies of the elementary excitations and the
spectra of the generalized magnons consisting of these excitations. We also
discuss the complex 2-strings in the context of the Bethe ansatz solutions.Comment: Revtex, 9 pages, 3 figures; typos correcte
Optical investigation on the electronic structures of Y_{2}Ru_{2}O_{7}, CaRuO_{3}, SrRuO_{3}, and Bi_{2}Ru_{2}O_{7}
We investigated the electronic structures of the bandwidth-controlled
ruthenates, YRuO, CaRuO, SrRuO, and BiRuO, by optical conductivity analysis in a wide energy region of 5 meV
12 eV. We could assign optical transitions from the systematic changes
of the spectra and by comparison with the O 1 x-ray absorption data. We
estimated some physical parameters, such as the on-site Coulomb repulsion
energy and the crystal-field splitting energy. These parameters show that the
4 orbitals should be more extended than 3 ones. These results are also
discussed in terms of the Mott-Hubbard model.Comment: 12 pages (1 table), 3 figure
Orbital excitations in LaMnO
We study the recently observed orbital excitations, orbitons, and treat
electron-electron correlations and lattice dynamics on equal footing. It is
shown that the orbiton energy and dispersion are determined by both
correlations and lattice-vibrations. The electron-phonon coupling causes
satellite structures in the orbiton spectral function and the elementary
excitations of the system are mixed modes with both orbital and phonon
character. It is proposed that the satellite structures observed in recent
Raman-scattering experiments on LaMnO are actually orbiton derived
satellites in the phonon spectral function, caused by the phonon-orbiton
interaction.Comment: 4 pages, 3 figures embedde
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