1,507 research outputs found
Pseudogap and Mott Transition Studied by Cellular Dynamical Mean Field Theory
We study metal-insulator transitions between Mott insulators and metals. The
transition mechanism completely different from the original dynamical mean
field theory (DMFT) emerges from a cluster extension of it. A consistent
picture suggests that the quasiparticle weight remains nonzero through
metals and suddenly jumps to zero at the transition, while the gap opens
continuously in the insulators. This is in contrast with the original DMFT,
where continuously vanishes but the gap opens discontinuously. The present
results arising from electron differentiation in momentum space agree with
recent puzzling bulk-sensitive experiments on CaVO and SrVO.Comment: 5 pages, 4 figure
Suppressed Coherence due to Orbital Correlations in the Ferromagnetically Ordered Metallic Phase of Mn Compounds
Small Drude weight together with small specific heat coefficient
observed in the ferromagnetic phase of RAMnO (R=La, Pr, Nd, Sm;
A=Ca, Sr, Ba) are analyzed in terms of a proximity effect of the Mott
insulator. The scaling theory for the metal-insulator transition with the
critical enhancement of orbital correlations toward the staggered ordering of
two orbitals such as and symmetries may lead to the
critical exponents of and with
and . The result agrees with the experimental indications.Comment: 4 pages LaTeX using jpsj.sty. To appear in J. Phys. Soc. Jpn.
67(1998)No.
Quantum-number projection in the path-integral renormalization group method
We present a quantum-number projection technique which enables us to exactly
treat spin, momentum and other symmetries embedded in the Hubbard model. By
combining this projection technique, we extend the path-integral
renormalization group method to improve the efficiency of numerical
computations. By taking numerical calculations for the standard Hubbard model
and the Hubbard model with next nearest neighbor transfer, we show that the
present extended method can extremely enhance numerical accuracy and that it
can handle excited states, in addition to the ground state.Comment: 11 pages, 7 figures, submitted to Phys. Rev.
Multi-wavelength spectroscopic observation of EUV jet in AR 10960
We have studied the relationship between the velocity and temperature of a
solar EUV jet. The highly accelerated jet occurred in the active region NOAA
10960 on 2007 June 5. Multi-wavelength spectral observations with EIS/Hinode
allow us to investigate Doppler velocities at the wide temperature range. We
analyzed the three-dimensional angle of the jet from the stereoscopic analysis
with STEREO. Using this angle and Doppler velocity, we derived the true
velocity of the jet. As a result, we found that the cool jet observed with
\ion{He}{2} 256 \AA is accelerated to around which is over the upper limit of the chromospheric evaporation. The
velocities observed with the other lines are under the upper limit of the
chromospheric evaporation while most of the velocities of hot lines are higher
than that of cool lines. We interpret that the chromospheric evaporation and
magnetic acceleration occur simultaneously. A morphological interpretation of
this event based on the reconnection model is given by utilizing the
multi-instrumental observations.Comment: Accepted for publication in Ap
Quantification of propagation modes in an astronomical instrument from its radiation pattern
Context. Understanding complex phenomena and unsolved problems in modern
astronomy requires wider-bandwidth observations. The current technique for
designing and fabricating an astronomical instrument potentially provides such
observations with higher efficiency and precision than in the past.
Higher-order modes in an instrument associated with wider bandwidths have been
reported, which may degrade observation precision. Aims. To reduce the
unfavorable degradation, we need to quantify the higher-order propagation
modes, though their power is too difficult to measure directly. Instead of the
direct mode measurement, we aim at developing a method based on measurable
radiation patterns from an instrument of interest. Method. Assuming a linear
system, whose radiated field is determined as a superposition of the mode
coefficients in an instrument, we obtain a coefficient matrix connecting the
inside modes and the outside radiated field and calculate the pseudo-inverse
matrix. To understand the estimation accuracy of the proposed method, we
demonstrate two cases with numerical simulations, axially-corrugated horn case
and offset Cassegrain antenna case, and investigate the effect of random errors
on the accuracy. Results. Both cases showed the estimated mode coefficients
with a precision of 10e-6 with respect to the maximum mode amplitude and 10e-3
degrees in phase, respectively. The calculation errors were observed when the
random errors were smaller than 0.01 percent of the maximum radiated field
amplitude. The demonstrated method works independently of the details of a
system. Conclusions. The method can quantify the propagation modes inside an
instrument and will be applicable to most of linear components and antennas.
This method can be employed for a general purpose, such as diagnosis of feed
alignment and higher-performance feed design.Comment: 7 pages, 11 figure
Competition between spin exchange and correlated hopping
The ground-state phase diagram is numerically studied for an electronic model
consisting of the spin exchange term (J) and the correlated hopping term (t_3:
the three-site term). This model has no single-particle hopping and the ratio
of the two terms is controlled by a parameter \alpha \equiv 4 t_3 / J. The case
of \alpha=1 corresponds to complete suppression of single-particle hopping in
the strong-coupling limit of the Hubbard model. In one dimension, phase
separation takes place below a critical value \alpha_c = 0.36-0.63 which
depends on the electron density. Spin gap opens in the whole region except the
phase-separated one. For \alpha \gsim 1.2 and low hole densities,
charge-density-wave correlations are the most dominant, whereas singlet-pairing
correlations are the most dominant in the remaining region. The possibility of
superconductivity in the two-dimensional case is also discussed, based on
equal-time pairing correlations.Comment: 4 pages including 5 figures. Proceedings of ISSP-Kashiwa 2001
(submitted to J. Phys. Chem. Solids
Doping-driven Mott transition in La_{1-x}Sr_xTiO_3 via simultaneous electron and hole doping of t2g subbands
The insulator to metal transition in LaTiO_3 induced by La substitution via
Sr is studied within multi-band exact diagonalization dynamical mean field
theory at finite temperatures. It is shown that weak hole doping triggers a
large interorbital charge transfer, with simultaneous electron and hole doping
of t2g subbands. The transition is first-order and exhibits phase separation
between insulator and metal. In the metallic phase, subband compressibilities
become very large and have opposite signs. Electron doping gives rise to an
interorbital charge flow in the same direction as hole doping. These results
can be understood in terms of a strong orbital depolarization.Comment: 4 pages, 5 figure
Cluster dynamical mean-field study of the Hubbard model on a 3D frustrated hyperkagome lattice
We study the Hubbard model on a geometrically-frustrated hyperkagome lattice
by a cluster extension of the dynamical mean field theory. We calculate the
temperature () dependences of the specific heat () and the spin-lattice
relaxation time () in correlated metallic region. shows a peak at
and rapidly decreases as . On the other hand, has a
peak at a higher temperature than , and largely decreases
below , followed by the Korringa law as . Both
peak temperatures are suppressed and the peaks become sharper as electron
correlation is increased. These behaviors originate from strong renormalization
of the energy scales in the peculiar electronic structure in this frustrated
system; a pseudo-gap like feature, the van-Hove singularity, and the flat band.
The results are discussed in comparison with the experimental data in the
hyperkagome material, NaIrO.Comment: 4 pages, 4 figures, Conference proceedings for Highly Frustrated
Magnetism 200
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