1,284 research outputs found
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.
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
Drude Weight of the Two-Dimensional Hubbard Model -- Reexamination of Finite-Size Effect in Exact Diagonalization Study --
The Drude weight of the Hubbard model on the two-dimensional square lattice
is studied by the exact diagonalizations applied to clusters up to 20 sites. We
carefully examine finite-size effects by consideration of the appropriate
shapes of clusters and the appropriate boundary condition beyond the imitation
of employing only the simple periodic boundary condition. We successfully
capture the behavior of the Drude weight that is proportional to the squared
hole doping concentration. Our present result gives a consistent understanding
of the transition between the Mott insulator and doped metals. We also find, in
the frequency dependence of the optical conductivity, that the mid-gap
incoherent part emerges more quickly than the coherent part and rather
insensitive to the doping concentration in accordance with the scaling of the
Drude weight.Comment: 9 pages with 10 figures and 1 table. accepted in J. Phys. Soc. Jp
Optical Conductivity of the Two-Dimensional Hubbard Model
Charge dynamics of the two-dimensional Hubbard model is investigated.
Lanczs-diagonalization results for the optical conductivity and
the Drude weight of this model are presented. Near the Mott transition, large
incoherence below the upper-Hubbard band is obtained together with a remarkably
suppressed Drude weight in two dimensions while the clearly coherent character
is shown in one dimension. The two-dimensional results are consistent with
previous results from quantum Monte Carlo calculations indicating that the Mott
transition in this two-dimensional model belongs to the universality class
characterized by the dynamical exponent of .Comment: 4 pages LaTeX including 2 PS figures, to appear in J. Phys. Soc. Jp
Quantum Transition between an Antiferromagnetic Mott Insulator and Superconductor in Two Dimensions
We consider a Hubbard model on a square lattice with an additional
interaction, , which depends upon the square of a near-neighbor hopping. At
half-filling and a constant value of the Hubbard repulsion, increasing the
strength of the interaction drives the system from an antiferromagnetic
Mott insulator to a superconductor. This conclusion is reached
on the basis of zero temperature quantum Monte Carlo simulations on lattice
sizes up to .Comment: 4 pages (latex) and 4 postscript figure
On the nature of spectral line broadening in solar coronal dimmings
We analyze the profiles of iron emission lines observed in solar coronal
dimmings associated with coronal mass ejections, using the EUV Imaging
Spectrometer on board Hinode. We quantify line profile distortions with
empirical coefficients (asymmetry and peakedness) that compare the fitted
Gaussian to the data. We find that the apparent line broadenings reported in
previous studies are likely to be caused by inhomogeneities of flow velocities
along the line of sight, or at scales smaller than the resolution scale, or by
velocity fluctuations during the exposure time. The increase in the amplitude
of Alfv\'en waves cannot, alone, explain the observed features. A
double-Gaussian fit of the line profiles shows that, both for dimmings and
active region loops, one component is nearly at rest while the second component
presents a larger Doppler shift than that derived from a single-Gaussian fit.Comment: 16 pages, 11 figures - Accepted for publication in Ap
Superconductivity from Flat Dispersion Designed in Doped Mott Insulators
Routes to enhance superconducting instability are explored for doped Mott
insulators. With the help of insights for criticalities of metal-insulator
transitions, geometrical design of lattice structure is proposed to control the
instability. A guideline is to explicitly make flat band dispersions near the
Fermi level without suppressing two-particle channels. In a one-dimensional
model, numerical studies show that our prescription with finite-ranged hoppings
realizes large enhancement of spin-gap and pairing dominant regions. We also
propose several multi-band systems, where the pairing is driven by intersite
Coulomb repulsion.Comment: 4 pages, to be published in Phys. Rev. Let
Magnetic and Metal-Insulator Transitions through Bandwidth Control in Two-Dimensional Hubbard Models with Nearest and Next-Nearest Neighbor Transfers
Numerical studies on Mott transitions caused by the control of the ratio
between bandwidth and electron-electron interaction () are reported. By
using the recently proposed path-integral renormalization group(PIRG)
algorithm, physical properties near the transitions in the ground state of
two-dimensional half-filled models with the nearest and the next-nearest
neighbor transfers ( and , respectively) are studied as a prototype of
geometrically frustrated system. The nature of the bandwidth-control
transitions shows sharp contrast with that of the filling-control transitions:
First, the metal-insulator and magnetic transitions are separated each other
and the metal-insulator (MI) transition occurs at smaller , although the
both transition interactions increase with increasing . Both
transitions do not contradict the first-order transitions for smaller
while the MI transitions become continuous type accompanied by emergence of
{\it unusual metallic phase} near the transition for large . A
nonmagnetic insulator phase is stabilized between MI and AF transitions. The
region of the nonmagnetic insulator becomes wider with increasing . The
phase diagram naturally connects two qualitatively different limits, namely the
Hartree-Fock results at small and speculations in the strong coupling
Heisenberg limit.Comment: 30 pages including 20 figure
Quantum criticality around metal-insulator transitions of strongly correlated electrons
Quantum criticality of metal-insulator transitions in correlated electron
systems is shownto belong to an unconventional universality class with
violation of Ginzburg-Landau-Wilson(GLW) scheme formulated for symmetry
breaking transitions. This unconventionality arises from an emergent character
of the quantum critical point, which appears at the marginal point between the
Ising-type symmetry breaking at nonzero temperatures and the topological
transition of the Fermi surface at zero temperature. We show that Hartree-Fock
approximations of an extended Hubbard model on square latticesare capable of
such metal-insulator transitions with unusual criticality under a preexisting
symmetry breaking. The obtained universality is consistent with the scaling
theory formulated for Mott transition and with a number of numerical results
beyond the mean-field level, implying that the preexisting symmetry breaking is
not necessarily required for the emergence of this unconventional universality.
Examinations of fluctuation effects indicate that the obtained critical
exponents remain essentially exact beyond the mean-field level. Detailed
analyses on the criticality, containing diverging carrier density fluctuations
around the marginal quantum critical point, are presented from microscopic
calculations and reveal the nature as quantum critical "opalescence". Analyses
on crossovers between GLW type at nonzero temperature and topological type at
zero temperature show that the critical exponents observed in (V,Cr)2O3 and
kappa-ET-type organic conductor provide us with evidences for the existence of
the present marginal quantum criticality.Comment: 24 pages, 19 figure
Tricritical Behavior in Charge-Order System
Tricritical point in charge-order systems and its criticality are studied for
a microscopic model by using the mean-field approximation and exchange Monte
Carlo method in the classical limit as well as by using the Hartree-Fock
approximation for the quantum model. We study the extended Hubbard model and
show that the tricritical point emerges as an endpoint of the first-order
transition line between the disordered phase and the charge-ordered phase at
finite temperatures. Strong divergences of several fluctuations at zero
wavenumber are found and analyzed around the tricritical point. Especially, the
charge susceptibility chi_c and the susceptibility of the next-nearest-neighbor
correlation chi_R are shown to diverge and their critical exponents are derived
to be the same as the criticality of the susceptibility of the double occupancy
chi_D0. The singularity of conductivity at the tricritical point is clarified.
We show that the singularity of the conductivity sigma is governed by that of
the carrier density and is given as
|sigma-sigma_c|=|g-g_c|^{p_t}Alog{|g-g_{c}|}+B), where g is the effective
interaction of the Hubbard model, sigma_c g_c represents the critical
conductivity(interaction) and A and B are constants, respectively. Here, in the
canonical ensemble, we obtain p_t=2beta_t=1/2 at the tricritical point. We also
show that p_t changes into p_{t}'=2beta=1 at the tricritical point in the
grand-canonical ensemble when the tricritical point in the canonical ensemble
is involved within the phase separation region. The results are compared with
available experimental results of organic conductor (DI-DCNQI)2Ag.Comment: 20 pages, 32 figures, to appear in J. Phys. Soc. Jpn.
Vol.75(2006)No.
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