1,854 research outputs found
Quantum Phase Transitions to Charge Order and Wigner Crystal Under Interplay of Lattice Commensurability and Long-Range Coulomb Interaction
Relationship among Wigner crystal, charge order and Mott insulator is studied
by the path-integral renormalization group method for two-dimensional lattices
with long-range Coulomb interaction. In contrast to Hartree-Fock results, the
solid stability drastically increases with lattice commensurability. The
transition to liquid occurs at the electron gas parameter for the
filling showing large reduction from in the continuum
limit. Correct account of quantum fluctuations are crucial to understand
charge-order stability generally observed only at simple fractional fillings
and nature of quantum liquids away from them.Comment: 4 pages including 7 figure
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
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
Absence of long-range superconducting correlations in the frustrated 1/2-filled band Hubbard model
We present many-body calculations of superconducting pair-pair correlations
in the ground state of the half-filled band Hubbard model on large anisotropic
triangular lattices. Our calculations cover nearly the complete range of
anisotropies between the square and isotropic triangular lattice limits. We
find that the superconducting pair-pair correlations decrease monotonically
with increasing onsite Hubbard interaction U for inter-pair distances greater
than nearest neighbor. For the large lattices of interest here the distance
dependence of the correlations approaches that for noninteracting electrons.
Both these results are consistent with the absence of superconductivity in this
model in the thermodynamic limit. We conclude that the effective 1/2-filled
band Hubbard model, suggested by many authors to be appropriate for the
kappa-(BEDT-TTF)-based organic charge-transfer solids, does not explain the
superconducting transition in these materials.Comment: 9 pages, 7 figures. Revised version to appear in Phys. Rev.
Ab-initio procedure for effective models of correlated materials with entangled band structure
We present a first-principles method for deriving effective low-energy models
of electrons in solids having entangled band structure. The procedure starts
with dividing the Hilbert space into two subspaces, the low-energy part ("
space'') and the rest of the space (" space''). The low-energy model is
constructed for the space by eliminating the degrees of freedom of the
space. The thus derived model contains the strength of electron correlation
expressed by a partially screened Coulomb interaction, calculated in the
constrained random-phase-approximation (cRPA) where screening channels within
the space, , are subtracted. One conceptual problem of this
established downfolding method is that for entangled bands it is not clear how
to cut out the space and how to distinguish from the total
polarization. Here, we propose a simple procedure to overcome this difficulty.
In our scheme, the subspace is cut out from the Hilbert space of the Kohn
Sham eigenfunctions with the help of a procedure to construct a localized
Wannier basis. The subspace is constructed as the complementary space
orthogonal to the subspace. After this disentanglement, becomes well
defined. Using the disentangled bands, the effective parameters are uniquely
determined in the cRPA. The method is successfully applied to 3 transition
metals.Comment: 14 pages, 4 figure
A revised checklist of Hawaiian mosses
A revised and updated literature-based checklist of Hawaiian mosses is presented. Geographic coverage includes the eight main Hawaiian Islands; the Northwestern Hawaiian Islands are excluded. The checklist is alphabetically ordered by scientific names; the family is noted for each genus. Synonyms and misapplied names are cross-referenced to the accepted names. A bibliography of supporting references is included
Gapped spin liquid states in a one-dimensional Hubbard model with antiferromagnetic exchange interaction
We study the phase diagram of a one-dimensional extended Hubbard model with
antiferromagnetic exchange interaction analytically and numerically. The
bosonization and transfer-matrix renormalization group methods are used in the
corresponding coupling regimes. At half-filling, the system is a Mott insulator
with a finite spin excitation gap if the on-site Coulomb repulsion is fairly
smaller than the antiferromagnetic exchange J. This Mott-insulator is
characterized by the bond-charge-density-wave order or spontaneously
dimerization. In the weak-coupling regime where the spin-charge separation
holds approximately, the critical point separating the gapless and gapped spin
liquid phases is U_c\sim J/2. However, as J increases, the spin-charge
couplings become important and the critical point U_c is significantly
suppressed and eventually tends to zero as J\to \infty. Away from half-filling,
the charge gap completely collapses but the spin gap persists.Comment: 5 pages, 2 figures, to appear in PR
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