503 research outputs found
A gapless charge mode induced by the boundary states in the half-filled Hubbard open-chain
We discuss the ground state and some excited states of the half-filled
Hubbard model defined on an open chain with L sites, where only one of the
boundary sites has a different value of chemical potential. We consider the
case when the boundary site has a negative chemical potential -p and the
Hubbard coupling U is positive. By an analytic method we show that when p is
larger than the transfer integral some of the ground-state solutions of the
Bethe ansatz equations become complex-valued. It follows that there is a
``surface phase transition'' at some critical value p_c; when p<p_c all the
charge excitations have the gap for the half-filled band, while there exists a
massless charge mode when p>p_c.Comment: Revtex, 25 pages, 3 eps figures; Full revision with Appendixes adde
Ground State Properties and Optical Conductivity of the Transition Metal Oxide
Combining first-principles calculations with a technique for many-body
problems, we investigate properties of the transition metal oxide from the microscopic point of view. By using the local density
approximation (LDA), the high-energy band structure is obtained, while screened
Coulomb interactions are derived from the constrained LDA and the GW method.
The renormalization of the kinetic energy is determined from the GW method. By
these downfolding procedures, an effective Hamiltonian at low energies is
derived. Applying the path integral renormalization group method to this
Hamiltonian, we obtain ground state properties such as the magnetic and orbital
orders. Obtained results are consistent with experiments within available data.
We find that is close to the metal-insulator transition.
Furthermore, because of the coexistence and competition of ferromagnetic and
antiferromgnetic exchange interactions in this system, an antiferromagnetic and
orbital-ordered state with a nontrivial and large unit cell structure is
predicted in the ground state. The calculated optical conductivity shows
characteristic shoulder structure in agreement with the experimental results.
This suggests an orbital selective reduction of the Mott gap.Comment: 38pages, 22figure
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
Absence of Translational Symmetry Breaking in Nonmagnetic Insulator Phase on Two-Dimensional Lattice with Geometrical Frustration
The ground-state properties of the two-dimensional Hubbard model with
nearest-neighbor and next-nearest-neighbor hoppings at half filling are studied
by the path-integral-renormalization-group method. The nonmagnetic-insulator
phase sandwiched by the the paramagnetic-metal phase and the
antiferromagnetic-insulator phase shows evidence against translational symmetry
breaking of the dimerized state, plaquette singlet state, staggered flux state,
and charge ordered state. These results support that the genuine Mott insulator
which cannot be adiabatically continued to the band insulator is realized
generically by Umklapp scattering through the effects of geometrical
frustration and quantum fluctuation in the two-dimensional system.Comment: 4 pages and 7 figure
Exact diagonalization study of Mott transition in the Hubbard model on an anisotropic triangular lattice
We study Mott transition in the two-dimensional Hubbard model on an
anisotropic triangular lattice. We use the Lanczos exact diagonalization of
finite-size clusters up to eighteen sites, and calculate Drude weight, charge
gap, double occupancy and spin structure factor. We average these physical
quantities over twisted boundary conditions in order to reduce finite-size
effects. We find a signature of the Mott transition in the dependence of the
Drude weight and/or charge gap on the system size. We also examine the
possibility of antiferromagnetic order from the spin structure factor.
Combining these information, we propose a ground-state phase diagram which has
a nonmagnetic insulating phase between a metallic phase and an insulating phase
with antiferromagnetic order. Finally, we compare our results with those
reported in the previous theoretical studies, and discuss the possibility of an
unconventional insulating state.Comment: 10 pages, 11 figure
Pump Built-in Hamiltonian Method for Pump-Probe Spectroscopy
We propose a new method of calculating nonlinear optical responses of
interacting electronic systems. In this method, the total Hamiltonian (system +
system-pump interaction) is transformed into a different form that (apparently)
does not have a system-pump interaction. The transformed Hamiltonian, which we
call the pump built-in Hamiltonian, has parameters that depend on the strength
of the pump beam. Using the pump built-in Hamiltonian, we can calculate
nonlinear responses (responses to probe beams as a function of the pump beam)
by applying the {\em linear} response theory. We demonstrate the basic idea of
this new method by applying it to a one-dimensional, two-band model, in the
case the pump excitation is virtual (coherent excitation). We find that the
exponent of the Fermi edge singularity varies with the pump intensity.Comment: 6 page
Variational Monte Carlo Study of Electron Differentiation around Mott Transition
We study ground-state properties of the two-dimensional Hubbard model at half
filling by improving variational Monte Carlo method and by implementing
quantum-number projection and multi-variable optimization. The improved
variational wave function enables a highly accurate description of the Mott
transition and strong fluctuations in metals. We clarify how anomalous metals
appear near the first-order Mott transition. The double occupancy stays nearly
constant as a function of the on-site Coulomb interaction in the metallic phase
near the Mott transition in agreement with the previous unbiased results. This
unconventional metal at half filling is stabilized by a formation of
``electron-like pockets'' coexisting with an arc structure, which leads to a
prominent differentiation of electrons in momentum space. An abrupt collapse of
the ``pocket'' and ``arc'' drives the first-order Mott transition.Comment: 4 pages, 3 figure
Control of Superconducting Correlations in High-Tc Cuprates
A strategy to enhance d-wave superconducting correlations is proposed based
on our numerical study for correlated electron models for high-Tc cuprates. We
observe that the pairing is enhanced when the single-electron level around
(pi,0) is close to the Fermi level E_F, while the d-wave pairing interaction
itself contains elements to disfavor the pairing due to shift of the
(pi,0)-level. Angle-resolved photoemission results in the cuprates are
consistently explained in the presence of the d-wave pairing interaction. Our
proposal is the tuning of the (pi,0)-level under the many-body effects to E_F
by optimal design of band structure.Comment: 4 pages, 6 eps figure
First-Principles Computation of YVO3; Combining Path-Integral Renormalization Group with Density-Functional Approach
We investigate the electronic structure of the transition-metal oxide YVO3 by
a hybrid first-principles scheme. The density-functional theory with the
local-density-approximation by using the local muffin-tin orbital basis is
applied to derive the whole band structure. The electron degrees of freedom far
from the Fermi level are eliminated by a downfolding procedure leaving only the
V 3d t2g Wannier band as the low-energy degrees of freedom, for which a
low-energy effective model is constructed. This low-energy effective
Hamiltonian is solved exactly by the path-integral renormalization group
method. It is shown that the ground state has the G-type spin and the C-type
orbital ordering in agreement with experimental indications. The indirect
charge gap is estimated to be around 0.7 eV, which prominently improves the
previous estimates by other conventional methods
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