206 research outputs found
Orbital Magnetism of Bloch Electrons III. Application to Graphene
The orbital susceptibility for graphene is calculated exactly up to the first
order with respect to the overlap integrals between neighboring atomic
orbitals. The general and rigorous theory of orbital susceptibility developed
in the preceding paper is applied to a model for graphene as a typical two-band
model. It is found that there are contributions from interband, Fermi surface,
and occupied states in addition to the Landau--Peierls orbital susceptibility.
The relative phase between the atomic orbitals on the two sublattices related
to the chirality of Dirac cones plays an important role. It is shown that there
are some additional contributions to the orbital susceptibility that are not
included in the previous calculations using the Peierls phase in the
tight-binding model for graphene. The physical origin of this difference is
clarified in terms of the corrections to the Peierls phase.Comment: 13 pages, 4 figure
Mean-Field Analysis of Electric Field Effect on Charge Orders in Organic Conductors
In order to investigate charge ordering phenomena under electric field,
static nonequilibrium Hartree approximation (SNHA) method is formulated on the
basis of the nonequilibrium Green's functions introduced by Keldysh. By
applying the SNHA to the 3/4-filling extended Hubbard model on anisotropic
triangular lattice, we study the stabilities and amplitudes of 3-fold and
horizontal charge orders in and -(BEDT-TTF) salts
under the electric field. The obtained results show that the electric field
stabilizes the 3-fold state in comparison to the horizontal state. The
amplitude of the 3-fold state tends to decrease by the field, whereas that of
the horizontal state does not change.Comment: 4pages, 6figures, and 1tabl
Meissner Effect of Dirac Electrons in Superconducting State due to Inter-band Effect
Dirac electrons in solids show characteristic physical properties due to
their linear dispersion relation and two-band nature. Although the transport
phenomena of Dirac electrons in a normal state have intensively been studied,
the transport phenomena in a superconducting state have not been fully
understood. In particular, it is not clear whether Dirac electrons in a
superconducting state show Meissner effect (ME), since a diamagnetic term of a
current operator is absent as a result of the linear dispersion. We investigate
the ME of three dimensional massive Dirac electrons in a superconducting state
on the basis of Kubo formula, and clarify that Meissner kernel becomes finite
by use of the inter-band contribution. This mechanism of the ME for Dirac
electrons is completely different from that for the electrons in usual metals.
Our result shows that the Meissner kernel remains finite even when the
superconducting gap vanishes. This is an unavoidable problem in the Dirac
electron system as reported in the previous works. Thus, we use a prescription
in which we subtract the normal state contribution. In order to justify this
prescription, we develop a specific model where the Meissner kernel is obtained
by the prescription. We also derive the result for the electron gas by taking
the non-relativistic limit of Dirac Hamiltonian, and clarify that the
diamagnetic term of the Meissner kernel can be regarded as the inter-band
contribution between electrons and positrons in terms of the Dirac model.Comment: 7 pages, 5 figures. To be published in J. Phys. Soc. Jp
One-Dimensional t-J Model from a Variational Viewpoint
The one-dimensional (1D) - model is investigated by using a
Gutzwiller-Jastrow-type variation method and the exact diagonalization of small
systems. Variational expectation values are estimated by the variational Monte
Carlo method with sufficient accuracy. First, we represent the diagonalization
results. Physical quantities like momentum distribution and some correlation
functions show some behaviors which are not expected in repulsive models, as
the value of increases. These properties as well as energy are well
understood by introducing intersite correlation factors into wave functions.
The phase transition to a separated phase in large- region can be
described by an attractive Jastrow wave function in quantitative agreement with
the exact results. On the other hand, for the supersymmetric case () the
original Gutzwiller wave function becomes an extremely good trial function for
all the range of electron density. Here a kind of \lq\lq free electron" state
is realized, particularly in the low electron density. Next, the above wave
functions are compared with the Tomonaga-Luttinger-liquid-type wave function
proposed by Hellberg and Mele. It is found that the correlation factors in
short distances control bulk quantities like energy and the magnitude of
correlation functions, while the long-range part of correlataion factors
determines the critical behavior of correlation functions. Lastly, using these
functions, charge and spin susceptibilities and magnetization curve are
estimated, which agree with the exact results. It is shown that the Mott
transition in 1D - model is quite different from the Brinkman-Rice
transition.Comment: plain TeX, 37 pages. Hard copies of 25 figures are available on
request. Submitted to Phys. Rev.
Doublon-Holon Binding Effects on Mott Transitions in Two-Dimensional Bose Hubbard Model
A mechanism of Mott transitions in a Bose Hubbard model on a square lattice
is studied, using a variational Monte Carlo method. Besides an onsite
correlation factor, we introduce a four-body doublon-holon factor into the
trial state, which considerably improves the variational energy and can
appropriately describe a superfluid-insulator transition. Its essense consists
in binding (and unbinding) of a doublon to a holon in a finite short range,
identical with the cases of fermions. The features of this transition are
qualitatively different from those of Brinkman-Rice-type transitions.Comment: 5 pages, 6 figures, proceedings of SNS200
Competition between spin fluctuations in CaSrRuO around
We study the static susceptibilities for charge and spin sectors in
paramagnetic states for CaSrRuO in
within random phase approximation on the basis of an effective Ru
orbital Hubbard model. We find that several modes of spin fluctuation around
\boldq=(0,0) and \boldq\sim(0.797\pi,0) are strongly enhanced for the model
of . This enhancement arises from the increase of the corresponding
susceptibilities for the orbital due to the rotation-induced
modifications of the electronic structure for this orbital (i.e., the
flattening of the bandwidth and the increase of the density of state near the
Fermi level). We also find that the ferromagnetic spin fluctuation becomes
stronger for a special model than for the model of , while the
competition between the modes of spin fluctuation at \boldq=(0,0) and around
\boldq\sim (\pi,0) is weaker for the special model; in this special model,
the van Hove singularity (vHs) for the orbital is located on the Fermi
level. These results indicate that the location of the vHs for the
orbital, which is controlled by substitution of Ca for Sr, is a parameter to
control this competition. We propose that the spin fluctuations for the
orbital around \boldq=(0,0) and \boldq\sim (\pi,0) play an
important role in the electronic states around other than the
criticality approaching the usual Mott transition where all electrons are
localized.Comment: 12 pages, 9 figures, accepted for publication in Phys. Rev.
Solitons in the Crossover between Band Insulator and Mott Insulator: Application to TTF-Chloranil under Pressure
Based on the Phase Hamiltonian, two types of solitons are found to exist in
the crossover region between band insulator and Mott insulator in
one-dimension. Both of these solitons have fractional charges but with
different spins, zero and 1/2, respectively. The results are in accord with the
experimental results by Kanoda et al. for TTF-Chloranil under pressure.Comment: Submitted to J. Phys. Soc. Japan, 8 pages, 4 figure
Unconventional Spin Hall Effect and Axial Current Generation in a Dirac Semimetal
We investigate electrical transport in a three-dimensional massless Dirac
fermion model that describes a Dirac semimetal state realized in topological
materials. We derive a set of interdependent diffusion equations with eight
local degrees of freedom, including the electric charge density and the spin
density, that respond to an external electric field. By solving the diffusion
equations for a system with a boundary, we demonstrate that a spin Hall effect
with spin accumulation occurs even though the conventional spin current
operator is zero. The Noether current associated with chiral symmetry, known as
the axial current, is also discussed. We demonstrate that the axial current
flows near the boundary and that it is perpendicular to the electric current.Comment: 5 pages, 2 figure
Crossover from dilute-Kondo system to heavy-fermion system
Ground state properties of a Kondo lattice model with random configuration of
electrons are investigated with a variational Monte Carlo method. We show
that the crossover from a dilute-Kondo system to a heavy-fermion system occurs
when the density of and electrons (, ) become comparable,
. In the heavy-fermion region, the correlation between
electrons is strong and the electrons themselves greatly contribute to the
screening of other -electron moments. We propose that the character of Kondo
screening changes from "individual" to "collective" across the crossover.Comment: 5 pages, 4 figures, submitted to Phys. Rev. Let
Microscopic Identification of the D-vector in Triplet Superconductor Sr_2RuO_4
Triplet superconductivity in Sr_2RuO_4 is investigated with main interest on
its internal degree of freedom. We perform a microscopic calculation to
investigate how the chiral state d(k) = (k_x \pm ik_y)z is realized among the
underlying six degenerate states. Starting from the three band Hubbard model
with spin-orbit interaction, we use a perturbation theory in order to calculate
the pairing interaction. The p-wave superconductivity with T_c \sim 1.5K is
obtained in the moderately weak coupling region. It is shown that the orbital
dependent superconductivity (ODS) robustly appears in Sr_2RuO_4. We determine
the stabilized state by solving the Eliashberg equations. It is found that the
Hund coupling term as well as the spin-orbit interaction is necessary for the
``symmetry breaking interaction''. The main result is that the chiral state is
stabilized in case of the p-wave symmetry with the main \gamma-band, which is
obtained in the perturbation theory. When we assume the other pairing symmetry
including the f-wave state, the symmetry breaking interaction gives the other
D-vector. The electronic structure constructed from the t_2g-orbitals is
essential for this result.Comment: To appear in J. Phys. Soc. Jpn. Vol.72 (2003) No.
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