402 research outputs found
Luttinger parameters and momentum distribution function for the half-filled spinless fermion Holstein model: A DMRG approach
We reexamine the nature of the metallic phase of the one-dimensional
half-filled Holstein model of spinless fermions. To this end we determine the
Tomonaga-Luttinger-liquid correlation parameter by large-scale
density-matrix renormalisation-group (DMRG) calculations, exploiting (i) the
leading-order scaling relations between the ground-state energy and the
single-particle excitation gap and (ii) the static charge structure factor in
the long-wavelength limit. While both approaches give almost identical results
for intermediate-to-large phonon frequencies, we find contrasting behaviour in
the adiabatic regime: (i) (attractive) versus (ii)
(repulsive). The latter result for the correlation exponent is corroborated by
data obtained for the momentum distribution function , which puts the
existence of an attractive metallic state in the spinless fermion Holstein
model into question. We conclude that the scaling relation must be modified in
the presence of electron-phonon interactions with noticeable retardation.Comment: 6 pages, 5 figures, revised versio
Role of Interchain Hopping in the Magnetic Susceptibility of Quasi-One-Dimensional Electron Systems
The role of interchain hopping in quasi-one-dimensional (Q-1D) electron
systems is investigated by extending the Kadanoff-Wilson renormalization group
of one-dimensional (1D) systems to Q-1D systems. This scheme is applied to the
extended Hubbard model to calculate the temperature () dependence of the
magnetic susceptibility, . The calculation is performed by taking
into account not only the logarithmic Cooper and Peierls channels, but also the
non-logarithmic Landau and finite momentum Cooper channels, which give relevant
contributions to the uniform response at finite temperatures. It is shown that
the interchain hopping, , reduces at low temperatures,
while it enhances at high temperatures. This notable
dependence is ascribed to the fact that enhances the
antiferromagnetic spin fluctuation at low temperatures, while it suppresses the
1D fluctuation at high temperatures. The result is at variance with the
random-phase-approximation approach, which predicts an enhancement of by over the whole temperature range. The influence of both the
long-range repulsion and the nesting deviations on is further
investigated. We discuss the present results in connection with the data of
in the (TMTTF) and (TMTSF) series of Q-1D organic
conductors, and propose a theoretical prediction for the effect of pressure on
magnetic susceptibility.Comment: 17 pages, 19figure
Anharmonicity in one-dimensional electron-phonon system
We investigate the effect of anharmonicity on the one-dimensional half-filled
Holstein model by using the determinant quantum Monte Carlo method. By
calculating the order parameters we find that with and without anharmonicity
there is always an transition from a disorder phase to a dimerized phase.
Moreover, in the dimerized phase a lattice dimerization and a charge density
wave coexist. The anharmonicity represented by the quartic term suppresses the
dimerization as well as the charge density wave, while a double-well potential
favors the dimerization. In addition, by calculating the correlation exponents
we show that the disorder phase is metallic with gapless charge excitations and
gapful spin excitations while in the dimerized phase both excitations are
gapful.Comment: 5 page
Finite-Temperature Properties across the Charge Ordering Transition -- Combined Bosonization, Renormalization Group, and Numerical Methods
We theoretically describe the charge ordering (CO) metal-insulator transition
based on a quasi-one-dimensional extended Hubbard model, and investigate the
finite temperature () properties across the transition temperature, . In order to calculate dependence of physical quantities such as the
spin susceptibility and the electrical resistivity, both above and below
, a theoretical scheme is developed which combines analytical
methods with numerical calculations. We take advantage of the renormalization
group equations derived from the effective bosonized Hamiltonian, where Lanczos
exact diagonalization data are chosen as initial parameters, while the CO order
parameter at finite- is determined by quantum Monte Carlo simulations. The
results show that the spin susceptibility does not show a steep singularity at
, and it slightly increases compared to the case without CO because
of the suppression of the spin velocity. In contrast, the resistivity exhibits
a sudden increase at , below which a characteristic dependence
is observed. We also compare our results with experiments on molecular
conductors as well as transition metal oxides showing CO.Comment: 9 pages, 8 figure
Theoretical Aspects of Charge Ordering in Molecular Conductors
Theoretical studies on charge ordering phenomena in quarter-filled molecular
(organic) conductors are reviewed. Extended Hubbard models including not only
the on-site but also the inter-site Coulomb repulsion are constructed in a
straightforward way from the crystal structures, which serve for individual
study on each material as well as for their systematic understandings. In
general the inter-site Coulomb interaction stabilizes Wigner crystal-type
charge ordered states, where the charge localizes in an arranged manner
avoiding each other, and can drive the system insulating. The variety in the
lattice structures, represented by anisotropic networks in not only the
electron hopping but also in the inter-site Coulomb repulsion, brings about
diverse problems in low-dimensional strongly correlated systems. Competitions
and/or co-existences between the charge ordered state and other states are
discussed, such as metal, superconductor, and the dimer-type Mott insulating
state which is another typical insulating state in molecular conductors.
Interplay with magnetism, e.g., antiferromagnetic state and spin gapped state
for example due to the spin-Peierls transition, is considered as well. Distinct
situations are pointed out: influences of the coupling to the lattice degree of
freedom and effects of geometrical frustration which exists in many molecular
crystals. Some related topics, such as charge order in transition metal oxides
and its role in new molecular conductors, are briefly remarked.Comment: 21 pages, 19 figures, to be published in J. Phys. Soc. Jpn. special
issue on "Organic Conductors"; figs. 4 and 11 replaced with smaller sized
fil
From Gapped Excitons to Gapless Triplons in One Dimension
Often, exotic phases appear in the phase diagrams between conventional
phases. Their elementary excitations are of particular interest. Here, we
consider the example of the ionic Hubbard model in one dimension. This model is
a band insulator (BI) for weak interaction and a Mott insulator (MI) for strong
interaction. Inbetween, a spontaneously dimerized insulator (SDI) occurs which
is governed by energetically low-lying charge and spin degrees of freedom.
Applying a systematically controlled version of the continuous unitary
transformations (CUTs) we are able to determine the dispersions of the
elementary charge and spin excitations and of their most relevant bound states
on equal footing. The key idea is to start from an externally dimerized system
using the relative weak interdimer coupling as small expansion parameter which
finally is set to unity to recover the original model.Comment: 18 pages, 10 figure
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