584 research outputs found
Giant infrared intensity of the Peierls mode at the neutral-ionic phase transition
We present exact diagonalization results on a modified Peierls-Hubbard model
for the neutral-ionic phase transition. The ground state potential energy
surface and the infrared intensity of the Peierls mode point to a strong,
non-linear electron-phonon coupling, with effects that are dominated by the
proximity to the electronic instability rather than by electronic correlations.
The huge infrared intensity of the Peierls mode at the ferroelectric
transition is related to the temperature dependence of the dielectric constant
of mixed-stack organic crystals.Comment: 4 pages, 4 figure
Correlation Effect on Peierls Transition
The effect of correlation on Peierls transition, which is accompanied by a
dimerization, t_d, of a bond alternation for transfer energy, has been examined
for a half-filled one-dimensional electron system with on-site repulsive
interaction (U). By applying the renormalization group method to the
interaction of the bosonized Hamiltonian, the dimerization has been calculated
variationally and self-consistently with a fixed electron-phonon coupling
constant (\lambda) and it is shown that t_d takes a maximum as a function of U.
The result is examined in terms of charge gap and spin gap and is compared with
that of the numerical simulation by Hirsch [Phys. Rev. Lett 51 (1983) 296].
Relevance to the spin Peierls transition in organic conductors is discussed.Comment: 4 pages, 4 figures, to be published in J. Phys. Soc. Jpn. 71 No.3
(2002
Structural and Electronic Instabilities in Polyacenes: Density Matrix Renormalization Group Study of a Long--Range Interacting Model
We have carried out Density Matrix Renormalization Group (DMRG) calculations
on the ground state of long polyacene oligomers within a Pariser-Parr-Pople
(PPP) Hamiltonian. The PPP model includes long-range electron correlations
which are required for physically realistic modeling of conjugated polymers. We
have obtained the ground state energy as a function of the dimerization
and various correlation functions and structure factors for
. From energetics, we find that while the nature of the Peierls'
instabilityin polyacene is conditional and strong electron correlations enhance
the dimerization. The {\it cis} form of the distortion is favoured over the
{\it trans} form. However, from the analysis of correlation functions and
associated structure factors, we find that polyacene is not susceptible to the
formation of a bond order wave (BOW), spin density wave (SDW) or a charge
density wave (CDW) in the ground state.Comment: 31 pages, latex, 13 figure
Dynamical Correlation Functions using the Density Matrix Renormalization Group
The density matrix renormalization group (DMRG) method allows for very
precise calculations of ground state properties in low-dimensional strongly
correlated systems. We investigate two methods to expand the DMRG to
calculations of dynamical properties. In the Lanczos vector method the DMRG
basis is optimized to represent Lanczos vectors, which are then used to
calculate the spectra. This method is fast and relatively easy to implement,
but the accuracy at higher frequencies is limited. Alternatively, one can
optimize the basis to represent a correction vector for a particular frequency.
The correction vectors can be used to calculate the dynamical correlation
functions at these frequencies with high accuracy. By separately calculating
correction vectors at different frequencies, the dynamical correlation
functions can be interpolated and pieced together from these results. For
systems with open boundaries we discuss how to construct operators for specific
wavevectors using filter functions.Comment: minor revision, 10 pages, 15 figure
DMRG study of scaling exponents in spin-1/2 Heisenberg chains with dimerization and frustration
In conformal field theory, key properties of spin-1/2 chains, such as the
ground state energy per site and the excitation gap scale with dimerization
delta as delta^alpha with known exponents alpha and logarithmic corrections.
The logarithmic corrections vanish in a spin chain with nearest (J=1) and next
nearest neighbor interactions (J_2), for J_2c=0.2411. DMRG analysis of a
frustrated spin chain with no logarithmic corrections yields the field
theoretic values of alpha, and the scaling relation is valid up to the
physically realized range, delta ~ 0.1. However, chains with logarithmic
corrections (J_2<0.2411 J) are more accurately fit by simple power laws with
different exponents for physically realized dimerizations. We show the
exponents decreasing from approximately 3/4 to 2/3 for the spin gap and from
approximately 3/2 to 4/3 for the energy per site and error bars in the exponent
also decrease as J_2 approaches to J_2c.Comment: 9 pages including two figures; added standard deviations of various
fitting parameters such as exponents, and several references to earlier wor
Molecular crystal approach for pi-conjugated polymers: from PPP Hamiltonian to Holstein model for polaron states
Starting from the -electron Pariser-Parr-Pople (PPP) Hamiltonian which
includes both strong electron-phonon and electron-electron interactions, we
propose some strongly correlated wave functions of increasing quality for the
ground state of conjugated polymers. These wavefunctions are built by combining
different finite sets of local configurations extended at most over two
nearest-neighbour monomers. With this picture, the doped case with one
additional particle is expressed in terms of quasi-particle. Thus, the polaron
formation problem goes back to the study of a Holstein like model.Comment: 27 pages, 6 eps figs, Revtex; enlarged version. Submitted to Journal
of Physics: Condensed Matte
Analytical solutions to the third-harmonic generation in trans-polyacetylene: Application of dipole-dipole correlation on the single electron models
The analytical solutions for the third-harmonic generation (THG) on infinite
chains in both Su-Shrieffer-Heeger (SSH) and Takayama-Lin-Liu-Maki (TLM) models
of trans-polyacetylene are obtained through the scheme of dipole-dipole ()
correlation. They are not equivalent to the results obtained through static
current-current () correlation or under polarization operator
. The van Hove singularity disappears exactly in the analytical forms,
showing that the experimentally observed two-photon absorption peak (TPA) in
THG may not be directly explained by the single electron models.Comment: 10 pages, 4 figures, submitted to Phys. Rev.
Electron Spin Resonance in S=1/2 antiferromagnetic chains
A systematic field-theory approach to Electron Spin Resonance (ESR) in the
quantum antiferromagnetic chain at low temperature (compared to the
exchange coupling ) is developed. In particular, effects of a transverse
staggered field and an exchange anisotropy (including a dipolar
interaction) on the ESR lineshape are discussed. In the lowest order
of perturbation theory, the linewidth is given as and
, respectively. In the case of a transverse staggered
field, the perturbative expansion diverges at lower temperature;
non-perturbative effects at very low temperature are discussed using exact
results on the sine-Gordon field theory. We also compare our field-theory
results with the predictions of Kubo-Tomita theory for the high-temperature
regime, and discuss the crossover between the two regimes. It is argued that a
naive application of the standard Kubo-Tomita theory to the
Dzyaloshinskii-Moriya interaction gives an incorrect result. A rigorous and
exact identity on the polarization dependence is derived for certain class of
anisotropy, and compared with the field-theory results.Comment: 53 pages in REVTEX, 7 figures in EPS included; revised version with
missing references and correction
Effects of Lattice and Molecular Phonons on Photoinduced Neutral-to-Ionic Transition Dynamics in Tetrathiafulvalene--Chloranil
For electronic states and photoinduced charge dynamics near the neutral-ionic
transition in the mixed-stack charge-transfer complex
tetrathiafulvalene--chloranil (TTF-CA), we review the effects of Peierls
coupling to lattice phonons modulating transfer integrals and Holstein
couplings to molecular vibrations modulating site energies. The former
stabilizes the ionic phase and reduces discontinuities in the phase transition,
while the latter stabilizes the neutral phase and enhances the discontinuities.
To reproduce the experimentally observed ionicity, optical conductivity and
photoinduced charge dynamics, both couplings are quantitatively important. In
particular, strong Holstein couplings to form the highly-stabilized neutral
phase are necessary for the ionic phase to be a Mott insulator with large
ionicity. A comparison with the observed photoinduced charge dynamics indicates
the presence of strings of lattice dimerization in the neutral phase above the
transition temperature.Comment: 9 pages, 7 figures, accepted for publication in J. Phys. Soc. Jp
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