547 research outputs found
Bond-order wave phase, spin solitons and thermodynamics of a frustrated linear spin-1/2 Heisenberg antiferromagnet
The linear spin-1/2 Heisenberg antiferromagnet with exchanges ,
between first and second neighbors has a bond-order wave (BOW) phase that
starts at the fluid-dimer transition at and is particularly
simple at . The BOW phase has a doubly degenerate singlet ground
state, broken inversion symmetry and a finite energy gap to the lowest
triplet state.
The interval has large and small finite size
corrections. Exact solutions are presented up to spins with either
periodic or open boundary conditions and for thermodynamics up to . The
elementary excitations of the BOW phase with large are topological
spin-1/2 solitons that separate BOWs with opposite phase in a regular array of
spins. The molar spin susceptibility is exponentially small for and increases nearly linearly with to a broad maximum. ,
spin chains approximate the magnetic properties of the BOW phase of
Hubbard-type models and provide a starting point for modeling alkali-TCNQ
salts.Comment: 10 pages, 12 figure
Bond order wave (BOW) phase of the extended Hubbard model: Electronic solitons, paramagnetism, coupling to Peierls and Holstein phonons
The bond order wave (BOW) phase of the extended Hubbard model (EHM) in one
dimension (1D) is characterized at intermediate correlation by exact
treatment of -site systems. Linear coupling to lattice (Peierls) phonons and
molecular (Holstein) vibrations are treated in the adiabatic approximation. The
molar magnetic susceptibility is obtained directly up to .
The goal is to find the consequences of a doubly degenerate ground state (gs)
and finite magnetic gap in a regular array. Degenerate gs with broken
inversion symmetry are constructed for finite for a range of near the
charge density wave (CDW) boundary at where is large. The electronic amplitude of the BOW in the regular array
is shown to mimic a tight-binding band with small effective dimerization
. Electronic spin and charge solitons are elementary excitations
of the BOW phase and also resemble topological solitons with small
. Strong infrared intensity of coupled molecular vibrations in
dimerized 1D systems is shown to extend to the regular BOW phase, while its
temperature dependence is related to spin solitons. The Peierls instability to
dimerization has novel aspects for degenerate gs and substantial that
suppresses thermal excitations. Finite implies exponentially small
at low temperature followed by an almost linear increase with .
The EHM with is representative of intermediate correlations in
quasi-1D systems such as conjugated polymers or organic ion-radical and
charge-transfer salts. The vibronic and thermal properties of correlated models
with BOW phases are needed to identify possible physical realizations.Comment: 12 pages, 10 figure
Symmetry crossover and excitation thresholds at the neutral-ionic transition of the modified Hubbard model
Exact ground states, charge densities and excitation energies are found using
valence bond methods for N-site modified Hubbard models with uniform spacing.
At the neutral-ionic transition (NIT), the ground state has a symmetry
crossover in 4n, 4n+2 rings with periodic and antiperiodic boundary conditions,
respectively. The modified Hubbard model has a continuous NIT between a
diamagnetic band insulator on the paired side and a paramagnetic Mott insulator
on the covalent side. The singlet-triplet (ST), singlet-singlet (SS) and charge
gaps for finite N indicate that the ST and SS gaps close at the NIT with
increasing U and that the charge gap vanishes only there. Finite-N excitations
constrain all singularities to about 0.1t of the symmetry crossover. The NIT is
interpreted as a localized ground state (gs) with finite gaps on the paired
side and an extended gs with vanishing ST and SS gaps on the covalent side. The
charge gap and charge stiffness indicate a metallic gs at the transition that,
however, is unconditionally unstable to dimerization.Comment: 12 pages, including 8 figure
Anomalous dispersion of optical phonons at the neutral-ionic transition: Evidence from diffuse X-ray scattering
Diffuse X-ray data for mixed stack organic charge-transfer crystals
approaching the neutral-ionic phase transition can be quantitatively explained
as due to the softening of the optical phonon branch. The interpretation is
fully consistent with vibrational spectra, and underlines the importance of
electron-phonon coupling in low-dimensional systems with delocalized electrons.Comment: 4 pages, 4 figure
Efficient Density Matrix Renormalization Group algorithm to study Y-Junctions with integer and half-integer spin
An efficient density matrix renormalization group (DMRG) algorithm is
presented and applied to Y-junctions, systems with three arms of sites that
meet at a central site. The accuracy is comparable to DMRG of chains. As in
chains, new sites are always bonded to the most recently added sites and the
superblock Hamiltonian contains only new or once renormalized operators.
Junctions of up to sites are studied with
antiferromagnetic (AF) Heisenberg exchange between nearest-neighbor spins
or electron transfer between nearest neighbors in half-filled Hubbard
models. Exchange or electron transfer is exclusively between sites in two
sublattices with . The ground state (GS) and spin densities at site are quite different for junctions with =
1/2, 1, 3/2 and 2. The GS has finite total spin for even (odd)
and for in the spin manifold, at sites
of the larger (smaller) sublattice. = 1/2 junctions have delocalized states
and decreasing spin densities with increasing . = 1 junctions have four
localized states at the end of each arm and centered on the
junction, consistent with localized states in = 1 chains with finite
Haldane gap. The GS of = 3/2 or 2 junctions of up to 500 spins is a spin
density wave (SDW) with increased amplitude at the ends of arms or near the
junction. Quantum fluctuations completely suppress AF order in = 1/2 or 1
junctions, as well as in half-filled Hubbard junctions, but reduce rather than
suppress AF order in = 3/2 or 2 junctions.Comment: 11 pages, 11 Figures and submitted to PR
Accounting for both electron--lattice and electron--electron coupling in conjugated polymers: minimum total energy calculations on the Hubbard--Peierls hamiltonian
Minimum total energy calculations, which account for both electron--lattice
and electron--electron interactions in conjugated polymers are performed for
chains with up to eight carbon atoms. These calculations are motivated in part
by recent experimental results on the spectroscopy of polyenes and conjugated
polymers and shed light on the longstanding question of the relative importance
of electron--lattice vs. electron--electron interactions in determining the
properties of these systems.Comment: 6 pages, Plain TeX, FRL-PSD-93GR
Electron-electron interaction effects on the photophysics of metallic single-walled carbon nanotubes
Single-walled carbon nanotubes are strongly correlated systems with large
Coulomb repulsion between two electrons occupying the same orbital.
Within a molecular Hamiltonian appropriate for correlated -electron
systems, we show that optical excitations polarized parallel to the nanotube
axes in the so-called metallic single-walled carbon nanotubes are to excitons.
Our calculated absolute exciton energies in twelve different metallic
single-walled carbon nanotubes, with diameters in the range 0.8 - 1.4 nm, are
in nearly quantitative agreement with experimental results. We have also
calculated the absorption spectrum for the (21,21) single-walled carbon
nanotube in the E region. Our calculated spectrum gives an excellent fit
to the experimental absorption spectrum. In all cases our calculated exciton
binding energies are only slightly smaller than those of semiconducting
nanotubes with comparable diameters, in contradiction to results obtained
within the {\it ab initio} approach, which predicts much smaller binding
energies. We ascribe this difference to the difficulty of determining the
behavior of systems with strong on-site Coulomb interactions within theories
based on the density functional approach. As in the semiconducting nanotubes we
predict in the metallic nanotubes a two-photon exciton above the lowest
longitudinally polarized exciton that can be detected by ultrafast pump-probe
spectroscopy. We also predict a subgap absorption polarized perpendicular to
the nanotube axes below the lowest longitudinal exciton, blueshifted from the
exact midgap by electron-electron interactions
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