425 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
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
Tuning the bond order wave (BOW) phase of half-filled extended Hubbard models
Theoretical and computational studies of the quantum phase diagram of the
one-dimensional half-filled extended Hubbard model (EHM) indicate a narrow bond
order wave (BOW) phase with finite magnetic gap for on-site repulsion , the critical point, and nearest neighbor interaction
near the boundary of the charge density wave (CDW) phase. Potentials with more
extended interactions that retain the EHM symmetry are shown to have a less
cooperative CDW transition with higher and wider BOW phase. Density
matrix renormalization group (DMRG) is used to obtain directly as the
singlet-triplet gap, with finite marking the BOW boundary . The
BOW/CDW boundary is obtained from exact finite-size calculations that
are consistent with previous EHM determinations. The kinetic energy or bond
order provides a convenient new estimate of based on a metallic point at
for . Tuning the BOW phase of half-filled Hubbard models with
different intersite potentials indicates a ground state with large charge
fluctuations and magnetic frustration. The possibility of physical realizations
of a BOW phase is raised for Coulomb interactions.Comment: 9 pages, 7 figure
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
Charge fluctuations and electron-phonon coupling in organic charge-transfer salts with neutral-ionic and Peierls transitions
The first-order transition of the charge-transfer complex TTF-CA
(tetrathiafulvalene-chloranil) is both a neutral-ionic and a Peierls
transition. In related organic charge transfer complexes, cooling at ambient
pressure increases the ionicity in strikingly different ways, and is
sometimes accompanied by a dielectric peak, that we relate to lattice
stiffness, to structural and energetic disorder, and to the softening of the
Peierls mode in the far-IR. The position operator for systems with periodic
boundary conditions makes possible a systematic treatment of electron-phonon
interactions in extended donor-acceptor stacks in terms of correlated
Peierls-Hubbard models. The IR intensity of the Peierls mode peaks at the
Peierls transition at small in soft lattices, where the dielectric
constant also has a large peak. In dimerized stacks, the IR intensity of
totally symmetric, Raman active, molecular vibrations is related to charge
fluctuations that modulate site energies. Combination bands of molecular and
Peierls modes are identified in regular TTF-CA stacks above Tc. Energetic
disorder can suppress the Peierls transition and rationalize a continuous
crossover from small to large . The TTF-CA scenario of a neutral-regular
to ionic-dimerized transition must be broadened considerably in view of charge
transfer salts that dimerize on the neutral side, that become ionic without a
structural change, or that show vibrational evidence for dimerization at
constant .Comment: 26 pages including figure
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
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