22 research outputs found
Thermodynamic properties and thermal correlation lengths of a Hubbard model with bond-charge interaction
We investigate the thermodynamics of a one-dimensional Hubbard model with
bond-charge interaction X using the transfer matrix renormalization group
method (TMRG). Numerical results for various quantities like spin and charge
susceptibilities, particle densities, specific heat and thermal correlation
lengths are presented and discussed. We compare our data also to results for
the exactly solvable case X/t=1 as well as to bosonisation results for weak
coupling X/t << 1, which shows excellent agreement. We confirm the existence of
a Tomonaga-Luttinger and a Luther-Emery liquid phase, in agreement with
previous studies at zero temperature. Thermal singlet-pair correlation lengths
are shown to dominate density and spin correlations for finite temperatures in
certain parameter regimes.Comment: 13 pages, revte
Concurrence and entanglement entropy in a dimerized spin-1/2 two-leg ladder
We consider the isotropic spin-1/2 two-leg ladders with the dominant
spatially modulated rung exchanges.
We study the effect of a uniform magnetic field on the ground state phase
diagram of the model using the perturbation theory and the numerical Lanczos
method. The ground state phase diagram consists of two gapless Luttinger liquid
(LL) and three gapped phases. Numerically, we calculate the concurrence between
two spins and entanglement entropy between legs. Numerical experiment shows
that in principle the gapless LL phases are different. In the first LL phase,
only spins on rungs are entangled, but in the second LL phase the spins on legs
are long-distance entangled. Therefore the concurrence between spins on legs
can be considered as a function to distinguish the LL phases.Comment: 7 pages, 7 figure
Finite-temperature properties of the Hubbard chain with bond-charge interaction
We investigate the one-dimensional Hubbard model with an additional
bond-charge interaction, recently considered in the description of compounds
that exhibit strong 1D features above the temperature of ordered phases. The
partition function of the model is exactly calculated for a value of the
bond-charge coupling; the behavior of the specific heat and spin susceptibility
as a function of temperature is derived at arbitrary filling, and particularly
discussed across the occurring metal-insulator transition. The results show
that the bond-charge terms weaken the spin excitations of the system.Comment: 5 pages, 3 eps figure
Ground-state phase diagram of the one-dimensional half-filled extended Hubbard model
We revisit the ground-state phase diagram of the one-dimensional half-filled
extended Hubbard model with on-site (U) and nearest-neighbor (V) repulsive
interactions. In the first half of the paper, using the weak-coupling
renormalization-group approach (g-ology) including second-order corrections to
the coupling constants, we show that bond-charge-density-wave (BCDW) phase
exists for U \approx 2V in between charge-density-wave (CDW) and
spin-density-wave (SDW) phases. We find that the umklapp scattering of
parallel-spin electrons disfavors the BCDW state and leads to a bicritical
point where the CDW-BCDW and SDW-BCDW continuous-transition lines merge into
the CDW-SDW first-order transition line. In the second half of the paper, we
investigate the phase diagram of the extended Hubbard model with either
additional staggered site potential \Delta or bond alternation \delta. Although
the alternating site potential \Delta strongly favors the CDW state (that is, a
band insulator), the BCDW state is not destroyed completely and occupies a
finite region in the phase diagram. Our result is a natural generalization of
the work by Fabrizio, Gogolin, and Nersesyan [Phys. Rev. Lett. 83, 2014
(1999)], who predicted the existence of a spontaneously dimerized insulating
state between a band insulator and a Mott insulator in the phase diagram of the
ionic Hubbard model. The bond alternation \delta destroys the SDW state and
changes it into the BCDW state (or Peierls insulating state). As a result the
phase diagram of the model with \delta contains only a single critical line
separating the Peierls insulator phase and the CDW phase. The addition of
\Delta or \delta changes the universality class of the CDW-BCDW transition from
the Gaussian transition into the Ising transition.Comment: 24 pages, 20 figures, published versio
Meissner effect in a bosonic ladder
We investigate the effect of a magnetic field on a bosonic ladder. We show
that such a system leads to the one dimensional equivalent of a vortex lattice
in a superconductor. We investigate the physical properties of the vortex
phase, such as vortex density and vortex correlation functions and show that
magnetization has plateaus for some commensurate values of the mag netic field.
The lowest plateau corresponds to a true Meissner to vortex transition at a
critical field that exists although the system has no long range
superconducting order. Implications for experimental realizations such as
Josephson junction arrays are discussed.Comment: 4 pages, 2 Encapsulated Postscript figures, RevTe
Pairing and Density Correlations of Stripe Electrons in a Two-Dimensional Antiferromagnet
We study a one-dimensional electron liquid embedded in a 2D antiferromagnetic
insulator, and coupled to it via a weak antiferromagnetic spin exchange
interaction. We argue that this model may qualitatively capture the physics of
a single charge stripe in the cuprates on length- and time scales shorter than
those set by its fluctuation dynamics. Using a local mean-field approach we
identify the low-energy effective theory that describes the electronic spin
sector of the stripe as that of a sine-Gordon model. We determine its phases
via a perturbative renormalization group analysis. For realistic values of the
model parameters we obtain a phase characterized by enhanced spin density and
composite charge density wave correlations, coexisting with subleading triplet
and composite singlet pairing correlations. This result is shown to be
independent of the spatial orientation of the stripe on the square lattice.
Slow transverse fluctuations of the stripes tend to suppress the density
correlations, thus promoting the pairing instabilities. The largest amplitudes
for the composite instabilities appear when the stripe forms an antiphase
domain wall in the antiferromagnet. For twisted spin alignments the amplitudes
decrease and leave room for a new type of composite pairing correlation,
breaking parity but preserving time reversal symmetry.Comment: Revtex, 28 pages incl. 5 figure
Phase diagram of the Hubbard chain with two atoms per cell
We obtain the quantum phase diagram of the Hubbard chain with alternating
on-site energy at half filling. The model is relevant for the ferroelectric
perovskites and organic mixed-stack donor-acceptor crystals. For any values of
the parameters, the band insulator is separated from the Mott insulator by a
dimer phase. The boundaries are determined accurately by crossing of excited
levels with particular discrete symmetries. We show that these crossings
coincide with jumps of charge and spin Berry phases with a clear geometrical
meaning.Comment: 5 pages including 2 figures To be published in Phys. Rev. B (Rapid
Communications
Charge dynamics in the Mott insulating phase of the ionic Hubbard model
We extend to charge and bond operators the transformation that maps the ionic
Hubbard model at half filling onto an effective spin Hamiltonian. Using these
operators we calculate the amplitude of the charge density wave in different
dimensions. In one dimension, the charge-charge correlations at large distance
d decay as 1/(d^3 ln^{3/2}d), in spite of the presence of a charge gap, as a
consequence of remaining charge-spin coupling. Bond-bond correlations decay as
(-1)^d 1/(d ln^{3/2}d) as in the usual Hubbard model.Comment: 4 pages, no figures, submitted to Phys. Rev. B printing errors
corrected and some clarifications adde
Elementary Excitations in Dimerized and Frustrated Heisenberg Chains
We present a detailed numerical analysis of the low energy excitation
spectrum of a frustrated and dimerized spin Heisenberg chain. In
particular, we show that in the commensurate spin--Peierls phase the ratio of
the singlet and triplet excitation gap is a universal function which depends on
the frustration parameter only. We identify the conditions for which a second
elementary triplet branch in the excitation spectrum splits from the continuum.
We compare our results with predictions from the continuum limit field theory .
We discuss the relevance of our data in connection with recent experiments on
, , and .Comment: Corrections to the text + 1 new figure, will appear in PRB (august
98
Strongly correlated hopping and many-body bound states
We study a system in which the quantum dynamics of electrons depend on the
particle density in their neighborhood. For any on-site repulsive interaction,
we show that the exact two-body and three-body ground states are bound states.
We also discuss the finite density case in a mean-field framework and we show
that the system can undergo an unusual transition from an effective attractive
interaction to a repulsive one, when varying the electron density.Comment: 6 pages, 6 EPS figures, minor modifications and references adde