307 research outputs found
Series Expansions for Excited States of Quantum Lattice Models
We show that by means of connected-graph expansions one can effectively
generate exact high-order series expansions which are informative of low-lying
excited states for quantum many-body systems defined on a lattice. In
particular, the Fourier series coefficients of elementary excitation spectra
are directly obtained. The numerical calculations involved are straightforward
extensions of those which have already been used to calculate series expansions
for ground-state correlations and susceptibilities in a wide variety of
models. As a test, we have reproduced the known elementary excitation spectrum
of the transverse-field Ising chain in its disordered phase.Comment: 9 pages, no figures, Revtex 3.0 The revised version corrects the
incorrect (and unnecessary) statement in the original that H and H^eff are
related by a unitary transformation; in fact they are related by via a
similarity transformation. This has no implications for the calculations of
spectra, but is important for matrix element
Spin-wave excitation spectra and spectral weights in square lattice antiferromagnets
Using a recently developed method for calculating series expansions of the
excitation spectra of quantum lattice models, we obtain the spin-wave spectra
for square lattice, Heisenberg-Ising antiferromagnets. The calculated
spin-wave spectrum for the Heisenberg model is close to but noticeably
different from a uniformly renormalized classical (large-) spectrum with the
renormalization for the spin-wave velocity of approximately . The
relative weights of the single-magnon and multi-magnon contributions to neutron
scattering spectra are obtained for wavevectors throughout the Brillouin zone.Comment: Two postscript figures, 4 two-column page
Spin-S bilayer Heisenberg models: Mean-field arguments and numerical calculations
Spin-S bilayer Heisenberg models (nearest-neighbor square lattice
antiferromagnets in each layer, with antiferromagnetic interlayer couplings)
are treated using dimer mean-field theory for general S and high-order
expansions about the dimer limit for S=1, 3/2,...,4. We suggest that the
transition between the dimer phase at weak intraplane coupling and the Neel
phase at strong intraplane coupling is continuous for all S, contrary to a
recent suggestion based on Schwinger boson mean-field theory. We also present
results for S=1 layers based on expansions about the Ising limit: In every
respect the S=1 bilayers appear to behave like S=1/2 bilayers, further
supporting our picture for the nature of the order-disorder phase transition.Comment: 6 pages, Revtex 3.0, 8 figures (not embedded in text
Simple Vortex States in Films of Type-I Ginzburg-Landau Superconductor
Sufficiently thin films of type-I superconductor in a perpendicular magnetic
field exhibit a triangular vortex lattice, while thick films develop an
intermediate state. To elucidate what happens between these two regimes,
precise numerical calculations have been made within Ginzburg-Landau theory at
and 0.25 for a variety of vortex lattice structures with one flux
quantum per unit cell. The phase diagram in the space of mean induction and
film thickness includes a narrow wedge in which a square lattice is stable,
surrounded by the domain of stability of the triangular lattice at thinner
films/lower fields and, on the other side, rectangular lattices with
continuously varying aspect ratio. The vortex lattice has an anomalously small
shear modulus within and close to the square lattice phase.Comment: 21 pages, 6 figure
Phase Transitions in the Symmetric Kondo Lattice Model in Two and Three Dimensions
We present an application of high-order series expansion in the coupling
constants for the ground state properties of correlated lattice fermion
systems. Expansions have been generated up to order for and
for for certain properties of the symmetric Kondo lattice
model. Analyzing the susceptibility series, we find evidence for a continuous
phase transition from the ``spin liquid'' phase characteristic of a ``Kondo
Insulator'' to an antiferromagnetically ordered phase in dimensions as
the antiferromagnetic Kondo coupling is decreased. The critical point is
estimated to be at for square lattice and
for simple-cubic lattice.Comment: 12 pages, Revtex, replace previous corrupted fil
The role of microtubule movement in bidirectional organelle transport
We study the role of microtubule movement in bidirectional organelle
transport in Drosophila S2 cells and show that EGFP-tagged peroxisomes in cells
serve as sensitive probes of motor induced, noisy cytoskeletal motions.
Multiple peroxisomes move in unison over large time windows and show
correlations with microtubule tip positions, indicating rapid microtubule
fluctuations in the longitudinal direction. We report the first high-resolution
measurement of longitudinal microtubule fluctuations performed by tracing such
pairs of co-moving peroxisomes. The resulting picture shows that
motor-dependent longitudinal microtubule oscillations contribute significantly
to cargo movement along microtubules. Thus, contrary to the conventional view,
organelle transport cannot be described solely in terms of cargo movement along
stationary microtubule tracks, but instead includes a strong contribution from
the movement of the tracks.Comment: 24 pages, 5 figure
Perturbation Theory for Spin Ladders Using Angular-Momentum Coupled Bases
We compute bulk properties of Heisenberg spin-1/2 ladders using
Rayleigh-Schr\"odinger perturbation theory in the rung and plaquette bases. We
formulate a method to extract high-order perturbative coefficients in the bulk
limit from solutions for relatively small finite clusters. For example, a
perturbative calculation for an isotropic ladder yields an
eleventh-order estimate of the ground-state energy per site that is within
0.02% of the density-matrix-renormalization-group (DMRG) value. Moreover, the
method also enables a reliable estimate of the radius of convergence of the
perturbative expansion. We find that for the rung basis the radius of
convergence is , with defining the ratio between
the coupling along the chain relative to the coupling across the chain. In
contrast, for the plaquette basis we estimate a radius of convergence of
. Thus, we conclude that the plaquette basis offers the
only currently available perturbative approach which can provide a reliable
treatment of the physically interesting case of isotropic spin
ladders. We illustrate our methods by computing perturbative coefficients for
the ground-state energy per site, the gap, and the one-magnon dispersion
relation.Comment: 22 pages. 9 figure
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