565 research outputs found
High Order Perturbation Theory for Spectral Densities of Multi-Particle Excitations: S=1/2 Two-Leg Heisenberg Ladder
We present a high order perturbation approach to quantitatively calculate
spectral densities in three distinct steps starting from the model Hamiltonian
and the observables of interest. The approach is based on the perturbative
continuous unitary transformation introduced previously. It is conceived to
work particularly well in models allowing a clear identification of the
elementary excitations above the ground state. These are then viewed as
quasi-particles above the vacuum. The article focuses on the technical aspects
and includes a discussion of series extrapolation schemes. The strength of the
method is demonstrated for S=1/2 two-leg Heisenberg ladders, for which results
are presented.Comment: 21 pages, 14 figures included; to appear in Eur. Phys. J. B All
technical details for the computation of spectral densities by perturbative
CUTs Minor misprints removed, references update
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Spectral Properties of Magnetic Excitations in Cuprate Two-Leg Ladder Systems
This article summarizes and extends the recent developments in the
microscopic modeling of the magnetic excitations in cuprate two-leg ladder
systems. The microscopic Hamiltonian comprises dominant Heisenberg exchange
terms plus an additional four-spin interaction which is about five times
smaller. We give an overview over the relevant energies like the one-triplon
dispersion, the energies of two-triplon bound states and the positions of
multi-triplon continua and over relevant spectral properties like spectral
weights and spectral densities in the parameter regime appropriate for cuprate
systems. It is concluded that an almost complete understanding of the magnetic
excitations in undoped cuprate ladders has been obtained as measured by
inelastic neutron scattering, inelastic light (Raman) scattering and infrared
absorption.Comment: 26 pages, 10 figures, review for Mod. Phys. Lett.
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Despite the substantial structural change in agriculture there is interest to run an organic farm by highly qualified people, who do not come from a farm. The process of starting a farm is complex and requires a high intensity of consulting and coaching. The main topics are: How to find a suitable object, conception/planning of the farm, financing, age safety, special legal questions of taxes and inheritance
Multi-particle excitations and spectral densities in quantum spin-systems
The excitation spectrum of the 2-leg S=1/2 Heisenberg ladder is examined
perturbatively. Using an optimally chosen continuous unitary transformation we
expand the Hamiltonian and the Raman operator about the limit of isolated rungs
leading to high order series expansions allowing to calculate spectral
densities quantitatively. The 2-particle sector is examined for total momentum
k=0. We show that triplet-triplet interaction gives rise to a band splitting.Comment: 2 pages, 1 figure; submitted to the proceedings of the SCES2001
conference (Physica B
Excitations in one-dimensional S=1/2 quantum antiferromagnets
The transition from dimerized to uniform phases is studied in terms of
spectral weights for spin chains using continuous unitary transformations
(CUTs). The spectral weights in the S=1 channel are computed perturbatively
around the limit of strong dimerization. We find that the spectral weight is
concentrated mainly in the subspaces with a small number of elementary triplets
(triplons), even for vanishing dimerization. So, besides spinons, triplons may
be used as elementary excitations in spin chains. We conclude that there is no
necessity to use fractional excitations in low-dimensional, undoped or doped
quantum antiferromagnets.Comment: 4 pages, 1 figure include
Continuous similarity transformation for critical phenomena: easy-axis antiferromagnetic XXZ model
We apply continuous similarity transformations (CSTs) to the easy-axis
antiferromagnetic XXZ-model on the square lattice. The CST flow equations are
truncated in momentum space by the scaling dimension so that all
contributions with are taken into account. The resulting quartic
magnon-conserving effective Hamiltonian is analyzed in the zero-, one-, and
two-magnon sector. In this way, a quantitative description of the ground-state
energy, the one-magnon dispersion and its gap as well as of two-magnon bound
states is gained for anisotropies ranging from the gapped Ising model to the
gapless Heisenberg model. We discuss the critical properties of the gap closing
as well as the evolution of the one-magnon roton mininum. The excitation
energies of two-magnon bound states are calculated and their decay into the
two-magnon continuum is determined via the inverse participation ratio.Comment: 12 pages, 8 figure
High and low states of the system AM Herculis
Context: We investigate the distribution of optically high and low states of
the system AM Herculis (AM Her).
Aims: We determine the state duty cycles, and their relationships with the
mass transfer process and binary orbital evolution of the system.
Methods: We make use of the photographic plate archive of the Harvard College
Observatory between 1890 and 1953 and visual observations collected by the
American Association of Variable Star Observers between 1978 and 2005. We
determine the statistical probability of the two states, their distribution and
recurrence behaviors.
Results: We find that the fractional high state duty cycle of the system AM
Her is 63%. The data show no preference of timescales on which high or low
states occur. However, there appears to be a pattern of long and short duty
cycle alternation, suggesting that the state transitions retain memories. We
assess models for the high/low states for polars (AM Her type systems). We
propose that the white-dwarf magnetic field plays a key role in regulating the
mass transfer rate and hence the high/low brightness states, due to variations
in the magnetic-field configuration in the system.Comment: 8 pages, 5 figures, accepted for publication in A&
One-dimensional fermionic systems after interaction quenches and their description by bosonic field theories
We show that the dynamics of quenches in one dimension far off equilibrium
can be described by power laws, but with exponents differing from the fully
renormalized ones at lowest energies. Instead they depend on the initial state
and its excitation energy. Furthermore, we found that for quenches to strong
interactions unexpected similarities between systems in one and in infinite
dimensions occur, indicating the dominance of local processes.Comment: This is a distinctly revised version which is focussed on the
description of the dynamics by bosonization technique
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