2,579 research outputs found
Renormalization by Continuous Unitary Transformations: One-Dimensional Spinless Fermions
A renormalization scheme for interacting fermionic systems is presented where
the renormalization is carried out in terms of the fermionic degrees of
freedom. The scheme is based on continuous unitary transformations of the
hamiltonian which stays hermitian throughout the renormalization flow, whereby
any frequency dependence is avoided. The approach is illustrated in detail for
a model of spinless fermions with nearest neighbour repulsion in one dimension.
Even though the fermionic degrees of freedom do not provide an easy starting
point in one dimension very good results are obtained which agree well with the
exact findings based on Bethe ansatz.Comment: 17 pages, figures included some revisions concerning the comparison
to other methods, references update
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
Keeping a Quantum Bit Alive by Optimized -Pulse Sequences
A general strategy to maintain the coherence of a quantum bit is proposed.
The analytical result is derived rigorously including all memory and
back-action effects. It is based on an optimized -pulse sequence for
dynamic decoupling extending the Carr-Purcell-Meiboom-Gill (CPMG) cycle. The
optimized sequence is very efficient, in particular for strong couplings to the
environment.Comment: 4 pages, 2 figures; revised version with additional references for
better context, more stringent discussio
On the Divergence Phenomenon in HermiteâFejĂ©r Interpolation
AbstractGeneralizing results of L. Brutman and I. Gopengauz (1999, Constr. Approx.15, 611â617), we show that for any nonconstant entire function f and any interpolation scheme on [â1, 1], the associated HermiteâFejĂ©r interpolating polynomials diverge on any infinite subset of C\[â1, 1]. Moreover, it turns out that even for the locally uniform convergence on the open interval ]â1, 1[ it is necessary that the interpolation scheme converges to the arcsine distribution
Constraints on Lorentz Invariance Violation using INTEGRAL/IBIS observations of GRB041219A
One of the experimental tests of Lorentz invariance violation is to measure
the helicity dependence of the propagation velocity of photons originating in
distant cosmological obejcts. Using a recent determination of the distance of
the Gamma-Ray Burst GRB 041219A, for which a high degree of polarization is
observed in the prompt emission, we are able to improve by 4 orders of
magnitude the existing constraint on Lorentz invariance violation, arising from
the phenomenon of vacuum birefringence.Comment: 5 pages, 3 figures, accepted for publication as a Rapid Communication
in Physical Review
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.
Metabolites that confirm induction and release of dormancy phases in sweet cherry buds
Here we report on metabolites found in a targeted profiling of âSummitâ flower buds for nine years, which could be indicators for the timing of endodormancy release (t1) and beginning of ontogenetic development (t1*). Investigated metabolites included chrysin, arabonic acid, pentose acid, sucrose, abscisic acid (ABA), and abscisic acid glucose ester (ABA-GE). Chrysin and water content showed an almost parallel course between leaf fall and t1*. After âswollen budâ, water content raised from ~60 to ~80% at open cluster, while chrysin content decreased and lost its function as an acetylcholinesterase inhibitor. Both parameters can be suitable indicators for t1*. Arabonic acid showed a clear increase after t1*. Pentose acid would be a suitable metabolite to identify t1 and t1*, but would not allow describing the ecodormancy phase, because of its continuously low value during this time. Sucrose reached a maximum during ecodormancy and showed a significant correlation with air temperature, which confirms its cryoprotective role in this phase. The ABA content showed maximum values during endodormancy and decreased during ecodormancy, reaching 50% of its content t1 at t1*. It appears to be the key metabolite to define the ecodormancy phase. The ABA-GE was present at all stages and phases and was much higher than the ABA content and is a readily available storage pool in cherry buds.Peer Reviewe
Metabolites in Cherry Buds to Detect Winter Dormancy
Winter dormancy is still a âblack boxâ in phenological models, because it evades simple observation. This study presents the first step in the identification of suitable metabolites which could indicate the timing and length of dormancy phases for the sweet cherry cultivar âSummitâ. Global metabolite profiling detected 445 named metabolites in flower buds, which can be assigned to different substance groups such as amino acids, carbohydrates, phytohormones, lipids, nucleotides, peptides and some secondary metabolites. During the phases of endo- and ecodormancy, the energy metabolism in the form of glycolysis and the tricarboxylic acid (TCA) cycle was shut down to a minimum. However, the beginning of ontogenetic development was closely related to the up-regulation of the carbohydrate metabolism and thus to the generation of energy for the growth and development of the sweet cherry buds. From the 445 metabolites found in cherry buds, seven were selected which could be suitable markers for the ecodormancy phase, whose duration is limited by the date of endodormancy release (t1) and the beginning of ontogenetic development (t1*). With the exception of abscisic acid (ABA), which has been proven to control bud dormancy, all of these metabolites show nearly constant intensity during this phase.Peer Reviewe
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