1,413 research outputs found
The density matrix renormalization group method. Application to the PPP model of a cyclic polyene chain
The density matrix renormalization group (DMRG) method introduced by White
for the study of strongly interacting electron systems is reviewed; the method
is variational and considers a system of localized electrons as the union of
two adjacent fragments A, B. A density matrix rho is introduced, whose
eigenvectors corresponding to the largest eigenvalues are the most significant,
the most probable states of A in the presence of B; these states are retained,
while states corresponding to small eigenvalues of rho are neglected. It is
conjectured that the decreasing behaviour of the eigenvalues is gaussian. The
DMRG method is tested on the Pariser-Parr-Pople Hamiltonian of a cyclic polyene
(CH)_N up to N=34. A Hilbert space of dimension 5 x 10^+18 is explored. The
ground state energy is 10^-3 eV within the full CI value in the case N=18. The
DMRG method compares favourably also with coupled cluster approximations. The
unrestricted Hartree-Fock solution (which presents spin density waves) is
briefly reviewed, and a comparison is made with the DMRG energy values.
Finally, the spin-spin and density-density correlation functions are computed;
the results suggest that the antiferromagnetic order of the exact solution does
not extend up to large distances but exists locally. No charge density waves
are present.Comment: 8 pages, RevTex, 2 figures, to be published in the Journal of
Chemical Physic
Near Infrared properties of 12 Globular Clusters toward the inner Bulge of the Galaxy
We present near-IR Colour-Magnitude diagrams and physical parameters for a
sample of 12 galactic globular clusters located toward the inner Bulge region.
For each cluster we provide measurements of the reddening, distance,
photometric metallicity, luminosity of the horizontal branch red clump, and of
the red giant branch bump and tip. The sample discussed here together with that
presented in Valenti, Ferraro & Origlia (2007) represent the largest
homogeneous catalog of Bulge globular clusters (comprising ~ 80% of the entire
Bulge cluster population) ever studied. The compilation is available in
electronic form on the World Wide Web (http://www.bo.astro.it/~GC/ir_archive)Comment: 2 pages, 13 figures, 4 tables, accepted for publication in MNRA
Italian networking of public experimental sites working on Organic Farming: an experience of networking in research
Research in organic farming, especially with an agro-ecological approach, is strictly linked to local conditions and interactions among several variables. The paper will present an example of a process developed in the Italian context to integrate and harmonize research in organic farming from different research stations, with different local conditions. Collecting similar and comparable data from different research stations participating in the network can have an interesting impact in results of organic farming experiments, due to high number and variability of data. ARSIA Toscana, with the aim of creating an Italian National Network of public experimental stations working in organic farming, involved FIRAB as a facilitator of the participatory process. The proposed process follows two parallel paths: individual questionnaire to single experimental sites to know the specific activities in place on organic farming and a series of meetings with direct involvement and exchange among researchers and policy makers from different regions to comment results of the questionnaire and future development of organic research in public experimental stations. The main outcome of the process has been the choice of a specific transversal topic to build the network: the development of synthetic agro-environmental indicators
Homogeneous and inhomogeneous magnetic phases of constrained dipolar bosons
We study the emergence of several magnetic phases in dipolar bosonic gases
subject to three-body loss mechanism employing numerical simulations based on
the density matrix renormalization group(DMRG) algorithm. After mapping the
original Hamiltonian in spin language, we find a strong parallelism between the
bosonic theory and the spin-1 Heisenberg model with single ion anisotropy and
long-range interactions. A rich phase diagram, including ferromagnetic,
antiferromagnetic and non-local ordered phases, emerges in the half-filled
one-dimensional case, and is preserved even in presence of a trapping
potential.Comment: v2: 9 pages, 15 figures, extended version, new numerical calculations
on the BKT transition, accepted for pubblication in PR
An Accurate, Easy to Use Abundace Scale for Globular Clusters Based on 2.2um Spectra of Giant Stars
We present a new method for the determination of [Fe/H] for globular
clusters. This new method is based on moderate resolution (R~1500) near-IR
spectroscopy in the K-band of 6 to 10 of the brightest giants in a cluster. Our
calibration is derived from spectra of 105 stars in 15 globular clusters. From
measurements of the equivalent widths of three features in these spectra, Na,
Ca, and CO, we are able to reproduce the Zinn & West (1984) abundance scale as
updated by Harris (1996) to better than 0.10 dex for clusters with near solar
[Fe/H] down to an [Fe/H] of -1.8. Three advantages of this method are that it
can be used for metal rich, heavily reddened globulars in crowded fields, it
does not require any knowledge of any other cluster or stellar parameters such
as reddening, distance, or luminosity, and it requires only minimal telescope
time. If stellar (J-K)0 and MK values are available as well, the accuracy of
the [Fe/H] estimate is further improved. Observations of as few as three stars
per cluster still gives an [Fe/H] estimate wich is nearly as reliable as that
based on two to three times as many stars.
The accuracy of an [Fe/H] value based on observations of CO absorption alone
is significantly less than that which results from the three spectroscopic
indices. However, we predict that space-based observations of this feature in
the integrated light of stellar systems will prove to be of great value for
abundance determinations at distances as far as the Coma cluster of galaxies.Comment: 53 pages, 16 Postscript figures. Submitted to the A
Frequency-dependent Thermal Response of the Charge System and Restricted Sum Rules in La(2-x)Sr(x)CuO(4)
By using new and previous measurements of the -plane conductivity
of LaSrCuO (LSCO) it is shown that
the spectral weight
obeys the same law which holds for a conventional
metal like gold, for 's below the plasma frequency. However
, which measures the "thermal response" of the charge system, in
LSCO exhibits a peculiar behavior which points towards correlation effects. In
terms of hopping models, is directly related to an energy scale
, smaller by one order of magnitude than the full bandwidth .Comment: 4 pages with 3 fig
V, J, H and K Imaging of the Metal Rich Globular Cluster NGC 6528
New near-infrared observations of NGC6528 are presented. The JHK observations
complement a previous HST/NICMOS data set by Ortolani et al. (2001), in that
they sample a larger area, contain a more numerous sample of red giant stars,
and include the K band. Also, archival HST data sets (separated by 6.093 years)
were used to proper-motion decontaminate the near-infrared sample and extract a
clean VJHK catalogue. Using the present wide colour baseline, we compared the
cleaned colour-magnitude diagrams of NGC6528 with those of NGC 6553 and NGC104
and derived new estimates of reddening and distance, E(B-V)=0.55 and
(m-M)o=14.44 (7.7 kpc). Moreover, the morphology and location of the cleaned
red giant branch were used to derive a photometric estimate of the cluster
metallicity. The average of 10 metallicity indicators yields a mean value of
[M/H] ~ 0.0, and [Fe/H] ~-0.20 and +0.08 on the Zinn & West (1984) and Carretta
& Gratton (1997) revised metallicity scale, respectively. The best isochrone
fit to the cleaned K,V-K diagram is obtained for a 12.6 Gyr and Z=0.02
isochrone, i.e. the derived metallicity of NGC6528 turns out to be very close
to the mean of stars in the Baade's Window. Five AGB variable star candidates,
whose membership has to be confirmed spectroscopically, are bolometrically as
bright as the known long period variable stars in NGC6553. As discussed in
Guarnieri et al. (1997) for NGC6553, this may indicate that an `intermediate
age' population is not needed to account for the brightest stars in external
galaxies such as M32.Comment: 11 pages, 9 figures, A&A accepte
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