1,462 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
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
On critical phases in anisotropic spin-1 chains
Quantum spin-1 chains may develop massless phases in presence of Ising-like
and single-ion anisotropies. We have studied c=1 critical phases by means of
both analytical techniques, including a mapping of the lattice Hamiltonian onto
an O(2) nonlinear sigma model, and a multi-target DMRG algorithm which allows
for accurate calculation of excited states. We find excellent quantitative
agreement with the theoretical predictions and conclude that a pure Gaussian
model, without any orbifold construction, describes correctly the low-energy
physics of these critical phases. This combined analysis indicates that the
multicritical point at large single-ion anisotropy does not belong to the same
universality class as the Takhtajan-Babujian Hamiltonian as claimed in the
past. A link between string-order correlation functions and twisting vertex
operators, along the c=1 line that ends at this point, is also suggested.Comment: 9 pages, 3 figures, svjour format, submitted to Eur. Phys. J.
DMRG Simulation of the SU(3) AFM Heisenberg Model
We analyze the antiferromagnetic Heisenberg chain by means of
the Density Matrix Renormalization Group (DMRG). The results confirm that the
model is critical and the computation of its central charge and the scaling
dimensions of the first excited states show that the underlying low energy
conformal field theory is the Wess-Zumino-Novikov-Witten
model.Comment: corrections and improvements adde
Magnetic properties of commensurate Bose-Bose mixtures in one-dimensional optical lattices
We investigate magnetic properties of strongly interacting bosonic mixtures
confined in one dimensional geometries, focusing on recently realized Rb-K
gases with tunable interspecies interactions. By combining analytical
perturbation theory results with density-matrix-renormalization group
calculations, we provide quantitative estimates of the ground state phase
diagram as a function of the relevant microscopic quantities, identifying the
more favorable experimental regimes in order to access the various magnetic
phases. Finally, we qualitatively discuss the observability of such phases in
realistic setups when finite temperature effects have to be considered.Comment: 9 pages, 7 figures, to be published in EPJ ST special issue on "Novel
Quantum Phases and Mesoscopic Physics in Quantum Gases
Phase separation and pairing regimes in the one-dimensional asymmetric Hubbard model
We address some open questions regarding the phase diagram of the
one-dimensional Hubbard model with asymmetric hopping coefficients and balanced
species. In the attractive regime we present a numerical study of the passage
from on-site pairing dominant correlations at small asymmetries to
charge-density waves in the region with markedly different hopping
coefficients. In the repulsive regime we exploit two analytical treatments in
the strong- and weak-coupling regimes in order to locate the onset of phase
separation at small and large asymmetries respectively.Comment: 13 pages, RevTeX 4, 12 eps figures, some additional refs. with
respect to v1 and citation errors fixe
A Near-Infrared Photometric Study of the Low Latitude Globular Clusters Liller 1, Djorgovski 1, HP 1, and NGC 6528
Images recorded through J, H, Ks, 2.2 micron continuum, and 2.3 micron CO
filters are used to investigate the stellar contents of the low Galactic
latitude globular clusters NGC 6528, Liller 1, Djorgovski 1, and HP 1, as well
as surrounding bulge fields. Metallicities are estimated for the latter three
clusters by comparing the colors and CO indices of giant branch stars with
those in other clusters and the bulge, while reddenings are estimated from the
colors of bright bulge stars in the surrounding fields. In some cases the
metallicities and reddenings are significantly different from previous
estimates.Comment: 32 pages, and 28 figure
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