119 research outputs found
Spin dynamics of hole doped Y2BaNiO5
Starting from a multiband Hamiltonian containing the relevant Ni and O
orbitals, we derive an effective Hamiltonian for the low energy
physics of doped YBaNiO For hole doping, describes O
fermions interacting with S=1 Ni spins in a chain, and cannot be further
reduced to a simple one-band model. Using numerical techniques, we obtain a
dynamical spin structure factor with weight inside the Haldane gap. The nature
of these low-energy excitations is identified and the emerging physical picture
is consistent with most of the experimental information in Y%
CaBaNiOComment: 4 pages, 2 figure
Multimorbidity as a predictor of health service utilization in primary care: a registry-based study of the Catalan population
Background: Multimorbidity is highly relevant for both service commissioning and clinical decision-making. Optimization of variables assessing multimorbidity in order to enhance chronic care management is an unmet need. To this end, we have explored the contribution of multimorbidity to predict use of healthcare resources at community level by comparing the predictive power of four different multimorbidity measures. Methods: A population health study including all citizens ≥18 years (n = 6,102,595) living in Catalonia (ES) on 31 December 2014 was done using registry data. Primary care service utilization during 2015 was evaluated through four outcome variables: A) Frequent attendants, B) Home care users, C) Social worker users, and, D) Polypharmacy. Prediction of the four outcome variables (A to D) was carried out with and without multimorbidity assessment. We compared the contributions to model fitting of the following multimorbidity measures: i) Charlson index; ii) Number of chronic diseases; iii) Clinical Risk Groups (CRG); and iv) Adjusted Morbidity Groups (GMA). Results: The discrimination of the models (AUC) increased by including multimorbidity as covariate into the models, namely: A) Frequent attendants (0.771 vs 0.853), B) Home care users (0.862 vs 0.890), C) Social worker users (0.809 vs 0.872), and, D) Polypharmacy (0.835 vs 0.912). GMA showed the highest predictive power for all outcomes except for polypharmacy where it was slightly below than CRG. Conclusions: We confirmed that multimorbidity assessment enhanced prediction of use of healthcare resources at community level. The Catalan population-based risk assessment tool based on GMA presented the best combination of predictive power and applicability
Electron-Doped Manganese Perovskites: The Polaronic State
Using the Lanczos method in linear chains we study the ground state of the
double exchange model including an antiferromagnetic super-exchange in the low
concentration limit. We find that this ground state is always inhomogeneous,
containig ferromagnetic polarons. The extention of the polaron spin distortion,
the dispersion relation and their trapping by impurities, are studied for
diferent values of the super exchange interaction and magnetic field. We also
find repulsive polaron polaron interaction.Comment: 4 pages, 6 embedded figure
Spectral density for a hole in an antiferromagnetic stripe phase
Using variational trial wave function based on the string picture we study
the motion of a single mobile hole in the stripe phase of the doped
antiferromagnet. The holes within the stripes are taken to be static, the
undoped antiferromagnetic domains in between the hole stripes are assumed to
have alternating staggered magnetization, as is suggested by neutron scattering
experiments. The system is described by the t-t'-t''-J model with realistic
parameters and we compute the single particle spectral density.Comment: RevTex-file, 9 PRB pages with 15 .eps and .gif files. To appear in
PRB. Hardcopies of figures (or the entire manuscript) can be obtained by
e-mail request to: [email protected]
Time-related efficacy of liver cell isografts in fulminant hepatic failure
We and others have reported that dispersed liver cells transplanted into the
spleen parenchyma of syngeneic rats remained functional and viable for a long
time. This report describes our results with hepatocellular transplantation as a
therapeutic method in a model of fulminant hepatic failure (FHF) in the rat. 60
male Sprague-Dawley rats weighing 200-250 g were used. The FHF was reached
through an Eck's fistula with 2/3 hepatectomy at the same time. This model
produced lethal hepatic failure in a highly reproducible manner. Liver cells were
isolated by the collagenase method. 40 X 10(6) hepatocytes suspended in Hanks'
balanced salt solution were transplanted into the spleen parenchyma 24 hr before
(group 1), at the same time as (group 2), and 24 hr after (group 3) FHF was
achieved. Additional sham-operated animals (groups 4 and 5) and a control group
(group 6) were used. The hepatocellular transplantation markedly increased the
survival of the animals with induced FHF to 80% (group 1) and 60% (group 2)--but
not in group 3 (20%),--compared with 10% in the control group. This study shows
that dispersed liver cells transplanted into the spleen can provide sufficient
support to allow animals with lethal hepatic failure to survive and recover.
Nevertheless the efficacy of transplantation is a time-related phenomenon with
the FHF induction
Charge and spin ordering, and charge transport properties in a two-dimensional inhomogeneous t-J model
We study a two-dimensional t-J model close to the Ising limit in which charge
inhomogeneity is stabilized by an on-site potential e_s, by using
diagonalization in a restricted Hilbert space and finite temperature Quantum
Monte Carlo. Both site and bond centered stripes are considered and their
similitudes and differences are analyzed. The amplitude of charge inhomogeneity
is studied as e_s -> 0. Moreover, we show that the anti-phase domain ordering
occurs at a much lower temperature than the formation of charge inhomogeneities
and charge localization. Hole-hole correlations indicate a metallic behavior of
the stripes with no signs of hole attraction. Kinetic energies and current
susceptibilities are computed and indications of charge localization are
discussed. The study of the doping dependence in the range 0.083 < x < 0.167
suggests that these features are characteristic of the whole underdoped region.Comment: minor changes, to be published in Physical Review
Ferromagnetic Polarons in Manganites
Using the Lanczos method in linear chains we study the double exchange model
in the low concentration limit, including an antiferromagnetic super-exchange
K. In the strong coupling limit we find that the ground state contains
ferromagnetic polarons whose length is very sensitive to the value of K/t. We
investigate the dispersion relation, the trapping by impurities, and the
interaction between these polarons. As the overlap between polarons increases,
by decreasing K/t, the effective interaction between them changes from
antiferromagnetic to ferromagnetic. The scaling to the thermodynamic limit
suggests an attractive interaction in the strong coupling regime (J_h > t) and
no binding in the weak limit (J_h \simeq t).Comment: 12 pages, accepted in PRB, to be published in Novembe
Charge and spin excitations of insulating lamellar copper oxides
A consistent description of low-energy charge and spin responses of the
insulating Sr_2CuO_2Cl_2 lamellar system is found in the framework of a
one-band Hubbard model which besides includes hoppings up to 3^{rd}
nearest-neighbors. By combining mean-field calculations, exact diagonalization
(ED) results, and Quantum Monte Carlo simulations (QMC), we analyze both charge
and spin degrees of freedom responses as observed by optical conductivity,
ARPES, Raman and inelastic neutron scattering experiments. Within this
effective model, long-range hopping processes flatten the quasiparticle band
around . We calculate also the non-resonant A_{1g} and B_{1g} Raman
profiles and show that the latter is composed by two main features, which are
attributed to 2- and 4-magnon scattering.Comment: 6 pages, 3 figures, To be published in PRB (july
Stripes and holes in a two-dimensional model of spinless fermions and hardcore bosons
We consider a Hubbard-like model of strongly-interacting spinless fermions
and hardcore bosons on a square lattice, such that nearest neighbor occupation
is forbidden. Stripes (lines of holes across the lattice forming antiphase
walls between ordered domains) are a favorable way to dope this system below
half-filling. The problem of a single stripe can be mapped to a spin-1/2 chain,
which allows understanding of its elementary excitations and calculation of the
stripe's effective mass for transverse vibrations. Using Lanczos exact
diagonalization, we investigate the excitation gap and dispersion of a hole on
a stripe, and the interaction of two holes. We also study the interaction of
two, three, and four stripes, finding that they repel, and the interaction
energy decays with stripe separation as if they are hardcore particles moving
in one (transverse) direction. To determine the stability of an array of
stripes against phase separation into particle-rich phase and hole-rich liquid,
we evaluate the liquid's equation of state, finding the stripe-array is not
stable for bosons but is possibly stable for fermions.Comment: 24 pages, 18 figure
Optical properties of an effective one-band Hubbard model for the cuprates
We study the Cu and O spectral density of states and the optical conductivity
of CuO_2 planes using an effective generalized one-band Hubbard model derived
from the extended three-band Hubbard model. We solve exactly a square cluster
of 10 unit cells and average the results over all possible boundary conditions,
what leads to smooth functions of frequency. Upon doping, the Fermi energy
jumps to Zhang-Rice states which are connected to the rest of the valence band
(in contrast to an isolated new band in the middle of the gap). The transfer of
spectral weight depends on the parameters of the original three-band model not
only through the one-band effective parameters but also through the relevant
matrix elements. We discuss the evolution of the gap upon doping. The optical
conductivity of the doped system shows a mid-infrared peak due to intraband
transitions, a pseudogap and a high frequency part related to interband
transitions. Its shape and integrated weight up to a given frequency (including
the Drude weight) agree qualitatively with experiments in the cuprates for low
to moderate doping levels, but significant deviations exist for doping .Comment: 11 pages (tex), 14 figures (ps
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