73 research outputs found
Neural network ensembles: Evaluation of aggregation algorithms
Ensembles of artificial neural networks show improved generalization
capabilities that outperform those of single networks. However, for aggregation
to be effective, the individual networks must be as accurate and diverse as
possible. An important problem is, then, how to tune the aggregate members in
order to have an optimal compromise between these two conflicting conditions.
We present here an extensive evaluation of several algorithms for ensemble
construction, including new proposals and comparing them with standard methods
in the literature. We also discuss a potential problem with sequential
aggregation algorithms: the non-frequent but damaging selection through their
heuristics of particularly bad ensemble members. We introduce modified
algorithms that cope with this problem by allowing individual weighting of
aggregate members. Our algorithms and their weighted modifications are
favorably tested against other methods in the literature, producing a sensible
improvement in performance on most of the standard statistical databases used
as benchmarks.Comment: 35 pages, 2 figures, In press AI Journa
Superconductivity and incommensurate spin fluctuations in a generalized t-J model for the cuprates
We consider the slave-fermion Schwinger-boson decomposition of an effective
model obtained through a systematic low-energy reduction of the three-band
Hubbard Hamiltonian. The model includes a three-site term t'' similar to that
obtained in the large-U limit of the Hubbard model but of opposite sign for
realistic or large O-O hopping. For parameters close to the most realistic ones
for the cuprates, the mean-field solution exhibits d+s superconductivity
(predominantly d_{x^2-y^2}) with a dependence on doping x very similar to the
experimentally observed. We also obtained incommensurate peaks at wave vectors
near in the spin structure factor, which also agree with
experiment.Comment: 9 pages, latex, 2 figures, to appear in Europhys. Let
Schwinger-boson approach to quantum spin systems: Gaussian fluctuactions in the "natural" gauge
We compute the Gaussian-fluctuation corrections to the saddle-point
Schwinger-boson results using collective coordinate methods. Concrete
application to investigate the frustrated J1-J2 antiferromagnet on the square
lattice shows that, unlike the saddle-point predictions, there is a quantum
nonmagnetic phase for 0.53 < J2/J1 < 0.64. This result is obtained by
considering the corrections to the spin stiffness on large lattices and
extrapolating to the thermodynamic limit, which avoids the infinite-lattice
infrared divergencies associated to Bose condensation. The very good agreement
of our results with exact numerical values on finite clusters lends support to
the calculational scheme employed.Comment: 4 pages, Latex, 3 figures included as eps files,minor correction
Stability of homogeneous magnetic phases in a generalized t-J model
We study the stability of homogeneous magnetic phases in a generalized t-J
model including a same-sublattice hopping t' and nearest-neighbor repulsion V
by means of the slave fermion-Schwinger boson representation of spin operators.
At mean-field order we find, in agreement with other authors, that the
inclusion of further-neighbor hopping and Coulomb repulsion makes the
compressibility positive, thereby stabilizing at this level the spiral and Neel
orders against phase separation. However, the consideration of Gaussian
fluctuation of order parameters around these mean-field solutions produces
unstable modes in the dynamical matrix for all relevant parameter values,
leaving only reduced stability regions for the Neel phase. We have computed the
one-loop corrections to the energy in these regions, and have also briefly
considered the effects of the correlated hopping term that is obtained in the
reduction from the Hubbard to the t-J model.Comment: 5 pages, 5 figures, Revte
The Heisenberg model on the 1/5-depleted square lattice and the CaV4O9 compound
We investigate the ground state structure of the Heisenberg model on the
1/5-depleted square lattice for arbitrary values of the first- and
second-neighbor exchange couplings. By using a mean-field Schwinger-boson
approach we present a unified description of the rich ground-state diagram,
which include the plaquette and dimer resonant-valence-bond phases, an
incommensurate phase and other magnetic orders with complex magnetic unit
cells. We also discuss some implications of ours results for the experimental
realization of this model in the CaV4O9 compound.Comment: 4 pages, Latex, 7 figures included as eps file
Rotational invariance and order-parameter stiffness in frustrated quantum spin systems
We compute, within the Schwinger-boson scheme, the Gaussian-fluctuation
corrections to the order-parameter stiffness of two frustrated quantum spin
systems: the triangular-lattice Heisenberg antiferromagnet and the J1-J2 model
on the square lattice. For the triangular-lattice Heisenberg antiferromagnet we
found that the corrections weaken the stiffness, but the ground state of the
system remains ordered in the classical 120 spiral pattern. In the case of the
J1-J2 model, with increasing frustration the stiffness is reduced until it
vanishes, leaving a small window 0.53 < J2/J1 < 0.64 where the system has no
long-range magnetic order. In addition, we discuss several methodological
questions related to the Schwinger-boson approach. In particular, we show that
the consideration of finite clusters which require twisted boundary conditions
to fit the infinite-lattice magnetic order avoids the use of ad hoc factors to
correct the Schwinger-boson predictions.Comment: 9 pages, Latex, 6 figures as ps files, fig.1 changed and minor text
corrections, to appear in Phys.Rev.
Nuclear Magnetic Relaxation in the Ferrimagnetic Chain Compound NiCu(C_7_H_6_N_2_O_6_)(H_2_O)_3_2H_2_O: Three-Magnon Scattering?
Recent proton spin-lattice relaxation-time (T_1_) measurements on the
ferrimagnetic chain compound NiCu(C_7_H_6_N_2_O_6_)(H_2_O)_3_2H_2_O are
explained by an elaborately modified spin-wave theory. We give a strong
evidence of the major contribution to 1/T_1_ being made by the three-magnon
scattering rather than the Raman one.Comment: J. Phys.: Condens. Matter 16, No. 49, 9023 (2004
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