4,762 research outputs found
Resonating valence bond wave function for the two dimensional fractional spin liquid
The unconventional low-lying spin excitations, recently observed in neutron
scattering experiments on , are explained with a spin
liquid wave function. The dispersion relation as well as the wave vector of the
incommensurate spin correlations are well reproduced within a projected BCS
wave function with gapless and fractionalized spin-1/2 excitations around the
nodes of the BCS gap function. The proposed wave function is shown to be very
accurate for one-dimensional spin-1/2 systems, and remains similarly accurate
in the two-dimensional model corresponding to , thus
representing a good ansatz for describing spin fractionalization in two
dimensions.Comment: 5 pages, 4 figures. To appear in Phys. Rev. Let
Vertex routing models
A class of models describing the flow of information within networks via
routing processes is proposed and investigated, concentrating on the effects of
memory traces on the global properties. The long-term flow of information is
governed by cyclic attractors, allowing to define a measure for the information
centrality of a vertex given by the number of attractors passing through this
vertex. We find the number of vertices having a non-zero information centrality
to be extensive/sub-extensive for models with/without a memory trace in the
thermodynamic limit. We evaluate the distribution of the number of cycles, of
the cycle length and of the maximal basins of attraction, finding a complete
scaling collapse in the thermodynamic limit for the latter. Possible
implications of our results on the information flow in social networks are
discussed.Comment: 12 pages, 6 figure
Intrinsic adaptation in autonomous recurrent neural networks
A massively recurrent neural network responds on one side to input stimuli
and is autonomously active, on the other side, in the absence of sensory
inputs. Stimuli and information processing depends crucially on the qualia of
the autonomous-state dynamics of the ongoing neural activity. This default
neural activity may be dynamically structured in time and space, showing
regular, synchronized, bursting or chaotic activity patterns.
We study the influence of non-synaptic plasticity on the default dynamical
state of recurrent neural networks. The non-synaptic adaption considered acts
on intrinsic neural parameters, such as the threshold and the gain, and is
driven by the optimization of the information entropy. We observe, in the
presence of the intrinsic adaptation processes, three distinct and globally
attracting dynamical regimes, a regular synchronized, an overall chaotic and an
intermittent bursting regime. The intermittent bursting regime is characterized
by intervals of regular flows, which are quite insensitive to external stimuli,
interseeded by chaotic bursts which respond sensitively to input signals. We
discuss these finding in the context of self-organized information processing
and critical brain dynamics.Comment: 24 pages, 8 figure
Real time localization of Gamma Ray Bursts with INTEGRAL
The INTEGRAL satellite has been successfully launched in October 2002 and has
recently started its operational phase. The INTEGRAL Burst Alert System (IBAS)
will distribute in real time the coordinates of the GRBs detected with
INTEGRAL. After a brief introduction on the INTEGRAL instruments, we describe
the main IBAS characteristics and report on the initial results. During the
initial performance and verification phase of the INTEGRAL mission, which
lasted about two months, two GRBs have been localized with accuracy of about
2-4 arcmin. These observations have allowed us to validate the IBAS software,
which is now expected to provide quick (few seconds delay) and precise (few
arcmin) localization for about 10-15 GRBs per year.Comment: 6 pages, latex, 3 figures, submitted to Adv. Sp. Res., Proceedings of
the 34th COSPAR Scientific Assembly, Houston, 10-19 October 200
Breakdown of the Luttinger sum-rule at the Mott-Hubbard transition in the one-dimensional t1-t2 Hubbard model
We investigate the momentum distribution function near the Mott-Hubbard
transition in the one-dimensional t1-t2 Hubbard model (the zig-zag Hubbard
chain), with the density-matrix renormalization-group technique. We show that
for strong interactions the Mott-Hubbard transition occurs between the
metallic-phase and an insulating dimerized phase with incommensurate spin
excitations, suggesting a decoupling of magnetic and charge excitations not
present in weak coupling. We illustrate the signatures for the Mott-Hubbard
transition and the commensurate-incommensurate transition in the insulating
spin-gapped state in their respective ground-state momentum distribution
functions
The de Rham-Witt and Z_p-cohomologies of an algebraic variety
We show how the Z_p(r)-cohomologies of a smooth projective algebraic variety
can be obtained via its de Rham-Witt complex.Comment: 6 page
Quantum antiferromagnetism and high superconductivity: a close connection between the t-J model and the projected BCS Hamiltonian
A connection between quantum antiferromagnetism and high
superconductivity is theoretically investigated by analyzing the t-J model and
its relationships to the Gutzwiller-projected BCS Hamiltonian. After numerical
corroboration via exact diagonalization, it is analytically shown that the
ground state of the t-J model at half filling (i.e., the 2D antiferromagnetic
Heisenberg model) is entirely equivalent to the ground state of the
Gutzwiller-projected BCS Hamiltonian with strong pairing. Combined with the
high wavefunction overlap between the ground states of the t-J model and the
projected BCS Hamiltonian at moderate doping, this equivalence provides strong
support for the existence of superconductivity in the t-J model. The
relationship between the ground state of the projected BCS Hamiltonian and
Anderson's resonating valence bond state (i.e., the projected BCS ground state)
is discussed.Comment: 18 pages, 9 figures, the final version published in Phys. Rev.
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