5,877 research outputs found
Active Estimation of Distance in a Robotic Vision System that Replicates Human Eye Movement
Many visual cues, both binocular and monocular, provide 3D information. When an agent moves with respect to a scene, an important cue is the different motion of objects located at various distances. While a motion parallax is evident for large translations of the agent, in most head/eye systems a small parallax occurs also during rotations of the cameras. A similar parallax is present also in the human eye. During a relocation of gaze, the shift in the retinal projection of an object depends not only on the amplitude of the movement, but also on the distance of the object with respect to the observer. This study proposes a method for estimating distance on the basis of the parallax that emerges from rotations of a camera. A pan/tilt system specifically designed to reproduce the oculomotor parallax present in the human eye was used to replicate the oculomotor strategy by which humans scan visual scenes. We show that the oculomotor parallax provides accurate estimation of distance during sequences of eye movements. In a system that actively scans a visual scene, challenging tasks such as image segmentation and figure/ground segregation greatly benefit from this cue.National Science Foundation (BIC-0432104, CCF-0130851
Time-Dependent Mean Field Theory for Quench Dynamics in correlated electron systems
A simple and very flexible variational approach to the out-of-equilibrium
quantum dynamics in strongly correlated electron systems is introduced through
a time-dependent Gutzwiller wavefunction. As an application, we study the
simple case of a sudden change of the interaction in the fermionic Hubbard
model and find at the mean field level an extremely rich behaviour. In
particular, a dynamical transition between small and large quantum quench
regimes is found to occur at half-filling, in accordance with the analysis of
Eckstein {\sl et al.}, Phys. Rev. Lett. {\bf 103}, 056403 (2009), obtained by
dynamical mean field theory, that turns into a crossover at any finite doping.Comment: 4 pages, 2 figures, published versio
Disordered Flat Phase in a Solid on Solid Model of Fcc(110) Surfaces and Dimer States in Quantum Spin-1/2 Chains
We present a restricted solid on solid hamiltonian for fcc (110) surfaces. It
is the simplest generalization of the exactly solvable BCSOS model which is
able to describe a missing-row reconstructed surface. We study
this model by mapping it onto a quantum spin-1/2 chain of the Heisenberg type,
with second and third neighbor couplings. The ground state phase
diagram of the spin-chain model is studied by exact diagonalization of finite
chains up to sites, as well as through analytical techniques. We find
four phases in the phase diagram: two ordered phases in which the spins have a
N\'eel-type of long range order (an unreconstructed and a missing-row
reconstructed phase, in the surface language), a spin liquid phase
(representing a rough surface), and an intermediate dimer phase which breaks
translational invariance and has a doubly degenerate ground state,
corresponding to a disordered flat surface. The transition from the reconstructed phase to the disordered flat phase belongs to the Ising
universality class. A critical (preroughening) line with varying exponents
separates the unreconstructed phase from the disordered flat phase. The
possible experimental signatures of the disordered flat phase are discussed.Comment: 20 pages (10 Figures available upon request), REVTEX, SISSA PREPRINT
1/94/CM/S
Gutzwiller description of non-magnetic Mott insulators: a dimer lattice model
We introduce a novel extension of the Gutzwiller variational wavefunction
able to deal with insulators that escape any mean-field like description, as
for instance non-magnetic insulators. As an application, we study the Mott
transition from a paramagnetic metal into a non-magnetic Peierls, or
valence-bond, Mott insulator. We analyze this model by means of our Gutzwiller
wavefunction analytically in the limit of large coordination lattices, where we
find that: (1) the Mott transition is first order; (2) the Peierls gap is large
in the Mott insulator, although it is mainly contributed by the electron
repulsion; (3) singlet-superconductivity arises around the transition.Comment: 15 pages, 9 figure
One-Dimensional Multi-Band Correlated Conductors and Anderson Impurity Physics
A single Anderson impurity model recently predicted, through its unstable
fixed point, the phase diagram of a two band model correlated conductor, well
confirmed by Dynamical Mean Field Theory in infinite dimensions. We study here
the one dimensional version of the same model and extract its phase diagram in
this opposite limit of reduced dimensionality. As expected for one dimension,
the Mott metal-insulator transition at half filling is replaced by a dimerized
insulator-undimerized Mott insulator transition, while away from half filling
the strongly correlated superconductivity for inverted Hund's rule exchange in
infinite dimensions is replaced by dominant pairing fluctuations. Many other
aspects of the one dimensional system, in particular the field theories and
their symmetries are remarkably the same as those of the Anderson impurity,
whose importance appears enhanced.Comment: 4 pages, 1 figur
Superconductivity in the doped bilayer Hubbard model
We study by the Gutzwiller approximation the melting of the valence bond
crystal phase of a bilayer Hubbard model at sufficiently large inter-layer
hopping. We find that a superconducting domain, with order parameter
, being the inter-layer direction and the intra-layer one,
is stabilized variationally close to the half-filled non-magnetic Mott
insulator. Superconductivity exists at half-filling just at the border of the
Mott transition and extends away from half-filling into a whole region till a
critical doping, beyond which it gives way to a normal metal phase. This result
suggests that superconductivity should be unavoidably met by liquefying a
valence bond crystal, at least when each layer is an infinite coordination
lattice and the Gutzwiller approximation becomes exact. Remarkably, this same
behavior is well established in the other extreme of two-leg Hubbard ladders,
showing it might be of quite general validity.Comment: 9 pages, 5 figure
Quasiparticle conductivities in disordered d-wave superconductors
We study the quasiparticle transport coefficients in disordered d-wave
superconductors. We find that spin and charge excitations are generally
localized unless magnetic impurities are present. If the system is close to a
nesting point in the impurity-scattering unitary limit, the tendency towards
localization is reduced while the quasiparticle density of states gets enhanced
by disorder. We also show that the residual repulsive interaction among
quasiparticles has a delocalizing effect and increases the density of states.Comment: 13 pages, no figure
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