120,502 research outputs found
Long-range spin-pairing order and spin defects in quantum spin-1/2 ladders
For w-legged antiferromagnetic spin-1/2 Heisenberg ladders, a long-range
spin-pairing order can be identified which enables the separation of the space
spanned by finite-range (covalent) valence-bond configurations into w+1
subspaces. Since every subspace has an equivalent counter subspace connected by
translational symmetry, twofold degeneracy, breaking traslational symmetry is
found except for the subspace where the ground state of w=even belongs to. In
terms of energy ordering, (non)degeneracy and the discontinuities introduced in
the long-range spin-pairing order by topological spin defects, the differences
between even and odd ladders are explained in a general and systematic way.Comment: 16 pages, 7 figures, 2 tables. To be publish in The European Physical
J.
Honoring, Training, and Building a Statewide Network of Elder Activists: The California Senior Leaders Program (2002-2012)
This case study explores the first decade (2002-2012) of the California Senior Leaders Program (CSLP), including participants' creation of a formal advocacy group, the California Senior Leaders Alliance. Grounded in concerns with ageism and invisibility, the CSLP provides recognition and support for diverse California elders engaged in community building and social justice work. This study employs qualitative analysis of data from participant interviews, event evaluations, program documents, video footage, and participant observation. Findings show emotional, learning, and networking benefits for participants, intergenerational influences, collective capacity and coalition building, and contributions to policy. Program challenges are described, and future directions discussed. © 2013 Copyright Taylor and Francis Group, LLC
Bosonic versus fermionic pairs of topological spin defects in monolayered high-T_c superconductors
The energy associated with bosonic and fermionic pairs of topological spin
defects in doped antiferromagnetic quantum spin-1/2 square lattice is estimated
within a resonating valence bond scenario, as described by a t-t'-J-like model
Hamiltonian, plus a t-perpendicular, responsible of a three-dimensional
screening of the electrostatic repulsion within the bosonic pairs. For
parameters appropriate for monolayered high-T_c superconductors, both fermionic
and bosonic pairs show x^2-y^2 symmetry. We find a critical value of doping
such that the energy of the bosonic pairs goes below twice the energy of two
fermionic pairs at their Fermi level. This finding could be related to the
onset of high-T_c superconductivity.Comment: 10 pages, 6 figures. To be published in Phys. Rev.
Unsupervised Emergence of Egocentric Spatial Structure from Sensorimotor Prediction
Despite its omnipresence in robotics application, the nature of spatial knowledgeand the mechanisms that underlie its emergence in autonomous agents are stillpoorly understood. Recent theoretical works suggest that the Euclidean structure ofspace induces invariants in an agent’s raw sensorimotor experience. We hypothesizethat capturing these invariants is beneficial for sensorimotor prediction and that,under certain exploratory conditions, a motor representation capturing the structureof the external space should emerge as a byproduct of learning to predict futuresensory experiences. We propose a simple sensorimotor predictive scheme, applyit to different agents and types of exploration, and evaluate the pertinence of thesehypotheses. We show that a naive agent can capture the topology and metricregularity of its sensor’s position in an egocentric spatial frame without any a prioriknowledge, nor extraneous supervision
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