13,870 research outputs found
A Neural Circuit for Coordinating Reaching with Grasping: Autocompensating Variable Initial Apertures, Perturbations to Target Size, and Perturbations to Target Orientation
A neural network model is presented, that extends principles of the VITE (vector integration to end-point) model [1, 2, 3, 4] of primate reaching to the more complex case of reach-grasp coordination. The main new planning problem addressed by the model is how to simulate human data on temporal coordination between reaching and grasping, while at the same time remaining stable and compensating for altered initial apertures and perturbations of object size and object location/ orientation. Simulations of the model replicate key features of four different experimental protocols with a single set of parameters. The proposed circuit computes reaching to grasp trajectories in real-time, by continuously updating vector positioning commands, and with no precomputation of total or component movement times. The model consists of three generator channels: transport, which generates a reaching trajectory; aperture, which controls distance between thumb and index finger; and orientation, which controls hand orientation vis-a-vis target's orientation.CONACYT of Mexico; Defense Advanced Research Projects Agency and Office of Naval Research (N00014-95-1-0409
A Neural Circuit for Coordinating Reaching with Grasping: Autocompensating Variable Initial Apertures, Perturbations to Target Size, and Perturbations to Target Orientation
A neural network model is presented, that extends principles of the VITE (vector integration to end-point) model [1, 2, 3, 4] of primate reaching to the more complex case of reach-grasp coordination. The main new planning problem addressed by the model is how to simulate human data on temporal coordination between reaching and grasping, while at the same time remaining stable and compensating for altered initial apertures and perturbations of object size and object location/ orientation. Simulations of the model replicate key features of four different experimental protocols with a single set of parameters. The proposed circuit computes reaching to grasp trajectories in real-time, by continuously updating vector positioning commands, and with no precomputation of total or component movement times. The model consists of three generator channels: transport, which generates a reaching trajectory; aperture, which controls distance between thumb and index finger; and orientation, which controls hand orientation vis-a-vis target's orientation.CONACYT of Mexico; Defense Advanced Research Projects Agency and Office of Naval Research (N00014-95-1-0409
Zeeman splitting of shallow donors in GaN
The Zeeman splitting of the donor spectra in cubic- and hexagonal-GaN are
studied using an effective mass theory approach. Soft-core pseudopotentials
were used to describe the chemical shift of the different substitutional
dopants. The donor ground states calculated range from 29.5 to 33.7 meV, with
typically 1 meV higher binding in the hexagonal phase. Carbon is found to
produce the largest donor binding energy. The ionization levels and excited
states are in excellent agreement with Hall and optical measurements, and
suggest the presence of residual C in recent experiments.Comment: REVTEX file - 2 figure
Aharonov-Bohm phase as quantum gate in two-electron charge qubits
We analyze the singlet-triplet splitting on a planar array of quantum dots
coupled capacitively to a set of external voltage gates. The system is modelled
using an extended Hubbard Hamiltonian keeping two excess electrons on the
array. The voltage dependence of the low-energy singlet and triplet states is
analyzed using the Feshbach formalism. The formation of a well decoupled
two-level system in the ground state is shown to rely on the fact of having two
particles in the system. Coherent operation of the array is studied with
respect to single quantum bit operations. One quantum gate is implemented via
voltage controls, while for the necessary second quantum gate, a uniform
external magnetic field is introduced. The Aharonov-Bohm phases on the closed
loop tunnel connections in the array are used to effectively suppress the
tunneling, despite a constant tunneling amplitude in the structure. This allows
one to completely stall the qubit in any arbitrary quantum superposition,
providing full control of this interesting quantum system.Comment: 6 pages, 5 figures (submitted to PRB
Magnon spin Hall magnetoresistance of a gapped quantum paramagnet
Motivated by recent experimental work, we consider spin transport between a
normal metal and a gapped quantum paramagnet. We model the latter as the
magnonic Mott-insulating phase of an easy-plane ferromagnetic insulator. We
evaluate the spin current mediated by the interface exchange coupling between
the ferromagnet and the adjacent normal metal. For the strongly interacting
magnons that we consider, this spin current gives rise to a spin Hall
magnetoresistance that strongly depends on the magnitude of the magnetic field,
rather than its direction. This Letter may motivate electrical detection of the
phases of quantum magnets and the incorporation of such materials into
spintronic devices.Comment: 5 pages, 5 figure
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