464 research outputs found
Imaging of an early memory trace in the Drosophila mushroom body
Extensive molecular, genetic, and anatomical analyses have suggested that olfactory memory is stored in the mushroom body (MB), a higher-order olfactory center in the insect brain. The MB comprises three subtypes of neurons with axons that extend into different lobes. A recent functional imaging study has revealed a long-term memory trace manifested as an increase in the Ca(2+) activity in an axonal branch of a subtype of MB neurons. However, early memory traces in the MB remain elusive. We report here learning-induced changes in Ca(2+) activities during early memory formation in a different subtype of MB neurons. We used three independent in vivo and in vitro preparations, and all of them showed that Ca(2+) activities in the axonal branches of alpha'/beta' neurons in response to a conditioned olfactory stimulus became larger compared with one that was not conditioned. The changes were dependent on proper G-protein signaling in the MB. The importance of these changes in the Ca(2+) activity of alpha'/beta' neurons during early memory formation was further tested behaviorally by disrupting G-protein signaling in these neurons or blocking their synaptic outputs during the learning and memory process. Our results suggest that increased Ca(2+) activity in response to a conditioned olfactory stimulus may be a neural correlate of early memory in the MB
Spin Driven Jahn-Teller Distortion in a Pyrochlore system
The ground-state properties of the spin-1 antiferromagnetic Heisenberg model
on the corner-sharing tetrahedra, pyrochlore lattice, is investigated. By
breaking up each spin into a pair of 1/2-spins, the problem is reduced to the
equivalent one of the spin-1/2 tetrahedral network in analogy with the valence
bond solid state in one dimension. The twofold degeneracy of the spin-singlets
of a tetrahedron is lifted by a Jahn-Teller mechanism, leading to a cubic to
tetragonal structural transition. It is proposed that the present mechanism is
responsible for the phase transition observed in the spin-1 spinel compounds
ZnVO and MgVO.Comment: 4 pages, 3 eps figures, REVTeX, to appear in Phys. Rev. Let
Metal-insulator Crossover Behavior at the Surface of NiS_2
We have performed a detailed high-resolution electron spectroscopic
investigation of NiS and related Se-substituted compounds
NiSSe, which are known to be gapped insulators in the bulk at all
temperatures. A large spectral weight at the Fermi energy of the room
temperature spectrum, in conjunction with the extreme surface sensitivity of
the experimental probe, however, suggests that the surface layer is metallic at
300 K. Interestingly, the evolution of the spectral function with decreasing
temperature is characterized by a continuous depletion of the single-particle
spectral weight at the Fermi energy and the development of a gap-like structure
below a characteristic temperature, providing evidence for a metal-insulator
crossover behavior at the surfaces of NiS and of related compounds. These
results provide a consistent description of the unusual transport properties
observed in these systems.Comment: 12 pages, 3 figure
Soft x-ray magnetic circular dichroism study of weakly ferromagnetic ZnVO thin film
We performed a soft x-ray magnetic circular dichroism (XMCD) study of a
ZnVO thin film which showed small ferromagnetic moment. Field and
temperature dependences of V 2 XMCD signals indicated the coexistence of
Curie-Weiss paramagnetic, antiferromagnetic, and possibly ferromagnetic V ions,
quantitatively consistent with the magnetization measurements. We attribute the
paramagnetic signal to V ions substituting Zn sites which are somewhat
elongated along the c-axis
Indication of intrinsic room-temperature ferromagnetism in Ti1-xCoxO2-d thin film: An x-ray magnetic circular dichroism study
Soft x-ray magnetic circular dichroism (XMCD) measurements at the Co L2,3
edges of Co doped rutile TiO2 at room temperature have revealed clear multiplet
features characteristic of ferromagnetic Co2+ ions coordinated by O2- ions,
being in sharp contrast to the featureless XMCD spectrum of Co metal or
metallic clusters. The absorption and XMCD spectra agree well with a full
atomic-multiplet calculation for the Co2+ high-spin state in the D2h-symmetry
crystal field at the Ti site in rutile TiO2. The results indicate that the
ferromagnetism arises from the Co2+ ions substituting the Ti4+ ions.Comment: 11 pages including 3 figure
High-energy spectroscopic study of the III-V nitride-based diluted magnetic semiconductor GaMnN
We have studied the electronic structure of the diluted magnetic
semiconductor GaMnN ( = 0.0, 0.02 and 0.042) grown on Sn-doped
-type GaN using photoemission and soft x-ray absorption spectroscopy. Mn
-edge x-ray absorption have indicated that the Mn ions are in the
tetrahedral crystal field and that their valence is divalent. Upon Mn doping
into GaN, new state were found to form within the band gap of GaN, and the
Fermi level was shifted downward. Satellite structures in the Mn 2 core
level and the Mn 3 partial density of states were analyzed using
configuration-interaction calculation on a MnN cluster model. The deduced
electronic structure parameters reveal that the - exchange coupling in
GaMnN is stronger than that in GaMnAs.Comment: 6pages, 10figures. To be published to Phys. Rev.
Clustering transitions in vibro-fluidized magnetized granular materials
We study the effects of long range interactions on the phases observed in
cohesive granular materials. At high vibration amplitudes, a gas of magnetized
particles is observed with velocity distributions similar to non-magnetized
particles. Below a transition temperature compact clusters are observed to form
and coexist with single particles. The cluster growth rate is consistent with a
classical nucleation process. However, the temperature of the particles in the
clusters is significantly lower than the surrounding gas, indicating a
breakdown of equipartition. If the system is quenched to low temperatures, a
meta-stable network of connected chains self-assemble due to the anisotropic
nature of magnetic interactions between particles.Comment: 4 pages, 5 figure
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