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 ZnV$_2$O$_4$ and MgV$_2$O$_4$.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$_2$ and related Se-substituted compounds NiS$_{2-x}$Se$_x$, 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$_2$ 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 Ca_1-xSr_xRuO_3 across the ferromagnetic quantum phase transition

Ca_1-xSr_xRuO_3, which is ferromagnetic for Sr concentration x > 0.3, has been studied by x-ray magnetic circular dichroism (XMCD) in Ru 3p and O 1s core-level x-ray absorption. XMCD signals appear at x ~ 0.3 and monotonically increases with x in the ferromagnetic phase. While the monotonic increase of the XMCD signals with x is of a typical Stoner-type, the absence of appreciable change in the spectral line shapes of both the Ru 3p and O 1s XMCD spectra indicate that the itinerant-electron ferromagnetism in Ca_1-xSr_xRuO_3 is influenced by strong electron correlation.Comment: 5 pages, 4 figures, accepted in Phys. Rev. B 1 page, correct the 4th affiliation 5 page, modifiy 9th referenc

Phase Separation and the Low-Field Bulk Magnetic Properties of Pr0.7Ca0.3MnO3

We present a detailed magnetic study of the perovskite manganite Pr0.7Ca0.3MnO3 at low temperatures including magnetization and a.c. susceptibility measurements. The data appear to exclude a conventional spin glass phase at low fields, suggesting instead the presence of correlated ferromagnetic clusters embedded in a charge-ordered matrix. We examine the growth of the ferromagnetic clusters with increasing magnetic field as they expand to occupy almost the entire sample at H ~ 0.5 T. Since this is well below the field required to induce a metallic state, our results point to the existence of a field-induced ferromagnetic insulating state in this material.Comment: 15 pages with figures, submitted to Physical Review

Spin-Peierls phases in pyrochlore antiferromagnets

In the highly frustrated pyrochlore magnet spins form a lattice of corner sharing tetrahedra. We show that the tetrahedral ``molecule'' at the heart of this structure undergoes a Jahn-Teller distortion when lattice motion is coupled to the antiferromagnetism. We extend this analysis to the full pyrochlore lattice by means of Landau theory and argue that it should exhibit spin-Peierls phases with bond order but no spin order. We find a range of Neel phases, with collinear, coplanar and noncoplanar order. While collinear Neel phases are easiest to generate microscopically, we also exhibit an interaction that gives rise to a coplanar state instead.Comment: REVTeX 4, 14 pages, 12 figures (best viewed in color