לוח שנה משולב

Topological insulators represent an emergent research field attracting
a lot of attention of experimentalists and theoreticians. These are
bulk insulators with delocalized (topologically protected) states on
their surface. In this talk, I will first review a full symmetry
classification of topological insulators. I will then focus on 2D and
3D topological insulators (and on topologically protected metals on
their boundaries) in systems with strong spin-orbit interaction
("symplectic symmetry class"). I will analyze the field theories of
these systems in the presence of disorder. A non-trivial topological
nature of these theories leads to topological protection of boundary
states from Anderson localization. I will also discuss an analogy with
graphene where the same topological protection is operative as long as
the intervalley scattering can be neglected.

Further, I will analyze the effect of Coulomb interaction on transport
in topological insulators. While the Coulomb interaction does not
affect the topological protection, it leads to emergence of a novel
quantum critical state with a conductivity ~e^2/h on the surface of a
3D topological insulator. Remarkably, this critical state emerges
without any adjustable parameters. Such a ``self-organized quantum
criticality'' is a novel concept in the field of interacting
disordered systems. Finally, we predict a quantum spin-Hall
transition between the normal and topological insulator phases in 2D
that occurs via a similar (or identical) quantum critical point.

Abstract: In many regions of the brain, neurons form an ordered representation of the outside world. For example, the 'homunculus' of the somatosensory cortex is a point-to-point topographic map of the body surface onto the brain surface. The spatially organized convergence of sensory inputs often leads to similar response properties in target neurons that are in close vicinity. Whether their individual information content is redundant or independent depends on the circuit architecture (the interplay between common input, lateral signals and feedback from other brain areas) and the computational goals of the network.

In the mammalian olfactory bulb (OB), sensory neurons expressing the same type of olfactory receptor (~10,000) converge in tight focus, forming clusters of synapses called glomeruli (~2,000). From each glomerulus, a few dozen mitral cells (principal output neurons of the OB) carry the output further to the cortex. The mitral cells, typically have only one primary dendrite that projects to a single glomerulus, but can sample inputs on their primary and secondary dendrites from functionally diverse glomeruli via several types of interneurons. Thus, a few dozen mitral cells share input from the same parent glomerulus, but may have different inhibitory surrounds.
In the first part of this talk, I will discuss the topographic layout of glomeruli on the bulb - the olfactory map. How precise is this map within and across two species: mouse and rat? How does its structure relate to odor processing? Do glomeruli that are responsive to structurally similar odor molecules have a tendency to lie next to each other? In other words, is there a chemotopic map?
In the second part of the talk, I will focus on probing the odor response properties of mitral cells using extracellular recordings and an optogenetic strategy to ask whether the OB is more than a relay station. Do mitral cells receiving common input from the same parent glomerulus carry redundant information about odors to cortex?
I will conclude by describing novel strategies that allow monitoring the input-output transfer function of the OB via multi-photon microscopy imaging of bulb neurons activity in the same animal, in different states of the circuit.
Link for further information:
http://www.cshl.edu/public/SCIENCE/albeanu.html