## Events - Daily

**November 19, 2012**>>

Monday,
November 19 |
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Biophysics Seminar Series - Fred Wolf (Max Planck Institute for Dynamics and Self-Organization) "Dynamical entropy production in recurrent neuronal circuits" Neurons in the cerebral cortex fire action potentials in highly irregular, seemingly random sequences. Since neurons in isolation reliably respond to the repeated injection of identical temporally varying inputs, irregular activity in the cortex is not believed to result from a randomness in the spike generating mechanism, but rather from strongly fluctuating synaptic inputs. Several explanations for the origin of such fluctuating inputs have been proposed. The prevailing explanation is a dynamic balance between excitatory and inhibitory inputs, also known as the balanced state of cortical networks. Such a balance in neuronal circuits has been demonstrated experimentally in vitro and in vivo. Its statistical characteristics have been extensively studied theoretically. The dynamical nature of the balanced state, however, remained controversial and poorly understood. Here, we characterize the dynamical properties of balanced model circuits compose of different single neuron models. We obtain the complete spectrum of Lyapunov exponents, the Kolgomorov-Sinai entropy rate and attractor dimension and establish the extensive nature of these properties. In stark contrast to statistical properties of balanced state networks (such as distributions of spike train cv and unit firing rates), their collective dynamics turns out to be extremely sensitive to minute details of the single neuron dynamics. Intriguingly, we find that modifying the time scale of action potential initiation can reduce dynamical entropy production in the network by orders of magnitude. In models of identical topological circuit structure, dynamical entropy production can be tuned from about one bit of information loss per neuron and spike to arbitrary small values. Very crisp action potential generators can even render the irregular network dynamics formally stable. Our results indicate that information flow through complex neural circuits should be expected extremely sensitive to minute features of the single neuron dynamics. Joseph Henry Room · 12:00 p.m.– 1:00 p.m. |

Condensed Matter Seminar - Michael Fuhrer, University of Maryland - "Surface Conduction of Topological Dirac Electrons in Bulk Insulating Bi2Se3" The three dimensional strong topological insulator (STI) is a new phase of electronic matter which is distinct from ordinary insulators in that it supports on its surface a conducting two-dimensional surface state whose existence is guaranteed by topology. I will discuss experiments on the STI material Bi2Se3, which has a bulk bandgap of 300 meV, much greater than room temperature, and a single topological surface state with a massless Dirac dispersion. Field effect transistors consisting of thin (3-20 nm) Bi2Se3 are fabricated from mechanically exfoliated from single crystals, and electrochemical and/or chemical gating methods are used to move the Fermi energy into the bulk bandgap, revealing the ambipolar gapless nature of transport in the Bi2Se3 surface states. The minimum conductivity of the topological surface state is understood within the self-consistent theory of Dirac electrons in the presence of charged impurities. The intrinsic finite-temperature resistivity of the topological surface state due to electron-acoustic phonon scattering is measured to be 60 times larger than that of graphene largely due to the smaller Fermi and sound velocities in Bi2Se3[2], which will have implications for topological electronic devices operating at room temperature. In the thinnest Bi2Se3 samples (~3 nm) we observe the opening of a bandgap due to coupling of the top and bottom surfaces which hybridize to form a conventional two-dimensional insulator[3], and by controllably thinning regions of Bi2Se3 samples we achieve quantum dots with gate-tunable insulating barriers[4]. [1] D. Kim et al., Nature Physics 8, 460 (2012). [2] D. Kim et al., Phys. Rev. Lett. 109, 166801 (2012). [3] S. Cho et al., Nano Letters 11, 1925 (2011). [4] S. Cho et al., Nano Letters 12, 469 (2012). PCTS Seminar Room · 1:15 p.m.– 2:30 p.m. |

High Energy Theory Seminar - IAS - David Marsh, Cornell University - “On Sequestering and De-Coupling in Stabilized String Models” I will describe recent efforts to understand the mediation of supersymmetry breaking in stabilized compactifications of type IIB string theory. By geometrically separating the visible sector from the supersymmetry breaking effects one may hope to achieve sequestered supersymmetry breaking and much ameliorated constraints from bounds on flavor changing neutral currents. However, in this talk I will discuss how non-perturbative superpotential cross-couplings between the visible sector and the Khler moduli may spoil sequestering and introduce a sensitivity to the global details of the compactification. As a simple example, I will describe the structure of these `de-sequestering operators for a class of visible sectors realized by D-branes probing an orbifold singularity, and I will discuss their importance in the KKLT and LVS moduli stabilization scenarios. Bloomberg Lecture Hall - Institute for Advanced Study · 2:30 p.m.– 3:30 p.m. |