Weakly nonlinear response of noisy neurons

We calculate the instantaneous firing rate of a stochastic integrate-and-fire neuron driven by an arbitrary time-dependent signal up to second order in the signal amplitude. For cosine signals, this weakly nonlinear theory reveals: (i) a frequency-dependent change of the time-averaged firing rate reminiscent of frequency locking in deterministic oscillators; (ii) higher harmonics in the rate that may exceed the linear response; (iii) a strong nonlinear response to two cosines even when the response to a single cosine is linear. We also measure the second-order response numerically for a neuron model with excitable voltage dynamics and channel noise, and find a strong similarity to the second-order response that we obtain analytically for the leaky integrate-and-fire model. Finally, we illustrate how the transition of neural dynamics from the linear rate response regime to a mode-locking regime is captured by the second-order response. Our results highlight the importance and robustness of the weakly nonlinear regime in neural dynamics.Bundesministerium für Bildung und Forschung https://doi.org/10.13039/501100002347Deutsche Forschungsgemeinschafthttps://doi.org/10.13039/501100001659Peer Reviewe

Unraveling the 0νββ decay mechanisms

We discuss the possibilities of distinguishing among different mechanisms of neutrinoless double beta decay arising in the effective field theory framework. Following the review and detailed investigation of the particular ways of discrimination, we conclude that the 32 different low-energy effective operators can be split into multiple groups that are in principle distinguishable from each other by measurements of the phase-space observables and by comparison of the decay rates obtained using different isotopes. This would require not only a substantial experimental precision but necessarily also a considerable improvement of the current theoretical knowledge of the underlying nuclear physics. Specifically, the limiting aspect in our approach turns out to be the currently unknown or uncertain values of low-energy constants. Besides the study adopting the effective field theory language we also look into several typical UV models

Nanowire Spin Torque Oscillator Driven by Spin Orbit Torques

Spin torque from spin current applied to a nanoscale region of a ferromagnet can act as negative magnetic damping and thereby excite self-oscillations of its magnetization. In contrast, spin torque uniformly applied to the magnetization of an extended ferromagnetic film does not generate self-oscillatory magnetic dynamics but leads to reduction of the saturation magnetization. Here we report studies of the effect of spin torque on a system of intermediate dimensionality - a ferromagnetic nanowire. We observe coherent self-oscillations of magnetization in a ferromagnetic nanowire serving as the active region of a spin torque oscillator driven by spin orbit torques. Our work demonstrates that magnetization self-oscillations can be excited in a one-dimensional magnetic system and that dimensions of the active region of spin torque oscillators can be extended beyond the nanometer length scale.Comment: The link to the published version is http://www.nature.com/ncomms/2014/141205/ncomms6616/full/ncomms6616.htm

Optical fiber sensors for materials and structures characterization

The final technical report on Optical Fiber Sensors for Materials and Structures Characterization, covering the period August 1990 through August 1991 is presented. Research programs in the following technical areas are described; sapphire optical fiber sensors; vibration analysis using two-mode elliptical core fibers and sensors; extrinsic Fabry-Perot interferometer development; and coatings for fluorescent-based sensor. Research progress in each of these areas was substantial, as evidenced by the technical publications which are included as appendices

Neutrino Physics: Open Theoretical Questions

We know that neutrino mass and mixing provide a window to physics beyond the Standard Model. Now this window is open, at least partly. And the questions are: what do we see, which kind of new physics, and how far "beyond"? I summarize the present knowledge of neutrino mass and mixing, and then formulate the main open questions. Following the bottom-up approach, properties of the neutrino mass matrix are considered. Then different possible ways to uncover the underlying physics are discussed. Some results along the line of: see-saw, GUT and SUSY GUT are reviewed.Comment: 17 pages, latex, 12 figures. Talk given at the XXI International Symposium on Lepton and Photon Interactions at High Energies, ``Lepton Photon 2003", August 11-16, 2003 - Fermilab, Batavia, IL US

Dynamics of flexible fibers in viscous flows and fluids

International audienceThe dynamics and deformations of immersed flexible fibers are at the heart of important industrial and biological processes, induce peculiar mechanical and transport properties in the fluids that contain them, and are the basis for novel methods of flow control. Here we focus on the low Reynolds number regime where advances in studying these fiber-fluid systems have been especially rapid. On the experimental side this is due to new methods of fiber synthesis, microfluidic flow control, and of microscope based tracking measurement techniques. Likewise, there have been continuous improvements in the specialized mathematical modeling and numerical methods needed to capture the interactions of slender flexible fibers with flows, boundaries, and each other