Recording from two neurons: second order stimulus reconstruction from spike trains and population coding

We study the reconstruction of visual stimuli from spike trains, recording simultaneously from the two H1 neurons located in the lobula plate of the fly Chrysomya megacephala. The fly views two types of stimuli, corresponding to rotational and translational displacements. If the reconstructed stimulus is to be represented by a Volterra series and correlations between spikes are to be taken into account, first order expansions are insufficient and we have to go to second order, at least. In this case higher order correlation functions have to be manipulated, whose size may become prohibitively large. We therefore develop a Gaussian-like representation for fourth order correlation functions, which works exceedingly well in the case of the fly. The reconstructions using this Gaussian-like representation are very similar to the reconstructions using the experimental correlation functions. The overall contribution to rotational stimulus reconstruction of the second order kernels - measured by a chi-squared averaged over the whole experiment - is only about 8% of the first order contribution. Yet if we introduce an instant-dependent chi-square to measure the contribution of second order kernels at special events, we observe an up to 100% improvement. As may be expected, for translational stimuli the reconstructions are rather poor. The Gaussian-like representation could be a valuable aid in population coding with large number of neurons

Temperature effect on (2+1) experimental Kardar-Parisi-Zhang growth

We report on the effect of substrate temperature (T) on both local structure and long-wavelength fluctuations of polycrystalline CdTe thin films deposited on Si(001). A strong T-dependent mound evolution is observed and explained in terms of the energy barrier to inter-grain diffusion at grain boundaries, as corroborated by Monte Carlo simulations. This leads to transitions from uncorrelated growth to a crossover from random-to-correlated growth and transient anomalous scaling as T increases. Due to these finite-time effects, we were not able to determine the universality class of the system through the critical exponents. Nevertheless, we demonstrate that this can be circumvented by analyzing height, roughness and maximal height distributions, which allow us to prove that CdTe grows asymptotically according to the Kardar-Parisi-Zhang (KPZ) equation in a broad range of T. More important, one finds positive (negative) velocity excess in the growth at low (high) T, indicating that it is possible to control the KPZ non-linearity by adjusting the temperature.Comment: 6 pages, 5 figure

Magnetic phases evolution in the LaMn1-xFexO3+y system

We have investigated the crystal structure and magnetic properties for polycrystalline samples of LaMn1-xFexO3+y, in the whole range x=0.0 to x=1.0, prepared by solid state reaction in air. All samples show the ORT-2 orthorhombic structure that suppresses the Jahn-Teller distortion, thus favoring a ferromagnetic (FM) superexchange (SE) interaction between Mn^{3+}-O-Mn^{3+}. For x=0.0 the oxygen excess (y ~ 0.09) produces vacancies in the La and Mn sites and generates a fraction around 18% of Mn^{4+} ions and 82% of the usual Mn^{3+} ions, with possible double exchange interaction between them. The Fe doping in this system is known to produce only stable Fe^{3+} ions. We find an evolution from a fairly strong FM phase with a Curie temperature T_{C} ~ 160 K, for x=0.0, to an antiferromagnetic (AFM) phase with T_{N} = 790 K, for x=1.0, accompanied by clear signatures of a cluster-glass behavior. For intermediate Fe contents a mixed-phase state occurs, with a gradual decrease (increase) of the FM (AFM) phase, accompanied by a systematic transition broadening for 0.2 < x < 0.7. A model based on the expected exchange interaction among the various magnetic-ion types, accounts very well for the saturation-magnetization dependence on Fe doping.Comment: 27 pages, 9 figure

Spin-glass behaviour on random lattices

The ground-state phase diagram of an Ising spin-glass model on a random graph with an arbitrary fraction $w$ of ferromagnetic interactions is analysed in the presence of an external field. Using the replica method, and performing an analysis of stability of the replica-symmetric solution, it is shown that $w=1/2$, correponding to an unbiased spin glass, is a singular point in the phase diagram, separating a region with a spin-glass phase ($w<1/2$) from a region with spin-glass, ferromagnetic, mixed, and paramagnetic phases ($w>1/2$)

Accuracy of a teleported trapped field state inside a single bimodal cavity

We propose a simplified scheme to teleport a superposition of coherent states from one mode to another of the same bimodal lossy cavity. Based on current experimental capabilities, we present a calculation of the fidelity that can be achieved, demonstrating accurate teleportation if the mean photon number of each mode is at most 1.5. Our scheme applies as well for teleportation of coherent states from one mode of a cavity to another mode of a second cavity, both cavities embedded in a common reservoir.Comment: 4 pages, 2 figures, in appreciation for publication in Physical Review

Diluted antiferromagnet in a ferromagnetic enviroment

The question of robustness of a network under random ``attacks'' is treated in the framework of critical phenomena. The persistence of spontaneous magnetization of a ferromagnetic system to the random inclusion of antiferromagnetic interactions is investigated. After examing the static properties of the quenched version (in respect to the random antiferromagnetic interactions) of the model, the persistence of the magnetization is analysed also in the annealed approximation, and the difference in the results are discussed

Charge asymmetries of top quarks: a window to new physics at hadron colliders

With the next start of LHC, a huge production of top quarks is expected. There are several models that predict the existence of heavy colored resonances decaying to top quarks in the TeV energy range. A peak in the differential cross section could reveal the existence of such a resonance, but this is experimentally challenging, because it requires selecting data samples where top and antitop quarks are highly boosted. Nonetheless, the production of such resonances might generate a sizable charge asymmetry of top versus antitop quarks. We consider a toy model with general flavour independent couplings of the resonance to quarks, of both vector and axial-vector kind. The charge asymmetry turns out to be a more powerful observable to detect new physics than the differential cross section, because its highest statistical significance is achieved with data samples of top-antitop quark pairs of low invariant masses