Unidirectional Rotation Neurones in the Optomotor System of the Crab, Carcinus

1. Among the optomotor fibres to the eye muscles in Carcinus a class was found which responds to unidirectional fast rotations around various body axes. All had large signals and are therefore of large diameter. 2. In one set of these fibres which fires especially for rotations around the dorsoventral axis, it could be shown that discharges take place especially during accelerations and that, when a rotation in the null direction is suddenly stopped, a short discharge occurs. The fibres for other axes behave in a similar manner. 3. For rotations around the ventro-dorsal axis, but not for other directions, mediumsized fibres are present which, in contrast to the fast fibres, respond to visual stimulation, as well as to body rotations in darkness, thus combining the input properties of the unidirectional fast rotatory and the unidirectional purely optokinetic small fibres. Their sensitivity to visual input is for low rotation velocities, to body rotations is for high rotation velocities

Factors Regulating the Discharge Frequency in Optomotor Fibres Of Carcinus Maenas

The influence of the excited state of the animal on various motor neurone discharges and accompanying muscle action potentials was studied in the eyestalk of the crab, Carcinus maenas. In most cases large increases in firing frequency could be obtained during such states. An exception is the tonic eye-withdrawal system in which an inhibitory effect is caused. A pronounced difference in habituation to constant stimuli between spring and summer was found for the position fibres; in spring it was slow and in summer much quicker

On the Decoupling of the Homogeneous and Inhomogeneous Parts in Inhomogeneous Quantum Groups

We show that, if there exists a realization of a Hopf algebra $H$ in a $H$-module algebra $A$, then one can split their cross-product into the tensor product algebra of $A$ itself with a subalgebra isomorphic to $H$ and commuting with $A$. This result applies in particular to the algebra underlying inhomogeneous quantum groups like the Euclidean ones, which are obtained as cross-products of the quantum Euclidean spaces $R_q^N$ with the quantum groups of rotation $U_qso(N)$ of $R_q^N$, for which it has no classical analog.Comment: Latex file, 27 pages. Final version to appear in J. Phys.

Thermodynamics of collisional models for Brownian particles: General properties and efficiency

We introduce the idea of {\it collisional models} for Brownian particles, in which a particle is sequentially placed in contact with distinct thermal environments and external forces. Thermodynamic properties are exactly obtained, irrespective the number of reservoirs involved. In the presence of external forces, entropy production presents a bilinear form in which Onsager coefficients are exactly calculated. Analysis of Brownian engines based on sequential thermal switchings is proposed and considerations about their efficiencies are investigated taking into account distinct external forces protocols. Our results shed light to a new (and alternative) route for obtaining efficient thermal engines based on finite times Brownian machines.Comment: to appear in Phys. Rev. Res. (2020). Revised version contains and extensive manuscript revision and the inclusion of an extra figur

Controlling the charge environment of single quantum dots in a photonic-crystal cavity

We demonstrate that the presence of charge around a semiconductor quantum dot (QD) strongly affects its optical properties and produces non-resonant coupling to the modes of a microcavity. We first show that, besides (multi)exciton lines, a QD generates a spectrally broad emission which efficiently couples to cavity modes. Its temporal dynamics shows that it is related to the Coulomb interaction between the QD (multi)excitons and carriers in the adjacent wetting layer. This mechanism can be suppressed by the application of an electric field, making the QD closer to an ideal two-level system.Comment: 12 pages, 4 figure

Baryon loading and the Weibel instability in gamma-ray bursts

The dynamics of two counter-streaming electron-positron-ion unmagnetized plasma shells with zero net charge is analyzed in the context of magnetic field generation in GRB internal shocks due to the Weibel instability. The effects of large thermal motion of plasma particles, arbitrary mixture of plasma species and space charge effects are taken into account. We show that, although thermal effects slow down the instability, baryon loading leads to a non-negligible growth rate even for large temperatures and different shell velocities, thus guaranteeing the robustness and the occurrence of the Weibel instability for a wide range of scenarios.Comment: 6 pages, 4 figures. Accepted for publication in MNRA

Hydration and anomalous solubility of the Bell-Lavis model as solvent

We address the investigation of the solvation properties of the minimal orientational model for water, originally proposed by Bell and Lavis. The model presents two liquid phases separated by a critical line. The difference between the two phases is the presence of structure in the liquid of lower density, described through orientational order of particles. We have considered the effect of small inert solute on the solvent thermodynamic phases. Solute stabilizes the structure of solvent, by the organization of solvent particles around solute particles, at low temperatures. Thus, even at very high densities, the solution presents clusters of structured water particles surrounding solute inert particles, in a region in which pure solvent would be free of structure. Solute intercalates with solvent, a feature which has been suggested by experimental and atomistic simulation data. Examination of solute solubility has yielded a minimum in that property, which may be associated with the minimum found for noble gases. We have obtained a line of minimum solubility (TmS) across the phase diagram, accompanying the line of maximum in density (TMD). This coincidence is easily explained for non-interacting solute and it is in agreement with earlier results in the literature. We give a simple argument which suggests that interacting solute would dislocate TmS to higher temperatures

Ionized Absorbers in AGN: the Role of Collisional Ionization and Time-Evolving Photoionization

In this paper we explore collisional ionization and time-evolving photoionization in the, X-ray discovered, ionized absorbers in Seyfert galaxies. These absorbers show temporal changes inconsistent with simple equilibrium models. We develop a simple code to follow the temporal evolution of non-equilibrium photoionized gas. As a result several effects appear that are easily observable; and which, in fact, may explain otherwise paradoxical behavior. Specifically we find that: 1) In many important astrophysical conditions pure collisional and photoionization equilibria can be distinguished with moderate spectral resolution observations, due to a strong absorption structure between 1 and 3 keV. 2) In time-evolving non-equilibrium photoionization models the response of the ionization state of the gas to sudden changes of the ionizing continuum is smoothed and delayed at low gas densities, even when the luminosity increases. 3) If the changes of the ionizing luminosity are not instantaneous, and the electron density is low enough (the limit depends on the average ionization state of the gas), the ionization state of the gas can continue to increase while the source luminosity decreases, so a maximum in the ionization state of a given element may occur during a minimum of the ionizing intensity (the opposite of the prediction of equilibrium models). 4) Different ions of different elements reach their equilibrium configuration on different time-scales. These properties are similar to those seen in several ionized absorbers in AGN, properties which had hitherto been puzzling. We applied these models to a high S/N ROSAT PSPC observation of the Seyfert 1 galaxy NGC 4051.Comment: 36 pages, 10 figures, accepted for publication on Apj, in pres