24,937 research outputs found

    Electronic structure and the Fermi surface of UTGa_{5} (T=Fe, Co, Rh)

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    The relativistic energy-band calculations have been carried out for UFeGa_{5}, UCoGa_{5} and URhGa_{5} under the assumption that 5f-electrons are itinerant. A hybridization between the U 5f state and Ga 4p state occurs in the vicinity of the Fermi level. The Fermi surface of UCoGa_{5} is quite similar to that of URhGa_{5}, which are all small in size and closed in topology. UFeGa_{5} has the quasi-two-dimensional Fermi surface which looks like a lattice structure.Comment: 2 pages, 3 figures, LT23auth.cls, elsart.cls. submitted to conference LT2

    Fast Zonal Field Dynamo in Collisionless Kinetic Alfven Wave Turbulence

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    The possibility of fast dynamo action by collisionless kinetic Alfven Wave turbulence is demonstrated. The irreversibility necessary to lock in the generated field is provided by electron Landau damping, so the induced electric field does not vanish with resistivity. Mechanisms for self-regulation of the system and the relation of these results to the theory of alpha quenching are discussed. The dynamo-generated fields have symmetry like to that of zonal flows, and thus are termed zonal fields

    Synchronizations in small-world networks of spiking neurons: Diffusive versus sigmoid couplings

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    By using a semi-analytical dynamical mean-field approximation previously proposed by the author [H. Hasegawa, Phys. Rev. E, {\bf 70}, 066107 (2004)], we have studied the synchronization of stochastic, small-world (SW) networks of FitzHugh-Nagumo neurons with diffusive couplings. The difference and similarity between results for {\it diffusive} and {\it sigmoid} couplings have been discussed. It has been shown that with introducing the weak heterogeneity to regular networks, the synchronization may be slightly increased for diffusive couplings, while it is decreased for sigmoid couplings. This increase in the synchronization for diffusive couplings is shown to be due to their local, negative feedback contributions, but not due to the shorten average distance in SW networks. Synchronization of SW networks depends not only on their structure but also on the type of couplings.Comment: 17 pages, 8 figures, accepted in Phys. Rev. E with some change

    Mechanism of Ambipolar Field-Effect Carrier Injections in One-Dimensional Mott Insulators

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    To clarify the mechanism of recently reported, ambipolar carrier injections into quasi-one-dimensional Mott insulators on which field-effect transistors are fabricated, we employ the one-dimensional Hubbard model attached to a tight-binding model for source and drain electrodes. To take account of the formation of Schottky barriers, we add scalar and vector potentials, which satisfy the Poisson equation with boundary values depending on the drain voltage, the gate bias, and the work-function difference. The current-voltage characteristics are obtained by solving the time-dependent Schr\"odinger equation in the unrestricted Hartree-Fock approximation. Its validity is discussed with the help of the Lanczos method applied to small systems. We find generally ambipolar carrier injections in Mott insulators even if the work function of the crystal is quite different from that of the electrodes. They result from balancing the correlation effect with the barrier effect. For the gate-bias polarity with higher Schottky barriers, the correlation effect is weakened accordingly, owing to collective transport in the one-dimensional correlated electron systems.Comment: 21 pages, 10 figures, to appear in J. Phys. Soc. Jp

    The pseudogap in Bi2212 single crystals from tunneling measurements: a possible evidence for the Cooper pairs above Tc

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    We present electron-tunneling spectroscopy of slightly overdoped Bi2212 single crystals with Tc = 87 - 90 K in a temperature range between 14 K and 290 K using a break-junction technique. The pseudogap which has been detected above Tc appears at T* = 280 K. The analysis of the spectra shows that there is a contribution to the pseudogap above Tc, which disappears approximately at 110 - 115 K. We associate this contribution with the presence of incoherent Cooper pairs.Comment: 12 pages including 4 figures, to be published in Europhysics Letter

    Ion Charge States in the Fast Solar Wind: New Data Analysis and Theoretical Refinements

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    We present a further investigation into the increased ionization observed in element charge states in the fast solar wind compared to its coronal hole source regions. Once ions begin to be perpendicularly heated by ion cyclotron waves and execute large gyro-orbits, density gradients in the flow can excite lower hybrid waves that then damp by heating electrons in the parallel direction. We give further analysis of charge state data from polar coronal holes at solar minimum and maximum, and also from equatorial coronal holes. We also consider further the damping of lower hybrid waves by ions and the effect of non-Maxwellian electron distribution functions on the degree of increased ionization, both of which appear to be negligible for the solar wind case considered here. We also suggest that the density gradients required to heat electrons sufficiently to further ionize the solar wind can plausibly result from the turbulent cascade of MHD waves.Comment: 27 pages, accepted by Ap

    Classical small systems coupled to finite baths

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    We have studied the properties of a classical NSN_S-body system coupled to a bath containing NBN_B-body harmonic oscillators, employing an (NS+NB)(N_S+N_B) model which is different from most of the existing models with NS=1N_S=1. We have performed simulations for NSN_S-oscillator systems, solving 2(NS+NB)2(N_S+N_B) first-order differential equations with NS‚ČÉ1‚ąí10N_S \simeq 1 - 10 and NB‚ČÉ10‚ąí1000N_B \simeq 10 - 1000, in order to calculate the time-dependent energy exchange between the system and the bath. The calculated energy in the system rapidly changes while its envelope has a much slower time dependence. Detailed calculations of the stationary energy distribution of the system fS(u)f_S(u) (uu: an energy per particle in the system) have shown that its properties are mainly determined by NSN_S but weakly depend on NBN_B. The calculated fS(u)f_S(u) is analyzed with the use of the őď\Gamma and qq-őď\Gamma distributions: the latter is derived with the superstatistical approach (SSA) and microcanonical approach (MCA) to the nonextensive statistics, where qq stands for the entropic index. Based on analyses of our simulation results, a critical comparison is made between the SSA and MCA. Simulations have been performed also for the NSN_S-body ideal-gas system. The effect of the coupling between oscillators in the bath has been examined by additional (NS+NBN_S+N_B) models which include baths consisting of coupled linear chains with periodic and fixed-end boundary conditions.Comment: 30 pages, 16 figures; the final version accepted in Phys. Rev.
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