Resistance effects due to magnetic guiding orbits

The Hall and magnetoresistance of a two dimensional electron gas subjected to a magnetic field barrier parallel to the current direction is studied as function of the applied perpendicular magnetic field. The recent experimental results of Nogaret {\em et al.} [Phys. Rev. Lett. {\bf 84}, 2231 (2000)] for the magneto- and Hall resistance are explained using a semi-classical theory based on the Landauer-B\"{u}ttiker formula. The observed positive magnetoresistance peak is explained as due to a competition between a decrease of the number of conducting channels as a result of the growing magnetic field, from the fringe field of the ferromagnetic stripe as it becomes magnetized, and the disappearance of snake orbits and the subsequent appearance of cycloidlike orbits.Comment: 7 pages, 7 figure

Effect of turbulence on electron cyclotron current drive and heating in ITER

Non-linear local electromagnetic gyrokinetic turbulence simulations of the ITER standard scenario H-mode are presented for the q=3/2 and q=2 surfaces. The turbulent transport is examined in regions of velocity space characteristic of electrons heated by electron cyclotron waves. Electromagnetic fluctuations and sub-dominant micro-tearing modes are found to contribute significantly to the transport of the accelerated electrons, even though they have only a small impact on the transport of the bulk species. The particle diffusivity for resonant passing electrons is found to be less than 0.15 m^2/s, and their heat conductivity is found to be less than 2 m^2/s. Implications for the broadening of the current drive and energy deposition in ITER are discussed.Comment: Letter, 5 pages, 5 figures, for submission to Nuclear Fusio

PAHs and star formation in the HII regions of nearby galaxies M83 and M33

We present mid-infrared (MIR) spectra of HII regions within star-forming galaxies M83 and M33. Their emission features are compared with Galactic and extragalactic HII regions, HII-type galaxies, starburst galaxies, and Seyfert/LINER type galaxies. Our main results are as follows: (i) the M33 and M83 HII regions lie in between Seyfert/LINER galaxies and HII-type galaxies in the 7.7/11.3 - 6.2/11.3 plane, while the different sub-samples exhibiting different 7.7/6.2 ratios; (ii) Using the NASA Ames PAH IR Spectroscopic database, we demonstrate that the 6.2/7.7 ratio does not effectively track PAH size, but the 11.3/3.3 PAH ratio does; (iii) variations on the 17 $\mu$m PAH band depends on object type; however, there is no dependence on metallicity for both extragalactic HII regions and galaxies; (iv) the PAH/VSG intensity ratio decreases with the hardness of the radiation field and galactocentric radius (Rg), yet the ionization alone cannot account for the variation seen in all of our sources; (v) the relative strength of PAH features does not change significantly with increasing radiation hardness, as measured through the [NeIII]/[NeII] ratio and the ionization index; (vi) We present PAH SFR calibrations based on the tight correlation between the 6.2, 7.7, and 11.3 $\mu$m PAH luminosities with the 24 $\mu$m luminosity and the combination of the 24 $\mu$m and H$\alpha$ luminosity; (vii) Based on the total luminosity from PAH and FIR emission, we argue that extragalactic HII regions are more suitable templates in modeling and interpreting the large scale properties of galaxies compared to Galactic HII regions.Comment: 26 pages, 24 figures, 6 tables. Accepted for publication in MNRA

Scattering of a Dirac electron on a mass barrier

The interaction of a wave packet (and in particular the wave front) with a mass barrier is investigated in one dimension. We discuss the main features of the wave packet that are inherent to two-dimensional wave packets, such as compression during reflection, penetration in the case when the energy is lower than the height of the barrier, waving tails, precursors, and the retardation of the reflected and penetrated wave packets. These features depend on the wave-packet envelope function which we demonstrate by considering the case of a rectangular wave packet with sharp front and trailing edges and a smooth Gaussian wave packet. The method of Fourier integral for obtaining the nonstationary solutions is used.Comment: 12 pages, 9 figure

Herschel PACS and SPIRE spectroscopy of the Photodissociation Regions associated with S 106 and IRAS 23133+6050

Photodissociation regions (PDRs) contain a large fraction of all of the interstellar matter in galaxies. Classical examples include the boundaries between ionized regions and molecular clouds in regions of massive star formation, marking the point where all of the photons energetic enough to ionize hydrogen have been absorbed. In this paper we determine the physical properties of the PDRs associated with the star forming regions IRAS 23133+6050 and S 106 and present them in the context of other Galactic PDRs associated with massive star forming regions. We employ Herschel PACS and SPIRE spectroscopic observations to construct a full 55-650 {\mu}m spectrum of each object from which we measure the PDR cooling lines, other fine- structure lines, CO lines and the total far-infrared flux. These measurements are then compared to standard PDR models. Subsequently detailed numerical PDR models are compared to these predictions, yielding additional insights into the dominant thermal processes in the PDRs and their structures. We find that the PDRs of each object are very similar, and can be characterized by a two-phase PDR model with a very dense, highly UV irradiated phase (n $\sim$ 10^6 cm^(-3), G$_0$ $\sim$ 10^5) interspersed within a lower density, weaker radiation field phase (n $\sim$ 10^4 cm^(-3), G$_0$ $\sim$ 10^4). We employed two different numerical models to investigate the data, firstly we used RADEX models to fit the peak of the $^{12}$CO ladder, which in conjunction with the properties derived yielded a temperature of around 300 K. Subsequent numerical modeling with a full PDR model revealed that the dense phase has a filling factor of around 0.6 in both objects. The shape of the $^{12}$CO ladder was consistent with these components with heating dominated by grain photoelectric heating. An extra excitation component for the highest J lines (J > 20) is required for S 106.Comment: 20 pages, 10 figures, A&A Accepte

A Dust Twin of Cas A: Cool Dust and 21-micron Silicate Dust Feature in the Supernova Remnant G54.1+0.3

We present infrared (IR) and submillimeter observations of the Crab-like supernova remnant (SNR) G54.1+0.3 including 350 micron (SHARC-II), 870 micron (LABOCA), 70, 100, 160, 250, 350, 500 micron (Herschel) and 3-40 micron (Spitzer). We detect dust features at 9, 11 and 21 micron and a long wavelength continuum dust component. The 21 micron dust coincides with [Ar II] ejecta emission, and the feature is remarkably similar to that in Cas A. The IRAC 8 micron image including Ar ejecta is distributed in a shell-like morphology which is coincident with dust features, suggesting that dust has formed in the ejecta. We create a cold dust map that shows excess emission in the northwestern shell. We fit the spectral energy distribution of the SNR using the continuous distributions of ellipsoidal (CDE) grain model of pre-solar grain SiO2 that reproduces the 21 and 9 micron dust features and discuss grains of SiC and PAH that may be responsible for the 10-13 micron dust features. To reproduce the long-wavelength continuum, we explore models consisting of different grains including Mg2SiO4, MgSiO3, Al2O3, FeS, carbon, and Fe3O4. We tested a model with a temperature-dependent silicate absorption coefficient. We detect cold dust (27-44 K) in the remnant, making this the fourth such SNR with freshly-formed dust. The total dust mass in the SNR ranges from 0.08-0.9 Msun depending on the grain composition, which is comparable to predicted masses from theoretical models. Our estimated dust masses are consistent with the idea that SNe are a significant source of dust in the early Universe.Comment: MNRAS: accepted on June 28, 2018 and published on July 4, 201

Classical double-layer atoms: artificial molecules

The groundstate configuration and the eigenmodes of two parallel two-dimensional classical atoms are obtained as function of the inter-atomic distance (d). The classical particles are confined by identical harmonic wells and repel each other through a Coulomb potential. As function of d we find several structural transitions which are of first or second order. For first (second) order transitions the first (second) derivative of the energy with respect to d is discontinuous, the radial position of the particles changes discontinuously (continuously) and the frequency of the eigenmodes exhibit a jump (one mode becomes soft, i.e. its frequency becomes zero).Comment: 4 pages, RevTex, 5 ps figures, to appear in Phys.Rev.Let

Polaron effects in electron channels on a helium film

Using the Feynman path-integral formalism we study the polaron effects in quantum wires above a liquid helium film. The electron interacts with two-dimensional (2D) surface phonons, i.e. ripplons, and is confined in one dimension (1D) by an harmonic potential. The obtained results are valid for arbitrary temperature ($T$), electron-phonon coupling strength ($\alpha$), and lateral confinement ($\omega_{0}$). Analytical and numerical results are obtained for limiting cases of $T$, $\alpha$, and $\omega_{0}$. We found the surprising result that reducing the electron motion from 2D to quasi-1D makes the self-trapping transition more continuous.Comment: 6 pages, 7 figures, submitted to Phys. Rev.