Hasil untuk "astro-ph.EP"

M. Katsnelson, K. Novoselov, A.K. Geim

The so-called Klein paradox—unimpeded penetration of relativistic particles through high and wide potential barriers—is one of the most exotic and counterintuitive consequences of quantum electrodynamics. The phenomenon is discussed in many contexts in particle, nuclear and astro-physics but direct tests of the Klein paradox using elementary particles have so far proved impossible. Here we show that the effect can be tested in a conceptually simple condensed-matter experiment using electrostatic barriers in single- and bi-layer graphene. Owing to the chiral nature of their quasiparticles, quantum tunnelling in these materials becomes highly anisotropic, qualitatively different from the case of normal, non-relativistic electrons. Massless Dirac fermions in graphene allow a close realization of Klein’s gedanken experiment, whereas massive chiral fermions in bilayer graphene offer an interesting complementary system that elucidates the basic physics involved.

L. Potters, L. Gaspar, B. Kavanagh et al.

Michael Z Su, A. Kneebone, H. Woo

M. Ingle

T. Miyazawa, A. Furuzawa, Yasufumi Kanou et al.

A. Marconi, M. Pistore, P. Traverso

U. Schmidt, K. Michel

Frederik L. Schodt

M. Kokubun, K. Abe, Y. Ezoe et al.

T. Hansen, D. Møller

A. Hanlon, G. Hanks

K. Boyce, M. D. Audley, R. Baker et al.

M. Kawaharada, Soojing Hong, M. Murashima et al.

H. Kunieda, M. Ishida, Takao Endo et al.

M. Bautz, S. Kissel, G. Prigozhin et al.

G. Kriss, A. Davidsen, Wei Zheng et al.

We observed NGC 4151 on six separate occasions at intervals of 1 to 3 days using the Hopkins Ultraviolet Telescope during the flight of Astro-2 aboard the space shuttle Endeavour in 1995 March. The far-UV spectra cover the spectral range from the interstellar cutoff at 912 to 1840 Å with a resolution of 2-4 Å. The mean spectrum, representing 4752 s of integration time, has a signal-to-noise ratio of approximately 30, and it shows profound differences compared to that obtained during the flight of Astro-1 in 1990 December. The continuum, 5.3 × 10-13 erg cm-2 s-1 Å-1 at 1455 Å, is 5 times brighter, the brightest UV flux ever observed for NGC 4151. All high-ionization absorption lines have strengthened considerably—S VI λλ933, 945, C III λ977, O VI λλ1032, 1038, N V λλ1239, 1243, Si IV λλ1394, 1403, and C IV λλ1548, 1551. The Lyman series absorption lines have also increased in strength, with the bulk of the absorption requiring a neutral hydrogen column density of 5 × 1017 cm-2 covering only 78% of the UV source with an effective Doppler parameter of 350 km s-1. However, up to 5 × 1020 cm-2 of neutral hydrogen that fully covers the source could be present in gas with a thermal Doppler parameter of 20 km s-1. Single-zone photoionization models of warm absorbing gas are unable to account for both the X-ray absorbing material and the UV-absorbing gas, largely because of the wide range of ionization states present in the UV and the X-ray.

A. Davidsen

H. Carey

H. Murakami

C. Stahle, C. Allen, K. Boyce et al.