13,132 research outputs found
Radio Galaxies at VHE energies
Radio Galaxies have by now emerged as a new -ray emitting source
class on the extragalactic sky. Given their remarkable observed
characteristics, such as unusual gamma-ray spectra or ultrafast VHE
variability, they represent unique examples to probe into the nature and
physics of AGN in general. This review provides a compact summary of their
observed characteristics at very high -ray energies (VHE; GeV)
along with a discussion of their possible physics implications. A particular
focus is given to a concise overview of fundamental concepts concerning the
origin of variable VHE emission, including recent developments in black hole
gap physics.Comment: Invited review article, submitted to Galaxies; review, 21 pages, 14
figures; small typos corrected and references fixed to match accepted versio
Orbital magnetoelectric coupling in band insulators
Magnetoelectric responses are a fundamental characteristic of materials that
break time-reversal and inversion symmetries (notably multiferroics) and,
remarkably, of "topological insulators" in which those symmetries are unbroken.
Previous work has shown how to compute spin and lattice contributions to the
magnetoelectric tensor. Here we solve the problem of orbital contributions by
computing the frozen-lattice electronic polarization induced by a magnetic
field. One part of this response (the "Chern-Simons term") can appear even in
time-reversal-symmetric materials and has been previously shown to be quantized
in topological insulators. In general materials there are additional orbital
contributions to all parts of the magnetoelectric tensor; these vanish in
topological insulators by symmetry and also vanish in several simplified models
without time-reversal and inversion those magnetoelectric couplings were
studied before. We give two derivations of the response formula, one based on a
uniform magnetic field and one based on extrapolation of a long-wavelength
magnetic field, and discuss some of the consequences of this formula.Comment: 13 page
Diffuse emission in the presence of inhomogeneous spin-orbit interaction for the purpose of spin filtration
A lateral interface connecting two regions with different strengths of the
Bychkov-Rashba spin-orbit interaction can be used as a spin polarizer of
electrons in two dimensional semiconductor heterostructures. [Khodas \emph{et
al.}, Phys. Rev. Lett. \textbf{92}, 086602 (2004)]. In this paper we consider
the case when one of the two regions is ballistic, while the other one is
diffusive. We generalize the technique developed for the solution of the
problem of the diffuse emission to the case of the spin dependent scattering at
the interface, and determine the distribution of electrons emitted from the
diffusive region. It is shown that the diffuse emission is an effective way to
get electrons propagating at small angles to the interface that are most
appropriate for the spin filtration and a subsequent spin manipulation.
Finally, a scheme is proposed of a spin filter device, see Fig. 9, that creates
two almost fully spin-polarized beams of electrons.Comment: 11 pages, 9 figure
Unmasking the Pollution Haven Effect
This paper uses both theory and empirical work to examine the effect of environmental regulations on trade flows. We develop a simple economic model to demonstrate how unobserved heterogeneity, endogeneity and aggregation issues bias measurements of the relationship between regulatory costs and trade. We apply an estimating equation derived from the model to data on U.S. regulations and net trade flows among the U.S., Canada, and Mexico, for 130 manufacturing industries from 1977 to 1986. Our results indicate that industries whose abatement costs increased most experienced the largest increases in net imports. For the 20 industries hardest hit by regulation, the change in net imports we ascribe to the increase in regulatory costs amounts to more than half of the total increase in trade volume over the period.
Quantum dot dephasing by edge states
We calculate the dephasing rate of an electron state in a pinched quantum
dot, due to Coulomb interactions between the electron in the dot and electrons
in a nearby voltage biased ballistic nanostructure. The dephasing is caused by
nonequilibrium time fluctuations of the electron density in the nanostructure,
which create random electric fields in the dot. As a result, the electron level
in the dot fluctuates in time, and the coherent part of the resonant
transmission through the dot is suppressed
Quantized adiabatic quantum pumping due to interference
Recent theoretical calculations, demonstrating that quantized charge transfer
due to adiabatically modulated potentials in mesoscopic devices can result
purely from the interference of the electron wave functions (without invoking
electron-electron interactions) are reviewed: (1) A new formula is derived for
the pumped charge Q (per period); It reproduces the Brouwer formula without a
bias, and also yields the effect of the modulating potential on the Landauer
formula in the presence of a bias. (2) For a turnstile geometry, with
time-dependent gate voltages V_L(t) and V_R(t), the magnitude and sign of Q are
determined by the relative position and orientation of the closed contour
traversed by the system in the {V_L-V_R} plane, relative to the transmission
resonances in that plane. Integer values of Q (in units of e) are achieved when
a transmission peak falls inside the contour, and are given by the winding
number of the contour. (3) When the modulating potential is due to surface
acoustic waves, Q exhibits a staircase structure, with integer values,
reminiscent of experimental observations.Comment: Invited talk, Localization, Tokyo, August 200
Unmasking the Pollution Haven Hypothesis
This paper uses both theory and empirical work to examine the effect of environmental regulations on trade flows. We develop a simple economic model to demonstrate how unobserved heterogeneity, endogeneity and aggregation issues bias measurements of the relationship between regulatory costs and trade. We apply an estimating equation derived from the model to data on U.S. regulations and net trade flows among the U.S., Canada, and Mexico, for 130 manufacturing industries from 1977 to 1986. Our results indicate that industries whose abatement costs increased most experienced the largest increases in net imports. For the 20 industries hardest hit by regulation, the change in net imports we ascribe to the increase in regulatory costs amounts to more than half of the total increase in trade volume over the period.
Electron-phonon bound states in graphene in a perpendicular magnetic field
The spectrum of electron-phonon complexes in a monolayer graphene is
investigated in the presence of a perpendicular quantizing magnetic field.
Despite the small electron-phonon coupling, usual perturbation theory is
inapplicable for calculation of the scattering amplitude near the threshold of
the optical phonon emission. Our findings beyond perturbation theory show that
the true spectrum near the phonon emission threshold is completely governed by
new branches, corresponding to bound states of an electron and an optical
phonon with a binding energy of the order of where
is the electron-phonon coupling and the phonon energy.Comment: To be published in Phys. Rev. Lett., 5 pages, 3 figures, 1 tabl
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