10,107 research outputs found
Transport and Dissipation in Quantum Pumps
This paper is about adiabatic transport in quantum pumps. The notion of
``energy shift'', a self-adjoint operator dual to the Wigner time delay, plays
a role in our approach: It determines the current, the dissipation, the noise
and the entropy currents in quantum pumps. We discuss the geometric and
topological content of adiabatic transport and show that the mechanism of
Thouless and Niu for quantized transport via Chern numbers cannot be realized
in quantum pumps where Chern numbers necessarily vanish.Comment: 31 pages, 10 figure
Climate model simulation of winter warming and summer cooling following the 1991 Mount Pinatubo volcanic eruption
We simulate climate change for the 2-year period following the eruption of Mount Pinatubo in the Philippines on June 15, 1991, with the ECHAM4 general circulation model (GCM). The model was forced by realistic aerosol spatial-time distributions and spectral radiative characteristics calculated using Stratospheric Aerosol, and Gas Experiment II extinctions and Upper Atmosphere Research Satellite-retrieved effective radii. We calculate statistical ensembles of GCM simulations with and without volcanic aerosols for 2 years after the eruption for three different sea surface temperatures (SSTs): climatological SST, El Nino-type SST of 1991-1993, and La Nina-type SST of 1984-1986. We performed detailed comparisons of calculated fields with observations, We analyzed the atmospheric response to Pinatubo radiative forcing and the ability of the GCM to reproduce it with different SSTs. The temperature of the tropical lower stratosphere increased by 4 K because of aerosol absorption of terrestrial longwave and solar near-infrared radiation. The heating is larger than observed, but that is because in this simulation we did not account for quasi-biennial oscillation (QBO) cooling and the cooling effects of volcanically induced ozone depletion. We estimated that both QBO and ozone depletion decrease the stratospheric temperature by about 2 K. The remaining 2 K stratospheric warming is in good agreement with observations. By comparing the runs with the Pinatubo aerosol forcing with those with no aerosols, we find that the model calculates a general cooling of the global troposphere, but with a clear winter warming pattern of surface air temperature over Northern Hemisphere continents. This pattern is consistent with the observed temperature patterns. The stratospheric heating and tropospheric summer cooling are directly caused by aerosol radiative effects, but the winter warming is indirect, produced by dynamical responses to the enhanced stratospheric latitudinal temperature gradient. The aerosol radiative forcing, stratospheric thermal response, and summer tropospheric cooling do not depend significantly on SST. The stratosphere-troposphere dynamic interactions and tropospheric climate response in winter are sensitive to SST
Dynamics of a classical Hall system driven by a time-dependent Aharonov--Bohm flux
We study the dynamics of a classical particle moving in a punctured plane
under the influence of a strong homogeneous magnetic field, an electrical
background, and driven by a time-dependent singular flux tube through the hole.
We exhibit a striking classical (de)localization effect: in the far past the
trajectories are spirals around a bound center; the particle moves inward
towards the flux tube loosing kinetic energy. After hitting the puncture it
becomes ``conducting'': the motion is a cycloid around a center whose drift is
outgoing, orthogonal to the electric field, diffusive, and without energy loss
Mapping the spin-dependent electron reflectivity of Fe and Co ferromagnetic thin films
Spin Polarized Low Energy Electron Microscopy is used as a spin dependent
spectroscopic probe to study the spin dependent specular reflection of a
polarized electron beam from two different magnetic thin film systems:
Fe/W(110) and Co/W(110). The reflectivity and spin-dependent
exchange-scattering asymmetry are studied as a function of electron kinetic
energy and film thickness, as well as the time dependence. The largest value of
the figure of merit for spin polarimetry is observed for a 5 monolayer thick
film of Co/W(110) at an electron kinetic energy of 2eV. This value is 2 orders
of magnitude higher than previously obtained with state of the art Mini-Mott
polarimeter. We discuss implications of our results for the development of an
electron-spin-polarimeter using the exchange-interaction at low energy.Comment: 5 pages, 4 figure
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