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AFM Dissipation Topography of Soliton Superstructures in Adsorbed Overlayers
In the atomic force microscope, the nanoscale force topography of even
complex surface superstructures is extracted by the changing vibration
frequency of a scanning tip. An alternative dissipation topography with similar
or even better contrast has been demonstrated recently by mapping the
(x,y)-dependent tip damping but the detailed damping mechanism is still
unknown. Here we identify two different tip dissipation mechanisms: local
mechanical softness and hysteresis. Motivated by recent data, we describe both
of them in a onedimensional model of Moire' superstructures of incommensurate
overlayers. Local softness at "soliton" defects yields a dissipation contrast
that can be much larger than the corresponding density or corrugation contrast.
At realistically low vibration frequencies, however, a much stronger and more
effective dissipation is caused by the tip-induced nonlinear jumping of the
soliton, naturally developing bistability and hysteresis. Signatures of this
mechanism are proposed for experimental identification.Comment: 5 pages, 5 figures, Phys Rev B 81, 045417 (2010
Observable non-gaussianity from gauge field production in slow roll inflation, and a challenging connection with magnetogenesis
In any realistic particle physics model of inflation, the inflaton can be
expected to couple to other fields. We consider a model with a dilaton-like
coupling between a U(1) gauge field and a scalar inflaton. We show that this
coupling can result in observable non-gaussianity, even in the conventional
regime where inflation is supported by a single scalar slowly rolling on a
smooth potential: the time dependent inflaton condensate leads to amplification
of the large-scale gauge field fluctuations, which can feed-back into the
scalar/tensor cosmological perturbations. In the squeezed limit, the resulting
bispectrum is close to the local one, but it shows a sizable and characteristic
quadrupolar dependence on the angle between the shorter and the larger modes in
the correlation. Observable non-gaussianity is obtained in a regime where
perturbation theory is under control. If the gauge field is identified with the
electromagnetic field, the model that we study is a realization of the
magnetogenesis idea originally proposed by Ratra, and widely studied. This
identification (which is not necessary for the non-gaussianity production) is
however problematic in light of a strong coupling problem already noted in the
literature.Comment: 28 pages, no figures. Final versio
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