366 research outputs found
On the generalized entropy pseudoadditivity for complex systems
We show that Abe's general pseudoadditivity for entropy prescribed by thermal
equilibrium in nonextensive systems holds not only for entropy, but also for
energy. The application of this general pseudoadditivity to Tsallis entropy
tells us that the factorization of the probability of a composite system into
product of the probabilities of the subsystems is just a consequence of the
existence of thermal equilibrium and not due to the independence of the
subsystems.Comment: 8 pages, no figure, RevTe
Nonlinear Acoustics at GHz Frequencies in a Viscoelastic Fragile Glass Former
Using a picosecond pump-probe ultrasonic technique, we study the propagation
of high-amplitude, laser-generated longitudinal coherent acoustic pulses in the
viscoelastic fragile glass former DC704. We observe an increase of almost ten
percent in acoustic pulse propagation speed of its leading shock front at the
highest optical pump fluence which is a result of the supersonic nature of
nonlinear propagation in the viscous medium. From our measurement we deduce the
nonlinear acoustic parameter of the glass former in the GHz frequency range
across the glass transition temperature.Comment: 5 pages, 3 figure
Time-domain Brillouin scattering assisted by diffraction gratings
Absorption of ultrashort laser pulses in a metallic grating deposited on a
transparent sample launches coherent compression/dilatation acoustic pulses in
directions of different orders of acoustic diffraction. Their propagation is
detected by the delayed laser pulses, which are also diffracted by the metallic
grating, through the measurement of the transient intensity change of the first
order diffracted light. The obtained data contain multiple frequency components
which are interpreted by considering all possible angles for the Brillouin
scattering of light achieved through the multiplexing of the propagation
directions of light and coherent sound by the metallic grating. The emitted
acoustic field can be equivalently presented as a superposition of the plane
inhomogeneous acoustic waves, which constitute an acoustic diffraction grating
for the probe light. Thus, the obtained results can also be interpreted as a
consequence of probe light diffraction by both metallic and acoustic gratings.
The realized scheme of time-domain Brillouin scattering with metallic grating
operating in reflection mode provides access to acoustic frequencies from the
minimal to the maximal possible in a single experimental configuration for the
directions of probe light incidence and scattered light detection. This is
achieved by monitoring of the backward and forward Brillouin scattering
processes in parallel. Applications include measurements of the acoustic
dispersion, simultaneous determination of sound velocity and optical refractive
index, and evaluation of the samples with a single direction of possible
optical access.Comment: 21 pages, 4 figures, 1 tabl
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