[Todos] Coloquios del Departamento de Física (jueves)

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                          COLOQUIOS DEL DEPARTAMENTO DE FÍSICA FCEyN - UBA

                                     http://coloquios.df.uba.ar/

                                        Charla, café y galletitas
                                   En el Aula Federman, 1er piso Pabellón I,
                                           Ciudad Universitaria


                                      Jueves 8 de julio, 14hs,


                       Metamateriales, Magnetismo a Altas Frecuencias
                              y el Argumento de Landau-Lifshitz


                                         Roberto Merlin

                                     University of Michigan




Homogeneous composites, or metamaterials, made of dielectric or  
metallic particles are known to show magnetic properties that  
contradict arguments by Landau and Lifshitz [Electrodynamics of  
Continuous Media (Pergamon Press, Oxford, 1960 ), p. 251] indicating  
that the magnetization and, thus, the permeability loses its meaning  
at relatively low frequencies. Here we show that these arguments do  
not apply to composites made of substances with ks = Im (sqrt(eps_s))  
 >> lambda/l or ns = Re(sqrt(eps_s)) sim lambda/l  (eps_s and l are  
the complex permittivity and the characteristic length of the  
particles, ks and ns are, respectively, the extinction coefficient and  
the refractive index, and lambda is the vacuum wavelength). Our  
general analysis is supported by studies of split-rings, one of the  
most common constituents of electromagnetic metamaterials, and  
spherical inclusions. An analytical solution is given to the problem  
of scattering by a small and thin split ring of arbitrary  
permittivity. Results reveal a close relationship between eps_s and  
the dynamic magnetic properties of metamaterials. For sqrt(eps_s) <<  
lambda/a  (a is the ring cross-sectional radius), the composites  
exhibit very weak magnetic activity, consistent with the  
Landau-Lifshitz argument and similar to that of molecular crystals. In  
contrast, large values of the permittivity lead to strong diamagnetic  
or paramagnetic behavior characterized by susceptibilities whose  
magnitude is significantly larger than that of natural substances. The  
double constraint ks >> lambda/l >> 1 (or ns sim lambda/l >> 1 if ks  
<< ns ) poses severe limitations for attaining magnetism at  
arbitrarily high frequencies. Because they have a large extinction  
coefficient, metals are to be favored at optical frequencies. Since  
eps_s sim -wp^2/w^2  for w tau >> 1, the constraint becomes lambda >>  
l >> lambda_p . The measured values of the permittivity for noble  
metals indicate that magnetism can coexist with the effective-medium  
condition for frequencies up to ~ 1.5 10^14 Hz (lambda ~ 2.5 um).



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