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We study the ergodic -- non-ergodic transition in a generalized Dicke model with independent co- and counter rotating light-matter coupling terms. By studying level statistics, the average ratio of consecutive level spacings, and the quantum butterfly effect (out-of-time correlation) as a dynamical probe, we show that the ergodic -- non-ergodic transition in the Dicke model is a consequence of the proximity to the integrable limit of the model when one of the couplings is set to zero. This can be interpreted as a hint for the existence of a quantum analogue of the classical Kolmogorov-Arnold-Moser theorem. Besides, we show that there is no intrinsic relation between the ergodic -- non-ergodic transition and the precursors of the normal -- superradiant quantum phase transition., 5 pages, 4 figures

We investigate the hydrodynamic interaction in suspensions of charged colloidal silica spheres. The volume fraction as well as the range of the electrostatic repulsion between the spheres is varied. Using a combination of dynamic x-ray scattering, cross-correlated dynamic light scattering, and small angle x-ray scattering, the hydrodynamic function Hq is determined experimentally. The effective hydrodynamic interactions are found to be screened, if the range of the direct interaction is relatively long and the static density correlations are strong. This observation of effective hydrodynamic screening is in marked contrast to hard-sphere-like systems. Colloidal suspensions have intriguing static and dynamic properties due to their composite nature and the mesoscopic length scales involved. Colloids interact by direct forces, such as electrostatic or hard-sphere repulsions, and hydrodynamic forces mediated by the suspending fluid [1,2]. Whereas the static properties of colloidal suspensions are strikingly similar to those of molecular fluids, the important role of the suspending medium leads to significant differences in the dynamical behavior. The complex viscoelastic and diffusional properties of colloidal suspensions depend on the interplay between direct and hydrodynamic interactions. This interplay is governed by the ratio of the different length scales in the system [3], such as particle radius, interparticle distance, and the range of the interactions. Unraveling these fascinating effects is

r 3 divergence at the origin. To incorporate these microscopic interactions local-field corrections are introduced. The approach is still macroscopic in nature, but the electric field at the radiating dipole, the local field, is different from the macroscopic field to incorporate local interactions. Macroscopic derivations of the local-field correction often imply the use of a cavity around the radiating dipole. The specific choice of the cavity is subtle matter [10], greatly complicating the interpretation of these models. Two limiting cases for a local-field model have been proposed. The empty-cavity model [11], in which the atom is inside a real empty spherical cavity in the dielectric, leads to

We present results of experiments on the transport of light through thin random media. Total transmission and time-resolved propagation measurements were performed using strongly scattering samples of varying thickness $L$. For $L/\ensuremath{\ell}l8$, where $\ensuremath{\ell}$ is the transport mean free path, the observed decay times from the long-time exponential behavior exhibit strong deviations from diffusion theory and radiative transport theory, whereas the total transmission measurements do not. For the thinnest sample $(L/\ensuremath{\ell}\ensuremath{\simeq}2)$ a reduction of the diffusion coefficient with a factor of 2 was observed.

We present theoretical and experimental results of the leading order angular correlations of second harmonic light generated inside random media, for both the transmission and reflection geometries. We find the striking result that correlations in reflection of the second harmonic light scales with the sample thickness L, in sharp contrast to the corresponding short-range correlation function in reflection of the fundamental light in the linear scattering regime, which scales with mean free path ${\mathit{l}}^{\mathrm{*}}$.

Light scattering from resonant two-level atoms on three-dimensional lattices can be described by a classical energy conserving t-matrix. The optical band structure of lattices filled with such atoms are calculated exactly in scalar approximation and displays, e.g., the formation of polariton gaps. The Einstein coefficient for spontaneous emission in a system with an inhomogeneous dielectric constant is shown to be proportional to part of the total density of states. This part is calculated for the dipolar lattice model.

We experimentally study the effects of correlations in the propagation of ultrasonic waves in water from a multielement source to a multielement detector through a strongly scattering system of randomly placed vertical rods. Due to the strong scattering, the wave transport in the sample is in the diffusive regime. The correlation between the waves is induced when the distance between transducer elements is within the coherence region of the scattered sound. We measure the multichannel transfer matrix H, each element of which represents the signal strength between the m individual transmitters and n receivers. The observed eigenvalue distribution of the matrix HH dagger clearly shows the effect of correlations between channels and can be interpreted using random matrix theory. These results are of practical importance in many areas, e.g., for evaluating the information transfer capacity of such a complex scattering system.