Vibrational dynamics and surface structure of amorphous selenium.

In contrast to crystalline solids in which structural order governs dynamics and thermodynamics, the lack of long-range periodicity in amorphous materials is responsible for several anomalies. Although the relation between these anomalies and the 'bulk structure' is generally understood, the surface structure and the corresponding vibrational spectrum of amorphous solids is practically an unexplored theme. In this study, we resolve the differences in vibrational dynamics and atomic structure between bulk and surface (top 5  nm) atoms of amorphous selenium.

Collective excitations in supercritical fluids: analytical and molecular dynamics study of "positive" and "negative" dispersion.

The approach of generalized collective modes is applied to the study of dispersion curves of collective excitations along isothermal lines of supercritical pure Lennard-Jones fluid. An effect of structural relaxation and other nonhydrodynamic relaxation processes on the dispersion law is discussed. A simple analytical expression for the dispersion law in the long-wavelength region of acoustic excitations is obtained within a three-variable viscoelastic model of generalized hydrodynamics.

Bacterial ratchet motors.

Self-propelling bacteria are a nanotechnology dream. These unicellular organisms are not just capable of living and reproducing, but they can swim very efficiently, sense the environment, and look for food, all packaged in a body measuring a few microns. Before such perfect machines can be artificially assembled, researchers are beginning to explore new ways to harness bacteria as propelling units for microdevices.

Competing interactions in arrested States of colloidal clays.

Using experiments, theory and simulations, we show that the arrested state observed in a colloidal clay at high concentrations is stabilized by screened Coulomb repulsion (Wigner glass). Dilution experiments allow us to distinguish this disconnected state, which melts upon addition of water, from a low-concentration gel state, which does not melt. Theoretical modeling and simulations at high concentrations reproduce the measured small angle x-ray scattering static structure factors and confirm the long-range electrostatic nature of the arrested structure.

Collective thermal diffusion of silica colloids studied by nonlinear optics.

We investigate the collective thermal diffusion of silica charged spheres in Sulpho-Rhodamine B/water solution at different concentrations by measuring time-dependent thermal and Soret lensings. We show a significant concentration-dependence of the thermal diffusion coefficient D(T), not previously reported. Moreover, the results clearly show that both mass diffusion and Soret coefficient are collective quantities being strongly dependent on the particles' packing fraction.

Comparison of Faxén's correction for a microsphere translating or rotating near a surface.

Boundary walls in microfluidic devices have a strong influence on the fluid flow and drag forces on moving objects. The Stokes drag force acting on a sphere translating in the fluid is increased by the presence of a neighboring wall by a factor given by Faxén's correction. A similar increase in the rotational drag is expected when spinning close to a wall.

Colloidal attraction induced by a temperature gradient.

Colloidal crystals are of extreme importance for applied research and for fundamental studies in statistical mechanics. Long-range attractive interactions, such as capillary forces, can drive the spontaneous assembly of such mesoscopic ordered structures. However, long-range attractive forces are very rare in the colloidal realm.

Observation of a gradient catastrophe generating solitons.

We investigate the propagation of a dark beam in a defocusing medium in the strong nonlinear regime. We observe for the first time a shock fan filled with noninteracting one-dimensional gray solitons that emanates from a gradient catastrophe developing around a null of the optical intensity. This scenario turns out to be very robust, persisting also when the material nonlocal response averages the nonlinearity over dimensions much larger than the emerging soliton filaments.

Phase diagram and complexity of mode-locked lasers: from order to disorder.

We investigate mode-locking processes in lasers displaying a variable degree of structural randomness. By a spin-glass theoretic approach, we analyze the mean-field Hamiltonian and derive a phase diagram in terms of pumping rate and degree of disorder. Paramagnetic (noisy continuous wave emission), ferromagnetic (standard passive mode locking), and spin-glass phases with an exponentially large number of configurations are identified.

Self-starting micromotors in a bacterial bath.

Micromotors pushed by biological entities, such as motile bacteria, constitute a fascinating way to convert chemical energy into mechanical work at the micrometer scale. Here we show, by using numerical simulations, that a properly designed asymmetric object can be spontaneously set into the desired motion when immersed in a chaotic bacterial bath. Our findings open the way to conceive new hybrid microdevices exploiting the mechanical power production of bacterial organisms.

Time-dependent nonlinear optical susceptibility of an out-of-equilibrium soft material.

We investigate the time-dependent nonlinear optical absorption of a clay dispersion (Laponite) in an organic dye (rhodamine B) water solution displaying liquid-arrested state transition. Specifically, we determine the characteristic time tauD of the nonlinear susceptibility buildup due to the Soret effect. By comparing tauD with the relaxation time provided by standard dynamic light scattering measurements we report on the decoupling of the two collective diffusion times at the two very different length scales during the aging of the out-of-equilibrium system.

Autoimmunity as a possible cause of growth hormone deficiency.

A possible autoimmune aggression to pituitary somatotrophs has been suggested by the occurrence of antipituitary antibodies (APA) directed against GH-secreting cells in some cases of GH deficiency (GHD) both in adults and in children and in some patients with autoimmune poliendocrine syndrome. We also detected APA in some patients with idiopathic short stature (ISS) and suggested that the presence of these antibodies could identify those of them prone to develop GHD. In fact, patients with ISS, resulted positive for APA at the first observation, during a longitudinal follow-up showed an impaired GH response to the stimuli in subsequent years suggestive of acquired GHD.

Prigogine-Defay ratio for an ionic glass-former: molecular dynamics simulations.

The pressure dependence of the glass-transition temperature, Tg(P), of the ionic glass-former 2Ca(NO3)2 x 3KNO3, CKN, has been obtained by molecular dynamics (MD) simulations The liquid-glass difference of thermal expansivity, deltaalpha, heat capacity, deltaCp, and isothermal compressibility, deltak, have been calculated as a function of pressure. It has been found that the Ehrenfest relation dTg/dP = TVdeltaalpha/deltaCp predicts the pressure dependence of Tg, but the other Ehrenfest relation, dTg/dP = deltakappa/deltaalpha, does not. Consequently, the Prigogine-Defay ratio, pi = deltaCpdeltakappa/TVdeltaalpha2, is n pi approximately 1.

High frequency dynamics in liquids and supercritical fluids: A comparative inelastic x-ray scattering study.

The microscopic dynamics of four prototype systems (water, ammonia, nitrogen, and neon) across the critical temperature has been investigated by means of high-resolution inelastic x-ray scattering. The experimental line shape has been described using a model based on the memory function formalism. Two main relaxations, the thermal and the structural one, were observed in all the investigated systems.

Ultrashort pulse propagation and the Anderson localization.

We investigate the dynamics of a 10 fs light pulse propagating in a random medium by the direct solution of the three-dimensional Maxwell equations. Our approach employs molecular dynamics to generate a distribution of spherical scatterers and a parallel finite-difference time-domain code for the vectorial wave propagation. We calculate the disorder-averaged energy velocity and the decay time of the transmitted pulse versus the localization length for an increasing refractive index.

Condensation in disordered lasers: theory, 3D+1 simulations, and experiments.

The complex processes underlying the generation of a coherent emission from the multiple scattering of photons and wave localization in the presence of structural disorder are still mostly unexplored. Here we show that a single nonlinear Schrödinger equation, playing the role of the Schwalow-Townes law for standard lasers, quantitatively reproduces experimental results and three-dimensional time-domain parallel simulations of a colloidal laser system.

Hydrodynamic interactions in two dimensions.

We measure hydrodynamic interactions between colloidal particles confined in a thin sheet of fluid. The reduced dimensionality, compared to a bulk fluid, increases dramatically the range of couplings. Using optical tweezers we force a two body system along the eigenmodes of the mobility tensor and find that eigenmobilities change logarithmically with particle separation.

Phase diagram of a solution undergoing inverse melting.

The phase diagram of alpha -cyclodextrin/water/4-methylpyridine solutions, a system undergoing inverse melting, has been studied by differential scanning calorimetry, rheological methods, and x-ray diffraction. Two different fluid phases separated by a solid region have been observed in the high alpha -cyclodextrin concentration range (c > or =150 mg/ml) . Decreasing c , the temperature interval where the solid phase exists decreases and eventually disappears, and a first-order phase transition is observed between the two different fluid phases.

Free-energy transition in a gas of noninteracting nonlinear wave particles.

We investigate the dynamics of a gas of noninteracting particlelike soliton waves, demonstrating that phase transitions originate from their collective behavior. This is predicted by solving exactly the nonlinear equations and by employing methods of the statistical mechanics of chaos. In particular, we show that a suitable free energy undergoes a metamorphosis as the input excitation is increased, thereby developing a first-order phase transition whose measurable manifestation is the formation of shock waves.

Dynamically correlated regions and configurational entropy in supercooled liquids.

When a liquid is cooled below its melting temperature, if crystallization is avoided, it forms a glass. This phenomenon, called glass transition, is characterized by a marked increase of viscosity, about 14 orders of magnitude, in a narrow temperature interval. The microscopic mechanism behind the glass transition is still poorly understood.

Role of saddles in topologically driven phase transitions: the case of the d -dimensional spherical model.

Analyzing the d -dimensional spherical model, we show that underlying saddles, defined through a map in the configuration space, play a central role in driving the phase transition. At the phase transition point the underlying saddle energy reaches its lowest value, corresponding to the trivial boundary topological singularity.

High frequency dynamics in liquid nickel: An inelastic x-ray scattering study.

Owing to their large relatively thermal conductivity, peculiar, nonhydrodynamic features are expected to characterize the acousticlike excitations observed in liquid metals. We report here an experimental study of collective modes in molten nickel, a case of exceptional geophysical interest for its relevance in earth interior science. Our result shed light on previously reported contrasting evidences: In the explored energy-momentum region, no deviation from the generalized hydrodynamic picture describing nonconductive fluids is observed.

Mode competitions and dynamical frequency pulling in Mie nanolasers: 3D ab-initio Maxwell-Bloch computations.

We investigate the process of light matter interaction in a spherical Mie nanolaser. We derive a rigorous theory based on a three dimensional vector set of Maxwell-Bloch equations and solve the resulting equations through a parallel Finite-Difference Time-Domain Maxwell- Bloch (FDTD-MB) code. Our results predicts a rich physical scenario, ranging from nontrivial vectorial energy matter interplay in the pre-lasing regime to mode competitions and dynamical frequency pulling phenomena.

Nonlinear optics in the X-ray regime: nonlinear waves and self-action effects.

We study the nonlinear refraction of X-rays in highly ionized condensed matter by using a classical model of a cold electron plasma in a lattice of still ions coupled with Maxwell equations. We discuss the existence and stability of nonlinear waves. As a real-world example, we consider beam self-defocusing in crystalline materials (B, C, Li, Na).