Semianalytical solution of the problem of converging shock waves

An approach to the determination of the self-similarity parameter in the problem of converging strong shock waves is suggested. This approach allows one to obtain analytical expressions that approximate the numerical solution. For adiabatic constants gamma = 6/5-7, the values of the obtained self-similarity parameter differ by <1% from the values determined by the numerical procedure. In addition, accurate analytical characteristics of the reflected shock wave are obtained.     

Excitation of a d-Density wave in an optical lattice with driven tunneling

Quantum phases with unusual symmetries may play a key role in the understanding of solid state systems at low temperatures. We propose a realistic scenario, well in reach of present experimental techniques, which should permit us to produce a stationary quantum state with d x2-y2 symmetry in a two-dimensional bosonic optical square lattice. This state, characterized by alternating rotational flux in each plaquette, arises from driven tunneling implemented by a stimulated Raman scattering process. We discuss bosons in a square lattice; however, more complex systems involving other lattice geometries appear possible.     

Interplay between spinodal decomposition and glass formation in proteins exhibiting short-range attractions

We investigate the competition between spinodal decomposition and dynamical arrest using aqueous solutions of the globular protein lysozyme as a model system for colloids with short-range attractions. We show that quenches below a temperature Ta lead to gel formation as a result of a local arrest of the protein-dense phase during spinodal decomposition. The rheological properties of these gels allow us to use centrifugation experiments to determine the local densities of both phases and to precisely locate the gel boundary and the attractive glass line close to and within the unstable region of the phase diagram.     

Correlation between the superconducting transition temperature and anisotropic quasiparticle scattering in Tl2Ba2CuO6+delta

Angle-dependent magnetoresistance measurements are used to determine the isotropic and anisotropic components of the transport scattering rate in overdoped Tl2Ba2CuO6+delta for a range of Tc values between 15 and 35 K. The size of the anisotropic scattering term is found to scale linearly with Tc, establishing a link between the superconducting and normal state physics. A comparison with results from angle resolved photoemission spectroscopy indicates that the transport and quasiparticle lifetimes are distinct.     

Translations and rotations are correlated in granular gases

In a granular gas of rough particles the axis of rotation is shown to be correlated with the translational velocity of the particles. The average relative orientation of angular and linear velocities depends on the parameters which characterize the dissipative nature of the collision. We derive a simple theory for these correlations and validate it with numerical simulations for a wide range of coefficients of normal and tangential restitution. The limit of smooth spheres is shown to be singular: even an arbitrarily small roughness of the particles gives rise to orientational correlations.     

Antiferromagnetic interactions in single crystalline Zn1-xCoxO thin films

In a rather contradictory situation regarding magnetic data on Co-doped ZnO, we have succeeded in fabricating high-quality single crystalline Zn(1-x)Co(x)O (x=0.003-0.07) thin films. This gives us the possibility, for the first time, to examine the intrinsic magnetic properties of ZnO:Co at a quantitative level and therefore to address several unsolved problems, the major one being the nature of the Co-Co interaction in the ZnO structure.     

Cl insertion on Si(100)-(2x1): etching under conditions of supersaturation

We use scanning tunneling microscopy to show that Cl2 dosing of Cl-saturated Si(100)-(2x1) surfaces at elevated temperature leads to uptake beyond "saturation" and allows access to a new etching pathway. This process involves Cl insertion in Si-Si dimer bonds or backbonds, diffusion of the inserted Cl, and ultimately desorption of SiCl2. Investigations into the etch kinetics reveal that insertion occurs via a novel form of Cl2 dissociative chemisorption that is mediated by dangling bond sites. Upon dissociation, one Cl atom adsorbs at the dangling bond while the other can insert.     

Towards a parameter-free characterization of charge transfer via hopping: the case of tris(8-hydroxyquinolato) aluminum

By using calculations based on density functional theory, we investigate the physical factors determining the elemental charge transfer in Alq3, taken as a prototype of molecular systems in condensed phase. The effect of the environment on the hopping of the charge carrier is evaluated self-consistently in a model in which an Alq3 dimer is embedded in an ensemble of permanent and polarizable dipoles, including orientational disorder and the presence of impurities. The results indicate that the origin of the activation barrier is mainly extrinsic and that the local orientation of the dipole moments plays a major role. The influence of nonadiabaticity is also studied and found to be more important for a hole than for an electron.     

Static and dynamic heterogeneities in a model for irreversible gelation

We study the structure and the dynamics in the formation of irreversible gels by means of molecular dynamics simulation of a model system where the gelation transition is due to the random percolation of permanent bonds between neighboring particles. We analyze the heterogeneities of the dynamics in terms of the fluctuations of the self-intermediate scattering functions: in the sol phase close to the percolation threshold, we find that this dynamic susceptibility increases with the time until it reaches a plateau. At the gelation threshold this plateau scales as a function of the wave vector k as k(eta-2), with eta being related to the decay of the percolation pair connectedness function. At the lowest wave vector, approaching the gelation threshold it diverges with the same exponent gamma as the mean cluster size. These findings suggest an alternative way of measuring critical exponents in a system undergoing chemical gelation.     

Spectral-fluctuations test of the quark-model baryon spectrum

We study the low-lying baryon spectrum (up to 2.2 GeV) provided by experiments and different quark models using statistical tools which allow us to postulate the existence of missing levels in spectra. We confirm that the experimental spectrum is compatible with random matrix theory, the paradigmatic model of quantum chaos, and we find that the quark models are more similar to a Poisson distribution, which is not compatible with what should be expected in a correlated spectrum. From our analysis it stems that the spectral fluctuation properties of quark-model spectra are incompatible with experimental data. This result can be used to enlighten the problem of missing resonances.