Simultaneous interaction of methyl radicals and atomic hydrogen with amorphous hydrogenated carbon films, as investigated with optical in situ diagnostics

Methyl radicals (CH₃) and atomic hydrogen (H) are dominant radicals in low-temperature plasmas from methane. The surface reactions of these radicals are believed to be key steps leading to deposition of amorphous hydrogenated carbon (a-C:H) films or polycrystalline diamond in these discharges. The underlying growth mechanism is studied, by exposing an a-C:H film to quantified radical beams of H and CH₃. The deposition or etching rate is monitored via ellipsometry and the variation of the stoichiometry is monitored via isotope labeling and infrared spectroscopy. It was shown recently that, at 320 K, methyl radicals have a sticking coefficient of 10^-4 on a-C:H films, which rises to 10^-2 if an additional flux of atomic hydrogen is present. This represents a synergistic growth mechanism between H and CH₃. From the interpretation of the infrared data, a reaction scheme for this type of film growth is developed: atomic hydrogen creates dangling bonds by abstraction of bonded hydrogen within a surface layer corresponding to the range of H in a-C:H films. These dangling bonds serve at the physical surface as adsorption sites for incoming methyl radicals and beneath the surface as radicalic centers for polymerization reactions leading to carbon–carbon bonds and to the formation of a dense a-C:H film. Received: 18 July 2000 / Accepted: 12 December 2000 / Published online: 3 April 2001     

Cluster growth processes by direct simulation monte carlo method

Thin films obtained by cluster deposition have attracted strong attention both as a new manufacturing technique to realize high-density magnetic recording media and to create systems with unique magnetic properties. Because the film’s features are influenced by the cluster properties during the flight path, the relevant physical scale to be studied is as large as centimeters. In this paper, a new model of cluster growth processes based on a combination of the Direct Simulation Monte Carlo (DSMC) method and the cluster growth model is introduced to examine the effects of experimental conditions on cluster growth by an adiabatic expansion process. From the macroscopic viewpoint, we simulate the behavior of clusters and inert gas in the flight path under different experimental conditions. The internal energy of the cluster, which consists of rotational and vibrational energies, is limited by the binding energy which depends on the cluster size. These internal and binding energies are used as criteria of the cluster growth. The binding energy is estimated by surface and volume terms. Several types of size distribution of generated clusters under various conditions are obtained by the present model. The results of the present numerical simulations reveal that the size distribution is strongly related to the experimental conditions and can be controlled. Received: 23 January 2001 / Accepted: 3 May 2001 / Published online: 30 August 2001     

The model of ultrafast light-induced insulator-metal phase transition in VO2

Ultrafast light-induced insulator-metal phase transitions (PT) in VO₂ thin films was studied with use of a pump-probe technique. The theoretical and experimental study of PT kinetics shows that the PT could be realized via an intermediate state. The relaxation processes after optical pumping are dependent on pump energy. The excitonic controlled model for such type of PT is proposed. The main channel for the ultrafast light-induced PT is the resonant transition between excited states of correlated vibronic Wannier-Mott excitons (WME) in insulator phase and the unoccupied excited states in metallic phase. During this process an equilibrium local distortion occurred. According to the proposed model the experimental observation of the drastic temperature- and pump power- dependent relaxation processes could be interpreted.     

A novel type of domain walls with two-dimensional magnetization distribution in magnetic triaxial films

On the basis of numerical minimization of total energy in magnetic triaxial ferromagnetic films with a surface of a (1 1 0)-type, we investigated two-dimensional structures of domain walls within a rigorous micromagnetic approach that takes into account all the main interactions including the dipole–dipole one. Novel two-vortex and three-vortex domain wall structures are established to exist. The profiles of domain wall structures and their stability regions are studied.     

Determination of cross over effects in lattice models from the local height difference distribution

Growth of interfaces during vapor deposition is analyzed on a discrete lattice. It leads to finding distribution of local heights, measurable for any lattice model. Invariance in the change of this distribution in time is used to determine the cross over effects in various models. The analysis is applied to the discrete linear growth equation and Kardar-Parisi-Zhang (KPZ) equation. A new model is devised that shows early convergence to the KPZ dynamics. Various known conservative and non conservative models are tested on a one dimensional substrate by comparing the growth results with the exact KPZ and linear growth equation results. The comparison helps in establishing the condition that determines the presence of cross over effect for the given model. The new model is used in (2+1) dimensions to predict close to the true value of roughness constant for KPZ equation.     

Magnetization anomalies of nanosize zinc ferrite particles prepared using electrodeposition

Nanosize zinc ferrite particles have been prepared for the first time using electrodeposition. Zinc and iron are deposited on the cathode from a common bath containing the salts of zinc and iron. The deposited materials were forced to undergo electrochemical oxidation in a strong alkaline solution (1 M KOH) to convert them into oxides. Crystallization in ZnFe₂O₄ structure was obtained by heating the deposited material at appropriate temperature. X-ray diffraction pattern confirmed that the procedure leads to the formation of pure phase of ZnFe₂O₄. The magnetization value for the smallest size ZnFe₂O₄ is much smaller than that for the ZnFe₂O₄ made by most of the other methods although it shows a nice hysteresis shape. The magnetization shows very little variation with size in the range studied.     

Catalytic behaviors of ruthenium dioxide films deposited on ferroelectrics substrates, by spin coating process

Catalytic ruthenium dioxide films were deposited by spin-coating process on ferroelectric films mainly constituted of SrBi₂Ta₂O₉ (SBT) and Ba₂NaNb₅O₁₅ (BNN) phases. After thermal treatment under air, these ferroelectric-catalytic systems were characterized by X-ray diffraction and scanning electron microscopy (SEM). SEM images showed that RuO₂ film morphology depended on substrate nature. A study of CH₄ conversion into CO₂ and H₂O was carried out using these catalytic-ferroelectric multilayers: the conversion was analyzed from Fourier transform infrared (FTIR) spectroscopy, at various temperatures. Improved catalytic properties were observed for RuO₂ films deposited on BNN oxide layer.     

A simple model of DNA denaturation and mutually avoiding walks statistics

Recently Garel, Monthus and Orland [Europhys. Lett. 55, 132 (2001)] considered a model of DNA denaturation in which excluded volume effects within each strand are neglected, while mutual avoidance is included. Using an approximate scheme they found a first order denaturation. We show that a first order transition for this model follows from exact results for the statistics of two mutually avoiding random walks, whose reunion exponent is c > 2, both in two and three dimensions. Analytical estimates of c due to the interactions with other denaturated loops, as well as numerical calculations, indicate that the transition is even sharper than in models where excluded volume effects are fully incorporated. The probability distribution of distances between homologous base pairs decays as a power law at the transition. Received 8 July 2002 / Received in final form 25 July 2002 Published online 17 September 2002     

Random quantum states: recent developments and applications

We review the methods and use of random quantum states with particular emphasis on recent theoretical developments and applications in various fields. The guiding principle of the review is the idea that random quantum states can be understood as classical probability distributions in the Hilbert space of the associated quantum system. We show how this central concept connects questions of physical interest that cover different fields such as quantum statistical physics, quantum chaos, mesoscopic systems of both non-interacting and interacting particles, including superconducting and spin–orbit phenomena, and stochastic Schrödinger equations describing open quantum systems.     

Growth of attached actin filaments

In several studies of actin-based cellular motility, the barbed ends of actin filaments have been observed to be attached to moving obstacles. Filament growth in the presence of such filament-obstacle interactions is studied via Brownian dynamics simulations of a three-dimensional energy-based model. We find that with a binding energy greater than 24k _B T and a highly directional force field, a single actin filament is able to push a small obstacle for over a second at a speed of half of the free filament elongation rate. These results are consistent with experimental observations of plastic beads in cell extracts. Calculations of an external force acting on a single-filament-pushed obstacle show that for typical in vitro free-actin concentrations, a 3pN pulling force maximizes the obstacle speed, while a 4pN pushing force almost stops the obstacle. Extension of the model to treat beads propelled by many filaments suggests that most of the propulsive force could be generated by attached filaments.