Martensitic transformations: first-principles calculations combined with molecular-dynamics simulations

Results are presented of first-principles total-energy calculations and molecular-dynamics simulations of structural transformations in magnetic transition metal alloys like Fe_1-xNi_x. While first-principles calculations allow to identify those structures having the lower total energy, molecular-dynamics simulations can be used to trace out the dependence of the transformation on temperature, composition, concentration of defects etc. We have used the method of the semi-empiric embedded-atom potential in the molecular-dynamics simulations which yields remarkable good results for the structural changes. Received: 3 February 1998 / Revised: 4 May 1998 / Accepted: 24 June 1998     

General plasma behavior in the energy relaxation of electrons in highly p-doped semiconductors

We present a systematic study of the dependence of the energy relaxation of photo-excited minority electrons on the doping concentration in highly p-doped GaAs. A nonmonotonic dependence is found in the region where the characteristics of the carrier-carrier interaction changes from plasmon-mediated to quasistatically screened. Using a detailed Monte-Carlo study we are able to attribute this observation to a general property of plasmas at high density. Received: 1st April 1998 / Revised: 6 May 1998 / Accepted: 18 May 1998     

VUV-spectroscopy of the plasma light emission generated by the pulsed arc cluster ion source (PACIS)

A pinhole grid spectrometer is used to measure the light emission from the plasma of the pulsed arc cluster ion source (PACIS). Spectra of various metals and carbon have been measured between 20 and 100 nm. In the case of carbon the average electron temperature is estimated to about 0.69 eV. Higher temperatures up to 0.79 eV are measured when inserting seeding gas which flushes the discharge volume with approx. one atmosphere of helium. An operation under this source conditions leads to the generation of an intense charged cluster beam. The application of the source as a bright light source in the VUV region is discussed. Received: 26 February 1998 / Revised: 12 May 1998 / Accepted: 14 May 1998     

Fluctuation-dissipation ratio in lattice-gas models with kinetic constraints

We investigate by Monte-Carlo simulation the linear response function of three dimensional structural glass models defined by short-range kinetic constraints and a trivial equilibrium Boltzmann-Gibbs measure. The breakdown of the fluctuation-dissipation theorem in the glassy phase follows the prediction of mean field low temperature mode-coupling theory. Received: 24 April 1998 / Received in final form: 8 May 1998 / Accepted: 11 May 1998     

Classical dynamical simulation of spontaneous alloying

“Spontaneous alloying” observed by Yasuda, Mori et al. for metallic small clusters is simulated using classical Hamiltonian dynamics. Very rapid alloying occurs homogeneously and cooperatively starting from the solid phase of the cluster if the heat of solution is negative and the size of cluster is less than a critical size. Analysis of 2D models reveals that the alloying rate obeys an Arrhenius-type law, which predicts the alloying time much less than second at room temperature. Evidences manifesting that the spontaneous alloying proceeds in the solid phase without melting are also presented. The simulation reproduces the essential features of the experiments. Received: 2 March 1998 / Revised: 21 May 1998 / Accepted: 28 May 1998     

Energies and widths of triply excited states of lithiumlike oxygen and neon

Seven low-lying triply exited states of lithium-like oxygen and neon are calculated with the multichannel saddle-point and saddle-point complex-rotation methods. The term energies are given for these excited states, along with level shifts and partial Auger widths from dominant decay channels. The mass polarization effect and relativistic corrections are included. The radiative transition rates are also calculated. These results are compared with other theoretical data in the literature. Received: 25 May 1998 / Revised: 28 July 1998 / Accepted: 25 August 1998     

A bound polaron in a spherical quantum dot

The binding energy of a bound polaron in a spherical quantum dot has been investigated by using the variational method. The influence of LO and SO phonons have taken into consideration. Result shows that the phonon contribution to the binding energy is dependent on the size of the quantum dot as well as the position of the impurity in the quantum dot. Numerical calculation on the ZnSe quantum dot shows that such contribution is about 5% to 20% of the total binding energy. Received: 13 October 1997 / Revised: 4 March 1998 / Accepted: 26 May 1998     

Nuclear fusion in charge-asymmetric muonic molecules

Nuclear fusion reactions in hydrogen-lithium muonic molecules, (where h=p,d,t) are considered and fusion rates from rotational states J=0 of the molecules are presented. Results obtained depend on the isotopic composition of the molecules and range between and . The upper limit for fusion rates from rotational states J=0 of hydrogen-helium muonic molecules, and , equal , is also found. Received: 4 December 1997 / Revised: 30 April 1998 / Accepted: 7 May 1998     

New considerations on the role of covalency in ferroelectric niobates and tantalates

We consider the interaction of an homogeneous polyelectrolyte with an obstacle during electrophoretic drift. We explicitly take into account the hydrodynamic interactions generated by this mechanical trapping, and we evaluate their influence on the unhooking process. Important qualitative effects are pointed out in low and moderate field regimes. However, numerical simulations indicate that, in strong field, the existing simpler local force models, which neglect these hydrodynamic interactions, are quantitatively acceptable. Received: 18 March 1998 / Received in final form and accepted: 20 May 1998     

Local field dynamics in a resonant quantum tunneling system of magnetic molecules

We observed non-exponential relaxation for a quantum tunneling molecular magnetic system at very low temperatures and argue that it results from evolving intermolecular dipole fields. At the very beginning of the relaxation, the magnetization follows a square-root time dependence. A simple model is developed for the intermediate time range that is in good agreement with the data over 4 decades in time. Detailed numerical calculations as well as measurements are presented which indicate unusual correlation effects in these systems. Received: 15 May 1998 / Revised: 10 July 1998 / Accepted: 11 July 1998     

Photo induced collisions with laser cooled He* atoms

This paper presents an experimental investigation of cold collisions between metastable helium atoms in an optical trap at 1 mK. Penning (PI) and associative (AI) ionization reactions are distinguished using a mass spectrometer and studied under influence of near resonant laser light. Dramatic enhancement, by more than a factor 15, of the ion rate is observed when the laser is tuned close to resonance. Experimental findings are well-described, on an absolute scale, by model predictions. Received: 9 February 1998 / Revised: 4 May 1998 / Accepted: 14 May 1998     

On the finite-field transition dipole moment calculations by effective Hamiltonian methods

A simple finite-field scheme of calculations on electronic transition dipole moments in molecules by effective Hamiltonian methods is presented and discussed. The reliability of underlying approximations is analyzed by means of the quasidegenerate perturbation theory and corroborated by the results of pilot numerical applications. Received: 11 May 1998 / Revised and Accepted: 20 July 1998     

A new simulation method for the determination of forces in polymer/colloid systems

We introduce a new simulation method, which we call the contact-distribution method, for the determination of the Helmholtz potential for polymer/colloid systems from lattice Monte-Carlo simulations. This method allows one to obtain forces between finite or semi-infinite objects of any arbitrary shape and dimensions in the presence of polymer chains in solution or physisorbed or chemisorbed at interfaces. We illustrate the application of the method using two examples: (i) the interaction between the tip of an atomic force microscope (AFM) and a single, end-grafted polymer chain and (ii) the interaction between an AFM tip and a polymer brush. Numerical results for the first two cases illustrate how the method can be used to confirm and extend scaling laws for forces and Helmholtz potentials, to examine the effects of the shapes and sizes of the objects and to examine conformational transitions in the polymer chains. Received: 15 May 1998 / Revised: 11 June 1998 / Accepted: 12 June 1998     

Crystallized vortex crystals

Numerical simulations of the equations of motion of 300 charged particles confined to a plane with an additional magnetic field orthogonal to the plane reproduce recently observed self-organization of non-neutral plasmas into a small number of interacting vortices. In the presence of damping we observe crystallized vortices, i.e. vortices with regular internal structure. We also observe crystallized vortex crystals, i.e. geometric patterns of crystallized vortices. Fractal vortex arrangements are investigated and found to be stable. Our results are relevant for quantum dots and artificial atoms. Received: 24 February 1998 / Revised: 4 March 1998 / Accepted: 4 May 1998     

Ascending air bubbles in protein solutions

We report measurements of the ascending velocity of air bubbles in protein (bovine serum albumin) solutions. We show that, because of the protein molecules adsorbed on their surface, the terminal ascending velocity of bubbles is strongly reduced compared to the terminal velocity in pure water: protein- covered bubbles behave hydrodynamically as solid spheres. From the evolution of the ascending velocity with time, we can derive the amount of protein needed to immobilize the bubble interface which is 0.5 mg m^-2, i.e. only one fifth of the amount adsorbed at equilibrium in the range of used bulk concentrations. Received: 6 March 1998 / Revised and accepted: 6 May 1998