Electro-optical properties of excitons in polydiacetylene chains

We show how to compute the optical functions (reflectivity, transmission, and absorption) of polydiacetylene chains diluted in their monomer matrix exposed to a uniform electric field in the chain direction, in the excitonic energy region. Adopting a model electron-hole potential, we derived an analytical expression for the effective chain susceptibility, which gives the optical functions. The resulting absorption shows excitonic peaks below the gap and Franz-Keldysh oscillations above the gap. The method has been applied for a 3BCMU polydiacetylene chain, showing a good agreement with experimental spectra. Received 5 November 1998 and Received in final form 23 February 1999     

Photoinduced transformations and holographic recording in nanolayered a-Se/As2S3 and AsSe/As2S3 films

Received: 13 October 1998 / Accepted: 9 December 1998 / Published online: 24 February 1999     

Quantum-Monte-Carlo simulations of polyethylene

The diffusion Quantum-Monte-Carlo method of solving the Schr?dinger equation is applied to the vibrational ground state of a polyethylene molecule. The results for the ground state energy show good agreement with normal mode analysis. In addition to stretching, bending and torsional interaction van-der-Waals interaction is applied to a single chain showing a decrease of the energy of 5%. The decrease for a polyethylene system of 5 chains with 10 atoms per molecule at the positions of a unit cell is determined to be 4.8% per molecule. Finally first steps towards simulating excited states were performed. Received: 9 February 1998 / Revised: 2 April 1998 / Accepted: 23 April 1998     

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     

Interaction-assisted propagation of Coulomb-correlated electron-hole pairs in disordered semiconductors

A two-band model of a disordered semiconductor is used to analyze dynamical interaction induced weakening of localization in a system that is accessible to experimental verification. The results show a dependence on the sign of the two-particle interaction and on the optical excitation energy of the Coulomb-correlated electron-hole pair. Received 4 September and Received in final form 26 November 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     

Characterization of high-quality MBE-grown GaN films on intermediate-temperature buffer layers

High-quality GaN thin films are grown by rf-plasma assisted molecular beam epitaxy. The quality of the GaN epitaxial layer is significantly improved by using an intermediate-temperature GaN buffer layer (ITBL) in addition to a conventional 20-nm-thick low-temperature buffer layer. The GaN epitaxial layers demonstrate systematic improvements in the electron mobility increasing from 82 cm² V^-1 s^-1, for films grown with just the low-temperature buffer layer, to about 380 cm² V^-1 s^-1 for films grown with an ITBL of thickness 800 nm. The photoluminescence also indicates systematic improvements in the intensity and the full-width-half-maximum with the use of ITBL. Photoreflectance spectra are measured from the GaN films. Detailed analyses of the excitonic transition energy demonstrate that the residual strain relaxes rapidly with the use of ITBL, which is attributed to the observed improvements in the mobility and the PL spectra. Received: 30 November 2000 / Accepted: 4 December 2000 / Published online: 9 February 2001     

The lamellar-disorder interface: one-dimensional modulated profiles

We study interfacial behavior of a lamellar (stripe) phase coexisting with a disordered phase. Systematic analytical expansions are obtained for the interfacial profile in the vicinity of a tricritical point. They are characterized by a wide interfacial region involving a large number of lamellae. Our analytical results apply to systems with one dimensional symmetry in true thermodynamical equilibrium and are of relevance to metastable interfaces between lamellar and disordered phases in two and three dimensions. In addition, good agreement is found with numerical minimization schemes of the full free energy functional having the same one dimensional symmetry. The interfacial energy for the lamellar to disordered transition is obtained in accord with mean field scaling laws of tricritical points. Received: 28 March 1997 / Revised: 6 February 1998 / Accepted: 16 February 1998     

Longitudinal Stern-Gerlach effect for slow cesium atoms

The mechanical Stern-Gerlach effect is investigated in the case of a slow atomic cloud falling through an inhomogeneous magnetic field featuring a strong longitudinal gradient. The resulting Zeeman sublevel state selection is demonstrated under various experimental conditions. Longitudinal spatial separations are in agreement with numerical simulations that take into account the gravitational acceleration and both the transverse and axial magnetic forces. Since separations greater than 20 cm are obtained, potential applications in atom optics are outlined. Received: 16 February 1998 / Received in final form: 1 April 1998 / Accepted: 6 April 1998     

Interaction versus dimerization in one-dimensional Fermi systems

In order to study the effect of interaction and lattice distortion on quantum coherence in one-dimensional Fermi systems, we calculate the ground state energy and the phase sensitivity of a ring of interacting spinless fermions on a dimerized lattice. Our numerical DMRG studies, in which we keep up to 1000 states for systems of about 100 sites, are supplemented by analytical considerations using bosonization techniques. We find a delocalized phase for an attractive interaction, which differs from that obtained for random lattice distortions. The extension of this delocalized phase depends strongly on the dimerization induced modification of the interaction. Taking into account the harmonic lattice energy, we find a dimerized ground state for a repulsive interaction only. The dimerization is suppressed at half filling, when the correlation gap becomes large. Received: 11 February 1998 / Revised: 1st April 1998 / Accepted: 30 April 1998     

Electron detachment and fragmentation in collisions between 1.25 keV/carbon and clusters and

The destruction cross-section for 22.5 and 50 keV C^1- , for 10 and 50 keV C₈ ^1- and for 50 and 75 keV C₆₀ ^1- clusters in collisions with H₂ has been measured by an attenuation method. The destruction of the cluster anions is dominated by electron detachment rather than fragmentation and is of the order of the geometric cross-section. The cross-sections vary little with bombarding energy. Received: 16 September 1998 / Received in final form: 23 February 1999     

Coulomb blockade of the persistent current in a one-dimensional ballistic Luttinger liquid ring

The effect of an electrostatic energy (in the geometrical capacitance approach) on a persistent current is considered. It is shown that at high temperatures the current amplitude shows periodic dips as a function of the potential difference between a ring and a reservoir. These dips correspond to a lift of the Coulomb blockade. In a minimum of a dips a current is periodic in a magnetic flux with a period at any temperatures. Received: 6 April 1998 / Revised: 5 June 1998 / Accepted: 21 July 1998     

Truncated Lévy laws and 2D turbulence

We study the probability distribution functions and scaling properties of truncated Lévy processes with sharp cut-offs. We find that they display features analog to those observed in some 2D numerical simulations of turbulence. Received: 29 October 1997 / Revised: 12 February 1998 / Accepted: 10 April 1998     

Shear modulus and compliance in the range of the dynamic glass transition for metallic glasses

High-resolution synchrotron-radiation powder diffraction experiments were performed to observe structural changes induced by hydrogen loading in rapidly-quenched Ti-Zr-Ni alloy ribbons with dominant icosahedral character. Lattice expansion effects due to hydrogen storage in Ti-Zr-Ni quasicrystals as well as phonon and phason disorder coefficients are obtained from an analysis of diffraction linewidths. Received: 26 August 1997 / Revised: 8 January 1998 / Accepted: 10 February 1998     

Melting of columnar hexagonal DNA liquid crystals

The persistence length DNA hexagonal-cholesteric phase transition upon dilution and/or increase in solvent ionic strength is investigated with polarized light microscopy. The ionic strength dependence of the transition follows Lindemann criterion , i.e., the hexagonal lattice melts when the root-mean-square fluctuations in transverse order exceed 10% of the interaxial spacing. The spacings are derived from density and the fluctuations are estimated with a theory of undulation enhanced electrostatic interactions. Additional support for this theory is given by the DNA equation of state and anisotropic neutron radiation scattering from magnetically aligned cholesteric samples just below the phase transition. Received: 17 November 1997 / Revised: 21 January 1998 / Accepted: 25 February 1998     

A Fourier optics model of two-beam scanning laser interferometers

The article illustrates the use of Fourier optics to describe the operation of two-beam scanning laser interferometers. It deals with the effect of diffraction on the spatial periodicity of a monochromatic and coherent beam. Particular attention is given to the analysis of systematic errors in high-accuracy laser metrology. The article reviews the special case of plane wave and Gaussian illuminations, examines how beam truncation affects the period of traveling fringes and presents a general relation between the relative wavelength deviation and the impulse standard deviation of the photons. Received: 18 February 1998 / Revised: 21 October 1998 / Accepted: 16 November 1998     

The sharp peak-flat trough pattern and critical speculation

We find empirically a characteristic sharp peak-flat trough pattern in a large set of commodity prices. We argue that the sharp peak structure reflects an endogenous inter-market organization, and that peaks may be seen as local “singularities” resulting from imitation and herding. These findings impose a novel stringent constraint on the construction of models. Received: 2 February 1998 / Revised and accepted: 22 February 1998     

Quantum wires and quantum dots for neutral atoms

By placing changeable nanofabricated structures (wires, dots, etc.) on an atom mirror one can design guiding and trapping potentials for atoms. These potentials are similar to the electrostatic potentials which trap and guide electrons in semiconductor quantum devices like quantum wires and quantum dots. This technique will allow the fabrication of nanoscale atom optical devices. Received: 28 October 1997 / Revised: 17 February 1998 / Accepted: 17 July 1998     

Stability and fragmentation of multiply ionized clusters

The stability of neutral, singly and multiply ionized silicon clusters, (N = 2-7, M = 0, , , ), has been investigated using an ab initio density functional method. We show that the fragmentation effect significantly affects the structure of mass-spectra of multiply ionized silicon clusters. For clusters, the clusters with a large fragmentation energy are found to correspond to the high peaks at N = 4 and 6 in mass-spectra. For clusters, a peak at N = 5 in mass-spectra has been predicted to be especially high. Received: 9 June 1997 / Revised: 8 January 1998 / Accepted: 25 February 1998     

Photoluminescence in amorphous TiO2-PbO systems

Photoluminescence (PL) at room temperature has been achieved in amorphous thin films and powders of the TiO₂–PbO system. They were prepared by the polymeric precursor method with [PbO]/[TiO₂] molar ratios ranging from 0.0 to 1.0. The energy position of maximum PL emission and the PL intensity showed dependence on Pb concentration. The Pb addition suggests an increase in the number of non-bridging oxygens (NBO) in the amorphous TiO₂ network. These results support the relationship between photoluminescence and structure in TiO₂-based amorphous materials. Received: 7 February 2001 / Accepted: 8 February 2001 / Published online: 27 June 2001