Optical potential discrete variable representation method in the adiabatic representation

The optical potential discrete variable representation method (OP-DVR) has been applied recently to calculate resonances in the framework of the diabatic representation [J. Chem. Phys. 101, 7580 (1994)]. This method is based on the conjoint use of the discrete variable representation (DVR) method and the properties of a complex absorbing potential (CAP). The OP-DVR method is the DVR version of the CAP stabilization method initially proposed by Jolicard and Austin [Chem. Phys. Lett. 121, 106 (1985)]. In the present study, we show that this efficient and accurate method can also be applied within the adiabatic representation since it allows one to overcome in a simple way, numerical difficulties associated with the first derivative operator which appears in the expression of non adiabatic couplings. Within the OP-DVR method, the choice of the representation (diabatic or adiabatic) is governed by physical arguments and by the fact that the potentials and the couplings are known in one or the other of these two representations. In the case where the potentials and the couplings are obtained in the adiabatic representation, we show in this paper that the transformation into the diabatic framework is not necessary. We demonstrate that the discrete variable representation can be a simple and an efficient way to deal with the adiabatic representation. Received: 30 April 1998 / Revised: 29 September 1998 /Accepted: 21 October 1998     

The smooth structural change in mesoscopic Peierls chains

We investigate the Peierls transition in finite chains by exact (Lanczos) diagonalization and within a seminumerical method based on the factorization of the electron-phonon wave function (Adiabatic Ansatz, AA). AA can be applied for mesoscopic chains up to micrometer sizes and its reliability can be checked self-consistently. Our study demonstrates the important role played for finite systems by the tunneling in the double well potential. The chains are dimerized only if their size N exceeds a critical value N_c which increases with increasing phonon frequency. Quantum phonon fluctuations yield a broad transition region. This smooth Peierls transition contrasts not only to the sharp mean field transition, but also with the sharp RPA soft mode instability, although RPA partially accounts for quantum phonon fluctuations. For weak coupling the dimerization disappears below micrometer sizes; therefore, this effect could be detected experimentally in mesoscopic systems. Received: 3 January 1998 / Revised: 13 March 1998 / Accepted: 3 April 1998     

Floquet theory for short laser pulses

We develop adiabatic perturbation theory for quantum systems responding to short laser pulses, with or without a frequency chirp. Our approach rests on lifting the time-dependent Schr?dinger equation to an extended Hilbert space, then applying standard perturbational techniques to Floquet states in this extended space, and finally projecting back to the physical Hilbert space. The same strategy also allows us to construct superadiabatic bases for monitoring the quantum evolution in the course of a pulse. These bases provide a diagnostic tool for improving the efficiency of pulse-induced population transfer. The formalism is applied to the selective excitation of molecular vibrational states by chirped laser pulses, which exploit either successive single-photon resonances or a multiphoton resonance, and by a STIRAP-like process. Received: 23 June 1998 / Revised: 18 August 1998 / Accepted: 25 August 1998     

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     

Correlations of conductance peaks and transmission phases in deformed quantum dots

We investigate the Coulomb blockade resonances and the phase of the transmission amplitude of a deformed ballistic quantum dot weakly coupled to leads. We show that preferred single-particle levels exist which stay close to the Fermi energy for a wide range of values of the gate voltage. These states give rise to sequences of Coulomb blockade resonances with correlated peak heights and transmission phases. The correlation of the peak heights becomes stronger with increasing temperature. The phase of the transmission amplitude shows lapses by between the resonances. Implications for recent experiments on ballistic quantum dots are discussed. Received 17 July 1998     

Novel examples of cluster-matter produced by LUCAS, a new laser cluster source

In this paper a detailed experimental study of Coherent Population Trapping (CPT) effect on sodium induced by a dye-laser operating in a three-mode regime is presented and a detailed analysis of the role of velocity changing collisions is made. These collisions show a very small relaxation effect on the dark resonances which are visible even at high pressures. For the first time we demonstrate the persistence of the ground state coherence to pressures up to one atmosphere for a relatively “heavy” buffer gas like argon. The experimental results have been compared with theory and a very good agreement has been obtained. Preliminary results on the effect of Na-N₂ collisions on Coherent Population Trapping are presented. Received: 5 October 1998 / Received in final form: 3 December 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     

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     

Identification of baryon resonances in central heavy-ion collisions at energies between 1 and 2 AGeV

The mass distributions of baryon resonances populated in near-central collisions of Au on Au and Ni on Ni are deduced by defolding the p_t spectra of charged pions by a method which does not depend on a specific resonance shape. In addition the mass distributions of resonances are obtained from the invariant masses of (p, π^±) pairs. With both methods the deduced mass distributions are shifted by an average value of −60 MeV/c² relative to the mass distribution of the free Δ(1232) resonance, the distributions descent almost exponentially towards mass values of 2000 MeV/c². The observed differences between (p, π⁻) and (p, π⁺) pairs indicate a contribution of isospin I = 1/2 resonances. The attempt to consistently describe the deduced mass distributions and the reconstructed kinetic energy spectra of the resonances leads to new insights about the freeze out conditions, i.e. to rather low temperatures and large expansion velocities. Received: 26 June 1998 / Revised version: 2 September 1998     

Resonant widths, line intensities and lineshapes for MQDT models with two or more open channels

Approximate analytical formulae describing the energy variation of line intensities, autoionization widths and lineshape asymmetries, are derived for a Phase-Shifted Multichannel Quantum Defect Theory model composed of two closed interacting channels coupled to two effective continua. This is accomplished by putting the two compatibility equation solutions, for the common phase shifts of the two open channels, in such a form so the resonant behavior is attributed to one of them, the other accounting for an energy dependent background. Then, the well-known procedures for the simpler case where only one continuum is considered are applied, using only the resonant solution. The method is quite general and applicable to any MQDT model with two or more open channels. The resulting analytical formulae are tested on experimental spectra of Sr, Ba and Cu and it is shown that they are valid as long as: i) The resonances are non-overlapping, ii) The direct closed channel coupling is much stronger than the indirect one through the continua and (when excitation matrix elements are involved) iii) The open channels excitation strength is smaller or at least comparable to the closed channels one. Received: 26 May 1998 / Accepted: 1st July 1998     

Influence of hydrodynamics on the dynamics of a homopolymer

We present an analytical approach of the dynamics of a polymer when it is quenched from a solvent into a good or bad solvent. The dynamics is studied by means of a Langevin equation, first in the absence of hydrodynamic effect, then taking into account the hydrodynamic interactions with the solvent. The variation of the radius of gyration is studied as a function of time. In both cases, for the first stage of collapse or swelling, the evolution is described by a power law with a characteristic time proportional to N ^4/3 (N), where N is the number of monomers, without (with) hydrodynamic interactions. At larger times, scaling laws are derived for the diffusive relaxation time. Received: 10 March 1998 / Received in final form: 15 September 1998 / Accepted: 25 September 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     

DC-electric-field-modified second-harmonic generation at the Si(100) surface

1 resonances for clean and H covered surfaces shift as a function of the dc field in agreement with experiment. This suggests the presence of built-in electric fields whose strength depends on the H coverage, and which are strongly localized in the subsurface region. Received: 20 September 1998     

Adiabatical stability of density distributions under Vlasov dynamics

It is shown using Vlasov dynamics that the density distribution corresponding to a mean field Bose condensate in an external time dependent potential is adiabatically stable whereas density distributions corresponding to finite temperature are not. Received: 27 February 1998 / Revised: 20 April 1998 / Accepted: 23 April 1998     

Magnetic and transport properties of sputtered Fe/Si multilayers

The structural, magnetic and transport properties of sputtered Fe/Si multilayers were studied. The analyses of the data of the X-ray diffraction, resistance and magnetic measurements show that heavy atomic interdiffusion between Fe and Si occurs, resulting in multilayers of different complicated structures according to different sublayer thicknesses. The nominal Fe layers in the multilayers generally consist of Fe layers doped with Si, ferromagnetic Fe-Si silicide layers and nonmagnetic Fe-Si silicide interface layers, while the nominal Si spacers turn out to be Fe-Si compound layers with additional amorphous Si sublayers only under the condition either for the series or for the series multilayers. A strong antiferromagnetic (AFM) coupling and negative magnetoresistance (MR) effect, about 1%, were observed only in multilayers with iron silicide spacers and disappeared when -Si layers appear in the spacers. The dependences of MR on and on bilayer numbers N resemble the dependence of AFM coupling. The increase of MR ratio with increasing N is mainly attributed to the improvement of AFM coupling for multilayers with N. The dependence of MR ratio is similar to that in metal/metal system with predominant bulk spin dependent scattering and is fitted by a phenomenological formula for GMR. At 77 K both the MR effect and saturation field increase. All these facts suggest that the mechanisms of the AFM coupling and MR effect in sputtered Fe/Si multilayers are similar to those in metal/metal system. Received: 11 February 1998 / Revised: 9 March 1998 / Accepted: 9 March 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     

Photoemission, inverse photoemission and superconducting correlations in Hubbard and t-J ladders: role of the anisotropy between legs and rungs

Several experiments in the context of ladder materials have recently shown that the study of simple models of anisotropic ladders (i.e. with different couplings along legs and rungs) is important for the understanding of these compounds. In this paper Exact Diagonalization studies of the one-band Hubbard and t-J models are reported for a variety of densities, couplings, and anisotropy ratios. The emphasis is given to the one-particle spectral function which presents a flat quasiparticle dispersion at the chemical potential in some region of parameter space. This is correlated with the existence of strong pairing fluctuations, which themselves are correlated with an enhancement of the bulk-extrapolated value for the two-hole binding energy as well as with the strength of the spin-gap in the hole-doped system. Part of the results for the spectral function are explained using a simple analytical picture valid when the hopping along the legs is small. In particular, this picture predicts an insulating state at quarter filling in agreement with the metal-insulator transition observed at this special filling for increasing rung couplings. The results are compared against previous literature, and in addition pair-pair correlations using extended operators are also here reported. Received: 22 April 1998 / Revised: 23 July 1998 / Accepted: 30 July 1998     

A dynamic study of onion phases under shear flow: size changes

It has been shown that lyotropic lamellar phases under shear flow form structures corresponding to a close packed assembly of monodisperse multilamellar vesicles (onions). The size, which is fixed by the shear rate, can vary from a few microns to a tenth of a micron. In this study, we investigate for the first time the transient behaviour of size changes of onions under shear flow by means of small angle light scattering, direct microscopic observations, and conductivity measurements. We evidence two regimes: continuous and discontinuous. The nature of which (continuous or discontinuous) depends on the initial and final shear rate, and can be described by a dynamic phase diagram. Received: 14 November 1997 / Received in final form: 2 March 1998 / Accepted: 9 March 1998     

Theory of resonant Raman scattering from low-energy collective excitations of interacting electrons in quantum wires

The Raman spectra of quantum wires in the region of electronic intra-band excitations are investigated using one- and two-band models based on the Luttinger approximation with spin. Structures related to charge and spin density modes are identified, and analyzed with respect to their behavior with photon energy and temperature. It is found that the low-energy peaks in the polarized spectra, close to resonance that are commonly assigned to “single particle excitations”, can be interpreted as the signature of spin density excitations. A broad structure in the resonant depolarized spectrum is predicted above the frequency of the spin density excitations. This is due to simultaneous but independent propagation of spin and charge density modes. The results, when compared with experiment, show, that the electronic collective excitations of quantum wires at low energies are characteristic for a non-Fermi liquid. Received: 25 March 1998 / Accepted: 3 June 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