A theory of double exchange in infinite dimensions

This paper gives a simplified model of the double exchange which is a kind of indirect exchange interaction between localized magnetic moments. The presented model is solved exactly in the case of infinite - dimensional space. Equations for single-particle Green's function and magnetization of the localized spins subsystem are obtained. It is shown that our simple double exchange model reveals an instability to the ferromagnetic ordering of localized moments. Magnetic and electric properties of this system on Bethe lattice with are investigated in detail. Received: 24 January 1997 / Revised: 14 February 1997 / Received in final form: 18 August 1997 / Accepted: 25 August 1997     

Special interaction between quasiperiodic grain boundaries and lattice dislocations in crystalline solids

A theoretical model is suggested which describes phason imperfections (specific excitations) in a quasiperiodic grain boundary in a polycrystalline solid as dilatation flexes. In the framework of the model, an elastic stress field of the quasiperiodic grain boundary is calculated as a stress field created by an ensemble of dilatation flexes (phason imperfections) located in the boundary. It is shown that there is a special elastic interaction between crystal lattice defects and quasiperiodic grain boundaries comprising phason imperfections. The strengthening effect in plastically deformed polycrystalline solids is quantitatively described which is related to the special elastic interaction between lattice dislocations and quasiperiodic grain boundaries. Received: 29 October 1996 / Revised: 22 August 1997 / Accepted: 13 November 1997     

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     

Metastability of a circular o-ring due to intrinsic curvature

An o-ring takes spontaneously the shape of a chair when strong enough torsion is applied in its tangent plane. This state is metastable, since work has to be done on the o-ring to return to the circular shape. We show that this metastable state exists in a Hamiltonian where curvature and torsion are coupled via an intrinsic curvature term. If the o-ring is constrained to be planar (2d case), this metastable state displays a kink-anti-kink pair. This state is metastable if the ratio is less than , where C and A are the torsion and the bending elastic constants [#!landau!#]. In three dimensions, our variational approach shows that . This model can be generalized to the case where the bend is induced by a concentration field which follows the variations of the curvature. Received: 27 August 1997 / Revised: 23 October 1997 / Accepted: 12 November 1997     

Polymer adsorption on heterogeneous surfaces

The adsorption of a single ideal polymer chain on energetically heterogeneous and rough surfaces is investigated using a variational procedure introduced by Garel and Orland (Phys. Rev. B 55, 226 (1997)). The mean polymer size is calculated perpendicular and parallel to the surface and is compared to the Gaussian conformation and to the results for polymers at flat and energetically homogeneous surfaces. The disorder-induced enhancement of adsorption is confirmed and is shown to be much more significant for a heterogeneous interaction strength than for spatial roughness. This difference also applies to the localization transition, where the polymer size becomes independent of the chain length. The localization criterion can be quantified, depending on an effective interaction strength and the length of the polymer chain. Received: 29 October 1997 / Revised: 19 December 1997 / Accepted: 6 March 1998     

Buckling in a solid Langmuir monolayer: light scattering measurements and elastic model

Above a surface pressure threshold , we detect a buckled state in the low temperature solid phase of a phospholipid monolayer spread at the air-formamide interface. Stable ripples are observed with a Brewster angle microscope, and light scattering provides measurements mNm^-1, wavelength m and amplitude of a few nm for the deformation. A model, coupling the monolayer thickness and elongation, and consistent with the monolayer texture, is also presented. Received: 18 August 1997 / Revised: 9 December 1997 / Accepted: 29 January 1998     

Inelastic neutron scattering from H2 on vapour deposited CO2

Inelastic neutron scattering spectra at 17 meV and 68 meV incident energies of molecular hydrogen adsorbed on vapour deposited substrates of pure CO₂, 90:10 and 50:50 CO₂:Kr are reported. The ortho-para transition is shifted from 14.7 meV in the free H₂-molecule to 9.4 meV in a presumably commensurate ortho-H₂ monolayer on the CO₂ surface. The quadrupole-quadrupole interaction of ortho-H₂ molecules with the CO₂ substrate results in a strongly anisotropic potential. In addition to rotations the dynamics of this layer comprise a local Einstein mode and phonons in resonance with the substrate, giving rise to intense multiphonon transitions. Quasielastic scattering on warmer samples is assigned to a liquidlike adsorption layer, in which the H₂ rotations are strongly perturbed. Received: 15 October 1996 / Revised: 25 August 1997 / Accepted: 24 October 1997     

Broad histogram Monte Carlo

We propose a new Monte Carlo technique in which the degeneracy of energy states is obtained with a Markovian process analogous to that of Metropolis used currently in canonical simulations. The obtained histograms are much broader than those of the canonical histogram technique studied by Ferrenberg and Swendsen. Thus we can reliably reconstruct thermodynamic functions over a much larger temperature scale also away from the critical point. We show for the two-dimensional Ising model how our new method reproduces exact results more accurately and using less computer time than the conventional histogram method. We also show data in three dimensions for the Ising ferromagnet and the Edwards Anderson spin glass. Received: 8 August 1997 / Revised: 11 August 1997 / Accepted: 30 October 1997     

Apparent slip at a polymer-polymer interface

We consider a planar interface between strongly-segregated homopolymers subjected to steady shear in the plane of the interface. We develop a constitutive equation for stress relaxation in an inhomogeneous system for chains obeying Rouse dynamics. Using this equation, the interfacial viscosity for a symmetric blend is found to be in agreement with a scaling prediction due to de Gennes, where is the bead friction coefficient, b is the segment length, is the segment volume and is the Flory-Huggins interaction parameter driving the phase separation. We generalize our results to asymmetric blends and describe a phenomenological extension to entangled melts. Received: 18 August 1997 / Received in final form: 1 December 1997 / Accepted: 2 December 1997     

Line group theory of commensurate and incommensurate modulations

It is shown that the line group formalism proposed is suitable to describe both commensurate and incommensurate modulations. Symmetry groups of modulated crystal lattices can completely be characterized by symmetry transformations existing in real space, without any application of the formalism based on reciprocal space. As typical examples of the method elaborated, the fundamental invariance and symmetry properties of spin density functions and the soliton lattice are determined. Received: 20 July 1997 / Accepted: 25 August 1997     

Rare-gas-induced broadening and shift of two-photon transitions to intermediate Rydberg states of atomic thallium*

Results of broadening and shift measurements of Doppler-free two-photon lines for transitions from the ground state to the Rydberg states with intermediate principle quantum numbers of atomic thallium perturbed by rare gases are reported. The rates show a distinct behaviour in this range of principle quantum numbers and significant dependence on the total angular momentum of the upper nP state. The experimental results are compared with calculations using a Van der Waals potential and a superposition of polarization and Fermi potentials. Additionally, broadening and shift rates of the transition Tl 6P _3/2 -9P _3/2 have been measured for quadrupolar as well as for mainly scalar excitation. The rates for both kinds of excitation coincide within the limits of error reflecting the small perturbation of the 6P _3/2 state compared to that of the upper 9P _3/2 state. Received: 6 August 1997 / Revised in final form: 29 November 1997 / Accepted: 11 December 1997     

Dynamic behaviour of electron interference in the two-mode SU(1,1) coherent state field

Aharonov-Bohm-type electron interference in the presence of the two-mode SU(1,1) coherent state(CS) field is investigated. The visibility of the time-averaged interference pattern is discussed for this state, and a comparison with classical cases is made. It is shown that the time evolution of the intensity of electron interference exhibits collapse and revival (CR) phenomenon for this state. The fluctuation in electron interference, and its relation to CR phenomenon are also discussed. Received: 22 August 1997 / Accepted: 15 January 1998     

Micelle-smectic phase coexistence: Origin of the maximum swelling of a mixed lamellar phase

By use of a mean-field approach the spin-wave dispersion of the Cu degrees of freedom in the undoped high-T _C material Nd₂CuO₄is investigated. The experimentally observed sharp decrease of the Cu spin-wave gap with increasing temperature in the range is explained by a paramagnetic-like susceptibility of the Nd spins which couple to the Cu subsystem. The degeneracy of the “in-plane” and “out-of-plane” polarized Cu spin-wave branches is shown to be lifted by the uniaxial anisotropy of the Cu-Cu nearest-neighbor interaction. Received: 13 August 1997 / Revised: 4 December 1997 / Accepted: 11 December 1997     

Ionization of metal clusters by ions in the Fermi velocity range

We simulate excitation of metal clusters by highly charged, energetic ions, analyzing electron emission in terms of discrete ionization probabilities. Our test case is the collision of on the cluster at velocities around the electronic Fermi velocity of bulk sodium. The calculations are performed with a density-functional approach, using the time-dependent local density approximation. We find that ionization takes place on an extremely short time scale of less than 5 fs. The preferred final charge state depends sensitively on the impact parameter. High ionization can easily be achieved in sufficiently close collisions. Direct trapping through the by-passing ion is found to be of little importance at the velocities considered. Received: 28 July 1997 / Received in final form: 23 December 1997 / Accepted: 8 January 1998     

Investigation of ripple structures of thin polycrystalline Co films by magnetic force microscopy

Received: 14 August 1997/Accepted: 14 August 1997     

Exact two-loop vacuum polarization correction to the Lamb shift in hydrogenlike ions

We present a calculation scheme for the two-loop vacuum polarization correction of order to the Lamb shift of hydrogenlike high-Z atoms. The interaction with the external Coulomb field is taken into account to all orders in . By means of a modified potential approach the problem is reduced to the evaluation of effective one-loop vacuum polarization potentials. An expression for the energy shift is deduced within the framework of partial wave decomposition performing appropriate subtractions. Exact results for the two-loop vacuum polarization contribution to the Lamb shift of K- and L-shell electron states in hydrogenlike Lead and Uranium are presented. Received: 10 August 1997 / Revised: 31 October 1997 / Accepted: 18 November 1997     

Interplay between chains of S = 5/2 localised spins and two-dimensional sheets of organic donors in the synthetically built magnetic multilayer λ ‒ (BETS)2FeCl4

In order to understand the magnetic field-induced restoration of a highly conductive state in , static (SQUID) and dynamic (ESR and AFR) magnetization measurements were performed on polycrystalline samples and single crystals, respectively. In addition, cantilever and resistivity measurements under steady fields were performed. While the metal-insulator transition curve of the () phase diagram exhibits a first order character, a “spin-flop” transition line divides the insulating state when the magnetic field is applied along the easy axis of magnetization. The effects of a RKKY-type indirect exchange and of applied magnetic field are described within the framework of a generalized Kondo lattice, namely two chains of localised spins coupled through the itinerant spins of the 2D sheets of BETS. The calculations, which can incorporate intramolecular electron correlations within a mean field theory, are in qualitative agreement with the field induced transition from the antiferromagnetic insulating ground state to a canted one, i.e. a not fully oriented paramagnetic, but metallic state. Received: 6 August 1997 / Received: 5 November 1997 / Accepted: 10 November 1997     

OZI rule violation in p¯p annihilation into φππ by two step processes

The φπ⁺π⁻ production in pˉp annihilation at rest is strongly enhanced by a two step mechanism with intermediate KˉKππ states. The relative yield of the φ production due to the resonant final state interaction decreases with increasing total energy of the pˉp system. Received: 4 August 1997     

Canonical dressing in the multimode two-quantum Jaynes-Cummings model

Based upon the hidden Lie SU(1,1) symmetry, we have constructed the unitary decoupling transformation which diagonalizes the multimode two-quantum Jaynes-Cummings model and provides us with an extremely convenient basis to gain a deeper understanding of the dressing processes present in the matter-field interaction. This canonical transformation approach is very simple and can be easily extended to other generalized Jaynes-Cummings models. Received: 5 July 1997 / Revised: 3 November 1997 / Accepted: 14 November 1997