Sensitivity to initial conditions in the Bak-Sneppen model of biological evolution

We consider biological evolution as described within the Bak and Sneppen 1993 model. We exhibit, at the self-organized critical state, a power-law sensitivity to the initial conditions, calculate the associated exponent, and relate it to the recently introduced nonextensive thermostatistics. The scenario which here emerges without tuning strongly reminds of that of the tuned onset of chaos in say logistic-like one-dimensional maps. We also calculate the dynamical exponent z. Received: 5 November 1997 / Received in final form: 11 November 1997 / Accepted: 19 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     

Controlling nonspreading wavepackets

We show how a static electric field can be used to control the localization of nonspreading wavepackets of a hydrogen atom driven by a linearly polarized microwave field. This allows for creation of wavepackets localized on fully stable resonance islands which, at the same time, can easily be excited by a direct optical transition from a low lying state. A semiclassical analysis is used to predict the energies and properties of such states. Received: 27 November 1997 / Accepted: 27 January 1998     

Amplification of atomic L-R asymmetries by stimulated emission: experimental demonstration of sensitivity enhancement valuable for parity violation measurements

While all Atomic Parity Violation experiments on highly forbidden transitions in a Stark field have used the detection of fluorescence signals, our group is engaged in an experiment on the cesium transition that uses a pump-probe scheme. The role of the probe beam is to detect the 7S state by stimulated emission. The detected Left-Right asymmetry () appears directly on the transmitted probe beam and the technique relies on differential-mode atomic polarimetry. We present here experimental results which illustrate two essential features of this approach. First, is amplified when the optical thickness for the probe beam is increased, hence it is an increasing function of the Stark field. Secondly, the experimental sensitivity to is simultaneously increase d, as demonstrated by our measurements of the signal-to-noise ratio. We emphasize also the advantage of choosing a probe transition that involves a “dark” state: the amplification is preserved at high levels of the probe intensity because saturation effects are greatly reduced. Received: 8 october 1997 / Accepted: 21 November 1997     

Mn-site doping induced CMR properties in calcium rich manganites Pr1‒xCaxMnO3(0.6 ≤ x ≤ 0.7)

Doping of Mn-site by chromium, cobalt and nickel has been investigated in the calcium rich manganites and . Whatever the nature of the doping element, a rapid disappearance of the charge ordered (CO) state is observed, decreasing as the doping rate increases. But the most important result concerns the Cr-doped compounds for which a re-entrant insulator to metal transition at 90-120 K is observed for x =0.10-0.12, in a zero magnetic field. The possibility to induce colossal magnetoresistance (CMR) properties for high (IV) contents ((IV)/) for x =0.07-0.12 is shown for the first time, the resistance ratios reaching at 30 K. This study also shows differently that the small size of the A-site cation (Pr, Ca) is not a redhibitory obstacle to the appearance of CMR properties in manganites, in contrast with previously established phase diagrams. Received: 18 June 1997 / Revised: 4 August 1997 / Accepted: 10 November 1997     

Transport and thermodynamic properties of CeAuAl3

We present measurements of the specific heat, the electrical resistivity, the Hall effect, and the magnetic susceptibility of CeAuAl₃, a new heavy-electron compound that crystallizes in an ordered derivative of the tetragonal BaAl₄-type structure. For comparison we have also done some of these measurements on the isostructural non-magnetic reference compound LaAuAl₃, which appears to be a simple metal. Below K, CeAuAl₃ orders antiferromagnetically and below 1K, we encounter Fermi liquid behaviour with considerably enhanced effective masses, i.e., a quadratic temperature dependence of the resistivity with a large prefactor and a sizable linear-in-T contribution to the specific heat. This linear-in-T contribution increases by more than a factor 50 from its value at to its value at . Consequently CeAuAl₃ develops a heavy-electron ground state, coexisting with antiferromagnetic order. The small energy scales involved in the problem make CeAuAl₃ a good candidate for tuning it, by varying external parameters, towards a quantum critical point. At high temperatures we observe local moment behaviour. From the temperature dependence of the magnetic susceptibility and the specific heat we have derived the crystalline-electric-field-split level scheme of the J =5/2 multiplet. Distinct features in the electrical resistivity provide additional evidence for this level splitting. Received: 6 November 1997 / Accepted: 17 November 1997     

Phase diagram of randomly polymerized membrane

Using a replica formalism, a generalization of a recent mean field model corresponding to the observed wrinkling transition in randomly polymerized membranes is presented. In this model we study the effects of global fluctuations of the surface normals to the flat membrane, which can be introduced by a random local field. In absence of these global fluctuations, we show that, the model exhibits both continuous and discontinuous transitions between flat and wrinkled phases, contrary to what has been predicted by Bensimon et al. and Attal et al. Phase diagrams both in replica symmetry and in breaking of replica symmetry in sense of Almeida and Thouless are given. We have also investigated the effects of global fluctuations on the replica symmetry phase diagram. We show that, the wrinkled phase is favored and the flat phase is unstable. For large global fluctuations, the transition between wrinkled and flat phases becomes first order. Received: 3 December 1997 / Revised: 31 March 1998 / Accepted: 3 August 1998     

Quantum fluctuations and ground state of weakly interacting helix spin chains in a magnetic field

Ferromagnetic spin chains of a hexagonal lattice coupled by a weak antiferromagnetic interaction J₁ develop a helix arrangement if the intrachain antiferromagnetic NNN exchange J₂ is sufficiently large. We show that the classical minimum energy spin configuration is an umbrella when an external magnetic field is applied. The scenario is dramatically changed by quantum fluctuations. Indeed we find that the zero point motion forces the spins in a plane containing the magnetic field so that classical expectation is deceptive for our model. Our result is obtained by controlled expansion in the low field-long wavelength modulation limit. Received: 9 September 1997 / Revised: 15 October 1997 / Accepted: 17 November 1997     

Effect of a strong electric field on a nematogen: evidence for polar short range order

We present experimental studies on the effect of strong electric fields on the nematogen p-cyanophenyl p-n-heptyl benzoate which has the strongly polar cyano end group and a large positive dielectric anisotropy. We use a local temperature measurement to take into account heating effects and an electrical impedance analysis to determine both the dielectric constant () and the resistance (R) of the sample. We also measure the higher harmonic responses of the medium. The new results obtained in this study are: (i) a detailed temperature dependence of the terms which describe (a) the quenching of macroscopic thermal fluctuations of the nematic director and (b) the enhancement of the orientational order parameter due to Kerr effect, (ii) clear evidence for the critical divergence of susceptibility as reflected in the third harmonic signal, (iii) an unusual enhancement of the conductivity which shows a large peak just below the critical point , (iv) a significant peak in the second harmonic signal at and (v) evidence for a field induced nematic-nematic transition well inside the nematic range. We argue that the results (iii)-(v) indicate the presence of polar short range order in the medium and hence support a molecular model in which such an order has been proposed. Received: 15 July 1997 / Received in final form: 24 September 1997 / Accepted: 29 October 1997     

Spin correlations in electron liquids: Analytical results for the local-field correction in two and three dimensions

We study the spin correlations in two- and three-dimensional electron liquids within the sum-rule version of the self-consistent field approach of Singwi, Tosi, Land, and Sj?lander. Analytic expressions for the spin-antisymmetric static structure factor and the corresponding local-field correction are obtained with density dependent coefficients. We calculate the spin-dependent pair-correlation functions, paramagnon dispersion, and static spin-response function within the present model, and discuss the spin-density wave instabilities in double-layer electron systems. Received: 22 September 1997 / Revised: 1 December 1997 / Accepted: 4 December 1997     

Axialisation, cooling and quenching of ions in a Penning trap

On the basis of a macroscopic ground state population it was argued recently that Bose-Einstein condensation should occur in a one-dimensional harmonic potential. We examine this situation by drawing analogies to bosons in a two-dimensional box, where the thermodynamic limit is well-defined. We show that in both systems although the ground state populations show sharp onsets at the critical temperature, the behaviour of the specific heat is analytic, which proves the absence of a phase transition in these systems. Received: 17 February 1997 / Revised: 3 September 1997 / Accepted: 13 October 1997     

Monte Carlo simulations of high-moment - low-moment transitions in Invar alloys

We have reproduced magneto-volume effects typical for Invar alloys by examining a spin-analogous model which describes coupled spatial and magnetic degrees of freedom and, additionally, chemical disorder. Constant pressure Monte Carlo simulations of this model show an almost vanishing thermal expansion over a broad range of temperatures below ,a softening of the bulk modulus and the absence of a sharp peak in the specific heat at the magnetic phase transition as observed in Fe₆₅Ni₃₅ Invar. Received: 9 September 1996 / Revised: 23 May and 23 September 1997 / accepted: 3 September 1997     

Scaling with respect to disorder in time-to-failure

We revisit a simple dynamical model of rupture in random media with long-range elasticity to test whether rupture can be seen as a first-order or a critical transition. We find a clear scaling of the macroscopic modulus as a function of time-to-rupture and of the amplitude of the disorder, which allows us to collapse neatly the numerical simulations over more than five decades in time and more than one decade in disorder amplitude onto a single master curve. We thus conclude that, at least in this model, dynamical rupture in systems with long-range elasticity is a genuine critical phenomenon occurring as soon as the disorder is non-vanishing. Received: 11 July 1997 / Revised: 6 November 1997 / Accepted: 10 November 1997     

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     

Charge transfers in complex transition metal alloys ()

We introduce a new Non-Orthogonal Tight-Binding model, for complex alloys, in which electronic structure is characterized by charge transfers. We give the analytic calculation of a charge transfer, in which overlapping two-center terms are rigorously taken into account. Then, we apply numerically this result to an approximant phase of a quasicrystal of alloy. This model is more particularly adapted to transition metals, and gives realistic densities of states. Received: 7 July 1997 / Revised: 10 March 1998 / Accepted: 3 April 1998     

Edge of disorder

Polyanilines belong to a family of electronic polymers. Due to a wide range of variable parameters in the synthesis of polyanalines, a variety of samples with different physical properties can be achieved. Different investigators have identified the degree of crystallinity for different varieties of polyanilines. They have classified them into two classes of emeraldine forms. In these structural investigations, the traditional “concept of the two phase system” was used for differentiating the crystalline phase. Recently, it has been shown that washing the emeraldine base (EB) by NMP or THF will establish two different X-ray patterns. The X-ray pattern of the sample washed with THF shows a limited number of Bragg lines superimposed on the broad diffuse halos. Other patterns only indicated broad diffuse halos. These experimental facts do not advocate the “two phase system model”, especially with scarce Bragg reflections. We have set-up an elegant prototype for X-ray diffractometry which allows us to obtain a high resolution X-ray pattern from polymers with coherent intrinsic backgrounds. Formally, this coherent background was excluded for polymer structural identification. Nevertheless, we have introduced a new method of separating the reflected lines from a coherent background. The interference functions F(K) of the coherent background were obtained applying this novel technique. The advantage of our method of separation is verified by comparing the background of F(K) with the F(K) of the sample washed with NMP. Fourier inversion is used to identify the short range order of the atoms responsible for the diffuse scattering. This result enabled us to choose a standard state of disorder in polymers. This standard state of randomness can be considered as the edge of disorder. By defining this reference state of randomness, it can be seen that the atomic arrangements of long and fast changing structural chains correspond best to the random array model rather than the two phase system model. The novel experiment and analysis technique presented here offer a possibility of application of structural studies to many common polymers, all of which have the characteristic of the diffuse halo underlying the diffraction lines. Received: 7 May 1997 / Revised: 2 July 1998 / Accepted: 25 September 1998     

Statistical analysis of the transition to turbulence in plane Couette flow

We argue on general grounds that the transition to turbulence in plane Couette flow is best studied experimentally at a statistical level. We present such a statistical analysis of experimental data guided by a parallel investigation of a simple coupled map lattice model for spatiotemporal intermittency. We confirm that this generic type of spatiotemporal chaos is relevant in the context of plane Couette flow, where the linear stability of the laminar regime at all Reynolds numbers insures the necessary local subcriticality. Using large ensembles of similar experiments, we show the existence of a well-defined threshold Reynolds number above which a unique, turbulent, intermittent attractor coexists with the laminar flow. Furthermore, our data reveals that this transition to spatiotemporal intermittency is discontinuous, i.e. akin to a first-order phase transition. Received: 10 April 1998 / Revised: 22 June 1998 / Accepted: 24 June 1998     

Viable improvement of the full potential linearized augmented plane wave (FLAPW) method

A characteristic feature of the Full Potential Linearized Augmented Plane Wave (FLAPW)-method consists in the spatial subdivision of the charge density analogous to that of the one-particle wavefunctions, i.e. into a portion that is expanded in terms of spherical harmonics Y_lm inside the muffin-tin spheres and into a plane wave expansion of the interstitial charge density. To obtain the Hartree potential inside the spheres one is hence forced to solve a boundary value problem at the sphere surface. In addition, in all non-equivalent spheres each (l,m)-component of the charge density is mapped onto 300-400 radial grid points. To ensure an accelerated convergence of the calculation, the pertinent schemes require this rather large data set to be stored and mixed within 3-6 iteration steps. We show and illustrate for the example of a spin-polarized Ni-film with and without an oxygen overlayer and for bulk Si that this data set can be compressed by at least two orders of magnitude if one partitions the charge density in a different way so that the relevant portion determining the interatomic bonding can be Fourier expanded throughout the lattice cell. One thereby arrives at a modified FLAPW-scheme that combines favorable features of the original method with virtues of the pseudopotential method which consist in the simple construction of the Hartree potential and the efficient way of achieving self-consistency. These advantages can be exploited to the fullest by using Fast Fourier Transform. Moreover, forces that atoms experience in off-equilibrium positions attain a particularly simple form in terms of the charge density expansion coefficients. Received: 31 May 1997 / Revised: 12 December 1997 / Accepted: 18 December 1997     

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