Two-colour coherent control of multiphoton ionization： a comparison between long-range and short-range potential model atoms

Using the numerical solution of the time-dependent Schr\"odinger equation of a one-dimensional model atom in a two-colour laser field, we have investigated the effects of the potential models on coherent control of atomic multiphoton ionization. It is found that the photoelectron spectra are obviously different for the long-range (Coulomb-like) and short-range (with no excited bound states) potential model atoms, which are produced by two-colour coherent control of atomic multiphoton ionization in a few laser cycles. Our results indicate that two-colour coherent control of atomic multiphoton ionization can be observed in simulations, depending on the choice of the model potentials.     

A convenient and accurate wide-range parameter relationship between Buckingham and Morse potential energy functions

This study explores the close-range, short-range and long-range relationships between the parameters of the Morse and Buckingham potential energy functions. The results show that the close-range and short-range relationships are valid for bond compression and for very small changes in bond length, respectively, while the long-range relationship is valid for bond stretching. A wide-range relationship is proposed to combine the comparative advantages of the close-range, short-range and long-range parameter relationships. The wide-range relationship is useful for replacing the close-range, short-range and long-range parameter relationships, thereby preventing the undesired effects of potential energy jumps resulting from functional switching between the close-range, short-range and long-range interaction energies.     

The short-range order and ordering energies in gold-copper polycrystalline alloys that are rich in gold

The concentration dependence of the short-range order parameters in gold-rich polycrystalline disordered solid gold-copper solutions was investigated using the X-ray diffuse-scattering method. On the X-ray diffuse-scattering patterns of disordered alloys with 75, 83, and 90% gold, diffuse maximums that are typical of short-range order were absent in the region of the possible superstructure reflex (100), but in the region of the possible reflex (110), an intensity rise on the scattering patterns was revealed that noticeably depended on the gold concentration. The application of the method, taking the effects of static displacements on the considered coordination spheres into account, allowed the determination of short-range order parameters for nine coordination spheres. The spectra of short-range order parameters, which was calculated using the least-squares method, had the long-range sign-alternating representation that is inherent in the Cu₃Au superstructure. The absolute values of short-range order parameters were detected to decrease with an increase of the gold content in the alloys. The concentration dependence and small values of ordering energy of investigated polycrystalline alloys were revealed on the first coordination sphere using the Clapp-Moss method.     

Short-range order in Fe-Rh solid solutions

We have used diffuse scattering of x-rays (DSX) and a model potential to show short-range order exists in iron-rhodium alloys. After the alloy Fe-25 at. % Rh was quenched following annealing at 900°C for 1 h, a clearly visible diffuse maximum (characteristic of short-range order) appeared in the diffraction pattern, in addition to reflections from the α- and γ-phases. For the single-phase, dilute solid solution Fe-2.5 at. % Rh, starting with the effects of static displacements, we have calculated the short-range order parameters (α₁ = −0.02, α₂ = −0.09, α₃ = 0.02, α₄ = 0.05). The elastic constants of Fe-2.5 at. % Rh needed to compute the short-range order parameters from the DSX data were calculated using a model potential. Theoretical calculations of the order energy and the short-range order parameters, performed using a model potential for these alloys, agree with the experimental data. M. V. Lomonosov Moscow State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 71–77, March, 1998.     

Short-Range Correlations and Meson Exchange Currents in Photonucleon Emission

One-nucleon emission processes induced by photon absorption are studied by considering short-range correlation effects. At energies above the giant resonance region the validity of the direct knock-out model has been tested by comparison with continuum random phase approximation results. Nucleon re-scattering effects have been considered by using an optical potential. The role of the electromagnetic convection, magnetization, and meson exchange currents has been investigated as a function of both excitation energy and momentum transfer. The short-range correlation effects have been studied by using various correlation functions. We found that the nucleon photo-emission cross section is rather sensitive to the presence of short-range correlations at large values of nucleon emission angle. In this region, however, the effects of meson exchange currents are even larger than those produced by short-range correlations.     

Positron and electron scattering on atoms and molecules–modified effective range theory revisited

New experiments on the very low-energy electron and positron scattering allow to verify the old question on applicability of modified effective range theory (MERT). We perform it using an analytical solution of the Schrödinger equation with the long-range polarization potential. In this work two atomic (He, Ar) and molecular (H₂, CH₄) target are studied using this approach. Total cross sections were used for obtaining parameters characterizing the scattering phase shifts related to the short-range interaction potential; differential cross sections were used for comparison. Differently from previous works, we conclude that MERT with few (2-3) partial waves applies very well up to energies of few eV in all four targets studied. For positrons, reliable experimental data indicate occurrence of zeros in the s-wave phase shifts for all four targets. This should be recognized as Ramsauer-Townsend minima.     

New multiple histogram method for studying phase transitions

For temperature zero the effects of disorder for interacting bosons are considered. The disorder induced superfluid-insulator transition in thed-dimensional disordered Bogoliubov model is discussed. Results for a short-range and a long-range random potential are given. For short-range disorder we argue that ford<4 arbitrarily small disorder localizes the Bose condensate for vanishing interaction potential. Ford>4 a certain strength of the disorder potential is necessary in order to localize the condensate. For the three-dimensional Bogoliubov model our results are in agreement with a recent calculation. We compare our theoretical predictions with numerical experiments for a disordered boson Hubbard model.     

Role of elastic scattering in high-order above threshold ionization

We investigate the target and intensity dependence of plateau in high-order above threshold ionization(HATI) by simulating the two-dimensional(2D) momentum distributions and the energy spectra of photoelectrons in HATI of rare gas atoms through using the quantitative rescattering model. The simulated results are compared with the existing experimental measurements. It is found that the slope of the plateau in the HATI photoelectron energy spectrum highly depends on the structure of elastic scattering differential cross section(DCS) of laser-induced returning electron with its parent ion. The investigations of the long- and short-range potential effects in the DCSs reveal that the short-range potential, which reflects the structure of the target, plays an essential role in generating the HATI photoelectron spectra.     

Charge asymmetry and neutron-neutron scattering

The uncertainty in the subtraction of electromagnetic effects from S-wave proton-proton scattering potentials is studied in two models using unitarily transformed potentials. Restrictions on these models caused by theoretical, off-shell and deuteron constraints are imposed. The probable uncertainty of physical interest is found to be small. It is typically only a few parts per thousand of the one-pion exchange potential. It has relatively little effect on Coulomb displacement energies. Phenomenological charge-symmetry-breaking potentials are constructed which both fit the experimental neutron-neutron (nn) scattering length (?16.4 fm). and give roughly the needed Coulomb displacement energies. These phenomenological potentials are found to contain a short-range repulsion and a strong long-range attraction for a neutron pair. The need for experimental information on the shape and state dependence of the nn force is emphasized. These can probably be obtained from precise nn scattering measurements.     

Scaling of dynamics with the range of interaction in short-range attractive colloids

We numerically study the dependence of the dynamics on the range of interaction Delta for the short-range square well potential. We find that, for small Delta, dynamics scale exactly in the same way as thermodynamics, both for Newtonian and Brownian microscopic dynamics. For interaction ranges from a few percent down to the Baxter limit, the relative location of the attractive-glass line and the liquid-gas line does not depend on Delta. This proves that, in this class of potentials, disordered arrested states (gels) can be generated only as a result of a kinetically arrested phase separation.     

Electronic localization in disordered systems

A brief review is given of the current understanding of the electronic structure, transport properties and the nature of the electronic states in disordered systems. A simple explanation for the observed exponential behaviour in the density of states (Urbach tails) based on short-range Gaussian fluctuations is presented. The theory of Anderson localization in a disordered system is reviewed. Basic concepts, and the physics underlying the effects of weak localization, are discussed. The scaling as well as the self-consistent theory of localization are briefly reviewed. It is then argued that the problem of localization in a random potential within the so-called ladder approximation is formally equivalent to the problem of finding a bound state in a shallow potential well. Therefore all states are exponentially localized in d=1 and d=2. The fractal nature of the states is also discussed. Scaling properties in highly anisotropic systems are also discussed. A brief presentation of the recently observed metal-to-insulator transition in dequals;2 is given and, finally, a few remarks about interaction effects in disordered systems are presented.     

Scaling and universality of integer quantum Hall plateau-to-plateau transitions

We have investigated the integer quantum Hall plateau-to-plateau transition in two-dimensional electrons confined to AlxGa(1-x)As-Al0.33Ga0.67As heterostructures over a broad range of Al concentration x. For x between 0.65% and 1.6%, where the dominant contribution to disorder is from the short-range alloy potential fluctuations, we observe a perfect power-law scaling in the temperature range from 30 mK to 1 K with a critical exponent kappa = 0.42 +/- 0.01.     

Modification of short-range repulsive interactions in ReaxFF reactive force field for Fe-Ni-Al alloy

The short-range repulsive interactions of any force field must be modified to be applicable for high energy atomic collisions because of extremely far from equilibrium state when used in molecular dynamics (MD) simulations. In this work, the short-range repulsive interaction of a reactive force field (ReaxFF), describing Fe-Ni-Al alloy system, is well modified by adding a tabulated function form based on Ziegler-Biersack-Littmark (ZBL) potential. The modified interaction covers three ranges, including short range, smooth range, and primordial range. The short range is totally predominated by ZBL potential. The primordial range means the interactions in this range is the as-is ReaxFF with no changes. The smooth range links the short-range ZBL and primordial-range ReaxFF potentials with a taper function. Both energies and forces are guaranteed to be continuous, and qualified to the consistent requirement in LAMMPS. This modified force field is applicable for simulations of energetic particle bombardments and reproducing point defects' booming and recombination effectively.     

Influence of surface affinity of planar walls on effective interaction between the wall and colloids

Using density functional theory, we investigate the effective interaction between a big colloid immersed in a sea of small colloids and a wall which has different affinity to the small colloids. Steele 10-4-3 potential is introduced to mimic both short-range repulsive and long-range attractive interactions between the wall and the small colloids. It is found that the surface affinity of the wall has a significant influence on the effective interaction. In the short-range repulsive case, the repulsion greatly enhances the big colloid-wall effective attraction, which sensitively depends on the concentration of small colloids, and is not sensitive to the repulsive strength. In the long-range attractive case, both the concentration of small colloids and the attractive strength have great effect on the effective interaction, and with an increase of the attractive strength, a strong repulsion may be induced when the big colloid is close to the wall. In low density limit of small colloids, the present results agree well with those of the Asakura and Oosawa(AO) approximation.     

An accurate theory for the square-well potential: from long to short well width

Density function theory (DFT) is combined with the first-order mean spherical approximation (FMSA) to study the radial distribution function (RDF) of the square-well (SW) potential. The combination (DFT + FMSA) is based on the direct correlation function (DCF) of the FMSA. Upon comparison with computer simulation data, DFT + FMSA is shown to give better performance than FMSA for mid- and long-range attractions. For short-range and very short-range attractions, the theory successfully corrects the deficiencies of the original FMSA. Comparisons include the evaluation of contact values, gap height at a discontinuity and profiles of the RDF. This work provides an accurate and consistent way to handle the SW potential.     

Polyelectrolyte multilayer formation: Electrostatics and short-range interactions

We investigate the phenomenon of multilayer formation via layer-by-layer deposition of alternating charged polyelectrolytes. Using mean-field theory, we find that a strong short-range attraction between the two types of polymer chains is essential for the formation of multilayers. For strong enough short-range attraction, the adsorbed amount per layer increases (after an initial decrease), and finally it stabilizes in the form of a polyelectrolyte multilayer that can be repeated hundreds of times. For weak short-range attraction between any two adjacent layers, the adsorbed amount (per added layer) decays as the distance from the surface increases, until the chains stop adsorbing altogether. The dependence of the threshold value of the short-range attraction as function of the polymer charge fraction and salt concentration is calculated.     

Modulation and incommensurability in a superionic conductor

The structure factor S(q) is calculated for the Frenkel-Kontorova model at high temperatures. Application to the 1-d ionic conductor K-Hollandite gives an explanation of diffuse X-ray scattering in terms of a modulation phenomenon. The positions of the diffuse sheets are predicted precisely from the stoichiometric composition. It is confirmed thereby that the mobile K⁺ ions behave like a chain with spacing b = 1/n where n is the concentration. The disorder is characterized as short-range crystalline order of wavelength b, competing with and modulated by the framework potential with a different period a.     

ATOMIC SHORT-RANGE ORDER OF AMORPHOUS Ta-Cu ALLOYS PREPARED BY MECHANICAL ALLOYING

In the present work, the atomic short-range ordering structure of the amorphous Ta-Cu alloys formed by mechanical alloying was investigated through radial distribution function (RDF) analysis. The results suggest that the elemental powders of Ta and Cu were well mixed by mechanical alloying at atomic level. The regions of short-range order (r_s) increase with Cu concentration in Ta-Cu alloys, This means that the short-range ordering becomes stronger in those alloys with higher Cu content. The coordination number was estimated to be 12-13 from the area under the first maximum of the RDF (r) curves for all the Ta-Cu amorphous alloys. This result indicates that the amorphous alloys formed by mechanical alloying are also of topologically dense-packing structure.     

Possible quantum critical point in La(2/3)Ca(1/3)Mn(1-x)Ga x O3

We study the magnetic ground state in La(2/3)Ca(1/3)Mn(1-x)Ga x O3 manganites, where a quantum critical point (QCP) has been theoretically predicted. The metallic ferromagnetic ground state for low Ga doping breaks down for x > or = 0.11, an insulating state being established at low temperatures. Long-range ferromagnetism coexists with short-range magnetic correlations in the concentration range 0.11 < or = x < or = 0.145 while only the short-range correlations survive for x > or = 0.16. We discuss the implications of such a QCP to the physics of manganites and compare to other QCP systems.     

Ionization and stabilization of a three-dimensional system with a short-range potential in a strong laser field

The interaction of a three-dimensional atomic system in a short-range potential with intense laser radiation is investigated by the direct numerical integration of the nonstationary Schrödinger equation. The calculations helped to discover a stabilization regime, which is interpreted as a result of forming a Kramers-Henneberger atom “dressed” in a field. Dynamics of the energy spectrum of photoelectrons depending on the increase of the laser field intensity is investigated, and conditions of a photodetachment of an electron from a bound state of the Kramers-Henneberger potential are analyzed. These results reveal specific features of the stabilization process of the three-dimensional system with a short-range potential compared to the similar process in a system with a long-range (Coulomb) potential.