Repulsive Core Research Articles

Density of impurity states in doped spherical quantum dots

We analyze the electronic states in spherical quantum dots (SQDs) with randomly distributed neutral, D0 and negatively charged D− donors by using the recently developed fractal dimension method, in which the problems of the D0 and the D− are reduced to similar ones for a hydrogen-like atom and a negative-hydrogen-like ion, respectively, in an isotropic effective space with the fractal dimension which depends on the electron–ion donor separation. In order to solve these problems in the new effective space, we use the numerical trigonometric sweep method and the three-parameter Hylleraas trial function. We present novel curves for the density of D0 impurity states for different potentials with and without the repulsive core. We find that the curves of the density of states with repulsive core have additional peaks, whose position and intensity depends on the width and the height barrier at the center of the dot. Our results are in a good agreement with those obtained previously for donor spectra in SQD with square-well potential shape.

THE EFFECT OF THE RANGE OF THE POTENTIAL ON THE MULTI-STEP CLUSTER MELTING PROCESS

By using the microcanonical molecular dynamics simulation, the melting processes of the clusters bound by Morse potential are investigated. It is found that these clusters show a multi-step melting process as long as the range of the Morse potential is a suitable value. The origins of this multi-step process are analyzed. I find that not only the repulsive core of the potential but also the attractive tail range of the potential influences the melting process. Moreover, the occurrence of the multi-step melting process is more sensitive to the tail region of the Morse potential.

A simple SCOZA for simple fluids

A simple version of the self-consistent Ornstein–Zernike approach (SCOZA) is investigated quantitatively. The simplification consists of retaining thermodynamic self-consistency between the compressibility and energy routes to thermodynamics from the pair distribution function g( r). However, we do not require that g( r) be identically zero within the hard repulsive core of the models we consider here. The resulting theory lends itself to analysis of Hamiltonian models to which the earlier SCOZA formulation cannot be readily applied as well as providing a convenient means of analyzing simpler models. We choose the hard-core Yukawa fluid as an illustrative example to calibrate the accuracy of the new version. The results are compared with our earlier SCOZA and simulation results, and a suggestion is given to systematically improve the accuracy of the theory in the applications that warrant so doing.

Density of impurity states in doped spherical quantum dots

We analyze the electronic states in spherical quantum dots (SQDs) with randomly distributed neutral, D 0 and negatively charged D − donors by using the recently developed fractal dimension method, in which the problems of the D 0 and the D − are reduced to similar ones for a hydrogen-like atom and a negative-hydrogen-like ion, respectively, in an isotropic effective space with the fractal dimension which depends on the electron–ion donor separation. In order to solve these problems in the new effective space, we use the numerical trigonometric sweep method and the three-parameter Hylleraas trial function. We present novel curves for the density of D 0 impurity states for different potentials with and without the repulsive core. We find that the curves of the density of states with repulsive core have additional peaks, whose position and intensity depends on the width and the height barrier at the center of the dot. Our results are in a good agreement with those obtained previously for donor spectra in SQD with square-well potential shape.

Θ+(1540)as a heptaquark with the overlap of a pion, a kaon, and a nucleon

We study the very recently discovered Theta+ (1540) at SPring-8, at ITEP and at CLAS-Thomas Jefferson Lab. We apply the same RGM techniques that already explained with success the repulsive hard core of nucleon-nucleon, kaon-nucleon exotic scattering, and the attractive hard core present in pion-nucleon and pion-pion non-exotic scattering. We find that the K-N repulsion excludes the Theta+ as a K-N s-wave pentaquark. We explore the Theta+ as a heptaquark, equivalent to a N+pi+K borromean bound-state, with positive parity and total isospin I=0. We find that the kaon-nucleon repulsion is cancelled by the attraction existing both in the pion-nucleon and pion-kaon channels. Although we are not yet able to bind the total three body system, we find that the Theta^+ may still be a heptaquark state. We conclude with predictions that can be tested experimentally.

A new scheme of many-body potentials for hcp metals

A nonspherical symmetric model of embedding atom potential is schemed for hexagonal close-packed (hcp) metals. The empirical embedding energy function and core–core repulsion used in this paper are based on the embedded atom theories of Daw–Baskes (D–B) and Finnis–Sinclair (F–S). The cohesive energy, vacancy formation energy, bulk modulus and elastic constants of five hcp metals are calculated and compared with the experimental data. The results show that it is useful in the calculations of mechanical and cohesive properties of hcp metals and can be used in computer simulations of the study of material properties with molecular dynamic method.

Density of electronic states in impurity-doped quantum well wires

We analyze the electronic states in the cylindrical quantum well-wire (QWW) with randomly distributed neutral, D 0 and negatively charged D − donors. In order to calculate the ground state energies of the off-center donors D 0 and D − as a function of the distance from the axis of QWW we use the recently developed fractal dimension method in which the problems are reduced to a similar one for a hydrogen-like atom and a negative-hydrogen-like ion respectively, in an isotropic effective space with variable fractional dimension. The numerical trigonometric sweep method and the three-parameter Hylleraas trial function are used to solve these problems. We present a novel curves for the density of impurity states in a cylindrical QWWs with square-well, parabolic and soft-edge barrier potentials. Additionally we analyze the effect of the repulsive core on the density of the impurity states.

Large Mass Enhancement in3He Monolayer Fluids Adsorbed on Graphite Preplated with4He

A fluid monolayer of 3He adsorbed on a graphite surface is an ideal twodimensional (2D) Fermi system. We have observed very large mass enhancement of the quasiparticles (m*/m=20±5) in preliminary heat-capacity measurements for high density 3He fluids adsorbed on graphite preplated with a 4He monolayer (3He/4He/gr). This could be divergence of m* toward the critical density for the 4/7 phase with a triangular lattice, which is a registered phase commensurate against the first layer. The observed m* is larger than those reported for other 2D 3He systems, e.g., m*/m=13 for 3He/HD/HD/gr system, presumably because of the higher critical density. The apparent divergence of m* toward the different Critical densities strongly suggests the Mott–Hubbard type transition between the 2D Fermi fluid phase and the registered phase under a strong onsite hard core repulsion between the 3He atoms.

Testing electronic structure methods for describing intermolecular H...H interactions in supramolecular chemistry.

In this article a wide variety of computational approaches (molecular mechanics force fields, semiempirical formalisms, and hybrid methods, namely ONIOM calculations) have been used to calculate the energy and geometry of the supramolecular system 2-(2'-hydroxyphenyl)-4-methyloxazole (HPMO) encapsulated in beta-cyclodextrin (beta-CD). The main objective of the present study has been to examine the performance of these computational methods when describing the short range H. H intermolecular interactions between guest (HPMO) and host (beta-CD) molecules. The analyzed molecular mechanics methods do not provide unphysical short H...H contacts, but it is obvious that their applicability to the study of supramolecular systems is rather limited. For the semiempirical methods, MNDO is found to generate more reliable geometries than AM1, PM3 and the two recently developed schemes PDDG/MNDO and PDDG/PM3. MNDO results only give one slightly short H...H distance, whereas the NDDO formalisms with modifications of the Core Repulsion Function (CRF) via Gaussians exhibit a large number of short to very short and unphysical H...H intermolecular distances. In contrast, the PM5 method, which is the successor to PM3, gives very promising results. Our ONIOM calculations indicate that the unphysical optimized geometries from PM3 are retained when this semiempirical method is used as the low level layer in a QM:QM formulation. On the other hand, ab initio methods involving good enough basis sets, at least for the high level layer in a hybrid ONIOM calculation, behave well, but they may be too expensive in practice for most supramolecular chemistry applications. Finally, the performance of the evaluated computational methods has also been tested by evaluating the energetic difference between the two most stable conformations of the host(beta-CD)-guest(HPMO) system.

Fusion, resonances and scattering in12C+12C reaction

The variation of fusion cross-section (σJfus) with energy in the12C+12C collision is linked to the underlying resonance phenomenon through the behavior of reaction cross-section (σ) of which σfus is taken as a part. The calculation of σfus is done through an energy-dependent imaginary potential in the optical model potential (OMP). Through dispersion relation, such an imaginary potential gives rise to energy-dependent real potential which is incorporated in the OMP. In our calculation, a form of potential for the nuclear part which has a soft repulsive in-built core is introduced based on similar works done earlier. The calculated results of σfus are used to explain the oscillatory structure, astrophysical S-factor and the decreasing trend at higher energies of the experimental σfus data in the case of12C+12C system with remarkable success. The potential used for fusion calculation is tested for fitting elastic scattering data at some energies and is found good in forward angles. Further improvement of the fitting of these data is obtained by incorporating a coupling potential in the surface region. About twenty resonances are observed in our calculation in the specific partial waves and some of them are found close to the experimentally identified resonances in12C+12C reaction. Thus, we provide an integrated and comprehensive analysis of fusion, resonance and scattering data in the best studied case of12C+12C reaction within the framework of optical potential model.

Elastic nucleus-nucleus scattering and incompressibility of nuclear matter

The goal of this report is to investigate the repulsive core and incompressibility of the nuclear matter analysing 16O+ 16O elastic scattering.

Theoretical study of the molecular motion of liquid water under high pressure

The pressure effects on the molecular dynamics of liquid water are investigated using the site–site generalized Langevin modified mode-coupling theory. The calculations are performed for temperatures from 273 to 373 K and densities from 0.9 to 1.2 g/cm3. The static structure factor required as input is obtained from the reference interaction-site model hypernetted chain integral equation. The shear viscosity, the dielectric relaxation time, the translational diffusion coefficient, and the first-rank reorientational relaxation times are evaluated. All these quantities show unusual pressure dependence in the low-density, low-temperature region in that the molecular mobility is enhanced by applying the pressure. The magnitude of the enhancement is larger on the reorientational motions than on the translational ones. These tendencies are consistent with experimental observations, although the quantitative agreement is not so good. An analysis of the theory indicates that the decrease in the dielectric friction on the collective polarization at small wave numbers upon increasing pressure is the principal reason for the pressure-induced enhancement of the dielectric relaxation and the decrease in the dielectric relaxation time affects other motions. The decrease in the dielectric friction is caused by the decrease in the number-density fluctuation around the low-wave-number edge of the first peak of the structure factor by compression. The comparison between the results for water and acetonitrile extracts two characteristic features of water that are important for the anomalous pressure effect on its molecular motion. The first one is the small collisional friction on the reorientation due to the spherical repulsive core, and the second one is the strong short-range Coulombic interaction caused by the formation of the hydrogen bonding. A theoretical calculation on a model diatomic liquid consisting of oxygen and hydrogen atoms proposes that the above two characteristic properties of water are sufficient for the emergence of the anomalous pressure dependence. This conclusion is also supported by the molecular dynamics simulation performed on the same model diatomic liquid.

The Fisher-Widom line for systems with low melting temperature

We present Monte Carlo results for the pair distribution function of three simple fluid models, with pairwise interactions, which have low triple point temperatures and mimic some aspects of Na and Hg liquid metals. The results are then used to get the direct correlation function, by numerical solution of the Ornstein-Zernike equation, and to characterize the decay modes of any density distribution towards the bulk fluid. The Fisher-Widom line is obtained from the crossing of the two lowest inverse decay lengths, associated to monotonic and to oscillatory decay modes. For the pair potential models with a soft repulsive core, the Fisher-Widom line appears well below the critical temperature and has positive slope of the temperature with respect to the density, contrary to previous results for the Lennard-Jones and square-well potentials which had located that line quite close to T c and with negative slope.

Study of the ferromagnetic transition in a positionally frozen Heisenberg spin system

Using extensive Monte Carlo simulations with both particle and cluster orientational moves, in conjunction with finite size scaling and histogram reweighting techniques, we have determined the Curie temperature for two models of positionally frozen Heisenberg spin systems: a system with spatial correlations corresponding to a hard sphere fluid and a spatially random system. We find that the results for the positionally frozen hard sphere Heisenberg system are fairly similar to those previously obtained for the Heisenberg spin fluid and quantitatively agree with the mean field theory estimates. The random system undergoes the ferromagnetic transition at a higher temperature since the lack of core repulsion increases the spin correlations. In this case however the mean field theory overestimates by far the critical temperature.

Medium effects to the N(1535) resonance and η mesic nuclei

The structure of η-nucleus bound systems (η mesic nuclei) is investigated as one of the tools to study in-medium properties of the N(1535) ( N ∗ ) resonance. We show that, as a general consequence, the η-nucleus potential has a repulsive core at the nuclear center with an attractive part at the nuclear surface, if sufficient reduction of the mass difference of N and N ∗ stems from the in-medium effects to N ∗ . The (d, 3He) spectra are evaluated for the formation of these bound states to investigate the experimental feasibility.

Molecular versus squared Woods–Saxon α-nucleus potentials in the 27Al(α, t)28Si reaction

The differential cross-section of the 27Al(α, t)28Si reaction for 64.5 MeV incident energy has been reanalysed in DWBA with full finite range using a squared Woods–Saxon (Michel) α-nucleus potential with the modified value of the depth parameter α = 2.0 as reported in a comment article by Michel and Reidemeister. This new value produces significant improvement in fitting the data of the reaction with its overall performance, in some cases, close to that previously observed for the molecular potential. Although the non-monotonic shallow molecular potential with a soft repulsive core and the Michel potentials produce the same quality fits to the elastic scattering and non-elastic processes, they are not phase equivalent. The two types of potential produce altogether different cross-sections, particularly at large reaction angles. The importance of the experimental cross-sections at large angles for both elastic scattering and non-elastic processes is elucidated.

Do Ordinary Nuclei Contain Exotic States of Matter?

The strongly repulsive core of the short-range nucleon-nucleon interaction leads to the existence of high-momentum nucleons in nuclei. Inclusive electron scattering can be used to probe these high-momentum nucleons and study the nature of the corresponding short-range correlations in nuclei. With recent data from Jefferson Lab we have begun to map out the strength of two-nucleon correlations in nuclei, while upcoming experiments should allow us to isolate the presence of multi-nucleon correlations. In addition to their importance in describing nuclear structure, these configurations of correlated nucleons represent high density 'droplets' of hadronic matter. As the density of hadronic matter increases there should be a weakening of quark confinement, similar to the onset of deconfinement expected at extremely high temperatures. While there have been hints of non-hadronic structure in nuclei, future measurements will allow us to directly probe the quark distributions of high density configurations in nuclei. A modified quark structure in these closely packed nucleons would provide a clear signature of exotic components to the structure of nuclei.

NN interaction derived from the Nambu–Jona-Lasinio model

The nucleon–nucleon potential is calculated within the Nambu–Jona-Lasinio (NJL) model. The effect of the quark–meson coupling constant, g, on the energy per baryon in quark matter, medium quark mass, medium pion and sigma meson masses and finally on the nucleon–nucleon potential is investigated. It is found that the potential obtained is attractive and the attraction raises with increasing coupling constant g, due to the increases in the scalar interaction. The medium and long range parts of the potential are well reproduced with g=4.6, where a better agreement with the phenomenological Paris potential is obtained. When the short range part is added to the NJL potential, through the one gluon exchange potential, the potential obtained exhibits all the qualitative features of the distance-dependence of the nucleon–nucleon interaction viz., a repulsive soft core followed by an attractive part at the right positions.

Two-dimensional arrangements of magnetic nanoparticles

Arrangements of magnetic nanoparticles in a plane interacting through hard core repulsion and magnetic dipole-dipole interaction are deduced from Monte Carlo relaxations. At low density, magnetic particles are arranged in sinuous lines and rings, while at higher densities lines of magnetic moments aggregate in spirals and vortices. An in-plane magnetic field tends to make lines parallel to the field and to break rings, while a perpendicular-to-the-plane field tends to arrange the particles in a glassy state.

Understanding congruent melting in binary solids: Molecular models of benzene–hexafluorobenzene mixtures

We present a Monte Carlo computer simulation study of solid–fluid and solid–solid equilibrium in molecular models representative of binary mixtures of benzene and hexafluorobenzene. The models considered treat the molecular shape with hard sphere interaction site potentials and the electrostatic interactions via quadrupole–quadrupole potentials. Monte Carlo simulations are used to calculate the free energy of the pure solids and the 1:1 solid compound that appear in the phase diagram, and also the solution thermodynamics in the fluid phase. In common with the real fluid phase mixtures of benzene and hexafluorobenzene the fluid phase of the model is close to an ideal mixture. The model also exhibits solid–fluid and solid–solid equilibrium similar to that seen experimentally.The results indicate that enhanced stability of the benzene-hexafluorobenzene 1:1 solid compound can be understood in terms of the cooperative effects of electrostatic interactions and the packing of the anisotropic repulsive cores of the molecules.