Competition between ferromagnetic and antiferromagnetic spin ordering in nuclear matter

The possibility of ferromagnetic and antiferromagnetic phase transitions in symmetric nuclear matter is analyzed in Fermi liquid theory with the Skyrme effective interaction. The density dependence of the ferromagnetic and antiferromagnetic parameters of spin polarization at zero temperature is obtained for SkM* and SGII effective potentials. In the density region where both solutions of self-consistency equations exist, the ferromagnetic spin state is preferable over the antiferromagnetic spin state.     

Coulomb instability of charged clusters

A model different from the Rayleigh model for Coulomb instability of charged metallic clusters is proposed. The two-component model of a metallic cluster in the quasi-classical approximation offers different critical charges depending on the type of charged particles. For small-sized parallelepiped clusters, the quantization of the electronic spectrum is taken into account. The critical sizes of Ag _N ^2? and Ag _N ^3? clusters are calculated in the framework of the proposed model. The results of calculations are in good agreement with experimental data. The Coulomb explosion of positively charged clusters Na _N ⁿ⁺ at 3≤n≤5 is explained qualitatively.     

Topological defect motifs in two-dimensional Coulomb clusters

We study the distribution of topological defects in two-dimensional Coulomb clusters with parabolic lateral confinement. The minima hopping algorithm based on molecular dynamics is used to efficiently locate the ground-?and low-energy metastable states, and their structure is analysed by means of the Delaunay triangulation. The size, structure and distribution of geometry-induced lattice imperfections strongly depends on the system size and the energetic state. Besides isolated disclinations and dislocations, classification of defect motifs includes defect compounds-grain boundaries, rosette defects, vacancies and interstitial particles. Proliferation of defects in metastable configurations destroys the orientational order of the Wigner lattice.     

Competition between pairing and deformation in finite Fermi systems

The problem of the possible deformation of finite Fermi systems with particle numbers of ≤10⁴ is studied within the Bardeen-Cooper-Schrieffer model. It is shown that for slight deformations of a spherical system consisting of one valence l shell (l ≫ 1), the results of the quasi-particle theory and the exact Richard- son theory virtually coincide. Analysis of the equilibrium deformation of a cluster shows that the system does not keep its spherical form even at a low number of valence particles.     

New form for the Coulomb potential in ordering alloys

A method of constructing the Coulomb potential in substitutional alloys with arbitrary positions of atoms of different species in the crystal lattice sites is suggested. The electron density distribution in the crystal is written as the sum of spherical distributions of electrons in the free atom. The work is carried out for longrange ordered alloys, though it can be easily generalized to short-range ordered alloys.     

Competition between binary reactions and fusion in heavy-ion collisions at the Coulomb barrier

Mass and charge distributions for binary reaction channels have been measured for the reactions⁸⁶Kr with⁷⁶Ge,¹⁰⁴Ru and¹³⁰Te at the Coulomb barrier using chemical separations and-ray spectroscopy. These systems span the region where dynamical hindrance to complete fusion sets in. The binary reactions can be subdivided into two components associated withi) reflection from the outer potential barrier (quasielastic), andii) reseparation after passing the barrier (complex reactions). The sum of complex-reaction channels and evaporation residues from complete fusion can be reproduced by a barrier passing calculation. The fraction of the barrier passing flux leading to reseparation increases from 26±10% for the lightest system to more than 90% for the heaviest system. The data indicate that fusion hindrance is primarily caused by reseparation shortly after passage of the barrier before Swiatecki's conditional saddlepoint is overcome, resulting in partitions close to the entrance channel configuration. In addition, for the heaviest system, a quasifission component representing somewhat less than 20% of the barrier-passing flux was observed. From the missing masses of fragment pairs we can deduce that the reseparating complex-reaction products have kinetic energies well below the fusion barrier and share the excitation energy in a way similar to the sawtooth-like curve known from low-energy fission. The quasielastic, predominantly one- and two-nucleon transfer channels, have strongly varying cross sections for the three systems despite similar effectiveQ-values. A systematics of one-neutron transfer cross sections at the Coulomb barrier is established and shown to differ considerably from the smooth behaviour observed at energies 20–30% above the barrier. The connection to nuclear polarization phenomena and orbit matching is pointed out.Nuclear reactions:⁷⁶Ge,¹⁰⁴Ru,¹³⁰Te(⁸⁶Kr, X).E=3.22 MeV/u, (3.64) 3.84 MeV/u, 3.96 MeV/u; enriched targets; catcher foil technique, chemical separations,-ray spectroscopy; deduced mass and charge distributions for binary reactions; competition with complete fusion     

Oscillation modes in Coulomb clusters with variable charges

The mode spectra of small N=3 and N=7 clusters of dust particles with fluctuating charges are analyzed. It is demonstrated that the main effect related to the charge fluctuations is a splitting of the spectrum modes. The splitting depends the value of the charge variance. It is found that the most affected mode is the fundamental pure rotational mode. The least affected modes are pure translational ones while other modes demonstrate mixed response to the variance of the charge fluctuations.     

Competition between the Coulomb and phonon fields: superconductivity in a quiescent Fermi gas

Co-existence of and competition between the screened Coulomb and phonon fields are fundamental facts in superconductors. In the conventional superconductors the strong screening greatly suppresses the Coulomb interaction and the phonon field that mediates a given attractive interaction between electrons may thus play a leading role. A quiescent Fermi sea hence applies, as proposed by Cooper and adopted in BCS.In this model a graphical derivative technique, based on Ward's identity, is developed to consider the contributions to the interparticle interaction. This technique allows one to evaluate the effective two-particle interaction (T-matrix). It is shown that the conventional Cooper instability in a quiescent Fermi gas is only caused by agivenattractive interaction. Of more importance is that the methodology presented in this work can be extended to the investigation of the intermediate and strong coupling cases where the well-known Migdal theorem is not applicable and the vertex corrections must be evaluated.     

Stationary entanglement between two nanomechanical oscillators induced by Coulomb interaction

We propose a scheme for entangling two nanomechanical oscillators by Coulomb interaction in an optomechanical system. We find that the steady-state entanglement of two charged nanomechanical oscillators can be obtained when the coupling between them is stronger than a critical value which relies on the detuning. Remarkably, the degree of entanglement can be controlled by the Coulomb interaction and the frequencies of the two charged oscillators.     

Coulomb explosion patterns of fast C60 clusters in solids

The molecular dynamics method is used to simulate Coulomb explosion of fast C60 ion clusters in an Al target, taking into account dynamical screening of interionic interactions by the electron gas of the target. It is found that the wake forces in the medium are strong enough, depending on cluster speed, to stabilize the whole cluster against Coulomb explosion, or to compress the trailing part of the cluster, for prolonged times of penetration through the target. This is encouraging news for such cluster-ion beams applications where massive energy depositions in small volumes of targets are desired.     

Ionic recoil energies in the Coulomb explosion of metal clusters

The photoionization of metal clusters in intense femtosecond laser fields has been studied. In contrast to an experiment on atoms, the interaction in this case leads to a very efficient and high charging of the particle where tens of electrons per atom are ejected from the cluster. The recoil energy distribution of the atomic fragment ions was measured which in the case of lead clusters exceeds 180 keV. Enhanced charging efficiency which we observed earlier for specific pulse conditions is not reflected in the recoil energy spectra. Both the average and the maximum energies decrease with increasing laser pulse width. This is in good agreement with molecular dynamics calculations. Received 20 December 2000     

Long-range ferromagnetic dipolar ordering of high-spin molecular clusters

We report the first example of a transition to long-range magnetic order in a purely dipolarly interacting molecular magnet. For the magnetic cluster compound Mn6O4Br4(Et2dbm)6, the anisotropy experienced by the total spin S = 12 of each cluster is so small that spin-lattice relaxation remains fast down to the lowest temperatures, thus enabling dipolar order to occur within experimental times at T(c) = 0.16 K. In high magnetic fields, the relaxation rate becomes drastically reduced and the interplay between nuclear- and electron-spin lattice relaxation is revealed.     

Structures, stabilities and ordering in Ni-Al nanoalloy clusters

A detailed study is made of the structures and relative stabilities of nickel and aluminium clusters and nickel-aluminium "nanoalloy" clusters, with up to 55 atoms, modelled by the many-body Gupta potential. Random search and genetic algorithms are used to find the lowest energy isomers (both geometrical and permutational). For the pure Al and Ni clusters, the lowest energy structures are identical for most nuclearities but different structures are found for clusters with 15-18 atoms -- the Al clusters having non-icosahedral structures. For these nuclearities, we investigate the effect of doping Al atoms into pure Ni clusters and vice versa, finding that the replacement of a single atom by a dopant atom is often sufficient to change the cluster geometry. The lowest energy isomers of nanoalloy clusters with the approximate composition "Ni₃Al" generally have structures based on icosahedral packing, though truncated octahedral (fcc packing) motifs are also observed. In agreement with previous studies, the atom ordering in the mixed Ni-Al clusters is found to depend on the maximization of the number of Ni-Al interactions, the minimization of the cluster surface energy, and atom size effects.Received: 17 January 2003, Published online: 17 July 2003PACS: 36.40.Mr Spectroscopy and geometrical structure of clusters - 61.46.+w Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals - 61.66.Dk AlloysM.S. Bailey: Present address: Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, NY 14853-1301, USA.     

Ultrafast Coulomb explosion imaging of molecules and molecular clusters

Taking an image of their structure and a movie of their dynamics of small quantum systems have always been a dream of physicists and chemists. Laser-induced Coulomb explosion imaging (CEI) provides a great opportunity to make this dream a reality for small molecules or their aggregation —— clusters. The method is unique for identifying the atomic locations with ångstrom spatial resolution and capturing the structural evolution with a femtosecond time scale, in particular for imaging transient state products. This review summarizes the determination of three-dimensional equilibrium geometry of molecules and molecular cluster system through the reconstruction from the fragments momenta, and also shows that the dissociation dynamics on the complex potential energy surface can be tracked in real-time with the ultrafast CEI (UCEI). Furthermore, the detailed measurement and analysis procedures of the CEI, theoretical methods, exemplary results, and future perspectives of the technique are described.     

Competition between two kinds of information among random-walking individuals

A model is proposed to describe the competition between two kinds of information among N random-walking individuals in an L × L square, starting from a half-and-half mixture of two kinds of information. Individuals remain or change their information according to their neighbors’ information. When the moving speed of individuals v is zero, the two kinds of information typically coexist, and the ratio between them increases with L and decreases with N. In the dynamic case (v>0), only one information eventually remains, and the time required for one information being left scales as Td～vαLβNγ.     

Competition between SFG and two SHGs in broadband type-I QPM

In this paper, we have studied the characteristics of second-order nonlinear interactions with band-overlapped type-I quasi-phase-matching (QPM) second harmonic generation (SHG) and sum-frequency generation (SFG), and predicted a blue-shift with a band-narrowing of their bands and a sunken response in the SFG curve, which are due to the phase-matching-dependent competition between band-overlapped SHG and SFG processes. This prediction is then verified by the experiment in an 18-mm-long bulk MgO-doped periodically poled lithium niobate crystal (MgO:PPLN) and may provide the candidate solution to output controlling for flexible broadcast wavelength conversion, channel-selective wavelength conversion and all-optical logic gates by cascaded QPM second-order nonlinear processes.     

Application of finite statistical ensembles in atomic ordering theory

Fluctuation relations are obtained which correspond to finite ensembles of different techniques in atomic ordering theory: Gorskii-Bragg-Williams, Kirkwood, quasichemical, the Bogolyubov-Born-Green-Kirkwood-Ivon equations, and Green's functions. The compatibility of the equilibrium equations of the aforementioned techniques with the corresponding fluctuation relations is verified.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 25–31, December, 1986.