Search Results (198 total)

The decay of the ²⁴⁶Bk^* nucleus, formed in entrance channel reactions ¹¹B+²³⁵U and ¹⁴N+²³²Th at different incident energies, is studied by using the dynamical cluster-decay model (DCM) extended to include the deformations and orientations of nuclei. The main decay mode here is fission. The other (weaker) decay channels are the light particles evaporation (A≤4) and intermediate mass fragments (5≤A≤20). All decay products are calculated as emissions of preformed clusters through the interaction barriers. The calculated fission cross sections σ_fiss, taken as a sum of the energetically favored symmetric and near symmetric fragments (A_CN/2±7 and A=106-110 plus complementary fragments) show an excellent agreement with experimental data at all experimental incident c.m. energies for both reactions, except for the top three energies in the case of the ¹¹B+²³⁵U reaction. The disagreement between the DCM calculations and data at higher incident c.m. energies for the ¹¹B+²³⁵U entrance channel is associated with the presence of additional effects of noncompound, quasifission (qf) components, in contradiction with the measured anisotropy effects which indicate the other entrance channel ¹⁴N+²³²Th to contain the noncompound nucleus contribution. The prediction of two fission windows, the symmetric fission (SF) and near symmetric or heavy mass fragments (HMFs), suggests the presence of a fine structure of fission fragments, which also need an experimental verification. The only parameter of the model is the neck length parameter ▵R whose value is shown to depend strongly on limiting angular momentum, which in turn depends on the use of sticking or nonsticking moment of inertia for angular momentum effects.

We address the occurrence of conduction-band crossover in III-V self-assembled quantum dots solely due to misfit strain. Band structure analysis in terms of standard deformation-potential theory shows that Γ-X crossover can occur in the dot, while both Γ-X and Γ-L crossovers are possible in the matrix at the interface. Crossover changes the nature of the fundamental band gap in the heterostructure, which may dramatically affect the optical properties. The implications of this are studied for a realistic InSb∕GaSb (001) heterostructure, where Γ-L crossover renders the ground-state optical transition indirect in k space. Our calculations and photoluminescence data are in remarkable agreement.

Results are presented of single crystal structural, thermodynamic, and reflectivity measurements of the double-perovskite Ba₂NaOsO₆. These characterize the material as a 5d¹ ferromagnetic Mott insulator with an ordered moment of ∼0.2μ_B per formula unit and T_C=6.8(3)  K. The magnetic entropy associated with this phase transition is close to Rln⁡2, indicating that the quartet ground state anticipated from consideration of the crystal structure is split, consistent with a scenario in which the ferromagnetism is associated with orbital ordering.

The semiclassical formulation of the Skyrme energy density functional for spin-orbit density part of the interaction potential is compared with the microscopic shell model formulation, at both the ground state and finite temperatures. The semiclassical spin-orbit interaction potential is shown to contain exactly the same shell effects as are there in the microscopic shell model, provided a normalization of all semiclassical results to the spin-saturated case (for one or both nuclei as spin-saturated) is made. On the other hand, the α nucleus structure present in microscopic shell model is found absent in semiclassical approach. The role of temperature is found not to change the behavior of shell or α nucleus structure effects up to about 3 MeV, and increase or decrease the height of the (normalized) barriers in accordance with the shell structure of nuclei. Calculations are made for three two-nucleon transfer reactions forming the α-nucleus A=4n,N=Z compound systems ⁵⁶Ni^* and ⁴⁸Cr^* and the non-α-nucleus compound system ⁵²Cr^*, and for Skyrme forces SIII and SLy4. The two parameter Fermi density, with its parameters fitted to experiments and made temperature dependent in a model way, is used for the nuclear density in semiclassical calculations, and the same in microscopic shell model is achieved via the Fermi-Dirac occupation of shell model states and particle number conservation.

Density functional calculations are used to calculate the structural and electronic properties of BaReH₉ and to analyze the bonding in this compound. The high coordination in BaReH₉ is due to bonding between Re 5d states and states of d-like symmetry formed from combinations of H s orbitals in the H₉ cage. This explains the structure of the material, its short bond lengths, and other physical properties, such as the high band gap. We compare with results for hypothetical BaMnH₉, which we find to have similar bonding and cohesion to the Re compound. This suggests that it may be possible to synthesize (MnH₉)²⁻ salts. Depending on the particular cation, such salts may have exceptionally high hydrogen contents, in excess of 10 wt %.

Iron self-diffusion in nanocomposite FeZr alloy has been investigated using a neutron reflectometry technique as a function of applied compressive stress. A composite target of Fe+Zr and ⁵⁷Fe+Zr was alternatively sputtered to deposit chemically homogeneous multilayer (CHM) structure [^naturalFe₇₅Zr₂₅∕⁵⁷Fe₅₇Zr₂₅]₁₀. The multilayers were deposited onto a bent Si wafer using a three-point bending device. Post-deposition, the bending of the substrate was released which results in an applied compressive stress on to the multilayer. In the as-deposited state, the alloy multilayer forms an amorphous phase, which crystallizes into a nanocomposite phase when heated at 373 K. Bragg peaks due to isotopic contrast were observed from CHM, when measured by neutron reflectivity, while x-ray reflectivity showed a pattern corresponding to a single layer. Self-diffusion of iron was measured with the decay of the intensities at the Bragg peaks in the neutron reflectivity pattern after thermal annealing at different temperatures. It was found that the self-diffusion of iron slows down with an increase in the strength of applied compressive stress.

Nonperturbative results for improvement and renormalization constants needed for on-shell and off-shell O(a) improvement of bilinear operators composed of Wilson fermions are presented. The calculations have been done in the quenched approximation at β=6.0, 6.2, and 6.4. To quantify residual discretization errors we compare our data with results from other nonperturbative calculations and with one-loop perturbation theory.

Based on fragmentation theory extended to include the orientation degrees of freedom and higher multipole deformations up to hexadecapole deformations, the compactness of ⁴⁸Ca induced reactions on various actinides is studied for Ds (Z=110) to 118 nuclei. It is shown that the reactions leading to Z≥114 nuclei are “compact” hot fusion reactions at θ=90^° orientation angles (equatorial compact or ec; collisions that are in the direction of the minor axis of the deformed reaction partner), but the ones for Z<114 nuclei are compact at θ<90^° (not-equatorial compact or nec). The phenomenon of “barrier distribution in orientation degrees of freedom” is observed for the first time to be related to the magnitudes of both the quadrupole and hexadecapole deformations of the deformed reaction partner. The ec configurations are obtained for the cases of quadrupole deformation alone and with small (including negative values) hexadecapole deformations. The presence of large (positive) hexadecapole deformations result in the nec configurations. These results are found to be quite general, applicable also to other lighter targets such as W and Ra with the ⁴⁸Ca beam and to Pb based reactions. Furthermore, for compact hot fusion reactions, in addition to the ⁴⁸Ca reaction valley, a number of other new reaction valleys (target-projectile combinations) are obtained, the most important one (next to ⁴⁸Ca) being the ⁵⁴Ti nucleus used previously in Pb based cold fusion reaction studies but now proposed with deformed actinide nuclei such as ²²⁶Ra, ²³²Th, ²³⁸U, and ²⁴²Pu.

We present a magnetic-field- and pressure-dependent Raman scattering study of the complex orbital, magnetic, and conducting phases of Ca₃Ru₂O₇, which result from a rich interplay between the orbital, spin, and electronic degrees of freedom. The Raman-active phonon and magnon excitations in Ca₃Ru₂O₇ convey sufficient information to map out the orbital, magnetic, and conducting (H,T) and (P,T) phase diagrams of this material. We find that quasihydrostatic pressure causes a linear suppression of the orbital-ordering temperature (T_OO=48 K at P=0), up to a T=0 critical point near P^*∼55 kbar, above which the material is in a metallic, orbital-degenerate phase. We associate this pressure-induced collapse of the antiferromagnetic orbital-ordered phase with a suppression of the RuO₆ octahedral distortions that are responsible for orbital-ordering. We also find that an applied magnetic field at low temperatures induces a change from an orbital-ordered to orbital-degenerate phase for fields aligned along the in-plane b-axis (H∥hard axis), but induces a reentrant orbital-ordered to orbital-disordered to orbital-ordered phase change for fields aligned along the in-plane a-axis (H∥easy axis). This complex magnetic field dependence betrays the importance of spin-orbit coupling in this system, which makes the field-induced phase behavior highly sensitive to both the applied magnetic-field magnitude and direction. It is further shown that rapid field-induced changes in the structure and orbital populations are responsible for the highly field-tunable conducting properties of Ca₃Ru₂O₇, and that the most dramatic magnetoconductivities are associated with an “orbital disordered” phase regime in which there is a random mixture of a- and b-axis oriented Ru moments and d-orbital populations on the Ru ions.

We have investigated the magnetic-field- and pressure-induced structural and magnetic phases of the triple-layer ruthenate Sr₄Ru₃O₁₀. Magnetic-field-induced changes in the phonon spectra reveal dramatic spin-reorientation transitions and strong magnetoelastic coupling in this material. Further, we are able to deduce key magnetoelastic coupling parameters, and evidence that the magnetic moments are localized on the Ru sites. Additionally, pressure-dependent Raman measurements at different temperatures reveal an anomalous negative Gruneisen parameter associated with the B_1g mode (∼380  cm^-1) at low temperatures (T<75  K), which can be explained consistently with the field-dependent Raman data.

We describe the extension of the improvement program for bilinear operators composed of Wilson fermions to nondegenerate dynamical quarks. We consider two, three and four flavors, and both flavor nonsinglet and singlet operators. We find that there are many more improvement coefficients than with degenerate quarks, but that, for three or four flavors, nearly all can be determined by enforcing vector and axial Ward identities. The situation is worse for two flavors, where many more coefficients remain undetermined.

The proximity potential is obtained in the form of the generalized “pocket formula” for a collision between any two symmetric or asymmetric mass, deformed and noncoplanar (including also the case of coplanar) nuclei, having the fixed orientations θ₁ and θ₂ and any azimuthal angle ϕ(=0^°-90^°). The method is applied first to some illustrative axially symmetric noncoplanar nuclei with the known “gentle”- and “hugging”-fusion configurations (θ₁=θ₂=90^°,ϕ=90^°). The very general case of noncoplanar nuclei having any orientation and azimuthal angles is also discussed. Application of the method to a specific reaction that has been used in experiments for synthesizing a superheavy nucleus is also made.

Intermetallic compounds based on hydrogen absorbing elements usually form stable hydrides. This is the case for PdZr₂. Surprisingly, ZrPd₂ does not absorb hydrogen although both compounds have the same crystal structure and satisfy the empirical geometrical criteria for hydride formation. Results of ab initio calculations reveal an unanticipated purely electronic origin. These results have implications in the search for new intermetallics for hydrogen storage.

Within the fragmentation theory, extended to include the orientations degrees of freedom and hexadecupole deformations, for optimized orientations, the ⁴⁸Ca+²⁴⁴Pu→²⁹²114^* reaction is shown to be a “compact” hot fusion reaction. The barrier is highest (hot fusion) and interaction radius smallest (compact), which occur for the collisions in the direction of the minor axis of the deformed reaction partner (i.e. for 90^° orientation of ²⁴⁴Pu). In addition to the ⁴⁸Ca+²⁴⁴Pu reaction valley, a number of other new reaction valleys (target-projectile combinations) are shown to arise for the “optimally oriented hot” fusion process, the ⁴⁸Ca+²⁴⁴Pu being the best (lowest barrier) and ⁵⁴Ti+²³⁸U as the next possible best reaction for forming the cold compound nucleus ²⁹²114^*. A similar reaction valley for ⁴⁸Ca+²⁴⁴Pu is found absent in the “optimally oriented cold” fusion process.

Thin films of iron and permalloy (Ni₈₀Fe₂₀) were prepared using an Ar+N₂ mixture with a magnetron sputtering technique at ambient temperature. The nitrogen partial pressure during the sputtering process was varied in the range of 0⩽R_N2⩽100%, keeping the total gas flow at constant. At lower nitrogen pressures (R_N2⩽33%), both Fe and NiFe first form a nanocrystalline structure, and an increase in R_N2 results in the formation of an amorphous structure. At intermediate nitrogen partial pressures, nitrides of Fe and NiFe were obtained, while at even higher nitrogen partial pressures, nitrides themselves became nanocrystalline or amorphous. The surface, structural, and magnetic properties of the deposited films were studied using x-ray reflection and diffraction, transmission electron microscopy, polarized neutron reflectivity, and using a dc extraction magnetometer. The growth behavior for amorphous film was found to be different as compared with poly or nanocrystalline films. The soft-magnetic properties of FeN were improved on nanocrystallization, while those of NiFeN were degraded. A mechanism inducing nanocrystallization and amorphization in Fe and NiFe due to reactive nitrogen sputtering is discussed in the present article.

The clustering phenomenon in light, stable and exotic nuclei is studied within the relativistic mean field (RMF) approach. Numerical calculations are done by using the axially deformed harmonic oscillator basis. The calculated nucleon density distributions and deformation parameters are analyzed to look for the cluster configurations. The calculations explain many of the well-established cluster structures in both the ground and intrinsic excited states. Comparisons of our results with other model calculations and the available experimental information suggest that the RMF theory is well suited for studying clustering in light nuclei. A few discrepancies and their possible sources are also discussed.

Based on the UrQMD (ultrarelativistic quantum molecular dynamics) model, we have investigated the influence of the symmetry potential on the negatively and positively charged π and Σ hyperon production ratios in heavy ion collisions at the SIS (SchwerIonen Synchrotron) energies. We find that, in addition to π^-/π⁺ ratio, the Σ^-/Σ⁺ ratio can be taken as a sensitive probe for investigating the density dependence of the symmetry potential of nuclear matter at high densities (1–4 times normal baryon density). This sensitivity of the symmetry potential to both the π^-/π⁺ and Σ^-/Σ⁺ ratios is found to depend strongly on the incident beam energy. Furthermore, the Σ^-/Σ⁺ ratio is shown to carry the information about the isospin-dependent part of the Σ hyperon single-particle potential.

We study the improvement of staggered fermions using hypercubically smeared links. We calculate the strange quark mass and the kaon B parameter, B_K, in quenched QCD on a 16³×64 lattice at β=6.0. We find m_s(MS̅ ,2  GeV)=101.2±1.3±4  MeV and B_K(MS̅ ,2  GeV)=0.578±0.018±0.042, where the first error is from statistics and fitting, and the second from using one-loop matching factors. The scale (1/a=1.95  GeV) is set by M_ρ, and m_s is determined using the kaon mass. Comparing to quenched results obtained using unimproved staggered fermions and other discretizations, we argue that the size of discretization errors in B_K is substantially reduced by improvement.

The electronic structure and stability of the 20-electron complex hydride, Mg₃MnH₇ is studied using density functional calculations. The heat of formation is larger in magnitude than that of MgH₂. The deviation from the 18-electron rule is explained by the predominantly ionic character of the band structure and a large crystal-field splitting of the Mn d bands. In particular, each H provides one deep band accomodating two electrons, while the Mn t_2g bands hold an additional six electrons per formula unit.

The dynamical cluster-decay model (DCM) is developed further for the decay of hot and rotating compound nuclei (CN) formed in light heavy-ion reactions. The model is worked out in terms of only one parameter, namely the neck-length parameter, which is related to the total kinetic energy TKE(T) or effective Q value Q_eff(T) at temperature T of the hot CN and is defined in terms of the CN binding energy and ground-state binding energies of the emitted fragments. The emission of both the light particles (LP), with A≤4,Z≤2, as well as the complex intermediate mass fragments (IMF), with 4<A<20,Z>2, is considered as the dynamical collective mass motion of preformed clusters through the barrier. Within the same dynamical model treatment, the LPs are shown to have different characteristics compared to those of the IMFs. The systematic variations of the LP emission cross section σ_LP and IMF emission cross section σ_IMF calculated from the present DCM match exactly the statistical fission model predictions. A nonstatistical dynamical description is developed for the first time for emission of light particles from hot and rotating CN. The model is applied to the decay of ⁵⁶Ni^∗ formed in the ³²S+²⁴Mg reaction at two incident energies E_c.m.=51.6 and 60.5 MeV. Both the IMFs and average TKE̲ spectra are found to compare resonably well with the experimental data, favoring asymmetric mass distributions. The LPs' emission cross section is shown to depend strongly on the type of emitted particles and their multiplicities.

The stability and bonding of the ternary complex K₂PtCl₆-structure hydrides is discussed using first principles density functional calculations. The cohesion is dominated by ionic contributions, but ligand field effects are important, and are responsible for the 18-electron rule. Similarities to oxides are discussed in terms of the electronic structure. However, phonon calculations for Sr₂RuH₆ also show differences, particularly in the polarizability of the RuH₆ octahedra. Nevertheless, the yet to be made compounds Pb₂RuH₆ and Be₂FeH₆ are possible ferroelectrics. The electronic structure and magnetic properties of the decomposition product, FeBe₂ are reported. Implications of the results for H storage are discussed.

Magnetic-field- and temperature-dependent Raman scattering studies of Ca₃Ru₂O₇ reveal dramatic field-dependent properties arising from transitions between various complex orbital and magnetic phases, including a field-induced orbital-ordered to orbital-disordered transition (H_o‖  hard axis), and a reentrant orbital-ordered to orbital-disordered to orbital-ordered transition (H_o‖  easy axis). We find that the dramatic magnetic-field properties are most prevalent in a “mixed”-magnetic and -orbital phase regime, providing evidence for a strong connection between orbital phase inhomogeneity and “colossal” field effects in the ruthenates.

The “pocket formula” for the proximity potential is generalized for collisions of any two (equal or unequal) deformed nuclei, having any degree of (equal or unequal) orientations (from 0° to 180°) in space. The method is applied to axially symmetric coplaner nuclei. The case of noncoplaner nuclei will be dealt with in a separate publication.

A new semiempirical formula, with only three parameters, is proposed for cluster decay half-lives. The parameters of the formula are obtained by making a least squares fit to the available experimental cluster-decay data. The calculated half-lives are compared with the results of the earlier proposed model-independent scaling law and the empirically fitted analytical super-asymmetric fission model (ASAFM), showing more closeness to the ASAFM results.

Density-functional calculations within the weighted density approximation (WDA) are presented for YH₃ and LaH₃. We investigate some commonly used pair-distribution functions G. These calculations show that within a consistent density-functional framework a substantial insulating gap can be obtained while at the same time retaining structural properties in accord with experimental data. Our WDA band structures agree with those of GW approximation very well. It shows that there is no strong correlation in H 1s states, and the absence of self-interaction in H 1s is crucial to obtain correct band structures of these rare-earth trihydrites. But the calculated band gaps are still 1.0–2.0 eV smaller than experimental findings.