Search Results (208 total)

Using the capture cross-section data from ⁴⁸Ca+²³⁸U, ⁴⁸Ca+²⁴⁴Pu, and ⁴⁸Ca+²⁴⁸Cm reactions in the superheavy mass region, and fusion-evaporation cross sections from ⁵⁸Ni+⁵⁸Ni, ⁶⁴Ni+⁶⁴Ni, and ⁶⁴Ni+¹⁰⁰Mo reactions known for fusion hindrance phenomenon in coupled-channels calculations, the Wong formula is assessed for its angular momentum and barrier-modification effects at sub-barrier energies. The simple, ℓ=0 barrier-based Wong formula is shown to ignore the modifications of the barrier due to its inbuilt ℓ dependence via ℓ summation, which is found to be adequate enough to explain the capture cross sections for all the three above-mentioned ⁴⁸Ca-based reactions forming superheavy systems. For the capture (equivalently, quasifission) reactions, the complete ℓ-summed Wong formula is shown to be the same as the dynamical cluster-decay model expression, of one of us (R.K.G.) and collaborators, with the condition of fragment preformation probability P₀^ℓ=1 for all the angular momentum ℓ values. In the case of fusion-evaporation cross sections, however, a further modification of barriers is required for below-barrier energies, affected in terms of either the barrier “lowering” or barrier “narrowing” via the curvature constant. Calculations are made for use of nuclear proximity potential, with effects of multipole deformations included up to hexadecapole, and orientation degrees of freedom integrated for both the coplanar and noncoplanar configurations.

Based on the preformed cluster model (PCM) of Gupta and collaborators, we have extended our recent study on ground-state cluster decays to parent nuclei resulting in daughters other than spherical ²⁰⁸Pb, i.e., to deformed daughters, and the very new cases of ¹⁴C and ¹⁵N decays of ²²³Ac, and ³⁴Si decay of ²³⁸U, taking nuclei as spherical, quadrupole deformed (β₂) alone, and with higher multipole deformations up to hexadecapole (β₂, β₃, β₄) together with the “optimum” orientations of cold decay process. Except for ¹⁴C decays of ²²¹Fr, ^221-224,226Ra, and ²²⁵Ac where higher multipole deformations up to β₄ are found essential, the quadrupole deformation β₂ alone is found good enough to fit the experimental data. Because the PCM treats the cluster-decay process as the tunneling of a preformed cluster, the deformations and orientations of nuclei modify both the preformation probability P₀ and tunneling probability P, and hence the decay half-life, considerably.

We calculate the binding energy, root-mean-square radius, and quadrupole deformation parameter for the recent, possibly discovered superheavy element Z=122, using the axially deformed relativistic mean-field (RMF) and nonrelativistic Skyrme Hartree-Fock (SHF) formalisms. The calculation is extended to include various isotopes of Z=122 element, starting from A=282 to A=320. We predict highly deformed structures in the ground state for all the isotopes. A shape transition appears at about A=290 from a highly oblate to a large prolate shape, which may be considered as the superdeformed and hyperdeformed structures of the Z=122 nucleus in the mean-field approaches. The most stable isotope (largest binding energy per nucleon) is found to be ³⁰²122, instead of the experimentally observed ²⁹²122.

We calculate the equation of state in 2+1 flavor QCD at finite temperature with physical strange quark mass and almost physical light quark masses using lattices with temporal extent N_τ=8. Calculations have been performed with two different improved staggered fermion actions, the asqtad and p4 actions. Overall, we find good agreement between results obtained with these two O(a²) improved staggered fermion discretization schemes. A comparison with earlier calculations on coarser lattices is performed to quantify systematic errors in current studies of the equation of state. We also present results for observables that are sensitive to deconfining and chiral aspects of the QCD transition on N_τ=6 and 8 lattices. We find that deconfinement and chiral symmetry restoration happen in the same narrow temperature interval. In an appendix we present a simple parametrization of the equation of state that can easily be used in hydrodynamic model calculations. In this parametrization we include an estimate of current uncertainties in the lattice calculations which arise from cutoff and quark mass effects.

The role of deformations and orientations of nuclei is studied for the first time in cluster decays of various radioactive nuclei, particularly those decaying to doubly closed shell, spherical ²⁰⁸Pb daughter nucleus. Also, the significance of using the correct Q-value of the decay process is pointed out. The model used is the preformed cluster model (PCM) of Gupta and collaborators [R. K. Gupta , Proc. Int. Conf. on Nuclear Reactions Mechanisms, Varenna, 1988, p. 416; Phys. Rev. C 39, 1992 (1989); 55, 218 (1997); Heavy Elements and Related New Phenomena, edited by W. Greiner and R. K. Gupta, World Sc. 1999, Vol. II, p. 731]. In this model, cluster emission is treated as a tunneling of the confining interaction barrier by a cluster considered already preformed with a relative probability P₀. Since both the scattering potential and potential energy surface due to the fragmentation process in the ground state of the parent nucleus change significantly with the inclusion of deformation and orientation effects, both the penetrability P and preformation probability P₀ of clusters change accordingly. The calculated decay half-lives for all the cluster decays investigated here are generally in good agreement with measured values for the calculation performed with quadrupole deformations β₂ alone and “optimum” orientations of cold elongated configurations. In some cases, particularly for ¹⁴C decay of Ra nuclei, the inclusion of multipole deformations up to hexadecapole β₄ is found to be essential for a comparison with data. However, the available β₄-values, particularly for nuclei in the mass region 16≤A≤26, need be used with caution.

We study the ground and the first excited intrinsic states of ⁵³Co and its mirror nucleus ⁵³Fe, within the frameworks of the relativistic and nonrelativistic mean field formalisms. The analysis of the single-particle energy spectra of these nuclei show a competition of spins 1/2^- and 3/2^- in a low-lying excited state, which agrees well with the recent experimental observation [D. Rudolph , Eur. Phys. J. A 36, 131 (2008)] of spin and parity J^π=3/2^- for the isomeric configuration in ⁵³Co.

Application of the preformed clusters based dynamical cluster-decay model (DCM) is made to the recent data on decay of the compound systems ^118,122Ba^* at a relatively low bombarding energy of 5.5 MeV/A. The same model has been applied earlier to the intermediate mass fragment (IMF) data of ¹¹⁶Ba^*, observed at medium and higher incident energies. For the heavier ^118,122Ba^* systems, however, a complete mass fragmentation spectrum is observed experimentally. Except for a small narrow region of heavier mass fragments (8≤Z_L≤15), the DCM gives an overall reasonable description of the observed data on both the intermediate mass fragments and the fusion-fission cross-sections, whereas the statistical model calculations based on BUSCO and GEMINI codes describe the intermediate mass fragment data and the heavier mass fragment and fusion-fission data, respectively. Within the DCM (with preformation factor P₀=1), the possibility of non-compound-nucleus decay contributing to the region 8≤Z_L≤15 of heavier mass fragments is also explored. All three models use the maximum angular momentum ℓ_max as a fitting parameter, which in the DCM is fixed via a neck-length parameter for the penetrability P→1.

We show via first-principles calculations that the electronic structure of ZrPd₂, which does not form a hydride, can be modified by partial substitution of Fe for Pd, leading to a material that forms a hydride. We also show that PdZr₂, which forms a very stable hydride, can also be modified by Fe addition to lower the enthalpy of hydride formation. These results are explained in terms of electronic structure, specifically electronegativity, charge transfer, and s-d bonding, and clearly have implications in the search of new materials for hydrogen storage.

Langmuir monolayers of chiral liquid crystals on the surface of water exhibit orientational waves with complex spatiotemporal patterns. These patterns arise from a collective precession of the mesogenic molecules, driven by the evaporation of water through the monolayer. We investigate the behavior of these orientational waves around topological defects in the molecular orientation. Through Brewster angle microscopy, we find that the waves form a reversing spiral pattern, which rotates about the central vortex. With increasing relative humidity, the rotation slows and then stops. We model the system theoretically, and show that predicted patterns are in good agreement with the experiments.

Langmuir films at the air-water interface exhibit a variety of surface phases which arise primarily due to the molecular interaction governed by intermolecular separation. We have studied the thermodynamical aspects of Langmuir monolayers of amphiphilic functionalized gold nanoparticles (AGNs) at the air-water (A-W) interface. Interestingly, the AGN monolayer exhibits phases like gas, a low-ordered liquid (L₁), a high-ordered liquid (L₂), and a collapsed state. We find that the first-order phase transition between L₁ and L₂ vanishes above a critical temperature of 28.4  °C. Surprisingly, for a range of higher temperatures (≥29.4  °C and ≤36.3  °C), the L₁ phase undergoes a transition to a bilayer of the L₂ phase before entering into the collapsed state.

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.