Prolate Structures Research Articles

Effect of substitution on aniline in inducing growth of anionic micelles

Small-angle neutron scattering (SANS) measurements were carried out on sodium dodecyl sulfate (SDS) micelles in the presence of three different hydrophobic salts, i.e. aniline hydrochloride,o-toluidine hydrochloride andm-toluidine hydrochloride. All these salts induce a uniaxial growth of micelles to form prolate ellipsoidal structures. A progressive decrease in the surface charge of the micelles was observed with the addition of salts followed by a rapid growth of the micelles. The presence of a methyl substitution at the ortho position of aniline does not alter the growth behavior significantly. However, when the substitution is at meta position micellar growth is favored at lower salt concentration than that is observed for aniline. This can be explained in terms of the difference in the chemical environments of the substituents at the ortho and meta positions.

Stable structures of Ge n ( [formula omitted]) clusters

Using full-potential linear-muffin-tin-orbital molecular-dynamics (FP-LMTO-MD) method, we have investigated the geometric and electronic structures of neutral medium-sized Ge n ( n = 21 – 25 ) clusters. We have obtained not only all the low energy structures reported previously, but also some new structures with larger binding energies. Our calculations suggest that all the lowest energy structures are the stacked prolate structures including a tricapped trigonal prism (TTP) subunit at least. The structural characteristic is similar to that of the ground-state structures for Si clusters. However, the most stable structures for Ge and Si clusters are obviously different in the range 21–25.

Configuration mixing of angular momentum projected self-consistent mean-field states for neutron-deficient Pb isotopes

We study the low-lying collective excitation spectra of the neutron-deficient lead isotopes Pb182-Pb194 by performing a configuration mixing of angular-momentum and particle-number projected self-consistent mean-field states. The same Skyrme interaction SLy6 is used supplemented by a density-dependent zero-range pairing force. This study supports the interpretation of spectra made on the grounds of more schematic models in terms of coexisting spherical, oblate, prolate and superdeformed prolate structures. The model qualitatively reproduces the variation of the spectra with neutron number. Our results for E0 and E2 transition probabilities are compared with the few existing experimental data. Finally, we predict the presence of superdeformed bands at low excitation energy in the most neutron-deficient isotopes.

Shape transition of medium‐sized neutral silicon clusters

AbstractAddressing the shape transition of silicon clusters, indicated by mobility experiments on silicon cluster cations with 24 to 30 atoms, we investigate the structure of low energy neutral silicon clusters with 25, 29, and 35 atoms within a density‐functional based tight‐binding approach. Since there is strong evidence for several nearly degenerate low‐energy isomers for clusters of this size, we perform an extensive, but limited global search with Simulated Annealing and statistically analyze for each cluster size the 100 clusters with the lowest energy. We find different dominant shapes in the set of low energy clusters for each size. For neutral silicon clusters with 25 atoms, both prolate and spherical structures with low cohesive energies exist. For clusters containing 29 or 35 atoms, the low‐energy isomers exhibit a spherical shape. For each cluster size several stable isomers with similar shapes, and hence similar mobilities, but different bonding patterns exist. The most stable 25 atom cluster resulting from our global search has the lowest energy within DFT‐GGA known so far. Finally, we investigate the transition to diamond‐like bonding patterns expected for larger silicon clusters. Clusters with up to 239 atoms resemble amorphous silicon rather than the diamond structure and contain several highly coordinated atoms. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ground state structures of Si n ( n=11–25) clusters

Using full-potential linear-muffin-tin-orbital molecular-dynamics (FP-LMTO-MD) method based on single-parent evolution algorithm, we have investigated the geometric and electronic structures of neutral medium-sized Si n ( n=11–25) clusters. Some new isomers, which were not reported before, have been found. Some of them are essentially as stable as the corresponding ground state structures reported previously. Some of them are lower in energy than those reported before. Most of the most stable structures ( n=11–25) include a tricapped trigonal prism (TTP) at least. They would become prolate configurations as the atom number increases. Within the range of 11 to 25 atoms, the more compact cage-like geometries are not so stable as the prolate structures.

Structural transition of Si clusters and their thermodynamics

The lowest energy structures of Si n clusters up to 21 atoms are optimized with a genetic algorithm (GA) and density functional theory (DFT) with generalized gradient approximation (GGA). The structural transition from prolate cage-based structures to near-spherical configurations is found at n=17. Remarkable different melting behaviors of silicon clusters are obtained between those in prolate structures and near-spherical geometries. The structural transformation of near-spherical clusters is observed in the melting process. Two possible melting processes are proposed: near-spherical → prolate → subunits → molten and near-spherical → prolate → molten oblate.

Relativistic mean field description of the even-even proton drip line nuclei near Z=34.

The ground state properties of the even-even Zn, Ge, Se, Kr, and Sr nuclei are studied in the vicinity of the proton drip line, in a relativistic mean field approach. The knowledge of the properties of these nuclei is crucial in understanding the passage of the r process in astrophysics. The obtained binding energies are shown to be consistent with the measured masses wherever available or otherwise with the mass table of M\oller and Nix. The shape coexistence between prolate and oblate structures obtained for $^{70}\mathrm{Kr}$ and $^{72}\mathrm{Kr}$ isotopes is analyzed in detail.

Relative population of oblate and prolate structures in189Au

High spin states of189Au were populated via the174Yb (19F, 4n reaction at 86, 90, 95 and 100 MeV beam energies. The study of the relative population of oblate and prolate structures shows a striking disappearance of the prolate band relative to the oblate ones as the beam energy goes from 86 to 100 MeV.

Relative population of the oblate and prolate structures in187Au

The population of the oblate and the prolate structures in187Au has been studied using the “Château de Cristal” set-up through the172Yb(19F, 4n) reaction at 90, 95 and 100 MeV beam energy. γ — γ, γ-Fold and γ -Sum energy coincidence measurements were performed, γ-ray intensity measurements showed that as the beam energy increases the prolate system is less populated than the oblate one. Fold and Sum-energy associated with oblate and prolate structures showed a different behaviour with increasing beam energy. Two mechanisms are suggested to explain these results.

Electromagnetic properties of high-spin states in 154Dy

Liftimes and g-factors of high-spin states in 154Dy, populated in the 110Pd( 48Ca,4n) reaction, have been measured by the recoil-distance and the recoil-distance transient field methods, respectively. The experiments were carried out in coincidence mode using the multidetector arrays ESSA30 and TESSA3. A drastic loss of collectivity along the yrast line is observed, consistent with a change from prolate collective structures at low spin to predominantly oblate particle-aligned structures at high spin. The g-factors confirm the νi 13 2 alignment of the s-band and indicate contributions of proton alignment at higher spins.

High-spin structure of 189Tl: Role of h9/2 protons in the prolate minimum of light Hg isotopes.

High-spin states in $^{189}\mathrm{Tl}$ have been populated through the $^{165}\mathrm{Ho}$${(}^{28}$Si,4n) reaction and studied with in-beam \ensuremath{\gamma}-ray spectroscopy techniques. Both oblate and prolate structures associated with the i13/2 proton orbit are confirmed and extended to higher spins (19/2 and 41/2, respectively). However, only the oblate structure related to the \ensuremath{\pi}h9/2 has been observed meaning that the associated prolate structure is nonyrast contrary to expectation. This experimental result points clearly to a large amplitude of (\ensuremath{\pi}h9/2${)}^{2}$ in the wave function of the prolate minimum of $^{188}\mathrm{Hg}$.

Theory of helical structures of tilted chiral lipid bilayers.

Analogous to the theory of cholesteric liquid crystal, a theory for tilted chiral lipid bilayers (TCLB) [briefly reported in Phys. Rev. Lett. 65, 1679 (1990)] is developed. The tilt equation and the surface-shape-equilibrium equation have been derived. Application of the theory shows that on a cylindrical surface, the field of tilt directions of TCLB molecules forms a right-handed helix with a 45\ifmmode^\circ\else\textdegree\fi{} gradient angle for ${\mathit{k}}_{2}$g0 or a left-handed helix with -45\ifmmode^\circ\else\textdegree\fi{} gradient angle for ${\mathit{k}}_{2}$0, where ${\mathit{k}}_{2}$ is the chiral curvature modulus of the cholesteric liquid crystal. In addition, there exists another type of helical structure that looks like twisted strips or helicoids. It is also shown that on a spherical surface, the tilt field has at least two singular points. Based on these results, the succession of transitions from a vesicular dispersion to a phase involving helical structures and then to tubes observed in several laboratories can be reasonably explained. The general formula that explains the aggregation of narrow and prolate ribbon structures is derived. By including more terms of elasticity free energy into the theory, the size of the helical structure is obtained and is shown to agree with experimental observations.

Study of band structures in the γ-unstable nucleus 186Pt

High-spin states in the transitional nucleus 186Pt have been populated via the 188Os(α, 6n) reaction and studied with the OSIRIS spectrometer. The yrast band and the γ-band have been extended to I π = (26 +) and 10 +, respectively, and nine new side bands have been observed. The features of the γ-band suggest that the nucleus is extremely γ-soft. A coexistence of prolate, oblate and triaxial band structures has been established. The crossing observed in the bands built on the prolate nuclear shape is considered to result from the ( πh 9 2 ) 2 alignment. For oblate shapes a ( v i 13 2 ) 2 alignment has been found.

Shape coexistence in 187Au.

The level structure of odd-proton $^{187}\mathrm{Au}$ was studied using the $^{172}\mathrm{Yb}$${(}^{19}$F,4n) reaction. Gamma-gamma coincidence, angular distributions and \ensuremath{\gamma}-\ensuremath{\gamma}-time measurements were used to establish the energies, spins, parities, and lifetimes for the states in the decay scheme of $^{187}\mathrm{Au}$. Both prolate and oblate structures have been established. The prolate bands are built on the \ensuremath{\pi}${h}_{9/2}$ (\ensuremath{\alpha}=\ifmmode\pm\else\textpm\fi{}(1/2) and the \ensuremath{\pi}${i}_{13/2}$ (\ensuremath{\alpha}=(1/2) particle states. An oblate, strongly coupled band built on a 90\ifmmode\pm\else\textpm\fi{}10 ns isomeric state is reported for the first time. A second oblate structure built on the ${h}_{11/2}$ proton-hole state was established to much higher spin than before. The backbend present in the \ensuremath{\pi}${i}_{13/2}$ band has been interpreted as the alignment of a pair of ${h}_{9/2}$ protons.

Shape coexistence at high spin in187Au

High-spin states in187Au have been populated in the172Yb(19F, 4n) reaction and studied with in-beam spectroscopic techniques using the “Château de Cristal” 4π-multidetector array. A comprehensive level scheme of187Au has been established. Experimental band crossing frequencies and gains in alignment were deduced. Shape coexistence in187Au, well established at low spin, is found to survive up to spin 57/2, and manifests itself through well separated oblate and prolate structures.

High-spin excitations of 187, 188Hg

High-spin states in 187, 188Hg have been investigated via the 163, 164 Dy( 28 Si, x n) reactions by in-beam γ-ray spectroscopic techniques using the “Chateau de Cristal” 4π-multidetector array. Level schemes have been established. Spins and parities of the states have been deduced from transition multipolarities obtained from measurements of the angular distribution and linear polarization of the γ-rays. The shape coexistence of both prolate and oblate band structures has been observed for the first time in 187Hg. In 188Hg negative-parity bands are well developed. The band structure is interpreted within the frame of the Strutinsky method with the deformed Woods-Saxon average field. Pairing behaviour with increasing spin and general features of shape coexistence in mercury nuclei are illustrated.

Shape coexistence in187Hg

Excited states of187Hg have been investigated via the164Dy (28Si,5n) reaction by means of in- beam gamma-ray spectroscopy using the multidetector 4π away “Ckāteau de. Cristal”. Different bands were populated up to spin I- 45/2. Coexistence of both prolate and oblate structures is observed for the first time in an odd- A Hg isotope. The crossing of aligned and strongly coupled sequences of levels built on the vi13/2 orbital infers the near degeneracy of oblate and prolate structures in187Hg.

Discontinuities and prolate-oblate competition near spin 40 in the N=90 isotones

The systematics of irregularities in rotational sequences near I=40 are summarised for N=90 isotones. These discontinuities can be understood in terms of competition between prolate collective structures and oblate single-particle states. Such an interpretation implies a sizeable interaction between “prolate collective” states and “oblate single-particle” states which was unexpected.

Coexistence of prolate and oblate structures up to spin 40

A rotational band with a constant moment of inertia ${\mathcal{I}}^{(2)}={58\ensuremath{\hbar}}^{2}$ ${\mathrm{MeV}}^{\ensuremath{-}1}$ has been observed in $^{152}\mathrm{Dy}$ extending from spin ${18}^{+}$ to ${40}^{+}$. The band coexists with the known oblate yrast states although it is up to 1.5 MeV higher in excitation energy. It carries 4% of the decay strength and it is proposed to be a four-quasiparticle aligned structure. It is conjectured that the superdeformed states at the highest spins may decay via this band rather than via the yrast oblate states.

Shape coexistence and alignment processes in the light Pt and Au region

High-spin states were populated in 185Au via the 170Yb( 19F, 4n) 185Au reaction. The shape coexistence of both prolate and oblate band structures was observed. In the prolate bands the two-quasiparticle πh 9 2 and νi 13 2 alignment processes are seen to occur at similar frequencies. The upbend observed in 184Pt is then interpreted as resulting from a double crossing involving both of these processes. Strong evidence is presented for the first identification of a low-lying f 7 2 structure in nuclei below lead.