Search Results (124 total)

High spin excited states in the neutron-rich nuclei ¹³⁷I and ¹³⁹Cs were investigated from a study of the prompt γ rays emitted in the spontaneous fission of ²⁵²Cf with the Gammasphere detector array. Ten new excited levels with 18 new deexciting transitions were observed in ¹³⁹Cs and the level scheme of ¹³⁹Cs was extended up to 4670 keV. Spins and parities of levels in ¹³⁹Cs were firmly assigned up to 25/2⁺. Three new levels were found in ¹³⁷I. Shell model calculations were performed to interpret the experimental results. A good agreement between theory and experiment in both nuclei was found.

The excited states of the neutron-rich nuclei ^155,156Pm and ^91,92,93Rb were studied from the spontaneous fission of ²⁵²Cf. The γ-γ-γ and x(Pm)-γ-γ triple coincidence relations were applied to identify the γ transitions. Fourteen, six, three, twelve, and twelve new γ transitions from high-spin states were observed in ^155,156Pm, ^91,92Rb, and ⁹³Rb (first levels), respectively. The π5/2[532] rotational band in ¹⁵⁵Pm was extended up to 23/2^-. The πg_9/2 particle states and πf_5/2 particle states in ^91,93Rb weakly coupled to ^90,92Kr, respectively, are reported.

The multipole mixing ratios of ΔI=1 transitions between levels in rotational bands built on single-particle states in odd neutron nuclei are dependent on the configurations of the states. In particular, the mixing ratio can be used to distinguish between several possible single-particle configurations if interpreted with the particle-axial-rotor model. This work features the first determination of the ground-state configurations of ^109,111Ru. The single-particle structures of the ground states of ¹⁰¹Zr and ^103,105,107Mo as well as excited states in ^103,107Mo are also investigated, with a new result found in ¹⁰⁷Mo.

The mechanical behavior of a transformation based spherical cloak under wave illumination is derived. We show that the equatorial region of the cloak is subject to much higher stress than the polar regions, where the polar axis is defined along the wave propagation direction. These forces do not exist before transformation but stem from the squeezed electromagnetic space. The trajectory of the ray can be interpreted as a result of the recoil force that the cloak exerts upon the ray. The total radiation pressure on an ideal cloak is shown to be exactly zero, effecting a stationary cloak.

Patterns of shape-phase transition in the proton-neutron coupled systems are studied within the SD-pair shell model. The results show that some transitional patterns in the SD-pair shell model are similar to the U(5)-SU(3) and U(5)-SO(6) transitions with signatures of the critical point symmetry of the interacting boson model.

High spin states in ¹³⁴I were identified for the first time based on measurements of prompt γ rays from the spontaneous fission of ²⁵²Cf at Gammasphere. Five excited levels with five deexciting transitions were observed. The mass number was assigned based on the intensity of transitions in the complementary Rh fragments. Angular correlations for the first two transitions in ¹³⁴I and for high spin states in ^133,135,136I were performed, but were not sufficient to firmly assign the spins and parities in ¹³⁴I.

We use neutron spectroscopy to determine the nature of the magnetic excitations in superconducting BaFe_1.9Ni_0.1As₂(T_c=20 K). Above T_c the excitations are gapless and centered at the commensurate antiferromagnetic wave vector of the parent compound, while the intensity exhibits a sinusoidal modulation along the c axis. As the superconducting state is entered a spin gap gradually opens, whose magnitude tracks the T dependence of the superconducting gap as observed by angle-resolved photoemission. Both the spin-gap and magnetic-resonance energies are temperature and wave-vector dependent, but their ratio is the same within uncertainties. These results suggest that the spin resonance is a singlet-triplet excitation related to electron pairing and superconductivity.

The high spin states of neutron-rich ^106,107Tc nuclei have been reinvestigated by observing prompt γ rays from the spontaneous fission of ²⁵²Cf. In ¹⁰⁶Tc, a previously known collective band is expanded, and a new collective band is identified. In ¹⁰⁷Tc, a collective band based on the π5/2^-[303] orbital is confirmed and extended. Inconsistencies in the configuration assignments for positive parity bands in ^105,107Tc in the previous reports are clarified. The spins and parities as well as the configurations for the two bands in ¹⁰⁶Tc are assigned according to the angular momentum alignments and g-factor calculations. Other characteristics for the observed bands are discussed.

The driving force of density fluctuation of amorphous polymer films under annealing processes was evaluated synthetically in terms of thermodynamics on the basis of the logarithmic light intensity as a function of annealing time. The time dependence of ln(I) of polyethylene (terephthalate) (PET) as an example for characterizing an amorphous state was classified into three stages: the first stage (stage I), where ln(I) showed insignificant changes with time; the second stage (stage II), where ln(I) increased linearly; and the third stage (stage III), where the intensity deviated from the linear relationship and tended to level off. The density fluctuation by chain diffusion, termed quasi-spinodal decomposition, in stage II was analyzed in terms of an increase of trans-conformation of an amorphous chain in stage I. To provide conclusive evidence, the orientation function of chain segments was calculated by using a lattice model that accounts for entropic and energetic characters. The former character is associated with segmental orientation due to the effect of chain stiffness of Kuhn segments characterized by a rod with a relatively large length-to-width ratio x, whereas the latter is associated with thermotropic systems with anisotropic polarizabilities. By using the theoretical orientation function, Hv light scattering patterns were calculated by a statistical approach in which the optical axis of a PET chain segment was chosen along the direction perpendicular to the benzene ring. This selection was justified by comparison of the three principal refractive indexes of a PET chain. The calculated patterns provided a clear X-type lobe, when the correlation between optical elements concerning the rotational fluctuation became stronger. The calculated patterns were in good agreement with the patterns observed in stage III. The series of experimental and theoretical results indicated that the conversion from gauche- to trans-conformation plays an important role to derive the density fluctuation of amorphous polymer chains associated with the initiation of crystallization.

Low-spin states of ¹³⁰Xe have been populated by a fusion evaporation reaction followed by β-decay ¹²⁶Te(₃⁷Li,3n)¹³⁰Cs→EC¹³⁰Xe. Two candidates for 2_1,ms⁺-states could be found, by measuring the E2/M1 mixing ratio of the transitions of the 2₃⁺ and 2₄⁺, at 2150 keV and 2296 keV, to the first excited 2⁺ state. The experimental results are supported by a shell model calculation and are in a good agreement with interacting boson model-2 (IBM-2) calculations. The mixing of the 2_1,ms⁺ with a symmetric 2⁺-state and its fragmentation into the 2₃⁺ and 2₄⁺ states are discussed.

We use inelastic neutron scattering to study magnetic excitations of the FeAs-based superconductor BaFe_1.9Ni_0.1As₂ above and below its T_c (=20  K). In addition to gradually open a spin gap at the in-plane antiferromagnetic ordering wave vector (1, 0, 0), the effect of superconductivity is to form a three-dimensional resonance with clear dispersion along the c axis. The intensity of the resonance develops like a superconducting order parameter, and the mode occurs at distinctively different energies at (1, 0, 0) and (1, 0, 1). If the resonance energy is associated with the superconducting gap energy Δ, then Δ is dependent on the wave vector transfers along the c axis. These results suggest that one must be careful in interpreting the superconducting gap energies obtained by surface sensitive probes such as scanning tunneling microscopy and angle resolved photoemission.

The superconducting phase diagrams have been established by the measurements of electrical resistivity, magnetic susceptibility, and thermopower in cobalt-doped LFeAsO (L=La and Sm) systems. It is shown that the antiferromagnetic spin-density-wave order in the parent compounds is rapidly suppressed by Co doping, and superconductivity emerges at x=0.025 and 0.05 in LaFe_1−xCo_xAsO and SmFe_1−xCo_xAsO, respectively. The T_c(x) curves of both systems are domelike, with a maximum T_c of 13 K at x=0.075 in LaFe_1−xCo_xAsO and 17.2 K at x=0.1 in SmFe_1−xCo_xAsO. Thermopower measurement shows dominant electron-type transport for the Co-doped samples, in accordance with itinerant character of Co 3d electrons. We found a close correlation between T_c and the abnormally enhanced part of normal-state thermopower. The occurrence of superconductivity via the Fe-site doping in the iron-based oxyarsenide contrasts sharply with the destruction of superconductivity by the Cu-site (within CuO₂ planes) doping in high-temperature superconducting cuprates.

High-spin structures in the neutron-rich ¹⁰⁸Tc nucleus have been reinvestigated by measuring the prompt γ-rays from spontaneous fission of ²⁵²Cf. A previously known collective band has been extended up to higher spin states and a new side band has been identified. These doublet bands are proposed as pseudospin partner bands with configurations π1/2⁺[431]⊗ν[312~ 5/2⁺,3/2⁺], which is a first identification in A~100 region. The particle-rotor model (PRM) was applied to calculate levels and B(M1)/B(E2) ratios of the bands in ¹⁰⁸Tc. The calculated results are in good agreement with the experimental values.

The g-factor of the 4⁺ state in ¹³⁴Te has been measured for the first time by using a new technique developed for measuring angular correlations with Gammasphere. Also reported is the first measurement of the g-factor of the 15/2⁺ state in ¹³⁵I and the mixing ratios of several transitions. Furthermore, spins and parities are assigned to several levels in ¹³⁴Te and ¹³⁵I. The g-factor measurements are compared to shell model predictions and good agreement is found between experiment and theory.

The level scheme of ¹⁰²Zr has been expanded by measuring high-fold prompt γ-ray coincidence events in the spontaneous fission of ²⁵²Cf with the Gammasphere array. Previous identifications have been confirmed. A two-quasi-particle band with a band head energy of 2926.4 keV has been identified for the first time, as well as two rotational collective bands, with band head energies of 1386.3 and 1652.7 keV, respectively. Spins and parties are assigned based on level systematics and angular correlation measurements.

A new theoretical approach is presented that combines the Hartree-Fock variational scheme with the exact solution of the pairing problem in the finite orbital space. Using this formulation in the sd-space as an example, we show that the exact pairing significantly improves the results for the ground state energy.

A cone-shaped cloak whose cross section gradually increases along the axial direction (z direction) is proposed in this paper. We present full wave analysis of this cloak in response to electromagnetic waves, showing that a perfect conical cloak can support the propagation of any kind of fields. In addition, the reduced set of cloaking parameters is derived for azimuthally invariant (∂/∂φ=0) incident fields. The advantage of this simplified cloaking structure is that all the components of material parameters are spatially invariant with relative magnitude larger than one. Hence, conical cloak with small scattering is physically realizable within a wide band of frequency for this specific type of incident fields. Finally, we apply similar transformation to achieve a polarization rotator which can arbitrarily control the polarization of the electromagnetic wave getting through. The proposed design provides a practical way to realize invisible cloak and some other electromagnetic devices, especially in the conditions that the source distribution is rotationally symmetric.

The higher spin levels in neutron-rich ¹⁰⁴Nb have been investigated from a study of the prompt γ rays in spontaneous fission of ²⁵²Cf with the Gammasphere detector array. A new collective band has been identified for the first time. This band is proposed as a semidecoupled band based on the configuration π5/2^-[303]⊗ν1/2^-[541]. The other structural characteristics are discussed.

The high spin excited states of neutron-rich nuclei ¹⁵⁹Sm and ^89,91Kr are identified for the first time from the spontaneous fission of ²⁵²Cf. Six excited states and six γ transitions in ¹⁵⁹Sm are reported here for the first time. Five and two new γ transitions are identified in ⁸⁹Kr and ⁹¹Kr, respectively. Level schemes in ^89,91Kr are interpreted as the 2d_5/2 neutron hole configuration weakly coupled to the neighboring even-even Kr nuclei. A possible yrast spin sequence of 11/2^--9/2⁺-5/2⁺ is suggested in ⁸⁹Kr. A rotational band with the proposed configuration of ν5/2[523] is observed in ¹⁵⁹Sm.

Neutron-rich nuclei of ^153,155Nd were studied from the spontaneous fission of ²⁵²Cf. Eleven and nine new γ transitions were identified in ¹⁵³Nd and ¹⁵⁵Nd, respectively. Rotational bands with the configurations of ν5/2[642] and ν3/2[521] were identified in ¹⁵³Nd and ¹⁵⁵Nd, respectively. Excited states and γ transitions in ¹⁵⁵Nd are reported for the first time here.

The high-spin spectroscopy of neutron-rich ¹⁰⁹Ru is studied by measuring the prompt γ rays from the spontaneous fission fragments of ²⁵²Cf with 102 Compton-suppressed Ge detectors in the Gammasphere detector array. Previous band structures are confirmed and the ground state band is extended. A positive parity band based on a 332.5 keV level is newly identified. This band is proposed as a single-neutron excitation band built on the 7/2⁺[404] Nilsson orbital. Some band structural characteristics are discussed.

We investigate a general class of electromagnetic devices created with any continuous transformation functions by rigorously calculating the analytical expressions of the electromagnetic field in the whole space. Some interesting phenomena associated with these transformation devices, including the invisibility cloaks, concentrators, and field rotators, are discussed. By carefully choosing the transformation function, we can realize cloaks, which are insensitive to perturbations at both the inner and outer boundaries. Furthermore, we find that when the coating layer of the concentrator is realized with left-handed materials, energy will circulate between the coating and the core, and the energy transmitted through the core of the concentrator can be much bigger than that transmitted through the concentrator. Therefore, such concentrator is also a power flux enhancer. Finally, we propose a spherical field rotator, which functions as not only a wave vector rotator but also a polarization rotator, depending on the orientations of the spherical rotator with respect to the incident wave direction. The functionality of these transformation devices are all successfully confirmed by our analytical full-wave method, which also provides an alternate computational efficient validation method in contrast to numerical validation methods.

V(TCNE)_x, with TCNE=tetracyanoethylene and x∼2, is an organic-based molecular magnet with potential to be used in spintronic devices. With the aim of shedding light on the unoccupied frontier electronic structure of V(TCNE)_x we have studied pristine TCNE and sodium-intercalated TCNE by near edge x-ray absorption fine structure (NEXAFS) spectroscopy as well as with theoretical calculations. Sodium-intercalated TCNE was used as a model system of the more complex V(TCNE)_x and both experimental and theoretical results of the model compound have been used to interpret the NEXAFS spectra of V(TCNE)_x. By comparing the experimental and theoretical C K-edge of pristine TCNE, the contributions from the various carbon species (cyano and vinyl) could be disentangled. Upon fully sodium intercalation, TCNE is n doped with one electron per molecule and the features in the C and N K-edge spectra of pristine TCNE undergo strong modification caused by partially filling the TCNE lowest unoccupied molecular orbital (LUMO). When comparing the C and N K-edge NEXAFS spectra of fully sodium-doped TCNE with V(TCNE)_x, the spectra are similar except for broadening of the features which originates from structural disorder of the V(TCNE)_x films. The combined results from the model system and V(TCNE)_x suggest that the lowest unoccupied molecular orbital with density on the nitrogen atoms in V(TCNE)_x has no significant hybridization with vanadium and is similar to the so-called singly occupied molecular orbital of the TCNE anion. This suggests that the LUMO of V(TCNE)_x is TCNE⁻ or vanadiumlike, in contrast to the frontier occupied electronic structure where the highest occupied molecular orbital is a hybridization between V(3d) and cyano carbons. The completely different nature of the unoccupied and occupied frontier electronic structure of the material will most likely affect both charge injection and transport properties of a spintronic device featuring V(TCNE)_x.

Coordinate transformation approach has previously been used for designing a cloak which can shield an interior region in a homogeneous medium. In this paper, we consider the case where the background is no longer a homogeneous medium and determine the relative constitutive parameters of the cloak according to the multilayered and gradually changing background. We propose the parameters of cylindrical cloak structures working in multilayered and gradually changing media and the scheme of specifying these parameters could also be applied to the design of a cloak in an arbitrary isotropic background. The theoretical analysis based on coordinate transformation is given and numerical simulations are performed to illustrate these properties. The simulation results show that the cloaking with the proposed parameters performs well in these inhomogeneous background medium. Potential applications are also discussed.

Neutral-beam injection into plasmas with negative central shear produces a rich spectrum of toroidicity-induced and reversed-shear Alfvén eigenmodes in the DIII-D tokamak. The first application of fast-ion D_α (FIDA) spectroscopy to Alfvén-eigenmode physics shows that the central fast-ion profile is anomalously flat in the inner half of the discharge. Neutron and equilibrium measurements corroborate the FIDA data. The current density driven by fast ions is also strongly modified. Calculations based on the measured mode amplitudes do not explain the observed fast-ion transport.