Two-quasiparticle K-isomeric states in strongly deformed neutron-rich Nd and Sm isotopes: a projected shell-model analysis

Motivated by recent spectroscopy data from fission experiments, we apply the projected shell model to study systematically the structure of strongly deformed, neutron-rich, even-even Nd and Sm isotopes with neutron number from 94 to 100. We perform calculations for rotational bands up to spin I = 20 and analyze the band structure of low-lying states with quasiparticle excitations, with emphasis given to rotational bands based on various negative-parity two-quasiparticle (2-qp) isomers. Experimentally known isomers in these isotopes are described well. The calculations further predict proton 2-qp bands based on a 5(-) and a 7(-) isomer and neutron 2-qp bands based on a 4(-) and an 8(-) isomer. The properties for the yrast line are discussed, and quantities to test the predictions are suggested for future experiment.

Effect of configuration and conformation on the spin multiplicity in xylylene type biradicals

The singlet-triplet splitting energy gap DeltaE(S.T) = E-S - E-T is calculated for the ortho-, meta-, and para-xylylenes and their heteroatomic analogous by means of AM1-CI approach. It is shown that when the radical centers R-.(R-.=H2C.-,H2N.+- or HN.-) are twisted sufficiently Tar out of conjugation with the benzene ring, DeltaE(S.T) tends to zero or is negative, i.e, ortho-, meta-, and para-phenylenes turn into weak ferromagnetic or antiferromagnetic coupling unit, while they are strong ferromagnetic (meta-isomers) or antiferromagnetic (ortho-, para-isomers) coupling units under planar conformation. It is suggested that serious twisted conformation is not recommended candidate for the design of novel high-spin molecules with stable high-spin ground states by ortho- or para-phenylene coupling unit.

Spin-polarized ballistic transport in diluted magnetic semiconductor quantum wire systems

The energy dispersion of an electron in a double quantum wire with a diluted magnetic semiconductor barrier in between is calculated. An external magnetic field modifies significantly the energy dispersion of the electron which is different for the two spin states. The conductance exhibits many interesting peaks and dips which are directly related to the energy dispersions of the different electron spin states. These phenomena are attributed to the interwell coupling which can be tuned by the magnetic field due to the s-d exchange interaction.

Multi-quasiparticle gamma-band structure in neutron-deficient Ce and Nd isotopes

The newly developed multi-quasiparticle triaxial projected shell model approach is employed to study the high-spin band structures in neutron-deficient even-even Ce- and Nd-isotopes. It is observed that gamma-bands are built on each intrinsic configuration of the triaxial mean-field deformation. Due to the fact that a triaxial configuration is a superposition of several K-states, the projection from these states results in several low-lying bands originating from the same intrinsic configuration. This generalizes the well-known concept of the surface gamma-oscillation in deformed nuclei based on the ground-state to gamma-bands built on multi-quasiparticle configurations. This new feature provides an alternative explanation on the observation of two I = 10 aligning states in Ce-134 and both exhibiting a neutron character. (C) 2009 Elsevier B.V. All rights reserved.

Low-lying states of heavy nuclei within the nucleon pair approximation

In this article we perform systematic calculations on low-lying states of 33 nuclei with A=202-212, using the nucleon pair approximation of the shell model. We use a phenomenological shell-model Hamiltonian that includes single-particle energies, monopole and quadrupole pairing interactions, and quadrupole-quadrupole interactions. The building blocks of our model space include one J=4 valence neutron pair, and one J=4,6,8 valence proton pair, in addition to the usual S and D pairs. We calculate binding energies, excitation energies, electric quadrupole and magnetic dipole moments of low-lying states, and E2 transition rates between low-lying states. Our calculated results are reasonably consistent with available experimental data. The calculated quadrupole moments and magnetic moments, many of which have not yet been measured for these nuclei, are useful for future experimental measurements.

Study of quasiparticles bands in odd-odd Au-188

High-spin level structure of Au-188 has been studied via the Yb-173(F-19, 4n gamma) reaction using techniques of in-beam gamma-ray spectroscopy. Based on the experimental results, the level scheme of 188Au has been revised significantly. The previously reported positive parity levels have been modified and a new 20(+) level was proposed to feed the 18(+) states via two low-energy transitions. The existence of the 20(+) and the level structures above it are similar to those in the neighboring odd-odd Au-190,Au-192, therefore, the pi h(11/12)(-1)circle times-vi(13/2)(-2)h(9/2)(-1) configuration was assigned to the 20(+) state.

Rotation alignment in neutron-rich Cr isotopes: A probe of deformed single-particle levels across N=40

Recent experiments have reached the neutron-rich Cr isotope with N = 40 and confirmed enhanced collectivity near this subshell. The current data focus on low-spin spectroscopy only, with little information on the states where high-j particles align their spins with the system rotation. By applying the projected shell model, we show that rotation alignment occurs in neutron-rich even-even Cr nuclei as early as spin 8 (h) over bar h and, owing to shell filling, the aligning particles differ in different isotopes. It is suggested that observation of irregularities in moments of inertia is a direct probe of the deformed single-particle scheme in this exotic mass region.

Extracting single-particle information for the superheavy mass region by studying excited structure in transfermium nuclei

Recent experimental advances have made it possible to study spectroscopy in very heavy nuclei. We show that from the excited high-spin structure of transfermium isotopes, one may gain useful information on single-particle states for the superheavy mass region, which is the key to locating the anticipated 'island of stability'. In this work, we employ the Projected Shell Model for Cf, Fm, and No isotopes to study rotation alignment of the particles that occupy particular high-j intruder orbitals.

Using triazine as coupling unit for intramolecular ferromagnetic coupling of multiradicals

Novel high spin tri-, tetra-, pentaradicals, composed of triazine coupling units and cationic amino radical spin centers (+ . NH) under various configurations and linkages, are predicted from AM1-CI calculations. It is found that for charged planar multiradicals the stability of high spin ground states depends on both the molecular configuration and the number of end groups. Generally, cyclic 1,3-bridged charged multiradicals (S less than or equal to 5/2) possess more stable high spin ground states than their isomers under the branched 1,3,5,-bridged configuration. Therefore, it is suggested that in the design of planar high spin molecules with stable high spin ground states, less end groups and all the supposed spin centers and/or the coupling units should be under the same structural situation. (C) 1999 Elsevier Science B.V. All rights reserved.

SPIN AND VALENCE STATE EQUILIBRIA OF COBALT AND MAGNETIC-PROPERTIES OF LACOO3

On the basis of the spin and valence state equilibria and superexchange interaction of the various cobalt ions in LaCoO3, an approximate semiempirical formula has been proposed and used to calculate magnetic susceptibilities of LaCoO3 over a wide temperature range (100-1200 K). The results indicate that there are thermodynamic equilibria between the low spin state Co(III) (t2g6e(g)0) ion, the high spin state Co3+ (t2g4e(g)2) ion, the Co(II) (t2g6e(g)1) ion and the Co(IV) (t2g5e(g)0) ion in LaCoO3. The energy difference between the low spin state Co(III) and the high spin state Co3+ is about 0.006 eV. The content of the low spin state Co(III) ion is predominant in LaCoO3 and the content of the high spin state Co3+ ion varies with temperature, reaching a maximum at about 350 K, then decreasing gradually with increasing temperature. At low temperature the contents of the Co(II) ion and the Co(IV) ion in LaCoO3 are negligible, while above 200 K the contents of both the Co(II) ion and the Co(IV) ion increase with increasing temperature; however, the content of the Co(II) ion always is larger than that of the Co(IV) ion at any temperature. These calculated results are in good agreement with experimental results of the Mossbauer effect, magnetic susceptibility and electrical conductivity of LaCoO3.

MAGNETIC-PROPERTIES OF LANTHANUM MANGANITE AND VALENCE EQUILIBRIA OF MANGANESE

The paramagnetic susceptibility of lanthanum manganite has been measured over a wide temperature range (100-1073 K). On the basis of the thermodynamic equilibria between the various manganese ions with different valence and spin states and the magnetic interactions between the various manganese ions, a semiempirical formula has been proposed to calculate the paramagnetic susceptibilities of lanthanum manganite at different temperatures. The results indicate that most of the discrepancies between the calculated and experimental reciprocal susceptibilities of lanthanum manganite are less than 10% and that the relative contents of the various manganese ions in lanthanum manganite vary with temperature. The relative content of the trivalent manganese ion with a high spin state is dominant over the whole temperature range, while be relative content of the tetravalent manganese ion with a high spin state decreases monotonously with increasing temperature. At 300 K the calculated relative content of the tetravalent manganese ion in lanthanum manganite is about 34%, which is in good agreement with the experimental result (30%). There are some divalent manganese ions present in lanthanum manganite from low temperature to high temperature. The ratio of the relative contents of the tetravalent and divalent manganese ions in the compound varies with temperature. Above 750 K the relative content of the tetravalent manganese ion is less than that of the divalent manganese ion. The variation in the electrical resistivity of lanthanum manganite with temperature has also been interpreted reasonably.

Mechanisms of ultrafast laser-induced deep-subwavelength gratings on graphite and diamond

Deep-subwavelength gratings with periodicities of 170, 120, and 70 nm can be observed on highly oriented pyrolytic graphite irradiated by a femtosecond (fs) laser at 800 nm. Under picosecond laser irradiation, such gratings likewise can be produced. Interestingly, the 170-nm grating is also observed on single-crystal diamond irradiated by the 800-nm fs laser. In our opinion, the optical properties of the high-excited state of material surface play a key role for the formation of the deep-subwavelength gratings. The numerical simulations of the graphite deep-subwavelength grating at normal and high-excited states confirm that in the groove the light intensity can be extraordinarily enhanced via cavity-mode excitation in the condition of transverse-magnetic wave irradiation with near-ablation-threshold fluences. This field enhancement of polarization sensitiveness in deep-subwavelength apertures acts as an important feedback mechanism for the growth and polarization dependence of the deep-subwavelength gratings. In addition, we suggest that surface plasmons are responsible for the formation of seed deep-subwavelength apertures with a particular periodicity and the initial polarization dependence. Finally, we propose that the nanoscale Coulomb explosion occurring in the groove is responsible for the ultrafast nonthermal ablation mechanism.

Reversible Resistance Switching Effect in Amorphous Ge1Sb4Te7 Thin Films without Phase Transformation

We demonstrate a reversible resistance switching effect that does not rely on amorphous-crystalline phase transformation in a nanoscale capacitor-like cell using Ge1Sb4Te7 films as the working material. The polarity and amplitude of the applied electric voltage switches the cell resistance between low- and high-resistance states, as revealed in the current-voltage characteristics of the film by conductive atomic force microscopy (CAFM). This reversible SET/RESET switching effect is induced by voltage pulses and their polarity. The change of electrical resistance due to the switching effect is approximately two orders of magnitude.

Magnetic silicon fullerene

A metal-encapsulating silicon fullerene, Eu@Si-20, has been predicted by density functional theory to be by far the most stable fullerene-like silicon structure. The Eu@Si-20 structure is a dodecahedron with D-2h symmetry in which the europium atom occupies the center site. The calculated results show that the europium atom has a large magnetic moment of nearly 7.0 Bohr magnetons. In addition, it was found that a stable "pearl necklace" nanowire, constructed by concatenating a series of Eu@Si-20 units, with the central europium atom, retains the high spin moment. The magnetic structure of the nanowire indicates potential applications in the fields of spintronics and high-density magnetic storage.

Anisotropic Zeeman splitting and Stark shift of In1-yMnyAs1-xNx oblate quantum dots

The electronic structure, Zeeman splitting, and Stark shift of In1-yMnyAs1-xNx oblate quantum dots are studied using the ten-band k center dot p model including the sp-d exchange interaction between the carriers and the magnetic ion. The Zeeman splitting of the electron ground states is almost isotropic. The Zeeman splitting of the hole ground states is highly anisotropic, with an anisotropy factor of 918 at B=0.1 T. The Zeeman splittings of some of the electron and hole excited states are also highly anisotropic. It is because of the spin-orbit coupling which couples the spin states with the anisotropic space-wave functions due to the anisotropic shape. It is found that when the magnetic quantum number of total orbital angular momentum is nearly zero, the spin states couple with the space-wave functions very little, and the Zeeman splitting is isotropic. Conversely, if the magnetic quantum number of total orbital angular momentum is not zero, the space-wave functions in the degenerate states are different, and the Zeeman splitting is highly anisotropic. The electron and hole Stark shifts of oblate quantum dots are also highly anisotropic. The decrease of band gap with increasing nitrogen composition is much more obvious in the smaller radius case because the lowest conduction level is increased by the quantum confinement effect and is closer to the nitrogen level. (C) 2007 American Institute of Physics.