Crossing of shears bands in196Pb

High-spin states in¹⁹⁶Pb have been populated using the reaction¹⁷⁰Er(³⁰Si,4n). The previously observed shears bands in this nucleus have been extended and some of their transitions have been reordered. They now form regular bands with band crossings. One of the bands splits into two pathways at high spin.     

Crossing of shears bands in 196Pb

High-spin states in 196Pb have been populated using the reaction ¹⁷⁰Er(³⁰Si,4n). The previously observed shears bands in this nucleus have been extended and some of their transitions have been reordered. They now form regular bands with band crossings. One of the bands splits into two pathways at high spin.     

Superdeformed bands and ‘shears’ bands in 197Pb and 198Pb

Excited states in ¹⁹⁷Pb and ¹⁹⁸Pb, produced in the ¹⁸⁶W (¹⁸O,xn) reaction, were studied using the EUROGAM phase 2 γ ray spectrometer array. Two new superdeformed bands have been observed in ¹⁹⁷Pb, and the existence of a superdeformed band in ¹⁹⁸Pb has been confirmed. The increased statistics provided by the latest generation of arrays has resulted in the reordering of some of the previous known cascades of magnetic dipole transitions, with the first observation of the extremely weak E2 crossover transitions.     

Simultaneous GCSM description of the M1 state and the major collective bands

The generalized coherent state model is used to simultaneously describe the magnetic state 1⁺ and the collective bands: ground, beta and gamma. Effective operators for the M1 and the E2 transitions are derived. Also the M1 form factor for the (e, e′) scattering is treated in the plane wave Born approximation. The symmetry of the lowest gamma band is discussed. The application is made for ¹⁵⁴Gd, ¹⁵⁴Sm, ¹⁶⁴Dy, ¹⁶⁸Er, ¹⁷⁴Yb, ²³²Th, ²³⁸U. The agreement with the experiment is very good for both the 1⁺ state and the considered collective bands.     

Microscopic description of absolute values for B(E2) within IBM

Processes governing the parameters of the boson operator for quadrupole transitions in the IBM1 representation are considered. Absolute values for B(E2) are described without introducing effective charges.     

Quadrupole moment of the 11(-) intruder isomer in 196Pb and its implications for the 16(-) shears band head

The quadrupole moment of the 11(-) isomer in 196Pb has been measured by the level mixing spectroscopy method. This state has a pi(3s(-2)(1/2)1h(9/2)1i(13/2))11(-) configuration which is involved in most of the shears band heads in the Pb region. The first directly measured value of Q(s)(11(-)) = (-)3.41(66) b, coupled to the previously known quadrupole moment of the nu(1i(-2)(13/2))12(+) isomer allows us to estimate the quadrupole moment of the 16(-) shears band head as Q(s)(16(-)) = -0.32(10) b. The experimental values are compared to tilted axis cranking calculations, giving insight into the validity of the additivity approach to couple quadrupole moments and on the amount of deformation in the shears bands.     

Superdeformation in Pb: Observation of three additional excited bands

The superdeformed nucleus ¹⁹³Pb has been studied following a heavy-ion fusion-evaporation reaction with the Gammasphere spectrometer array. Three new superdeformed bands, two of them probably signature partners, have been observed, bringing the total number of superdeformed bands known in this nucleus to nine. An estimate of the g-factor associated with one of the previously known pairs of strongly coupled excited bands has been made, confirming their configuration. The properties of the bands are compared with the results of cranked shell model calculations; good agreement is found and configurations are assigned to each band. The two new signature partner bands are suggested to be based on the two signatures of the [512]5/2 neutron orbital. The third new band is probably based on the 7₃ intruder orbital. In addition, the high-energy region of the spectrum in coincidence with the yrast superdeformed bands has been carefully studied. The current data provide no evidence for the previously reported transitions linking the superdeformed and normal-deformed states with energies around 2.2–2.4 MeV.     

IBA,macroscopic SU(3) and microscopic description of ground state bands in rotational nuclei

The number of bosons deduced from variational calculations of spectra and intrinsic mass quadrupole moments of heavy deformed nuclei are compared with predictions of IBA in the SU(3) limit. The differenct between the two suggest the need for including l > 2 bosons in IBA.     

Towards a shell-model description of the low-energy structure of deformed nuclei II. Electromagnetic properties of collective M1 bands

A shell-model theory, called the pseudo SU(3) model, which was proposed previously for giving the structure of low-lying states in heavy deformed nuclei is used to predict the number of 1⁺ states with strong M1 transitions to ground states for the nuclei ¹⁵⁴Sm, ^{156 160}Gd, ¹⁶⁴Dy, ¹⁶⁸Er, ¹⁷⁴Yb of the rare earth region and the actinide species ²³²Th, ^{234 240}U, ²⁴²Pu. Results are also given for E2 and M3 transition strengths in these nuclei. The measures provide a rigorous test of the theory, which in reality is a many-particle Nilsson scheme, because the real M1, E2, and M3 operators are used in the calculations. It is found that the results for E2 strengths using the real quadrupole operator differ by less than 5% from those of calculations that use the operator which is a generator of the pseudo SU(3) symmetry. This is so even for weak interband transitions. To further test the theory additional experimental information on the 2_γ⁺ states is necessary. In particular, the 1⁺ → 2_γ⁺ decay strengths are needed to differentiate between theories for the structure of the giant M1 states.     

The 3B1-1A1 band system of sulfur dioxide: Rotational analysis of the (010), (100), and (110) bands

The (010)-(000) band of the ${}^3\text{B}{}_1\text{←}{}^1\text{A}{}_1$ system of sulfur dioxide has been rotationally analyzed and constants for the magnetic interactions in the (010) vibrational level determined. Partial rotational analyses are given for the rotationally perturbed (100)-(000) and (110)-(000) bands of the system. The extensive perturbations in this band system are provisionally attributed to: (i) vibronic interaction with a ³A₂ state, visible (a) as a large negative anharmonicity in higher bands of the ³B₁ system, and (b) as a ΔK = 0 resonance perturbation in low-lying vibrational levels of the ³B₁ state; and (ii) a K, K ± 1N-dependent perturbation possibly due to rotational-electronic coupling with a ³B₂ state, though evidence on this point is incomplete. Evidence bearing on these conclusions is discussed.     

The (1 × 1) structure of Si(111): Strong correlations in surface state bands?

It is shown that a model in which surface states in the semiconductor gap are described by the Hubbard model yields an interpretation of photoemission from the (1 × 1) structure of Si(111) which is compatible with the relaxed upper layer structure extracted from analyses of low energy electron diffraction intensities. The model predicts a low temperature antiferromagnetic ground state for the two-dimensional surface state electron fluid. This state should be observable experimentally.     

Systematic behavior ofB(E2) values in the yrast bands of doubly even nuclei

The experimental information onB(E2) transition rates in the yrast bands of doubly even nuclei (126≦A≦184) is systematized. The strength functionS _exp≡B(E2,I→I?2)×E(I→I?2) is found to reveal characteristic behavior significant for structure studies of yrast bands. The energy-weightedB(E2,I→I?2) values (S _exp) and 2?/?²(?: moment of inertia) are plotted versus the rotational frequency squared ?²ω² for each nucleus. In strongly deformed nuclei (N≧90), theS _exp curves smoothly increase for low rotational frequencies suggesting that up to spin valuesI≈8 the ratioQ ₀ ² ? is nearly constant (Q ₀: quadrupole moment). This is not the case in nuclei with a soft core (N≦88). In the relevant discussion, the hydrodynamical model as well as the CAP effect are considered. The results in the backbending region are qualitatively discussed in terms of the two-band crossing model. Evidence is found supporting the prediction of an oscillating behavior of the yrast-yrare interaction.     

Pb induced charge accumulation on InAs(1 1 1)B

The Pb/InAs(1 1 1)B interface has been studied by synchrotron radiation photoelectron spectroscopy (SR-PES) of valence band and In4d, As3d and Pb5d core levels. Room temperature deposition of ∼1 ML of Pb on InAs(1 1 1)B leads to an ordered overlayer that induces a metallic channel at the surface, as seen through a weak emission in the vicinity of the Fermi level. Its narrow localization in reciprocal space supports the formation of a two-dimensional free electron gas (2DEG) in the surface region. It is proposed that the adsorbed metal layer swaps the initial polarisation of the surface and thus pulls electrons back to the surface. This charge re-arrangement increases the charge density in the accumulation layer and reduces the screening length and thus the depth of the potential well at the surface.