Proton-neutron pairing in Z=N nuclei with A=76-96

The application of scanning tunnelling microscopy (STM) to semiconductor surfaces is reviewed. After a brief description of theoretical models for STM the experimental requirements are summarized. STM results will be shown and discussed for clean semiconductor surfaces (Si(111), Si(100), Si(110), high-index faces of Si, Ge and GaAs). Examples will be given for the more important reconstructions of these surfaces and for temperature dependent phase transitions, e.g., for the high-temperature transition of Si(111) and for the low-temperature c transition of Si(100). The properties of steps will be described for Si(111) and Si(100). The study of adsorption and growth processes is one of the main fields in current STM work on semiconductors and representative examples will be given for homoepitaxy, heteroepitaxy, cross-sectional studies, metal adsorption and adsorption of small molecules which give rise to surface reactions. Finally, more recent applications such as manipulation of semiconductor surfaces, ballistic-electron emission, potentiometry and electrochemistry will briefly be reviewed.

https://iopscience.iop.org/article/10.1088/0034-4885/59/6/001/pdf

Scanning tunnelling microscopy of semiconductor surfaces

The g{sub 9/2} shell of identical particles is the first one for which one can have seniority-mixing effects. We consider three interactions: A delta interaction that conserves seniority, a quadrupole-quadrupole (Q{center_dot}Q) interaction that does not, and a third one consisting of two-body matrix elements taken from experiment ({sup 98}Cd) that also leads to some seniority mixing. We deal with proton holes relative to a Z=50,N=50 core. One surprising result is that, for a four-particle system with total angular momentum I=4, there is one state with seniority v=4 that is an eigenstate of any two-body interaction--seniority conserving or not. The other two states are mixtures of v=2 and v=4 for the seniority-mixing interactions. The same thing holds true for I=6. Another point of interest is that, in the single-j-shell approximation, the splittings {delta}E=E(I{sub max})-E(I{sub min}) are the same for three and five particles with a seniority conserving interaction (a well-known result), but are equal and opposite for a Q{center_dot}Q interaction. We also fit the spectra with a combination of the delta and Q{center_dot}Q interactions. The Z=40,N=40 core plus g{sub 9/2} neutrons (Zr isotopes) is also considered, although it is recognized that the core is deformed.

Seniority conservation and seniority violation in the g9/2 shell

High spin states in /sup 81/Sr were investigated in the reactions /sup 56/Fe(/sup 28/Si,2pn) at 103.6 MeV and /sup 55/Mn(/sup 29/Si,p2n) at 95.2 MeV using the techniques of in-beam ..gamma..-ray spectroscopy. Measurements were made of ..gamma..-..gamma.. coincidences and ..gamma..-ray directional correlations from oriented nuclei. The lifetimes of 12 states and lifetime limits of 10 others were determined with the Doppler shift attenuation method. The data have been analyzed in terms of the Woods-Saxon cranking model. The first crossing observed in the yrast band is attributed to the alignment of a pair of g/sub 9/2/ protons. The irregularities observed in the J/sup (2)/ moment of inertia in the K/sup ..pi../ = (5/2/sup -/ and 1/2/sup (+)/ bands are interpreted as evidence for the alignment of a g/sub 9/2/ proton pair as well. The coexistence of prolate and oblate configurations at low spin is discussed in terms of the polarization of the soft even-even core by the odd neutron.

Shape coexistence effects and quasiparticle alignment in 81Sr.

High-spin states in {sup 74}Kr were studied using the {sup 58}Ni({sup 19}F,{ital p}2{ital n}){sup 74}Kr reaction at 62 MeV. States up to (14{sup +}) in the ground state band and a number of states in three side bands were placed in the level scheme. The level scheme of {sup 74}Kr closely resembles those of {sup 76}Kr and {sup 78}Kr. Quasiparticle alignment occurs at {h bar}{omega}{approx}0.65 MeV in agreement with a recent calculation. There is no evidence for an alignment at lower frequency as was previously reported. Mean lifetimes were measured for six states in the ground state band of {sup 74}Kr using the Doppler shift attenuation and recoil-distance methods. The lifetimes imply transition quadrupole moments of 3.2{minus}3.3 {ital e}b, the highest known among Kr isotopes. The corresponding axial deformation is {beta}{sub 2}{approx}0.41.

Quasiparticle alignment in the most deformed krypton isotope 74Kr.

The level structure of /sup 83/Zr was investigated via the fusion-evaporation reaction /sup 54/Fe(/sup 32/S,2pn)/sup 83/Zr at beam energies between 105 and 120 MeV using in-beam ..gamma..-ray spectroscopy techniques. ..gamma.. transitions from the decay of /sup 83/Zr were unambiguously identified by means of recoil-..gamma.., neutron-..gamma.., and x-ray-..gamma.. coincidences. The decay scheme was constructed on the basis of Compton suppressed ..gamma..-..gamma.. coincidences, delayed ..gamma..-..gamma.. coincidences, and angular distribution measurements. A total of 34 transitions were identified connecting 26 new energy levels and the previously known ground state. The lifetimes of two low-lying isomeric states were determined. A decoupled positive parity band built on the odd g/sub 9/2/ neutron configuration was observed up to spin (37/2h-dash-bar with the first band crossing due to proton alignment showing a sharp backbend at h-dash-bar..omega.. = 0.5 Mev. A strongly coupled negative parity band based on a (5/2/sup -/ bandhead was observed up to spin (35/2h-dash-bar. The angular momentum alignment is discussed and compared to neighboring nuclei. A theoretical analysis was performed using the Woods-Saxon cranking model. The results of the calculation are consistent with the experimental findings for /sup 83/Zr and its neighbors. Triaxial shapes are predicted for all observed bands in /sup 83/Zr. Atmore » higher angular momenta a transition to well-deformed prolate bands involving the h/sub 11/2/ intruder orbitals is expected.« less

Level structure of 83Zr.

Excited positive parity states up to spin ((41/2) have been studied in /sup 77/Kr using a 98 MeV /sup 28/Si beam on a natural chromium target deltaJ = 1 transitions have been identified throughout the positive parity band The energies, mixing ratios, and B(M1) transition rates for these transitions alternate in size as the spin increases A cranked shell model analysis was performed along with Strutinsky-Bogolyubov cranking calculations The observed decrease in the signature splitting of the nug/sub 9/2/ band has been attributed to a band crossing due to an aligning pair of g/sub 9/2/ protons Prolate quadrupole deformations of beta/sub 2/ = 034 for the one-quasiparticle band and beta/sub 2/ = 026 for the three-quasiparticle band are predicted This band crossing is associated with a shape change caused by the polarization effect of aligned quasiparticles

Shape changes and alignment properties in 77Kr.

Core polarization by an intruder orbital in81Sr

New high spin states in $^{25}\mathrm{Mg}$ (up to 13.3 MeV in excitation) have been located using the $^{12}\mathrm{N}$,p\ensuremath{\gamma}${)}^{25}$Mg reaction at a laboratory beam energy of 23.5 MeV. Gamma decay from 28 neutron unbound levels has been observed (24 of which are reported here for the first time) up to 7 MeV above the neutron drip line. Tentative identification of the yrast line up to ${(17/2}^{+}$ has been made, along with an extension of the ground-state rotational band up to ${(15/2}^{+}$ and the location of several other states with ${J}^{\ensuremath{\pi}}$${=(11/2}^{+}$ and ${(15/2}^{+}$. Extensive comparison with the s-d shell model and the cranked Nilsson-Strutinsky model has been made for the states in $^{25}\mathrm{Mg}$. Very good agreement is observed between the shell model and experimental level energies and \ensuremath{\gamma}-decay branches. Potential energy surfaces for ${J}^{\ensuremath{\pi}}$${=(11/2}^{+}$, ${(13/2}^{+}$, and ${(17/2}^{+}$ in the cranked Nilsson-Strutinsky model show a noncollective oblate local minimum at (${\ensuremath{\epsilon}}_{2}$,\ensuremath{\gamma})=(0.34,60\ifmmode^\circ\else\textdegree\fi{}) for ${(11/2}^{+}$ and prolate configurations at (0.2,-120\ifmmode^\circ\else\textdegree\fi{}) for ${(13/2}^{+}$ and ${(17/2}^{+}$. These states compete with the collective ground-state band members at \ensuremath{\gamma}\ensuremath{\approxeq}0\ifmmode^\circ\else\textdegree\fi{} and become yrast for ${(17/2}^{+}$. The stabilization of these prolate aligned states relative to more deformed, collective states of the same spin results from shell effects at high spin in the cranking model, and provides a description of the configuration in the yrast ${8}^{+}$; ${(11/2}^{+}$, ${(13/2}^{+}$, and ${(17/2}^{+}$; and ${4}^{+}$ states in $^{24}\mathrm{Mg}$, $^{25}\mathrm{Mg}$, and $^{26}\mathrm{Mg}$, respectively.

Gamma decay of high spin states in 25Mg above 6.1 MeV.

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