Electron–hole symmetry and spin–orbit splitting in IV–VI asymmetric quantum wells

It is shown that, due to the electron–hole symmetry of the fundamental gap of the lead–salts (PbTe, PbSe and PbS), the Rashba spin splitting in their flat band asymmetric quantum wells is much reduced with the usual equal conduction and valence band-offsets. Different from the III–V case, we find that the important structure inversion asymmetry for the Rashba splitting in IV–VI quantum wells with different left and right barriers is not a material property (i.e., barrier height, effective mass or band gap) but results from the band alignment. This is shown by specific envelope function calculations of the spin-dependent subband structure of Pb_1−xEu_xTe/PbTe/Pb_1−yEu_yTe asymmetric quantum wells (x≠y), based on a simple but accurate four-band kp model for the bulk band structure near the gap, which takes into account band anisotropy, nonparabolicity and multi-valley effects.     

The mechanical flexibility,electronic structure and carrier mobility of monolayer GeP: A first principles study

Inspired by successful exfoliation in experiment, we explore mechanical, electronic and transport properties of GeP monolayer using first-principles calculations. It is found that the cleavage energy of GeP monolayer is ∼0.39 J/m², verifying it can be feasibly extracted from its bulk form. The calculated stress-strain relation reveals that monolayer GeP can withstand a tensile stress and strain up to 14.88 GPa and 28%, respectively. The band structure calculations indicate the monolayer GeP possesses an indirect band gap ∼2.28 eV, which can be reduced to 0.43 eV and experiences an indirect-direct transition when axial strain is applied. Besides, the effective masses can be dramatically tuned by strain. The predicted carrier mobilities of GeP monolayer are directionally anisotropic and the electron mobility in x direction exhibits high carrier mobility up to 1242.09 cm² V⁻¹ S⁻¹. Therefore, GeP monolayer has great potential for applications in high performance flexible field-effect transistors and optoelectronic devices.     

Suppression of band crossing in the neutron-rich nuclei 172, 173Yb due to the absence of a static pair field

High-spin states in the neutron-rich nuclei ^172,173Yb have been populated in a ¹⁷⁰Er(⁷Li,(p,d,t)xn) incomplete-fusion reaction and the emitted γ-radiation was detected with the GASP array. The signature partners of the 7/2⁺[633] rotational band of the odd-N ¹⁷³Yb isotope have been newly established and were observed up to spin values of (45/2⁺) and (43/2⁺), respectively. The ground-state band of the even-even nucleus ¹⁷²Yb has been observed up to a spin value of (22⁺). No band crossings were found in these bands. To explain this observation, it is proposed that the static pair field is absent, considering that the neutron odd-even mass differences reach for these nuclei very small values and that the band crossing is absent in cranked shell modell calculations without pairing. The results indicate, however, that strong dynamic correlations are still present.     

(2 + 1) Resonance enhanced multiphoton ionization of hydrogen chloride in a pulsed supersonic jet: Vacuum wavenumbers of rotational lines with detailed band analysis for excited electronic states of HCl

A comprehensive study of the excited electronic states of HCl is reported. Using resonance enhanced multiphoton ionization ((2 + 1) REMPI) and time-of-flight mass spectrometry (TOFMS) over 120 band systems are analyzed. Supersonic jet techniques are used to prepare rotationally cold species for laser spectroscopy in the 77 000 to 96 000 cm⁻¹ region. At least 50 new electronic states are characterized as well as several features only tentatively assigned previously. A long vibrational progression consisting of 29 vibrational levels of the deeply bound V¹Σ⁺(0⁺) valence/ion-pair state is studied. We also extend the identification and analysis to high vibrational levels of low-lying Rydberg states. The assignments of [²Π_i] Rydberg state complexes are evaluated in terms of spin-orbit coupling and united-atom calculations. In several band systems, the spectra exhibit anomalous rotational line intensities and broadened linewidths which are consistent with predissociation. Multiphoton ionization with mass spectrometry permits the investigation of isotope effects as well as the appearance of fragment species associated with repulsive potential curves.     

Superdeformation in142Eu

A rotational band with 15 transitions has been observed in¹⁴²Eu in an experiment with the GASP Ge detector array by the¹¹⁰Pd(³⁷Cl,5_n) reaction. The average energy spacing between the neighbouring transitions is 60 keV, which is similar to the energy spacings in the superdeformed bands in the mass 140 region. The band has a constant dynamic moment of inertia as suggested by Cranked Shell Model calculations.     

Projected Shell Model Description of Positive Parity Band of <Superscript>130</Superscript>Pr Nucleus

Theoretical investigation of positive parity yrast band of odd-odd ¹³⁰Pr nucleus is performed by applying the projected shell model. The present study is undertaken to investigate and verify the very recently observed side band in ¹³⁰Pr theoretically in terms of quasi-particle (qp) configuration. From the analysis of band diagram, the yrast as well as side band are found to arise from two-qp configuration πh _11/2???νh _11/2. The present calculations are viewed to have qualitatively reproduced the known experimental data for yrast states, transition energies, and B(M1) / B(E2) ratios of this nucleus. The recently observed positive parity side band is also reproduced by the present calculations. The energy states of the side band are predicted up to spin 25⁺, which is far above the known experimental spin of 18⁺ and this could serve as a motivational factor for future experiments. In addition, the reduced transition probability B(E2) for interband transitions has also been calculated for the first time in projected shell model, which would serve as an encouragement for other research groups in the future.     

High spin states of120Ba

The very neutron deficient nucleus¹²⁰Ba has been investigated in a high-spin γ-spectroscopic study. The yrast band of¹²⁰Ba is extended up to spin 22? and one tentatively assigned negative-parity side band is observed up to spin 15?. The experimental results are compared with Total Routhian Surface calculations.     

Density functional studies on lanthanide (III) texaphyrins (Ln-Tex2+, Ln = La,Gd, Lu): structure,stability and electronic excitation spectrum

Density functional calculations have been performed to study the molecular structure and chemical properties of selected lanthanide(III) texaphyrins (Ln-Tex²⁺, Ln = La, Gd, Lu). The lanthanide element is found to reside above the mean N₅ texaphyrin plane, and the larger the cation, the greater the observed out-of-plane displacement. It is concluded that the lanthanide cation is tightly bound to the macrocyclic skeleton, yielding a stable structure. However, the chemical properties of Ln-Tex²⁺ are found to be only slightly affected by the substitution of the lanthanide element. A low-energy LUMO is found for the Ln-Tex²⁺ (Ln = La, Gd, Lu), which are therefore easily reduced in an electron-rich environment. Two characteristic bands are obtained in the calculated electronic excitation spectrum (a high-energy band at 454–462 nm and a low-energy band at 681–686 nm). The intensity of the high-energy band is much larger than that of the low-energy one, yielding a rather unique spectral feature.     

Study of the side band in 188Pt

Low-lying levels in ¹⁸⁸pt were studied using the ¹⁹¹Ir(p, 4nγ)¹⁸⁸Pt reaction and higher spin members of the quasi-γ band were proposed. The results resemble a slightly perturbed phonon scheme, from comparisons with macroscopic calculations which would remove the degeneracy of the phonon multiplets. The importance of mass parameter renormalization is pointed out.     

Band crossings in 170Os

Excited states in the neutron-deficient nucleus ¹⁷⁰Os were identified up to spin (24⁺) in the yrast band and to spin (23⁻) in the lowest negative-parity band. Deformation systematics implied by the 2⁺ state energies for the very light osmium isotopes are compared with theory. Band-crossing frequencies, alignments and alignment gains are compared with cranked shell-model calculations. Deformation changes are required to obtain detailed agreement. A three-band mixing approach is invoked to explain the low-spin yrast anomaly in ¹⁷²Os and to reproduce the yrast band in ¹⁷⁰Os. The excitation energy of the postulated “intruder” band in ¹⁷⁰Os and ¹⁷²Os is deduced.     

Electronic charge distribution of the polarizable O2− ion in MgO and CaO in contrast to the F− ion in NaF

The electronic charge densities of NaF, MgO, and CaO are obtained by self-consistent band structure calculations using the augmented plane wave (APW) method. The fact that F^– is stable, whilst O^2– is unstable as a free ion affects the radial charge density of fluorides and ionic oxides significantly. The Watson sphere model can simulate the stabilizing crystalline environment of an O^2– ion and provides an ionic density which, when superposed with the cation density, leads to a radial charge density in excellent agreement with the one obtained by our APW calculations. It is therefore meaningful to speak of an O^2– ion, although the corresponding wave functions are more extended for O^2– than for F^–. Furthermore, the Watson sphere model can account for the main differences of the oxygen radial density between MgO and CaO and demonstrates that the polarizable O^2– ion is strongly affected by its environment.     

Limits of Majorana neutrino mass from combined analysis of data from <Superscript>76</Superscript>Ge and <Superscript>136</Superscript>Xe neutrinoless double beta decay experiments

We present effective Majorana neutrino mass limits <m _ββ> obtained from the joint analysis of the recently published results of ⁷⁶Ge and ¹³⁶Xe neutrinoless double beta decay (0νββ) experiments, which was carried out by using the Bayesian calculations. Nuclear matrix elements (NMEs) used for the analysis are taken from the works, in which NMEs of ⁷⁶Ge and ¹³⁶Xe were simultaneously calculated. This reduced systematic errors connected with NME calculation techniques. The new effective Majorana neutrino mass limits <m _ββ> less than [85.4–197.0] meV are much closer to the inverse neutrino mass hierarchy region.     

Excited states in neutron-rich 188W produced by an 18O-induced 2-neutron transfer reaction

Excited states in neutron-rich ¹⁸⁸W have been populated using a ¹⁸⁶W(¹⁸O,¹⁶O) reaction. In-beam γ-rays were measured in coincidence with scattered particles detected by a high-resolution ΔE-E Si telescope. In this experiment, the ground-state band has been identified up to I ^π = 8⁺. The γ band, the K ^π = 2^- octupole band, and a 2-quasiparticle state were also observed. The results are compared with predictions of self-consistent HFB cranking calculations and blocked-BCS multi-quasiparticle calculations.     

Energy levels in the nucleus 156Dy through the 159Tb(p, 4nγ)156Dy reaction

Using the ¹⁵⁹Tb(p, 4nγ)¹⁵⁶Dy reaction at E_p = 27 to 51 MeV and standard on-line γ-ray spectroscopy methods, the energies and decay properties of members of various rotational bands in the nucleus ¹⁵⁶Dy have been investigated, i.e. the ground-state band up to 14_g⁺, the β-vibrational band up to 14_β⁺,the γ-vibrational band up to 11_γ⁺,and two other bands, one with odd-spin levels up to 11, the other with even-spin levels up to 10. The results are compared with various calculations in the framework of the collective model, and no satisfactory fit is obtained; possible improvements of the model to remove these discrepancies are suggested.     

The electronic structures of Mg,Al and Si in octahedral coordination with oxygen from SCF Xα MO calculations

Molecular orbital calculations performed using the SCF Xα Scattered Wave Cluster method are presented for the octahedral oxyanions MgO₆^?10, AlO₆^?9 and SiO₆^?8. The AlO₆^?9 results are used to assign and interpret the X-ray photoelectron spectra (XPS), X-ray emission (XES) and u.v. spectra of Al₂O₃. Agreement between calculation and experiment is good for valence band and fair for conduction band orbitais. The SCF Xα results for MgO₆^?10 are also in good agreement with observed valence band energies in MgO, but in this case the lowest energy features in the u.v. spectrum are not assignable in terms of either the calculations or the X-ray spectral results. The substantial increase in covalency expected between the Mg and Si oxides is evidenced in the calculations by an increase in valence region width from 2.6 to 5.3 eV and an increase in valence-conduction band separation from 5.2 to 10.0 eV. The calculated trends are in reasonable agreement with u.v. spectral data and with absolute valence orbital binding energies derived from X-ray spectra. A comparison of the SiO₆^?8 calculation with the analogous tetrahedral SiO₄^?4 calculation shows the valence band in the octahedral oxyanion to be much simpler in structure and somewhat narrower than that in the tetrahedral oxyanion. Using the orbital structure calculated for the valence bands of tetrahedral and octahedral oxides, a method is presented for calculating atomization energies directly from X-ray spectral data for SiO₂, Al₂O₃ and MgO. Results are in good agreement with experiment but the method involves an empirical parameter which is not presently understood in detail. Studies of trends in p-type bonding orbital binding energies derived from experimental data provide a qualitative explanation for the preferred coordination numbers in the Mg, Al and Si oxides.     

Non-yrast states of 132Ce polutated in β-decay

Non-yrast low spin states of the nucleus ¹³²Ce were studied by means of γ-spectroscopy following the β-decay of the ground and medium spin isomeric states of ¹³²Pr. The activity was produced with the reaction ¹¹⁷Sn(¹⁹F,4n) ¹³²Pr at the Cologne FN TANDEM accelerator. The γγ coincidences and singles spectra were measured with the OSIRIS cube spectrometer. The β-decay of ¹³²Pr populates states with spins up to 6ħ and excitation energies up to 4.4 MeV in ¹³²Ce. Besides ground and quasi-gamma bands, an excited band based on the 0₂⁺ state and many other low-lying states were observed. The γγ angular correlations were analyzed to assign spins and parities to the excited states, and to determine the multipolarities of the γ-transitions. We found dominant E2 transitions in the quasi-gamma band and from the quasi-gamma band to the ground band. The experimental data are compared with calculations using the Interacting Boson Model (IBM). Good agreement is reached in the vicinity of the O(6) limit.     

First-principles calculation of the insulator-to-metal transition pressure in CsI

The room temperature compression isotherm and electronic band structure of CsI have been computed with the nonrelativistic self-consistent augmented-plane-wave method. The isotherm is in excellent agreement with available static and ultrasonic measurements, and hence allows an improved extrapolation of Asaumi and Kondo's measurements of the absorption edge to gap closure in the range 38–46 ų and 65–105 GPa when uncertainties in the experimental determination of the band gap are accounted for. The calculated band structure suggests that the band gap is direct and it is in close agreement with the corresponding experimentally determined band gap when the Slater exchange potential is used. These consistencies between the measurements and calculations suggest that the insulator-to-metal transition occurs in the 100±10 GPa range, somewhat higher than Asaumi and Kondo's estimate of 70 GPa, but in good agreement with the prediction from Herzfeld's theory.     

Deformation and shape coexistence in medium mass nuclei

Emerging evidence for deformed structures in medium mass nuclei is reviewed. Included in this review are both nuclei that are ground state symmetric rotors and vibrational nuclei where there are deformed structures at excited energies (shape coexistence). For the first time. Nilsson configurations in odd-odd nuclei within the region of deformation are identified. Shape coexistence in nuclei that abut the medium mass region of deformation is also examined. Recent establishment of a four-particle, four-hole intruder band in the doublesubshell closure nucleus⁹⁶Zr₅₆ is presented and its relation to the Nuclear Vibron Model is discussed. Special attention is given to the N=59 nuclei where new data have led to the reanalysis of⁹⁷Sr and⁹⁹Zr and the presence of the [404 9/2] hole intruder state as isomers in these nuclei. The low energy levels of the N=59 nuclei from Z=38 to 50 are compared with recent quadrupole-phonon model calculations that can describe their transition from near-rotational to single closed shell nuclei. The odd-odd N=59 nuclei are discussed in the context of coexisting shape isomers based on the (p[303 5/2]n[404 9/2])2^– configuration. Ongoing in-beam (t.p conversion-electron) multiparameter measurements that have led to the determination of monopole matrix elements for even-even₄₂Mo nuclei are presented, and these are compared with initial estimates using lBA-2 calculations that allow mixing of normal and cross subshell excitations. Lastly, evidence for the neutron-proton³S₁ force's influence on the level structure of these nuclei is discussed within the context of recent quadrupole-phonon model calculations.Work supported by USDOE contract Nr.W-7405-Eng-48 and NATO Grant Nr.RGO565/82.     

Lifetime measurements in166Er

The lifetimes of five states in the ground band, from spin 6⁺ up to spin 14⁺, and of all even states in the gamma band up to spin 12⁺, have been measured in¹⁶⁶Er using the recoil distance method. The reduced electric quadrupole transition probabilities have been determined from the measured lifetimes using previously measured branching ratios, and the mixing between the ground band and theγ-band has been studied. The transitional electric quadrupole moments for the ground band and theγ-band have been deduced and are discussed.