Single-hole States Research Articles

Exciton transfer dynamics in CdTe/(Cd,Zn) Te asymmetric double quantum well structures

Time-resolved photoluminescence measurements on CdTe/(Cd,Zn)Te asymmetric double quantum wells (ADQWs) with electron and hole confinement potentials of some tens of meV are reported. The carrier transfer from the narrow into the wide well is evidenced by photoluminescence decay times of the narrow well as short as a few picoseconds. The observed tunneling times depend strongly upon the barrier and well widths of the investigated samples which determine the relative position of spatial direct and cross exciton transitions. Very efficient tunneling is found in ADQWs in which the transfer of spatial direct excitons under emission of LO phonons is possible. In any case, one has to consider excitonic rather than single particle electron or hole states in order to explain the observed behaviour of the tunneling times.

Energy-dependent single-particle state density effects in the hybrid model of pre-equilibrium nuclear reactions

Different energy dependences of the single-hole and excited-particle state densities, of Fermi-gas model type and a linear dependence respectively, are adopted in accordance with recent proofs of accurate shell-model Hamiltonian eigenvalues. The respective different values at the Fermi level also make possible both the correct prediction of the bound nucleon number, for a Fermi energy value of 40 MeV, and the agreement with the average neutron resonance spacing data. An attempt to distinguish between the surface effects and the influences attributable to the energy dependence of the single-particle state density is made within the geometry-dependent hybrid model of the pre-equilibrium emission. Comparisons with the large amount of data existing for fast-neutron reactions on the 51V and 56Fe target nuclei at incident energies up to 20 MeV support the model assumptions.

High-resolution monochromatized He II alpha -excited photoelectron spectrum of Ar between 28 and 40.8 eV.

The inner-valence-photoelectron spectrum of argon has been recorded between 28 and 40.8 eV at a resolution of 20 meV using monochromatized He ii alpha excitation. More than 40 lines are observed and assigned by comparison to optical data and theoretical results. Except for the 3s single-hole state at 29.240 eV, the spectrum is associated with transitions to electron-correlation states arising from 3${\mathit{p}}^{4}$ nl configurations, where n=3,4,5 and l=0--3. The complete manifold of ionic states is observed for most configurations. Final states with an angular momentum up to J=9/2 acquire a substantial intensity.

Hole strengths and spreading widths observed in [formula omitted] reaction at 65 Me V

Differential cross sections and analyzing powers have been measured for the 92 Mo( p, d ) 91 Mo reaction with a polarized proton beam of 65 MeV incident energy for investigating the level structure of 91Mo up to 10 MeV. The data analysis with the standard DWBA theory provides transferred angular momenta lj values and spectroscopic factors for 42 transitions to the single-hole 1 g 9 2 , 2 p 1 2 , 2 p 3 2 , 1 f 5 2 and 1 f 7 2 st analog states. The spreading widths are deduced and compared with theoretical predictions for discussing the damping mechanism of the single-hole states.

Near-threshold double photoionization in Zn and Hg.

Valence photoelectron spectra of Zn and Hg have been recorded by using monochromatized synchrotron radiation in ultraviolet region and monochromatized A1 Kα radiation (hv=1487 eV), respectively. In the region of the double-ionization thresholds, satellite states associated with the nd -1 single-hole states (n=3 for Zn and n=5 for Hg) are observed above the nd 10 1 S double-ionization threshold. The Auger decay of these satellites may account significantly in the formation of the 1 S state of the dication. In case of Zn, the asymmetric profile of the main satellite lying above the 3d 10 1 S double-ionization threshold suggests the existence of a coupling between the discrete satellite state with this underlying shake-off continua

40Ca(p,d)39Ca reaction at 65 MeV.

Cross sections and analyzing powers have been measured for the $^{40}\mathrm{Ca}$(p,d${)}^{39}$Ca reaction with polarized proton beams of 65 MeV incident energy. The data analysis with a standard distorted-wave Born approximation theory provides transferred angular momenta l, j, and spectroscopic factors for 58 transitions mainly to the 1${\mathit{d}}_{3/2}$, 2${\mathit{s}}_{1/2}$, and 1${\mathit{d}}_{5/2}$ hole states in $^{39}\mathrm{Ca}$. The occupation probabilities of the surface shells in $^{40}\mathrm{Ca}$ and the spreading widths of the hole states are determined. The damping mechanism of the single hole states is discussed.

Proton-90Zr mean field between -60 and +185 MeV from a dispersive optical model analysis.

Elastic scattering of protons from $^{90}\mathrm{Zr}$ in the energy range ${\mathit{E}}_{\mathit{p}}$=9.8 to 135 MeV is analyzed using a dispersive optical model potential (OMP). In this analysis, a dispersion relation connects the volume integrals of the imaginary and the real parts of the OMP. Best-fit dispersive OMP parameters are obtained from fits to experimental cross section and analyzing power data at each energy, while the volume integrals of the imaginary potential and dispersive correction terms are fixed at the empirical values obtained in the individual proton elastic scattering analyses from 9.8 to 135 MeV. Predictions of the cross section and analyzing power angular distributions from the best-fit dispersive OMP and conventional OMP are obtained, and give similar quality fits to data. A dispersive OMP with parameters that show a smooth energy dependence are determined from fits to the entire data set. Comparison of cross sections and analyzing powers calculated by the dispersive OMP with experimental data at 160 and 185 MeV is also presented. The dispersive OMP with a smooth energy dependence is extended to the negative energy region with the guidance of the known first single-particle and single-hole state energies near the Fermi energy, ${\mathit{E}}_{\mathit{F}}$=-6.8 MeV, to provide parameters for the shell model potential. This analysis also provides estimates for single-particle and hole energies ${\mathit{E}}_{\mathit{n}\mathit{l}\mathit{j}}$, root-mean-square radii ${\mathit{R}}_{\mathit{n}\mathit{l}\mathit{j}}$, expectat ion values of the effective mass 〈${\mathit{m}}^{\mathrm{*}}$/m${\mathrm{〉}}_{\mathit{n}\mathit{l}\mathit{j}}$, occupation probabilities ${\mathit{N}}_{\mathit{n}\mathit{l}\mathit{j}}$, absolute spectroscopic factors ${\mathit{S}}_{\mathit{n}\mathit{l}\mathit{j}}$, and spectral functions ${\mathrm{\ensuremath{\zeta}}}_{\mathit{n}\mathit{l}\mathit{j}}$(${\mathit{E}}_{\mathit{x}}$) for proton single-particle and single-hole orbits in $^{90}\mathrm{Zr}$. These estimates are compared with available experimental information.

High-lying resonances observed in heavy-ion transfer reactions.

High-lying single-particle and single-hole states were studied using ([sup 7]Li,[sup 6]Li), ([sup 7]Li,[sup 6]He), ([sup 12]C,[sup 13]C), and ([sup 12]C,[sup 13]N) reactions at 30 MeV/nucleon on the targets [sup 207]Pb, [sup 208]Pb, [sup 209]Bi, [sup 89]Y, [sup 90]Zr, and [sup 91]Zr. Broad resonancelike features were observed in both proton and neutron transfer reactions at excitation energies close to the excitation energies of the giant quadrupole resonances. The measured excitation energies and reaction [ital Q] values suggest that these features are predominantly single-particle states rather than collective giant resonance excitations. The existence of an extra particle or hole outside closed-shell nuclei does not change the strengths of the broad peaks significantly, but does change the excitation energies in some cases. Shell model calculations in the Pb region support this conclusion.

Hole pairs in the one-dimensional Hubbard model in a staggered field.

The one-dimensional Hubbard model in a staggered magnetic field h, introduced recently to simulate planar correlated electronic systems, is studied within the unrestricted Hartree-Fock approximation. The emphasis is on solutions for a few holes added to the reference antiferromagnet. Both analytical results, available at U,ht, and numerical results confirm the polaronic character of a single-hole state for all U,h, while bipolarons (bound pairs of holes) are stable for 0U${\mathit{U}}_{\mathit{c}}$(h). Since under the same conditions we do not find any evidence for stable hole clusters and the phase separation, the latter regime remains a candidate for hole pairing.

X-ray photoelectron spectroscopy and ab initio CI study of the core and valence ionisation of formamide

High resolution X-ray photoelectron spectra have been recorded of the core and valence levels of the formamide molecule using monochromatised Al Kα radiation for the excitation of the spectra. Ab initio CI calculations of the binding energies and intensities have been carried out in order to interpret the experimental results and to provide additional information on the electronic structure. The core level shake-up spectra are found to be similar to those observed earlier of the CO molecule, acetaldehyde and acetone. In particular, the substitution of a hydrogen atom by methyl or amino groups tends to give rise to an additional shake-up line corresponding to the additional valence orbital of π symmetry. The valence photoelectron spectrum reveals new structures compared to previous studies. This has enabled an assignment in terms of both single hole states and many-electron states and the study confirms that the orbital ordering in the outermost region should be given as 10a′ (n)2a″ (π)2a″ (π)9a′ (σ) in order of increasing binding energy. Ionisation from the 7a′ (σ) orbital at about 18 eV leads to two structures in the spectrum with almost equal intensity. The two strong photoelectron bands in the inner valence region are found to be composed of four- or five-component states, which implies a more or less complete breakdown of the orbital model in this region.

Search for 3s1/2 hole-strength fragments via the 208Pb(d, 3He)207Tl reaction

In an attempt to locate weak fragments of the 3s proton hole strength in 207Tl the 208Pb(d, 3He)207Tl reaction was studied at Ed=80 MeV. The spectrum is dominated by five single-hole states. A careful analysis of the remaining, weakly excited states below 5 Mev of excitation energy reveals no additional 3s strength. For the total excited 3s strength an upper limit of 5% of the ground-state strength is obtained.

Many-body calculations of the valence photoemission spectra of NiCO, PdCO and PtCO

The valence photoemission spectra of NiCO, PdCO and PtCO are calculated by the ab initio Green's function method using an extended basis set. The present ab initio many-body approach gives a reasonably good description of the valence photoemission spectra of CO adsorbed on Ni, Pd and Pt metal surfaces. A detailed analysis of the spectral features of the valence photoemission spectra of NiCO, PdCO and PtCO is made. For the 1π level of NiCO and PtCO the quasi-particle picture (QPP) breaks down, however, for PdCO the QPP is still valid. For the 4σ and 5σ levels of NiCO, the lowest energy state is the main line single-hole (1h) state not the CT (charge transfer) 2h 1p (two hole one particle) shakedown state. However, for PdCO and PtCO, the σ (π) to π∗ metal-ligand CT 2h 1p shakedown state of a non-negligible intensity becomes the lowest energy state. The main line state is still the 1h state and the QPP ios valid. For the PdCO and PtCo where the π bonding is considered to be weaker than for NiCo, the CT screening from the σ metal orbital becomes substantial because of a weaker π bonding.

Fast-neutron interaction with elemental zirconium, and the dispersive optical model.

Differential neutron elastic- and inelastic-scattering cross sections of elemental zirconium are measured from {approx} 1.5 to 10 MeV. Below 3 MeV the measurements are made at incident-neutron energy intervals of {approx} 100 keV, from 3 to 4 MeV at intervals of {approx} 200 keV, and at intervals of {approx} 500 keV at higher energies. the angular range of the measurements is {approx} 18{degrees} to 160{degrees}, with up to more than 100 differential values per distribution. this comprehensive data base, augmented with a 24-MeV elastic-scattering distribution from the literature, is used to develop two phenomenological optical-statistical models which both describe the data very well. First, the parameters of the conventional spherical optical model (SOM) are deduced. Secondly, the model in which the change in the real potential brought about by the dispersion relationship (DOM) is examined. the SOM parameters are consistent with systematics previously reported from this laboratory, and the volume integral-integral-per-nucleon of the real potential strength, J{sub v}, and the radius, r{sub v}, are energy dependent. When the DOM is used, a substantial part of the enrgy dependence of J{sub v}({approx} 30%) disappears. However, the change in the energy dependence of r{sub v} is small, so that a significant energymore » dependence remains when the DOM is used. Both models are extrapolated to the bound-state regime where they have modest success in predicting the binding energies of the single-particle and single-hole states in {sup 90}Zr. 55 refs., 20 figs., 7 tabs.« less

Exciton complexes in ZnSe layers: a tool for probing the strain distribution

Differently acting strain components caused by lattice mismatch or deviating thermal expansion coefficients of ZnSe and any substrate material yield a typical thickness-dependent strain profile in ZnSe epitaxial layers. For the analysis of strain in those layers, the investigation of free and bound excitons is very suitable. In the present work, samples are used exhibiting surface strain magnitudes ϵ between -5×10 -4 (compressive in-plane strain) and +15×10 -4 (tensile in-plane strain), determined from reflection loops of the strain-split free excitons X lh and X hh. Whereas reflection spectroscopy scans the strain situation at the layer surface only, excitation and resonant Raman spectroscopy, in particular of donor-bound excitons (D 0, X), give information about the strain distribution in the whole film. Based on the observed set of different (D 0, X) transitions I 2 i , originating from excited single-hole states, we are able to predict the strain profile and the distribution of the main donors in the film.

Coupling between the charge carriers and lattice distortions via modulation of the orbital angular momentum [formula omitted] of the 3d holes by polarized xas spectroscopy

The change of the orbital angular momentum M ℓ of the Cu 3d holes going from the insulating to the metallic phase has been studied in several families of high T c superconductors. The symmetry of the 3d 9 L states in the metallic phase has been studied by quantitative analysis of the variation of polarized Cu L 3 x-ray absorption spectra. At a doping level n h∼15% we have found 10% of Zhang-Rice singlets 3d x2 -y2 L(b 1) and 5% of 3d 3z2 -r2 L(a 1) states. Therefore the percentage of the 3d 3z2 −r2 L states on the total number of the 3d 9 L states is about 30% i.e. much larger than the probability of single hole states 3d 3z2 −r2 in the insulating phase. The EXAFS and XANES studies of the Cu site structure and dynamics in Bi 2Sr 2Ca 1−xY xCu 2O 8 system point toward the coupling of the charge carriers with distortions of the Cu sites driven by the Mℓ=0 character of the Cu 3d holes that can be called a 3d z2 −r2 polaron.

Evidence for parity dependence in the neutron-40Ar optical model potential.

The neutron dispersive optical model potential that was developed previously from an analysis of extensive data on the neutron${\mathrm{\ensuremath{-}}}^{40}$Ca system is adapted to the neutron${\mathrm{\ensuremath{-}}}^{40}$Ar system by adjusting the central depth of the local equivalent of the Hartree-Fock-type potential. The n${\mathrm{\ensuremath{-}}}^{40}$Ar data for this adjustment are the total cross section for 11E40 MeV and the centroid energy for the observed single-particle and single-hole states of the valence shells. The resulting potential for n${\mathrm{\ensuremath{-}}}^{40}$Ar yields good predictions for the particle-hole gap for the valence shells and for the energy-averaged s-wave and p-wave scattering functions in the resonance region. However, it gives a poor prediction for the total cross section from 2 to 11 MeV. A similar discrepancy was observed earlier for $^{40}\mathrm{Ca}$. A parity dependence in the surface imaginary potential is suggested from a more careful examination of the s-wave and p-wave scattering functions for $^{40}\mathrm{Ar}$; that parity dependence gives a good prediction of the total cross section for 2 to 11 MeV.

Single-hole state in the half-filled Hubbard model. The coupled-cluster method

We have investigated the ground state of a single hole in the half-filled Hubbard model on a 2D square lattice using the coupled- cluster method. In the strong-coupling limit we have obtained an analytical expression of the hole energy dispersion ϵ( k ) with several of its features consistent with earlier studies of the Hubbard model and t-J model. An appreciable discrepancy on the hole bandwidth is, however, found between the Hubbard model and the t-J model. We believe that this discrepancy is due to the absence of the three-site hopping term in the t-J model.

X-ray, ultraviolet, and synchrotron radiation excited inner-valence photoelectron spectra of CH4

The inner-valence electron states of the methane molecule have been studied by means of x-ray, synchrotron radiation, and UV-photoelectron spectroscopy. Five correlation satellites have been identified and a detailed study has been carried out of the 2a−11 single hole state. For this state a Franck–Condon analysis has been performed, suggesting an equilibrium bond distance of 1.279 Å. The vibrational lines have a Lorentzian shape and the linewidth increases gradually with the vibrational quantum number. This probably indicates a reduction of the lifetime of the vibrational states due to predissociation. A discussion of the potential curves related to the correlation satellites is included.

Single-hole state in half-filled 2D Hubbard model

We have investigated the ground state of a single hole in the half-filled Hubbard model on a 2D square lattice using the coupled-cluster method. In particular we obtained an analytical expression of the hole energy dispersion function ɛ(k) which is consistent with earlier studies on thet-J model in the strong-coupling limit. An appreciable discrepancy on the hole energy bandwidth is, however, observed between the Hubbard model and thet-J model. We believe that this discrepancy is due to the absence of the three-site interaction term in thet-J model.

Theory of Core Hole Decay Dynamics of Adsorbate on Metal Surfaces

A comparative study of a core hole decay dynamics is made for an adsorbate on a metal surface. On the basis of the available experimental results for a CO molecule chemisorbed on Cu(llO) as a prototype system, we investigate the elementary processes of the core hole decay via various new Auger channels open for adsorbates, i.e., the crossed Auger transition and the participation of an electron in the initially unoccupied level of the adsorbate created by charge transfer from the substrate before core hole decay or resonant core excitation. We calCulate the valence Auger spectrum and deexcitation spectrum following resonant excita­ tion. These spectra are compared with the direct valence photoemission, in particular with a single valence hole state screened by charge transfer from the metal. Absence of shake-up satellite in the deexcitation spectrum following resonant core to bound (e.g., 2Jr* of a CO molecule) level excitation and the mismatch of the binding energy of the screened final state between direct photoemission or normal Auger spectrum and resonantly excited Auger spectrum are explained in terms of the relaxation processes of the excited state before the Auger decay of the core hole. The electronic structures of the adsorbate on a metal surface, which is of eminent interest not only for its own right, but also for gaining deeper insight into the microscopic mechanisms of a wide variety of surface chemical reactions, have been studied by means of various surface electronic spectroscopies such as core (XPS) and valence (UPS) photoemission (PES) for the occupied states, inverse photoemission (IPS) for partially occupied or unoccupied states, and absorption (ABS) for the electronic transitions between the occupied and the unoccupied states. Auger elec­ tron spectroscopy (AES) commonly used to probe the chemical species at or near surfaces is also recognized as a useful tool to determine the energy levels and the bonding configurations of adsorbates, A comparison of these surface electron spectroscopies has been presented in a review article of Plummer et al. 1 ) for a CO molecule on various metal surfaces as a prototype system. In a series of works.. we have studied the ABS,