The formation of heavy-fermion states in non-Fermi-liquid impurity systems

A mechanism for the occurrence of heavy-fermion states in non-Fermi-liquid (NFL) metals with f-shell impurities is proposed. The impurity with an unstable valence is suggested to have an energy spectrum consisting of a deep f-level and quasicontinuum states (narrow band) in resonance with the Fermi energy. Depending on the impurity concentration, the single-site NFL states are generated by the two-channel Kondo scattering for the low concentration (the Kondo regime) or by the screening interaction for a relatively high concentration (the X-ray-edge regime). It is shown that the NFL states are unstable against the scattering of the NFL excitations by electron states of the narrow band. This scattering generates additional narrow Fermi-liquid (FL) resonances at/near the Fermi level in the Kondo regime and in the X-ray-edge regime. The mixed-valence states are shown to be induced by new FL resonances. The mixed valence mechanism is local and is related to the instability of single-site NFL states. The FL resonances lead to the existence of additional energy scales and of pseudogaps near the Fermi level in the mixed-valence states. They also considerably narrow the region with a nearly integer valence.     

Recombination parameters in low-resistivity gamma-irradiated η-type germanium

Studies of recombination in ～0.2 Ω-cm As- and Sb-doped Co⁶⁰ γ-irradiated Ge which yield energy levels and the temperature dependence of the electron and hole capture probabilities are reported. For Sb-doped material at 323°K, the recombination center energy level position (neglecting statistical weight) was found to be 0.361±0.005 eV above the valence band with a possible slight temperature dependence corresponding roughly to one-half the variation of band gap with temperature. The capture probability ratio at this same temperature was 740. For the Asdoped case, two different levels appear to dominate the recombination process in annealed and unannealed low resistivity material. The energy level positions relative to the valence band (neglecting statistical weight) are 0.327±0.005 eV and 0.37±0.01 eV at room temperature for the annealed and unannealed samples, respectively. The corresponding capture probability ratios are 650 and 810. As in the case of Sb-doping, the energy level appears to shift with temperature at about one-half the rate of the shift in band gap energy.     

A simulation study of vibrational relaxation of I− 3 in liquids

The temperature dependence of the vibrational relaxation of a flexible model of triiodide in a Lennard-Jones solvent (xenon) has been studied using equilibrium molecular dynamics simulations. The internal dynamics of the ion is calculated from a previously published semi-empirical valence bond model with a limited number of basis states. Vibrational decorrelation rates of the symmetric and antisymmetric stretching modes were found from the time correlation functions of the normal coordinate velocities and the vibrational energy relaxation rates from the time correlation functions of the kinetic energy in each mode. The vibrational dephasing rates and the energy relaxation rates decrease slowly as the temperature is lowered and do not show a discontinuity when the fluid solidifies, although the reorientational diffusion rates change rapidly at low temperatures. In order to interpret the results, perturbation theory expressions for the relaxation rates were evaluated for simulations of a rigid model of the ion and found to agree well with the direct observations. These showed that, unusually, both the solvent force and its derivative, the solvent potential curvature, contribute to the dephasing of the symmetric mode. The relevant fluctuation correlation times are very short, which may explain the insensitivity of the vibrational relaxation to the state of the solvent.     

Numerical simulation of the temperature dependence of the ionization energy of hydrogen-like impurities in semiconductors: Application to transmutation-doped Ge: Ga

An electrostatic model describing the dependence of the thermal ionization energy of impurities on their concentration, compensation factor, and temperature is developed. The model takes into account the screening of impurity ions by holes (electrons) hopping from impurity to impurity, the change in the impurity-band width, and its displacement with respect to the edge of the valence band for acceptors (conduction band for donors). The displacement of the impurity band is due to the functional dependence of the hole (electron) affinity of the ionized acceptor (donor) on the screening of the Coulomb field of the ions. The spatial distribution of the impurity ions over the crystal was assumed to be Poisson-like, and the energy distribution was assumed to be normal (Gaussian). For the relatively low doping levels under investigation, the behavior of the density of states at the edges of the valence and conduction bands was assumed to be the same as for the undoped crystal. The results of the numerical study are in agreement with the decrease in the ionization energy that is experimentally observed for moderately compensated Ge: Ga as the temperature and the doping level are decreased. It is predicted that the temperature dependence of the thermal ionization energy has a small anomalous maximum at small compensation factors.     

Influence of the Diagonal and Off-Diagonal Electron–Phonon Interactions on the Formation of Local Polarons and Their Band Structure in Materials with Strong Electron Correlations

For systems with strong electron correlations and strong electron–phonon interaction, we analyze the electron–phonon interaction in local variables. The effects of the mutual influence of electron–electron and electron–phonon interactions that determine the structure of local Hubbard polarons are described. Using a system containing copper–oxygen layers as an example, we consider the competition between the diagonal and off-diagonal interactions of electrons with the breathing mode as the polaron band structure is formed within a corrected formulation of the polaron version of the generalized tight-binding method. The band structure of Hubbard polarons is shown to depend strongly on the temperature due to the excitation of Franck–Condon resonances. For an undoped La₂CuO₄ compound we have described the evolution of the band structure and the spectral function from the hole dispersion in an antiferromagnetic insulator at low temperatures with the valence band maximum at point (π/2, π/2) to the spectrum with the maximum at point (π, π) typical for the paramagnetic phase. The polaron line width at the valence band top and its temperature dependence agree qualitatively with angle-resolved photoemission spectroscopy for undoped cuprates.     

Low energy electron attachment to C<Subscript>60</Subscript>

Low energy electron attachment to the fullerene molecule (C₆₀) and its temperature dependence are studied in a crossed electron beam–molecular beam experiment. We observe the strongest relative signal of C₆₀ anion near 0 eV electron energy with respect to higher energy resonant peaks confirming the contribution of s-wave capture to the electron attachment process and hence the absence of threshold behavior or activation barrier near zero electron energy. While we find no temperature dependence for the cross-section near zero energy, we observe a reduction in the cross-sections at higher electron energies as the temperature is increased, indicating a decrease in lifetime of the resonances at higher energies with increase in temperature.     

Luminescence of CuInS2: II. Exciton and near edge emission

The near-edge (exciton) emission of CuInS₂ is investigated for various material-compositions as a function of temperature. From these investigations the exciton ionization energy (20 meV) and the temperature dependence of the energy gap were determined. For the first time, recombination of the free exciton belonging to the deeper lying Γ₇ valence bands has been observed. Moreover, six different bound exciton emission lines and a donor to valence band transition were detected. These emissions could be assigned in terms of the defect-chemical model presented in Part I.     

Comparison of the electron energy loss spectrum of erbium metal with ErO2andErH2

Electron energy loss spectra of metallic erbium, Er under different exposures of oxygen at room temperature, and Er deposited in an atmosphere of H₂ are presented in both N(E) and $\text{dN}\text{dE}$ form for primary energies in the range 100–1000 eV. Resonant excitations associated with the 5p and 4d levels in Er show little environmental dependence, and are largely intraatomic in character. In contrast the main plasmon peak shifts to higher energy on exposure to oxygen or hydrogen, and the spectrum of one electron excitations at low energies alters with a decrease in metal losses around 3.5 eV accompanied by a build up of valence band transitions at 8–9 eV. There is no evidence of a stable chemisorption phase under oxygen exposure, but the results are consistent with rapid oxygen incorporation into subsurface layers and oxide formation.     

Resonance neutron capture in 56Fe

The neutron capture cross section of ⁵⁶Fe has been measured with 0.2–0.3% energy resolution from 2.5 keV up to the inelastic neutron scattering threshold. Results are compared with recent total cross-section data and average parameters are derived for s-, p- and d-wave resonances. The low correlation coefficient observed between the s-wave reduced neutron and radiative widths is consistent with the minor contribution of the valence capture mechanism as calculated in the framework of the optical model. Broad E1 and M1 doorway states for s-, p- and d-wave resonances are postulated to explain the cross-section data and γ-ray spectra up to 1 MeV.     

Optical absorption due to transitions between bands and local levels in CdCr2Se4

The theory of optical absorption due to transitions between a valence band and a hydrogen-like local level associated with a conduction band is modified to permit an arbitrary power-law dependence of energy on the magnitude of the wave-vector of carriers in the valence band. The observed absorption for photon energies below 1.6 eV in the ferromagnetic semiconductor CdCr₂Se₄ is discussed in terms of a combination of two types of terms. The first type of absorption is due to transitions to a local level from a band with two branches, in each of which there is an energy region with a width of 0.28 eV or more beginning 0.10–0.16 eV from the band edge, in which the energy measured from some origin near but not necessarily equal to the band-edge is approximately proportional to (wave-vector)^{($\text{1}\text{3}$)}. The second type of absorption has a dependence on photon energy ?ω of the form (?ω ? E₃)², where E₃ is a threshold energy probably connected with indirect transitions between bands as suggested by Sakai, Sugano and Okabe. After constraints on parameters appearing in the theory are imposed by use of results of these authors and of Shepherd, it is found that curves of Harbeke and Lehmann on optical absorption in CdCr₂Se₄ at 4.2, 78, 130 and 298 K in the photon-energy range 1.14–1.42 eV can be fitted to a mean accuracy of 3%, using an average of 3.75 adjustable parameters for each curve. The strength of the indirect band-to-band absorption does not have the temperature dependence expected for phonon-assisted indirect band-to-band transitions, but can be described by a term independent of temperature plus another term proportional to the square of the deviation of the magnetization from saturation. The fitting of the absorption curves requires that the ratio of the widths of the two branches of the bands varies from about 1.6 at low temperatures to 1.35 at 298 K and that the total width of the bands involved is less than 1 eV.     

Analysis of optical spectra of CdS and CdSe films deposited on a sapphire substrate by laser ablation technique

CdSe and CdS films, deposited on a sapphire substrate by means of pulsed laser ablation technique, have been investigated by means of reflectivity and photoluminescence measurements in order to study the effect of such a transparent substrate on the optical properties of the deposited epilayers. The reflectivity spectra at low temperature have been studied by means of an analytical model which permits one to obtain the energies of the excitonic resonances. The photoluminescence spectra show that our CdSe and CdS films present excitonic emission at low temperature, differently from the same films deposited on quartz. The temperature dependence of the excitonic energy has been analysed by taking into account the contribution of both the thermal expansion and electron-phonon interaction. The exciton linewidth has been analysed according to well known phenomenological models. Received 21 June 2001 and Received in final form 18 November 2001     

Microscopic Many-Body Effects on Intersubband Resonance in Quantum Well

Considering the Coulomb many-body interactions, we investigate the intersubband optical processes of the quantum well by using the semiconductor Bloch equations. We calculate the evolution of intersubband absorption spectral line shape as a function of lattice temperature and electron density. It is found that the coupling of intersubband plasmons can reduce and red-shift the lower energy resonance, simultaneously enhance and blue-shift the higher energy resonance. The dependence of cascading resonances on temperature and electron density is also discussed.     

The angular behaviour of electrons inelastically scattered from the shape resonance for CO chemisorbed on Ni(110)

Recent vibrational high resolution electron energy loss experiments (HREELS) have shown evidence for molecular shape resonances in the inelastic scattering of electrons from chemisorbed molecules. Such resonances arise from the capture of the incident electron in a quasibound state of the molecule, leading to the formation of a temporary negative ion. They are manifest as an enhancement in the intensity of a specific vibrational mode at a characteristic incident electron energy. In contrast to gaseous species, the alignment which the surface provides for the chemisorbed species, can be exploited to determine the angular characteristics of the resonant state. In this work, we show evidence for a shape resonance, centred at an incident energy of 18 eV, for CO/Ni(110). The angular dependence of the scattered electron intensity has been measured for the CO stretching vibration. The results are discussed in terms of the spherical harmonic components of the resonant state, modified by vibrational broadening caused by low frequency bending modes associated with the bonding of the CO molecule to the surface.     

Multiphonon resonances in the Debye-Waller factor of atom surface scattering

He atom surface scattering by dispersionless phonons is treated employing coupled channel (CC) calculations. At low energies, they predict a behavior opposite to perturbative Born or "exponentiated" Born approximation: strong resonant phonon stimulated elastic and inhibited inelastic scattering. The corresponding resonances have not been observed in earlier CC results since these have considered only the temperature dependence of the Debye-Waller factor at higher energy or omitted the attractive well. The resonances can be interpreted in terms of bound states in the attractive well with several excited vibrational quanta. They may be observable for, e.g., He scattering by a cold Xe/Cu surface.     

Laser initiation of a mixture of PETN with NiC nanoparticles at elevated temperatures

The temperature dependence of the probability of the explosion of pentaerythritol tetranitrate (PETN) with an admixture of NiC particles (0.3 wt %) initiated by laser pulses (1064 nm, 20 ns) was studied over the temperature range 295–450 K. At 295–350 K, a weak temperature dependence was observed. The determining contribution to explosion initiation was made by the absorption of laser radiation by nanoparticles. The threshold of explosive decomposition at 295 K decreased by ∼40 times compared with samples free of NiC nanoparticles. Over the temperature range 400–450 K, the threshold of the explosive decomposition of samples containing NiC nanoparticles decreased with the activation energy ∼0.4 eV. A decrease in the threshold of explosive decomposition with a ∼0.4 eV activation energy over the temperature range 340–440 K was also observed for laser action on PETN samples not containing NiC. A hypothesis was suggested according to which the absorption of a light quantum caused the transfer of an electron from the valence band of the crystal to a level in the forbidden band with subsequent thermal positive ion dissociation to the carbocation and NO₃ radical.     

Optical investigations of Zn,Hg and Li doped GaN

The use of infrared quenching of photoluminescence to study the spectral dependence of the photoionization cross-section in GaN doped with Zn, Hg, and Li is reported. It is shown that these impurities produce deep level centres 0.48 eV, 0.41 eV, and 0.75 eV, respectively, above the valence band. In addition, excitation spectra are investigated for Zn and Li doped GaN, giving values of 3.17 eV and 2.86 eV at 78 K for the energy distance of these levels from the conduction band. Finally, from the temperature dependence of the excitation spectra, it is concluded that the levels are probably pinned to the valence band.     

Resonant mechanisms of inelastic light scattering by low-dimensional electron gases

The contribution of elementary excitations in low-dimensional electron gases to resonant inelastic light scattering is found to be determined by interband transitions involving states at specific wave vectors. In modulation-doped GaAs/GaAlAs quantum wells, we detect only the single-particle excitations (SPE) at resonances with electron-hole transitions at the Fermi wave vector, and only plasmons at resonances with zone-center excitons. The plasmon cross section is comparable to the SPE when double electronic resonance is achieved by tuning the plasmon energy to a valence subband separation.     

Inspection of a magneto-optical trap via electromagnetically induced absorption

Electromagnetically induced absorption (EIA) was observed for the first time on a sample of ⁸⁵Rb in a magneto-optical trap using low intensity cw copropagating pump and probe optical fields. Narrow resonances revealing the dependence of the ground-state Zeeman sublevels energy structure on the quadrupolar magnetic field and the trapping optical field intensity at different trap positions, were observed. Coherence resonances as narrow as 30 kHz were obtained under low trapping field intensities. The use of EIA spectroscopy for the magnetic field mapping of cold atomic samples is illustrated.     

Microscopic description of giant resonances in highly excited nuclei

Giant resonances of general multipolarity in highly excited nuclei, which are produced in compound nuclear and deep inelastic heavy ion reactions, are described microscopically in the finite temperature linear response formalism. The linear response function is calculated in the finite temperature (FT) quasi-particle RPA approximation (FT-HFB-RPA) and is based on the corresponding self-consistent quasi-particle basis (FT-HFB). The theory is derived from the small amplitude limit of FT-TDHFB. The inclusion of cranking constraints allows the investigation of giant resonances in nuclei with large intrinsic excitation energy and high spin. A schematic model for the FT-HFB-RPA is developed and applied to the isovector giant dipole resonance in hot spherical nuclei. It is shown that the energy of the resonance depends only weakly on temperature in these systems. The experimentally observed lowering of the giant mode in highly excited nuclei is to be attributed to different effects. The descritpion of resonance damping lies beyond the scope of the random phase approximation. Possible extensions in this direction and qualitative features of the width of giant resonances at finite temperature are discussed.     

Influence of resonances on spectral formation of x-ray lines in Fe XVII

New theoretical results from large-scale relativistic close coupling calculations reveal the precise effect of resonances in collisional excitation of x-ray lines of Ne-like Fe XVII. By employing the Breit-Pauli R-matrix method and an 89-level eigenfunction expansion, including up to n = 4 levels, significant resonance enhancement of the collision strengths of forbidden and intercombination transitions is shown. The present results should help resolve long-standing discrepancies; in particular, the present line ratios of three benchmark diagnostic lines 3C, 3D, and 3E at 15.014, 15.265, and 15.456 A, respectively, are in excellent agreement with two independent measurements on electron-beam ion traps. Strong energy dependence in cross sections due to resonances is demonstrated.