Dipolar excitations at the LIII x-ray absorption edges of the heavy rare-earth metals

We report measured dipolar asymmetry ratios at the LIII edges of the heavy rare-earth metals. The results are compared with a first-principles calculation and excellent agreement is found. A simple model of the scattering is developed, enabling us to reinterpret the resonant x-ray scattering in these materials and to identify the peaks in the asymmetry ratios with features in the spin and orbital moment densities.     

Particle-hole excitations in nucleus-nucleus collisions

Nucleus-nucleus collisions lead to nuclear excitations which can be expressed in terms of particles and holes. An empirical formula is presented to calculate this initially degree of freedom. The formula is tested against data and found to have a large range of validity with respect to projectile masses and bombarding energies.     

d-d excitations in manganites probed by resonant inelastic x-ray scattering

We report a study of electronic excitations in manganites exhibiting a range of ground states, using resonant inelastic x-ray scattering (RIXS) at the Mn K edge. Excitations with temperature dependent changes correlated with the magnetism were observed as high as 10 eV. By calculating Wannier functions, and finite-q response functions, we associate this dependence with intersite d-d excitations. The calculated dynamical structure factor is found to be similar to the RIXS spectra.     

Effects of electron-hole excitations in ion-surface collisions

We investigate effects related to electron-hole pair production and atomic level shift in atom scattering at surfaces by using a recently proposed exactly soluble model. We show that electron-hole pair production weakens Stückelberg oscillations and enhances loss of memory of the initial atomic charge state for narrow bands because of the diffusion of an electron or hole captured by the band. Wide band materials tend to display memory loss at lower velocities than do narrow band materials. Allowing the atomic energy level to shift above the Fermi energy tends to reduce memory loss.     

Multiphonon excitations in heavy ion grazing collisions

A semiclassical model is used to study the excitation of giant resonances in heavy ion grazing collisions. The projectile is described as a moving Woods-Saxon potential with a fixed shape, and the evolution of the target state is calculated by time-dependent perturbation theory. Using random phase approximation wave functions, probabilities to excite various resonances are obtained. Multiple phonon excitations appear as the possible mechanism for the structures observed in heavy ion collisions.     

Theory of resonant inelastic x-ray scattering by collective magnetic excitations

I present a tractable theory for the resonant inelastic x-ray scattering (RIXS) of magnons. The low-energy transition operator is written as a product of local spin operators and fundamental x-ray absorption spectral functions. This leads to simple selection rules. The scattering cross section linear (quadratic) in spin operators is proportional to the fundamental magnetic circular (linear) dichroic spectral function. RIXS is a novel tool to measure magnetic quasiparticles (magnons) and the incoherent spectral weight, as well as multiple magnons up to very high energy losses, in small samples, thin films, and multilayers, complementary to neutron scattering.     

Line width of crystal-field excitations in metallic rare-earth systems

A theory is developed for the line width of crystal-field excitations in metallic rare-earth systems were the damping is due to conduction electron-hole excitations. First the single ion or dilute alloy case is studied. As an example the theory is applied to Ce in LaAl₂ and compared with recent experiments. Then the case of a regular lattice of rare-earth ions is considered. The theory is applied to the ₁ – ₄ system. Special attention is paid to the behaviour of the central peak near an induced-moment phase transition. It is shown that the dynamics of the conduction electrons leads to observable effects in the centralpeak intensity and can not be neglected as it is usually done. The theoretical predictions are in agreement with recent experimental findings on Pr₃Tl.     

Far-wing light absorption induced by resonant or near-resonant collisions

Summary We have studied the absorption of light induced by a resonant or near-resonant collision between two atoms. The calculations have been performed by taking into account also the magnetic sublevels, which makes our theoretical predictions more applicable to realistic cases. Analytical expressions for the far-wing absorption cross-section have been obtained.     

Double resonant charge exchange in ion-ion collisions

The process of resonant double charge exchange in low energy ion-ion collisions is considered. The effects of the Coulomb interaction of the nuclei are taken into account. Cross section calculations are performed both for completely and partially stripped projectile ions with 3? Z ? 10.     

Low energy excitations in amorphous metals

The low temperature properties of amorphous metals pertinent to the existence of low energy excitations are reviewed. In an introductory section, the currently accepted model for low energy excitations, i.e. the model of two level systems (TLS), is discussed. The treatment of amorphous metals focuses on the specific heat C, thermal conductivity κ, ultrasonic properties and electrical resistivity ?. Measurements of C and κ are often performed on amorphous superconductors in order to exclude the effect of conduction electrons. The TLS density of states and the TLS-phonon coupling in these materials as determined from C and κ respectively are surprisingly close to values for insulating glasses. Ultrasonic experiments carried out mostly on normal amorphous metals reveal a strongly enhanced TLS relaxation rate with respect to insulators. This can be attributed to TLS-electron coupling. The effect of this coupling on ? is uncertain at present. In order to obtain more insight into the microscopic origin of TLS in metallic and insulating glasses some examples of TLS in crystalline disordered solids are discussed. Finally, some formal analogies are drawn between spin glasses and real glasses.     

Low-field mobilities of rare-earth metals

The KLL Auger spectrum of Ni generated in the electron capture decay of radioactive ⁶⁴Cu in a solid state matrix was measured for the first time using a combined electrostatic electron spectrometer adjusted to a 7 eV instrumental resolution. Energies and relative intensities of the all nine basic spectrum components were determined and compared with data obtained from X-ray induced spectra of metallic Ni and with theoretical results as well. Absolute energy of 6562.5 ± 1.3 eV (related to the Fermi level) measured for the dominant KL₂L₃(¹D₂) than a value obtained from the X-ray induced spectra which is probably caused by the effects of chemical bonding and physico-chemical environment. Moreover, it is higher by 20.4 eV (16??) than a prediction of the semi-empirical calculations by Larkins which indicates an influence of the ??atomic structure effect?? on absolute energies of the Auger transitions following the electron capture decay and, possibly, some imperfections in the calculations. Good agreement of the measured and predicted KL₁L₂(³P₀/¹P₁) transition intensity ratios indicates perceptible influence of the relativistic effects on the KLL Auger spectrum even at Z = 28.     

Charge excitations in heavy electron metals

We show that the optical response of metals with strong electron-electron correlation consists of two excitations, a renormalized Drude response at zero energy and a mid-infrared peak occurring at frequencies around 2000 cm^-1. The latter originates from a dynamical, correlation-induced gap, as evinced from a many body theoretical approach based on the periodic Anderson model. At very low temperatures, it can be viewed as optical gap between two renormalized quasi-particle bands. The gap size is proportional to the geometric mean of the characteristic lattice Kondo temperature of the material and its bandwidth. Received 28 August 2000 and Received in final form 31 October 2000     

Afterglow of GaN wurtzite-structure crystals doped by rare-earth metals

The effect of impurity distribution and total defect concentration on the transport of nonequilibrium carriers has been estimated and specific features of afterglow in GaN and GaN〈Eu, Sm, Er〉 crystals revealed by measuring delayed near-edge photoluminescence spectra. A decrease in the total carrier concentration is shown to correlate with the afterglow in GaN wurtzite-structure crystals. The influence of additional illumination at a wavelength of 5145 Å on the evolution of delayed near-edge photoluminescence spectra was estimated in crystals with different Fermi level positions.     

Single and double orbital excitations probed by resonant inelastic x-ray scattering

The dispersion of the elusive elementary excitations of orbital ordered systems, orbitons, has escaped detection so far. The recent advances in resonant inelastic x-ray scattering (RIXS) techniques have made it, in principle, a powerful new probe of orbiton dynamics. We compute the detailed traces that orbitons leave in RIXS for an e{g} orbital ordered system, using the ultrashort core-hole lifetime expansion for RIXS. We observe that both single- and double-orbiton excitations are allowed, where the former, at lower energy, have sharper features. The rich energy- and momentum-dependent intensity variations that we observe make clear that RIXS is an ideal method to identify and map out orbiton dispersions.     

Nuclear time delay in resonant heavy ion collisions

From a quantum mechanical model for quasielastic nuclear scattering, involving a pocket in the internuclear potential, we derive a distribution of nuclear delay times. We show that coherent excitation of states in a rotational band of nuclear resonances leads to a lighthouse effect in the nuclear scattering cross section. Its influence on atomic excitations is shortly discussed for the case of positron creation in supercritical heavy ion collisions.     

Absorption of resonant electromagnetic radiation in electron-atom collisions

Nonrelativistic quantum theory is used to study the possibility of amplification of electromagnetic radiation in forced braking scattering of an electron beam on atoms. The interaction of the atom with the electromagnetic field is considered in the resonant approximation. Cases of large and small detuning from resonance are considered. It is shown that for any orientation of the electron beam relative to the field polarization vector, absorption of radiation occurs, with the major contribution being produced by atomic electrons.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 79–84, April, 1986.The authors are indebted to M. F. Fedorov and S. I. Yakovlenko for their valuable discussion and advice.