Localized electronic excitations in crystalline phenacite Be2SiO4

The results of coordinated spectroscopic studies of the nature and properties of electronic excitations localized at regular and defect sites of the Be₂SiO₄ lattice are presented. The methods employed are electron-beam-excited pulsed absorption spectroscopy, pulsed cathodoluminescence, and low-temperature VUV spectroscopy with selective excitation by synchrotron radiation. The bands in luminescence spectra of Be₂SiO₄ at 2.70 and 3.15 eV are assigned to [AlO₄]^5? and [SiO₄]^4? centers formed both in direct relaxation of electronic excitations at defect levels and through the formation of exciton-defect complexes. Disruptions of beryllium-oxygen bonds (short-lived defects in the form of beryllium vacancies V _Be ^? ) are considered as initiating the formation of optically active centers with characteristic absorption bands in the range 1.5–4.0 eV. The intrinsic luminescence of the Be₂SiO₄ crystal at 3.6 and 4.1 eV is attributed to radiative decay of self-trapped excitons of two types. A mechanism of exciton self-trapping at the [SiO₄] and [BeO₄] tetrahedral groups is proposed, which involves excitation transfer from a threefold-coordinated oxygen atom to neighboring silicon or beryllium atoms.     

Relaxation of electronic excitations in beryllium oxide: A time-resolved vacuum-UV spectroscopy study

Beryllium oxide crystals are studied by time-resolved optical and luminescence vacuum-UV spectroscopy. The low-temperature luminescence spectra and the luminescence decay kinetics (2.5–10 eV, 1–500 ns) upon selective photoexcitation, and also the luminescence excitation and reflectivity spectra (8–35 eV), are analyzed for BeO crystals with the optic axis aligned parallel and perpendicular to the electric vector of exciting polarized synchrotron radiation. It is found that the radiative relaxation of electronic excitations proceeds through a large number of channels. The excited states of self-trapped excitons are characterized by different multiplicity depending on the excitation energy and the sample orientation.     

On the magnetic excitations in nickel

By comparing measured and calculated versions of the generalized magnetic response function of nickel over a wide range of temperatures it is shown that the much-discussed simple model of an itinerant-electron ferromagnet based on a random-phase-approximation treatment of the Hubbard Hamiltonian accounts, in its essentials, for the magnetism of that substance. Thermal neutron inelastic scattering techniques were used to explore the whole Brillouin zone of wave vectors K in nickel up to energies ▄ω of some 2k _B T _C over the range of temperatures up to 2T _C; the computations were based on direct evaluation of the Lindhard expression for the generalized susceptibility, using a tight-binding band structure, followed by exchange enhancement of the spin part as per Izuyama, Kim and Kubo. The contribution to the neutron scattering from orbital motion of the electrons was estimated to be small, and in a special experimental investigation a satisfactorily low upper limit on this quantity for our present purposes was observed.

In the course of the article we attempt to illuminate with our computational results the theoretical content of the random-phase approximation, and to show how it accounts for spin waves, paramagnons and a variety of other phenomena. For instance at low temperatures, where the collective or spin-wave mode is the most prominent feature, the Stoner-mode states are so strongly mixed with the spin-wave states by the electron-electron interaction that little trace is left of the ‘band of Stoner modes’ often referred to in elementary accounts of this phenomenon. With increase of temperature, the severe broadening of the spin wave and the characteristic style of evolution of the magnetic response function through the critical temperature are correctly predicted, outside a narrow range of conditions about the critical point. The dramatic divergence of the susceptibility characteristic of the phase transition is confined much more closely towards the origin of K,ω space than could be accounted for by free-electron gas theory, and this is explained. On the other hand what appears to be a case of breakdown of the random-phase approximation is detected in the ferromagnetic state, where in a certain set of conditions at large K and small ω an excess of longitudinal spin correlation was measured over that expected theoretically.

Aside from the aforementioned aberrant region, however, absolute agreement between experiment and theory essentially within the statistical error is obtainable by assuming that the effective Coulomb interaction parameter I _eff(K) falls off gently between K=0 and K _max in a manner consistent with the predictions of Singwi et al. ; with increase of temperature, I _eff(0) declines gently and the amount of fall-off with K increases. It is shown that the theory of the Heisenberg ferromagnet fails entirely to account for the observations—indeed, fails to correlate any of the data on nickel—and that at least three-quarters of the known magnetic stiffness of nickel must be due to band-theory effects rather than to Heisenberg-type exchange integrals.     

Localized excitations in arrays of synchronized laser oscillators

We investigate the spatiotemporal dynamics of a large array of laser oscillators. The oscillators are locally coupled and their natural frequencies are randomly detuned. We show that synchronization of the array elements results in localized excitations wandering along well-defined trajectories.     

Magnetic excitations in cobalt oxide

Raman measurements of the temperature dependence of several low-lying magnetic excitations in CoO are reported. The large q = 0 value of the lowest excitation (143 cm^-1) and its q-independence are attributed to the large single-ion anisotropy of Co²⁺.     

Magnetic excitations in cobalt oxide

Raman measurements of the temperature dependence of several low-lying magnetic excitations in CoO are reported. The large q = 0 value of the lowest excitation (143 cm^?1) and its q-independence are attributed to the large single-ion anisotropy of Co²⁺.     

Localized spin excitations in magnets with triangular spin ordering

Some types of localized spin excitations in hexagonal antiferromagnets with triangular spin ordering are investigated. The character of relaxation of these spin excitations is studied.     

Surface photovoltage and XPS studies of electronic structure in defective nickel oxide powders

Surface photovoltage measurements of polycrystalline powder samples of nickel oxide have revealed differences in band bending (V_s) related to the defect concentrations of the oxide. The powders prepared at 700°C and 1450°C in air consist of small single crystals with low concentrations ( <10⁹ cm⁻²) of extended defects, but considerably different nickel vacancy concentrations. The defective NiO (700°C) gave V_s = 760 mV and the equilibrated NiO (1450°C) V_s = 230 mV under UV illumination, possibly due to Fermi level pinning of the Ni 3d⁸ band near V′_Ni and V″_Ni, respectively. These results are consistent with values of V_s (i.e. 630 and 240 mV, respectively) for these two samples obtained previously from XPS measurements of the dependence of surface charging on temperature. A visible transition at ≈ 2.5 eV, giving Δφ = 135 mV, is also found in the equilibrated NiO (1450°C) sample but not in the defective NiO (700°C) sample. The results are discussed in relation to other work on the electronic band structure and surface states of nickel oxide.     

Localized electronic states in nanoscale systems

Multiple scattering theory formulated on the basis of the Lippman-Schwinger integral equations was used to consider the special features of the electronic structure of nanosized clusters containing defects with special characteristics (monovacancies, impurity centers, or doping atoms). The notion of the amplitude of intracluster transitions was introduced into multiple scattering theory. These amplitudes established correlations of electron density distributions on various centers. They were used to suggest an original calculation scheme that could be used to describe localized electronic states close to defects. The electronic states distributed over the whole nanocluster volume were determined using equations simpler than initial ones.     

Relaxation of electronic excitations in strontium titanate

The transient absorption spectra and kinetics were studied for undoped, lead doped and high purity SrTiO 3 single crystals. The pulsed electron beam induced transient absorption is studied in all crystals. The strong absorption at 0.8 v eV was observed only in high purity SrTiO 3 . This absorption is suggested to arise from intrinsic electron polaron. The bound electron polarons are likely responsible for absorption band at 1.4 v eV. The main luminescence band under excitation pulse is observed at 2.75 v eV. The luminescence decay is faster than that of transient absorption.     

Multiplication of electronic excitations in AgCl crystals

The kinetics of pulse conduction in silver chloride single crystals is investigated in the temperature range 12–300 K upon picosecond excitation with x-ray bremsstrahlung. It is found that the experimentally observed concentration of conduction electrons is nearly twice as high as the concentration of electron-hole pairs generated by an exciting pulse. This effect is associated with the chain multiplication of band charge carriers.     

Localized excitations in polymer chain with an isotopically substituted host

The optical properties of the molecular chain with the guest being an isotopically substituted molecule has been considered. The energies of the localized excitations are determined by the use of a particular theoretical approach.     

Ionic displacement caused by electronic excitations

Electronic excitations are often accompanied by displacement of the ions from their ground-state equilibrium positions. This leads to line broadening of optical spectra, Stokes shifts, conformational changes, and photoinduced reactions. Here we discuss approaches to these features within ab-initio methods, in particular within many-body perturbation theory. A number of various systems, including molecules, point defects, polymers, and surfaces, are discussed to illustrate issues like localization and self-trapping that are relevant for a detailed understanding of the interrelation between excited states and geometrical structure. To cite this article: M. Rohlfing, C. R. Physique 10 (2009).     

Localized electronic excitations in NiO studied with resonant inelastic X-Ray scattering at the Ni M threshold: evidence of spin flip

We studied the neutral electronic excitations of NiO localized at the Ni sites by measuring the resonant inelastic x-ray scattering (RIXS) spectra at the Ni M2,3 edges. The good energy resolution allows an unambiguous identification of several spectral features due to excitations. The dependence of the RIXS spectra on the excitation energy gives evidence of local spin flip and yields a value of 125 +/- 15 meV for the antiferromagnetic exchange interaction. Accurate crystal field parameters are also obtained.     

Femtosecond mechanisms of electronic excitations in crystalline materials

The lattice dynamics of crystals is investigated in the course of high-power electronic excitation. It is revealed that, at W _e < W _eo , atoms and ions are displaced from their regular sites for 100–300 fs. Subsequent relaxation of the crystal lattice in response to a strong local electric field induced by the collisional displacement of ions occurs for 10–50 ns in an oscillatory manner with a period of 0.5–1.5 ps.     

On the computation of electronic excitations in solids

An approximation scheme is proposed for calculating electronic excitations in solids. It is described within a projection operator formalism of the Mori-Zwanzig type. The approximation consists in successively limiting the space of dynamical variables which are treated. The selection of the variables is done by making use of the local character of the correlation hole which is surrounding an electron. The method is distinct from a perturbation expansion and can be related to a variational ansatz via the Sauermann functional. The computation of the spectral density of the Green's function is reduced to the diagonalization of matrices. Their dimension depends on the required degree of accuracy of the calculations. The present method can be considered as an extension to excited states of a previously developed. Local Approach for ground state calculations.Dedicated to Prof. S. Methfessel on the occasion of his 60th birthday