A mechanism for photoinduced electronic reconstruction of the oxygen vacancy in doped quartz glass and its characteristics

A mechanism is proposed for electronic reconstruction of excited oxygen-deficient (OD) centers in doped quartz glass, associated with partial delocalization of one of the valence electrons. Using Falicov’s model Hamiltonian, the binding energy, radius, and criterion for an electron transition from a localized state to a state of intermediate radius are qualitatively determined. Densification of the glass matrix surrounding the OD centers is predicted as a possible consequence of the reconstruction of their valence electrons. An explanation is proposed for the irreversible photoinduced changes in refractive index, photoluminescence spectra, and electron paramagnetic resonance that are experimentally observed in doped quartz glasses after processing with large doses of ultraviolet light. Fiz. Tverd. Tela (St. Petersburg) 41, 1019–1025 (June 1999)     

Spin-peierls systems in magnetic field: Phase transition from dimerized to soliton lattice state

The phase diagram for the spin-Peierls system in high magnetic field is studied. The line of transition from dimerized (D) to soliton lattice (SL) phase is found, and it is shown that the SL phase is stable in fields h;h_c. Magnetization vs h is calculated as well as the sound velocity u_s of the domain walls oscillations. The possible experimental verification of the proposed theory is discussed also.     

Half-metallic spin polarized electron states in the chimney-ladder higher manganese silicides MnSi1−x (x = 1.75 − 1.73) with silicon vacancies

We define a numerical method that provides a non-parametric estimation of the kernel shape in symmetric multivariate Hawkes processes. This method relies on second order statistical properties of Hawkes processes that relate the covariance matrix of the process to the kernel matrix. The square root of the correlation function is computed using a minimal phase recovering method. We illustrate our method on some examples and provide an empirical study of the estimation errors. Within this framework, we analyze high frequency financial price data modeled as 1D or 2D Hawkes processes. We find slowly decaying (power-law) kernel shapes suggesting a long memory nature of self-excitation phenomena at the microstructure level of price dynamics.     

Third-harmonic generation in polyvinylpirrolidone-based metallic complexes

The third-order nonlinear susceptibilities of the aqueous solutions of polyvinylpirrolidone doped with iron and zinc were studied by the method of third-harmonic generation with 1064-nm Nd:YAG laser radiation. The efficiencies of transformation into the third harmonic in these metallic complexes were 8×10^?7 and 5×10^?7, respectively. To determine the role of the Kerr effect in the transformation process, the nonlinear refractive index of the iron-doped polyvinylpirrolidone solution was measured by the Z-scan method and a value of n ₂=?6.7×10^?13 cgs unit was obtained.     

Engineering near-surface electron states in three-dimensional topological insulators

Using the continual model of a semi-infinite three dimensional (3D) topological insulator (TI) we study the effect of the surface potential (SP) on the formation of helical topological states near the surface. The results reveal that spatial profile and spectrum of these states strongly depend on the SP type and strength. We pay special attention to the 3D TI substrate/non-magnetic insulating overlayer system to illustrate the principles of the topological near-surface states engineering.     

Effect of the spin short-range order on electron transport and neutron scattering in amorphous alloys Gd<Subscript>x</Subscript>Si<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>

Direct experimental evidence has been obtained for the existence of short-range ferromagnetic order in Gd_xSi_1?x alloys. Along with the standard magnetotransport measurements, a “local” experimental method of the elastic scattering of thermal neutrons on the magnetic moment of Gd is applied. The measurement results provide the conclusion that short-range ferromagnetic order is formed in nanoclusters with an increased content of Gd ions. We consider the amorphous alloy Gd_xSi_1?x as a new magnetic material with unique properties characteristic of both strongly inhomogeneous alloys and nanocomposite compounds.     

Low- and high-order nonlinear optical characterization of C60-containing media

Third- and higher-order nonlinear optical processes in fullerenes were studies to define the influence of low-order nonlinearities on the high-order harmonic generation in these media. We measured the nonlinear absorption coefficients of the C₆₀:toluene solution using the 532 and 1064 nm, 50 ps pulses. The high-order harmonic generation was studied during propagation of the 790 nm, 150 fs pulses through the plasmas produced on surfaces containing fullerene powder. These studies have shown that the low-order nonlinearities of fullerenes have no impact on the generation of harmonics in such mediums in the vacuum ultraviolet range at optimal intensity of laser radiation.     

Effect of higher order nonlinear optical processes on optical absorption in the photorefractive BSO and BGO crystals

The processes of nonlinear refraction and nonlinear absorption are studied in the photorefractive Bi₁₂SiO₂₀ (BSO) and Bi₁₂GeO₂₀ (BGO) crystals at a wavelength of a picosecond Nd:YAG laser of 1064 nm. The nonlinear refraction in the crystals is shown to be related to the Kerr effect, and the nonlinear absorption at this wavelength, to three-photon absorption. The three-photon absorption coefficients of the BSO and BGO crystals are equal, respectively, to (2.5±0.8) ×10^?20 and (4.4±1.3) ×10^?20 cm³W^?2.     

Contact-induced magnetism in nanostructures based on chromium with nonmagnetic metal monolayers

Magnetic ordering is studied in Cr/X (1 ML) layer structures with monatomic layers (ML) of nonmagnetic metals (X = Sn, V, Ag, Au) incorporated into a chromium matrix. The mechanism associated with a redistribution of the charge and spin densities near the Cr/X interface and through which a spin-density wave (SDW) is induced and modified in these systems is analyzed. A semiphenomenological model is considered in detail in terms of which SDW structure near a single planar nonmagnetic defect can be described qualitatively using the Ginzburg-Landau functional. The spatial SDW configuration in a [Cr(t/X(1 ML)] superlattice is calculated, and the dependences of the SDW parameters on the temperature T, type of metal X, and superlattice period t are established. Based on the results of the study, experimental Mössbauer spectroscopy data are interpreted.