Synthesis of Silver Nanoparticles by the Ion Implantation Method and Investigation of their Optical Properties

We have investigated the process of metal nanoparticle (NP) synthesis in SiO₂ by implanting Ag₊ ions with an energy of 30 keV depending on the dose ((2–8)·10¹⁶ cm⁻²) and the ionic current density (4–15 µA/cm²). Analysis of the composite materials formed was performed with the use of optical spectroscopy and atomic-force microscopy (AFM). The NPs synthesized in the glass demonstrate a characteristic absorption line associated with the surface plasma resonance effect. A correlation of the spectral shift of the lines caused by a change in the NP size with the diameter of the hemispherical asperities on the SiO₂ surface registered by the AFM method has been revealed. It has been found that for the case of a fixed current density in the ion beam the silver NP sizes remain practically unaltered with increasing ion dose up to a certain value (6·10¹⁶ cm⁻²), and only an increase in the concentration of NPs is observed thereby. However, a further increase in the dose causes a decrease in both the NP density and size. On the other hand, at a fixed high dose an increase in the ionic current density leads to a gradual enlargement of the NPs. We have considered the mechanisms explaining the change in the NP sizes with increasing dose and ionic current density and evaluated the possibilities of carrying out controlled synthesis by varying the implantation conditions.__________Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 2, pp. 218–223, March–April, 2005.     

The “SKATE” CO2 Gigawatt Laser for a Laser-Plasma Generator of Ions and Nuclei

A set-up of the CO₂ SKATE and its output characteristics are described. The system is based on a self-sustained gas discharge module with x-ray preionization that ensures reliability and stable repetition rate operation at 1 Hz for a long time. Measurement of the initial electron density in the discharge volume produced by an x-ray gun is presented for different gases and their mixtures, which is of independent interest for some applications of x-ray preionization. Data on measurements of the low-intensity signal gain and laser parameters for a free-running generator configuration are presented as well as long-term operation characteristics. This laser system has been used as a CO₂ generator with gigawatt peak intensity and high spatial–temporal characteristics. In particular, the laser can be used as a driver for a source of multicharged ions in contrast to the laboratory prototypes previously investigated.     

Electron spin resonance in the spin-1/2 quasi-one-dimensional antiferromagnet with Dzyaloshinskii-Moriya interaction BaCu2Ge2O7

We have investigated the electron spin resonance (ESR) on single crystals of BaCu2Ge2O7 at temperatures between 300 and 2 K and in a large frequency band, 9.6-134 GHz, in order to test the predictions of a recent theory, proposed by Oshikawa and Affleck (OA) [Phys. Rev. Lett. 82, 5136 (1999)]], which describes the ESR in a spin-1/2 Heisenberg chain with the Dzyaloshinskii-Moriya interaction. We find, in particular, that the ESR linewidth, Delta H, displays a rich temperature behavior. As the temperature decreases from T(max)/2 approximately 170 to 50 K, Delta H shows a rapid and linear decrease, Delta H approximately T. At low temperatures, below 50 K, Delta H acquires a strong dependence on the magnetic field orientation and for H axially c it shows a (h/T)(2) behavior which is due to an induced staggered field h, according to OA's prediction.     

Saturated absorption and nonlinear refraction of silicate glasses doped with silver nanoparticles at 532 nm

The nonlinear optical characteristics of silicate glasses doped with silver nanoparticles are investigated by Z-scan technique using second harmonic radiation of picosecond Nd:YAG laser (=532 nm, =55 ps). The real and imaginary parts of third-order nonlinear susceptibility of silver-contained glasses were measured. It was found that the sign of Im ⁽³⁾was negative due to saturated absorption and the sign of Re ⁽³⁾ was changed from negative (self-defocusing) to positive (self-focusing) with growth of laser radiation intensity. The mechanisms responsible for saturated absorption and nonlinear refraction are discussed.     

Seven Classes of Harmonic Diffeomorphisms

We deduce two necessary and sufficient conditions for a diffeomorphism $f : M \to \overline{M}$ of a Riemannian manifold (M,g) onto a Riemannian manifold $(\overline{M},\bar g)$ to be harmonic. Using the representation theory of groups, we define in an intrinsic way seven classes of such harmonic diffeomorphisms and partly describe the geometry of each class.     

Several Constructions of Exact Sequences

The article presents a construction of a 5-term relative exact sequence in purely categorical terms and the Mayer–Vietoris sequence for weak -groupoids of R. Street. In the first half of the paper, it is shown that several well-known exact sequences can be obtained, using the Z-diagram described at the beginning of the article. There are number of categories that are close to the category of topological spaces in the sense of homotopy theory. The objects of such a category can be viewed as weak -groupoids in a very intuitive way. Therefore, the language of weak -groupoids seems to be very convenient to make explicit constructions in categories of this type. The article presents a combinatorial construction of the path space of a weak -groupoid and applies the machinery of Sec. 1 to obtain the long exact sequence of a homotopy fiber. The construction is quite natural and gives information on the structure of the relative terms. In the last section, the Mayer–Vietoris sequence of a fiber square of weak -groupoids is obtained under some conditions. Of course, this construction makes sense for topological spaces or for any category mentioned above. But the statement of the conditions is more natural in terms of weak -groupoids. This sequence generalizes the sequence of fibration, and in this case the conditions given in the article are equivalent to the homotopy lifting property. Bibliography: 6 titles.     

Orbital frustration at the origin of the magnetic behavior in LiNiO2

We report on the ESR, magnetization, and magnetic susceptibility measurements performed over a large temperature range, from 1.5 to 750 K, on high-quality stoichiometric LiNiO2. We find that this compound displays two distinct temperature regions where its magnetic behavior is anomalous. With the help of a statistical model based on the Kugel'-Khomskii Hamiltonian, we show that below T(of) approximately 400 K, an orbitally frustrated state characteristic of the triangular lattice is established. This then gives a solution to the long-standing controversial problem of the magnetic behavior in LiNiO2.     

High-temperature electron paramagnetic resonance in magnets with the Dzyaloshinskii-Moriya interaction

We analyze the high-temperature electron paramagnetic resonance (EPR) absorption in a weakly anisotropic Heisenberg magnet having two distinct types of anisotropy, represented, respectively, by a symmetric term and the Dzyaloshinskii-Moriya (DM) term. Contrary to the widespread opinion that the latter is responsible for the excessive linewidth observed in the EPR spectra of many oxides, we prove that its contribution to the linewidth is only of the same level as that of the symmetric anisotropy. This gives a solution to the long-standing controversial problem of the high-temperature magnetic relaxation in quantum-spin systems with the DM interaction.     

Josephson effect in MgB(2) break junctions

We present the first observation of the dc and ac Josephson effect in MgB(2) break junctions. The junctions, obtained at 4.2 K in high-quality, high-density polycrystalline metallic MgB(2) samples, show a nonhysteretic dc Josephson effect. By irradiating the junctions with microwaves we observe clear Shapiro steps spaced by the ideal Delta V value. The temperature dependence of the dc Josephson current and the dependence of the height of the steps on the microwave power are obtained. These results directly prove the existence of pairs with charge 2e in MgB(2) and give evidence of the superconductor-normal metal-superconductor weak link character of these junctions.