Continuous-wave and Q-switched microchip laser performance of Yb : Y3Sc2Al3O12 crystals

Optical properties of Yb:Y3Sc2Al3O12 crystal were investigated and compared with those from Yb:YAG crystals. The broad absorption and emission spectra of Yb:Y3Sc2Al3O12 show that this crystal is very suitable for laser-diode pumping and ultrafast laser pulse generation. Laser-diode pumped continuous-wave and passively Q-switched Yb:Y3Sc2Al3O12 lasers with Cr4+: YAG crystals as saturable absorber have been demonstrated for the first time. Continuous-wave output power of 1.12 W around 1032 nm ( multi-longitudinal modes) was measured with an optical-to-optical efficiency of 30%. Laser pulses with pulse energy of over 31 mu J and pulse width of 2.5 ns were measured at repetition rate of over 12.7 kHz; a corresponding peak power of over 12 kW was obtained. The longitudinal mode selection by a thin plate of Cr4+: YAG as an intracavity etalon was also observed in passively Q-switched Yb:Y3Sc2Al2O12 microchip lasers. (C) 2008 Optical Society of America

Optical spectra, crystal-field parameters, and magnetic susceptibility of the new multiferroic NdFe3(BO3)4

We report high-resolution optical absorption spectra for NdFe3(BO3)4 trigonal single crystal which is known to exhibit a giant magnetoelectric effect below the temperature of magnetic ordering TN = 33 K. The analysis of the temperature-dependent polarized spectra reveals the energies and, in some cases, symmetries and exchange splittings of Nd3+ 84 Kramers doublets. We perform crystal-field calculations starting from the exchange-charge model, obtain a set of six real crystal-field parameters, and calculate wave functions and magnetic g-factors. In particular, the values g(perpendicular) = 2.385, g(parallel) = 1.376 were found for the Nd3+ ground-state doublet. We obtain Bloc=7.88 T and |JFN|= 0.48 K for the values of the local effective magnetic field at liquid helium temperatures at the Nd3+ site and the Nd - Fe exchange integral, respectively, using the experimentally measured Nd3+ ground-state splitting of 8.8 cm-1. To check reliability of our set of crystal field parameters we model the magnetic susceptibility data from literature. A dimer containing two nearest-neighbor iron ions in the spiral chain is considered to partly account for quasi-one-dimensional properties of iron borates, and then the mean-field approximation is used. The results of calculations with the exchange parameters for Fe3+ ions Jnn = -6.25 K (intra-chain interactions) and Jnnn = -1.92 K (inter-chain interactions) obtained from fitting agree well with the experimental data.Comment: 13 pages, 8 figures, 2 table

EXPERIMENTAL INVESTIGATION OF THE TRANSIENT CHARACTERISTICS AND MECHANICAL LOSSES IN Ni-Mn-Ga ALLOYS

The reported study was funded by RFBR, project number 20-32-90195

Solving the riddle of codon usage preferences: a test for translational selection

Translational selection is responsible for the unequal usage of synonymous codons in protein coding genes in a wide variety of organisms. It is one of the most subtle and pervasive forces of molecular evolution, yet, establishing the underlying causes for its idiosyncratic behaviour across living kingdoms has proven elusive to researchers over the past 20 years. In this study, a statistical model for measuring translational selection in any given genome is developed, and the test is applied to 126 fully sequenced genomes, ranging from archaea to eukaryotes. It is shown that tRNA gene redundancy and genome size are interacting forces that ultimately determine the action of translational selection, and that an optimal genome size exists for which this kind of selection is maximal. Accordingly, genome size also presents upper and lower boundaries beyond which selection on codon usage is not possible. We propose a model where the coevolution of genome size and tRNA genes explains the observed patterns in translational selection in all living organisms. This model finally unifies our understanding of codon usage across prokaryotes and eukaryotes. Helicobacter pylori, Saccharomyces cerevisiae and Homo sapiens are codon usage paradigms that can be better understood under the proposed model

Positive Magneto-Resistance in Quasi-1D Conductors

We present here a simple qualitative model that interpolates between the high and low temperature properties of quasi-1D conductors. At high temperatures we argue that transport is governed by inelastic scattering whereas at low temperatures the conductance decays exponentially with the electron dephasing length. The crossover between these regimes occurs at the temperature at which the elastic and inelastic scattering times become equal. This model is shown to be in quantitative agreement with the organic conductor $TTT_2I_{3-\delta}$. Within this model, we also show that on the insulating side, the positive magnetoresistance of the form $(H/T)^2$ observed in $TTT_2I_{3-\delta}$ and other quasi-1D conductors can be explained by the role spin-flip scattering plays in the electron dephasing rate.Comment: 4 pages, Latex, no figure

Composite Yb : YAG/Cr4+: YAG ceramics picosecond microchip lasers

Efficient laser-diode pumped picosecond self-Q-switched all-ceramic composite Yb: YAG/ Cr4+: YAG microchip lasers with 0.72 MW peak power has been developed. Lasers with nearly diffraction-limited beam quality (M-2 < 1.09), oscillate at stable single- and multi- longitudinal-modes due to the combined etalon effects in the Yb:YAG and Cr4(+:) YAG parts of its binary structure. (C) 2007 Optical Society of America

OBTAINING INTERMETALLIC COMPOUNDS IN Al–Ti–Zn SYSTEM

Binary intermetallic compounds – titanium aluminides (TiAl, Ti3Al) – when added to the alloys, significantly increase their strength and special properties. The most promising direction to produce intermetallic compounds are mechanochemical technologies, including mechanical alloy building. Mechanical alloying makes it possible to introduce much smaller particles into the metal matrix than can be achieved using standard powder metallurgy technologies. In addition to mechanical synthesis, aluminum-based intermetallic compounds were produced by self-propagating high-temperature synthesis (SHS) of solid chemical compounds. The synthesis was carried out according to a multistage scheme: preparation of titanium and aluminum powder, mixing; synthesis of the Al3Ti intermetallic compound by the SHS method in vacuum followed by mechanical activation of stoichiometric charges. The aim of the research was to study the dynamics of the development of nanodispersed phases in the process of synthesis during mechanical alloying. The power absorbed by the unit mass of the material for different processing times of the charge was calculated. When the level of the specific power (dose) of mechanical treatment was 3.5 kJ/g, the maximum content of intermetallic compound in the resulting material was achieved. Based on calculations and the data obtained during X-ray phase analysis, the dependence of the change in the content of ternary intermetallic compounds in the final product on the absorbed power was determined. As a result of the studies using raster electron microscopy and X-ray analysis, it was found that mechanical alloying of nanostructured intermetallic compounds Ti4ZnAl11 and Ti25Zn9Al66 with the size of nanodisperse phases less than 12 nm in the Al–Ti–Zn system, the weight ratio of proportion of the latter reaches 74 %

Eye-Safe Solid-State Quasi-CW Raman Laser with Millisecond Pulse Duration

We demonstrate the first quasi-CW (ms-long pulses, pump duty cycle of 10%) end-diode pumped solid state laser generating eye-safe radiation via intracavity Raman conversion. The output power at the first Stokes wavelength (1524 nm) was 250 mW. A theoretical model was applied to analyze the laser system and provide routes for optimization. The possibility of true CW operation was discussed.Comment: Preprint accepted for publication in Optics Communications on Feb 6, 201

Negative magneto-resistance of electron gas in a quantum well with parabolic potential

We have studied the electrical conductivity of the electron gas in parallel electric and magnetic fields directed along the plane of a parabolic quantum well (across the profile of the potential). We found a general expression for the electrical conductivity applicable for any magnitudes of the magnetic field and the degree of degeneration of the electron gas. A new mechanism of generation of the negative magnetoresistance has been revealed. It has been shown that in a parabolic quantum well with a non-degenerated electron gas the negative magnetoresistance results from spin splitting of the levels of the size quantization.Comment: 15 pages, 3 figure