High-pressure effect on the crystal and magnetic structure of LuMnO3: Correlation between a distortion of the triangular lattice and the symmetry of the magnetic state in hexagonal frustrated manganites

The effect of a high pressure (up to 6 GPa) on the crystal and magnetic structure of the hexagonal manganite LuMnO₃ is studied by neutron diffraction in the temperature range 10–295 K. It is found that, as the pressure increases, the ordered magnetic moment of Mn ions at T = 10 K decreases noticeably from 2.48 (0 GPa) to 1.98 μ_B (6 GPa). This decrease is due to an enhancement of the geometrical frustration effects on the triangular lattice. At the same time, the symmetry of the triangular antiferromagnetic state (the irreducible representation Γ₂) remains unchanged. A correlation is revealed between the distortion parameter of the triangular lattice formed by Mn ions and the symmetry of the antiferromagnetic state of hexagonal manganites RMnO₃. Based on this correlation, a generalized magnetic phase diagram of these compounds is constructed. The obtained phase diagram provides an explanation for the changes observed in the magnetic state of hexagonal manganites caused by high pressure and chemical substitution.     

On the stability of the output characteristics of a grazing incidence grating dye laser transversely pumped by a copper vapor laser

The output characteristics of a high repetition rate pulsed dye laser has both short-term fluctuations and long-term drift. In high power high repetition rate lasers flow induced variations dominate over those due to other factors. In this paper it is shown by dye laser measurements that bandwidth fluctuations can be traced to the effective changes of the resonator dispersion due to fluctuations in the penetration depth of the pump beam in the dye medium. Short-term wavelength fluctuations can be traced to instantaneous deflection of the dye laser axis by the refractive index changes due to absorption of the pump beam. The fluctuations in both the bandwidth and the wavelength decreases with increasing flow rates within a laminar region. A copper vapor laser operating at 5.6 kHz repetition rate pumped the Rhodamine 6G dye laser used. The wavelength fluctuation of ±0.0035, 0.0030, 0.0004 nm and the bandwidth fluctuation of ±710, 132, 45 MHz over approx. 60 minutes were observed at 1.2, 3.7, 5.5 lpm flow rates respectively.     

Representations of symmetric groups with maximal dimension

Results of numerical modeling connected with the study of the asymptotics of representations of the symmetric group S_N for largeN which are of maximal dimension are given.Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova, Akad. Nauk SSSR, Vol. 172, pp. 160–166, 1989.     

Mutual coupling of elements in finite microstrip antennas

The mutual coupling of the radiating elements in microstrip antennas is studied using a rigorous electrodynamic approach in which the field singularities at the radiator edges are taken into account with analytic accuracy. A spectral method is employed in combination with a method of semi-inversion by extraction of the Green-function singularity. The degree of interaction of the elements of microstrip structures at the fundamental and higher current harmonicsis determined, and the applicability of various approximations is assessed.Radio-Astronomy Institute, Academy of Sciences of Ukraine. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 35, No. 8, pp. 688–701, August, 1992.     

Simulation of pumping beams self focusing and defocusing in NH3 Raman laser

High power IR pumping beams self focusing and defocusing in NH₃ Raman laser is investigated by numerical methods on the basis of the solution of nonlinear quasioptical equations. The analysis of self focusing have been done for a wide range of parameters of radiation and of active medium. Spatial evolution of the beam configuration have been investigated for different regimes of nonlinear diffraction. Developed model enables to allow exactly for the spectroscopic features of active molecules and may be applied to the analysis of the properties of powerful optically pumped FIR lasers.     

Passivation of an Al acceptor in silicon by hydrogen or muonium

We report on calculations of the electric field gradient at the site of the Al nucleus in the Si-H-Al complex in silicon. As a model for the surroundings of the complex we used a cluster consisting of 43 silicon and one aluminum atoms. The geometry of this cluster was fully optimized at the Hartree-Fock level. To discuss the dependence of our results on the level of theory, we studied a subcluster of four silicon and one aluminum atoms at the Hartree-Fock and CASSCF level.     

Necessary and sufficient conditions for existence of exponential dichotomy of impulsive equations

Necessary and sufficient conditions for the existence of an exponential dichotomy of impulsive differential equations in a Hilbert space are found.     

151Eu Mössbauer studies on Eu1+x Ba2−x Cu3O7+δ oxide systems

Mössbauer parameters of¹⁵¹Eu in Eu_1+x Ba_2?x Cu₃O_7+δ have been studied as a function of temperature, forx=0.15 andx=0.25. The areas of the absorption spectra for the two samples are almost equal. This could mean that thef-Mössbauer factor for¹⁵¹Eu at the barium sites is much smaller than that for¹⁵¹Eu at the rare earth sites, i.e. we only observe the spectra of the latter sites. According to this, the fits of the spectra with a single quadrupolar pattern and with two quadrupolar patterns have given equivalent χ² values.