Rare-earth Metal Borosilicides R9Si15–xB3 (R = Tb,Yb): New Ordered Structures Derived from the AlB2 Structure Type

Two novel ternary borosilicides R₉Si_15–xB₃ (R = Tb, x = 1.80, R = Yb, x = 1.17) were synthesized from the initial elements using tin flux method. Their crystal structures were determined by means of X-ray single crystal diffraction. Both refer to space group R32, Z = 1: a = 6.668(2) Å, c = 12.405(4) Å [R₁ = 0.027, wR₂ = 0.031 for 1832 reflections with I_o > 2σ (I_o)] for Tb₉Si_15–xB₃, and a = 6.5796(3) Å, c = 12.2599(5) Å [R_F = 0.052, wR = 0.090 for 1369 reflections with I_o > 2σ (I_o)] for Yb₉Si_15–xB₃. The structures represent a new structure type, derived from that of AlB₂, with ordering in the metalloid sublattice resulting in distorted [Si₅B] hexagons. The presence or absence of boron in this ordered structure is discussed on the basis of difference Fourier syntheses, interatomic distances, structural analysis, and theoretical calculations in relation with the parent structures of the binaries AlB₂ and Yb₃Si₅ (Th₃Pd₅ type of structure). Theoretical calculations show substantial covalent interactions between the metal and nonmetal elements. The small percentage of silicon atoms, which are missing in these nonstoichiometric compounds, probably allows strengthening boron-metal and boron-silicon bonding.     

Study of double pomeron exchange in pp collisions at √s=31GeV

The reaction pp→ppπ⁺π^? at √s=31GeV was studied at the Split Field Magnet Facility of the CERN-ISR. Selecting events with two leading protons of x>0.9 and a rapidity gap Δy>2 between the protons and both pions a sample of 720 events is obtained representing a cross section of 25±10μb. The mass distribution of the dipion system, x and p_T distributions for the protons are presented and analyzed for the presence of double pomeron exchange.     

Long pulse ultraviolet laser emissions in an aerodynamically cooled electron-beam excited Ne-NF3 mixture

Long pulse laser action has been achieved at 3378.28 and 3481.96 Å in an electron beam excited supersonic flow. These laser emissions, attributed to NeII transitions, have been obtained when a mixture of Ne and NF₃ at a pressure of 0.2 atm. and aerodynamically cooled to 80 K was excited by a medium intensity electron beam. Laser pulse duration, about 350 ns fwhm for both lines, was only limited by the electron beam pulse length. The second of these laser lines had not been previously reported.     

Modeling the frequency dependence (5-120 MHz) of ultrasound backscattering by red cell aggregates in shear flow at a normal hematocrit

The frequency dependence of the ultrasound signal backscattered by blood in shear flow was studied using a simulation model. The ultrasound backscattered signal was computed with a linear model that considers the characteristics of the ultrasound system and tissue acoustic properties. The tissue scattering properties were related to the position and shape of the red blood cells (RBCs). A 2D microrheological model simulated the RBC dynamics in a Couette shear flow system. This iterative model, described earlier [Biophys. J. 82, 1696-1710 (2002)], integrates the hydrodynamic effect of the flow, as well as adhesive and repulsive forces between RBCs. RBC aggregation was simulated at 40% hematocrit and shear rates of 0.05-2 s(-1). The RBC aggregate sizes ranged, on average, from 3.3 RBCs at 2 s(-1) to 33.5 cells at 0.05 s(-1). The ultrasound backscattered power was studied at frequencies between 5-120 MHz and insonification angles between 0-180 degrees. At frequencies below approximately 30 MHz, the ultrasound backscattered power increased as the shear rate was decreased and the size of the aggregates was raised. A totally different scattering behavior was noted above 30 MHz. Typical spectral slopes of the backscattered power (log-log scale) between 5-25 MHz equaled 3.8, whereas slopes down to 0.6 were measured at 0.05 s(-1), between 40-60 MHz. The ultrasound backscattered power was shown to be angle dependent at low frequencies (5-25 MHz). The anisotropy persisted at high frequencies (>25 MHz) for small aggregates (at 2 s(-1)). In conclusion, this study sheds some light on the blood backscattering behavior with an emphasis on the non-Rayleigh regime. Additional experimental studies may be necessary to validate the simulation results, and to fully understand the relation between the ultrasound backscattered power, level of RBC aggregation, shear rate, frequency, and insonification angle.     

Element-selective nanosecond magnetization dynamics in magnetic heterostructures

We have developed a new original technique to study the magnetization reversal dynamics of thin films with element selectivity in the nanosecond time scale. X-ray magnetic circular dichroism measurements in pump-probe mode are carried out taking advantage of the time structure of synchrotron radiation. The dynamics of the magnetization reversal of each of the layers of complex heterostructures (like spin valves or tunnel junctions) can be probed independently. The interlayer coupling in the studied systems has been shown to play a key role in the determination of the magnetization reversal of each individual layer.     

Debye-Hückel limit of quantum coulomb systems

In this paper, we consider charge symmetric quantum Coulomb systems with Boltzmann statistics. We prove that the theory of screening of Debye and Hückel is a combined classical and mean field limit of these quantum Coulomb systems.Supported by the Swiss National Foundation for Scientific Research     

Magnetic interface polarization of the La-5d states in Fe/LaHx multilayers

For rare-earth (RE)-hydrides (REHx), a metal-to-insulator transition is observed if the hydrogen concentration is increased from the dihydride (x=2) towards the trihydride (x=3). This transition provides an object for studies of the magnetic interface polarization in layered ferromagnet/insulator structures. For different samples with a fixed Fe and varied LaH_x sublayer thickness t_LaHx ([15 ? Fe/t? LaH_x]xn), the H concentration x in the LaH_x sublayer is gradually increased in the experiment. Starting from the as prepared dihydride REH_2-δ, the modification of the magnetic polarization of the La-5d states at the Fe/La_x interface is studied across the transition by magnetic circular dichroism measurements at the La-L₂ and L₃ edges. The experiments reveal an induced magnetic polarization of the La-5d hole states on a length scale of 9 ? that is independent of the altered electronic structure of the La-5d states. Received: 23 May 2001 / Accepted: 4 July 2001 / Published online: 5 October 2001