A free convective boundary layer in a conducting fluid in the presence of a transverse magnetic field

The properties of free convection in a conducting fluid in laminar regime near a hot solid vertical w all in the presence of a transverse magnetic field are theoretically analyzed. The existence of two regimes of heat transfer from the wall to the fluid are established. In the first regime, at small heights x?x* where the magnetic field effect can be disregarded, heat transfer is described by the well-known results for a free convective boundary layer in a nonconducting fluid with the Nusselt number Nu_x∝x^3/4. In the second regime, at x? x* where the magnetic field plays a crucial role, the dependence of heat transfer on the height and field strength is \(Nu_x \propto {{\sqrt x } \mathord{\left/ {\vphantom {{\sqrt x } B}} \right. \kern-\nulldelimiterspace} B}\). The location of the boundary between these regimes strongly depends on the magnetic field, x*∝ B^?4.     

Static and switching characteristics of field-controlled integrated thyristor

The static parameters of and switching transients in an advanced high-voltage high-speed switching device—a MOSFET-controlled integrated thyristor—and their dependences on the geometry and electrophysical properties of base layers in the n ⁺ pnn’p ⁺ device structure are studied experimentally. It is shown that the residual resistance of the high-voltage (∼2.5 kV) microthyristor chip in the on-state is much smaller than that of the high-voltage IGBT with the speed of the former kept at a sufficiently high level. The new device is multifunctional and can operate under different operating conditions. Specifically, it can be turned off by shunting the n ⁺ p emitter by an add-on MOSFET, by applying a gate current pulse, and in the cascode turn-off regime. In these regimes, the ultimate value of the turn-off current noticeably grows.     

On a simple invariant of Turaev-Viro type

For 3-manifolds, we define an invariant t(M)=a+bε, where a,b are integers and . An advantage of the invariant is that it admits a very simple interpretation in terms of a fake surface and a simple geometric proof of the invariance. Actually, it coincides with the homologically trivial part of the Turaev-Viro invariant of degree r=5. Extensive tables for all closed irreducible orientable 3-manifolds of complexity less than or equal to six are explicitly presented. Similar tables for r=3,4 were composed by L. H. Kauffman and S. Lins. Bibliography: 8 titles. Published inZapiski Nauchnykh Seminarov POMI, Vol. 234, 1996, pp. 137–142.     

Renormalization of potential scattering in a weakly interacting one-dimensional electron gas

The conductance of a weakly interacting electron gas in the presence of a single scatterer is found at arbitrary strength of the scattering potential. At weak interaction one can use a simple renormalization group approach instead of the standard bosonization technique. For a model with spinless electrons this approach allows us to show explicitly the crossover from the Fermi-gas to the low-temperature Luttinger liquid behavior. Deviations from the Luttinger liquid theory are studied for a realistic model of spin- electrons.     

Quark-antiquark composite systems: The Bethe-Salpeter equation in the spectral-integration technique

The Bethe-Salpeter equations for the quark-antiquark composite systems, q\(\bar q\), are written in terms of spectral integrals. For the q\(\bar q\) mesons characterized by the mass M, spin J, and radial quantum number n, the equations are presented for the following (n, M²) trajectories: π _J , η _J , a _J , f _J , ρ _J , ω _J , h _J , and b _J .     

Nature of parity violation in neutron interaction with lead

The effect of parity violation in the interaction of thermal neutrons with lead was discovered in a number of studies. According to the existing theory, this effect is explained by the mixing of compound states characterized by different parities (s-and p-wave resonances). In view of the absence of a p-wave resonance in the region of thermal neutron energies, it is of importance to reveal a level below the neutron binding energy, a so-called negative resonance. The energy dependence of the cross section for radiative neutron capture on lead was measured in the present study, and it is shown that, for the ²⁰⁷Pb isotope, the results of this measurement deviate from the \({1 \mathord{\left/ {\vphantom {1 {\sqrt E }}} \right. \kern-\nulldelimiterspace} {\sqrt E }}\) law, thereby suggesting the presence of a strong negative resonance. The parameters of this resonance are estimated.     

Micellar Effects of Functional Detergents – 1-Cetyl-3-(2-hydroxyiminopropyl)imidazolium Halides – in Reactions with 4-Nitrophenyl Toluenesulfonate and 4-Nitrophenyl Diethyl Phosphate

An anomalously large increase was found in the rate of cleavage of 4-nitrophenyl 4-toluenesulfonate (13000 times) and diethylphosphate (550 times) by the zwitterionic micelles of 1-cetyl-3-(2-hydroxyiminopropyl)imidazolium chloride and bromide. Detailed kinetic and thermodynamic analysis of these processes made it possible to establish that the main factor responsible for the high rates of reaction in the micellar phase is the concentration of the reagents.     

g-factors of discrete levels in nanoparticles

Spin-orbit scattering suppresses Zeeman splitting of individual energy levels in small metal particles. This suppression becomes significant when the spin-orbit scattering rate tau(-1)(so) is comparable to the quantum level spacing delta. At small deltatau(so) the g-factor exhibits strong mesoscopic fluctuations. We find the shape of their distribution function using the random matrix theory, and express its parameters in terms of physical characteristics: tau(so), delta, the electron mean free path l, and the particle size L. At deltatau(so)-->0 the average g-factor levels off at a small value g approximately (l/L)(1/2). However, in 2D quantum dots the g-factor is strongly enhanced by spin-orbit coupling.     

Thermally polarized (1)H NMR microimaging studies of liquid and gas flow in monolithic catalysts

The feasibility of gas flow imaging in moderately high magnetic fields employing thermally polarized gases at atmospheric pressures is demonstrated experimentally. Two-dimensional spatial maps of flow velocity distributions for acetylene, propane, and butane flowing along the transport channels of shaped monolithic alumina catalysts were obtained at 7 T by (1)H NMR, with true in-plane resolution of 400 &mgr;m and reasonable detection times. The resolution is shown to be limited by the echo attenuation due to rapid molecular diffusion in the imaging gradients of magnetic field. All gas flow images exhibit flow patterns that are not fully developed, in agreement with the range of Reynolds numbers (190-570) and the length of the sample used in gas flow experiments. The flow maps reveal the highly nonuniform spatial distribution of shear rates within the monolith channels of square cross-section, the kind of information essential for evaluation and improvement of the efficiency of mass transfer in shaped catalysts. The water flow images were obtained at lower Re numbers for comparison. These images demonstrate the transformation of a transient flow pattern observed closer to the inflow edge of a monolith into a fully developed one further downstream. Copyright 2000 Academic Press.