Synthesis of a silene from 1,1-dilithiosilole and 2-adamantanone

Reaction of 1,1-dilithio-1-sila-2,3,4,5-tetraphenylsilole with 2-adamantanone produces a 5-silafulvene. This represents a new method for synthesis of silenes, leading to the first example of a silapentafulvene.     

Emission spectra of nitrous oxide supported acetylene flames at atmospheric pressure

The emission spectra of a premixed flame of acetylene supported by nitrous oxide have been recorded under different fuel-gas mixture conditions. The emission spectra in these flames of a series of metals, for which it is difficult to obtain a significant population of ground state atoms for atomic absorption spectroscopy in more conventional flames, have also been studied. The red secondary zone which is present in the fuel-rich flames shows emission attributable to long-lived CN and NH species which form a strongly reducing atmosphere to inhibit refractory oxide formation from elements such as molybdenum, titanium and aluminium introduced into the flame. An attempt has also been made to explain some of the reactions which may occur between the flame species above the primary reaction zone.     

The application of molecular theories to the behaviour of bulk polymers in steady shear and recovery

Summary The tangential and normal stress characteristics of a number of polystyrene, polyethylene and polydimethyl-siloxane polymers have been determined in steady shear flow using aWeissenberg Rheogoniometer. The influence of molecular weight, molecular weight distribution and temperature was studied and the results compared with the predictions of theRouseZimm and network theories of liquid behaviour. With the exception of certain types of polymer it was found that the behaviour observed was in reasonable agreement with the predictions of theRouse-Zimm model in association with the free volume theory. Attention is drawn how-ever to the fact that the network theory successfully described the optical and certain of the constrained recovery characteristics of polyisobutenes determined byBancroft andKaye, areas in which theRouse-Zimm theory has not yielded predictions.

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Zusammenfassung Mit Hilfe einesWeienberg-Rheogoniometers wurden die Tangential- und Normalspannungseigenschaften einer Anzahl von Polystyrol-, Polyäthylen- und Polydimethylsiloxan-Polymeren bei stationärer Scherströmung bestimmt. Der Einfluß von Molekulargewicht, Molekulargewichtsverteilung und Temperatur wurde untersucht und die dabei ermittelten Ergebnisse mit den Vorhersagen derRouse-Zimm-Theorie und Netzwerk-theorien zur Flüssigkeitsstruktur verglichen. Die Ergebnisse zeigen, daß mit Ausnahme bestimmter Arten von Polymeren das beobachtete Verhalten mit den durch dasRouse-Zimm-Modell bezüglich der Theorie des freien Volumens gemachten Voraussagen befriedigend übereinstimmte. Die Netzwerktheorie beschrieb dagegen die vonBancroft undKay ermittelten optischen sowie einige der erzwungenen Erholungseigenschaften von Polyisobutylenen.

     

Two-dimensional metal-insulator transition as a percolation transition in a high-mobility electron system

By carefully analyzing the low temperature density dependence of 2D conductivity in undoped high-mobility n-GaAs heterostructures, we conclude that the 2D metal-insulator transition in this 2D electron system is a density inhomogeneity driven percolation transition due to the breakdown of screening in the random charged impurity disorder background. In particular, our measured conductivity exponent of approximately 1.4 approaches the 2D percolation exponent value of 4/3 at low temperatures and our experimental data are inconsistent with there being a zero-temperature quantum critical point in our system.     

Spin transition in strongly correlated bilayer two-dimensional electron systems

Using a combination of heat pulse and nuclear magnetic resonance techniques, we demonstrate that the phase boundary separating the interlayer phase coherent quantum Hall effect at nu(T) = 1 in bilayer electron gases from the weakly coupled compressible phase depends upon the spin polarization of the nuclei in the host semiconductor crystal. Our results strongly suggest that, contrary to the usual assumption, the transition is attended by a change in the electronic spin polarization.     

Particle-hole symmetric Luttinger liquids in a quantum Hall circuit

We report current transmission data through a split-gate constriction fabricated onto a two-dimensional electron system in the integer quantum Hall (QH) regime. Split-gate biasing drives interedge backscattering and is shown to lead to suppressed or enhanced transmission, in marked contrast to the expected linear Fermi-liquid behavior. This evolution is described in terms of particle-hole symmetry and allows us to conclude that an unexpected class of gate-controlled particle-hole-symmetric chiral Luttinger liquids (CLLs) can exist at the edges of our QH circuit. These results highlight the role of particle-hole symmetry on the properties of CLL edge states.     

Circular-polarization-dependent study of the microwave photoconductivity in a two-dimensional electron system

The polarization dependence of the low field microwave photoconductivity and absorption of a two-dimensional electron system has been investigated in a quasioptical setup in which linear and any circular polarization can be produced in situ. The microwave induced resistance oscillations and the zero resistance regions are notably immune to the sense of circular polarization. This observation is discrepant with a number of proposed theories. Deviations between different polarizations occur only near the cyclotron resonance where an unprecedented large resistance response is observed.     

Evidence for skyrmion crystallization from NMR relaxation experiments

A resistively detected NMR technique was used to probe the two-dimensional electron gas in a GaAs/AlGaAs quantum well. The spin-lattice relaxation rate (1/T(1)) was extracted at near complete filling of the first Landau level by electrons. The nuclear spin of (75)As is found to relax much more efficiently with T --> 0 and when a well developed quantum Hall state with R(xx) approximately 0 occurs. The data show a remarkable correlation between the nuclear spin relaxation and localization. This suggests that the magnetic ground state near complete filling of the first Landau level may contain a lattice of topological spin texture, i.e., a Skyrmion crystal.     

Quiescent double barrier regime in the DIII-D tokamak

A new sustained high-performance regime, combining discrete edge and core transport barriers, has been discovered in the DIII-D tokamak. Edge localized modes (ELMs) are replaced by a steady oscillation that increases edge particle transport, thereby allowing particle control with no ELM-induced pulsed divertor heat load. The core barrier resembles those usually seen with a low (L) mode edge, without the degradation often associated with ELMs. The barriers are separated by a narrow region of high transport associated with a zero crossing in the E x B shearing rate.     

Plateau Behavior in the Chiral Luttinger Liquid Exponent

The current-voltage power law exponent, alpha, for electron tunneling into chiral Luttinger liquids at the fractional quantum Hall edge is found to exhibit a plateaulike structure at alpha close to 3 as the filling factor, nu, is varied. The presence of a plateau near alpha = 3 strongly suggests a fundamental connection between alpha and the structure of the underlying quantum ground states associated with the robust incompressible nu = 1/3 Hall fluid. However, the position in the inverse filling factor where the plateau occurs can vary between samples and appears shifted to values higher than expected from theory.