Density-induced interchange of anisotropy axes at half-filled high Landau levels

We observe density-induced 90 degrees rotations of the anisotropy axes in transport measurements at half-filled high Landau levels in the two dimensional electron system, where stripe states are proposed ( nu = 9/2, 11/2, etc.). Using a field effect transistor, we find the transition density to be 2.9x10(11) cm(-2) at nu = 9/2. Hysteresis is observed in the vicinity of the transition. We construct a phase boundary in the filling factor magnetic field plane in the regime 4.4相似文献   

A comparison of atomic force microscopy (AFM) and dynamic light scattering (DLS) methods to characterize nanoparticle size distributions

This paper compares the accuracy of conventional dynamic light scattering (DLS) and atomic force microscopy (AFM) for characterizing size distributions of polystyrene nanoparticles in the size range of 20–100 nm. Average DLS values for monosize dispersed particles are slightly higher than the nominal values whereas AFM values were slightly lower than nominal values. Bimodal distributions were easily identified with AFM, but DLS results were skewed toward larger particles. AFM characterization of nanoparticles using automated analysis software provides an accurate and rapid analysis for nanoparticle characterization and has advantages over DLS for non-monodispersed solutions.     

Large P4-free graphs with bounded degree

Let ex * (D; H) denote the maximum number of edges in a connected graph with maximum degree D and no induced subgraph isomorphic to H. We prove that this is finite only when H is a disjoint union of paths,m in which case we provide crude upper and lower bounds. When H is the four-vertex path P₄, we prove that the complete bipartite graph K_D,D is the unique extremal graph. Furthermore, if G is a connected P₄-free graph with maximum degree D and clique number ω, then G has at most D² ? D(ω ? 2)/2 edges. © 1993 John Wiley & Sons, Inc.     

Some morphological observations on polycaprolactam filament

Examination of longitudinal, cross, and skew sections of extruded polycaprolactam filaments with the polarizing light microscope reveal morphological features not previously described. In cross-sectional view, three distinct layers enclose a “homogeneous” central matrix. The outermost layer is characterized by large, birefringent, truncated cones extending upward from the surface. Associated with the cones and an entity in its own right is what appears to be a second layer of ultrafibrils. Below the latter is a fine-structured layer of submicroscopic spherulites. A combination of the above features forms the transcrystalline region of stressed melts. Row nucleation is evident on the surface of the filaments.     

An ultrasonic velocity borehole logger for use in rock

An ultrasonic velocity borehole logger has been developed for use in small diameter boreholes drilled in rock for probing ahead of tunnels. The paper gives details of its design and construction and examples of its use in logging boreholes drilled in a laboratory trial and in an underground stone quarry. Two special features of the logging tool are that the transducers are mechanically pressed against the rock in the borehole wall and therefore no liquid is needed in the borehole to provide a coupling for the signal, and that the whole tool has been made compact enough to fit into a 56 mm diameter borehole. The instrument measures the compressional wave velocity of the rock, which may be interpreted to provide a qualitative indication of rock properties. In addition a method of deriving the rock strength from the velocity is suggested and demonstrated for the two test boreholes. Recommendations for developing the prototype into an instrument for routine use are made.     

Some observations on the mechanisms of atomization in atomic absorption spectrometry with atom-trapping and electrothermal techniques

The mechanisms of collection and release of sixteen elements in atom-trapping atomic absorption spectrometry with a water-cooled silica trap in an air-acetylene flame are examined. Ag, Au, Cd, Co, Cu, Fe, Ni, Pb, Se and Zn appear to accumulate as metals whilst K, Li, Na, Cr, Mg and Mn are trapped as silicates or oxides. Al and V are also trapped as oxides, but were not studied further. No evidence could be found that the surface temperature of the trap exceeds 1700 K during the release cycle. Plots of appearance time of atoms vs. m.p. suggest that while direct evaporation can play a part in atomization, sputtering by energetic species in the scouring flame gases may explain the appearance of gaseous atoms at the relatively low temperatures involved. The atomization phenomena are related to those observed with electrothermal atomizers based on carbon and tantalum. It is suggested that sputtering processes may also be involved in such atomizers.