Two-dimensional character

Both structures are maintained over thicknesses of up to several micrometers and confer a two-dimensional character to the medium. Charge transport is favored in the direction parallel to the film, that is, perpendicular to the molecular axis. 

The two-dimensional character of the Tf NMR spectrum (Fig. 3.15) makes analysis by examination impossible. 

Besides electron-phonon coupling, the shape of the Fermi surface is important in deciding CDW formation. Fermi surfaces of systems containing linear or planar arrays of atoms reflect their one- or two-dimensional character. Since the Fermi surfaces connect many states with the same wave vector, Q, a periodic distortion having the wave vector Q will produce gaps at those portions connected by Q. The energy gained... 

There are several papers on the energy-band structure of SrTi03, e.g. Soules et al, (1972). Wolfram (1972) emphasized the two-dimensional character of the Ti d-band (the conduction band), giving a rapid rise of N( ) with . This will facilitate polaron formation. 

The lipid part of the membrane is essentially a two-dimensional liquid in which the other materials are immersed and to which the cytoskeleton is anchored. This last statement is not totally correct, as some membrane bound enzymes require the proximity of particular lipids to function properly and are thus closely bound to them. Simple bilayers formed from lipids in which both hydrocarbon chains are fully saturated can have a highly ordered structure, but for this reason tend to be rigid rather than fluid at physiological temperatures. Natural selection has produced membranes which consist of a mixture of different lipids together with other amphiphilic molecules such as cholesterol and some carboxylic acids. Furthermore, in many naturally occurring lipids, one hydrocarbon chain contains a double bond and is thus kinked. Membranes formed from a mixture of such materials can retain a fluid structure. The temperature at which such membranes operate determines a suitable mixture of lipids so that a fluid but stable structure results at this temperature. It will be seen that the lipid part of a membrane must, apart from its two-dimensional character, be disordered to do its job. However, the membrane bound proteins have a degree of order, as will be discussed below. 

As already illustrated by a previous paper (5 ) on the structure of high modulus fibers, electron microscopy can be very successful when applied to these beam-resistant materials, and so constitutes an essential complement to x-ray studies. In the present work, electron diffraction coupled with BF and DF imaging has allowed detection of the best ordered zones within PBT fibers which illustrates the structure possibly obtainable by fiber processing refinement. The well ordered structures observed thus far compare rather well, with the exception of their fibrillar texture, to the structure of PPT high-modulus fibers. The two dimensional character of the crystallites is likely due to the freedom of axial translation of the molecules. Future work should determine if this feature is a direct consequence of the chemical structure of the PBT molecule or is simply the result of non-optimized processing conditions. 

The stack of BEDT is of the so-called a-phase. The sulphur-sulphur contacts between the stacks (S- S distances s= 3.47 A, shorter than the sum of the van der Waals radii of 3.6 A) and within the stack (S S distances w4.l-4.2A) give the layers a decidedly two-dimensional character. The BEDT donors are related by a two-fold screw axis, and therefore have no real chiral nature (the molecules have pseudo-centre symmetry). There are several hydrogen bonds between the hydrogen atoms of the ethylene groups of BEDT and the carboxylate groups of the counter-ion. The material is a semiconductor, where the conductivity falls as the temperature is lowered, from about 1 S cm 1 at room temperature. 

Figure 6 shows transmitted x-ray diffraction patterns for the POD films with progressive HTT. The characteristic (101) and (112) reflections are absent for HTT s below 2200 C and suddenly appear at 2500 C. This result implies that the three-dimensional order of the graphite lattice is not established at lower HTT s, namely the condensed aromatic layers exist as individuals and have a highly two-dimensional character. This may have come from the absence of cross-linked network structures at lower HTT s and may have facilitated the the recrystallization of graphite at such low temperatures and without pressure. 

The structure of KEuPSe4 has two-dimensional character. Looking down the a-axis in Figure 63(b), one can see layers of [EuPSe4] separated by K+ cations that have a distorted bicapped trigonal prismatic coordination. The layers are... 

Thus far, quantum chaos theory has mainly concentrated on the statistical description of real energy levels and their spacings (see Section 4.1.1). The complex helium resonances with their two-dimensional character require a different approach. It may be based on Ginibre s ensemble of complex random matrices (Ginibre (1965)). The investigation of the statistical properties of the helium resonances in the complex plane is certainly a good subject for future research. 

The calculated values of the /i,2 and /ps exchange interaction integrals are negligible as a consequence of the essentially two-dimensional character of crystalline Tg (see Fig. 6-3). This leads [(see Eq. (6.1a)-(6.1d)] to the degeneracy of the and crystal components with Og and bg, respectively. We note that the lowest component (a ) is formally allowed. However it is calculated, within the oriented gas model (OGM), to be vanishing as a consequence of the relative ori-... 

The curve of Cj, versus for an infinitely long cylinder normal to the flow is much like that for a sphere, but at low Reynolds numbers, does not vary inversely with because of the two-dimensional character of the flow around the cylinder. For short cylinders, such as catalyst pellets, the drag coefficient falls between the values for spheres and long cylinders and varies inversely with the Reynolds number at very low Reynolds numbers. Disks do not show the drop in drag coefficient at a critical Reynolds number, because once the separation occurs at the edge of the disk, the separated stream does not return to the back of the disk and the wake does not shrink when the boundary layer becomes turbulent. Bodies that show this type of behavior are called bluff bodies. For a disk the drag coefficient Cj, is approximately unity at Reynolds numbers above 2000. 

Figure A3.10.8 schematically depicts a Si(lOO) surface (a) being etched to yield a rough surface (b) and a more regular surface (c). The surfaces shown here are seen to consist of steps, terraces and kinks, and clearly have a three-dimensional character, rather than the two-dimensional character of an ideally flat, smooth surface. The general etching mechanism is based on the use of halogen molecules, the principal etchants used in dry etching. Upon adsorption on silicon at room temperature, Br2 dissociates to form bromine atoms, which react with surface silicon atoms. Then, if an external source of energy is provided, for example by heating Si...

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