Elimination of dislocations in heteroepitaxial MBE and RTCVD Ge x Si1-x grown on patterned Si substrates

We have grown Ge _x Si_1-x (0 <x < 0.20,1000–3000Å thick) on small growth areas etched in the Si substrate. Layers were grown using both molecular beam epitaxy (MBE) at 550° C and rapid thermal chemical vapor deposition (RTCVD) at 900° C. Electron beam induced current images (EBIC) (as well as defect etches and transmission electron microscopy) show that 2800Å-thick, MBE Ge_0.19Si_0.81 on 70-μm-wide mesas have zerothreading and nearly zero misfit dislocations. The Ge_0.19Si{0.81} grown on unpatterned, large areas is heavily dislocated. It is also evident from the images that heterogeneous nucleation of misfit dislocations is dominant in this composition range. 1000Å-thick, RTCVD Ge_0.14Si_0.86 films deposited on 70 μm-wide mesas are also nearly dislocation-free as shown by EBIC, whereas unpatterned areas are more heavily dislocated. Thus, despite the high growth temperatures, only heterogeneous nucleation of misfit dislocations occurs and patterning is still effective. Photoluminescence spectra from arrays of GeSi on Si mesas show that even when the interface dislocation density on the mesas is high, growth on small areas results in a lower dislocation density than growth on large areas.     

Undefining life's biochemistry: implications for abiogenesis

In the mid-twentieth century, multiple Nobel Prizes rewarded discoveries of a seemingly universal set of molecules and interactions that collectively defined the chemical basis for life. Twenty-first-century science knows that every detail of this Central Dogma of Molecular Biology can vary through either biological evolution, human engineering (synthetic biology) or both. Clearly the material, molecular basis of replicating, evolving entities can be different. There is far less clarity yet for what constitutes this set of possibilities. One approach to better understand the limits and scope of moving beyond life''s central dogma comes from those who study life''s origins. RNA, proteins and the genetic code that binds them each look like products of natural selection. This raises the question of what step(s) preceded these particular components? Answers here will clarify whether any discrete point in time or biochemical evolution will objectively merit the label of life''s origin, or whether life unfolds seamlessly from the non-living universe.     

13—THE SHRINK-RESIST TREATMENT OF WOOL BY EXHAUSTION OF POLYMERS PART I: SYNTHAPPRET BAP–VERSAMINE 552 COACERVATES

An exhaustion method to shrink-resist unchlorinated wool fabrics is described. This method uses complex coacervate dispersions prepared by mixing dilute aqueous solutions of Synthappret BAP (a poly(carbamoyl sulphonate)) and the acetate salt of Versamine 552 (a C₃₆-diamine derived from dimerized fatty acids). The conditions for the exhaustion of the coacervate dispersions onto wool were quite critical, but the range of conditions could be extended by the addition of neutral polyacrylamide flocculants. Particle-size measurements indicated that coalescence occurred during exhaustion. The level of shrink-resistance depended on both the exhaustion conditions and the curing conditions.

A simple titration method was adapted for the analysis of the cationic and anionic polymers used in this study.     

Full bulk spin polarization and intrinsic tunnel barriers at the surface of layered manganites.

Transmission of information using the spin of the electron as well as its charge requires a high degree of spin polarization at surfaces. However, at surfaces this degree of polarization can be quenched by competing interactions. Using a combination of surface-sensitive X-ray and tunnelling probes, we show for the quasi-two-dimensional bilayer manganites that only the outermost Mn-O bilayer is affected: it is a 1-nm-thick insulator that exhibits no long-range ferromagnetic order, whereas the next bilayer displays the full spin polarization of the bulk. Such an abrupt localization of the surface effects is due to the two-dimensional nature of the layered manganite, and the loss of ferromagnetism is attributed to weakened double exchange in the reconstructed surface bilayer and a resultant antiferromagnetic phase. The creation of a well-defined surface insulator atop a fully spin-polarized bulk demonstrates the ability of two of the most demanding components of an ideal magnetic tunnel junction to self-assemble naturally.     

Analysis of low count time series data by poisson autoregression

Abstract.  This study provides new methods of assessing the adequacy of the Poisson autoregressive time-series model for count data. New expressions are given for the score function and the information matrix and these lead to the construction of new types of residuals for this model. However, these residuals often need to be supplemented by formal statistical procedures and an overall test of the model adequacy is given via the information matrix equality that holds for correctly specified models. The techniques are applied to a monthly count data set of claimants for wage loss benefit, in order to estimate the the expected duration of claimants in the system.     

The evolution of primate malaria parasites based on the gene encoding cytochrome b from the linear mitochondrial genome

We report a phylogenetic analysis of primate malaria parasites based on the gene encoding the cytochrome b protein from the mitochondrial genome. We have studied 17 species of Plasmodium, including 14 parasitic in primates. In our analysis, four species were used for rooting the Plasmodium phylogenetic tree: two from closely related genera (Hepatocystis sp. and Haemoproteus columbae) and two other Apicomplexa (Toxoplasma gondii and Theileria parva). We found that primate malaria parasites form a monophyletic group, with the only exception being the Plasmodium falciparum-Plasmodium reichenowi lineage. Phylogenetic analyses that include two species of non-Plasmodium Haemosporina suggest that the genus Plasmodium is polyphyletic. We conclude that the biologic traits, such as periodicity and the capacity to relapse, have limited value for assessing the phylogenetic relationships among Plasmodium species. For instance, we found no evidence that would link virulence with the age of the host-parasite association. Our studies also reveal that the primate malaria parasites originated in Africa, which contradicts the presently held opinion of Southeast Asia as their center of origin. We propose that the radiation of Asian monkey parasites is a recent event where several life history traits, like differences in periodicity, appeared de novo.     

Book Review

Forecasting and Time Series Analysis. Douglas C. Montgomery and Lynwood A. Johnson, McGraw-Hill Book Co., 1221 Avenue of the Americas, New York, NY 10020. (304 pages; $14.50) 1976.

Integer Programming: Theory, Applications, and Computations. Hamdy A. Taha Academic Press, Inc., 11 1 Fifth Avenue, New York, New York 1003 (380 pages, $1 9.50) 1975.

Queueing Systems. Vol. 2, Computer Applications. Leonard Kleinrock John Wiley & Sons, Inc., 605 Third Avenue, New York, NY 1001 6. (549 pages; $24.95) 1976.     

Evolutionary consequences of eating:Trichosurus vulpecula (marsupialia) and the genusEucalyptus

While being somewhat specialized on a diet of matureEucalyptus leaves (66% of feeding time), wildTrichosurus vulpecula consume an average of three different foods per night. Usually, these foods are two different species ofEucalyptus leaves, and “ground feeding.” Laboratory feeding experiments indicate that this ingestion of a variety of foods is due to severe limitations on the quantity ofEucalyptus leaves a possum is capable of consuming. It is argued that the limitation is due to plant toxins (volatile oils, phenols) present inEucalyptus leaves, rather than to “normal” nutritional factors. We hypothesize thatEucalyptus toxins indirectly regulate possum populations at levels that afford theEucalyptus trees some degree of protection from possum predation. In addition, we suggest that the “New Zealand phenomenon” can be explained by a lack, and acquisition, of plant chemical defenses against herbivores introduced from chemically more complex environments.