Anomalous scattering study of the bi distribution in the 2212 superconductor: implications for cu valency

The distribution of the bismuth atoms over the cation sites in the 2212 Bi-Sr-Ca-Cu-O superconductor has been determined by anomalous scattering synchrotron crystallography. The analysis of reflection pairs measured at wavelengths of 0.9243 and 0.9600 angstrom shows a delocalization of the bismuth atoms over the calcium and strontium sites. The "mixed" plane between the CuO(2) layers contains 6.0(1.4) percent bismuth (where the number in brackets represents the statistical standard deviation derived from the least-squares refinement of the data), and a much smaller amount of strontium than often assumed. The strontium deficiency is charge-compensated by the creation of electron holes in the CuO(2) layer. The result supports the view that neither extra oxygen nor overlap of the bismuth 6p and copper 3d bands is needed to account for the holes, which are an essential feature of the superconductivity mechanism.     

The incommensurate modulation of the 2212 bi-sr-ca-cu-o superconductor

The incommensurate modulation evident in the diffraction pattern of the superconductor Bi(2)Sr3-xCa(x)Cu(2)O8+y consists of almost sinusoidally varying displacements of up to 0.4A of the Bi and Sr atoms in the a-and c-directions of the unit cell, and of up to 0.3 A of the Cu atoms in the c direction only. Thus, a newly discovered feature of the Bi(2)Sr3-xCaxCu(2)O8+y structure is sizable Cu displacement, which is related to static wave formation in the Cu-O sheets. Reported thermal parameters give evidence that similar distortions occur on cooling of the thallium-containing superconductors.     

Naturally occurring parvovirus-associated feline hypogranular cerebellar hypoplasia-- A comparison to experimentally-induced lesions using immunohistology

Three cases of feline cerebellar hypoplasia are presented. At the time of examination, the ages of the cats ranged from 2 months to 1 year. Necropsy revealed cerebellar and pons hypoplasia. Polymerase chain reaction for parvoviral deoxyribonucleic acid was positive in cerebellar tissue. Cell-specific immunolabeling was used to characterize the lesions, which were characterized into 2 types. In type 1 lesions, the cortex was nearly agranular, with an extremely thin molecular layer; the Purkinje cells were randomly placed and oriented, and their stunted main dendrite produced a thorn-covered atrophic dendritic tree; the basket cell axons ran randomly and had dysmorphic endings; and myelinated fibers were severely reduced in folia axes. In type 2 lesions, the cortex was hypogranular; the Purkinje cells were linearly organized, but their main dendrite extended too far in the molecular layer before giving up smooth, bent secondary dendrites; many basket cells were located along the cerebellar surface, and their axons ran at right angle to the surface; myelinated fibers were moderately reduced. Defects in climbing fiber synapse translocation and elimination were evident in both types of lesion. This immunohistologic study allowed a comparison between lesions in these spontaneous cerebellar hypoplasia cases with those documented when using silver impregnation studies after perinatal experimental cerebellar damage. Such a comparison is consistent with viral infection that occurs before birth in all 3 cases. Progress in parvovirus biology knowledge suggests that viral NS1 protein cytotoxicity might explain degenerative changes in the Purkinje cells that were present, in addition to the development defect.     

Evaluation of sample pre-treatment and potential contamination of soluble carbon in soil extracts during lyophilisation

Lyophilisation of K₂SO₄ soil extracts has been proposed as a sample preparation technique before elemental analysis of carbon or nitrogen. However, previous measurements, based on wet oxidation or catalytic combustion, indicated that C measurements in lyophilised samples not always proved to be accurate. To determine whether the C analysis was affected by the lyophilisation process, an exploratory study was conducted to investigate the potential effects of the sample pre-treatment and of the lyophilisation process itself. This paper puts forward that the use of soil extracts, previously stored at −20 °C, may affect the recovery of salt in the samples and that contamination of the soluble carbon with exogenous C during lyophilisation is feasible. Therefore we recommend to use freshly prepared soil extracts for lyophilisation and always to include an internal standard among the unknown samples to account for a possible contamination.