No description available.

The formation of the corpus luteum, its maintenance throughout pregnancy and the secretory activity of this transient endocrine gland receive special emphasis because of its role in the secretion of progesterone. An inadequate luteal phase has been associated with an increased incidence of congenital abnormalities. Anything which interferes with the hormonal environment may lead to fetal death or abnormal embryogenesis. Retinoic acid was administered to mice early in pregnancy to induce neural tube defects. Plasma concentrations were determined by high pressure liquid chromatography. Distribution of the retinoic acid was determined by wholebody autoradiography. The hormonal profile during pregnancy was determined in control and retinoic acid treated dams by specific radioimmunoassay for progesterone and 20 alpha dihydroprogesterone. There was no significant effect of retinoic acid on the hormonal profile of treated dams. This suggests that the teratogenic effects of retinoic acid are not due to a change in the hormonal environment. Progesterone and 20 alpha dihydroprogesterone concentrations were significantly increased on days 10 and 10.5 of pregnancy in both treated and control dams. This is the point when pseudopregnancy would end and pregnancy maintenance comes under the control of luteinizing hormone. The increase in 20 alpha dihydroprogesterone concentration at this point in pregnancy was similar to that observed after the administration of prostaglandin F_2alpha (PGF_2alpha). It seems likely that PGF_2alpha and luteinizing hormone act antagonistically during midpregnancy. Concomitant treatment revealed an interaction resulting in an increased progesterone secretion and the conversion of progesterone to 20 alpha dihydroprogesterone. Hence, PGF_2alpha may be released at the point in pregnancy when luteinizing hormone takes over as the major luteotrophin. Luteinizing hormone thus maintains pregnancy in the mouse in the face of an endogenous PGF_2alpha release.

No description available.

No description available.

The aim of this thesis is to study topological soliton solutions in classical field theories, called sigma models, on a three-dimensional space. In chapter 1 we review the general field-theoretical framework of classical soliton solutions and exemplify it on the main features of the 0(3) σ-model and the Abehan Higgs model in (2+1) dimensions. In chapter 2 a U(l)-gauged 0(3) σ-model is discussed, where the behaviour of the gauge field is determined by a Chern-Simons term in the action. We find numerical solutions for radially symmetric fields and discuss those of degree one and two. They carry a non-vanishing angular momentum and can be interpreted as classical anyons. A similar model is studied in chapter 3. Here the potential is of Higgs-type and chosen to produce a Bogomol'nyi model where the energy is bounded from below by a linear combination of the topological degree of the matter fields and the local U(l)-charge. Depending on internal parameters, the solutions are solitons or vortices. We study them numerically and prove for a certain range of the matter field's vacuum value that there cannot be a 1-soliton.In chapter 4 we discuss a modified 0(3) σ-model in (3+0) dimensions. The topological stability of the solitons is here imphed by the degree of the map S(^3) → S(^2), which provides a lower boundon the potential energy of the configuration. Numerical solutions are obtained for configurations of azimuthal symmetry and the spectrum of slowly rotating solitons is approximated. Chapter 5 deals with a theory where the fields are maps IR(^2+1) → CP(^2). The Lagrangian includes a potential and a fourth-order term in the field-gradient. We find a family of static analytic solutions of degree one and study the 2-soIiton configuration numerically by using a gradient-flow equation on the moduli space of solutions. We conclude this thesis with a brief summary and give an outlook to open questions.

This thesis is primarily concerned with the characterisation of the native defects present in CdTe crystals grown from the vapour phase using two different methods. The principal characterisation techniques used were TEM, SEM and etching/optical microscopy. Crystal defects in bulk CdTe are of great importance since this material is in demand as a substrate for the epitaxial growth of Cd(_x)Hg(_1-x)Te for use in large area infra-red imaging devices. Large single crystals grown using the 'Durham' technique invariably contain first order twin boundaries lying on {111} planes and these are of the 'ortho'-type. Twin boundaries on other planes were characterised using the coincidence site lattice models of twin boundaries in sphalerite which were developed during this work. First order lateral twin boundaries lying on {511}-{111}, {112}-{112}, {001}-{221} and {110}-{114} and second order twin boundaries lying on {111}-{115} {114}-{114} and {22l}-{22l} are discussed. Twinning in CdTe is thought to be due to post-growth stress rather than to a growth phenomenon. The dislocations which are present in crystals grown by the 'Durham’ method are predominantly concentrated into well-formed arrays which are sometimes associated with other crystal defects. These arrays were shown by EBIC studies to be electrically active and the average sub-grain dimension was found to be ~ 150 µm. It is thought that the arrays form as a result of dislocation polygonisation during the latter stages of growth. Precipitates, which were identified as being comprised of Te, are found in the bulk of the crystals although they are more often seen on twin boundaries. The relationship between boundary type and the density of precipitates is discussed. Sub-grain boundaries and precipitates were also examined in a small number of CdTe crystals which had been grown by a modified 'Piper-Polich' technique. The differences between the defects in this material and those in CdTe crystals grown by the 'Durham' method are fully explained in terms of the differences in the crystal growth conditions employed in the two techniques. Networks of dislocations in Cd(_0.95) Zn(_0.0 5) Te crystals grown by the 'Durham' technique were also investigated. A mechanochemical polishing machine which was developed during the course of this work and which is capable of producing high quality chemically polished surfaces is described. Also the use of chemical etchants to determine the crystallographic polarity of {111} oriented surfaces is clarified. Important features of this work include; the development of coincidence site lattice models of first order lateral and second order twin boundaries in the sphalerite structure and the thorough characterisation of these boundaries in as-grown CdTe crystals, and the comparison of the sub-grain boundaries and precipitates present in material grown from the vapour phase by two different techniques.

DLTS, ODLTS and DLOS have been used to characterise the main deep level trapping centres in some II-VI semiconductors; these were single crystal CdS, (ZnCd)S, CdSe, CdTe and ZnS, and polycrystalline CdS films. Undoped, single crystal CdS contained four electron traps as detected by DLTS, at 0.29eV, 0.41eV, 0.61eV and 0.74eV below the conduction band (CB). The first two were observed in all samples and were due to native defects. The two states of highest energy were found only in material that had been annealed in S or Cd vapours. The 0.61ev level could be photoinduced by illumination at photon energies greater than about 1eV. It decayed in the dark with an activation energy of 0.25eV. The 0.61eV and 0.74eV centres were associated with electrically active extended defects (subgrain boundaries Such samples had dislocation densities of about 10(^10) cm(^-2). Copper was found to be a residual impurity in CdS. It produced two deep hole traps resulting from a crystal field splitting of the Cu d(^9) state. They were detected by ODLTS and DLOS and were found at 0.35eV and 1.lev above the valence band (VB).Introduction of the isoelectronic impurity tellurium into CdS induced a hole repulsive centre at 0.21eV above the VB. This is thought to be an inportant radiative recombination centre. The main electron trap in CdS at 0.41eV was found to shift to higher energy with incorporation of Zn. Replacement of 20% of the Cd with Zn shifted the energy to 0.63eV. The level appeared fixed to the VB and had a similar functional dependence on composition as the band gap. The activation energies of the copper centres observed in CdS remained unchanged with incorporation of Zn up to the composition (^Zn)0.45 (^cd)0.55(^s) showed that the crystal field splitting was constant and that these levels were also pinned to the VB. During the fabrication process of the (ZnCd)S/Cu(_2)S solar cell, a deep level was induced at about 1.2eV below the CB. This is thought to be a recombination centre and one of the contributory factors to the reduction observed in the current collection efficiency of these devices. Polycrystalline CdS films were prepared by silk screen printing (SP) and evaporation. The SP films were annealed at various times and temperatures to improve the crystallinity of the layers. At 640C for 1hr, deep states at 0.16eV and 0.48eV were detected. The levels disappeared when annealed at 670C-700C and a new level was observed at 0.13eV. CdS/Cu(_2)S heterojunctions were prepared on the material sintered at 670C; this induced a further trapping level at 1.1eV and one that was poorly resolved. Copper diffused into the CdS during the fabrication of the device so the states associated with copper were detected at 0.35eV and 1.1eV, The evaporated CdS layers showed that the defect signature was sensitive to the type of substrate. Using Ag instead of the usual SnO(_x), deep states were induced at 0.48eV and 0.98eV below the CB. These Ag-associated impurity centres prevent the indiffusion of Cu during the optimising heat treatment of the CdS/Cu(_2)S heterojunction. This maintains the stoichicmetry of the Cu(_2)S layer, thereby, preventing degradation of the devices. CdSe and copper doped CdSe were found to contain several important defect centres: a native sensitising centre (0.64eV from the VB), a class I recombination centre (0.9eV from the CB), a copper impurity centre (0.2eV from the CB) and two native defects (0.16eVand 0.45eV from the CB). n-type CdTe grown by the Piper-Polich technique contained6 electron traps at 0.15eV, 0.21eV, 0.40eV, 0.47eV, 0.53eV and 0.63eV. Their presence was shown to be dependent upon the method of growth of the crystal by comparing with material grown by other techniques. One or more of these states were thought to be due to extended defects or Te precipitates. Low resistivity ZnS contained two deep electron traps at 0.25eV and O.50eV as detected by DLTS. In addition DLOS showed the presence of four further states at 1.25eV, 1.37eV, 1.89eV and 2.19eV below the CB. The first two are thought to be the strong luminescence centres observed by other workers.

No description available.

No description available.

Thesis (Ph. D.)--Virginia Polytechnic Institute and State University, 1992. / Vita. Abstract. Includes bibliographical references (leaves 203-208). Also available via the Internet.