Exchange coupling in CaMnO3_3 and LaMnO3_3: configuration interaction and the coupling mechanism

The equilibrium structure and exchange constants of CaMnO$_3$ and LaMnO$_3$ have been investigated using total energy unrestricted Hartree-Fock (UHF) and localised orbital configuration interaction (CI) calculations on the bulk compounds and Mn$_2$O$_{11}^{14-}$ and Mn$_2$O$_{11}^{16-}$ clusters. The predicted structure and exchange constants for CaMnO$_3$ are in reasonable agreement with estimates based on its N\'eel temperature. A series of calculations on LaMnO$_3$ in the cubic perovskite structure shows that a Hamiltonian with independent orbital ordering and exchange terms accounts for the total energies of cubic LaMnO$_3$ with various spin and orbital orderings. Computed exchange constants depend on orbital ordering. UHF calculations tend to underestimate exchange constants in LaMnO$_3$, but have the correct sign when compared with values obtained by neutron scattering; exchange constants obtained from CI calculations are in good agreement with neutron scattering data provided the Madelung potential of the cluster is appropriate. Cluster CI calculations reveal a strong dependence of exchange constants on Mn d e$_g$ orbital populations in both compounds. CI wave functions are analysed in order to determine which exchange processes are important in exchange coupling in CaMnO$_3$ and LaMnO$_3$.Comment: 25 pages and 9 postscript figure

Effects of Cation Vacancy Distribution in Doped LaMnO3+d Perovskites

In this paper we report studies on the correlation between the presence and distribution of cation vacancies in doped manganites (La,M)MnO3+delta (where M = Na, Ca) and their magnetic properties. Results indicate that cation vacancies are distributed differently for the different crystal structures and dopant ion type. In particular it is shown that knowledge of the total vacancies concentration alone is not enough to fully characterize the physical properties of manganites and that their distribution between the A and B sites of the perovskite structure plays a crucial role which should be taken into account in future studies.Comment: 27 pages, 5 figure. To appear in J. Solid State Che

Maternal mortality due to cardiovascular disease in the Netherlands:a 21-year experience

Objective Cardiovascular disorders are the leading cause of indirect maternal mortality in Europe. The aim of this study is to present an extensive overview concerning the specific cardiovascular causes of maternal death and to identify avoidable contributing care factors related to these deaths. Methods We assessed all cases of maternal death due to cardiovascular disorders collected by a systematic national confidential enquiry of maternal deaths published by the Dutch Maternal Mortality and Morbidity Committee on behalf of the Netherlands Society of Obstetrics and Gynaecology over a 21-year period (1993-2013) in the Netherlands. Results There were 96 maternal cardiovascular deaths (maternal mortality rate due to cardiovascular diseases 2.4/100,000 liveborn children). Causes were aortic dissection (n & x202f;= 20, 21%), ischaemic heart disease (n & x202f;= 17, 18%), cardiomyopathies (including peripartum cardiomyopathy and myocarditis, n & x202f;= 20, 21%) and (unexplained) sudden death (n & x202f;= 27, 28%). Fifty-five percent of the deaths occurred postpartum (n & x202f;= 55, 55%). Care factors that may have contributed to the adverse outcome were identified in 27 cases (28%). These factors were patient-related in 40% (pregnancy against medical advice, underestimation of symptoms) and healthcare-provider-related in 60% (symptoms not recognised, delay in diagnosis, delay in referral). Conclusion The maternal cardiovascular mortality ratio is low in the Netherlands and the main causes of maternal cardiovascular mortality are in line with other European reports. In a minority of cases, care factors that were possibly preventable were identified. Women with cardiovascular disease should be properly counselled about the risks of pregnancy and the symptoms of complications. Education of care providers regarding the incidence, presentation and diagnosis of cardiovascular disease during pregnancy is recommended

Design of a freeform two-reflector system to collimate and shape a point source distribution

In this paper we propose a method to compute a freeform reflector system for collimating and shaping a beam from a point source. We construct these reflectors such that the radiant intensity of the source is converted into a desired target. An important generalization in our approach compared to previous research is that the output beam can be in an arbitrary direction. The design problem is approached by using a generalized Monge-Amp\`ere equation. This equation is solved using a least-squares algorithm for non-quadratic cost functions. This algorithm calculates the optical map, from which we can then compute the surfaces. We test our algorithm on two cases. First we consider a uniform source and target distribution. Next, we use the model of a laser diode light source and a ring-shaped target distribution

Moreel klimaat van scholen voor voortgezet onderwijs: De perceptie van de leerlingen

Wetensch. publicatieFaculteit der Sociale Wetenschappe