Patterns of twinning for Swedish women, 1961-1999

The Nordic population registers provide a unique possibility to study the demographic behavior of small population groups and rare events. In this paper, we study the childbearing behavior of Swedish mothers of twins between 1961 and 1999, inclusive. The twinning rate has increased since the mid-1970s in response to a growing use of fertility-stimulating treatments such as in-vitro fertilization. Such medical procedures are applied mainly to women beyond prime childbearing ages. Nevertheless, we find no simple age pattern in twinning rates. They do not just increase with the woman´s age. Our most consistent finding is that mothers of twins wait noticeably longer than women with singletons before they have another child. This apart, mothers with twins at their first birth have next-birth fertility patterns very similar to women who have two singletons at their first two births. This commonality in childbearing behavior does not extend to higher-order births, though. For mothers with a singleton and a pair of twins, the progression to a third birth depends very much on whether the twins came first or second. We also discover that at parities beyond 3, twinning rates increase with parity, especially at very short durations since the previous birth.vital events

Childbearing patterns for Swedish mothers of twins, 1961-1999

The Nordic population registers provide a unique possibility to study the demographic behavior of very small population groups and rare events. In this paper, we study the childbearing behavior of Swedish mothers of twins between 1961 and 1999, inclusive. Our most consistent finding is that mothers of twins wait noticeably longer than women with singletons before they have another child. This apart, mothers with twins at their first birth have next-birth fertility patterns very similar to women who have two singletons at their first two births. This commonality in child-bearing behavior does not extend to higher-order births. For mothers with a singleton and a pair of twins, the progression to a third birth depends very much on whether the twins came first or second. Beside these main results our fascinating material also provides a number of descriptive findings. The Swedish twinning rate has increased since the mid-1970s in response to a growing use of fertility-stimulating treatments such as in-vitro fertilization, in parallel with similar developments in many other countries. Such medical procedures are applied mainly to women beyond prime childbearing ages. Nevertheless, we find no simple age pattern in twinning rates. Even in recent years they do not just in-crease with the woman’s age. By way of contrast, at parities beyond 3 twinning rates increase with parity when we control for calendar period, time since last previous birth, and (NB) the woman’s own age.Sweden, twin studies

Priorities for sustainable turfgrass management: a research and industry perspective

This paper provides a brief review and assessment of the key environmental, regulatory and technical issues facing the turfgrass sector with specific reference to the European context. It considers the range of externalities or ‘drivers for change' facing the industry, and the challenges and opportunities available for promoting and achieving more sustainable turfgrass management within the sports, landscape and amenity sectors. The analysis confirms that there are a number of key areas where a concerted research and industrial effort is required. These include responding to the pressures from government demands for greater environmental regulation, the increasing pressure on natural resources (notably water, energy and land), the emerging role of turf management in supporting ecosystem services and enhancing biodiversity, the continued need to promote integrated pest management, and the looming challenges posed by a changing climate, and urgent need to adapt. Whilst many of these externalities appear to be risks to the sports turf industry, there will also be significant opportunities, for those where the labour, energy and agronomic costs are minimized and where the drive to adopt a multifunctional approach to sportsturf management is embraced

Chern number spins of Mn acceptor magnets in GaAs

We determine the effective total spin $J$ of local moments formed from acceptor states bound to Mn ions in GaAs by evaluating their magnetic Chern numbers. We find that when individual Mn atoms are close to the sample surface, the total spin changes from $J = 1$ to $J = 2$, due to quenching of the acceptor orbital moment. For Mn pairs in bulk, the total $J$ depends on the pair orientation in the GaAs lattice and on the separation between the Mn atoms. We point out that Berry curvature variation as a function of local moment orientation can profoundly influence the quantum spin dynamics of these magnetic entities.Comment: 4 pages, 3 figure

Magnetic properties of substitutional Mn in (110) GaAs surface and subsurface layers

Motivated by recent STM experiments, we present a theoretical study of the electronic and magnetic properties of the Mn-induced acceptor level obtained by substituting a single Ga atom in the (110) surface layer of GaAs or in one of the atoms layers below the surface. We employ a kinetic-exchange tight-binding model in which the relaxation of the (110) surface is taken into account. The acceptor wave function is strongly anisotropic in space and its detailed features depend on the depth of the sublayer in which the Mn atom is located. The local-density-of-states (LDOS) on the (110) surface associated with the acceptor level is more sensitive to the direction of the Mn magnetic moment when the Mn atom is located further below the surface. We show that the total magnetic anisotropy energy of the system is due almost entirely to the dependence of the acceptor level energy on Mn spin orientation, and that this quantity is strongly dependent on the depth of the Mn atom.Comment: 14 pages, 13 figure

Magnetic interactions of substitutional Mn pairs in GaAs

We employ a kinetic-exchange tight-binding model to calculate the magnetic interaction and anisotropy energies of a pair of substitutional Mn atoms in GaAs as a function of their separation distance and direction. We find that the most energetically stable configuration is usually one in which the spins are ferromagnetically aligned along the vector connecting the Mn atoms. The ferromagnetic configuration is characterized by a splitting of the topmost unoccupied acceptor levels, which is visible in scanning tunneling microscope studies when the pair is close to the surface and is strongly dependent on pair orientation. The largest acceptor splittings occur when the Mn pair is oriented along the symmetry direction, and the smallest when they are oriented along . We show explicitly that the acceptor splitting is not simply related to the effective exchange interaction between the Mn local moments. The exchange interaction constant is instead more directly related to the width of the distribution of all impurity levels -- occupied and unoccupied. When the Mn pair is at the (110) GaAs surface, both acceptor splitting and effective exchange interaction are very small except for the smallest possible Mn separation.Comment: 25 figure

Microwave Spectroscopy

Contains reports on two research projects.Joint Services Electronics Programs (U.S. Army, U. S. Navy, and U. S. Air Force) under Contract DAAB07-71-C-030

Magnetic Anisotropy of Isolated Cobalt Nanoplatelets

Motivated in part by experiments performed by M.H. Pan et al. (nanoletters, v.5, p 83, 2005), we have undertaken a theoretical study of the the magnetic properties of two-monolayer thick Co nanoplatelets with an equilateral triangular shape. The analysis is carried out using a microscopic Slater-Koster tight-binding model with atomic exchange and spin-orbit interactions designed to realistically capture the salient magnetic features of large nanoclusters containing up to 350 atoms. Two different truncations of the FCC lattice are studied, in which the nanoplatelet surface is aligned parallel to the FCC (111) and (001)crystal planes respectively. We find that the higher coordination number in the (111) truncated crystal is more likely to reproduce the perpendicular easy direction found in experiment. Qualitatively, the most important parameter governing the anisotropy of the model is found to be the value of the intra-atomic exchange integral J. If we set the value of J near the bulk value in order to reproduce the experimentally observed magnitude of the magnetic moments, we find both quasi-easy-planes and perpendicular easy directions. At larger values of J we find that the easy-axis of magnetization is perpendicular to the surface, and the value of the magnetic anisotropy energy per atom is larger. The possible role of hybridization with substrate surface states in the experimental systems is discussed.Comment: 15 pages, 13 figure