Origin of Spin Ice Behavior in Ising Pyrochlore Magnets with Long Range Dipole Interactions: an Insight from Mean-Field Theory

Recent experiments suggest that the Ising pyrochlore magnets ${\rm Ho_{2}Ti_{2}O_{7}}$ and ${\rm Dy_{2}Ti_{2}O_{7}}$ display qualitative properties of the ferromagnetic nearest neighbor spin ice model proposed by Harris {\it et al.}, Phys. Rev. Lett. {\bf 79}, 2554 (1997). The manifestation of spin ice behavior in these systems {\it despite} the energetic constraints introduced by the strength and the long range nature of dipole-dipole interactions, remains difficult to understand. We report here results from a mean field analysis that shed some light on the origin of spin ice behavior in (111) Ising pyrochlores. Specifically, we find that there exist a large frustrating effect of the dipolar interactions beyond the nearest neighbor, and that the degeneracy established by effective ferromagnetic nearest neighbor interactions is only very weakly lifted by the long range interactions. Such behavior only appears beyond a cut-off distance corresponding to $O(10^2)$ nearest neighbor. Our mean field analysis shows that truncation of dipolar interactions leads to spurious ordering phenomena that change with the truncation cut-off distance.Comment: 7 Color POSTSCRIPT figures included. To appear in Canadian Journal of Physics for the Proceedings of the {\it Highly Frustrated Magnetism 2000 Conference}, Waterloo, Ontario, Canada, June 11-15, 2000 Contact: [email protected]

Learning Professional Knowledge: Bachelor Nursing Students’ Experiences in Learning and Knowledge Quality Outcomes in a Competence-Based Curriculum

Since decades, nursing education struggles with a persistent gap between the theoretical knowledge offered in the study program and its application in professional practice. To bridge this gap competence-based curricula were developed with instructional designs as authentic learning contexts and self-directed learning. In this project we explored final year Bachelor Nursing (BN) students’ experiences in learning in a newly developed curriculum, and their knowledge quality outcomes and the degree of agreement with knowledge requirements. An instrumental multiple case study was conducted with interviews, concept mapping and a domain knowledge list. Results show that a third of the participants had positive learning experiences and got high appraisals for their knowledge quality. Similar to the medium and low scoring participants, they developed instrumental knowledge but integrated other forms of learning into a system of meaning, which is needed to solve non-routine problems in future practice. Medium and low scoring participants did not profit from learning in authentic contexts and self-directed learning. In conclusion, developing sufficient professional knowledge in a constructivist competence-based curriculum is influenced by students’ intrinsic motivation to build a strong knowledge base, by their perception of how to learn and use professional knowledge, and their expectations of the degree of supervision and guidance by the teacher. It is recommended to evaluate the extent to which the intended curriculum is being taught

Long Range Order at Low Temperature in Dipolar Spin Ice

Recently it has been suggested that long range magnetic dipolar interactions are responsible for spin ice behavior in the Ising pyrochlore magnets ${\rm Dy_{2}Ti_{2}O_{7}}$ and ${\rm Ho_{2}Ti_{2}O_{7}}$. We report here numerical results on the low temperature properties of the dipolar spin ice model, obtained via a new loop algorithm which greatly improves the dynamics at low temperature. We recover the previously reported missing entropy in this model, and find a first order transition to a long range ordered phase with zero total magnetization at very low temperature. We discuss the relevance of these results to ${\rm Dy_{2}Ti_{2}O_{7}}$ and ${\rm Ho_{2}Ti_{2}O_{7}}$.Comment: New version of the manuscript. Now contains 3 POSTSCRIPT figures as opposed to 2 figures. Manuscript contains a more detailed discussion of the (i) nature of long-range ordered ground state, (ii) finite-size scaling results of the 1st order transition into the ground state. Order of authors has been changed. Resubmitted to Physical Review Letters Contact: [email protected]

Inotropic effects of propofol, thiopental, midazolam, etomidate, and ketamine on isolated human atrial muscle

Background: Cardiovascular instability after intravenous induction of anesthesia may be explained partly by direct negative inotropic effects. The direct inotropic influence of etomidate, ketamine, midazolam, propofol, and thiopental on the contractility of isolated human atrial tissue was determined. Effective concentrations were compared with those reported clinically. Methods: Atrial tissue was obtained from 16 patients undergoing coronary bypass surgery. Each fragment was divided into three strips, and one anesthetic was tested per strip in increasing concentrations (10 -6 to 10 - 2 M). Strips were stimulated at 0.5 Hz, and maximum isometric force was measured. Induction agents were studied in two groups, group 1 (n = 7) containing thiopental, midazolam, and propofol, and group 2 (n = 9) consisting of etomidate, ketamine, and propofol. Results: The tested anesthetics caused a concentration-dependent depression of contractility resulting in complete cessation of contractions at the highest concentrations. The IC 50s (mean ± SEM; μM) for inhibition of the contractility were: thiopental 43 ± 7.6, propofol 235 ± 48 (group 1), and 246 ± 42 (group 2), midazolam 145 ± 54, etomidate 133 ± 13, and ketamine 303 ± 54. Conclusions: This is the first study demonstrating a concentration-dependent negative inotropic effect of intravenous anesthetics in isolated human atrial muscle. NO inhibition of myocardial contractility was found in the clinical concentration ranges of propofol, midazolam, and etomidate. In contrast, thiopental showed strong and ketamine showed slight negative inotropic properties. Thus, negative inotropic effects may explain in part the cardiovascular depression on induction of anesthesia with thiopental but not with propofol, midazolam, and etomidate. Improvement of hemodynamics after induction of anesthesia with ketamine cannot be explained by intrinsic cardiac stimulation

Local moment formation in zinc doped cuprates

We suggest that when zinc is substituted for copper in the copper oxide planes of high $T_{c}$ superconductors, it does not necessarily have a valency of 2+. Rather, the valency of a zinc impurity should be determined by its surrounding medium. In order to study this hypothesis, we examine the effect of static impurities inducing diagonal disorder within a one band Hubbard model coupled to a localised state. We use this model to discuss the physics of zinc doping in the cuprates. Specifically, we discuss the formation of local moments near impurity sites and the modification of the transverse spin susceptibility in the antiferromagnetic state.Comment: 7 pages RevTex, includes 4 figure

Ordering of the pyrochlore Ising model with the long-range RKKY interaction

The ordering of the Ising model on a pyrochlore lattice interacting via the long-range RKKY interaction, which models a metallic pyrochlore magnet such as Pr_2Ir_2O_7, is studied by Monte Carlo simulations. Depending on the parameter k_F representing the Fermi wavevector, the model exhibits rich ordering behaviors

Ordered Phase of the Dipolar Spin Ice under [110]-Magnetic Fields

We find that the true ground state of the dipolar spin ice system under [110]-magnetic fields is the ``Q=X'' structure, which is consistent with both experiments and Monte Carlo simulations. We then perform a Monte Carlo simulation to confirm that there exists a first order phase transition under the [110]-field. In particular this result indicates the existence of the first order phase transition to the ``Q=X'' phase in the field above 0.35 T for Dy2Ti2O7. We also show the magnetic field-temperature phase diagram to summarize the ordered states of this system.Comment: 4 pages, 5 figures, in RevTex4, submitted to J. Phys. Soc. Jp

Socio-economic inequalities in injury incidence in the Netherlands

Background: Interventions to reduce socio-economic inequalities in injury incidence should be tailored to specific priority areas that may be identified by descriptive studies. We aimed to provide an overview of exist