Competing Antiferromagnetic and Spin-Glass Phases in a Hollandite Structure

We introduce a simple lattice model with Ising spins to explain recent experimental results on spin freezing in a hollandite-type structure. We argue that geometrical frustration of the lattice in combination with nearest-neighbour antiferromagnetic (AFM) interactions is responsible for the appearance of a spin-glass phase in presence of disorder. We investigate this system numerically using parallel tempering. The model reproduces the magnetic behaviour of oxides with hollandite structure, such as $\alpha-\text{MnO}_2$ and presents a rich phenomenology: in absence of disorder three types of ground states are possible, depending on the relative strength of the interactions, namely AFM ordered and two different disordered, macroscopically degenerate families of ground states. Remarkably, for sets of AFM couplings having an AFM ground state in the clean system, there exists a critical value of the disorder for which the ground state is replaced by a spin-glass phase while maintaining all couplings AFM. To the best of our knowledge this is the only existing model that presents this kind of transition with short-range AFM interactions. We argue that this model could be useful to understand the relation between AFM coupling, disorder and the appearance of a spin-glass phase.Comment: 8 pages, 7 figure

Incommensurate, helical spin ground states on the Hollandite lattice

We present a model of classical Heisenberg spins on a Hollandite lattice, which has been developed to describe the magnetic properties of $\alpha$-MnO$_2$ and similar compounds. The model has nearest neighbor interacting spins, however the strength and the sign of spin-spin interactions is anisotropic and depends on the nature of the bonds. Our analysis shows that the Hollandite lattice supports four different incommensurate and helical magnetic ground states depending on the relative strengths and signs of spin-spin interactions. We show that the incommensurate helical ground states appear due to the geometrical frustration present in the model. We demonstrate that each of the four helical incommensurate magnetic phases are continuously connected to four different collinear antiferromagnetic ground states as the strength of spin-spin interaction along some bonds is increased. The present results give support to the presence of helical states that have been previously suggested experimentally for Hollandite compounds. We provide an in-depth analysis of the magnetic form factors for each helical phase and describe how it could be used to identify each of these phases in neutron diffraction experiments.Comment: 11 pages, 8 figure

Meningkatkan Hasil Belajar IPA Melalui Media Gambar Pada Siswa Kelas Iia Sdn 78 Pekanbaru

Natural Sciences (IPA) is one of the important lesson in learning. Even for the upper secondary level or high school level, this subject to one program penjurusan which will determine the students majoring in College. In addition, this science subjects also joined the national final examination (UAN) who became the main requirements for students graduating. The research design used in this research is the class action research (PKT). The aspect that is observed on each cycle and is the result of the learning and teaching process subjects source of energy and its benefits using media pictures. The subject of the learning improvement activities are the students of class 2 SDN 78 Pekanbaru with the number of students 39 people with the details of 20 students of male and female students 19 people. The results of research on the pre cycle, the value of the results of the learning science students, the average 43,08 and there are no students who achieve KKM. Then on the cycle I the results of learning science students an average of 1.807 and students who achieve KKM 9 students (23%). At cycle II, the value of the average student learning results reach 82,56 and students who achieve KKM 39 students (100%). The implementation of the media image can improve the results of learning science students on the subjects of energy sources and benefits. The learning process explains the source of energy and its benefits the sequence on each cycle apply media image and be led by the researchers and observers

Pedestrian level of interaction on platform conflict areas by real-scale laboratory experiments

The objective of this work was to develop a new method to measure the interaction of passengers boarding and alighting at metro stations. This method included the Level of Interaction (LOI) as more precise indicator compared to the Level of Service (LOS). The method consisted of building a mock-up of a metro car and a series of simulation experiments in University College London’s Pedestrian Accessibility Movement Environmental Laboratory (PAMELA) based on observation at two London Underground station. This mock-up included Platform Edge Doors (PEDs) and a new space defined as Platform Conflict Area in front of the train doors in which the density of passengers was high. Results of the laboratory experiments were expressed according to the types of queues, formation of lanes, density by layer, and distance between passengers, in which the interaction followed a Logarithmic Distribution and no statistical differences were found with PEDs. These results are helpful for traffic engineers and policy makers to measure the interaction and use the LOI as a new indicator for the design of spaces in metro systems

Ab initio study of element segregation and oxygen adsorption on PtPd and CoCr binary alloy surfaces

The segregation behavior of the bimetallic alloys PtPd and CoCr in the case of bare surfaces and in the presence of an oxygen ad-layer has been studied by means of first-principles modeling based on density-functional theory (DFT). For both systems, change of the d-band filling due to charge transfer between the alloy components, resulting in a shift of the d-band center of surface atoms compared to the pure components, drives the surface segregation and governs the chemical reactivity of the bimetals. In contrast to previous findings but consistent with analogous PtNi alloy systems, enrichment of Pt atoms in the surface layer and of Pd atoms in the first subsurface layer has been found in Pt-rich PtPd alloy, despite the lower surface energy of pure Pd compared to pure Pt. Similarly, Co surface and Cr subsurface segregation occurs in Co-rich CoCr alloys. However, in the presence of adsorbed oxygen, Pd and Cr occupy preferentially surface sites due to their lower electronegativity and thus stronger oxygen affinity compared to Pt and Co, respectively. In either cases, the calculated oxygen adsorption energies on the alloy surfaces are larger than on the pure components when the more noble components are present in the subsurface layers

Implementasi Supervisi Klinis Oleh Pengawas Untuk Meningkatkan Kinerja Guru Tk Berbasis Permendiknas No. 58 Tahun 2009 Pada Gugus III Kecamatan Buleleng

Penelitian Tindakan Sekolah ini bertujuan untuk meningkatkan kemampuan para guru TK se Gugus III Kecamatan Buleleng dalam menyusun RKM, RKH, melaksanakan proses pembelajaran, dan melaksanakan penilaian yang sesuai dengan Permendiknas No. 58 Tahun 2009 melalui supervisi klinis oleh pengawas. Penelitian dilaksanakan dua siklus dengan subjek sebanyak 27 orang guru. Pengumpulan data dilakukan dengan metode observasi berdasarkan Permendiknas No. 58 Tahun 2009. Data dianalisis secara deskriptif. Indikator keberhasilan penelitian ini adalah 75% dalam kategori baik. Hasil penelitian menunjukkan bahwa terjadi peningkatan dalam menyusun RKM, RKH, melaksanakan proses pembelajaran, dan melaksanakan penilaian yang sesuai dengan Permendiknas No. 58 Tahun 2009. Hal ini terlihat dari tingkat kemampuan guru dalam merencanakan pembelajaran yang meliputi menyusun RKM dan RKH pada siklus I sebesar 75,06% yang tergolong baik, meningkat pada siklus II menjadi 93,25% yang tergolong sangat baik. Tingkat kemampuan guru dalam melaksanakan pembelajaran pada siklus I sebesar 73,57% yang tergolong baik, meningkat pada siklus II menjadi 90,86% yang tergolong sangat baik. Tingkat kemampuan guru dalam melaksanakan penilaian pada siklus I sebesar 74,76% yang tergolong baik, meningkat pada siklus II menjadi 94,13% yang tergolong sangat baik.Kata Kunci : supervisi klinis, kinerja guru, permendiknas no. 58 This classroom action research aims at improving kindergarten teachers\u27 performance in Cluster III of Buleleng Subdistric in making RKM, RKH, implementing teaching-learning process, and doing assessment which are in line with Permendiknas No. 58 Tahun 2009 through clinical supervision. It was done in two cycles with the subject of 27 teachers. The data were collected through observation based on Permendiknas No. 58 Tahun 2009. The data were analyzed descriptively. The expected criteria of this research were 75% or categorized as good. The result of the research shows that there is an improvement in making RKM, RKH, implementing teaching-learning process, and doing assessment which are in line with Permendiknas No. 58 Tahun 2009. It can be seen on the teachers\u27 performance in making RKM and RKH in cycle I which is 75,06% and can be categorized as good improves to 93,25% in cycle II which can be categorized as very good. Teachers\u27 performance in implementing teaching-learning process in cycle I which is 73,57% and can be categorized as good improves to 90,86 in cycle II which can be categorized as very good. Teachers\u27 performance in doing assessment in cycle I which is 74,76% and can be categorized as good improves to 94,13% in cycle II which can be categorized as very good

Workspace optimization for a planar cable-suspended direct-driven robot

The present work is inspired by an industrial task, i.e. spray painting a large area by means of a robotic system consisting in a Cable-Driven Parallel Robot (CDPR). In many cases, the area of the robot workspace is smaller than the area to be painted. For this reason, the base of the robot has to be shifted several times during the painting process. These robots are referred to as Repetitive Workspace Robots (RWR). In other words, in order to accomplish the whole task, they need to be moved after they have completed a sub-task locally. A cable suspended CDPR is an ideal candidate for such tasks; it can be thin, light, flexible and cost-efficient. The question is: which is the best shape of the local workspace in these conditions? In fact, not always a larger area of the local workspace guarantees an efficient painting process. This is because the efficiency relies mainly on the shape rather than on the local workspace area itself. In this work we employ an index [Seriani S, Gallina P, Gasparetto A, 2014] to evaluate the efficiency of the workspace of a 2-link CDPR. Finally, we show how the index value changes in relation to some geometrical parameters of the robot, thus laying the foundations for a general design methodology

First-principles simulations of the oxidation of methane and CO on platinum oxide surfaces and thin films

The catalytic oxidation activity of platinum particles in automobile catalysts is thought to originate from the presence of highly reactive superficial oxide phases which form under oxygen-rich reaction conditions. The thermodynamic stability of platinum oxide surfaces and thin films was studied, as well as their reactivities towards oxidation of carbon compounds by means of first-principles atomistic thermodynamics calculations and molecular dynamics simulations based on density functional theory. On the Pt(111) surface the most stable superficial oxide phase is found to be a thin layer of alpha-PtO2, which appears not to be reactive towards either methane dissociation or carbon monoxide oxidation. A PtO-like structure is most stable on the Pt(100) surface at oxygen coverages of one monolayer, while the formation of a coherent and stress-free Pt3O4 film is favoured at higher coverages. Bulk Pt3O4 is found to be thermodynamically stable in a region around 900 K at atmospheric pressure. The computed net driving force for the dissociation of methane on the Pt3O4(100) surface is much larger than on all other metallic and oxide surfaces investigated. Moreover, the enthalpy barrier for the adsorption of CO molecules on oxygen atoms of this surface is as low as 0.34 eV, and desorption of CO2 is observed to occur without any appreciable energy barrier in molecular dynamics simulations. These results, combined, indicate a high catalytic oxidation activity of Pt3O4 phases that can be relevant in the contexts of Pt-based automobile catalysts and gas sensors