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

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

Rhodium Nanoparticle Shape Dependence in the Reduction of NO by CO

The shape dependence of the catalytic reduction of nitric oxide by carbon monoxide on rhodium nanopolyhedra and nanocubes was studied from 230 to 270 degrees C. The nanocubes are found to exhibit higher turnover frequency and lower activation energy than the nanopolyhedra. These trends are compared to previous studies on Rh single crystals.Chemistry, PhysicalSCI(E)EI21ARTICLE3-4317-32213

Dioxygen at Biomimetic Single Metal-Atom Sites: Stabilization or Activation? The Case of CoTPyP/Au(111)

By means of a combined experimental and computational approach, we show that a 2D metal\u2013organic framework self-assembled at the Au(111) termination is able to mimic the O2 stabilization and activation mechanisms that are typical of the biochemical environment of proteins and enzymes. 5,10,15,20-tetra(4-pyridyl)21H,23H-porphyrin cobalt(III) chloride (CoTPyP) molecules on Au(111) bind dioxygen forming a covalent bond at the Co center, yielding charge injection into the ligand by exploiting the surface trans-effect. A weakening of the O\u2013O bond occurs, together with the development of a dipole moment, and a change in the molecule\u2019s magnetic moment. Also the bonding geometry is similar to the biological counterpart, with the O2 molecule sitting on-top of the Co atom and the molecular axis tilted by 118\ub0. The ligand configuration lays between the oxo- and the superoxo-species, in agreement with the observed O\u2013O stretching frequency measured in situ at near-ambient pressure conditions

The Influence of the Density Inside a Train Carriage on Passenger Boarding Rate

Passenger alighting and boarding time is an important factor for crowded metros with high-frequency operations. Some operators leave the train doors open until the last passenger boards or passengers voluntarily stop boarding, while others regulate the boarding process. To evaluate whether it is effective to regulate the passenger while boarding or not, this research explores how the density inside the train would influence the passenger boarding rate by conducting a laboratory experiment. The results showed that the passenger boarding rate increased as the density increased, up to approximately 2.5–3.0 passengers/m2. Beyond that point, our results suggested that the boarding rate may have decreased, but the evidence was not conclusive. It is deduced that, under the experimented density (up to around 5 passengers/m2), the density does not have any apparent negative effects on the boarding rate, which implies that there is no strong evidence to recommend passenger boarding regulation

Density functional theory study of platinum oxides: From infinite crystals to nanoscopic particles

For over a century, platinum oxides find technologically relevant applications in various fields ranging from catalysis to electrochemistry and nanoelectronics. We have performed a density functional theory study of the PtO, Pt3O4, and PtO2 bulk oxide phases. In our calculations, PtO and Pt3O4 present metallic character at the simple generalized gradient approximation level. The application of Hubbard corrections to the Kohn-Sham Hamiltonian opens a small gap in the electronic band structure of PtO, but not of Pt3O4, in which metallic Pt-Pt bonds are revealed by a Bader analysis of the calculated electronic structure. These results, together with the noninteger oxidation number of the Pt ions, are indicative of metallicity of the Pt3O4 phase which may be consistent with the known metallic character of platinum bronzes. Moreover, we have calculated the relative thermodynamic stabilities of platinum oxide Wulff's particles and discussed the results in the context of catalysis. Finally, we have predicted that the formation of alpha-PtO2 nanotubes could be energetically feasible. This result is of potential interest both for nanotechnological and catalytic applications and may explain the formation of curled alpha-PtO2 sheets observed in high-resolution transmission electron microscopy images

A microkinetic model of ammonia decomposition on a Pt overlayer on Au(111)

International audienceAmmonia decomposition is important for a series of technological applications. For developing efficient catalysts for this reaction, basic understanding of underlying mechanisms is fundamental. We have investigated ammonia decomposition on platinum (11 1) and (1 0 0) surfaces, and on a platinum overlayer on a (1 1 1) gold substrate within a microkinetic model. The kinetic parameters in the corresponding rate equations have been estimated on the basis of ab initio calculations of reaction and activation energies for the adsorption and dehydrogenation processes on the catalyst surface. Steady-state coverages of species participating in the decomposition have been determined as solution of the rate equations in the limiting case of small ammonia concentrations of an N(2)-NH(3) model gas mixture under flow conditions. Calculated turnover frequencies of ammonia decomposition as a function of the temperature reproduce characteristic features reported for platinum wires. The highest turnover frequency and hydrogen coverage have been obtained for the platinum overlayer on gold. Potential application of this system as ammonia-sensing device is discussed

Exploring the Effect of Boarding and Alighting Ratio on Passengers’ Behaviour at Metro Stations by Laboratory Experiments

The objective of this work was to study the effect of the ratio between passengers boarding and alighting on the passengers’ behaviour at metro stations. A mock-up of a vehicle and the relevant portion of the platform was built to run a series of simulation experiments at University College London’s Pedestrian Accessibility and Movement Environment Laboratory (PAMELA). Different scenarios were tested based on the next generation London Underground trains. The scenarios were classified according to different load conditions. Four types of behaviour are described. In most cases boarding is first, and passengers compete for space to enter the train. In the case of alighting, first passengers are faster than the rest of alighters due to the space available on the platform as boarding passengers give way to those who are getting off the train. In addition, alighters form lanes of flow depending on the number of passengers waiting to board the train on the platform. With respect to the train, if the density inside the train is higher than 4 passengers per square metre, then the flow at the doors starts to decrease. More experiments are needed to study the relationship between platform density and boarding and alighting time