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

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

A new mechanism for soft landing in robotic space exploration

Landing safely is the key to successful exploration of the solar system; the mitigation of the connected effects of collision in mechanical systems relies on the conversion of kinetic energy into heat or potential energy. An effective landing-system design should minimize the acceleration acting on the payload. In this paper, we focus on the application of a special class of nonlinear preloaded mechanisms, which take advantage of a variable radius drum (VRD) to produce a constant reactive force during deceleration. Static and dynamic models of the mechanism are presented. Numerical results show that the system allows for very efficient kinetic energy accumulation during impact, approaching the theoretical limit

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

A new family of magnetic adhesion based wall-climbing robots

This paper is devoted to climbing robots that adhere to the wall through permanent magnetic elements. If the surface on which they adhere is not ferromagnetic, it is necessary for the system to be composed of two subsystems (master and follower carts), arranged in a sandwich configuration, with the surface to climb interposed between the two. However, this configuration does not allow the robot to descend from the wall where it is climbing and to move freely on the floor (because of the presence of the follower). This paper shows how to remove this limitation. In fact, the system is able to automatically detach the follower when the robot has to move on the floor and recover it when it has to climb

Exploring the Effect of Train Design Features on the Boarding and Alighting Time by Laboratory Experiments

The objective of this work is to study the effect of design features such as door width, vestibule setback and vertical gap on passengers’ boarding and alighting time (BAT) at metro stations. Simulated experiments were performed at University College London’s Pedestrian Accessibility Movement Environment Laboratory (PAMELA). The mock-up included a hall or entrance to the train and a relevant portion of the platform in front of the doors. Different scenarios were tested based on existing stations. Results were compared to observations at Green Park Station of the London Underground (LU). Results from PAMELA showed that wider doors (1.80 m), larger vestibule setback (800 mm) and smaller vertical gap (50 mm) reduced the average boarding time. However, the average alighting time presented no significant differences due to other phenomenon such as congestion or formation of lines of flow at doors. The observation at LU presented a reduction of the BAT when a small vertical gap (170 mm) was presented. More experiments are needed at PAMELA to test the effect of the design features for different densities and types of passengers

PENGARUH PAJAK HOTEL, PAJAK RESTAURAN, RETRIBUSI OBYEK WISATA, BEA PROLEHAN HAK ATAS TANAH DAN BANGUNAN (BPHTB), TERHADAP PENDAPATAN ASLI DAERAH

This study aims to determine the effect of hotel taxel, restaurant taxes, tourist attraction fees, land and building rights acquisition fees on local revenue in West Manggarai Regency.  This type of research is quantitative research. The population in this study is the report of West Manggarai Regency Original Revenue and Tourist Object Retribution in 2016-2019. The sample of this study used the purposive sampling method. The data source used is secondary data. Hypothesis testing in this study using multiple linear regression analysis method with SPSS program.  The results showed thad hotel tax had a positive but not significant effect on local revenue. Restaurant tax has a positive but not significant effect on local revenue. Retribution for tourism objects has a positive but not significant effect on local revenue. The cost of acquiring land and building has a positive but not significant effect on local revenue