Classical particle scattering for power-law two-body potentials

We present a rigorous study of the classical scattering for anytwo-body inter-particle potential of the form $v(r)=g/r^\gamma$, with\gamma\textgreater{}0, for repulsive (g\textgreater{}0) and attractive (g\textless{}0)interactions. We give a derivation of the complete power series of thedeflection angle in terms of the impact factor for the weak scatteringregime (large impact factors) as well as the asymptotic expressionsfor the hard scattering regime (small impact factors). We see a verydifferent qualitative and quantitative behavior depending whether theinteraction is repulsive or attractive. In the latter case, thefamilies of trajectories depend also strongly on the value of$\gamma$. We also study carefully the modifications of the resultswhen a regularization is introduced in the potential at small scales.We check and illustrate all the results with the exact integration ofthe equations of motion.Comment: 23 pages, 17 figure

Particle linear theory on a self-gravitating perturbed cubic Bravais lattice

Discreteness effects are a source of uncontrolled systematic errors of N-body simulations, which are used to compute the evolution of a self-gravitating fluid. We have already developed the so-called "Particle Linear Theory" (PLT), which describes the evolution of the position of self-gravitating particles located on a perturbed simple cubic lattice. It is the discrete analogue of the well-known (Lagrangian) linear theory of a self-gravitating fluid. Comparing both theories permits to quantify precisely discreteness effects in the linear regime. It is useful to develop the PLT also for other perturbed lattices because they represent different discretizations of the same continuous system. In this paper we detail how to implement the PLT for perturbed cubic Bravais lattices (simple, body and face-centered) in a cubic simulation box. As an application, we will study the discreteness effects -- in the linear regime -- of N-body simulations for which initial conditions have been set-up using these different lattices.Comment: 9 pages, 4 figures and 4 tables. Minor corrections to match published versio

Computational Essays: An Avenue for Scientific Creativity in Physics

Computation holds great potential for introducing new opportunities for creativity and exploration into the physics curriculum. At the University of Oslo we have begun development of a new class of assignment called computational essays to help facilitate creative, open-ended computational physics projects. Computational essays are a type of essay or narrative that combine text and code to express an idea or make an argument, usually written in computational notebooks. During a pilot implementation of computational essays in an introductory electricity and magnetism course, students reported that computational essays facilitated creative investigation at a variety of levels within their physics course. They also reported finding this creativity as being both challenging and motivating. Based on these reflections, we argue that computational essays are a useful tool for leveraging the creative affordances of programming in physics education.Comment: Accepted to the 2019 Physics Education Research Conference Proceeding

Understanding Student Computational Thinking with Computational Modeling

Recently, the National Research Council's framework for next generation science standards highlighted "computational thinking" as one of its "fundamental practices". 9th Grade students taking a physics course that employed the Modeling Instruction curriculum were taught to construct computational models of physical systems. Student computational thinking was assessed using a proctored programming assignment, written essay, and a series of think-aloud interviews, where the students produced and discussed a computational model of a baseball in motion via a high-level programming environment (VPython). Roughly a third of the students in the study were successful in completing the programming assignment. Student success on this assessment was tied to how students synthesized their knowledge of physics and computation. On the essay and interview assessments, students displayed unique views of the relationship between force and motion; those who spoke of this relationship in causal (rather than observational) terms tended to have more success in the programming exercise.Comment: preprint to submit to PERC proceedings 201

Detecting photon-photon scattering in vacuum at exawatt lasers

In a recent paper, we have shown that the QED nonlinear corrections imply a phase correction to the linear evolution of crossing electromagnetic waves in vacuum. Here, we provide a more complete analysis, including a full numerical solution of the QED nonlinear wave equations for short-distance propagation in a symmetric configuration. The excellent agreement of such a solution with the result that we obtain using our perturbatively-motivated Variational Approach is then used to justify an analytical approximation that can be applied in a more general case. This allows us to find the most promising configuration for the search of photon-photon scattering in optics experiments. In particular, we show that our previous requirement of phase coherence between the two crossing beams can be released. We then propose a very simple experiment that can be performed at future exawatt laser facilities, such as ELI, by bombarding a low power laser beam with the exawatt bump.Comment: 8 pages, 6 figure

Interaction effects and quantum phase transitions in topological insulators

We study strong correlation effects in topological insulators via the Lanczos algorithm, which we utilize to calculate the exact many-particle ground-state wave function and its topological properties. We analyze the simple, noninteracting Haldane model on a honeycomb lattice with known topological properties and demonstrate that these properties are already evident in small clusters. Next, we consider interacting fermions by introducing repulsive nearest-neighbor interactions. A first-order quantum phase transition was discovered at finite interaction strength between the topological band insulator and a topologically trivial Mott insulating phase by use of the fidelity metric and the charge-density-wave structure factor. We construct the phase diagram at $T = 0$ as a function of the interaction strength and the complex phase for the next-nearest-neighbor hoppings. Finally, we consider the Haldane model with interacting hard-core bosons, where no evidence for a topological phase is observed. An important general conclusion of our work is that despite the intrinsic nonlocality of topological phases their key topological properties manifest themselves already in small systems and therefore can be studied numerically via exact diagonalization and observed experimentally, e.g., with trapped ions and cold atoms in optical lattices.Comment: 13 pages, 12 figures. Published versio

Long-range gravitational-like interaction in a neutral atomic cold gas

A quasi-resonant laser induces a long-range attractive force within a cloud of cold atoms. We take advantage of this force to build in the laboratory a system of particles with a one-dimensional gravitational-like interaction, at a fluid level of modeling. We give experimental evidences of such an interaction in a cold Strontium gas, studying the density profile of the cloud, its size as a function of the number of atoms, and its breathing oscillations.Comment: 4 pages, 4 figures. Published in PRA 87, 013401 (2013

Differential Attraction Of Drosophilids To Banana Baits Inoculated With Saccharomyces Cerevisiae And Hanseniaspora Uvarum Within A Neotropical Forest Remnant

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Yeasts are a necessary requisite in the diet of most Drosophila species that, in turn, may vector their dispersal in natural environments. Differential attractiveness experiments and the isolation of yeasts consumed by Drosophila may be informative for characterizing this association. Hanseniaspora uvarum is among the most common yeast species isolated from Drosophila crops, with high attractiveness to drosophilids. Saccharorrtyces cerevisiae has been widely used to collect flies, and it allows broad sampling of almost. all local Drosophila species. Pronounced differences in the field concerning Drosophila attractivity to baits seeded with these yeast species have been previously reported. However, few explicit generalizations have. been. set. Since late fifties, no field experiments of. Drosophila attractivity were carried out. in.the Neotropical region, which is facing shifts in abiotic and biotic factors.. Our objective is to characterize preference behavior that mediates the interaction in the wild among Neotropical Drosophila species and yeasts associated with them. We want to set a broad generalization about drosophilids attracted to the se yeasts. Here we present the results of a differential attractiveness experiment we carried out in a natural Atlantic Rainforest fragment to assess the preferences of Drosophila species groups to baits inoculated with H. uvarum and S. cerevisiae. Methods. Both yeast species were cultured in GYMP broth and separately poured in autoclaved mashed banana that was left fermenting. In the field, we collected drosophilids over five arrays of three different baits: non-inoculated autoclaved banana and banana inoculated with each yeast. In the laboratory the drosophilids were sorted to five sets according to their external morphology and/or genitalia: tripunctata; guarani; exotic; and the remaining flies pooled in others. Results and Conclusions. Uninoculated banana baits attracted virtually no flies. We found significant departures from random distribution over the other two baits (1:1 proportion) for all sets, except the pooled others. Flies of the sets willistoni and exotic preferred H. uvarum over S. cerevisiae, while the remaining sets were more attracted to S. cerevisiae. Previously various authors reported similar patterns in attraction experiments with S. cerevisiae and H. uvarum. It is also noteworthy that both yeast species have been isolated from natural substrates and crops of Drosophila species. Taken together, these results suggest that the preferences among Drosophila species groups may be reflecting deep and stable relations with yeast species in natural environments. They can be summarized as: forest dwelling species from subgenus Drosophila (such as tripunctata and guarani groups) are attracted to banana baits seeded with S. cerevisiae; while exotic (as D. melanogaster) and subgenus Sophophora species are preferentially attracted to baits seeded with H. uvarum.5Coordenacao de Aperfeicoamento de Pessoal do Ensino Superior (CAPES) [PRODOC 2709/2010]Conselho Nacional de Desenvolvimento Cientifico e Tecnologico [308285/2014-0, 457499/2014-1, 312066/2014-7]Fundo de Apoio ao Ensino, a Pesquisa e Extensao da Unicamp (FAEPEX-PEC2012) [1578/12]Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP) [2012/03144-0, 2014/03791-1]Fundacao de Amparo a Pesquisa do Estado de Minas Gerais (FAPEMIG) [APQ-01525-14]Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP

Finite-frequency counting statistics of electron transport: Markovian Theory

We present a theory of frequency-dependent counting statistics of electron transport through nanostructures within the framework of Markovian quantum master equations. Our method allows the calculation of finite-frequency current cumulants of arbitrary order, as we explicitly show for the second- and third-order cumulants. Our formulae generalize previous zero-frequency expressions in the literature and can be viewed as an extension of MacDonald's formula beyond shot noise. When combined with an appropriate treatment of tunneling, using, e.g. Liouvillian perturbation theory in Laplace space, our method can deal with arbitrary bias voltages and frequencies, as we illustrate with the paradigmatic example of transport through a single resonant level model. We discuss various interesting limits, including the recovery of the fluctuation-dissipation theorem near linear response, as well as some drawbacks inherent of the Markovian description arising from the neglect of quantum fluctuations.Comment: Accepted in New Journal of Physics. Updated tex