Probing the Fermi surface by positron annihilation and Compton scattering

Positron annihilation and Compton scattering are important probes of the Fermi surface. Relying on conservation of energy and momentum, being bulk sensitive and not limited by short electronic mean-free-paths, they can provide unique information in circumstances when other methods fail. Using a variety of examples, their contribution to knowledge about the electronic structure of a wide range of materials is demonstrated

Pagodas, polities, period and place: a data led exploration of the regional and chronological context of Liao dynasty architecture

The Liao dynasty (907-1125) was a dominant force in the political landscape of East Asia for a period of over two centuries. Despite this, when placed within the framework of Chinese history, the Liao polity and its associated architecture are forced to the periphery. This study aims to re-centre the Liao by exploring the pagodas constructed under this polity within a wider regional and chronological framework. To achieve this end, extant pagodas from China, North Korea, South Korea and Japan were recorded together in a database for the first time. The HEAP (Historical East Asian Pagoda) Database logs the date, location and feature set of each pagoda it contains and provides a means to compare Liao examples to those from other polities, places and periods. Through analysis and visualisations of this data, the Liao are identified as a polity that produced unique pagoda designs and a distinct visual style. While Liao pagodas played a major role in the wider design trends of the period, it is the influence they had at a more local level that may be of most significance, potentially making us rethink the way we frame and construct histories of architecture in China and East Asia

Fermi surface of the colossal magnetoresistance perovskite La_{0.7}Sr_{0.3}MnO_{3}

Materials that exhibit colossal magnetoresistance (CMR) are currently the focus of an intense research effort, driven by the technological applications that their sensitivity lends them to. Using the angular correlation of photons from electron-positron annihilation, we present a first glimpse of the Fermi surface of a material that exhibits CMR, supported by ``virtual crystal'' electronic structure calculations. The Fermi surface is shown to be sufficiently cubic in nature that it is likely to support nesting.Comment: 5 pages, 5 PS figure

Experimental determination of the state-dependent enhancement of the electron-positron momentum density in solids

The state-dependence of the enhancement of the electron-positron momentum density is investigated for some transition and simple metals (Cr, V, Ag and Al). Quantitative comparison with linearized muffin-tin orbital calculations of the corresponding quantity in the first Brillouin zone is shown to yield a measurement of the enhancement of the s, p and d states, independent of any parameterizations in terms of the electron density local to the positron. An empirical correction that can be applied to a first-principles state-dependent model is proposed that reproduces the measured state-dependence very well, yielding a general, predictive model for the enhancement of the momentum distribution of positron annihilation measurements, including those of angular correlation and coincidence Doppler broadening techniques

Zut! J'ai renversé ma pédagogie...

Une équipe de professeurs de physique du Cégep John Abbott qui favorisait déjà l’apprentissage collaboratif et par les pairs dans ses classes cherchait un moyen de rendre ces stratégies pédagogiques encore plus efficaces. Car les discussions entre les étudiants, quoique très efficaces et appréciées de tous, exigeaient beaucoup de temps en classe et limitaient la quantité de contenus pouvant y être abordés. Les professeurs ont d’abord exploré la méthode Just-in-time Teaching (« enseignement juste-à-temps »). Celle-ci leur a permis de cibler les discussions et les exercices portant sur des aspects difficiles de la matière étudiée et pouvant être faits en classe, grâce à une préparation préalable des étudiants et grâce à un ajustement des contenus effectué « juste à temps » pour le cours. Cependant, les étudiants n’arrivaient pas toujours bien préparés et ne réalisaient pas tous les lectures assignées. Les auteurs de cet article présentent comment, en voulant ajuster la méthode « juste-à-temps », ils en sont venus à inverser leur pédagogie par inadvertance. Ce faisant, ils expliquent qu’ils ont aussi réalisé, à leur plus grande surprise, que l’exposé magistral pouvait être très efficace dans certaines conditions

Fermi Surface as the Driving Mechanism for Helical Antiferromagnetic Ordering in Gd-Y Alloys

The first direct experimental evidence for the Fermi surface (FS) driving the helical antiferromagnetic ordering in a gadolinium-yttrium alloy is reported. The presence of a FS sheet capable of nesting is revealed, and the nesting vector associated with the sheet is found to be in excellent agreement with the periodicity of the helical ordering.Comment: 4 pages, 4 figure