Les fonds patrimoniaux de la Section genevoise du Club Alpin Suisse: une collection à évaluer, réorganiser, préserver et valoriser

Ce travail est le résultat d’un mandat donné par la Section genevoise du Club Alpin Suisse (C.A.S.), qui souhaite avoir une meilleure vision de son fonds documentaire, dont la connaissance s’est perdue au fil des ans. Il n’est que l’un des mandats d’une série de travaux réalisés sur différentes parties de la collection, et se concentre essentiellement sur les fonds spéciaux, c’est à dire les documents rangés dans la partie inférieure des meubles de la bibliothèque de la Section. Ces documents constituent une collection hétérogène par sa typologie : périodiques, monographies, cartes et panoramas, documents d’archives et manuscrits. Un premier inventaire a permis de déterminer que presque tous concernent la montagne et qu’une moitié d’entre eux sont des productions documentaires du C.A.S., qu’il s’agisse de la Section genevoise ou d’autres sections. En examinant leur accessibilité dans d’autres bibliothèques et centres de documentation suisse, ainsi que de leur proximité avec la Section, ce travail conclut quelle fonds possède une réelle valeur patrimoniale, pour la Section même et pour l’alpinisme genevois dans son ensemble. Plusieurs solutions de valorisation sont proposées. La collection dans son ensemble, c’est à dire les fonds spéciaux examinés ici et les divers autres fonds possédés par la Section, pourrait être versée à un partenaire externe qui se chargerait alors de sa!mise en valeur. Plusieurs partenaires possibles sont proposés, dont le plus approprié serait la! Bibliothèque de Genève (BGE), qui a pour mission d’assurer la sauvegarde du patrimoine genevois. Alternativement, la Section peut décider de gérer elle-même sa collection. Dans ce cas, il est nécessaire de protéger les documents plus fragiles en les conditionnant de manière appropriée, puis de réaliser un inventaire détaillé de l’ensemble de la collection en y incluant les parties non couvertes par le présent travail. La communication de cet inventaire permettra alors de présenter le fonds documentaire tout entier et de faire apparaître sa réelle valeur. Certains documents individuels pourraient être exploités par des chercheurs, en particulier dans le domaine des sciences humaines

Superconducting fluctuations in a thin NbN film probed by the Hall effect

We present a comprehensive study of how superconducting fluctuations in the normal state contribute to the conductivity tensor in a thin (119 $\AA$) film of NbN. It is shown how these fluctuations drive a sign change in the Hall coefficient $R_\mathrm{H}$ for low magnetic fields near the superconducting transition. The scaling behaviours as a function of distance to the transition $\epsilon=\ln(T/T_\mathrm{c})$ of the longitudinal ($\sigma_\mathrm{xx}$) and transverse ($\sigma_\mathrm{xy}$) conductivity is found to be consistent with Gaussian fluctuation theory. Moreover, excellent quantitative agreement between theory and experiment is obtained without any adjustable parameters. Our experimental results thus provide a case study of the conductivity tensor originating from short-lived Cooper pairs.Comment: 6 pages, 4 figure

Assistance énergétique à base de supercondensateurs pour véhicules à propulsion électrique et hybride

So far in the Twenty-first century the field of people transportation has had numerous setbacks, of these some are related to air pollution and some to the rarefaction of petroleum sources. Studies have been undertaken within this domain for many years now. Hybrid vehicles in which the conventional energy source is kept and an on-board energy source is added are showing themselves to be a potentially good solution in the short term. In these hybrids the vehicle autonomy is assumed by the first energy source and the power constraints are taken upon by the second. Moreover, the storage element adds the possibility to recuperate the braking energy in an optimal way. During the last decade, an important development has come about in the field of energy storage elements. The supercapacitors newly appearing on the scene are power components well suited for an application in transportation domain: their lifetime is over 500,000 cycles and their power density (W/kg) is much higher than for batteries. The present work is concerned with air pollution and energy storage elements and presents the details of using a power assistance system for vehicles. In this case, the auxiliary power energy storage element is on board and is made of supercapacitors. The decreased energy consumption of the vehicle is directly dependant on the vehicle's type, the route driven and the size of the storage element within the vehicle. An optimal method of sizing the energy storage element is developed. An application of the principles is presented in three different categories of vehicle: a transportation network fed by catenaries, a diesel-electric vehicle and a light electrical vehicle. In all three cases, the size of the storage elements, the on-board energy control system and the reduction of the vehicle consumption are defined. Following the different methods developed here, it is possible to define the conditions for when a power assistance system can give a real decrease in the vehicle's energy consumption. When an on-board storage element is added in a vehicle, a static converter has to be used. Its main role is to adapt the voltage level between the storage element and the other vehicle propulsion equipment and to control the energy flow on board the vehicle. An interleaved mutichannel continuous-continuous converter operating in a discontinuous conduction mode is especially dedicated to mobile applications. This type of converter is lighter and smaller in volume, yet its efficiency is greater

Charge-Stripe Order and Superconductivity in Ir1−xPtxTe2\mathrm{Ir_{1-x}Pt_xTe_2}

A combined resistivity and hard x-ray diffraction study of superconductivity and charge ordering in $\mathrm{Ir_{1-x}Pt_xTe_2}$, as a function of Pt substitution and externally applied hydrostatic pressure, is presented. Experiments are focused on samples near the critical composition $x_c\sim 0.045$ where competition and switching between charge order and superconductivity is established. We show that charge order as a function of pressure in $\mathrm{Ir_{0.95}Pt_{0.05}Te_{2}}$ is preempted - and hence triggered - by a structural transition. Charge ordering appears uniaxially along the short crystallographic (1,0,1) domain axis with a $\mathrm{(\frac{1}{5},0,\frac{1}{5})}$ modulation. Based on these results we draw a charge-order phase diagram and discuss the relation between stripe ordering and superconductivity.Comment: 8 pages, 4 figures: Accepted in Scientific Report

Magnetism and anomalous transport in the Weyl semimetal PrAlGe: Possible route to axial gauge fields

In magnetic Weyl semimetals, where magnetism breaks time-reversal symmetry, large magnetically sensitive anomalous transport responses are anticipated that could be useful for topological spintronics. The identification of new magnetic Weyl semimetals is therefore in high demand, particularly since in these systems Weyl node configurations may be easily modified using magnetic fields. Here we explore experimentally the magnetic semimetal PrAlGe, and unveil a direct correspondence between easy-axis Pr ferromagnetism and anomalous Hall and Nernst effects. With sizes of both the anomalous Hall conductivity and Nernst effect in good quantitative agreement with first principles calculations, we identify PrAlGe as a system where magnetic fields can connect directly to Weyl nodes via the Pr magnetization. Furthermore, we find the predominantly easy-axis ferromagnetic ground state co-exists with a low density of nanoscale textured magnetic domain walls. We describe how such nanoscale magnetic textures could serve as a local platform for tunable axial gauge fields of Weyl fermions.Comment: 42 pages, 5 figure

Magnetotransport of dirty-limit van Hove singularity quasiparticles

Tuning of electronic density-of-states singularities is a common route to unconventional metal physics. Conceptually, van Hove singularities are realized only in clean two-dimensional systems. Little attention has therefore been given to the disordered (dirty) limit. Here, we provide a magnetotransport study of the dirty metamagnetic system calcium-doped strontium ruthenate. Fermi liquid properties persist across the metamagnetic transition, but with an unusually strong variation of the Kadowaki-Woods ratio. This is revealed by a strong decoupling of inelastic electron scattering and electronic mass inferred from density-of-state probes. We discuss this Fermi liquid behavior in terms of a magnetic field tunable van Hove singularity in the presence of disorder. More generally, we show how dimensionality and disorder control the fate of transport properties across metamagnetic transitions

Damped spin excitations in a doped cuprate superconductor with orbital hybridization

A resonant inelastic x-ray scattering study of overdamped spin excitations in slightly underdoped La2−x Srx CuO4 (LSCO) with x = 0.12 and 0.145 is presented. Three high-symmetry directions have been investigated: (1) the antinodal (0,0) → ( 1 ,0), (2) the nodal (0,0) → ( 1 , 1 ), and (3) the zone-boundary direction 2 4 4 ( 1 1 1 2 ,0) → ( 4 ,4 ) connecting these two. The overdamped excitations exhibit strong dispersions along (1) and (3), whereas a much more modest dispersion is found along (2). This is in strong contrast to the undoped compound La2CuO4 (LCO) for which the strongest dispersions are found along (1) and (2). The t − t i − t ii − U Hubbard model used to explain the excitation spectrum of LCO predicts—for constant U/t —that the dispersion along (3) scales with (t i/t )2. However, the diagonal hopping t i extracted on LSCO using single-band models is low (t i/t ∼ −0.16) and decreasing with doping. We therefore invoked a two-orbital (dx2 −y2 and dz2 ) model which implies that t i is enhanced. This effect acts to enhance the zone-boundary dispersion within the Hubbard model. We thus conclude that hybridization of dx2 −y2 and dz2 states has a significant impact on the zone-boundary dispersion in LSCO

Direct observation of orbital hybridisation in a cuprate superconductor

The minimal ingredients to explain the essential physics of layered copper-oxide (cuprates= materials remains heavily debated. Effective low energy single-band models of the copper-oxygen orbitals are widely used because there exists no strong experimental evidence supporting multiband structures. Here we report angle-resolved photoelectron spectroscopy experiments on La-based cuprates that provide direct observation of a two-band structure. This electronic structure, qualitatively consistent with density functional theory, is parametrised by a two-orbital ($d_{x^2-y^2}$ and $d_{z^2}$) tight-binding model. We quantify the orbital hybridisation which provides an explanation for the Fermi surface topology and the proximity of the van-Hove singularity to the Fermi level. Our analysis leads to a unification of electronic hopping parameters for single-layer cuprates and we conclude that hybridisation, restraining d-wave pairing, is an important optimisation element for superconductivity.Comment: supplementary material available on reques