Surface Charge Density Wave Transition in NbSe3_3

The two charge-density wave (CDW) transitions in NbSe$_3$ %at wave numbers at $\bm{q_1}$ and $\bm{q_2}$, occurring at the surface were investigated by scanning tunneling microscopy (STM) on \emph{in situ} cleaved $(\bm{b},\bm{c})$ plane. The temperature dependence of first-order CDW satellite spots, obtained from the Fourier transform of the STM images, was measured between 5-140 K to extract the surface critical temperatures (T$_s$). The low T CDW transition occurs at T$_{2s}$=70-75 K, more than 15 K above the bulk T$_{2b}=59$K while at exactly the same wave number. %determined by x-ray diffraction experiments. Plausible mechanism for such an unusually high surface enhancement is a softening of transverse phonon modes involved in the CDW formation.% The large interval of the 2D regime allows to speculate on % %the special Berezinskii-Kosterlitz-Thouless type of the surface transition expected for this incommensurate CDW. This scenario is checked by extracting the temperature dependence of the order % %parameter correlation functions. The regime of 2D fluctuations is analyzed according to a Berezinskii-Kosterlitz-Thouless type of surface transition, expected for this incommensurate 2D CDW, by extracting the temperature dependence of the order parameter correlation functions.Comment: 5 pages, 2 figure

Bisguanidine, Bis(2-aminoimidazoline), and Polyamine Derivatives as Potent and Selective Chemotherapeutic Agents against Trypanosoma brucei rhodesiense. Synthesis and in Vitro Evaluation

The in vitro screening for trypanocidal activity against Trypanosoma brucei rhodesiense of an in-house library of 62 compounds [i.e. alkane, diphenyl, and azaalkane bisguanidines and bis-(2-aminoimidazolines)], which were chosen for their structural similarity to the trypanocidal agents synthalin (1,10-decanediguanidine) and 4,4′-diguanidinodiphenylmethane and the polyamine N1-(3-amino-propyl)propane-1,3-diamine, respectively, is reported. The original synthetic procedure for the preparation of 21 of these compounds is also reported. Most compounds displayed low micromolar antitrypanosomal activity, with five of them presenting a nanomolar inhibitory action on the parasite: 1,9-nonanediguanidine (1c), 1,12-dodecanediguanidine (1d), 4,4′ -bis[1,3-bis(tert-butoxycarbonyl)-2-imidazolidinylimino]diphenylamine (28a), 4,4′-bis(4,5-dihydro-1H-2-imidazolylamino)diphenylamine (28b), and 4,4′-diguanidino-diphenylamine (32b). Those molecules that showed an excellent in vitro activity as well as high selectivity for the parasite [e.g. 1c (IC50 = 49 nM; SI > 5294), 28b (IC50 = 69 nM; SI = 3072), 32b (IC50 = 22 nM; SI = 29.5), 41b (IC50 = 118 nM; SI = 881)] represent new antitrypanosomal lead compounds.Peer Reviewe

Isolated pairs of Majorana zero modes in a disordered superconducting lead monolayer

Majorana zero modes are fractional quantum excitations appearing in pairs, each pair being a building block for quantum computation . Some possible signatures of these excitations have been reported as zero bias peaks at endpoints of one-dimensional semiconducting wires and magnetic chains. However, 1D systems are by nature fragile to a small amount of disorder that induces low-energy excitations, hence obtaining Majorana zero modes well isolated in a hard gap requires extremely clean systems. Two-dimensional systems offer an alternative route to get robust Majorana zero modes. Indeed, it was shown recently that Pb/Co/Si(111) could be used as a platform for generating 2D topological superconductivity with a strong immunity to local disorder. While 2D systems exhibit dispersive chiral edge states, they can also host Majorana zero modes located on local topological defects. According to predictions, if an odd number of zero modes are located in a topological domain an additional zero mode should appear all around the domain's edge. Here we use scanning tunneling spectroscopy to characterize a disordered superconducting monolayer of Pb coupled to underlying Co-Si magnetic islands meant to induce a topological transition. We show that pairs of zero modes are stabilized: one zero mode positioned at a point in the middle of the magnetic domain and its zero mode partner extended all around the domain. The zero mode pair is remarkably robust, it is isolated within a hard superconducting energy gap and it appears totally immune to the strong disorder present in the Pb monolayer. Our theoretical scenario supports the protected Majorana nature of this zero mode pair, highlighting the role of magnetic or spin-orbit coupling textures. This robust pair of Majorana zero modes offers a new platform for theoretical and experimental study of quantum computing

Dynamical Coulomb Blockade Observed in Nano-Sized Electrical Contacts

Electrical contacts between nano-engineered systems are expected to constitute the basic building blocks of future nano-scale electronics. However, the accurate characterization and understanding of electrical contacts at the nano-scale is an experimentally challenging task. Here we employ low-temperature scanning tunneling spectroscopy to investigate the conductance of individual nano-contacts formed between flat Pb islands and their supporting substrates. We observe a suppression of the differential tunnel conductance at small bias voltages due to dynamical Coulomb blockade effects. The differential conductance spectra allow us to determine the capacitances and resistances of the electrical contacts which depend systematically on the island--substrate contact area. Calculations based on the theory of environmentally assisted tunneling agree well with the measurements.Comment: 5 pages, 3 figures, to appear in PR

LES of the turbulent compressible flow spatially developing in a plane channel

Large eddy simulation of the turbulent flow which spatially develops in a plane channel with a length of 88h will be presented for Re=4880 and M=0.7. The effect of realistic inflow conditions and low reflective outflow conditions will be analysed and discussed. Supersonic channel flow case and distorsion of the flow by mean of adverse pressure gradient will be further considered

Yu-Shiba-Rusinov bound states versus topological edge states in Pb/Si(111)

There is presently a tremendous activity around the field of topological superconductivity and Majorana fermions. Among the many questions raised, it has become increasingly important to establish the topological or non-topological origin of features associated with Majorana fermions such as zero-bias peaks. Here, we compare in-gap features associated either with isolated magnetic impurities or with magnetic clusters strongly coupled to the atomically thin superconductor Pb/Si(111). We study this system by means of scanning tunneling microscopy and spectroscopy (STM/STS). We take advantage of the fact that the Pb/Si(111) monolayer can exist either in a crystal-ordered phase or in an incommensurate disordered phase to compare the observed spectroscopic features in both phases. This allows us to demonstrate that the strongly resolved in-gap states we found around the magnetic clusters in the disordered phase of Pb have a clear topological origin.Comment: 11 pages, 5 figures. To be published in European Physical Journal Special Topics.dedicated to the conference FQMT'1

MODELISATION 3D DE ROUTES PAR TELEMETRIE LASER EMBARQUEE POUR LA MESURE DE DISTANCE DE VISIBILITE

National audienceLa plateforme LARA-3D, développée au Centre de Robotique de Mines Paris, est un système de télémétrie laser embarquée sur véhicule permettant de numériser des environnements routiers ou urbains à la vitesse du déplacement, et d'obtenir ainsi des nuages de points 3D denses. Nous présentons l'utilisation de ce système à des fins de mesure de distances de visibilité sur des routes, effectuée dans le cadre du projet SARI/VIZIR, en partenariat avec le LRPC de Strasbourg

Local density of state fluctuations in 2D superconductor as a probe of quantum diffusion

The interplay of superconductivity and disorder generates a wealth of complex phenomena. In particular, the peculiar structure of diffusive electronic wavefunctions is predicted to increase the superconducting critical temperature in some range of disorder. In this work, we use an epitaxial monolayer of lead showing a simple band structure and homogenous structural disorder as a model system of a 2D superconductor in the weak-antilocalization regime. Then, we perform an extensive study of the emergent fluctuations of local density of states (LDOS) and spectral energy gap in this material and compare them with both analytical results and numerical solution of the attractive Hubbard model. We show that mesoscopic LDOS fluctuations allow to probe locally both the elastic and inelastic scattering rates which are notoriously difficult to measure in transport measurements.Comment: Accepted for publication in PRB - 13 pages, 6 figure

Etude expérimentale et numérique des structures de Kelvin-Helmholtz se développant dans les couches cisaillées décollées autour d'un cylindre à section carrée à 500 ≤ Re ≤ 200000

International audienceOn s'intéresse à l'analyse des échelles de temps caractéristiques de l'écoulement turbulent décollé autour d'un cylindre à section carrée afin d'en étudier la dépendance au nombre de Reynolds dans la gamme Re = 50 à Re = 200000. L'utilisation de la Simulation des Grandes Echelles de la turbulence (SGE) à nombre de Reynolds intermédiaire et de l'Anémométrie LASER Doppler (LDA) à nombre de Reynolds élevé permet de caractériser les structures cohérentes présentes dans les couches cisaillées du cylindre. Au delà de Re ≈ 1000, la signature des structures de type Von Kármán (VK) et Kelvin-Helmholtz (KH) est présente simultanément dans le signal de vitesse. On étudie la combinaison de leurs fréquences caractéristiques à l'aide de transformée en ondelettes

Etude expérimentale et numérique des structures de Kelvin-Helmholtz se développant dans la couche cisaillée décollée au dessus d'un cylindre à section carrée à 500 < Re < 200,000

L'objet de l'étude présente est de déterminer et d'analyser les différentes échelles de temps caractéristiques de l'écoulement turbulent décollé au dessus d'un cylindre à section carrée, et d'en étudier la dépendance au nombre de Reynolds. Le nombre de Reynolds de l'écoulement varie dans la gamme Re=50 à Re=200,000. La mise en oeuvre d'outils de Simulation des Grandes Echelles de la turbulence (SGE) à nombre de Reynolds intermédiaire et de techniques d'Anémométrie LASER Doppler (LDA) à nombre de Reynolds élevé a permis de caractériser les structures cohérentes présentes dans la couche cisaillée décollée et dans le sillage du cylindre. Au delà de Re=1,000, la signature des structures de type Von Karman (VK) et Kelvin-Helmholtz (KH) est présente simultanément dans le signal de vitesse. On étudie la combinaison de leurs fréquences caractéristiques à l'aide de transformée en ondelettes