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

Two-dimensional topological superconductivity in Pb/Co/Si(111)

Just like insulators can host topological Dirac states at their edges, superconductors can also exhibit topological phases characterized by Majorana edge states. Remarkable zero-energy states have been recently observed at the two ends of proximity induced superconducting wires, and were interpreted as localized Majorana end states in one-dimensional (1D) topological superconductor. By contrast, propagating Majorana states should exist at the 1D edges of two-dimensional (2D) topological superconductors. Here we report the direct observation of dispersive in-gap states surrounding topological superconducting domains made of a single atomic layer of Pb covering magnetic islands of Co/Si(111). We interpret the observed continuous dispersion across the superconducting gap in terms of a spatial topological transition accompanied by a chiral edge mode and residual gaped helical edge states. Our experimental approach enables the engineering and control of a large variety of novel quantum phases. This opens new horizons in the field of quantum materials and quantum electronics where the magnetization of the domains could be used as a control parameter for the manipulation of topological states.Comment: 12 pages, 3 figure

Electrochemical Shot-Noise of a Redox Monolayer

Redox monolayers are the base for a wide variety of devices including high-frequency molecular diodes or biomolecular sensors. We introduce a formalism to describe the electrochemical shot-noise of such monolayer, confirmed experimentally at room temperature in liquid. The proposed method, carried out at equilibrium, avoids parasitic capacitance, increases the sensitivity and allows to obtain quantitative information such as the electronic coupling (or standard electron transfer rates), its dispersion and the number of molecules. Unlike in solid-state physics, the homogeneity in energy levels and transfer rates in the monolayer yields a Lorentzian spectrum. This first step for shot-noise studies in molecular electrochemical systems opens perspectives for quantum transport studies in liquid environment at room temperature as well as highly sensitive measurements for bioelectrochemical sensors.Comment: 6 pages, 4 Figures (main) + 15 pages, 11 Figures (SM

Structural and magnetic properties of molecular beam epitaxy (MnSb2Te4)x(Sb2Te3)1-x topological materials with exceedingly high Curie temperature

Tuning magnetic properties of magnetic topological materials is of interest to realize elusive physical phenomena such as quantum anomalous hall effect (QAHE) at higher temperatures and design topological spintronic devices. However, current topological materials exhibit Curie temperature (TC) values far below room temperature. In recent years, significant progress has been made to control and optimize TC, particularly through defect engineering of these structures. Most recently we showed evidence of TC values up to 80K for (MnSb2Te4)x(Sb2Te3)1-x, where x is greater than or equal to 0.7 and less than or equal to 0.85, by controlling the compositions and Mn content in these structures. Here we show further enhancement of the TC, as high as 100K, by maintaining high Mn content and reducing the growth rate from 0.9 nm/min to 0.5 nm/min. Derivative curves reveal the presence of two TC components contributing to the overall value and propose TC1 and TC2 have distinct origins: excess Mn in SLs and Mn in Sb2-yMnyTe3QLs alloys, respectively. In pursuit of elucidating the mechanisms promoting higher Curie temperature values in this system, we show evidence of structural disorder where Mn is occupying not only Sb sites but also Te sites, providing evidence of significant excess Mn and a new crystal structure:(Mn1+ySb2-yTe4)x(Sb2-yMnyTe3)1-x. Our work shows progress in understanding how to control magnetic defects to enhance desired magnetic properties and the mechanism promoting these high TC in magnetic topological materials such as (Mn1+ySb2-yTe4)x(Sb2-yMnyTe3)1-x

Développement d'une microscopie électrochimique à force atomique SECM-AFM à médiateur lié à la sonde en vue de son application à l'étude du fonctionnement d'une molécule d'enzyme unique

PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

Développement d'une microscopie électrochimique (SECM) à l'échelle du nanomètre par couplage avec la microscopie à Force Atomique (AFM) (application à l'étude de la compression et de la dynamique de chaînes linéaires de polyethylène glycol immobilisées par une de leur extrémité à la surface d'un substrat d'or)

PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

Vers l'adressage de biomolécules uniques par microscopie électrochimique à force atomique à médiateur lié Mt/AFM-SECM

Le but de ce travail de thèse était de faire évoluer les performances de la microscopie électrochimique à force atomique à médiateur lié (Mt/AFM-SECM), vers la détection de molécules uniques. Il a tout d'abord fallu finaliser le développement du mode d'imagerie tapping de la microscopie Mt/AFM-SECM, mode indispensable à la localisation de nano-objets et molécules individuelles. Dans ce but, le comportement oscillant des sondes AFM-SECM fabriquées au laboratoire, qui se distinguent des sondes commerciales de par leur matériau constitutif et leurs dimensions, a été analysé et quantifié. Ce travail a permis de définir les conditions optimales d'imagerie Mt/AFM-SECM permettant l'acquisition simultanée de la topographie du substrat et de la réponse électrochimique de macromolécules à marqueur redox. La résolution de cette imagerie a ensuite été portée à l'échelle du nanomètre grâce à la mise au point d'un nouveau protocole de fabrication de sondes combinées à extrémité de ~10 nm de rayon contre 100 nm auparavant. Nos efforts ont ensuite porté sur la conception de substrats nano-structurés. Deux approches expérimentales ont été menées de front pour la fabrication de ce type de substrat. La première a fait appel à la litographie électronique pour former un réseau d'électrodes nanométriques à bandes d'or sur surfaces de silicium. La seconde a consisté à utiliser l'auto-assemblage de nanoparticules d'or de ~ 20 nm de diamètre sur couche d'alcane-thiol. Dans ce dernier cas l'adressage par Mt/AFM-SECM de nanoparticules individuelles, portant des macromolécules redox modèles, des chaines polyéthylène glycol à tête ferrocène, a été démontré.The aim of the present work was to improve the performances of Molecule-touching Atomic Force Electrochemical Microscopy (Mt/AFM-SECM), aiming at single molecule sensitivity. The first task was to finalize the development of the imaging mode of Mt/AFM-SECM, which is carried out in tapping mode and is indispensible for locating individual nano-objects and molecules onto surfaces. The oscillating behavior of our hand-made AFM-SECM probes, which greatly differ from conventional probes due to their unusual dimensions and material, was fully characterized. This work allowed us to define the optimal experimental conditions for stable Mt/AFM-SECM imaging of surfaces where the substrate topography and electrochemical response of surface-attached redox-labeled macromolecules are simultaneously acquired. The lateral resolution of this new type of microscopy was improved down to the nanometer range thanks to the development of a new protocol for fabricating AFM-SECM combined probes characterized by a tip radius of ~ 10 nm (against 100 nm for previous generation probes). The problem of designing and fabricating nano-structured substrates, was then tackled. Two experimental approaches have been carried out to fabricate such substrates. The first one relied on electronic lithography to fabricate an array of ~ 20 nanometer wide band electrodes onto a silicon surface. The second one consisted in forming a random array of gold nanoparticles assembled on an alkane-thiol layer borne by a gold surface. In this later case probing of individual nanoparticles bearing ferrocenylated polyethylene glycol chains as model redox macromolecules was demonstrated.PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF