119 research outputs found
Detection of Magnetic moments using a NanoSQUID : Limits of resolution and sensitivity in near-field SQUID magnetometry
International audienceWe investigate the coupling efficiency of a localized magnetic moment placed at a distance z from a DC-SQUID magnetometer of loop radius a with nanobridges of cross section r^2. Using simple magnetostatic considerations, we show that there exist two detection regimes: the usual far-field regime (z >> a) for which inductive coupling is achieved by the entire loop and a near-field regime (z ≪ a) where nanobridges become the active detecting elements. Simulation shows that the greatest coupling efficiency is obtained in the near-field regime (z ≪ a) when the magnetic moment sits directly on the nanobridge. The maximum coupling limit is given by: 1/2μ0 M . Using nanoscale weak links and typical noise performance of nano-SQUID, we conclude that the limit of single molecular magnet detection can be obtained with r ∼ 1 nm, a value reachable using carbon nanotube Josephson junction
Interférométrie supraconductrice dans un nanotube de carbone
Les progrès des techniques de nano-fabrication permettent aujourd'hui de connecter électriquement des molécules individuelles avec des contacts faiblement résistifs. Les nanotubes de carbone ont été parmi les premiers objets moléculaires à avoir pu bénéficier de ces avancées. Les circuits obtenus sont des conducteurs unidimensionnels quasi-idéaux. La transmission entre le nanotube et des électrodes supraconductrices, par exemple, est telle qu'à très basse température, un courant supraconducteur peut circuler au travers du nanotube. Dans cet article, nous présentons une expérience d'interférence de deux courants supraconducteurs circulant dans deux portions d'un même nanotube de carbone. Grâce à la miniaturisation ultime apportée par le nanotube, une telle interférométrie quantique permet de sonder avec précision des champs magnétiques à l'échelle nanométrique et ouvre la voie à la magnétométrie d'objets quantiques uniques comportant un petit nombre de spins tels des aimants moléculaires uniques
Optimizing the flux coupling between a nanoSQUID and a magnetic particle using atomic force microscope nanolithography
We present results of Niobium based SQUID magnetometers for which the
weak-links are engineered by the local oxidation of thin films using an Atomic
Force Microscope (AFM). Firstly, we show that this technique allows the
creation of variable thickness bridges with 10 nm lateral resolution. Precise
control of the weak-link milling is offered by the possibility to realtime
monitor weak-link conductance. Such a process is shown to enhance the magnetic
field modulation hence the sensitivity of the magnetometer. Secondly, AFM
lithography is used to provide a precise alignment of NanoSQUID weak-links with
respect to a ferromagnetic iron dot. The magnetization switching of the
near-field coupled particle is studied as a junction of the applied magnetic
field direction
Electric field-controlled rippling of graphene
International audienceMetal-graphene interfaces generated by electrode deposition induce barriers or potential modulations influencing the electronic transport properties of graphene based devices. However, their impact on the local mechanical properties of graphene is much less studied. Here we show that graphene near a metallic interface can exhibit a set of ripples self-organized into domains whose topographic roughness is controlled by the tip bias of a scanning tunneling microscope. The reconstruction from topographic images of graphene bending energy maps sheds light on the local electro-mechanical response of graphene under STM imaging and unveils the role of the stress induced by the vicinity of the graphene-metal interface in the formation and the manipulation of these ripples. Since microscopic rippling is one of the important factors that limit charge carrier mobility in graphene, the control of rippling with a gate voltage may have important consequences in the conductance of graphene devices where transverse electric fields are created by contactless suspended gate electrodes. This opens up also the possibility to dynamically control the local morphology of graphene nanomembranes
Large and Flat Graphene Flakes Produced by Epoxy Bonding and Reverse Exfoliation of Highly Oriented Pyrolitic Graphite
We present a fabrication method producing large and flat graphene flakes that
have a few layers down to a single layer based on substrate bonding of a thick
sample of highly oriented pyrolytic graphite (HOPG), followed by its controlled
exfoliation down to the few to single graphene atomic layers. As the graphite
underlayer is intimately bonded to the substrate during the exfoliation
process, the obtained graphene flakes are remarkably large and flat and present
very few folds and pleats. The high occurrence of single layered graphene
sheets having tens of micron wide in lateral dimensions is assessed by
complementary probes including spatially resolved Micro-Raman Spectroscopy,
Atomic Force Microscopy and Electrostatic Force Microscopy. This versatile
method opens the way of deposition of graphene on any substrates including
flexible ones.Comment: 15 pages 5 figure
Improving Neural Additive Models with Bayesian Principles
Neural additive models (NAMs) can improve the interpretability of deep neural
networks by handling input features in separate additive sub-networks. However,
they lack inherent mechanisms that provide calibrated uncertainties and enable
selection of relevant features and interactions. Approaching NAMs from a
Bayesian perspective, we enhance them in three primary ways, namely by a)
providing credible intervals for the individual additive sub-networks; b)
estimating the marginal likelihood to perform an implicit selection of features
via an empirical Bayes procedure; and c) enabling a ranking of feature pairs as
candidates for second-order interaction in fine-tuned models. In particular, we
develop Laplace-approximated NAMs (LA-NAMs), which show improved empirical
performance on tabular datasets and challenging real-world medical tasks
Deviation from the normal mode expansion in a coupled graphene-nanomechanical system
We optomechanically measure the vibrations of a nanomechanical system made of
a graphene membrane suspended on a silicon nitride nanoresonator. When probing
the thermal noise of the coupled nanomechanical device, we observe a
significant deviation from the normal mode expansion. It originates from the
heterogeneous character of mechanical dissipation over the spatial extension of
coupled eigenmodes, which violates one of the fundamental prerequisite for
employing this commonly used description of the nanoresonators' thermal noise.
We subsequently measure the local mechanical susceptibility and demonstrate
that the fluctuation-dissipation theorem still holds and permits a proper
evaluation of the thermal noise of the nanomechanical system. Since it
naturally becomes delicate to ensure a good spatial homogeneity at the
nanoscale, this approach is fundamental to correctly describe the thermal noise
of nanomechanical systems which ultimately impact their sensing capacity
Nanosystèmes graphitiques (cavités optiques ajustables et détection spectrale des contraintes dans un nanorésonateur mécanique)
Le graphène et les nanotubes de carbone, assimilés à des nano-systèmes graphitiques, partagent des propriétés mécaniques, optiques, électroniques et vibrationnelles uniques. Associant faible masse, grande rigidité et comportement semi-transparent, des membranes de 10 à 100 couches de graphène ont été suspendues au dessus d'un substrat réfléchissant, formant ainsi un résonateur mécanique couplé à une cavité optique. Le projet de cette thèse repose sur les diffusions élastiques et inélastiques de la lumière pour sonder les contraintes mécaniques et les effets thermiques dans ces nano-systèmes graphitiques. Ce type de mesure repose sur la spectroscopie Raman, sensible aux phonons optiques du matériau sondé. Un premier aspect du présent projet de thèse porte sur l'utilisation de cavités optiques à base de graphène comme élément de base pour constituer un système hybride. Après avoir déposé une couche de molécules à la surface de ces membranes, nous avons montré que le signal Raman des molécules est exalté par un effet d'interférences optiques constructives. Nous avons mis en évidence la possibilité de moduler ce signal en se déplaçant le long de l'échantillon, ou en variant la position de la membrane à l'aide d'une actuation électrostatique. De plus, on peut observer des effets thermiques importants associés aux phénomènes d'interférences optiques dans ces membranes à base de graphène. Le second axe de cette thèse est la détection du mouvement et des contraintes mécaniques dans un résonateur graphitique (membranes de graphène multicouche, nanotubes, etc.). Au travers d'expériences menées sur des membranes suspendues de graphène multicouche, nous avons détecté la résonance mécanique de deux façons : en analysant la modulation de la lumière réfléchie et en mesurant les variations de la réponse Raman du résonateur. Cette détection, reposant sur l'augmentation des contraintes mécaniques à résonance, associe le mouvement mécanique du résonateur à un décalage en énergie des photons Raman et représente un schéma original de couplage optomécanique.Graphitic nano systems, such as graphene or carbon nanotubes, share unique mechanical, optical, electrical and vibrational properties. Combining low mass, high rigidity and semi-transparent behavior, membranes made of 10 to 100 graphene layers have been suspended over a reflecting substrate. This results in a nanomechanical resonator coupled to an optical cavity. This Phd project is based on elastic and inelastic scattering of light in order to probe mechanical stress and thermal effects within graphitic nano systems. This type of measurement is made by Raman spectroscopy which is sensitive to optical phonons. A first part of this Phd project is about using graphene based optical cavities as a constitutive blocks to make a hybrid system. We have shown interferential enhancement of Raman signal of molecules grafted on the membrane surface. We have also demonstrated the possibility to tune this molecular Raman signal by moving along the suspended membrane, or by changing the membrane position using electrostatic actuation. Moreover, we have observed important thermal effects associated to optical interferences within these graphene based cantilevers. A second part of this Phd project is the detection of motion and mechanical stress within a graphitic nano resonator. Through experiments on suspended multilayer graphene membranes, we have detected the mechanical resonance by two different means : by analyzing the reflected light modulation, and by measuring the variations of the Raman signal of the resonator. This spectral detection, based on the increase of the mechanical stress at resonance, couples the mechanical motion of the resonator to a shift in energy of the Raman scattered photons. This represents an original scheme for optomechanical coupling.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF
Strain superlattices and macroscale suspension of Graphene induced by corrugated substrates
We investigate the organized formation of strain, ripples and suspended
features in macroscopic CVD-prepared graphene sheets transferred onto a
corrugated substrate made of an ordered arrays of silica pillars of variable
geometries. Depending on the aspect ratio and sharpness of the corrugated
array, graphene can conformally coat the surface, partially collapse, or lay,
fakir-like, fully suspended between pillars over tens of micrometers. Upon
increase of pillar density, ripples in collapsed films display a transition
from random oriented pleats emerging from pillars to ripples linking nearest
neighboring pillars organized in domains of given orientation.
Spatially-resolved Raman spectroscopy, atomic force microscopy and electronic
microscopy reveal uniaxial strain domains in the transferred graphene, which
are induced and controlled by the geometry. We propose a simple theoretical
model to explain the transition between suspended and collapsed graphene. For
the arrays with high aspect ratio pillars, graphene membranes stays suspended
over macroscopic distances with minimal interaction with pillars tip apex. It
offers a platform to tailor stress in graphene layers and open perspectives for
electron transport and nanomechanical applications
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