A subKelvin scanning probe microscope for the electronic spectroscopy of an individual nano-device

We present a combined scanning force and tunneling microscope working in a dilution refrigerator that is optimized for the study of individual electronic nano-devices. This apparatus is equipped with commercial piezo-electric positioners enabling the displacement of a sample below the probe over several hundred microns at very low temperature, without excessive heating. Atomic force microscopy based on a tuning fork resonator probe is used for cryogenic precise alignment of the tip with an individual device. We demonstrate the local tunneling spectroscopy of a hybrid Josephson junction as a function of its current bias

Spatially-Correlated Microstructure and Superconductivity in Polycrystalline Boron-Doped Diamond

Scanning tunneling spectroscopies are performed below 100~mK on nano-crystalline boron-doped diamond films characterized by Transmission Electron Microscopy and transport measurements. We demonstrate a strong correlation between the local superconductivity strength and the granular structure of the films. The study of the spectral shape, amplitude and temperature dependence of the superconductivity gap enables us to differentiate intrinsically superconducting grains that follow the BCS model, from grains showing a different behavior involving the superconducting proximity effect

The understanding of silicon sequential elutriation behaviour

During the fluidization of broad PSD (Particle Size Distribution) powders, elutriation can not be avoided, but has to be process controlled. Batch elutriations of continuous PSD powders were studied in a laboratory scale fluidized bed. The reference sample was metallurgical-grade silicon powder, with non-spherical shape. The smallest elutriable fines, namely superfines (\u3c10 µm) are entrained first. However, the largest elutriable particles (Ut ~ Ug) do not begin to be entrained simultaneously, but only after a delay that is as long as the time required for the superfines to leave the bed, thus inducing sequential elutriation (Figures 1). When no superfines were present, the entrainment was not delayed. This peculiar phenomenon was observed at all of the tested gas velocities (0.05-0.2 m/s). The superfines thus seem to strongly limit the elutriation of the larger elutriable particles. This sequential behaviour is particularly interesting to separate particles according to a small and narrow PSD (Figure 2). These phenomena are related to interparticle interactions within the bed and/or the freeboard and confirm the importance of polydispersity in the elutriation behavior. Thanks to the elutriation mathematical models developed in this study, the behavior that was thought to be explained by Silicon attrition can now be explained by sequential elutriation. Please click Additional Files below to see the full abstract

Adiabatischer Transport unter Quanten-Hall-Regime : Vergleich zwischen Transport- and Rastersondenuntersuchungen.

In this PhD work, the local potential distribution has been measured in high mobility 2DES under quantum Hall conditions. The 2DES embedded in a GaAs-AlGaAs heterostructure designed in a small Hall bar geometry shows intrinsic adiabatic transport features. Usually presented in the literature with the edge state picture, these features are the disappearance of peaks in the Shubnikov-de Haas oscillations, the extension of quantum Hall plateaus to lower magnetic fields and the existence of non-local resistances. Our local potential measurement via cryogenic scanning force microscopy presents another microscopic explanation of such adiabatic transport. The new picture is based on compressible and incompressible strips. An incompressible strip is a region in which the Fermi energy is located inside the energy gap (the electron density is constant and the electrostatic potential is changing) whereas a compressible strip occurs if the Fermi energy is pinning inside a Landau level (the electron density is changing and the electrostatic potential is screening). In previous work, the compressible and incompressible strips model has been successfully used to describe the quantum Hall effect. The present work demonstrates that the strips distribution accounts also for the adiabatic transport features observed on high mobility samples in the quantum Hall regime. Our research shows that in adiabatic situations, compressible regions with an unusual difference of electrochemical potential are found to coexist along the same edge due to an insulator-like incompressible strip in between and due to the lack of impurities scattering. Due to the high mobility and small size of the Hall bar, such non equilibrium survives along the complete length of the sample and determines the transport features. The insulator properties of incompressible strips in front of the alloyed ohmic contacts are found to be anisotropic with a dependency on the orientation of the contact borderline with respect to the crystal direction. The incompressible strips are broader -so more insulating- if they are located close to contact with an interface perpendicular to the [01-1] direction than if they are in front of contact with an interface parallel to the [01-1] direction. This finding gives a physical meaning to the term "non ideal contact" in the case of low resistive and ohmic contacts. Finally our results advertise that every 2DES is inhomogeneous. A 2DES is never a flat distribution of electron but it owns border with gradient of electron density even in front of metal contacts. These "Regular inhomogeneities" at the edges of the mesa and in front of contacts determines the insulator properties of the incompressible strips in high magnetic field and therefore the transport.Mikroskopische Modelle, um dem Quanten-Hall-Effekt zu erklären, wurden sehr kontrovers diskutiert. In unserer Gruppe konnte mit Hilfe eines Rasterkraftmikroskopes die Potentialverteilung in einer Quanten-Hall-Probe basierend auf einer GaAs-AlGaAs-Heterostruktur mit Erfolg messen. Dieses ortsaufgelöste Bild des Potentials und der Stromverteilung führte zur Entwicklung eines geschlossenes theoretischen Bildes zur Erklärung des Quanten-Hall-Effekts. An den Rändern eines zweidimensionalen Elektronensystems steigt die Elektronendichte von Null auf den Wert des Innern an. Entlang der Ränder bildet sich eine streifenartige Struktur der Elektronendichte aus, in der sich die Streifen konstanter Elektronendichte inkompressibel verhalten, diejenigen variierender Elektronendichte kompressibel. In den kompressiblen Bereichen existieren besetzte und unbesetzte Elektronenzustände am Fermi-Niveau, die eine isoenergetische Umbesetzung zur elektrischen Abschirmung möglich machen. In den inkompressiblen Bereichen liegen besetzte Zustände unterhalb der Fermi-Niveaus, unbesetzte oberhalb, sodass dort elektrische Felder durch Elektronenumbesetzung nicht abgeschirmt werden können. P. Weitz and E. Ahlswede entdeckten oberhalb ganzzahliger Landauniveau-Füllfaktoren Abfälle der Hall-Spannung nur an den innersten inkompressiblen Streifen symmetrisch an beiden Rändern. Aus diesem Umstand konnte geschlossen werden, dass der dissipationfrei Strom in inkompressiblen Streifen fliesst. In der vorliegenden Arbeit wurde mit dem gleichen Tieftemperatur-Rasterkraftmikroskop Hall-Potential-Profile in 2DES mit höherer Elektronenbeweglichkeit unter Quanten-Hall-Bedingungen gemessen. Überraschend konnte ich in diesen Proben sogenannte adiabatische Transportphänomene beobachten, d.h. zu niedrigen Magnetfeldern verschobene Hall-Plateaus, unterdrückte Längswiderstände und das Auftreten von nichtlokalen Magnetowiderständen. Diese Phänomene wurden in der Literatur mit dem Auftreten von einer Nichtgleichgewichtssituation zwischen Randkanälen interpretiert. Durch systematische Magnetotransport- und rasterkraftmikroskopische Untersuchungen gelang es mir, ein mikroskopische Beschreibung im Rahmen kompressibler und inkompressibler Streifen zu erhalten. So konnte ich zeigen, dass entlang der einlegierten Metallkontakte eine elektrostatische Verarmung im 2DES auftritt, die in hohen Magnetfeldern zum Auftreten eines inkompressiblen Streifen, and damit zu einer Isolation zwischen Rand und Innern des 2DES führt. Wichtig ist hierbei, dass die Isolationswirkung (wahrscheinlich aufgrund der Breite) des inkompressiblen Streifens von der Orientierung der Kontakt/2DES-Grenzlinie relativ zur Kristallorientierung der GaAs/AlGaAs-Heterostruktur abhängt. Erst diese Anisotropie in der Breite der Streifen erklärt die gemessenen Potentialverteilungen und Magnetotransportmessungen in diversen elektrischen Anordnungen und Hallstruktur-Orientierungen

ASLC 2016 Quatrièmes ateliers sur la contradiction Collection libres opinions Presses des mines

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ASLC 2016 Quatrièmes ateliers sur la contradiction Collection libres opinions Presses des mines

International audienc

ASLC 2016 Quatrièmes ateliers sur la contradiction

International audienc

Atomic Transport in Nano-Crystalline Silicides Studied by <i>In Situ</i> Auger Electron Spectroscopy: Interfacial Reaction Effect

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