Cryogenic AFM-STM for mesoscopic physics

Abstract

Electronic spectroscopy based on electron tunneling gives access to the electronic Density of States (DoS) in conductive materials, and thus provides detailed information about their electronic properties. During this thesis work, we have developed a microscope in order to perform spatially resolved (10 nm) tunneling spectroscopy, with an unprecedented energy resolution (10 µeV), on individual nanocircuits. This machine combines an Atomic Force Microscope (AFM mode) together with a Scanning Tunneling Spectroscope (STS mode), and functions at very low temperatures (30mK). In the AFM mode, the sample topography is recorded using a piezoelectric quartz tuning fork, which allows locating and imaging nanocircuits. Tunneling can then be performed on conductive areas of the circuit. With this microscope, we have measured the local DoS in a hybrid Superconductor-Normal metal-Superconductor (S-N-S) structure. In such circuit, the electronic properties of N and S are modified by the superconducting proximity effect. In particular, for short N wires, we have observed a minigap in the DoS of the N wire, independent of position. Moreover, when varying the superconducting phase difference between the S electrodes, we have measured the modification of the minigap, and its disappearance when the phase difference equals π. Our experimental results for the DoS, and its dependences (with phase, position, N length) are quantitatively accounted for by the quasiclassical theory of superconductivity. Some predictions of this theory are observed for the first time. CRYOGENIC AFM-STM FOR MESOSCOPIC PHYSICSH COVER:The front cover shows an AFM image of a small circuit fabricated by electronic lithography and double angle evaporation, comprising a 350 nm long, 30 nm thick silver wire (colorized in orange) and a 60 nm thick aluminium half loop (dark grey). This structure has been imaged using an atomic force sensor, and tunneling spectroscopy was subsequently performed at several positions. Curves on the background are tunneling spectra measured for different values of the superconducting phase difference across the normal wire (at the location of the tip represented on the AFM image).The graph on the backside of the cover is a 3D plot of the predicted local density of states along a 1D S-N-S structure, with a normal wire length L = 1.4 ξ (corresponding to 350 nm), and perfect interfaces. Merci infiniment à Daniel Esteve, mon directeur de thèse, pour avoir appuyé sans réserve ma candidature, et tout ce qui s'en est suivi : Travailler avec un « chef » comme toi est un enrichissement extrême, tant pour ton recul sur la compréhension des phénomènes envisagés, que pour ton esprit pratique qui nous permet de parvenir plus vite (et mieux bien sûr !) au résultat souhaité… Et puis tu n'es pas chef pour rien : ce don d'ubiquité qui te permet de consacrer à tous les projets simultanément la même attention exclusive force sincèrement l'admiration ! J'espère avoir mérité toute la confiance que m'a accordée Philippe Joyez en me laissant la responsabili...

Force-gradient-induced mechanical dissipation of quartz tuning fork force sensors used in atomic force microscopy

“…Equation (2) shows that a positive force gradient acting on prong 1 can be used to rebalance a tip loaded tuning fork. A similar strategy has been used in previous works to rebalance a tuning fork by increasing the prong stiffness [16,17]. As a consequence of the equivalence between mass load and force gradient, the response of tuning fork sensors to mass loads can be used to prove the validity of the coupled oscillators model.…”

Highly reproducible low temperature scanning tunneling microscopy and spectroscopy with in situ prepared tips

“…Nevertheless, most of in situ tip preparation procedures require dedicated setups such as ion milling, argon sputtering or UHV annealing [1][2][3] which are typically not available in some STM setups (especially in those operated at ultralow temperatures and high magnetic fields). Other common in situ preparation procedures rely on intentionally crashing the tip against the substrate to coat the tip with the substrate material [4]. However, this procedure has been only applied for a limited number of substrates such as gold, silver, lead, aluminum or copper.…”

“…Instituto Universitario de Ciencia de Materiales "Nicolás Cabrera". Campus de Cantoblanco, 28049 Madrid, Spain 4. Instituto Madrileño de Estudios Avanzados en Nanociencia IMDEA-Nanociencia, 28049 Madrid, Spain.…”

“…Andres Castellanos-Gomez 1,2 * Gabino Rubio-Bollinger 1,3, *, Manuela Garnica 1,4 , Sara Barja 1,4 , Amadeo L. Vázquez de Parga 1,3,4 , Rodolfo Miranda 1,3,4 and Nicolás Agraït 1,3,4 .…”