Effet tunnel

L'effet tunnel désigne la propriété que possède un objet quantique de franchir une barrière de potentiel même si son énergie est inférieure à l'énergie minimale requise pour franchir cette barrière. C'est un effet purement quantique, qui ne peut pas s'expliquer par la mécanique classique. Pour une telle particule, la fonction d'onde, dont le carré du module représente la densité de probabilité de présence, ne s'annule pas au niveau de la barrière, mais s'atténue à l'intérieur de la barrière (pratiquement exponentiellement pour une barrière assez large). Si, à la sortie de la barrière de potentiel, la particule possède une probabilité de présence non nulle, cela signifie qu'elle peut traverser cette barrière. Cette probabilité dépend des états accessibles de part et d'autre de la barrière ainsi que de l'extension spatiale de la barrière. Analyse Au niveau théorique, le comportement tunnel n'est pas fondamentalement différent du comportement classique de la particule quan

Magnetism and metal-organic self-assembly of rare-earth atoms on decoupling layers

Sébastien Reynaud

This thesis presents investigations of magnetic and structural properties of two dimensional metal-organic frameworks and of single atoms, both adsorbed on decoupling layers grown on metal surfaces, with focus on rare earth elements.We first report on a series of attempts at realizing self-assembled metal-organic networks on different decoupling layers, probed by scanning tunneling microscopy. The aim is to produce regular structures with high surface filling factors such that the magnetic properties of the rare earth atoms in the metal-organic networks can be investigated by X-ray ensemble measurements. The combinations of three molecular ligands (QDC, BDA, ZnTPyP) on three insulating layers (MgO, NaCl, graphene) with Tb as rare earth are reported. Of the three decoupling layers, only graphene is found to allow the synthesis of metal-organic structures. We successfully synthesized large islands of a rare earth-QDC coordination complex, with a majority of fivefold coordination of the rare earth atoms. The same structure is reproduced with Dy and Er.X-ray absorption spectroscopy, X-ray magnetic circular and linear dichroism measurements are then carried out on the Dy- and Er-QDC/gr/Ir(111) networks. Compared to single atoms on gr/Ir(111), both Dy and Er change their

4f

occupancy from 4

f^n

4f^{n-1}

and change their easy axis of magnetization. Both species are paramagnetic when incorporated into the metal-organic structure. The last part of the thesis is devoted to the study of Dy, Ho, and Gd on NaCl thin films. All three atoms display

4f

occupancy of the gas phase. The top-Cl adsorption site is determined with scanning tunneling microscopy. The magnetic properties are probed with X-ray absorption spectroscopy, X-ray magnetic circular and linear dichroism. Ho and Gd are paramagnetic. Dy has out-of-plane anisotropy with open hysteresis at non zero magnetic field. However, thermally assisted quantum tunneling of the magnetization prevents remanence at zero field.

EPFL2022

NMR investigation of the low-temperature dynamics of solid He-4 doped with He-3 impurities

The lattice dynamics of solid He-4 has been explored using pulsed NMR methods to study the motion of He-3 impurities in the temperature range (0.05-0.20 K) where experiments have revealed anomalies attributed to superflow or unexpected viscoelastic properties of the solid He-4 lattice. We report the results of measurements of the nuclear spin-lattice and spin-spin relaxation times that measure the fluctuation spectrum at high and low frequencies, respectively, of the He-3 motion that results from quantum tunneling in the He-4 matrix. The measurements were made for He-3 concentrations 16 < x(3) < 2000 ppm. For He-3 concentrations x(3) = 16 and 24 ppm, large changes are observed for both the spin-lattice relaxation time T-1 and the spin-spin relaxation time T-2 at temperatures close to those for which the anomalies are observed in measurements of torsional oscillator responses and the shear modulus. These changes in the NMR relaxation rates were not observed for higher He-3 concentrations.

American Physical Society2013

Magnetism and metal-organic self-assembly of rare-earth atoms on decoupling layers

Sébastien Reynaud

4f

occupancy from 4

f^n

4f^{n-1}

4f

EPFL2022

Electron Tunneling in symmetric and asymmetric alkanethiol junctions

Magnetism and metal-organic self-assembly of rare-earth atoms on decoupling layers

NMR investigation of the low-temperature dynamics of solid He-4 doped with He-3 impurities

Magnetism and metal-organic self-assembly of rare-earth atoms on decoupling layers

Electron Tunneling in symmetric and asymmetric alkanethiol junctions

NMR investigation of the low-temperature dynamics of solid He-4 doped with He-3 impurities