Liaison nucléaire | EPFL Graph Search

La liaison nucléaire est le phénomène qui assure la cohésion d'un noyau atomique. Le noyau atomique est composé de protons de charge électrique positive, et de neutrons de charge électrique nulle. La répulsion coulombienne tend à séparer les protons. C'est la force nucléaire qui permet d'assurer la stabilité du noyau. Énergie de liaison L'énergie de liaison E d'un noyau atomique est l'énergie qu'il faut fournir au noyau pour le dissocier en ses nucléons, qui s'attirent du fait de la force nucléaire, force qui correspond à l’interaction forte résiduelle. On définit aussi l'énergie de liaison par nucléon, E/A, où A désigne le nombre de masse du nucléide. Énergie nucléaire énergie nucléaire alt=Courbe d'Aston : énergie de liaison par nucléon en fonction du nombre de masse, pour les isotopes les plus courants.|upright=2|vignette|Courbe d'Aston : énergie de liaison par nucléon en fonction du nombre de masse, pour les isotopes

Kinks in the angle resolved photoemission and inelastic x-ray spectra of high temperature superconductors

Jeff Graf

Though the high superconducting transition temperature (Tc) is the most interesting technological aspect of high temperature superconductors, the complex way in which the spin, lattice and electronic degrees of freedom interplay, makes them of the highest scientific interest. This is illustrated by their rich phase diagram characterized by a variety of exotic groundstate including; Mott insulating, pseudogap and high temperature superconductivity. One of the most serious barriers that has prevented our understanding of the mechanism of high temperature superconductors thus far is the lack of information on how low energy Landau quasiparticles result from an organization of real particles. This organization, often colloquially referred to as "dressing", is a fundamental general concept in physics that explains a variety of physical phenomena, such as exotic particle formations and phase transitions. The role of the lattice in this dressing is particularly controversial. Phonons are quanta of lattice vibration energy, and play a crucial role in conventional superconductivity. They provide an attractive interaction allowing the electrons to condensate in superconducting Cooper pairs. However, high temperature superconductivity in the cuprates in achieved through hole doping in an antiferromagnetic Mott insulator. In this case, the antiferromagnetic background, the strong coulomb repulsion and the anisotropic superconducting gap all suggest a marginal role of the phonons. In order to assess experimentally the exact role of the phonon, angle resolved photoemission spectroscopy (ARPES) is the weapon of choice given its unique ability to probe the electronic structure of solids in an energy- and momentum-resolved manner. In this thesis, I will use ARPES to characterize the various form of dressing of the quasiparticles in the high temperature superconductor from the Fermi level all the way to the valence band complex. I'll show that when combined with isotope substitution and inelastic x-rays scattering, the strong electron-phonon interaction can be probed in vivid details. This thesis presents three fundamental results. 1) Using the isotope effect, the important and unusual role of the phonon is demonstrated, as well as the strong interplay between the magnetic and phonic degrees of freedom. 2) Using inelastic scattering, the intriguing interplay between the Fermi surface and the bond stretching phonon is exposed. And 3) By exploring the ARPES data up to high energy, two new energy scales at high binding energy as well as the spectral waterfalls phenomenon are revealed. When all these new elements are considered together, they clearly show the need to consider simultaneously the spin, lattice, Fermi surface topology and electronic degrees of freedom. The spectacular progress in ARPES techniques over the past two decades was critical for these results, and I will present in this thesis our current effort in pushing the techniques even further. In particular I'll present my efforts in building a laser based ARPES setup with a hemispherical analyzer and a spin resolved time of flight analyzer.

EPFL2008

Survival of rotational alignment in H-2 scattering from Si(100)

Christopher Scott Reilly

We report a state-prepared, state-resolved study of rotational scattering of a diatomic molecule from a solid surface. Specifically, H-2 molecules with 80 meV kinetic energy are rotationally aligned in the j = 3 rotational state via stimulated Raman pumping and then scattered from a Si(100) surface at normal incidence. The rotational alignment of the scattered molecules is determined by measuring, for both the incident and scattered molecules, the ionization yield of a probe laser, tuned to selectively ionize molecules in the j = 3 rotation level, as the probe laser polarization is rotated. The measurement is performed for two initial rotational alignments: a "helicoptering " alignment with the bonds constrained to lie primarily parallel to the surface and a "cartwheeling " alignment with the bonds lying primarily normal to the surface. For both initial alignments, the modulation of the probe ionization yield with laser polarization for the scattered molecules is pronounced, although significantly weaker than the modulation measured for the incident molecules. This indicates a significant modification but not a complete elimination of the initial alignment. The modulation is found to be stronger for the scattered molecules originating in the cartwheeling alignment than for the helicoptering alignment. These results contribute toward an improved understanding of the role of rotational motion in molecule-surface dynamics.

AIP Publishing2021

The design and synthesis of nanostructured hydrogen oxidation reaction electrocatalysts for hydroxide exchange membrane fuel cells

Weiyan Ni

Hydrogen fuel cell is a promising green technology that transforms the chemical energy in hydrogen into electricity. Currently, most of the efforts are focused on developing proton exchange membrane fuel cells (PEMFCs), however they are heavily dependent on rare and expensive platinum group metal (PGM) catalysts. In contrast, hydroxide exchange membrane fuel cells (HEMFCs) work in alkaline media, thus allow the use of cheaper and more abundant non-PGM catalysts. However, there is no materials found suitable to catalyze the anodic hydrogen oxidation reaction (HOR). Therefore, developing new, active and robust HOR catalysts in alkaline media is of paramount importance. Hereby, this dissertation presented my recent research works on preparation, characterization, and application of HOR catalysts for HEMFCs.Chapter 2 described the exploration of new earth-abundant HOR catalysts. A series of Ni-based compounds were synthesized and tested for HOR, among which Ni3N had the best activity. With further nano-engineering, the mass activity reached the highest at the time reported and the oxidation resistance of the supported version can be significantly improved. Spectroscopic evidence suggested that the downshift of the metal d band weakened the binding energies of the catalyst toward adsorbates, which accounted for the improved activity and stability of Ni3N/C.Chapter 3 demonstrated how to exploit strain-engineering for HOR catalysis. A series of Ni/C composites were prepared by pyrolyzing a Ni-based metal-organic framework (MOF). The optimal catalyst, Ni-H2-2%, has the highest mass activity (at the time reported) and abnormal high intrinsic activity. Characterizations using synchrotron XRD suggested that it was the distinct but relative optimal strain level of Ni-H2-2% that rendered it high activity.Chapter 4 is a complicated version of Chapter 3. The Ni catalysts were also prepared from the same Ni-based MOF with more complex synthesis conditions. The best catalyst, Ni-H2-NH3, possessed highest intrinsic activity among non-precious metals. Combining UPS, H2 chemisorption, isotopic studies, and electrochemical characterizations, the balanced hydrogen binding energy (HBE) and hydroxide binding energy (OHBE) is likely to be the origin of the high activity of Ni-H2-NH3. In the end, MEA measurements confirmed its excellent performance: for non-PGM MEA, the peak power density could reach 488 mW/cm2, 6.4 times higher than the previous record, which demonstrates the feasibility of efficient PGM-free HEMFCs.Chapter 5 exploited the interaction between PGMs and carbon support to improve their HOR activity. A porous N-doped carbon (pN-C) support was found to have prominent enhancement effect toward PGMs. Especially, the Pt0.25Ru0.75/pN-C had the up-to-date highest mass activity and intrinsic activity toward HOR. Using characterizations including XPS, UPS, H2 chemisorption, CO chemisorption, in situ surface-enhanced infrared adsorption spectroscopy (SEIRAS), and electrochemical characterizations, we concluded the superior HOR activity of Pt0.25Ru0.75/pN-C was a combination of interactions between Pt, Ru and the pN-C, with modulated the HBE and strengthened the H2O binding strength. The HEMFC using Pt0.25Ru0.75/pN-C also possessed great performance that far exceeded the U.S. Department of Energy (DOE) 2022 target for HEMFCs, even though the testing condition was harsher. This work demonstrated the feasibility of HEMFC replacing PEMFC.

EPFL2021