Paradoxe de Fermi

vignette|Vue d'artiste des trois étoiles de l'exoplanète HD 188753 Ab, à partir d'un hypothétique satellite de cette dernière. Le paradoxe de Fermi est le nom donné à une série de questions que s'est posées le physicien italien Enrico Fermi en 1950, alors qu'il débattait avec des amis de la possibilité d'une vie extraterrestre et d'une visite d'extraterrestres. Fermi, lauréat du prix Nobel en 1938, et alors qu'il est impliqué dans le projet Manhattan à Los Alamos aux États-Unis, déjeune avec plusieurs de ses amis et collègues (Emil Konopinski, Edward Teller et Herbert York). Lors du repas, il en vient à demander où sont les extraterrestres, et pose le principe du paradoxe qui porte son nom. Celui-ci consiste à se demander pourquoi l'Humanité n'a, jusqu'à présent, trouvé aucune trace de civilisations extraterrestres, alors que le Soleil est plus jeune que beaucoup d'étoiles situées dans notre galaxie. Selon Fermi, des civilisations plus avancées auraient dû apparaître parmi les systèm

Quantum oscillations in the type-II Dirac semi-metal candidate PtSe2

Philip Johannes Walter Moll, Hao Yang

Three-dimensional topological semi-metals carry quasiparticle states that mimic massless relativistic Dirac fermions, elusive particles that have never been observed in nature. As they appear in the solid body, they are not bound to the usual symmetries of space-time and thus new types of fermionic excitations that explicitly violate Lorentz-invariance have been proposed, the so-called type-II Dirac fermions. We investigate the electronic spectrum of the transition-metal dichalcogenide PtSe2 by means of quantum oscillation measurements in fields up to 65 T. The observed Fermi surfaces agree well with the expectations from band structure calculations, that recently predicted a type-II Dirac node to occur in this material. A hole- and an electron-like Fermi surface dominate the semi-metal at the Fermi level. The quasiparticle mass is significantly enhanced over the bare band mass value, likely by phonon renormalization. Our work is consistent with the existence of type-II Dirac nodes in PtSe2, yet the Dirac node is too far below the Fermi level to support free Dirac-fermion excitations.

2018

Photo- and Thermally- induced Effects on the Absorption Spectroscopy of Au/SiO2 and Au/TiO2 Core-shell Nanoparticles

Lijie Wang

Electrons generated by Landau damping of the plasmon excitation on gold nanoparticles that can be injected into an adjacent semiconductor e.g. anatase TiO2, enhancing the light harvesting capabilities of solar energy conversion devices. The understanding of the electron injection processes is crucial for the optimization of the devices. The first step is to have an understanding of the bare gold nanoparticle itself, we thus report on ultrafast transient absorption studies of surface plasmon-excited ~25 nm diameter gold nanoparticles in solution, monitoring the surface plasmon resonance response in the visible (1.7-3.0 eV) as well as the inter- and intra-band transitions in the near-to-deep-UV (3.4-4.5 eV). However, in order to distinguish the thermal effects in absorption spectrum on gold NPs, we present temperature-dependent (from room temperature to 80 °C) absorption spectra of Au/SiO2 core-shell nanoparticles (core diameter: ~25 nm) in the range from 1.5 eV to 4.5 eV, which encompasses the localized surface plasmon resonance and the interband transitions. The changes of absorption spectra are well reproduced by theoretical calculations based on the experimentally measured temperature-dependence of the real (e_1) and imaginary (e_2) part of the gold nanoparticle dielectric constant. We also compare the results to ultrafast photo-induced transient absorption spectra upon excitation of the plasmon band, and conclude that while the plasmon band can be used to monitor the heating of the nanoparticles, the interband region is little affected by thermal effects and is dominated by electronic ones. Our deep-UV probed transient results on gold nanoparticles show that after the initial ultrafast electron-electron and electron-phonon scattering processes, low-energy hot electrons above the Fermi level remain in equilibrium with the hot lattice for tens to hundreds of ps. Then we investigate the photo-induced charge carrier transfer and relaxation in Au/TiO2, our results show that the bleach at the exction resonance in Au/TiO2 has a significant slow decay speed upon plasmon excitation, and its transient absorption intensity at long term decay gives a quadratic dependence on excitation fluence, implying the appearance of two-photon absorption in anatase TiO2 that enhanced by the surface plasmon resonance of gold nanoparticles. Besides, in the Au/SiO2 and Au/TiO2 NPs, we both observe acoustic vibrations in the UV region for the first time but with significantly different oscillation frequencies which were attributed to the different elastic properties of the shell.

EPFL2022

Photo- and Thermally- induced Effects on the Absorption Spectroscopy of Au/SiO2 and Au/TiO2 Core-shell Nanoparticles

Lijie Wang

EPFL2022

Questioning Green paradox

Quantum oscillations in the type-II Dirac semi-metal candidate PtSe2

Photo- and Thermally- induced Effects on the Absorption Spectroscopy of Au/SiO2 and Au/TiO2 Core-shell Nanoparticles

Questioning Green paradox

Quantum oscillations in the type-II Dirac semi-metal candidate PtSe2

Photo- and Thermally- induced Effects on the Absorption Spectroscopy of Au/SiO2 and Au/TiO2 Core-shell Nanoparticles