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Public defence in Engineering Physics, M.Sc. (Tech) Ville Pyykkönen

Flat bands in a lattice struture potentially allow to increase superconducting critical temperatures. This dissertation studies the flat band transport phenomena in the cases of superconductivity and all-optical switching. Public defence from the Aalto University School of Science, Department of Applied Physics.
Doctoral hat floating above a speaker's podium with a microphone

Title of the doctoral thesis: Transport of interacting particles through a flat Bloch band: superconductivity and all-optical switching

Doctoral student: Ville Pyykkönen
Opponent: Prof. Antti-Pekka Jauho, Technical University of Denmark, Denmark
Custos: Prof. Päivi Törmä, Aalto University School of Science, Department of Applied Physics

Flat bands in a lattice struture potentially allow to increase superconducting critical temperatures. This dissertation studies the flat band transport phenomena in the cases of superconductivity and all-optical switching. 

Solid matter is formed by atoms, which are often ordered as lattice structures. The electrons move in the lattice as quantum mechanical waves following the so-called electron band structure. Flat bands are electron bands that confine single electrons to a few atoms by the destructive interference between the electrons. Recent studies have shown that flat bands are useful in increasing critical temperatures of superconductors, namely the temperatures below which they superconductivity occurs. A superconductor conducts electricity without resistance. A room-temperature superconductor would allow significant energy savings. 

This dissertation examines the transport phenomena of flat-band lattice systems. The central finding of the research is that interacting particles move in flat bands as pairs while single particles remain localized. In the case of superconductivity, we find that the Cooper pairs, which are the pairs of electrons responsible for the phenomenon, can carry the supercurrent while single-particle transport processes are suppressed. Often these single-particle processes are detrimental for applications of superconductivity, allowing flat bands to be useful in their design. 

The thesis also presents an all-optical switching principle based on the flat band localized states. Optical switching is an integral part of optical communication. Typically, switching also involves electronics, which causes energy losses. The presented flat band switching principle is based on photons passing only when coupled with a control photon to form a pair. In other words, a single photon can be used to control the passage of the other through the switch. The switch operates at the minimum energy. The dissertation opens new possibilities in the applications of flat bands in photonics.

Thesis available for public display 10 days prior to the defence at: https://aaltodoc.aalto.fi/doc_public/eonly/riiputus/

Contact details:


Doctoral theses in the School of Science: https://aaltodoc.aalto.fi/handle/123456789/52

Key words: superconductive transport, all-optical switching, flat band, Hubbard model, non-equilibrium Green's functions

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