Public defence in Engineering Physics, M.Sc. (Tech) Kukka-Emilia Huhtinen

Title of the doctoral thesis: Superconductivity and normal state properties in flat bands

Opponent: Professor Kristjan Haule, Rutgers University, USA
Custos: Professor Päivi Törmä, Aalto University School of Science, Department of Applied Physics

The doctoral thesis is publicly displayed 10 days before the defence in the publication archive Aaltodoc of Aalto University

Electronic thesis

Public defence announcement:

Superconductivity is a phenomenon in which electrical current can flow without resistance, and which when applied to various fields could lead to energy savings. However, its use is hampered by the low temperatures at which it occurs. One promising avenue for the realisation of superconductivity at higher temperatures is systems featuring flat energy bands. Such systems can facilitate the formation of exotic states of matter, since even small interactions can have remarkable effects.

This work theoretically investigates the properties of flat band systems, which are often determined by the geometric properties of the quantum states. It is shown that the minimal quantum metric plays a significant role in superconductivity in flat bands. In a flat band, single particles have an infinite effective mass, and can not move. Superconducting current instead arises from the movement of Cooper pairs, which have an effective mass proportional to the minimal quantum metric. A large minimal quantum metric is then related to stable superconductivity, which could persist up to higher temperatures. Moreover, it is shown that systems where a flat band touches other non-flat bands can feature enhanced superconducting transition temperatures when compared to systems where the flat band is isolated. This is promising for the search of high temperature superconductors, since flat bands in experiments and real materials are often not isolated.

In many high-temperature superconductors, peculiar properties occur even before the material is cooled down enough to enter the superconducting phase, in the so-called normal state. This work shows that this is also the case in flat bands. At low interactions, the effective mass of single particles diverges. As the interaction is increased, single particles become mobile and the normal state contains preformed pairs. The results of the thesis confirm the potential of flat band systems for the observation of exotic phases of matter.

Contact details of the doctoral student: [email protected], 0442133702