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Superconductivity is a state of electronic matter occurring at low temperatures, in which the resistivity of the material vanishes. Ferromagnetism, on the other hand, is a state of matter, in which the magnetic dipoles associated with the electron spin spontaneously organize so that each dipole points along the same direction. In macroscopic structures these two states of matter oppose each other, and as a result magnetism often quenches the superconductivity.

In nanoscale devices the situation is more complicated, as the so-called proximity effects become relevant on small length scales. In systems composed of magnetic and superconducting films, the magnetization can “leak” into the superconductor through the interface between the films. In this process, magnetic properties are injected into the superconductor.

“The resulting material can be regarded as the mixture of the two states of matter. It is partially a superconductor and partially a magnet”, tells Ojajärvi. What is particularly interesting about this combination is that in addition to the ordinary charge current, it is possible to transmit a spin current with superconducting properties through it. In one of the nine articles included in his dissertation, Ojajärvi demonstrated how this spin supercurrent affects the magnetic properties of the sample, and its other experimental signatures.

In the dissertation, Ojajärvi also studied superconductivity and magnetism on exotic systems, with flat electronic bands. Flat bands rose at the forefront of the physics research in 2018 when the researchers in Massachusetts Institute of Technology experimentally found that graphene, a two-dimensional carbon-based material, becomes superconducting when another graphene layer is placed on top of at a twist angle of one degree. At this angle, the electronic bands in the two graphene layers press against each other and become flat, and the effects due to interactions between electrons become pronounced. According to the study by Ojajärvi and others, it is possible to understand the occurrence of the superconductivity by combining the conventional theory of superconductivity, that is based on the interaction between electrons and phonons, with the peculiar properties of flat band systems.

The research has been supported by Jenny and Antti Wihuri foundation, and it has been conducted in the Nanoscience Center in Ģ��ֱ��.

The dissertation is published in JYU Dissertations series, number 500, Jyväskylä 2022. ISBN 978-951-39-9060-2 (PDF), URN:ISBN:978-951-39-9060-2, ISSN 2489-9003. Link to publication: 

M.Sc. Risto Ojajärvi defends his doctoral dissertation "Competition and interplay between magnetism and superconductivity" on Friday 8 April 2022 at 12 noon. Opponent Professor Daniel Loss (University of Basel, Switzerland) and Custos Professor Tero Heikkilä (Ģ��ֱ��). The doctoral dissertation is held in English.