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CODE Report Summary

Project ID: 340748
Funded under: FP7-IDEAS-ERC
Country: Finland

Mid-Term Report Summary - CODE (Condensation in designed systems)

Quantum coherent phenomena, especially macroscopic quantum coherence, are among the most striking predictions of quantum mechanics. They have led to remarkable applications like lasers and modern optical technologies, and in the future, such breakthroughs as quantum information processing are envisioned. Macroscopic quantum coherence is manifested in Bose-Einstein condensation (BEC), superfluidity, and superconductivity, which have been observed in a variety of systems and continue to be at the front line of scientific research. The objective of this project is to extend the realm of Bose-Einstein condensation into new conceptual and practical directions. The project focuses on the role of a hybrid character of the object that condenses and on the role of non-equilibrium in the BEC phenomenon. The work is mostly theoretical but also has an experimental component. We study two new types of fundamentally different hybrids. First, pairing and superfluidity of fermionic particles are considered. Experimental realization of a variety of lattice geometries is now feasible in ultracold Fermi gases, and the project provides predictions to such systems as well as new, general knowledge on superfluidity. Second, the project explores the possibility of finding novel hybrids of light and matter excitations that may display condensation; this part of the research involves both experimental and theoretical work within the group. By combining insight from these two cases, the goal is to understand how the hybrid and non-equilibrium nature can be exploited to design desirable properties, such as high critical temperatures.
At this stage of the project, the main achievement on the topic of fermionic superfluidity was the discovery of a connection between flat band superfluidity and the quantum metric (Peotta and Törmä, Nature Communications, 6, 8944 (2015)). Flat, that is dispersionless, bands have been predicted to show high critical temperatures of superconductivity, but the existence of finite supercurrent in such lattices where one intuitively expects no movement of the infinitely massive particles had not been properly solved. We showed that indeed such flat bands can carry supercurrent, but only if the band has suitable quantum geometrical properties. We derived a connection between superfluid weight and the quantum geometric tensor, producing a lower bound for the superfluid weight given by the Chern number that characterizes the band’s topological properties. These breakthrough results reveal a connection between superfluidity and geometrical as well as topological properties of the system that provide guidance in the quest for high temperature superconductivity. We are currently working on proposals to test the findings experimentally in collaborating groups.
Concerning the topic of light-matter hybrids, the main achievements constitute a series of publications in which we experimentally demonstrated that lattice modes in arrays of silver nanoparticles couple strongly with ensembles of emitters (Väkeväinen et al, Nano Letters 14, 1721 (2014), Shi et al, Physical Review Letters 112, 153002 (2014)) and found that nanoparticles made of a magnetic materials interplay with the lattice geometry due to the intrinsic spin-orbit coupling of the material (Kataja et al, Nature Communications 6, 7072 (2015)). A remarkable amount of effort was also dedicated to the general theory and literature on strong coupling in nanoplasmonics in the form of a review written by the PI with a colleague (Törmä and Barnes, Reports on Progress in Physics 78, 013901 (2015)). These achievements form a solid basis for the future goals of the project related to lasing and condensation phenomena of the strongly coupled hybrids.

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