Bose Einstein Condensates in Flat Bands: Topological Discovery

A team of researchers led by ICREA Prof. at ICFO Maciej Lewenstein, who is also the Dynamite project coordinator, reports in Physical Review Letters on the stability of Bose Einstein Condensates, where Bogoliubov excitation modes have non-trivial topological properties.

Topological Insulators and Flat Band Systems: A New Frontier in Condensed Matter Physics

The discovery of topological band structures has given rise to an exciting and rapidly evolving field that blends concepts from topology and condensed matter physics by means of new materials with unique properties. The most prominent feature of topological insulators is their edge or surface states, which are topologically protected from backscattering. This protection means that these surface states are robust against impurities and imperfections, making them of great interest for potential applications in electronic devices.

Cold atomic systems, like ultracold bosonic gases, have given us a fascinating platform to explore topological properties in bosonic matter. These experiments have pushed our understanding of topological band structures across various quantum systems.

In some quantum systems, the interactions between particles, not just the single-particle band structure, give rise to non-trivial topological features. The non-trivial topology doesn’t start in the single-particle bands but interactions induce it in the many-body system. Degenerate flat bands in magic-angle twisted bilayer graphene are a great example. These bands have zero or near-zero dispersion and can come from lattice structures or potentials. Interactions between particles, like bosons or electrons, in these flat bands can lead to exotic, interaction-driven topological states and other phenomena like unconventional superconductivity.

Optical lattices with Kagome geometries, consisting of interconnected triangles in a hexagonal pattern, are known for hosting the lowest flat band via artificial gauge fields, making them potential candidates for topological properties in the presence of interactions. However, the stability of Bose Einstein condensation in this lowest flat band is questionable due to the effects of zero-point fluctuations stemming from the suppression of kinetic energy and the presence of many-body interactions.

Topological Insulators and Flat Band Systems: A New Frontier in Condensed Matter Physics

In a recent study published in Physical Review Letters, ICFO researchers Zahra Jalali-mola, Tobias Grass, Valentin Kasper and Utso Bhattacharya, led by ICREA Prof. at ICFO and Dynamite project member Maciej Lewenstein, have reported on the stability of BEC in Kagome optical lattices in the lowest flat band. Furthermore, they found that the Bogoliubov excitation modes, as a result of broken time reversal symmetry in the Bogoliuobov-de-Gennes Hamiltonian, have non-trivial topological properties.

Flat band systems have garnered considerable attention in the search for high-temperature superconductors because they are natural hosts of strong interactions. In addition, their exotic quantum properties also make them ideal candidates for research on quantum information and quantum simulation. Therefore, considering a flat-band system in the context of Bose condensates, the team of scientists has found that these interactions indeed induce topological excitations. Moreover, by tuning these interactions, there exists a topological phase transition emerging as different topological properties of the Bogoliubov excitations, or more specifically, different topological Chern numbers.

Zahra Jalalimola together with the ICFO-QOT group led by ICREA Prof. at ICFO Maciej Lewenstein and with ex-Icfonians Tobias Grass and Valentin Kasper publishes this study in Phys. Rev. Lett. devoted to exotic topological properties of Bogoliubov Quasiparticles from Bose-Einstein Condensate in a Flat Band System.

Original article

Jalali-mola, Z., Grass, T., Kasper, V., Lewenstein, M., & Bhattacharya, U. (2023). Topological Bogoliubov Quasiparticles from Bose-Einstein Condensate in a Flat Band System. Physical Review Letters, 131(22), 226601. https://doi.org/10.1103/PhysRevLett.131.226601