Flat bands in condensed-matter systems – perspective for magnetism and superconductivity

There is a recent upsurge of interests in flat bands in condensed-matter systems and the consequences for magnetism and superconductivity. This article highlights the physics, where peculiar quantum-mechanical mechanisms for the physical properties such as flatband ferromagnetism and flatband superconductivity that arise when the band is not trivially flat but has a strange Hilbert space with non-orthogonalisable Wannier states, which goes far beyond just the diverging density of states. Peculiar wavefunctions come from a quantum-mechanical interference and entanglement. Interesting phenomena become even remarkable when many-body interactions are introduced, culminating in flatband superconductivity as well as flatband ferromagnetism. Flatband physics harbours a very wide range physics indeed, extending to non-equilibrium physics in laser illumination, where Floquet states for topologcial superconductivity is promoted in flatbands. While these are theorecially curious, possible candidates for the flatband materials are beginning to emerge, which is also described. These provide a wide and promising outlook.