  Flat bands in electronic band structures are intriguing platforms for strong
correlation and topological physics, primarily due to the suppressed kinetic
energy of electrons. Various methods have been developed to create flat bands,
utilizing lattice geometry or finely tuned parameters. Despite this, the
investigation of orbital symmetry in multiorbital materials is a relatively new
area of focus. In this work, we propose a site symmetry based systematic
approach to emerging multiorbital flat bands in lattices made of
corner-connecting motifs such as the kagome and pyrochlore lattices. As a
conceptual advance, the one-orbital flat bands are shown to originate as mutual
eigenstates of isolated molecular motifs. Further developing the mutual
eigenstate method for multiorbitals transforming differently under the site
symmetries such as mirror and inversion, we derive multiorbital flat bands from
the skew-symmetric interorbital Hamiltonian and introduce an isolated molecule
enabled group-theoretic description of the flat band wavefunctions.
Realizations of the multiorbital flatbands in relevant materials are shown to
be possible under the Slater-Koster formalism. Our findings provide new
directions for exploring flatband electronic structures for novel correlated
and topological quantum states.
