Engineering band topology of Dirac semimetal films via in-plane magnetic fields

Band structure engineering continues to be pursued in solid-state physics. Here, we study Dirac semimetal (DSM) thin films under in-plane magnetic fields and uncover an extrinsic mechanism for band control. In DSM films, quantum tunneling between surface states on opposing surfaces opens a hybridization gap. We demonstrate that the in-plane magnetic field and the intersurface hybridization intertwine to invert the band structure, driving a phase transition from a band insulator to a nodal-ring semimetal. Highlighting the key role of band inversion, we further analyze the magnetoresponse in the nodal-ring regime, and find the Landau levels experience a serpentine oscillation and exhibit multiple crossings. These crossings mark a rich variety of topological phases, encompassing the helical quantum Hall insulator, spin-filtered quantum Hall insulator, and normal band insulator. Our results establish a unique pathway of band inversion via external fields, providing a controllable approach to engineer energy bands and manipulate topological states in DSMs.