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 magneticfields and uncover an extrinsic mechanism for band control. In DSM films,quantum tunneling between surface states on opposing surfaces opens ahybridization gap. We demonstrate that the in-plane magnetic field and theintersurface hybridization intertwine to invert the band structure, driving aphase transition from a band insulator to a nodal-ring semimetal.Highlighting the key role of the band inversion, we further analyze themagnetoresponse in the nodal-ring regime, and find the Landau levelsexperience a serpentine oscillation and exhibit multiple crossings. Thesecrossings mark a rich variety of topological phases, encompassing the helicalquantum Hall insulator, spin-filtered quantum Hall insulator, and normal bandinsulator. Our results establish a unique pathway of band inversion viaexternal fields, providing a controllable approach to engineer energy bandsand manipulate topological states in DSMs.