Modeling strained Cd 3 As 2 thin films and their behavior in magnetic fields

We present a systematic analysis of the behavior of thin films of Cd$_3$As$_2$ under different strain profiles and in magnetic fields. In each case, we construct effective $k \cdot p$ models by considering the reduction of symmetry and all constraints imposed by the remaining symmetries. Our analysis naturally describes both in-plane biaxial and uniaxial strain. Biaxial strain is expected to preserve in-plane $C_4$ rotational symmetry while breaking inversion, allowing for a description in terms of the $4mm$ point group. Uniaxial strain, on the other hand, breaks $C_4$ symmetry. For this case, we consider two scenarios: one preserving inversion, described by the $mmm$ group, and one breaking it, leading to $2mm$ symmetry. After deriving the models, we examine the effects of out-of-plane magnetic fields, identifying two possible microscopic mechanisms that can account for the experimental results reported in Ahadi et al. (2025). Importantly, our analysis proposes a new method for differentiating between them. By incorporating the effects of multiple subbands along the confinement direction, we show that the opening of a gap in the lowest Landau level requires either reducing the symmetry down to $2mm$, breaking both inversion and $C_4$ rotations, or a topological transition of the band structure due to strain-induced band renormalization. Furthermore, we demonstrate that a two-dimensional Dirac semimetal phase can be induced by sufficiently large in-plane magnetic fields. This phase is highly sensitive to different strain profiles, with band touchings occurring when the field is applied perpendicular to preserved mirror planes, serving as a powerful probe of the material's strain profile.