EPJ B Highlight - Understanding 2D Dirac semimetals in tilted magnetic fields

Details

Published on 09 July 2025

Model reveals that within ultrathin Dirac semimetal films, the transport of quantised Dirac fermions can occur in two distinct ways, depending on how the film is tilted relative to an applied magnetic field

Dirac matter is an exotic phase of matter in which quasiparticles – arising from low-energy electron excitations behave like relativistic particles – obey the rules of both quantum mechanics and special relativity. Among these systems are materials called Dirac semimetals, which are characterised by discrete points where their conduction and valence bands touch, forming a linear energy–momentum relationship. Today, physicists are especially interested in the unusual topological phases that can emerge when Dirac semimetals are fabricated into ultrathin 2D films.

Through theoretical analysis published in EPJ B, Rui Min and Yi-Xiang Wang at Jiangnan University, China, investigate how Dirac fermions are transported in thin semimetal films under tilted magnetic fields. Their model reveals that the quantum Hall behaviour of these materials changes in distinct ways depending on the field’s orientation – offering new insights into the topological nature of Dirac matter. Their results could lead to applications in areas including quantum computing, low-power electronics, and spin-based information processing.

In recent experiments, physicists have fabricated thin films of cadmium arsenide (Cd₃As₂) just tens of nanometres thick. When a magnetic field is applied, the Dirac fermions in these films are confined to discrete energy levels via the quantum Hall effect.

In their study, Min and Wang used a complete model to analyse how these quantised energy levels change when the film is tilted at different angles relative to the applied magnetic field. When the field lies mostly in-plane with the film, their model shows that certain energy levels become ‘doubly degenerate’, meaning each level contains two independent quantum states with the same energy. This results in odd-numbered quantum Hall plateaus, where only every other energy level contributes a step in the Hall resistance.

However, when the film is tilted out of the plane, the spacing between these energy levels increases, breaking their degeneracy. The Hall steps then follow a more typical integer sequence; increasing stepwise with each filled level. Through these results, Min and Wang hope to support experimental progress toward exotic topological phases, which may arise in such ultrathin Dirac semimetal systems.