Direct observation of Landau level resonance and mass generation in Dirac semimetal Cd3As2 thin films

摘要

Three-dimensional topological Dirac semimetals have hitherto stimulatedunprecedented research interests as a new class of quantum materials. Breakingcertain types of symmetries has been proposed to enable the manipulation ofDirac fermions; and that was soon realized by external modulations such asmagnetic fields. However, an intrinsic manipulation of Dirac states, which ismore efficient and desirable, remains a significant challenge. Here, we reporta systematic study of quasi-particle dynamics and band evolution in Cd3As2 thinfilms with controlled Chromium (Cr) doping by both magneto-infraredspectroscopy and electrical transport. For the first time, we observesquare-root-B relation of inter-Landau-level resonance in undoped Cd3As2 Diracsemimetal, an important signature of ultra-relativistic Dirac stateinaccessible in previous optical experiments. A crossover from quantum toquasi-classical behavior makes it possible to directly probe the mass of Diracfermions. Importantly, Cr doping allows for a Dirac mass acquisition andtopological phase transition enabling a desired dynamic control of Diracfermions. Corroborating with the density-functional theory calculations, weshow that the mass generation is essentially driven by explicit C4 rotationsymmetry breaking and the resultant Dirac gap engineering through Crsubstitution for Cd atoms. The manipulation of the system symmetry and Diracmass in Cd3As2 thin films provides a tuning knob to explore the exotic statesstemming from the parent phase of Dirac semimetals.