Quantum oscillations in Dirac semimetals

Abstract

Dirac semimetals have recently gained interest due to their unique experimental signatures many of which can be explained by the topologically non-trivial band structure in these materials. Among the many candidates, the tellurides NbTe₄, TaPtTe₅ and TaNiTe₅ appear to provide a promising platform for the study of such topological properties. To probe their band structure and its corresponding Fermi surface, quantum oscillations have proven very useful as they provide a tool to directly measure the Berry phase under certain conditions. The (double) Dirac semimetal NbTe₄ has shown the occurrence of multiple charge-density-wave (CDW) phases, and as a result a complex band structure at low temperatures. Density functional theory (DFT) calculations predict the presence of an eight-fold degenerate band crossing whose topological properties are investigated in this thesis. The magnetoresistance in this material shows the onset of linear behaviour at 15 T which indicates the presence of Dirac-like band crossings, while the angular magnetoresistance (AMR) measurements also resemble results known from topological semimetals, however alternative explanations seem more likely to explain these results in NbTe₄. Experimental signatures expected from Dirac semimetals are further examined with a comprehensive study of the magnetic torque anomaly, and by measuring de Haas - van Alphen (dHvA) oscillations in the magnetic torque and Shubnikov - de Haas (SdH) oscillations in the magnetoresistance. Combined with DFT calculations, quantum oscillations allow to map the Fermi surface of this material. Furthermore, non-trivial Berry phases and low effective masses can be observed together with high frequencies that could either be the remains of a different CDW phase or arise from magnetic breakdown. Based on symmetry arguments of the crystal structure, the Dirac nodal-line semimetal candidate TaPtTe₅ is theoretically predicted to host four-fold degenerate lines with linear band dispersion in reciprocal space. The work presented here aims at confirming the theoretical predictions by providing a numerical investigation of the band structure and comparing that to results from dHvA oscillations in the magnetic torque. Closely related to TaPtTe₅, the structurally similar compound TaNiTe₅ is also predicted to host Dirac nodal-lines. Again, quantum oscillations in the magnetic torque are used to obtain insights into the morphology of the Fermi surface, while effective masses can be extracted from temperature-dependent dHvA measurements in the magnetisation. Unlike other semimetals reported in the literature, TaNiTe₅ furthermore shows enhanced oscillations in the magnetoresistance, even in the out-of-phase component of the applied alternating current. A thorough investigation of this effect is performed and a numerical model can explain the effect within the framework of classical electrodynamics without involvement of topological physics.