Abstract
This doctoral thesis focuses on the study of bulk (3D) materials with strongly non-parabolic/linear (Dirac) energy band structure and explores the validity of the relativistic models in the description of condensed matter systems. The first three chapters give the general base to the theoretical as well as experimental techniques used during the completion of the studies. The original new results are presented in the three last chapters.The first chapter provides a comprehensive exploration of topological fundamentals in condensed matter physics, including the concept of topology and various topological classes of materials such as topological insulators, Weyl semimetals, and Dirac semimetals. It emphasizes the characterization of material topology and how it influences electronic properties, while also considering the role of crystalline symmetries in determining topological properties.The second chapter delves into the behavior of solid-state matter in the presence of an external magnetic field, specifically focusing on optical response, optical transitions in crystalline solids, and Landau level formation. It examines how the Landau level structure affects the optical response, particularly emphasizing absorption and reflection spectra.In the third chapter, experimental techniques employed in this study are outlined. THz spectroscopy, including magneto-transmission experiments utilizing far-infrared molecular lasers and Fouriertransform infrared spectroscopy with a Michelson interferometer are discussed.The fourth chapter presents the results of THz magneto-optical spectroscopy analysis of Hg1−xCdxTe films under hydrostatic pressure, focusing on samples near critical values of x that undergo a topological phase transition from a semimetal to a semiconductor. Using a simplified Kane model with two free parameters, the study determines the hydrostatic deformation potentials ac−av at 4 K. By measuring negative and positive band gap values, the analysis reveals semi-metallic and semiconductor film behavior, respectively. The study derives the deformation potential ac−av at low temperatures for any x value through linear extrapolation based on values reported for CdTe, eliminating the need for band structure calculations for HgTe/HgCdTe superlattices.The fifth chapter explores the topological phase transition in (Cd1−xZnx)3As2, a system based on Cd3As2 with varying Zn content from 1 to 22%. Magnetooptics investigations demonstrate the expected behavior of a topological quantum phase transition by observing the evolution of the band gap.The sixth chapter investigates the properties of a new combined Weyl semimetal, Nb1−xTaxAs. This material exhibits characteristic features of Weyl theory and combines elements from NbAs and TaAs, which are well-known type-I Weyl semimetals. Through magneto-reflectivity and band structure analysis, the chapter confirms the discovery of this breakthrough material, Nb1−xTaxAs, providing evidence for the validity of Weyl’s theory and exhibiting down-dispersing lines similar to those observed in TaP.