Optical spectroscopic studies of the thermal effects on electronic structures of semimetallic/semiconducting small carrier system

Abstract

Recently, there has been a spate of investigations into low density metals or semimetals with multiband electronic structures, as these compounds are known to display intriguing physics. Due to high characteristic mobilities, multiple species of charge carriers, and, in some cases, novel electronic dispersions, these materials reveal a variety of spectacular transport properties and/or quantum states that have not been observed in conventional metals or insulators. In this dissertation, I will report optical spectroscopic studies of the thermal effects on electronic structures of semimetallic/semiconducting small carrier system. First, using temperature-dependent optical spectroscopy, we observe a large and unusual blueshift in the free carrier plasma frequency with reduced temperature in SrMnSb2. This contradicts the common understanding of free carrier dynamics in weakly correlated metals and semiconductors, as the plasma frequency is expected to be constant or else reduced at low temperatures. We also observed noticeable changes in the optical interband transitions and Hall coefficient. To explain these novel properties, we constructed a minimal three band model based on ab initio band structure calculations. The model includes two nearly degenerate conduction bands with significantly different masses. As temperature is raised, we find that a significant number of carriers are thermally redistributed between the two bands, leading to pronounced changes in the optical and transport properties. Remarkably, the simulation reproduces not only the qualitative features of the data but also quantitative values of the plasma frequency, the Hall coefficient and the optical gap as a function of temperature using parameters that are consistent with the ab initio results. Second, I will examine the temperature (T) evolution of the optical conductivity spectra of Sr3Ir2O7 over a wide range of 10–400 K. The system was barely insulating, exhibiting a small indirect bandgap of ~0.1 eV. The low-energy features of the optical d-d excitation (ħω < 0.3 eV) evolved drastically, whereas such evolution was not observed for the O K-edge X-ray absorption spectra. This suggests that the T evolution in optical spectra is not caused by a change in the bare (undressed) electronic structure, but instead, presumably originates from an abundance of phonon-assisted indirect excitations. We demonstrated that the low-energy excitations were dominated by phonon-absorption processes in which optical phonons, in particular, are involved. Our results showed that phonon-assisted processes significantly facilitate the charge dynamics in barely insulating Sr3Ir2O7. Finally, the electrodynamics of an anisotropic Dirac material SrMnBi2, which has an antiferromagnetic order, will be investigated by optical spectroscopy and first-principle calculations. Optical spectra also show a seemingly gap-like dip near 120 meV, similar to those due to spin-density wave formation. However, we found that this feature should be attributed to the direct optical transitions between the Dirac or the semimetallic bands, not to the magnetic origin. As the temperature increases, the scattering rates of both charge carriers rapidly increase and become comparable, and the seemingly gap-like feature is gradually filled, possibly due to the indirect optical transition assisted by phonon absorption.