THz charge carrier dynamics of two-dimensional electron gases in topological insulators and oxide heterostructure

Topological insulators and oxide heterostructures hosting two-dimensional electron gases (2DEGs) are new generation materials which have potential for a wide range of applications such as spintronics, quantum computing and in electronic devices such as sensors and transistors. Shape memory alloys define another fascinating class of materials characterized by shape memory effect and super-elasticity. They find a wide range of application from medical devices to advanced engineering equipment and structures. The main focus of the thesis is on THz properties and carrier relaxation dynamics of two-dimensional electron gases in topological insulators and oxide heterostructures. In addition, role of electrons during martensitic phase transition and phonon softening in pre-martensite phase in shape memory alloy is investigated. The topological insulators studied in the thesis are nanograined Bi2Te3 (bismuth telluride) pellets and SnTe (tin telluride) thin films. Terahertz time domain spectroscopy (THz-TDS) is used to investigate the individual contribution of bulk and surface charge carriers to the net conductivity in nanograined Bi2Te3 pellets of relative densities of 82% and 99%. The results reveal an enhanced surface contribution of 60% -80% to the net conductivity of the samples in a wide temperature range of 4 K to 300 K. In addition to linear spectroscopic studies, THz emission spectroscopy measurements are conducted on nanograined Bi2Te3 pellet. The sample when illuminated with NIR pulses emits THz radiation. Unlike nanograined Bi2Te3 pellets, it is not possible to disentangle the individual contribution of bulk and surface charge carriers to the net conductivity in SnTe thin films because the gap in their mobility values is not known. Although it is not possible to have an exact quantification of surface contribution in SnTe thin films, THz-TDS helps to get a generalized picture of surface transport in the material system. The oxide heterostructure studied in the thesis is composed of two insulating oxides: strontium titanate (STO) and lanthanum titanate (LTO). The heterostructure is designed in a layered structure of STO/LTO/STO/Si. Using THz-TDS, the carrier density and mobility of 2DEG at the interface of STO and LTO at different temperatures are estimated. The results reveal moderate carrier density and mobility of the interfacial 2DEG which results in increased conductivity of the 2DEG at the interface compared to the oxides in the bulk. In addition to THz-TDS, THz pump-THz probe measurements are carried out at the free-electron laser (FEL) facility FELBE at the Helmholtz-Zentrum Dresden Rossendorf. The measurements reveal very small non-linearities where the relative pump-induced change in transmission is around 0.04%-0.05% at a pump fluence of about 185 nJ/cm2. The shape memory alloy studied in the thesis is single crystal NiTi (nickel titanium). THz-TDS is used to study the changes in reflection of single crystal NiTi when it undergoes martensitic phase transition. This is compared to phase transition in polycrystalline NiTi sample. Our investigation show that phase transition is sharper in single crystal NiTi compared to that in polycrystalline NiTi. In addition, THz-TDS is used to study the role of phonons in the high temperature phase of single crystal NiTi. The results reveal evidence of phonon softening which hint towards the occurrence of R phase in the material.