Investigations of surface related electronic properties in SmB6 and LaAlO3/SrTiO3 heterostructures.

摘要

This dissertation reports research performed on two types of two-dimensional systems: SmB6 and LaAlO3/SrTiO3 (LAO/STO). SmB6 has been proposed to be a topological Kondo insulator at low temperature. In order to understand carriers/ lattice dynamics and their interactions, femtosecond pump-probe spectroscopy is performed in SmB6 single crystals and thin lms at variable temperatures. The collective oscillation modes in GHz - THz and the change of carrier relaxations is observed as a function of temperature. From the temperature dependent results for d hybridization, opening of the hybridization gap, phonon bottleneck", and th possible topological surface state formation is revealed. The topological surface state should support helical Dirac dispersion with momentum-spin lockage. This dissertation reports on current injection in SmB6 thin lm with circularly polarized light at oblique incidence. This spin polarized photocurrent is concluded to be a direct result of spin momentum lockage in SmB6. LAO/STO interface shows 2-dimensional electron gas (2DEG) at the interface when the thickness of LAO is more than 3 unit cell. Carrier properties at the LAO/STO interfaces are highly sensitive to the top surface termination of LAO. The spontaneous dissociation of water on LAO surface is systematically studied by density functional theory and experimental surface characterizations. Extrinsic effects from surface adsorbates were often ignored in the previous studies of the 2DEG. From the experiments, it is found that the dissociated water molecules, especially the surface protons, strongly aect the interface density of states, electron distributions and lattice distortions. The investigations also reveal the importance of additional molecular water layers. These additional water layers, through hydrogen bonds, provide an energetically feasible pathway for manipulating the surface-bonded protons and thus, the interface electrical characteristics.