Surface Optical and Bulk Acoustic Phonons in Topological Insulator Bi₂Se₂Te

Mukhopadhyay, Uditendu (2015) Surface Optical and Bulk Acoustic Phonons in Topological Insulator Bi₂Se₂Te. Masters thesis, Indian Institute of Science Education and Research Kolkata.

PDF (MS dissertation of Uditendu Mukhopadhyay (10MS01)) Uditendu_Mukhopadhyay-10MS01.pdf - Submitted Version Restricted to Repository staff only Download (9MB)

Abstract

The newly discovered 3D topological insulators (TIs) represent a class of exotic quantum state of matter with an insulating bulk and conducting surface states. In this thesis we want to study the non-equilibrium carrier and phonon dynamics of Bi₂Se₂Te (BST), a TI materials using optical methods. The project is divided into two parts. First part was devoted towards instrumentation and optimization of the setup followed by measurement and data analysis. In the first part of our experiment presence of a 2-D Auger recombination process is conclusively proven in the carrier dynamics of Si doped GaAs thin film using ultrafast pump-probe spectroscopy. The Auger coeficient comes out to be 1.81±0.15 cm³s⁻¹. Auger recombination, being a many-body phenomenon, is only observed when the carrier density is suficiently higher. The Auger timescale also varies with the initial carrier density. Auger recombination, mainly observed in semiconductor materials, plays a huge role in designing solar cells. The electrons tend to get back from conduction to valance band very quickly through Auger recombination. This creates a huge problem in designing solar cells. A careful analysis of Auger recombination can therefore help us find systems with minimum Auger effect. We can also try minimizing Auger process in different materials. For topological insulators, our experiments capture the optical and acoustic phonon modes through time resolved differential re ectivity measurements of BST thin films. Short lived fast oscillations, which are superimposed on the pump-probe signal are attributed to the A₁¹g optical phonon mode. The remaining slow oscillations correspond to modulation due to interference of bulk acoustic phonon modes. By studying films of different thickness, we have been able to conclusively establish that optical phonon modes are indeed a surface phenomenon with no contribution form the bulk. The slow oscillation corresponding to the acoustic phonon modes on the other hand is significantly affected by thickness which has been qualitatively explained in the light of dynamics of mechanical standing waves in elastic media. Form the perspective of device applications, though the surface electronic states are topologically protected against scattering from non-magnetic defects, acoustic phonons are still an impediment to the electron ow in the bulk. This is because, electron-phonon scattering will always be there at room temperature. Thus for good device applications, one needs to lower the temperature to minimise the electron-phonon coupling. However, this ability to tune the frequency of acoustic phonons with thickness can be of advantage. In addition to this the fact that we have managed to separate the contribution of bulk acoustic phonons and surface optical phonons has a bearing on improving the low power spintroninc device application in certain cases without having to cool the system. Thus the ability to tune the frequency of acoustic phonons with sample thickness can potentially help in further research on the room temperature low power spintronic device applications and in topological quantum computation.

Actions (login required)

View Item