Identifying electron-phonon interactions in 3D topological insulator nanoflakes by inelastic light scattering

Abstract

The 3D topological insulator (TI) materials Bi2Se3 and Bi2Te3 belong to a new class of materials that are characterized by insulating bulk properties but host quasi-relativistic states at their surface. These topological surface states form as a consequence of the differing topology of the underlying Hamiltonian of the material and air and have been shown to be very robust at ambient conditions as long as the topological character of the bulk is unchanged. In addition, the strong spin-momentum locking that is present in Bi2Se3 and Bi2Te3 leads to helical spin-polarization and protects the surface states against backscattering and enables dissipationless transport. Since their discovery these novel quantum states have lead to a vast interest in the condensed matter community for the realization of applications in spintronics, quantum computing and low-resistance materials at room temperature. This work presents studies on Bi2Se3 and Bi2Te3 in the form of 2D nanoflakes, which greatly reduces bulk contribution and enhances the topological surface properties. By the use of Raman spectroscopy this work investigates the interaction of the crystal lattice with the electronic degrees of freedom in the ultrathin limit. Hereby, the investigation of the phonon’s frequency and line shape is used to identify significant electron-phonon interactions. By investigating the 2D nanoflakes’ Raman response under the influence of different external parameters like low temperatures, strong magnetic fields, and the interface to a gold substrate, the changes in electron-phonon coupling are identified. These are used to deduct information on the nature and manipulation of the topological band structure in the ultrathin flakes.

In detail, high resolution temperature dependent Raman measurements in the range between 300K to 3K reveal for Bi2Se3 additional phonon self-energy corrections at low temperatures caused by interactions with an electronic susceptibility in the energy range of the phonons. This electronic susceptibility can be related to the quasi-relativistic electrons and shows the high contribution of topological surface states in the studied nanoflakes. The topological nature of the coupling electrons is confirmed in magnetic field dependent Raman measurements. The application of magnetic fields above 3T reveals further changes in the Bi2Se3 phonon’s self-energy that are shown to origin from the manipulation of the electronic susceptibility due to a gap opening in the topological surface states. For potential applications of Bi2Se3 and Bi2Te3 in devices, the influence of a gold-interface on the electronic surface band structure is studied. By investigating the Raman response of single nanoflakes on gold as a function of flake thickness carrier injection from the gold into the contacted surface is identified. Our study confirms the Au-interface induced downward band bending in Bi2Se3 and Bi2Te3 and demonstrates high contributions of trivial bulk electrons for nanoflakes below 10nm and 12 nm, respectively. The obtained results are finally discussed with respect to possible thickness limitations in accessing the topological transport regime.