Colin Howard

Electron-Phonon Coupling on the Surface of the Topological Insulator Bi2Se3 Determined from Surface-Phonon Dispersion Measurements

In this Letter, we report measurements of the coupling between Dirac fermion quasiparticles (DFQs) and phonons on the (001) surface of the strong topological insulator Bi2Se3. While most contemporary investigations of this coupling have involved examining the temperature dependence of the DFQ self-energy via angle-resolved photoemission spectroscopy measurements, we employ inelastic helium-atom scattering to explore, for the first time, this coupling from the phonon perspective. Using a Hilbert transform, we are able to obtain the imaginary part of the phonon self-energy associated with a dispersive surface-phonon branch identified in our previous work [Phys. Rev. Lett. 107, 186102 (2011)] as having strong interactions with the DFQs. From this imaginary part of the self-energy we obtain a branch-specific electron-phonon coupling constant of 0.43, which is stronger than the values reported from the angle-resolved photoemission spectroscopy measurements.

Interaction of phonons and dirac fermions on the surface of Bi2Se3: a strong Kohn anomaly.

We report the first measurements of phonon dispersion curves on the (001) surface of the strong three-dimensional topological insulator Bi2Se3. The surface phonon measurements were carried out with the aid of coherent helium beam surface scattering techniques. The results reveal a prominent signature of the exotic metallic Dirac fermion quasiparticles, including a strong Kohn anomaly. The signature is manifest in a low energy isotropic convex dispersive surface phonon branch with a frequency maximum of 1.8 THz and having a V-shaped minimum at approximately 2kF that defines the Kohn anomaly. Theoretical analysis attributes this dispersive profile to the renormalization of the surface phonon excitations by the surface Dirac fermions. The contribution of the Dirac fermions to this renormalization is derived in terms of a Coulomb-type perturbation model.

Helium Atom-Surface Scattering (HASS)

This chapter will focus on the HASS technique and its advantages over other surface probes. From there I will move on to describing the manner in which the atom–surface interaction is modeled. Lastly, I will describe the scattering processes that can occur and what information they carry about the structure and vibrational character of the surface.

HASS Results from the Surface of Bi2Se3 and Bi2Te3

This chapter contains the main experimental data presented in this thesis. I begin by presenting the elastic diffraction results that I used to determine the crystallographic orientation of the surface. From there, I move on to the measured inelastic data along with calculated surface phonon dispersion curves. Finally, I end this chapter with a calculation of the mode-specific electron–phonon coupling parameter λ ν (q) based upon the experimental results.

Experimental Apparatus and Technique

The main experimental apparatus of the Laboratory for Surface Physics and Electron Spectroscopies at Boston University consists of a series of HV and UHV chambers equipped with HASS capabilities and diagnostic tools including a LEED unit. An aerial schematic of the entire setup can be seen in Fig. 4.1. In the proceeding sections I will detail the different parts of the apparatus and their function. From there, I will present the sample preparation methodology and measurement techniques for both inelastic and elastic scattering.

Properties of Bi 2 Se 3 and Bi 2 Te 3

In this chapter I will present the fundamentals of the studied systems. I will begin by identifying the crystal structure of each system as well as their point and space group symmetries. From there I will move on to a review of measurements of the bulk vibrational structure using Raman, IR, and inelastic neutron scattering spectroscopies. Additionally, I will present recent ARPES measurements of the surface electronic structure that clearly indicate the presence of chiral DFQs, whose interaction with phonons is the main topic of this dissertation.

Pseudocharge Phonon Model

In order to identify the character and symmetry of the measured phonon events to be presented later, I employ empirical lattice dynamics calculations, based on the pseudocharge model (PCM) [1–3]. This model includes direct ion–ion interactions as well as indirect adiabatic coupling through the mediating electrons. Historically this model has been successful in reproducing surface phonon dispersion and HASS scattering amplitudes, derived from calculated surface charge deformations. Here I review some of the model’s basic characteristics and tabulate the parameters used in our realization of the model.

Conclusion and Future Directions

Having presented the totality of my research and findings, I will use this chapter to conclude my dissertation. First, I will present a brief summary of my work and discuss its implications. Then I will move on to discuss the future research direction for the Boston University Surface Laboratory.

Translating Between Electron and Phonon Perspectives

Having analyzed and quantified the DFQ–phonon interaction from the phonon perspective, I will now turn to the electron perspective. In this chapter I will show that the interaction Hamiltonian in Eq. ( 6.7) modifies the DFQ energies and lifetimes much like we have already seen in the case of the surface phonons. The matrix elements gq, ν of the interaction Hamiltonian along with the dressed phonon propagator, both of which have already been determined from the phonon data, can be used as input to a Matsubara Green function formalism to calculate the modifications to the electron dispersion.