Electronic Structure Manipulation of Topological Materials Probed by Angle-Resolved Photoemission

by Wanyu Chen (KTH Applied Physics)

Friday 5 Jun 2026, 14:00 → 17:00 Europe/Stockholm

Albano 3: 4204 - SU Conference Room (56 seats) (Albano Building 3)

Opponent: Professor Kevin Smith,

Supervisor: Oscar Tjernberg, Nordic Institute for Theoretical Physics NORDITA, Ljus och materiens fysik

Abstract

Topological quantum materials host electronic states protected by topology and symmetry, giving rise to robust surface or edge states and unconventional electronic properties. Understanding how their electronic band structures respond to external perturbations is essential for both fundamental physics and potential applications. This thesis investigates the electronic structure of topological materials and its evolution under controlled perturbations by combining static angle-resolved photoemission spectroscopy (ARPES) and time-resolved ARPES (tr-ARPES).

First, chemical doping is an effective method for manipulating the electronic structure of materials. Cl-doped Bi₂Se₃ is systematically studied here. Chlorine incorporation acts as an effective electron donor, shifting the Fermi level while preserving the topological band structure and reinforcing the intrinsic n-type character through heterovalent substitution at Se sites. The time evolution of the band structure reveals a strongly reduced energy shift under prolonged extreme ultraviolet radiation (XUV) exposure compared to pristine Bi₂Se₃, indicating suppressed adsorption-induced band bending and a modified near-surface defect landscape. Together, these results demonstrate that chemical doping can simultaneously tune the bulk carrier density and stabilize the surface electronic environment, providing a controlled strategy to engineer the electronic structure of topological insulators without compromising their topological character.

Second, optical excitation with femtosecond laser pulses is employed as a route to investigate topological phases in the topological crystalline insulator (TCI) Pb_1-xSn_xSe. By combining time-resolved ARPES and static X-ray diffraction, we demonstrate that the lattice constant serves as the fundamental control parameter of the normal-insulator-to-topological-crystalline-insulator transition. tr-ARPES measurements reveal that optical excitation, generating electronic temperatures far above the topological-to-normal transition temperature T_c, unexpectedly drives the system deeper into the TCI phase. Analysis of the transient electronic structure shows that the excitation induces an ultrafast lattice contraction on a sub-picosecond timescale. This contraction originates from an electronically induced strengthening of bonds in the inverted band-gap regime. These results show that the TCI phase is robust against optical excitation.

Finally, a spatially structured optical pump, realized using a transient optical grating geometry, is combined with tr-ARPES to investigate ultrafast dynamics in quasi-free-standing bilayer graphene. Although this approach enables spatially and temporally modulated excitation, efficient strain-wave generation requires high pump fluence, leading to enhanced space-charge and surface photovoltage effects. These effects distort photoelectron trajectories and shift measured energies, thereby limiting the sensitivity to subtle band-structure changes. This highlights important experimental constraints in high-fluence ultrafast photoemission measurements.

From a methodological perspective, we optimized the spot sizes of both the pump and probe beams in the tr-ARPES setup, thereby improving the spatial resolution and reducing the influence of sample inhomogeneity. A smaller spot size also increases the achievable upper limit of the pulse fluence for a given laser power, providing greater flexibility for different experimental conditions. In parallel, we employ ultrafast laser-assisted scribing to guide the cleavage process along a desired crystallographic plane, which enhances the reliability and reproducibility of sample preparation. In addition, comprehensive data processing and electron-optics-assisted conversion of raw data from a time-of-flight photoelectron analyzer are implemented to reconstruct the electronic structure in energy–momentum space.

Overall, this thesis demonstrates how static and time-resolved ARPES can be used to probe the electronic structure of topological materials and their evolution under controlled perturbations, highlighting general strategies for tuning electronic states as well as key experimental challenges in exploring the dynamic properties of quantum materials.