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SUMMARY:Electronic Structure Manipulation of Topological Materials Probed 
 by Angle-Resolved Photoemission
DTSTART:20260605T120000Z
DTEND:20260605T150000Z
DTSTAMP:20260604T024600Z
UID:indico-event-9722@indico.fysik.su.se
DESCRIPTION:Speakers: Wanyu Chen (KTH Applied Physics)\n\nOpponent: Profe
 ssor Kevin Smith\,\nSupervisor: Oscar Tjernberg\, Nordic Institute for Th
 eoretical Physics NORDITA\, Ljus och materiens fysik\nAbstract\nTopologica
 l quantum materials host electronic states protected by topology and symme
 try\, giving rise to robust surface or edge states and unconventional elec
 tronic properties. Understanding how their electronic band structures resp
 ond to external perturbations is essential for both fundamental physics an
 d potential applications. This thesis investigates the electronic structur
 e of topological materials and its evolution under controlled perturbation
 s by combining static angle-resolved photoemission spectroscopy (ARPES) an
 d time-resolved ARPES (tr-ARPES).\nFirst\, chemical doping is an effective
  method for manipulating the electronic structure of materials. Cl-doped B
 i2Se3 is systematically studied here. Chlorine incorporation acts as an e
 ffective electron donor\, shifting the Fermi level while preserving the to
 pological band structure and reinforcing the intrinsic n-type character th
 rough heterovalent substitution at Se sites. The time evolution of the ban
 d structure reveals a strongly reduced energy shift under prolonged extrem
 e ultraviolet radiation (XUV) exposure compared to pristine Bi2Se3\, indic
 ating suppressed adsorption-induced band bending and a modified near-surfa
 ce defect landscape. Together\, these results demonstrate that chemical do
 ping can simultaneously tune the bulk carrier density and stabilize the su
 rface electronic environment\, providing a controlled strategy to engineer
  the electronic structure of topological insulators without compromising t
 heir topological character.\nSecond\, optical excitation with femtosecond 
 laser pulses is employed as a route to investigate topological phases in t
 he topological crystalline insulator (TCI) Pb1-xSnxSe. By combining time-r
 esolved ARPES and static X-ray diffraction\, we demonstrate that the latti
 ce constant serves as the fundamental control parameter of the normal-insu
 lator-to-topological-crystalline-insulator transition. tr-ARPES measuremen
 ts reveal that optical excitation\, generating electronic temperatures far
  above the topological-to-normal transition temperature Tc\, unexpectedly
  drives the system deeper into the TCI phase. Analysis of the transient el
 ectronic structure shows that the excitation induces an ultrafast lattice 
 contraction on a sub-picosecond timescale. This contraction originates fro
 m an electronically induced strengthening of bonds in the inverted band-ga
 p regime. These results show that the TCI phase is robust against optical 
 excitation.\nFinally\, a spatially structured optical pump\, realized usin
 g a transient optical grating geometry\, is combined with tr-ARPES to inve
 stigate ultrafast dynamics in quasi-free-standing bilayer graphene. Althou
 gh this approach enables spatially and temporally modulated excitation\, e
 fficient strain-wave generation requires high pump fluence\, leading to en
 hanced space-charge and surface photovoltage effects. These effects distor
 t photoelectron trajectories and shift measured energies\, thereby limitin
 g the sensitivity to subtle band-structure changes. This highlights import
 ant experimental constraints in high-fluence ultrafast photoemission measu
 rements.\nFrom 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 pul
 se fluence for a given laser power\, providing greater flexibility for dif
 ferent experimental conditions. In parallel\, we employ ultrafast laser-as
 sisted scribing to guide the cleavage process along a desired crystallogra
 phic plane\, which enhances the reliability and reproducibility of sample 
 preparation. In addition\, comprehensive data processing and electron-opti
 cs-assisted conversion of raw data from a time-of-flight photoelectron ana
 lyzer are implemented to reconstruct the electronic structure in energy–
 momentum space.\nOverall\, this thesis demonstrates how static and time-re
 solved ARPES can be used to probe the electronic structure of topological 
 materials and their evolution under controlled perturbations\, highlightin
 g general strategies for tuning electronic states as well as key experimen
 tal challenges in exploring the dynamic properties of quantum materials.\n
 \nhttps://indico.fysik.su.se/event/9722/
LOCATION:Albano 3: 4204 - SU Conference Room (56 seats) (Albano Building 3
 )
URL:https://indico.fysik.su.se/event/9722/
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