# Metadata **Title:** Photoelectron spin-flipping and texture manipulation in a topological insulator **Authors:** Jozwiak, Chris; Park, Cheol-Hwan; Gotlieb, Kenneth; Hwang, Choongyu; Lee, Dung-Hai; Louie, Steven G.; Denlinger, Jonathan D.; Rotundu, Costel R.; Birgeneau, Robert J.; Hussain, Zahid; Lanzara, Alessandra **Year:** 2013 **Tags:** #TI #ARPES [URL](http://dx.doi.org/10.1038/nphys2572)|[Bookends](bookends://sonnysoftware.com/3629) --- # Conclusions * The [[Physics/Spin]] polarization of the [[photoemission|photoelectrons]] resulting from probing [[Topological Surface State|topological surface states]] can be manipulated in 3D by selecting the light polarization. This is due to [[Physics/Spin]]-dependent interaction of the [[Spin-Momentum Locking|helical]] TSS electrons with the light, an effect originating from strong [[spin-orbit coupling]]. The theory was developed in [[Park2012]]. * The results in this work observe situation where the spin polarization of the photo electrons is completely different from that of the originating original state. * For Au (111), the spin polarization of the photoelectrons is far less dependent on the light polarization, showing that the effects seen in TIs are not generic or trivial experimental artifacts. * Using p-polarization and probing the $\overline{M}$ point of e.g. Bi$_2$Te$_3$ ([[Bi2Te3]]), you will see the same spin-polarization of photoelectrons as the TSS. If you use s-polarization, you will get the opposite. If you probe the $\overline{K}$ point, the situation flips, s-polarization gives you the same photoelectron spin polarization as the TSS, p-polarization gives you the opposite. This is the most important conclusion of this paper. See [[#Figure 4]] for a summary. # Relevant Facts * Electrons in TSS are spin polarized with their spins governed by their momentum (i.e. [[spin-momentum locking]]), leading to a helical spin texture in momentum space. * Spin-ARPES is the only tool capable of directly observing this feature with simultaneous energy, momentum and spin sensitivity. * It has been a common assumption that electron spin is conserved in the photoemission process. # Discussion of Figures ## Figure 1 ![[Pasted image 20210118143501.png]] * In subfigures (a) and (b), we see the [[ARPES]] intensity map of Bi$_2$Se$_3$ ([[Bi2Se3]]), with the white arrows representing the helical [[spin texture|spin texture]] of the [[Topological Surface State|TSS]]. The $k_x$ direction is along the $\Gamma M$ direction. The circled electron spin here is at the k-vector $\pmb{k} = (-k_F, 0)$ that we will be discussing the EDCs for. * In subfigures (c) and (d) we see the energy distribution curves (EDCs) showing the spin polarization of the circled electron. The inset here shows the direction of the probed [[photoemission|photoelectron]] spin based on the EDCs.We note that for the $p$ polarization, the photoelectron has the same spin polarization as the TSS, while for the s-polarization it is opposite. * (e) shows the experimental setup * (f) shows what happens to the spin polarization when you rotate the polarization angle. Black line is fit based on the theory. ## Figure 2 ![[Pasted image 20210120130813.png]] * (a) and (e) show a schematic of the [[surface state|surface states]] of Bi$_2$Se$_3$ and Au (111). * (b) and (f) show spin-integrated ARPES intensity map. * (c), (g) show spin-ARPES results for both materials with p-polarization. The results are in agreement with the spin helicity of the surface states. * (d) and (h) show the same but using s-polarization. For Bi$_2$Se$_3$, the spin of the photoelectron is opposite to that of the TSS (and thus the p-polarization). For gold, however, there is no dependence of the spin of the photoelectron on the light polarization, showing this is a non-trivial effect. ## Figure 3 ![[Pasted image 20210118145524.png]] * (c) and (d) show the spin-polarization of the photoelectron when using LHCP and RHCP light. ## Figure 4 ![[Pasted image 20210118145849.png]] This is the most important figure in the paper. * (a) Spin of the TSS before photoemission * (b-g) Schematics of the spin of the photoelectrons using several polarizations