[Paper Review] High spin polarization and circular dichroism of topological surface states on Bi2Te3

M. R. Scholz, J. Sánchez‐Barriga+5 more|arXiv (Cornell University)|Aug 4, 2011

Topological Materials and Phenomena3 citations

TL;DR

This study demonstrates that the topological surface state (TSS) of Bi2Te3 exhibits exceptionally high spin polarization—up to 80% at the Fermi level—far exceeding previous measurements. Using spin- and angle-resolved photoemission spectroscopy combined with circular dichroism in photoelectron angular distribution (CDAD), the authors confirm that the TSS maintains strong spin polarization even when hybridized with bulk states, and that the observed CDAD asymmetry of ~20% directly correlates with the spin orientation, indicating a robust spin-momentum locking characteristic of topological insulators.

ABSTRACT

Topological insulators have been successfully identified by spin-resolved photoemission but the spin polarization remained low (~20%). We show for Bi2Te3 that the in-gap surface state is much closer to full spin polarization with measured values reaching 80% at the Fermi level. When hybridizing with the bulk it remains highly spin polarized which may explain recent unusual quantum interference results on Bi2Se3. The topological surface state shows a large circular dichroism in the photoelectron angle distribution with an asymmetry of ~20% the sign of which corresponds to that of the measured spin.

Motivation & Objective

• To determine whether the low spin polarization (~20%) previously reported in Bi2Te3's TSS is due to intrinsic limitations or extrinsic experimental factors.
• To investigate the spin polarization of the TSS in Bi2Te3 under high-quality cleaving conditions to minimize surface disorder and extrinsic effects.
• To examine the role of hybridization between topological surface states and bulk states in modifying spin polarization.
• To correlate circular dichroism in photoelectron angular distributions (CDAD) with the intrinsic spin texture of the TSS.
• To assess whether CDAD can serve as a high-throughput, spin-structure-probing alternative to spin-resolved photoemission.

Proposed method

• Single crystals of Bi2Te3 were grown via the Bridgman method and cleaved ex situ to expose pristine, high-quality surfaces.
• Spin- and angle-resolved photoemission spectroscopy (ARPES) was used to measure the spin polarization of the TSS across the Fermi level and Dirac point.
• Circular dichroism in the angular distribution of photoelectrons (CDAD) was measured using circularly polarized light (σ+ and σ−) to probe spin-dependent photoemission intensity asymmetries.
• The CDAD asymmetry was calculated as (Iσ+ − Iσ−)/(Iσ+ + Iσ−) to quantify the spin-dependent angular distribution.
• The spin texture and CDAD were correlated to confirm that the dichroism signal reflects the intrinsic spin polarization of the TSS.
• Theoretical analysis of matrix elements and spin-orbit coupling effects was used to interpret the origin of the large CDAD signal.

Experimental results

Research questions

• RQ1 Is the previously reported low spin polarization in Bi2Te3's TSS due to experimental artifacts or intrinsic limitations?
• RQ2 To what extent does hybridization with bulk states affect the spin polarization of the topological surface state?
• RQ3 Can circular dichroism in photoelectron angular distributions (CDAD) serve as a reliable, high-throughput proxy for spin polarization in topological insulators?
• RQ4 Does the observed CDAD asymmetry of ~20% correlate directly with the spin texture of the TSS, as predicted by theory?
• RQ5 What is the role of spin-orbit coupling in generating the large CDAD signal in Bi2Te3?

Key findings

• The spin polarization of the topological surface state in Bi2Te3 reaches up to 80% at the Fermi level, significantly higher than the previously reported ~20%.
• The spin polarization reverses between ±k∥, confirming the Kramers-degenerate, spin-momentum locked nature of the TSS.
• Even after hybridization with bulk states below the Dirac point, the TSS maintains high spin polarization, indicating robust topological protection.
• A circular dichroism asymmetry of ~20% is observed in the photoelectron angular distribution, with the sign matching the spin polarization direction.
• The CDAD signal remains robust across the Dirac point and correlates precisely with the spin texture, indicating that CDAD can indirectly probe spin structure.
• The CDAD method offers count rates 2–3 orders of magnitude higher than spin-resolved photoemission, making it a viable alternative for high-throughput spin texture mapping.