Topological Surface States Protected From Backscattering by Chiral Spin Texture

Abstract: Topological insulators are a new class of insulators in which a bulk gap for electronic excitations is generated by strong spin orbit coupling. These novel materials are distinguished from ordinary insulators by the presence of gapless metallic boundary states, akin to the chiral edge modes in quantum Hall systems, but with unconventional spin textures. Recently, experiments and theoretical efforts have provided strong evidence for both two- and three-dimensional topological insulators and their novel edge and surface states in semiconductor quantum well structures and several Bi-based compounds. A key characteristic of these spin-textured boundary states is their insensitivity to spin-independent scattering, which protects them from backscattering and localization. These chiral states are potentially useful for spin-based electronics, in which long spin coherence is critical, and also for quantum computing applications, where topological protection can enable fault-tolerant information processing. Here we use a scanning tunneling microscope (STM) to visualize the gapless surface states of the three-dimensional topological insulator BiSb and to examine their scattering behavior from disorder caused by random alloying in this compound. Combining STM and angle-resolved photoemission spectroscopy, we show that despite strong atomic scale disorder, backscattering between states of opposite momentum and opposite spin is absent. Our observation of spin-selective scattering demonstrates that the chiral nature of these states protects the spin of the carriers; they therefore have the potential to be used for coherent spin transport in spintronic devices.

• The paper demonstrates that chiral spin texture in Bi₁₋ₓSbₓ crystals robustly protects topological surface states from backscattering.
• It employs cryogenic STM and FT-STS along with ARPES to visualize electron interference, revealing a 95% correlation between QPI and spin scattering probability.
• The findings highlight promising applications in spintronics and fault-tolerant quantum computing through enhanced spin coherence.

Advancements in Observing Topological Surface States Using STM

This paper presents a detailed analysis of the topological surface states (TSS) in Bi$_{1-x}$Sb$_x$ single crystals, focusing on the protection against backscattering provided by the chiral spin texture. Utilizing an advanced combination of scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES), the study investigates the remarkable resilience of these surface states to spin-independent scattering, even amidst significant atomic-scale disorder induced by random alloying.

Key Findings

The experimental results demonstrate several important features:

1. Chiral Spin Texture: The surface states in Bi$_{1-x}$Sb$_x$ exhibit unique chiral spin arrangements, distinct from ordinary surface states with spin-orbit coupling. This spin structure ensures that backscattering, typically associated with Anderson localization, is suppressed.
2. Experimental Techniques and Observations: The researchers used STM operating at cryogenic temperatures to visualize the surface states and assess their scattering properties. The long-wavelength modulations in the local density of states across the sample, identified through STM topographies and tunneling spectroscopy, are dominated by the topological states.
3. Spin-Selective Scattering: Through Fourier transform scanning tunneling spectroscopy (FT-STS), the study highlights modulation in conductance corresponding to the interference patterns due to the surface state electrons. Cross-correlation between quasi-particle interference (QPI) patterns and ARPES-derived joint density of states (JDOS) and spin scattering probability (SSP) reveals that conventional backscattering is notably absent.
4. Correlation Analysis: A quantitative comparative analysis showed a 95% correlation between QPI and SSP, whereas the correlation with JDOS was 83%. This confirms that spin-dependent scattering matrix elements significantly account for the observed high-symmetry suppression of backscattering.
5. Scattering Matrix Element: The study postulates a spin-dependent scattering matrix element $T(q,k) = S(k) \cdot S(k + q)$, which effectively nullifies scattering between states of opposite spin alignments. This model aligns closely with the experimental data, providing clarity on how spin cohesion preserves the integrity of these states.

Implications and Future Directions

The high fidelity between experimental observations and theoretical predictions underscore the potential of using these surface states for applications requiring high spin coherence, such as spintronic devices and topological quantum computing.

Practical developments could explore incorporating magnetic scattering centers in future studies, aiming to manipulate spin-coupled states in topological insulators. Additionally, assessing how TSS behavior scales with increased disorder could yield further insights relevant to device fabrication and stability.

On a theoretical level, these findings may invigorate research into other Bi-based topological insulators or broader classes where robust spin-polarized states could provide avenues for exploiting quantum phenomena. The observed spin-momentum locking may also drive innovations in fault-tolerant quantum information processing systems.

In conclusion, this research explores a pivotal intersection of STM imaging and topological quantum phenomena, revealing salient features about spin-textured electronic states and offering a rigorous pathway for future explorations in enhanced spintronics and quantum computation using topological insulators.