Observation of in-gap surface states in the Kondo insulator SmB6 by photoemission (1306.5664v1)

Abstract: Kondo insulators (KIs) are strongly correlated materials in which the interactions between 4f and conduction electrons lead to a hybridization gap opening at low temperature 1-2. SmB6 is a typical KI, but its resistivity does not diverge at low temperatures, which was attributed to some in-gap states 3-10. However after several decades of research, the nature and origin of the in-gap states remain unclear. Recent band calculation and transport measurements suggest that the in-gap states could actually be ascribed to topological surface states. SmB6 thus might be the first realization of topological Kondo insulator (TKI) 13, the strongly correlated version of topological insulator (TI) 11,12. Here by performing angle-resolved photoemission spectroscopy (ARPES), we directly observed several dispersive states within the hybridization gap of SmB6, which cross the Fermi level and show negligible kz dependence, indicative of their surface origin. Furthermore, the circular dichroism (CD) ARPES results of the in-gap states suggest the chirality of orbital momentum, and temperature dependent measurements have shown that the in-gap states vanish simultaneously with the hybridization gap around 150 K. These strongly suggest their possible topological origin.

• The paper reveals direct ARPES evidence of dispersive in-gap surface states in SmB6 that support a topological origin.
• It identifies specific α and γ surface pockets with negligible k-dependence, persisting until the hybridization gap closes around 150 K.
• The study bridges strong electron correlations with topological behavior, offering insights for future quantum materials research.

Observation of In-gap Surface States in the Kondo Insulator SmB6 by Photoemission

The paper of Kondo insulators has been of significant interest due to their unique electronic properties arising from strong electron correlations. SmB$_6$, a prototypical Kondo insulator, exhibits a hybridization gap via interactions between 4f and conduction electrons. However, contrary to typical insulating behavior, its resistivity saturates at low temperatures, suggesting the presence of in-gap states. Despite many investigations, these states' nature had remained largely speculative, often linked to localized bulk states. Recent theoretical advancements proposed that these in-gap states might be topological surface states, prompting a reevaluation of SmB$_6$ as a potential topological Kondo insulator (TKI).

This paper utilizes angle-resolved photoemission spectroscopy (ARPES) to examine the electronic states on the surface of SmB$_6$, revealing several dispersive states crossing the Fermi level within the hybridization gap, characterized by negligible k-dependence, indicative of their surface origin. The observed states' persistence up to a critical temperature, coinciding with the hybridization gap closure, alongside the circular dichroism (CD) ARPES results demonstrating orbital momentum chirality, supports the hypothesis of their topological origin. These findings provide a compelling argument that SmB$_6$ manifests topological surface states, substantiating its categorization as a TKI.

Key Results and Analysis

The ARPES measurements uncovered explicit surface states within the hybridization gap of SmB$_6$. Fermi surfaces, nearly oval-shaped, identified around the $\Gamma$ and X points, lacked discernible variations in their spectral weight across different photon energies, confirming their surface localization rather than bulk provenance. The existence of an $\alpha$ pocket, spanning approximately 24% of the surface Brillouin zone with a Fermi velocity of 0.24 eV Å$^-1$, and a $\gamma$ pocket around $\Gamma$, further corroborates the manifestation of surface-only states.

The temperature-dependent analysis demonstrated that these surface states, specifically $\alpha$ and $\gamma$, diminish circa 150 K, alongside the hybridization gap closure. Such behavior aligns with the expectations for topologically protected surface states within TKIs, which dissipate once the bulk system reverts to a metallic phase upon reaching higher temperatures. Moreover, the orbital chirality detected via CD-ARPES solidifies these states' topological characteristics, echoing observations in other known topological insulators.

Implications and Future Directions

The detection of topological surface states in SmB$_6$ elucidates both its residual conductivity at low temperatures and the broader understanding of spin-orbit interaction impacts in strongly correlated electron systems. This insight into the strong correlation topological aspect marries the fields of condensed matter physics and quantum materials, opening new avenues for exploring correlated electron phenomena in other potential TKIs.

From a practical perspective, these findings not only enhance the foundational understanding of Kondo insulators but also suggest technological pathways for utilizing TKIs' unique electronic properties. The combination of correlation-driven phenomena with topological protection could lead to novel quantum computing applications or enhanced energy-efficient electronic devices.

Future research should focus on enhancing the resolution of ARPES measurements to distinguish between potentially overlapping surface and bulk contributions with greater precision. Moreover, integrating spin-resolved ARPES and further theoretical simulations could provide deeper insight into these states' spin textures, fortifying the topological interpretation.

In conclusion, the paper provides a systematic investigation of SmB$_6$, setting a precedent for research into TKIs and demonstrating the intricate interplay between topological surface states and underlying strong electron correlations. This work signifies an essential step toward leveraging these quantum materials' potential for next-generation quantum technologies.