Surface electronic structure of the topological Kondo insulator candidate correlated electron system SmB6 (1312.1979v2)

Abstract: The Kondo insulator SmB6 has long been known to exhibit low temperature transport anomalies whose origin is of great interest. Here we uniquely access the surface electronic structure of the anomalous transport regime by combining state-of-the-art laser- and synchrotron-based angle-resolved photoemission techniques. We observe clear in-gap states (up to 4 meV), whose temperature dependence is contingent upon the Kondo gap formation. In addition, our observed in-gap Fermi surface oddness tied with the Kramers' points topology, their coexistence with the two-dimensional transport anomaly in the Kondo hybridization regime, as well as their robustness against thermal recycling, taken together, collectively provide by-far the strongest evidence for protected surface metallicity with a Fermi surface whose topology is consistent with the theoretically predicted topological surface Fermi surface (TSS). Our observations of systematic surface electronic structure provide the fundamental electronic parameters for the anomalous Kondo ground state of the correlated electron material SmB6.

• The paper demonstrates in-gap states emerging up to 4 meV at low temperatures, reinforcing SmB₆’s robust topological phase.
• It maps an odd number of Fermi pockets around Kramers' points via ARPES, aligning with Z₂ topological insulating predictions.
• The study reveals 2D transport anomalies with resistivity saturation near 6 K, indicating a distinct surface contribution to conductivity.

Surface Electronic Structure in Topological Kondo Insulators: The Case of SmB$_6$

The paper "Surface electronic structure of the topological Kondo insulator candidate correlated electron system SmB$_6$" presents a detailed investigation of the surface electronic properties of the compound samarium hexaboride (SmB$_6$) using advanced angle-resolved photoemission spectroscopy (ARPES) techniques. The paper uncovers significant insights into the electronic structure that emerge when SmB$_6$ transitions into a topological Kondo insulator (TKI) regime, an area of substantial interest in condensed matter physics.

Key Findings

The salient features of the research can be summarized as follows:

1. Observation of In-gap States: The authors report the identification of in-gap electronic states appearing up to approximately 4 meV within the Kondo hybridization gap. These states exhibit temperature-dependent behavior, becoming more pronounced at low temperatures (around 6 K) and vanishing as the temperature increases toward the onset of Kondo coherence near 30 K.
2. Fermi Surface and Topology: The measured Fermi surface mapping reveals an odd number of Fermi pockets, encircling three of the four Kramers' points in the surface Brillouin zone. This characteristic is consistent with the predictions for a Z$_2$ TKI phase.
3. Temperature-dependent Robustness: The in-gap states demonstrate robustness against thermal cycling, reappearing consistently when the temperature returns to low values, thereby supporting their topological nature rather than resulting from non-reproducible surface states.
4. Dimensional Transport Anomalies: At low temperatures, the resistivity saturates, indicative of two-dimensional (2D) transport characteristics. This behavior aligns with the presence of surface metallic states within the Kondo insulating gap, suggesting a distinct surface contribution to the conductivity.

Theoretical and Practical Implications

The paper contributes to the broader understanding of topological matter by providing the most compelling evidence thus far for the existence of a Z$_2$ topological phase in SmB$_6$. The research illustrates how low-energy states within the Kondo gap can be probed using ARPES to reveal non-trivial topological properties, emphasizing the relevance of in-gap states in defining surface conductivity and potential device applications.

From a theoretical perspective, the paper reinforces the significance of spin-orbit coupling in materials with inversion symmetry, such as SmB$_6$, where it may induce band inversion necessary for topological insulating behavior. The observed Fermi pockets enclose an odd number of Kramers' points, providing further empirical support for the theoretical models predicting non-trivial topology in Kondo insulators.

Prospective Developments

This paper lays the groundwork for investigating other rare-earth compounds that might exhibit similar topological insulating properties. Moreover, fostering the development of materials with coexisting strong correlation effects and topological order opens avenues for exploring novel phases of quantum matter.

Future research could focus on epitaxial growth of similar TKIs on superconducting substrates, with the aim of leveraging the proximity effect to induce unconventional superconductivity. This approach could test the interplay between electron correlations inherent in Kondo systems and the topological order, potentially leading to new quantum phases of matter.

In conclusion, the thorough examination of the surface electronic structure of SmB$_6$ using state-of-the-art ARPES techniques constitutes an essential step toward uncovering the complex physics underlying TKIs. The findings provide a pivotal reference point for future work on the fascinating intersection of topological matter and strongly correlated electron systems.