Session M39: Invited Session: SmB6: A Possible Topological Kondo Insulator

Heavy Fermions, Rise of the Topologies

The electrons in Heavy fermion materials are subject to spin-orbit coupling interactions that greatly exceed their Kinetic energy. It has long been known that the spin orbit coupling stablizes new kinds of heavy fermion metals, superconductors and ``Kondo insulators'' against the competing state of magnetism. In this talk I will discuss the new realization that spin orbit copling can influence the ground state, changing its topology and giving rise to Topological Kondo insulators. We'll look at samarium hexaboride, SmB6, ``the worlds oldest topological insulator,'' a Kondo insulator discovered 45 years ago, predicted to be topological in 2011, and tentatively confirmed to be so in a series of hot new experimental studies of the past few months. I'll discuss a simple model for a topological Kondo insulator and introduce the most recent measurements, including ARPES, de Haas van Alphen and weak antilocalization that appear to support the idea that this is a strongly interacting topological insulator in which the surface conductance is carried by electrons on spin-orbit coupled Dirac cones. We'll also discuss the open unanswered questions surrounding this topic.   

Topological surface state in the Kondo insulator Samarium Hexaboride

Topological invariants of electron wave functions in condensed matter reveal many intriguing phenomena. The most exotic one is the topological insulator (TI) characterized by the Z2 group where an insulating bulk coexists with a metallic boundary state. Possible novel quantum states supporting coherent qubits using Majorana fermions with their potential for technological application have led to intense research into Bi based TIs with large band gap. However, the main complication concerning these Bi based materials is their considerable residual conductivity in the bulk, and only experimental techniques distinguishing bulk and surface clearly such as ARPES or STM can be used to explore the surface properties of these materials properly. Theories predict that a Kondo insulator SmB6, which evolves from a Kondo lattice metal to an insulator with a small gap as the temperature is lowered, could be a topological insulator. Although the insulating bulk and metallic surface separation has been demonstrated in recent transport measurements, these were not able to prove that the metallic surface state is topologically protected. we report careful thickness dependence transport measurements on doped SmB6 which show that magnetic and non-magnetic dopants in SmB6 exhibit clearly contrasting behavior supporting that that SmB6 is the first perfect 3D topological insulator with virtually zero residual bulk conductivity. We anticipate our results to be a starting point to explore the details of topological Kondo insulators and their potential applications toward scalable quantum information processing.   

Topological phases in mix valence compounds

In this talk, I will propose that the mix valence phenomena in some of the rare earth compounds will naturally lead to non-trivial topology in band structure.One of the typical example is SmB6, where the intermediate valence of Sm generates band inversion at the X point and the non-trivial Z2 index. Other than SmB6, YbB6 and YbB12 are both mix valence compounds. By applying LDA$+$Gutzwiller to these materials, we find that YbB6 has non-trivial Z2 index, indicating that YbB6 is another three dimensional topological insulator with strong correlation effects. Our calculation also finds that YbB12 is a trivial insulator in the sense of Z2 but it can be classified as topological crystalline insulator with non-zero mirror Chern number. The electronic structure at finite temperature has also been studied using LDA$+$DMFT, indicating YbB6 is still in the mix valence region while YbB12 is quite close to the Kondo limit.   

Topological Kondo Insulator (TKI) and related candidate materials: High-resolution ARPES studies

In this talk, I plan to present ARPES (synchrotron and laser-based) studies of several mix valence and Kondo insulator phenomena in some of the rare earth heavy fermion compounds in connection to their non-trivial topology of band structures. Focus will be on SmB6 which has been predicted to be a TKI recently. By combining low-temperature and high energy-momentum resolution of the laser-based ARPES technique, for the first time, we accessed the surface electronic structure of the anomalous conductivity regime. At low T, we observe in-gap states within a 4 meV energy window of the Fermi level, which lie clearly within the bulk insulating gap. The in-gap states are found to be suppressed and eventually disappear, as the temperature is raised in approaching the coherent Kondo lattice hybridization (30 K), which proves that the in-gap states strongly depend on the existence of Kondo lattice hybridization and the effective Kondo gap, in agreement with their theoretical predicted origin of topological surface states within the Kondo insulating gap . Our Fermi mapping at the energy corresponding to these in-gap states shows distinct Fermi pockets that enclose the three Kramers' points the surface Brillouin zone, which are remarkably consistent with the theoretically predicted topological surface Fermi surface in the topological Kondo insulating phase within the level of energy resolution. The observed Fermi surface topology of the in-gap states, their temperature dependence across the transport anomaly and Kondo lattice hybridization temperatures, as well as their robustness against repeated thermal recycling, collectively not only provide a unique insight illuminating the nature of the residual conductivity anomaly but also serve as a strong experimental evidence to the predicted topological Kondo insulator phase. I also plan to present results on YbB6 and YbB12 both of which are mix valence compounds. This work is in collaboration with \textbf{Madhab Neupane}, N. Alidoust, S.-Y. Xu, T. Kondo, Y. Ishida, D.-J. Kim, Chang Liu, I. Belopolski, T.-R. Chang, H.-T. Jeng, T. Durakiewicz, L. Balicas, H. Lin, A. Bansil, S. Shin and Z. Fisk and primarily supported by U.S. DOE and Princeton University.   

Hybridization Gap, Metallic Surface States and Quantum Transport in SmB$_6$

Topological insulators, with metallic boundary states protected against time-reversal-invariant perturbations, are a promising avenue for realizing exotic quantum states of matter. According to recent theoretical predictions, a topological insulating state can emerge not only from a weakly interacting system with strong spin-orbit coupling, but also in insulators driven by strong electron correlations. The Kondo insulator compound SmB$_6$ is an ideal candidate for realizing this exotic state of matter, with hybridization between itinerant conduction electrons and localized $f$-electrons driving an insulating gap that facilitates the emergence of topological surface states at low temperatures. In this talk I will discuss our point-contact spectroscopy studies of the bulk hybridization gap of SmB$_6$ and its relation to purported metallic surface states. I will also present milliKelvin magnetotransport studies that reveal both weak antilocalization and quantized conductance phenomena that provide strong evidence for topologically non-trivial surface states in SmB$_6$.