Session D14: Invited Session: Bulk and Surface Spectroscopy of Mixed Valence Topological Insulator Samarium Hexaboride

Quantum Oscillations in Kondo Insulator SmB$_6$

In Kondo insulator samarium hexaboride SmB$_6$, strong correlation and band hybridization lead to a diverging resistance at low temperature. The resistance divergence ends at about 3 Kelvin, a behavior recently demonstrated to arise from the surface conductance. However, questions remain whether and where a topological surface state exists. Quantum oscillations have not been observed to map the Fermi surface. We solve the problem by resolving the Landau Level quantization and Fermi surface topology using torque magnetometry. The observed angular dependence of the Fermi surface cross section suggests two-dimensional surface states on the (101) and (100) plane. Furthermore, similar to the quantum Hall states for graphene, the tracking of the Landau Levels in the infinite magnetic field limit points to -1/2, the Berry phase contribution from the 2D Dirac electronic state.   

Spin- and angle-resolved photoemission on the first topological Kondo Insulator: SmB$_{6}$

The concept of a topological Kondo insulator (TKI) has been brought forward as a new class of topological insulators in which non-trivial surface states appear in the bulk Kondo band gap at low temperatures due to the strong spin-orbit coupling. Theoretical considerations have shown that SmB$_{6}$ is a promising candidate for the first realization of TKIs. In this contribution we will present comprehensive studies of the bulk and surface electronic structures of SmB$_{6}$. Using high-resolution angle-resolved photoemission spectroscopy (ARPES) we revealed that the two dimensional surface states reside within a bulk Kondo gap and form three Fermi surfaces in the surface Brillouin zone [1]. The odd number of surface bands crossing the Fermi level fulfills the necessary condition of topologically nontrivial surface states and is in good agreement with the theoretical prediction. Applying spin-resolved ARPES to SmB$_{6}$, we show that the energy bands of the surface states around the X bar points are spin-polarized. The spins of the surface states are locked to crystal momentum and the spin-helical structure fulfills the requirement of time-reversal symmetry [2]. Our results prove that SmB$_{6}$ is the first realization of strongly correlated topological Kondo insulator. We will also show the evolution of the bulk electronic structure from a metallic state at high temperatures to a Kondo insulating phase, and how the non-trivial states appear in the system [3]. \\[4pt] [1] N. Xu \textit{et al}., Surface and bulk electronic structure of the strongly correlated system SmB$_{6}$ and implications for a topological Kondo insulator, Phys. Rev. B \textbf{88}, 121102 (rapid, editor's suggestion) (2013)\\[0pt] [2] N. Xu \textit{et al}., Direct observation of the spin texture in SmB$_{6}$ as evidence of the topological Kondo insulator, Nature Comms. \textbf{5}, 4566 (2014).\\[0pt] [3] N. Xu \textit{et al}., Exotic Kondo cross-over in a wide tempersture region in the toplogical Kondo insulator SmB$_{6}$ revealed by high-resolution ARPES, \quad Phys. Rev. B \textbf{90}, 085148 (2014)   

Interaction-driven sub-gap resonance in the topological Kondo insulator SmB6

Samarium hexaboride (SmB6) is a strongly correlated Kondo Insulator with a non-trivial band-structure topology. I will discuss recent neutron scattering experiments and analysis that expose a 14 meV resonant mode in SmB6 and relate it to the low energy insulating band structure. Repeating outside the first Brillouin zone, the mode is coherent with a 5$d$-like magnetic form factor. I will discuss how band inversion can be inferred from neutron scattering and show that a perturbative slave boson treatment of a hybridized 2 species ($d$/$f$) band structure within an Anderson model can produce a spin exciton with the observed characteristics. This analysis provides a detailed physical picture of how the SmB6 band topology arises from strong electron interactions, and accounts for the 14 meV resonant mode as a magnetically active exciton.   

Surface structure of SmB$_6$ investigated by STM and HAXPES

The intermediate-valence compound SmB$_6$ is typically considered a ``Kondo insulator'' albeit the concept of the Kondo effect does, in principle, not hold for an intermediate-valence material. Nonetheless, the hybridization between conduction electrons and the strongly interacting Sm $f$-electrons results in a gap at the Fermi energy and hence, an insulating ground state arises at temperatures below about 40 K. Recently, SmB$_6$ has become of enormous topical interest because it is a candidate material for hosting topologically protected surface states. The intermediate valence of Sm in SmB$_6$ was confirmed by HAXPES measurements down to 5 K. Such measurements conducted at high photon energies for improved probing depth are of importance in view of valency limits given for strong topological insulators [1]. Scanning tunneling microscopy (STM) has the unique capability of providing combined topographic and spectroscopic information. We conducted STM on numerous samples cleaved {\it in situ} at around 20 K [2]. Cleavage along the \{001\} plane of the cubic structure through breaking inter-octahedral B-B bonds gives rise to polar surfaces. In result, we found disordered chain-like as well as ordered ($2 \times 1$) surface reconstructions. Occasionally, we also observed patches of non-reconstructed surface areas of both, Sm and B termination. On such areas, we found indications for the Kondo effect being at play. Also, for non-reconstructed surface areas of some ten nanometers in size the d$I$/d$V$-curves can be well described by a Fano resonance. Thus, the hybridization picture typically considered for this material could be fully confirmed. All types of surfaces, reconstructed and non-reconstructed, displayed a finite zero-bias conductance of considerable magnitude. This finding, in spite of different surface topologies, confirms the robustness of the metallic states and is in line with the proposal of SmB$_6$ being a topological insulator.\\[4pt] [1] V. Alexandrov {\it et al.}, Phys. Rev. Lett. {\bf 111} (2013) 226403.\\[0pt] [2] S. R\"o\ss ler {\it et al.}, Proc. Natl. Acad. Sci. USA {\bf 111} (2014) 4798.   

Temperature dependence, termination dependence, and the spin chirality of the electronic structure of a mixed-valent topological insulator: SmB6

A great deal of recent effort has been paid to demonstrate the topological origin of a strongly correlated mixed-valent insulator SmB6. Despite extensive research, however, there is no consensus yet on the topological nature of the Kondo insulator SmB6. In this respect, the works reported so far need be reexamined carefully to see whether the reported results are relevant to the intrinsic property or not. For example, the termination-dependent physical properties of the surface in-gap states in SmB6 have hardly been examined seriously. Also it is important to investigate the spin chirality of the surface states in SmB6. Furthermore, if bulk SmB6 is really a Kondo insulator, its energy gap should have a many-body origin. Then the temperature-dependent evolution of electronic structure is expected in both the bulk and surface bands, which can be measured by ARPES. We have investigated the topological properties of SmB6, based on the dynamical mean-field theory (DMFT) calculations [1] and the density-functional theory (DFT) slab calculations [2], and compared them with those of a similar mixed-valent system of golden phase SmS and other hexaboride systems. \\[4pt] [1] J. D. Denlinger, J. W. Allen, J.-S. Kang, K. Sun, J.-W. Kim, J.H. Shim, B. I. Min, Dae-Jeong Kim, Z. Fisk, arXiv:1312.6637.\\[0pt] [2] Junwon Kim, Kyoo Kim, Chang-Jong Kang, Sooran Kim, Hong Chul Choi, J.-S. Kang, J. D. Denlinger, B. I. Min, Phys. Rev. B 90(7), 075131 (2014).