Nanotechnologies / Materials
Electronic materials, Energy materials

Control of the surface electronic structureof topological Kondo insulators
Department of Frontier Biosciences, Graduate School of Frontier Biosciences
Professor Shin-ichi Kimura
http://researchmap.jp/read0182671/?lang=en

Nanotechnologies / Materials

Abstract
Topological insulators (TIs) behave as insulators in bulk but havemetallic properties on their surfaces, which originate from the edge
state due to the different topology between the inside and outsideof materials. Since the surface state is protected by the topology, ithas been believed that the control of the topological surface state(TSS) is impossible by other methods. In this study, an anisotropicstructure with a symmetry different from that of the bulk was fabricated on the surface of samarium hexaboride (SmB6), a topologicalinsulator, in only a few atomic layers near the crystal surface. TheFermi surface observed by angle-resolved photoelectron spectroscopy suggests that the surface electronic state of TIs not only originates from the topology of the bulk electronic structure but also
reflects the state of the surface structure. This suggests the breakdown of the bulk-related in-plane rotation symmetry in the TSSs ofSmB6.

Fig. 1 a) Crystal structure of samarium hexaboride (SmB6) and the vicinal surface.
b) The electron diffraction pattern of the vicinal surface. The intensity difference
intensity between spots marked by solid and dashed lines reflects the anisotoropy.

▍ Background & Results
In the last decade, topological materials, which are classified according to the symmetry of their electronic state inside the crystal(bulk), have been the focus of intense research. Among these, TIs,
which were discovered at an early stage, have extremely promisingproperties as materials for next-generation devices: their bulk is an
insulator, but their surface is a metal with high electrical conductivity with spin polarization that depends on the direction of electronmotion.
The TSS has been determined by the topological classificationof the bulk electronic state. So, it is thought to be difficult to control
the TSS according to purpose.
In this study, a new surface superstructure was fabricated bypolishing the slightly tilted (vicinal) surface of the topological Kondo insulator, SmB6, from a square crystal plane with high symmetry[(001) orientation] (Fig. 1).
To observe the TSS of the vicinal surface, the electronic state
was observed by angle-resolved photoemission spectroscopy.
As shown in Fig. 2, a topological surface state with a bright Fermi
surface in only a diagonal direction was observed. This result suggests that a different topological surface state has been created byfabricating a slightly tilted surface structure.
The results show that the TSS does not reflect the bulk state in a‘robust’ manner, as previously thought, but is strongly influenced
by the surface atomic structure, including the superstructure, andis ‘flexible’.

▍ Significance of the research and Future perspective
It has been assumed that the surface electronic state of TI isdetermined solely by the topological nature of the bulk electronicstate, but it has been shown that it also reflects the state of thesurface structure. As the newly discovered method to control TSS,
engineering of surface atomic structure is expected to be applied
to next-generation devices with low power consumption and highspeed, and quantum computers.

Fig. 2 The Fermi surface of the surface electronic state obtained with an angleresolved photoelectron spectroscopy. The intensity is extended in a diagonal direction, which does not reflect to the four-fold symmetry of bulk crystal structure but
to the two-fold symmetry of the surface structure.

P a t e n t
Ohtsubo, Yoshiyuki; Kimura, Shin-ichi et al. Breakdown of bulk-projected isotropy in surface electronic states of topological Kondo insulator SmB6(001).
Nature Communications. 2022, 13, 5600. doi: 10.1038/s41467-022-33347-0
Ohtsubo, Yoshiyuki; Kimura, Shin-ichi et al. Non-trivial surface states of samarium hexaboride at the (111) surface. Nature Communications 2019, 10,
Tr e a t i s e2298. doi: 10.1038/s41467-019-10353-3
Hagiwara, Kenta; Kimura, Shin-ichi et al. Surface Kondo effect and non-trivial metallic state of the Kondo insulator YbB12. Nature Communications. 2016, 7,12690. doi: 10.1038/ncomms12690
UR
L http://www.kimura-lab.com
K e y w o r d topological insulator, Kondo insulator, angle-resolved photoelectron spectroscopy, electronic structure, SmB6