Electrical detection of the surface spin polarization of the candidate topological Kondo insulator $\mathrm{Sm}{\mathrm{B}}_{6}$

Electrical detection of the surface spin polarization of the candidate topological Kondo insulator $Sm B_{6}$

Jehyun Kim, Chaun Jang, Xiangfeng Wang, Johnpierre Paglione, Seokmin Hong, Jekwan Lee, Hyunyong Choi, and Dohun Kim

Phys. Rev. B 99, 245148 – Published 26 June 2019

Abstract

The Kondo insulator compound $Sm B_{6}$ has emerged as a strong candidate for the realization of a topologically nontrivial state in a strongly correlated system, a topological Kondo insulator, which can be a novel platform for investigating the interplay between nontrivial topology and emergent correlation-driven phenomena in solid-state systems. Electronic transport measurements on this material, however, so far showed only the robust surface-dominated charge conduction at low temperatures, lacking evidence of its connection to the topological nature by showing, for example, spin polarization due to spin-momentum locking. Here, we find evidence for surface-state spin polarization by electrical detection of a current-induced spin chemical-potential difference on the surface of a $Sm B_{6}$ single crystal. We clearly observe that a surface-dominated spin voltage, which is proportional to the projection of the spin polarization onto the contact magnetization, is determined by the direction and magnitude of the charge current and is strongly temperature dependent due to the crossover from surface to bulk conduction. We estimate the lower bound of the surface-state net spin polarization as 25% based on the quantum transport model, providing direct evidence that $Sm B_{6}$ supports metallic spin helical surface states.

Received 9 December 2018

DOI:https://doi.org/10.1103/PhysRevB.99.245148

Physics Subject Headings (PhySH)

Spintronics Topological insulators

Kondo insulators

Resistivity measurements

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Jehyun Kim¹, Chaun Jang², Xiangfeng Wang³, Johnpierre Paglione³, Seokmin Hong², Jekwan Lee⁴, Hyunyong Choi⁴, and Dohun Kim^1,*

¹Department of Physics and Astronomy and Institute of Applied Physics, Seoul National University, Seoul 08826, Korea
²Center for Spintronics, Korea Institute of Science and Technology, Seoul 02792, Korea
³Center for Nanophysics and Advanced Materials, Department of Physics, University of Maryland, College Park, Maryland 20742-4111, USA
⁴School of Electrical and Electronic Engineering, Yonsei University, Seoul 03722, Korea

^*Corresponding author: dohunkim@snu.ac.kr

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Vol. 99, Iss. 24 — 15 June 2019

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Figure 1
Surface spin texture and potentiometric spin measurement in $Sm B_{6}$ . (a) Anticlockwise spin texture for the surface band in $Sm B_{6}$ near the Fermi energy shown by spin and angle-resolved photoemission spectroscopy and first-principles calculations [2, 11]. (b) Degree of electron occupation in channel and spin dependent electrochemical potential under electrochemical bias. The length of the red and blue arrows indicates the degree of moving electrons with spin $S$ . (c) Measurement configuration for detecting spin voltage. Inset: Optical microscope image of the device. The scale bar is $100 μ m$ . (d) Electrochemical potential with respect to the position of the channel, illustrating concepts of the potentiometric spin measurement.
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Figure 2
Measurement of current-induced surface channel spin polarization. (a) Schematic of the transport measurements. A constant current $I_{b}$ is applied in the $x$ direction while sweeping a magnetic field in the $y$ direction, which is orthogonal to the bias current direction (colinear with the spin-polarization direction). (b, c) Magnetic field dependence of the voltage measured at the ferromagnetic contact for $I_{b}$ of (b) $+ 100 μ A$ and (c) $- 100 μ A$ . Inset: Black, red, and blue arrows indicate the direction of the electron momentum $I_{e}$ , $p$ , and $M$ , respectively. (d) Dependence of spin voltage $Δ V_{xy}$ on $I_{b}$ measured at 1.8 K.
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Figure 3
Magnetization orientation dependence of spin voltage. (a, b) $V_{xy}$ as a function of an external magnetic field swept (a) in the $y$ direction and (b) in the $x$ direction under $I_{b}$ of $100 μ A$ at 1.8 K. Inset: Black, red, and blue arrows show the direction of the electron momentum $I_{e}$ , $p$ , and $M$ , respectively. Detector magnetization $M$ is aligned (a) in the $y$ direction (perpendicular to current) and (b) in the $x$ direction (parallel to current).
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Figure 4
Temperature dependence of the spin voltage and resistance of $Sm B_{6}$ . (a) Electrical resistance of $Sm B_{6}$ as a function of temperature. Inset: Spin voltage $Δ V_{xy}$ as a function of temperature. (b–g) $V_{xy}$ measured by sweeping ${\vec{H}}_{ext}$ parallel to the $y$ axis under a bias current of $100 μ A$ at different temperatures: (b) 1.8 K, (c) 2.5 K, (d) 3 K, (e) 3.5 K, (f) 4 K, and (g) 4.5 K.
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Figure 5
Angle-resolved current-induced spin polarization. (a) Schematic measurement configuration for potentiometric measurement with rotating magnetic field in the y-z plane, where γ is the angle between the applied external magnetic field and the $y$ axis. (b) Polar plot of normalized $Δ V_{xy}$ as a function of γ showing the in-plane character of the current-induced spin polarization in $Sm B_{6}$ .
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Physical Review B

covering condensed matter and materials physics

Electrical detection of the surface spin polarization of the candidate topological Kondo insulator $Sm B_{6}$

Jehyun Kim, Chaun Jang, Xiangfeng Wang, Johnpierre Paglione, Seokmin Hong, Jekwan Lee, Hyunyong Choi, and Dohun Kim

Phys. Rev. B 99, 245148 – Published 26 June 2019

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Physical Review B

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Electrical detection of the surface spin polarization of the candidate topological Kondo insulator SmB6

Jehyun Kim, Chaun Jang, Xiangfeng Wang, Johnpierre Paglione, Seokmin Hong, Jekwan Lee, Hyunyong Choi, and Dohun Kim

Phys. Rev. B 99, 245148 – Published 26 June 2019

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Electrical detection of the surface spin polarization of the candidate topological Kondo insulator $Sm B_{6}$