Coexistence of Surface Superconducting and Three-Dimensional Topological Dirac States in Semimetal KZnBi

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Coexistence of Surface Superconducting and Three-Dimensional Topological Dirac States in Semimetal KZnBi

Junseong Song, Sunghun Kim, Youngkuk Kim, Huixia Fu, Jahyun Koo, Zhen Wang, Gyubin Lee, Jouhahn Lee, Sang Ho Oh, Joonho Bang, Taku Matsushita, Nobuo Wada, Hiroki Ikegami, Jonathan D. Denlinger, Young Hee Lee, Binghai Yan, Yeongkwan Kim, and Sung Wng Kim

Phys. Rev. X 11, 021065 – Published 28 June 2021

Abstract

We report the discovery of a new three-dimensional (3D) topological Dirac semimetal (TDS) material KZnBi, coexisting with a naturally formed superconducting state on the surface under ambient pressure. Using photoemission spectroscopy together with first-principles calculations, a 3D Dirac state with linear band dispersion is identified. The characteristic features of massless Dirac fermions are also confirmed by magnetotransport measurements, exhibiting an extremely small cyclotron mass of $m^{*} = 0.012$ $m_{0}$ and a high Fermi velocity of $v_{F} = 1.04 \times 10^{6} m / s$ . Interestingly, superconductivity occurs below 0.85 K on the (001) surface, while the bulk remains nonsuperconducting. The captured linear temperature dependence of the upper critical field suggests the possible non- $s$ -wave character of this surface superconductivity. Our discovery serves a distinctive platform to study the interplay between 3D TDS and the superconductivity.

Received 25 August 2020
Revised 13 March 2021
Accepted 7 May 2021

DOI:https://doi.org/10.1103/PhysRevX.11.021065

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Dirac fermions Superconductivity Topological phases of matter

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Junseong Song ^1,2,*, Sunghun Kim^3,*, Youngkuk Kim ^4,*, Huixia Fu⁵, Jahyun Koo⁵, Zhen Wang¹, Gyubin Lee³, Jouhahn Lee⁶, Sang Ho Oh ¹, Joonho Bang¹, Taku Matsushita ⁷, Nobuo Wada⁷, Hiroki Ikegami ⁸, Jonathan D. Denlinger⁹, Young Hee Lee^1,2, Binghai Yan^5,†, Yeongkwan Kim^3,10,‡, and Sung Wng Kim^1,2,§

¹Department of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of Korea
²Center for Integrated Nanostructure Physics, Institute for Basic Science, Suwon 16419, Republic of Korea
³Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon 34141, Republic of Korea
⁴Department of Physics, Sungkyunkwan University, Suwon 16419, Republic of Korea
⁵Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot 7610001, Israel
⁶Advanced Nano Surface Research Group, Korea Basic Science Institute, Daejeon 34133, Republic of Korea
⁷Department of Physics, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan
⁸RIKEN Center for Quantum Computing (RQC), RIKEN, Wako, Saitama 351-0198, Japan
⁹Advanced Light Source, Lawrence Berkeley National Laboratory, California 94720, USA
¹⁰Graduate School of Nanoscience and Technology, Korea Advanced Institute of Science and Technology, Daejeon 34141, Republic of Korea

^*These authors contributed equally to this work.
^†binghai.yan@weizmann.ac.il
^‡yeongkwan@kaist.ac.kr
^§kimsungwng@skku.edu

Popular Summary

The superconducting state in topological matter—systems with properties impervious to defects or perturbations—has attracted tremendous interest because of the possibility of realizing Majorana fermions, quasiparticles that are a desired component of quantum computation. Among the various topological systems, one compelling candidate for superconductivity is the 3D topological Dirac semimetal (3D TDS), an exotic material with novel electronic properties such as high electron mobility. However, superconductivity in 3D TDS has been induced only under certain conditions such as high pressure. In this work, we discover a new 3D TDS, KZnBi, which is constructed with a planar honeycomb lattice, exhibiting a naturally formed surface superconductivity under ambient pressure and inert conditions.

By using angle-resolved photoemission spectroscopy together with first-principles calculations, we demonstrate that signatures of a 3D TDS originate from the planar ZnBi honeycomb layer. Magnetotransport properties, meanwhile, reveal a characteristic feature of Dirac fermions, further supporting that the system is a 3D TDS. More interestingly, we verify that the superconductivity of the 3D TDS KZnBi occurs at the surface of the crystal, while the bulk remains in a nonsuperconducting topological Dirac state.

Our discovery provides a new 3D TDS that exhibits a surface superconducting state. We believe that if this surface superconducting state correlates with the topological nature of the bulk Dirac states, it would be interesting for the research and future applications of topological superconductivity.

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Vol. 11, Iss. 2 — April - June 2021

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Figure 1
Crystal structure of KZnBi. (a),(b) Atomic structure of KZnBi, which belongs to the $P 6_{3} / m m c$ space group (a). The solid lines represent a conventional hexagonal unit cell. The Zn (green) and Bi (purple) ions form honeycomb layers with an $A B$ stacking order while the K ion is located at the inversion center. The plane indicated by orange dashed lines ( $g$ ) denotes the glide mirror plane and the blue solid line shows the principal axis of the threefold rotation symmetry ( $C_{3 z}$ ) (b). (c) Image of high-quality KZnBi single crystal with a few millimeters size. (d) Single-crystal XRD patterns for the family of ( $00 l$ ) peaks (top) and periodic reflections of (112) plane (bottom). (e) High-angle annular dark-field (HAADF)-STEM images taken along [010] (top) and [001] (bottom) zone axes. (f) XPS spectra of K $2 p$ (top), Zn $2 p$ (middle), and Bi $4 f$ (bottom) orbitals.
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Figure 2
Calculated band structure of KZnBi. (a) Bulk and (010) projected surface BZ of KZnBi with high symmetric points. Two Dirac points (green dots, labeled “ $D$ ”) exist along the $Γ − A$ line. (b),(c) Calculated band structure of KZnBi along the high symmetry lines (b) and the expanded band structure along $Γ − A$ (c). The red and blue represent the Zn $s$ and Bi $p$ orbitals, respectively. (d) Dirac bands on the $k_{x} − k_{z}$ plane. (e) Anisotropic isoenergy surfaces of Dirac bands along the $k_{z}$ direction. A small pocket is forming when $E_{F}$ is located below $- 50 meV$ . (f) Calculated surface state for (010) plane. (g) Isoenergy surfaces of the surface band structure with Dirac points and Fermi arcs.
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Figure 3
Direct observation of 3D Dirac fermions from angle-resolved photoemission spectroscopy (ARPES). (a) ARPES spectra of the pristine sample along the $Γ − K$ direction at $k_{z} = 0$ (a1). The overlaid blue solid lines show the calculated band structure. $E_{F}$ is rising with surface Rb doping and the conduction band appears [(a2) and (a3)]. (b) Schematic of 3D Dirac cone with slices (gray planes) at different positions of $k_{z}$ . (c) Measured band dispersions after surface Rb doping along the $k_{y}$ direction at different $k_{z}$ positions from 0 to $π / c$ [(c1)−(c6)]. The $k_{z}$ positions of each panel correspond to those presented in (b) and are numbered accordingly. White lines represent the conduction band minima (CBM) and valance band maxima (VBM). The CBM energy in each panel was set to 0 eV after subtracting the Rb impurity level. (d) Stacked MDCs of (c). Overlaid gray solid lines are guidelines for the band dispersion. (e) Calculated band dispersions at $k_{z}$ positions corresponding to (c) and (d). The red and blue solid lines represent Zn $s$ and Bi $p$ orbitals, respectively.
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Figure 4
Quantum transport properties of 3D Dirac fermions. (a) $T$ dependence of electrical resistivity $ρ_{x x}$ under various magnetic fields along the $z$ direction. (b) $T$ dependence of carrier concentration $n_{H}$ (red dots) and mobility $μ_{H}$ (blue dots). (c) MR under $B$ of up to 9 T at various $T$ . SdH oscillations appear at low $T$ and vanish as $T$ increases. (d) SdH oscillation component $d R_{x x} / d B$ as a function of $1 / B$ at various $T$ from 2 to 150 K, extracted from MRs in (c) by subtracting backgrounds. The inset shows $T$ dependence of the normalized amplitude of the oscillations at $B = 3.75 T$ . The red solid line is fit to the Lifshitz-Kosevich formula. (e),(f) Angle-dependent MRs (e) and SdH oscillations (f) at 2 K. The measurement geometry is illustrated in the inset of (e). The inset of (f) shows that the measured periodicity $d (1 / B)$ (black circles) matches with the theoretically calculated values (red line) for the Dirac band. Schematic illustration shows the cross-sectional Fermi surface area corresponding to the various $B$ field directions, this is shown in Fig. S6(k) [20] with the real size of the Fermi surface in BZ.
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Figure 5
Surface superconductivity of KZnBi. (a) $T$ dependence of the normalized resistance in single-crystal samples (S1−S4) (Fig. S7 [20]). The inset shows the heat capacity (blue dots), $C / T$ , versus $T^{2}$ (left and bottom scales) which can be fitted to a linear function (purple dashed line) and the real part of the ac magnetic susceptibility with decreasing (black solid line) and increasing (orange solid line) $T$ (right and top scales), measured with the single-crystal samples S5 and S6, respectively (see also Fig. S8 [20]). (b) Characteristic current ( $I$ )−voltage ( $V$ ) curves of two samples with similar widths ( $w$ ) and different thicknesses ( $t$ ) (S7 and S8) at $T = 0.1 - 0.2 K$ . $I_{c}$ is indicated by the red and blue arrows for S7 and S8, respectively. S7 is 2.5 times thicker than S8. (c) Critical current density per unit width ( $a_{c} = I_{c} / w$ ) for S7–S11 samples. The deviation in $a_{c}$ is smaller than that of the critical current density per unit area ( $j_{c} = I_{c} / w t$ ) (Fig. S9 [20]). (d) $T$ dependence of the normalized resistance in S1 under $B$ field parallel (top) and perpendicular (bottom) to the $z$ -direction (for S2 and S3, see Fig. S7 [20]). The measurement geometries are illustrated in Fig. S7(g) [20]. (e) $T / T_{c}$ dependence of reduced critical field ( $h^{*}$ ) in samples S1–S3 for $B$ parallel (solid dots) and perpendicular (open dots) to the $z$ direction (Appendix). Red and black dashed lines are the WHH curve of $p$ -wave and $s$ -wave superconductors, respectively. Inset of (e) shows the linear $T$ dependence of $H_{c 2}$ for samples S1−S3.
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