Fermiology of a topological line-nodal compound ${\mathrm{CaSb}}_{2}$ and its implication to superconductivity: Angle-resolved photoemission study

Fermiology of a topological line-nodal compound ${CaSb}_{2}$ and its implication to superconductivity: Angle-resolved photoemission study

Chien-Wen Chuang, Seigo Souma, Ayumi Moriya, Kosuke Nakayama, Atsutoshi Ikeda, Mayo Kawaguchi, Keito Obata, Shanta Ranjan Saha, Hidemitsu Takahashi, Shunsaku Kitagawa, Kenji Ishida, Kiyohisa Tanaka, Miho Kitamura, Koji Horiba, Hiroshi Kumigashira, Takashi Takahashi, Shingo Yonezawa, Johnpierre Paglione, Yoshiteru Maeno, and Takafumi Sato

Phys. Rev. Materials 6, 104203 – Published 24 October 2022

Abstract

We performed angle-resolved photoemission spectroscopy with microfocused beam on a topological line-nodal compound ${CaSb}_{2}$ which undergoes a superconducting transition at the onset $T_{c} \sim$ 1.8 K, to clarify the Fermi-surface topology relevant to the occurrence of superconductivity. We found that a three-dimensional hole pocket at the $Γ$ point is commonly seen for two types of single-crystalline samples fabricated by different growth conditions. On the other hand, the carrier-doping level estimated from the position of the chemical potential was found to be sensitive to the sample fabrication condition. The cylindrical electron pocket at the $Y (C)$ point predicted by the calculations is absent in one of the two samples, despite the fact that both samples commonly show superconductivity with similar $T_{c}$ 's. This suggests a key role of the three-dimensional hole pocket to the occurrence of superconductivity, and further points to an intriguing possibility to control the topological nature of superconductivity by carrier tuning in ${CaSb}_{2}$ .

Received 13 July 2022
Revised 5 October 2022
Accepted 10 October 2022

DOI:https://doi.org/10.1103/PhysRevMaterials.6.104203

Physics Subject Headings (PhySH)

Electronic structure Fermi surface Superconductivity Symmetry protected topological states Topological superconductors

First-principles calculations Photoemission spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Chien-Wen Chuang ¹, Seigo Souma^2,3, Ayumi Moriya¹, Kosuke Nakayama ^1,4, Atsutoshi Ikeda ⁵, Mayo Kawaguchi⁶, Keito Obata⁶, Shanta Ranjan Saha⁵, Hidemitsu Takahashi⁶, Shunsaku Kitagawa⁶, Kenji Ishida⁶, Kiyohisa Tanaka^7,8, Miho Kitamura⁹, Koji Horiba^9,10, Hiroshi Kumigashira ¹¹, Takashi Takahashi¹, Shingo Yonezawa⁶, Johnpierre Paglione⁵, Yoshiteru Maeno ⁶, and Takafumi Sato ^1,2,3,12

¹Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
²Center for Science and Innovation in Spintronics (CSIS), Tohoku University, Sendai 980-8577, Japan
³Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, Sendai 980-8577, Japan
⁴Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), Tokyo 102-0076, Japan
⁵Maryland Quantum Materials Center and Department of Physics, University of Maryland, College Park, Maryland 20742-4111, USA
⁶Department of Physics, Kyoto University, Kyoto 606-8502, Japan
⁷UVSOR Synchrotron Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
⁸School of Physical Sciences, The Graduate University for Advanced Studies (SOKENDAI), Okazaki 444-8585, Japan
⁹Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan
¹⁰National Institutes for Quantum Science and Technology (QST), Sendai 980-8579, Japan
¹¹Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, Sendai 980-8577, Japan
¹²International Center for Synchrotron Radiation Innovation Smart (SRIS), Tohoku University, Sendai 980-8577, Japan

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Vol. 6, Iss. 10 — October 2022

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Figure 1
(a) Crystal structure (side view) of ${CaSb}_{2}$ . Two types of Sb atoms at different sites (Sb1 and Sb2) are indicated by green and orange circles. (b) Monoclinic bulk Brillouin zone (BZ) of ${CaSb}_{2}$ . Red lines indicate high-symmetry lines where the band calculation shown in (c) was carried out. $k_{x}$ , $k_{y}$ , and $k_{z}$ axes are also indicated. (c) Calculated band dispersions along high-symmetry lines in bulk BZ. (d) Calculated FSs in bulk BZ which incorporate (top panel) no $μ$ shift and (bottom panel) downward $μ$ shift of 210 meV. (e) X-ray Laue diffraction pattern of ${CaSb}_{2}$ single crystal. (f) EDC which includes the Sb $4 d$ and Ca $3 p$ core levels measured with $h ν$ = 90 eV. The EDC was obtained by integrating the ARPES spectra collected within angular window of $\pm 15^{\circ}$ around the surface normal. Inset shows an expanded view of the Ca $3 p$ core level. (g) Near- $E_{F}$ ARPES intensity crossing the $Γ$ point at $k_{z}$ = 0 measured at $T$ = 40 K with $h ν$ = 118 eV. (h) Corresponding calculated band structure along the $Γ Y$ cut ( $k_{z}$ = 0 plane) which takes into account the downward $μ$ shift of 210 meV. Dashed line indicates the original Fermi-level position $E_{F}$ '.
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Figure 2
(a) Bulk BZ of ${CaSb}_{2}$ with the measured k cuts and $k$ planes. Yellow rectangle represents the $k$ plane ( $k_{y} − k_{z}$ plane) where the $h ν$ -dependent out-of-plane FS mapping shown in (b) was performed. Red curves indicate the k cuts (cuts 1–6) where the ARPES measurement shown in (d) was carried out. The gray rectangles indicate the k slices where in-plane FS mappings shown in (e) were performed. The slices 7, 8, and 9 correspond to $k_{z} \sim$ 0, $- 0.5 π$ , and $- π$ , respectively. (b) Out-of-plane FS mapping in the $k_{y} − k_{z}$ plane obtained by sweeping $h ν$ . Dashed red curves are guide for the eyes to trace the $Γ$ -centered pocket. (c) Corresponding calculated FSs with (red) and without (blue) taking into account the downward $μ$ shift of 210 meV. (d) $h ν$ dependence of experimental band dispersions measured along the k cuts (1–6) shown in (a). ARPES data along cuts 1 and 6 nearly trace the same $k$ region in reduced BZ but measured with different $h ν$ 's (118 and 88 eV). (e) In-plane FS mapping obtained with different $h ν$ 's of 118 eV ( $k_{z} \sim$ 0: cut 7), 110 eV ( $k_{z} \sim - 0.5 π$ ; cut 8), and 103 eV ( $k_{z} \sim - π$ ; cut 9).
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Figure 3
(a) FS mapping at $T$ = 40 K measured with $h ν$ = 90 eV (corresponding to $k_{z} \sim 0$ plane) for sample B. Blue lines indicate high-symmetry lines in surface BZ. Red lines indicate representative k cuts (cuts A and B) where the ARPES intensity shown in (b) and (c) was obtained. [(b) and (c)] ARPES intensity as a function of wave vector and binding energy measured along cuts A and B, respectively. $k$ in horizontal axis is defined as the distance between the measured $k$ point and the intersection of red line and $k_{x}$ ( $k_{y}$ = 0) axis. (d) EDC around the Y point which signifies the peak and metallic Fermi-edge cut-off associated with the electron pocket.
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Fermiology of a topological line-nodal compound CaSb2 and its implication to superconductivity: Angle-resolved photoemission study

Phys. Rev. Materials 6, 104203 – Published 24 October 2022

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Fermiology of a topological line-nodal compound ${CaSb}_{2}$ and its implication to superconductivity: Angle-resolved photoemission study