Photon Factory Activity Report 2022 #40 (2023)

BL-28A/2021S2-001, 2022G652

μ-ARPES study of topological line-nodal compound CaSb2
Chien-Wen CHUANG1, Seigo SOUMA2,3,*, Kosuke NAKAYAMA1,4, Atsutoshi IKEDA5, Mayo
KAWAGUCHI6, Keito OBATA6, Shanta Ranjan SAHA5, Hidemitsu TAKAHASHI6, Shunsaku
KITAGAWA6, Kenji ISHIDA6, Miho KITAMURA7, Koji HORIBA8, Hiroshi KUMIGASHIRA9,Takashi TAKAHASHI1 , Shingo YONEZAWA6, Johnpierre PAGLIONE5, Yoshiteru MAENO6 andTakafumi SATO1,2,3,10**
1Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
2Center for Science and Innovation in Spintronics, 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, CollegePark, Maryland 20742-4111, USA
6Department of Physics, Kyoto University, Kyoto 606-8502, Japan
7Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK),Tsukuba, Ibaraki 305-0801, Japan
8National Institutes for Quantum Science and Technology (QST), Sendai 980-8579, Japan
9Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University,Sendai 980-8577, Japan
10International Center for Synchrotron Radiation Innovation Smart, Tohoku University, Sendai 980-8577, Japan
1 Introduction
Recently, the superconductivity with superconducting
transition temperature (Tc) of 1.7 K was discovered in aline-nodal compound CaSb2 [1]. This material crystalizes
in the monoclinic structure with P21/m space group withSb zigzag chains running along the b-axis and mirror
surfaces in the a-c plane, as shown in Fig. 1(a) [note thatthe bulk Brillouin zone (BZ) is shown in Fig. 1(b)]. Firstprinciples band-structure calculations of CaSb2 predicted
the compensated semimetallic band structure [Fig. 1(c)]consisting of a 3D hole pocket extending roughly along theΓX direction and quasi-two-dimensional (2D) cylindricalelectron pockets axially centered along the YC line [toppanel of Fig. 1(d)]. Band structure calculations predicted
the presence of several Dirac points at specific k points
such as Y, C, E, and A points (but not Γ and Z points which
are related to the 3D pocket) due to the nonsymmorphic
symmetry [2] [some of these Dirac points at the Y and C
points are indicated by black arrows in Fig. 1(c)]. Suchband crossings are connected to the line nodes at the ky = πplane [2] and one of them is associated with the shallow
electron pocket crossing EF, likely contributing to thetransport properties, while these line nodes and Dirac
nodes had to await experimental verification.
Magnetotransport measurements of CaSb2 suggests theexistence of line nodes [2], and specific heat measurements
signified the deviation from the BCS behavior [3],suggestiveof theunconventional natureofsuperconductivity. On the other hand, nuclear quadrupole
resonance measurements suggested an exponential decrease in the inverse relaxation rate 1/T1 at low
temperature, supportive of the conventional s-wave

superconductivity with a full gap [4]. The presence of
cylindrical electron pockets with line nodes has been
discussed in favor of topological superconductivity [1]associated with the dominant interorbital pairing
interaction with odd-parity pairing within the cylindricalFSs, similarly to the case of doped TIs [5] and doped Diracsemimetals [6,7]. To experimentally clarify the electronicstates of CaSb2 and to establish the interplay among nodal
electrons, superconductivity, and topology, we have
performed microfocused ARPES of CaSb2 and clarifoed
the 3D band structure and Fermi-surface (FS) topology [8].

Fig. 1 (a) Crystal structure (side view) of CaSb2. (b)Monoclinic bulk Brillouin zone (BZ) of CaSb2. (c)Calculated band dispersions along high-symmetry lines inbulk BZ. (d) Calculated FSs in bulk BZ which incorporate(top panel) no μ shift and (bottom panel) downward μ shiftof 210 meV.

Photon Factory Activity Report 2022 #40 (2023)

2 Experiment
We prepared two different types of single crystalline
samples for ARPES measurements, called here sample A
and sample B. The samples A and B were grown by a selfflux method. For sample A, Ca (Sigma-Aldrich, 99.99%),and Sb (Alfa Aesar, 99.9999%) were melted at 1000 ◦C in
an alumina crucible inside a sealed quartz tube, cooled
down to 740 ◦ C, and then cooled down to 610 ◦C at a rateof −1◦C/h. Crystals were filtered in a centrifuge at thistemperature. For sample B, Ca (Sigma-Aldrich, 99.99%)
and Sb (Rare Metallic, 99.99%) were heated in a similar
temperature profile but in a tungsten crucible. We foundthat typical size of the single-crystal domains is different
between sample A (more than 1 mm size) and sample B(typically a few tens of μm). From the magnetic
susceptibility measurements, we have confirmed the onset
Tc values of samples A and B to be ∼1.8 K [8].
We performed ARPES measurements at BL28A in
KEK- PF [9]. We used circularly polarized photons of 30–300 eV. The energy and angular resolution was ∼20 meVand 0.2◦, respectively. The beam size on sample was 10×12μm2 [9]. Samples were kept at 40 K during measurements.
The Fermi level (EF) of samples was referred to that of agold film deposited on the sample holder. A clear Lauepattern was observed for sample A, confirming the highsingle crystallinity of sample (note that it was difficult toobtain a clear Laue pattern for sample B due to the mixtureof multiple crystal domains. However, we could safely
determine the sample orientation for a small area of sample
B on which the micro-ARPES measurement wasperformed, by looking at the periodicity and symmetry ofobtained band structure). Since CaSb2 single crystal wasvery hard to cleave, we tried cleaving several times in an
ultrahigh vacuum (UHV), occasionally obtained a small
flat area on the crystal with a few tens μm square and thenfocused the micro photon beam on it. The cleaving planeis a-b plane (perpendicular to the c* axis) according to our
x-ray diffraction and ARPES data. We observed nosignature of aging or contamination of sample surface
during ARPES measurements.
3 Results and Discussion
Figure 2(a) shows the in-plane FS mapping at T = 40 K
measured for sample B at hν = 90 eV which corresponds
to the kz ∼ 0 plane. One can see a dominant intensity
around the Γ point elongated along the kx direction. Thisfeature originates from the Sb2-orbital-derived 3D hole
pocket [2] as in the case of sample A, but its intensity
distribution is different from that of sample A at kz ∼ 0 [8],likely due to the difference in their doping levels as
detailed below. The existence of a hole pocket isrecognized from the clear EF crossing of holelike band
along a representative k cut (cut A) shown in Fig. 2(b).
Besides the hole pocket, one can see in Fig. 2(a) a weakintensity around the Y point which is well separated fromthe hole pocket. Intriguingly, this feature is absent insample A [8]. The ARPES intensity along k cut crossing
this feature (cut B) in Fig. 2(c) signifies the existence of ashallow electron pocket around the Y point. The metallic
electron pocket is also confirmed by the energy distribution

curve (EDC) at the Y point shown in Fig. 2(d) in which apeak near EF associated with the electron-band bottom
accompanied with the Fermi edge cutoff is clearly seen.
We have confirmed the existence of electron pocket also athν = 120 eV, in line with its quasi-2D nature predicted fromthe calculation shown in Figs. 1(c) and 1(d). These results
indicate that sample B is relatively more electron doped
than sample A, and is situated in a semimetallic phase withboth hole and electron pockets as predicted by thecalculation for stoichiometric CaSb2 (i.e., the calculation
without μ shift). It is thus suggested that the absence ofelectron pocket in sample A [8] is not due to theexperimental artifacts such as the strong intensity
suppression associated with the matrix-element effect but
due to the difference in the doping levels. This conclusion
is also supported by the quantitative analysis of theexperimental hole-band dispersion, the photoemission
spectra in a wider energy range, and the Sb 4d core-level
energies [8].

Fig. 2 (a) FS mapping at T = 40 K measured with hν = 90eV (corresponding to kz ∼ 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 theARPES intensity shown in (b) and (c) was obtained. (b),(c)
ARPES intensity as a function of wave vector and binding
energy measured along cuts A and B, respectively. k inhorizontal axis is defined as the distance between themeasured k point and the intersection of red line and kx (ky= 0) axis. (d) EDC around the Y point which signifies thepeak and metallic Fermi-edge cut-off associated with theelectron pocket.
Our susceptibility measurements indicate that bothsamples A and B show superconductivity with almost thesame onset Tc of ∼1.8 K [8], despite the marked differencein the carrier-doping level. The fact that the FS of sample

Photon Factory Activity Report 2022 #40 (2023)

A is composed only of the Sb2-derived 3D hole pocket atthe Γ point suggests that the main player ofsuperconductivity is the hole carriers in this pocket. It isthus inferred that the electron carriers in the Sb1-derived
quasi-2D FS centered at the Y(C ) point is not essential forthe occurrence of superconductivity. This argument puts a
constraint on the microscopic origin of superconductivity
as well as its possible nontrivial nature in CaSb2, because
the occupancy of the electron band hosting the line nodes
was suggested to be important for promoting topologicalsuperconductivity associated with the dominantinterorbital pairing interaction with odd-parity pairing [1].
Also, it is worth noting that CaSb2 maintains
superconductivity regardless of the existence or absence ofthe electron pocket, which points to an intriguing
possibility that one can tune the topological nature ofsuperconductivity by simply controlling the carrier
concentration in CaSb2, although its validation needs
further experimental studies that directly connects thefermiology and the superconducting pairing symmetry. We
leave such an experiment as a challenge in futureAcknowledgements
This work was supported by JST-CREST (No.
JPMJCR18T1), JST-PREST (No. JPMJPR18L7), JSPS
KAKENHI Grants (No. JP21H04435, No. JP20H01847,
No. JP20H00130, No. JP20KK0061, No. JP21K18600,
and No. JP22H04933). C.-W. C. acknowledges support
from GP-Spin and JSPS.
References[1] A. Ikeda, M. Kawaguchi, S. Koibuchi, T. Hashimoto, T.
Kawakami, S. Yonezawa, M. Sato, and Y. Maeno, Phys.
Rev. Mater. 4, 041801(R) (2020).
[2] K. Funada, A. Yamakage, N. Yamashina, and H.
Kageyama, J. Phys. Soc. Jpn. 88, 044711 (2019).
[3] M. Oudah, J. Bannies, D. A. Bonn, and M. C. Aronson,Phys. Rev. B 105, 184504 (2022).
[4] H. Takahashi, S. Kitagawa, K. Ishida, M. Kawaguchi,
A. Ikeda, S. Yonezawa, and Y. Maeno, J. Phys. Soc. Jpn.
90, 073702 (2021).
[5] S. Sasaki, M. Kriener, K. Segawa, K. Yada, Y. Tanaka,
M. Sato, and Y. Ando, Phys. Rev. Lett. 107, 217001(2011).
[6] T. Kawakami, T. Okamura, S. Kobayashi, and M. Sato,Phys. Rev. X 8, 041026 (2018).
[7] S. Kobayashi and M. Sato, Phys. Rev. Lett. 115, 187001(2015).
[8] C.-W. Chuang, S. Souma, A. Moriya, K. Nakayama, A.
Ikeda, M. Kawaguchi, K. Obata, S. R. Saha, H.
Takahashi, S. Kitagawa, K. Ishida, K. Tanaka, M.
Kitamura, K. Horiba, H. Kumigashira, T. Takahashi, S.
Yonezawa, J. Paglione, Y. Maeno, and T. Sato, Phys.
Rev. Mater. 6, 104203 (2022).
[9] M. Kitamura, S. Souma, A. Honma, D. Wakabayashi,H. Tanaka, A.Toyoshima, K. Amemiya, T. Kawakami,
K. Sugawara, K. Nakayama, K. Yoshimatsu, H.
Kumigashira, T. Sato, and K. Horiba, Rev. Sci. Instrum.
93, 033906 (2022).

* s.souma@arpes.phys.tohoku.ac.jp** t-sato@arpes.phys.tohoku.ac.jp[1]