Photon Factory Activity Report 2022 #40 (2023)

BL-28A/2021S2-001
BL-2A/2022G652

μ-ARPES imaging of polar surface of kagome superconductor CsV3Sb5Takemi KATO1, Yongkai LI2,3, Kosuke NAKAYAMA1,4.*, Zhiwei WANG2,3, Seigo SOUMA5,6, Miho
KITAMURA7, Koji HORIBA8, Hiroshi KUMIGASHIRA9, Takashi TAKAHASHI1,5,6 and Takafumi
SATO1,5,6,11,**
1Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
2Centre for Quantum Physics, Key Laboratory of Advanced Optoelectronic Quantum Architectureand Measurement (MOE), School of Physics, Beijing Institute of Technology, Beijing 100081, China
3
Beijing Key Lab of Nanophotonics and Ultrafine Optoelectronic Systems, Beijing Institute ofTechnology, Beijing 100081, China
4Precursory Research for Embryonic Science and Technology (PRESTO),
Japan Science and Technology Agency (JST), Tokyo 102-0076, Japan
5Center for Science and Innovation in Spintronics, Tohoku University, Sendai 980-8577, Japan
6Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, Sendai 980-8577, Japan
7
Institute 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
CsV3Sb5 is a member of kagome metals AV3Sb5 (A = K,Rb, and Cs) which were recently discovered to showsuperconductivity with Tc = 0.9–2.5 K coexisting withcharge density wave (CDW) below TCDW = 78–103 K [1–3] with the in-plane 2×2 periodicity [4–11]. The crystalstructure of CsV3Sb5 consists of two building blocks, a Cs
layer, and a V3Sb5 layer, and the cleaved surface is
expected to be terminated either by the Cs or V 3Sb5 (Sb2)layer. Although the bonding between V and Sb1/Sb2 atoms
within the V3Sb5 layer may be intermetallic, the coupling
between the Cs and V3Sb5 layers is ionic because Cs isfully ionized (Cs1+) to donate one electron to the V3Sb5
layer [(V3Sb5)1-]. Such ionicity would lead to polar
instability. In fact, two different surface terminations havebeen identified by scanning-tunneling-microscopy (STM)
measurements, and intriguingly, there is growing
experimental evidence that such a surface hosts peculiar
quantum states distinct from those of the bulk as
exemplified by the formation of unidirectional 4×1 CDW[4,6–12], surface- dependent vortex-core states [7], and thepair-density wave [4]. However, the band structureassociated with the polar surface remains totally
unexplored in AV3Sb5.
Here, we report a spatially resolved ARPES study ofCsV3Sb5. By utilizing the microfocused photon beam fromthe synchrotron[13], we have succeeded in separately
observing two kinds of polar surfaces and uncovered
marked differences in the band structures between them.
The Cs-terminated surface shows the band doubling
associated with the three-dimensional (3D) nature of CDW
at low temperatures, whereas the Sb- terminated surface isstrongly hole doped, resulting in the suppression of CDW.
We discuss implications of the present results in relation to
the mechanism of the CDW and unconventional surface
properties [14].
2 ExperimentHigh-quality single crystals of CsV3Sb5 weresynthesized with the self-flux method. ARPES
measurements were performed using Scienta-Omicron
DA30 and SES2002 spectrometers at BL-28A and BL-2A
in Photon Factory, KEK. We used energy tunable photonsof hν = 85–350 eV. ARPES data were mainly obtained bycircularly polarized 106-eV photons (corresponding to theout-of-plane wave vector kz ∼ 0) with the beam spot sizeof 10×12 μm2 [13]. The energy resolution was set to be 25
meV. The samples were cleaved at the same temperaturesat which the ARPES measurements were performed (8 and120 K). All the data have been recorded within 8 hs after
the cleavages.
3 Results and Discussion
Figure 1(a) shows a mapping of the spatial distributionof the binding energy (EB) of Sb-4d5/2 core-level peak [seeFig. 1(c)] by sweeping the microfocused photon beam on
the surface of CVS. This is a good measure of the surface
doping level (note that the selection of the Sb-4d3/2 peak
reproduces the same result). As seen in Fig. 1(a), thesurface looks to consist of two regions (blue and redregions). The core-level spectrum in the red region [redcurve in Fig. 1(c)] is shifted as a whole toward higher EBby ∼130 meV with respect to that in the blue region (bluecurve), indicating that the red region is more electron

Photon Factory Activity Report 2022 #40 (2023)

doped than the blue region. Such a difference in the doping
level signifies different polarities of the two regions. Here
we assign the electron (hole)-rich red (blue) region to theCs (Sb)-terminated surface because Cs atoms on topsurface do not need to donate electrons to the missing
upper V3Sb5 layer which is removed by the cleaving so that
the V3Sb5 layer beneath the topmost Cs layer is more
electron doped for the Cs-terminated surface. Inversely, theV3Sb5 layer at the surface is hole doped compared with thebulk counterpart. This is also corroborated by the behaviorof the Cs-4d core level. Figure 1(d) shows the Cs-4d corelevel spectra measured at the same spatial location as inFig. 1(b) where we observe that each spin-orbit satellite ofCs-4d core levels (d5/2 or d3/2) consists of two peaks; a peakat lower EB originates from bulk Cs atoms whereas that at
higher EB is from surface Cs atoms as confirmed by theangular dependence of their intensity ratio. One canrecognize that the surface Cs peak is stronger than the bulk
one for the Cs-termination-dominant surface (red curve),
whereas it is weaker for the Sb-termination-dominant case(blue curve). The spatial image of the intensity ratio
between the surface and the bulk Cs peaks in Fig. 1(b)shows good agreement with that of the Sb-core-level EB in
Fig. 1(a). This supports our assignment of the Cs- and Sbtermination-dominant surfaces (hereafter, we simply callthem Cs- and Sb-terminated surfaces, respectively) as
schematically shown in Fig. 1(e).

Fig. 1 (a) Spatial map of the EB position of Sb-4d5/2 corelevel measured with hν = 106 eV at T = 8 K in the spatial
region enclosed by red dashed rectangle (area size:
600×800 μm2), obtained with the step size of 20 μm. (b)Spatial map of the intensity ratio of the surface/bulk Cs4d5/2 core-level peaks. (c) and (d) EDC in the Sb-4d andCs-4d core-level regions, respectively, for the Cs-(red) andSb-(blue) terminated surfaces. (e) Schematic of the cleaved
surface of CsV3Sb5.

Fig. 2 (a) and (b) ARPES intensity at T = 120 K for Csand Sb-terminated surfaces, respectively, measured withhν = 106 eV along the ΓKM cut.
Figs. 2(a) and 2(b) show the ARPES intensity at T=120K (above TCDW =93K) along the ΓKM cut (kz ∼0) for
the Cs- and Sb-terminated surfaces, respectively. One canrecognize overall similarity in the band structure between
the two, i.e., an electron band at the Γ point (α) with theSb-5pz character, linearly dispersive V-3dxz/yz bands (β, γ)
forming a Dirac cone near EF (marked by an arrow), the δband with the V-3dxy/x2−y2 character forming a saddle point(SP) slightly above EF, and the ε band that intersects the δband to form a Dirac point at the K point at EB ∼ 0.3 eV.
A closer look further reveals some intrinsic differences; theband structure of the Cs-terminated surface is shifted
downward as a whole with respect to that of the Sbterminated surface as recognized by a direct comparison of
surface-termination-dependent
ARPES
spectraat
representative k points along cut A-C in Figs. 2(a) and2(b). Numerical fittings to the peak position clarified theenergy shift to be 30–90 meV (note that a finite variationin the magnitude of the energy shift among different bands
and Sb-4d core levels suggests a non-rigid-band-type
energy shift. Moreover, whereas the Sb-terminated surface
exhibits a single α band bottomed at EB ∼ 0.5 eV, it splits
into two bands bottomed at EB ∼ 0.5 and 0.8 eV for the Csterminated surface. This suggests that not only polarity, butalso the band structure above TCDW is different between theCs- and the Sb-terminated surfaces. We have performed
ARPES measurements more than three times with different
samples for each surface termination and confirmed thereproducibility.
We further measured temperature dependence ofARPES data across TCDW and examined its termination
dependence [14]. We found that the Cs-terminated surface
shows intriguing doubling of V-derived bands at low
temperatures associated with the band folding due to thethree-dimensional CDW, whereas the Sb-terminated one
shows no band doubling or resultant CDW-gap opening
indicative of the suppression of bulk-originated CDW due
to polar charge. Such polar-surface-dependent band

Photon Factory Activity Report 2022 #40 (2023)

structures must be incorporated for understanding theorigin of unconventional superconducting and chargeorder at the surface of AV3Sb5 [14].
Acknowledgements
This work was supported by JST-CREST (No.
JPMJCR18T1), JST-PREST (No. JPMJPR18L7),JSPS KAKENHI Grants (No. JP21H04435 and
No. JP20H01847). T. K. acknowledges support from GPSpin at Tohoku University.
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* k.nakayama@arpes.phys.tohoku.ac.jp** t-sato@arpes.phys.tohoku.ac.jp