Photon Factory Activity Report 2022 #40 (2023)

BL-28A/2021S2-001

μ-ARPES study of Kagome superconductor Cs(V1-xNbx)3Sb5
Takemi KATO1, Yongkai LI2,3, Kosuke NAKAYAMA1,4.*, Zhiwei WANG2,3, Seigo SOUMA5,6,Fumihiro MATSUI7, Miho KITAMURA8, Koji HORIBA9, Hiroshi KUMIGASHIRA10, Takashi
TAKAHASHI1,5,6, Yugui YAO2,3, 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
3Beijing 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
7UVSOR Synchrotron Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
8Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK),Tsukuba, Ibaraki 305-0801, Japan
9National Institutes for Quantum Science and Technology (QST), Sendai 980-8579, Japan
10Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University,Sendai 980-8577, Japan
11International Center for Synchrotron Radiation Innovation Smart, Tohoku University, Sendai 980-8577, Japan
1 Introduction
Recently, a family of kagome metals AV3Sb5 (AVS; A =K, Rb, Cs) with a V kagome network has emerged as a newplatform to study the physics associated with the saddle
point (SP) singularity due to its proximity to EF, as revealed
by density-functional-theory calculations [1–10] andangle-resolved photoemission spectroscopy (ARPES) [11–18]. AVS commonly exhibits superconductivity (Tc = 0.9–2.5 K) and a charge density wave (CDW; TCDW = 78–103
K). The mechanism has been intensively discussed interms of the characteristic band structure with the SP at theM point of the BZ [2-4,5–18]. For instance, the scattering
connecting different SPs would promote unconventional
superconducting pairing, whereas the same scattering also
contributes to the energy gain to stabilize the chiral CDW
with an in-plane 2 × 2 periodicity.
A promising strategy to study the interplay amongsuperconductivity, a CDW, and electronic states is tomodulate the band structure by varying key physicalparameters that characterize the electronic phase diagram,
such as pressure and carrier concentration. Such attempts
have been made in AVS. For example, transportmeasurements under high pressure have clarified theanticorrelation between Tc and TCDW as well as an
unconventional double superconducting dome [19–22],
whereas the relevant band structure is under debate.
Chemical substitution in crystal is a useful method to tune
chemical pressure and carrier concentrations, but suchstudies are still limited in AVS.
Here, we report an μ-ARPES study of Nb-substituted
CVS, Cs(V1−xNbx)3Sb5 (x=0, 0.03, and 0.07), in which thesubstitution of V ions with isovalent Nb ions leads to Tc
enhancement and simultaneously TCDW reduction [23].
2 ExperimentHigh-quality single crystals of Cs(V1−xNbx)3Sb5 weresynthesized with the self-flux method. VUV-ARPES
measurements with micro-focused synchrotron light were
performed with a DA30 electron analyzer at BL28A [24],as well as SES2002 spectrometers at Tohoku University.
The energy resolution was set to be at 35 meV at Photon
factory and 7 meV at Tohoku University. The angularresolution was set to be 0.2-0.3˚. Crystals were cleaved insitu in an ultrahigh vacuum of ~ 1×10-10 Torr.
3 Results and Discussion
Figures 1(a) and 1(b) display a comparison of theARPES intensity between x = 0.07 and x = 0 measured
along the ΓKM cut at T = 120 K. In both measurements,
the electron-like α band at Γ, the linearly dispersive γ/εbands crossing EF, and the δ band forming a SP at M arecommonly resolved. This indicates that the overall band
structure is unchanged after the Nb substitution. To clarify
quantitative differences in the band position, we havedetermined the experimental band dispersion (red andblack crosses), and directly compared them in Fig. 1(c). At
first glance, both band structures for x = 0.07 and 0 welloverlap each other. However, a closer look reveals a finite
difference in the energy position of the α band, which shifts
downward ∼20 meV at x = 0.07 relative to that at x = 0.
The shift is also seen by a comparison of momentum
distribution curves (MDCs) at EF in Fig. 1(d) where thepeaks for x = 0.07 are located outside of the peaks for x =Photon Factory Activity Report 2022 #40 (2023)

0 (note that the peak width is slightly wider in x = 0.07 dueto the increase of impurity scattering by Nb substitution).
Moreover, as shown by the energy distribution curves(EDCs) at Γ in Fig. 1(e), the bottom of the α band moves
toward higher EB by ∼20 meV with Nb substitution, One
can also see from Fig. 1(c) that the ε-band bottom and theη-band top at M are shifted downward (by ∼20 meV) uponNb substitution. These results suggest that the Nb
substitution affects both the Sb- and V-derived bands. Incontrast to the downward shift of the α, ε, and η bands, theγ and δ bands near EF look relatively stationary.

substitution shifts the Sb-derived electron band at the Γpoint downward and simultaneously moves the V-derived
band around the M point upward to lift up the saddle point(SP) away from the Fermi level, leading to the reduction ofthe CDW-gap magnitude and TCDW. This indicates a
primary role of the SP density of states to stabilize theCDW. The present result also suggests that theenhancement of superconductivity by Nb substitution iscaused by the cooperation between the expansion of theSb-derived electron pocket and the recovery of the Vderived density of states at the Fermi level [23].
Acknowledgements
This work was supported by JST-CREST (No.
JPMJCR18T1), JST-PREST (No. JPMJPR18L7), JSPS
KAKENHI Grants (No. No. JP21H04435 and No.
JP20H01847). T. K. acknowledges support from GP-Spinat Tohoku University.

Fig. 1 (a) ARPES-intensity plot along the ΓKM cut for x =0.07 measured at T = 120 K with hν = 106 eV. Crosses
show experimental band dispersion extracted from thepeak position in EDCs and MDCs. (b) Same as (a) but forx = 0. (c) Comparison of band dispersions between x = 0.07
and x = 0 [same as red and black crosses in (a) and (b)].
(d),(e) Comparison of MDCs at EF and EDCs at ky = 0 [thecuts are indicated by white dashed lines in (a)],respectively, between x = 0.07 (red) and x = 0 (black). Thehorizontal axis in (d), ky/|kΓK|, is defined as the ky value inunits of the ΓK length where the Γ and K points correspondto 0 and 1, respectively. Red dashed lines in (d) are a guidefor the eye to trace the peak position of x = 0.07. Triangles
in (e) indicate the peak position.
In addition to the experiment at synchrotron, we also
performed high-resolution experiment to observe theCDW gap at Tohoku University. We found that the Nb

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* k.nakayama@arpes.phys.tohoku.ac.jp** t-sato@arpes.phys.tohoku.ac.jp