Photon Factory Activity Report 2022 #40 (2023)

BL-28A/2021S2-001

μ-ARPES study of charge density wave of Kagome superconductor CsV3Sb5
Kosuke NAKAYAMA1,2,*, Yongkai LI3,4, Takemi KATO1, Min LIU3,4, Zhiwei WANG3,4, Takashi
TAKAHASHI1,5,6, Yugui YAO3,4, and Takafumi SATO1,5,6,7**
1Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
2Precursory Research for Embryonic Science and Technology (PRESTO),
Japan Science and Technology Agency (JST), Tokyo 102-0076, Japan
3Centre for Quantum Physics, Key Laboratory of Advanced Optoelectronic Quantum Architectureand Measurement (MOE), School of Physics, Beijing Institute of Technology, Beijing 100081, China
4Beijing Key Lab of Nanophotonics and Ultrafine Optoelectronic Systems, Beijing Institute ofTechnology, Beijing 100081, China
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
7International Center for Synchrotron Radiation Innovation Smart, Tohoku University, Sendai 980-8577, Japan
1 Introduction
Recent discovery of superconductivity with the superconducting transition temperature Tc of 0.93–2.5 K in a
family of kagome metals AV3Sb5 (AVS) (A = K, Rb, andCs) [1–3] sparked a great deal of attention, because therealization of superconductivity in kagome metals is veryrare and may invoke an unconventional mechanism as
inferred from possible symmetry breaking [4–7]. A central
issue under intensive debate for the AVS kagome metals is
the relationship between the band structure and themechanism of CDW and superconductivity. Theoretically,
the saddle point can promote f- or d-wave superconducting
pairing associated with the scattering with the Q = (π, 0)vector connecting the saddle points, whereas this scattering
would also enhance an instability toward CDW (possiblychiral CDW) or unconventional density waves [8–14].
Thus, the interplay between CDW and superconductivity
is not straightforward and a fundamental question as towhether the saddle points play a more important role in
CDW or superconductivity needs to be answered. Given
the multiorbital character of AVS, it is also essential tounderstand how other bands or orbitals are involved in
CDW and superconductivity. In the experimental side, theobservation of a CDW gap [4–6,15,16], particularly thelarge gap on the saddle point of the V dxy/x2−y2 band [15,16],
supports that the saddle points play a certain role instabilizing CDW. However, the mechanism of CDW andsuperconductivity is still far from reaching a consensus.
For example, the influence of multiorbital character is yet
to be clarified. A promising route to clarify this issue is tocontrol the band energy position relative to EF throughcarrier doping because the CDW transition would besensitive to the saddle point energy and/or the band fillingin the simple nesting picture. Carrier doping may also
modify a charge balance among multiple orbitals andwould be useful to differentiate the contribution of eachorbital, as has been utilized to pin down the origin of
multiorbital superconductivity and density wave in Febased superconductors [17]. However, the carrier-doping
effect has been unexplored in AVS due to the lack of aneffective means of carrier doping.

Here, we report ARPES study on Cs-dosed CsV3Sb5(CVS) bulk single crystals to experimentally establish theevolution of a Fermi surface (FS) and band structure uponelectron doping, and to uncover the orbital-dependent
electron-doping effect and suppression of CDW
accompanied with the reduction of CDW gaps [18].
2 ExperimentHigh-quality single crystals of CVS were synthesizedwith the self-flux method. VUV-ARPES measurementswith micro-focused synchrotron light were performed witha DA30 electron analyzer at BL28A [19], as well asSES2002 spectrometers at Tohoku University. The energy
resolution was set to be at 35-60 meV at Photon factoryand 7 meV at Tohoku University. The angular resolution
was set to be 0.2-0.3˚. Crystals were cleaved in-situ in anultrahigh vacuum of ~ 1×10-10 Torr.
3 Results and Discussion
Figures 1(a) and 1(b) show the FS mapping for pristineand moderately Cs-dosed (labeled as Cs) samples obtained
at T = 120 K above TCDW of pristine CVS (91 K) with 106eV photons which probe the electronic states at kz ∼ 0. In
the pristine sample, a circular pocket formed by a parabolic
electron band centered at the Γ point [Fig. 1(d)] isrecognized. This pocket is attributed to the 5pz band of Sb1
atoms embedded in the kagome-lattice plane [inset of Fig.
1(a)]. At the Brillouin-zone boundary, two features arerecognized, one forms a large hexagonal FS centered at theΓ point with mainly V 3dxz/yz character of the kagomelattice band and the other forms a triangular FS centered at
the K point with mainly V 3dxy/x2−y2 character. A side-byside comparison of Figs. 1(a) and 1(b) reveals that the Γcentered pocket expands with Cs doping (dashed circles).
This is natural because each Cs atom adsorbed on thesurface would donate one electron to the CVS bands. The
FS expansion is also visualized by a direct comparison of
the parabolic band around the Γ point in Figs. 1(c) and 1(d)showing a clear downward shift with Cs dosing. By aquantitative analysis of the experimental band dispersions,the energy shift of the Cs sample with respect to the pristinePhoton Factory Activity Report 2022 #40 (2023)

one was estimated to be ∼0.24 eV, indicating a marked
influence of Cs dosing to the surface electronic states ofCVS. By determining the location of Fermi wave vector(kF) points in the 2D k space for the electron pocket [Fig.
1(e)], the FS for the Cs sample was found to be expandedby 65% relative to that of the pristine sample.

Fig. 1 (a),(b) ARPES-intensity map at EF plotted as afunction of kx and ky measured at T = 120 K (above TCDW)
with 106-eV photons (corresponding to the kz ∼ 0 plane)
for pristine CVS (pristine) and moderately Cs-dosed (Cs)samples, respectively. The inset of (a) shows the crystal
structure of CVS. (c),(d) Comparison of the ARPES
intensity around the Γ point between pristine and Cssamples. (e) Comparison of the kF points for the Γ-centeredpocket between the two samples.
To examine the energy shift of bands in more detail, weshow the APRES intensity along the ΓKM and ΓM cuts inpristine and Cs samples at T = 120 K in Figs. 2(a)–2(d).
The ARPES intensity for the pristine sample in Figs. 2(a)and 2(b) signifies the direct correspondence with thecalculated bands. For example, besides the Γ-centeredelectron band with the Sb pz orbital (labeled as α band) seenin Fig. 1(c), two linearly dispersive bands (β and γ bands)
with the V dxz/yz character forming a Dirac-cone-like
dispersion between the Γ and K points are identified in theexperiment [Fig. 2(a)]. Also, the calculated dxy/x2−y2 band (δband) forming a saddle point pinned almost at EF, called
here the saddle point band, is seen in the experiment as ashallow hole band at the M point. Another calculated dxy/x2−y2 band (ε band) that intersects the saddle point band at theK point to form a Dirac point (in the case of negligible
SOC) is also seen in the experiment. The ARPES intensity
also reproduces the calculated dxz/yz bands topped at ∼0.45
eV and near EF at the M point (ζ and η bands) along theKM cut [Fig. 2(a)], and a shallow electron band (ζ band)
and a holelike band with the top of dispersion slightly
below EF (ι band) at the M point along the ΓM cut [Fig.
2(b)]. These experimental results show a good agreementwith the calculation regarding the number and shape of theband dispersion. All the near-EF bands predicted in thecalculation are also resolved in the Cs sample [Figs. 2(c)and 2(d)] but with a band-dependent energy shift comparedwith the pristine sample. For instance, the crossing point
between the β and γ bands shifts downward by ∼0.1 eV
after Cs dosing, in disagreement with the downward shiftof 0.24 eV for the Γ-centered α band. Furthermore, thecrossing point between the δ and ε bands does not show aclear downward shift by Cs dosing [Fig. 2(c)], suggesting
the orbital- dependent band shift.

Fig. 2 (a),(b) ARPES intensity as a function of wave vector
and binding energy (EB) in a wide EB range, measured at T= 120 K along the ΓKM and ΓM lines, respectively, forpristine sample. (c),(d) Same as (a) and (b), respectively,
but for Cs sample.
We revealed that Cs dosing leads to an intriguing change
in the band structure that is beyond the explanation withthe rigid-band model, i.e., the orbital-selective electron
doping. We found that the CDW can be completely killed
when the dxz/yz-derived shallow electron band is pushed
away from EF while keeping the saddle point of the dxy/x2−
y2 orbital pined almost at EF, pointing to the importance ofthe multiorbital effect for understanding the CDW. The
present result opens a pathway toward manipulating theCDW and superconducting properties in AV3Sb5 throughcarrier tuning.
Acknowledgements
This work was supported by JST-CREST (No.
JPMJCR18T1), JST-PREST (No. JPMJPR18L7), JSPS
KAKENHI Grants (No. JP17H01139 and No.
JP18H01160 ). T. K. acknowledges support from GP-Spinat Tohoku University.
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H. Tanaka, A.Toyoshima, K. Amemiya, T. Kawakami,* k.nakayama@arpes.phys.tohoku.ac.jp** t-sato@arpes.phys.tohoku.ac.jp