# Evidence of topological nodal lines and surface states
in the centrosymmetric superconductor SnTaS2

Wenqing Chen  State Key Laboratory of Functional Materials for Informatics and
Center for Excellence in Superconducting Electronics, Shanghai Institute of
Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai
200050, China  College of Materials Science and Opto-Electronic Technology,
University of Chinese Academy of Sciences, Beijing 100049, China  Lulu Liu
National Laboratory of Solid State Microstructures, School of Physics, Nanjing
University, Nanjing 210093, China  Wentao Yang  State Key Laboratory of
Surface Physics, Department of Physics, and Advanced Materials Laboratory,
Fudan University, Shanghai 200438, China  Dong Chen  College of Physics,
Qingdao University, Qingdao 266071, China  Zhengtai Liu  State Key Laboratory
of Functional Materials for Informatics and Center for Excellence in
Superconducting Electronics, Shanghai Institute of Microsystem and Information
Technology, Chinese Academy of Sciences, Shanghai 200050, China  Yaobo Huang
Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai
201204, China  Tong Zhang  State Key Laboratory of Surface Physics, Department
of Physics, and Advanced Materials Laboratory, Fudan University, Shanghai
200438, China  Collaborative Innovation Center of Advanced Microstructures,
Nanjing University, Nanjing 210093, China  Haijun Zhang<EMAIL_ADDRESS>National Laboratory of Solid State Microstructures, School of Physics, Nanjing
University, Nanjing 210093, China  Collaborative Innovation Center of Advanced
Microstructures, Nanjing University, Nanjing 210093, China  Zhonghao Liu
<EMAIL_ADDRESS>State Key Laboratory of Functional Materials for
Informatics and Center for Excellence in Superconducting Electronics, Shanghai
Institute of Microsystem and Information Technology, Chinese Academy of
Sciences, Shanghai 200050, China  College of Materials Science and Opto-
Electronic Technology, University of Chinese Academy of Sciences, Beijing
100049, China  D. W. Shen<EMAIL_ADDRESS>State Key Laboratory of
Functional Materials for Informatics and Center for Excellence in
Superconducting Electronics, Shanghai Institute of Microsystem and Information
Technology, Chinese Academy of Sciences, Shanghai 200050, China  College of
Materials Science and Opto-Electronic Technology, University of Chinese
Academy of Sciences, Beijing 100049, China

###### Abstract

The discovery of signatures of topological superconductivity in
superconducting bulk materials with topological surface states has attracted
intensive research interests recently. Utilizing angle-resolved photoemission
spectroscopy and first-principles calculations, here, we demonstrate the
existence of topological nodal-line states and drumheadlike surface states in
centrosymmetric superconductor SnTaS2, which is a type-ii@ superconductor with
a critical transition temperature of about 3 K. The valence bands from Ta 5$d$
orbitals and the conduction bands from Sn 5$p$ orbitals cross each other,
forming two nodal lines in the vicinity of the Fermi energy without the
inclusion of spin-orbit coupling (SOC), protected by the spatial-inversion
symmetry and time-reversal symmetry. The nodal lines are gapped out by SOC.
The drumheadlike surface states, the typical characteristics in nodal-line
semimetals, are quite visible near the Fermi level. Our findings indicate that
SnTaS2 offers a promising platform for exploring the exotic properties of the
topological nodal-line fermions and helps in the study of topological
superconductivity.

##  I Introduction

The long-sought topological superconductor (TSC) with Majorana fermions
extends scientific territory and plays an important role in strategies for
topological quantum computing, though it still remains elusive in real
materials. A typical TSC can be found in a $p+ip$ system, where the core of
the superconducting vortex contains a localized quasiparticle with exactly
zero energy Qi and Zhang (2011). However, the properties of several
experimental candidates for $p$-wave superconductivity remain unclear
Mackenzie and Maeno (2003). Besides the $p$-wave pairing, the TSC could also
be realized in the system with an $s$-wave pairing plus the spin-orbit
coupling (SOC) and spin polarization, which is equivalent to the $p+ip$ system
with broken spin-rotation symmetry Fu and Kane (2008); Liu  _et al._ (2011).
Due to the proximity effect Fu and Kane (2008); Huang  _et al._ (2019), an
effective $p$-wave pairing can be realized on the interface of a
heterostructure between a strong topological insulator (TI) and an $s$-wave
superconductor (e.g., Bi2Se3/NbSe2, Bi2Te3/NbSe2, and Bi2Se3/Pb) Mourik  _et
al._ (2012); Wang  _et al._ (2012); Xu  _et al._ (2015); Sun  _et al._ (2016);
Qu  _et al._ (2012) or a full-gapped bulk superconductor with topologically
protected gapless surface (or edge) states (e.g., $\beta$-PdBi2, FeTe1-xSex,
CuxBi2Se3, Li0.84Fe0.16OHFeSe, and 2$M$-WS2) Sakano  _et al._ (2015); Hao and
Hu (2019); Sasaki  _et al._ (2011); Tao  _et al._ (2018); Wang  _et al._
(2018); Zhang  _et al._ (2018); Yin  _et al._ (2015); Liu  _et al._ (2018a);
Fang  _et al._ (2019); Yuan  _et al._ (2019). Based on the latter scenario,
type-ii@ superconductors with nontrivial surface states are promising
candidates for TSCs. The surface states and superconducting gaps can be
experimentally determined by angle-resolved photoemission spectroscopy (ARPES)
and quasiparticle scattering interference imaging combined with scanning
tunneling spectra. The scanning tunneling spectra can further be used to
search for Majorana zero modes in the vortex core of these materials.

In addition, noncentrosymmetric superconductors with strong SOC have been
proposed as a kind of TSC, because of the breaking of spin degeneracy by
asymmetric SOC Ali  _et al._ (2014). It has been reported that
noncentrosymmetric PbTaSe2 hosting topological nodal lines and nontrivial
surfaces states around the Fermi energy ($E\rm_{F}$) is a candidate for TSC
without the aid of doping or artificial heterostructures Bian  _et al._
(2016); Chang  _et al._ (2016); Wang  _et al._ (2016); Guan  _et al._ (2016);
Xu  _et al._ (2019); Ikeda  _et al._ (2020); Benia  _et al._ (2016). The
superconductivity on its surface states induced by the proximity effect can be
stronger than that in TI/$s$-wave superconductor heterostructures Chang  _et
al._ (2016). Combining its helical spin-polarized surface state Chang  _et
al._ (2016) with superconductivity, PbTaSe2 therefore would be a TSC even
without asymmetric SOC. Here, we find the centrosymmetric superconductor
SnTaS2, which is isoelectronic to PbTaSe2 and also has a crystal structure
similar to that of layered PbTaSe2. This centrosymmetric crystal structure is
protected by the spatial-inversion symmetry and the time-reversal symmetry
($T$$\cdot$$P$ symmetry) Chiu and Schnyder (2014); Yang  _et al._ (2018). The
previous calculations suggest that SnTaS2 has nodal-line structures with
drumheadlike surface states around $E\rm_{F}$ Chen  _et al._ (2019); Dijkstra
_et al._ (1989); Jin  _et al._ (2019), making it a good candidate to further
detect the TSCs and Majorana fermions. However, so far, direct ARPES
measurements of the low-energy electronic states are still absent.

Figure 1:  (a) Crystal structure of SnTaS2. (b) The top view of the crystal
structure shows centrosymmetric structure. (c) Three-dimensional bulk and
projected 2D BZs. (d) STM topography image of the cleaved SnTaS2 surface at
$T$ = 4 K. Set point condition: $V$ = 3 V, $I$ = 10 pA. Inset: Atomic-resolved
STM topography taken on TaS2 surface shows hexagonal atomic structure. Set
point condition: $V$ = -0.6 V, $I$ = 10 pA. (e) Scanning tunneling spectra
($dI/dV$) taken on the TaS2 surface at $T$ = 0.4 K under a magnetic field of 0
(blue) and 0.013 T (red) perpendicular to the sample surface. The
superconducting gap of about 0.35 meV is estimated by fitting the blue points
with the Dynes function.

In this paper, we demonstrate the existence of topological nodal-line states
and nontrivial surface states in a type-ii@ BCS superconductor SnTaS2 by
employing ARPES and scanning tunneling microscopy/spectroscopy (STM/S) in
combination with first-principles calculations. STS measurements indicate that
the superconducting gap of about 0.35 meV at $T$ = 0.4 K can be suppressed by
an applied magnetic field of 0.013 T perpendicular to the sample surface,
which is consistent with the transport measurements Chen  _et al._ (2019). The
calculations show that the nodal lines are formed by the crossings of the Ta
5$d$ orbitals and the Sn 5$p$ orbitals in the vicinity of $E\rm_{F}$ without
the inclusion of SOC. If SOC is considered, nodal lines are gapped out. The
surface bands connecting to each nodal line indicate the topological
nontrivial states. ARPES observations of the features are in good agreement
with the band calculations. The observed surface state can be clearly
distinguished from the bulk states near $E\rm_{F}$. These results uncover the
topological electronic states in superconducting SnTaS2, offering a new
platform for further studies into Dirac nodal-line physics as well as TSCs.

Figure 2:  (a) Total and orbital-projected DOS within $\pm$2 eV at $E\rm_{F}$.
The different colors indicate the different orbitals. (b) and (c) The
calculated bulk bands with their orbital characters along the high-symmetry
directions without and with SOC, respectively. (d),(e) Enlarged views of bulk
band structure without and with SOC on the $M$-$K$-$\Gamma$ plane around
$E\rm_{F}$, respectively, show nodal-line structure. The band crossings in (d)
are indicated by N1, N2, and N2′. The gapped nodes in (e) are indicated by D1,
D2, and D2′. (f) and (g) The calculated constant-energy surfaces without SOC
at $E\rm_{F}$ and $E\rm_{F}$ \- 0.39 eV, respectively, show nodal lines (NL1
and NL2) as indicated by the arrows. (h) and (i) Band structures with SOC for
the 30-unit-thick layer. The bands with separation and the gray shaded regions
present the bulk bands, and the others are surface bands. Projected band
structures for the Sn-terminated (red) and S-terminated (blue) (001) surfaces
of SnTaS2 are shown in (i). The surface states connecting the nodes are
indicated by the arrows.  Figure 3:  (a) Integrated intensity plots on the _k
z_-_k ∥_ plane at $E_{F}\pm$10 meV. (b) Corresponding second-derivative plots
of (a). The red line indicates a band with weak $k_{z}$ dispersion. (c)
Bottom: Photoemission intensity plots taken along the $A$-$H$ direction with
48-eV photons ($k_{z}\sim$ $\pi$). Top: MDC is taken along the red solid line,
and the peaks are indicated by the turquoise arrows. (d) Corresponding second-
derivative plots of (c). The observed surface state is indicated by the arrow.
(e) Projected band structures for the two terminated (001) surfaces along
$\bar{\Gamma}-\bar{K}$. (f)-(h) Integrated intensity plots at $E_{F}\pm$10 meV
taken with 60-eV ($k_{z}\sim$ 0), 72-eV ($k_{z}\sim$ $\pi$), and 84-eV
($k_{z}\sim$ 0) photons, respectively.  Figure 4:  (a)-(c) Photoemission
intensity plots along the $\Gamma$-$K$, $K$-$M$, and $\Gamma$-$M$ directions,
respectively. (d)-(f) DFT-projected bulk bands and surface bands of the Sn-
terminated (001) surface along the $\Gamma$-$K$, $K$-$M$, and $\Gamma$-$M$
directions, respectively, corresponding to (a)-(c). The surface states and the
Dirac nodes are indicated by the black arrows and the transparent dots,
respectively. (g) MDCs around the nodal-line feature near the $K$ point along
$\Gamma$-$K$, as indicated by the dashed rectangle in (a). (h) MDCs around the
nodal-line feature near the $K$ point along $K$-$M$, as indicated by the
dashed rectangle in (b). The dashed lines are guides to the bands.

##  II Methods

The single crystals were grown by the chemical vapor transport method as
described elsewhere Chen  _et al._ (2019). STM experiments were carried out
with a Unisoku low-temperature STM system. SnTaS2 single crystals were cleaved
$in$ $situ$ at $T$ = 77 K under ultrahigh vacuum and transferred immediately
to a STM head under a vacuum of 2$\times$10-10 Torr. STS measurements were
done using a standard lock-in technique with a bias modulation of 0.1 mV at a
frequency of 741 Hz. ARPES measurements were performed at the Dreamline and
03U beamlines of Shanghai Synchrotron Radiation Facilities (SSRF). Samples
smaller than $1\times 1$ mm2 were cleaved $in$ $situ$, exposing flat,
mirrorlike (001) surfaces. During measurements, the temperature was kept at
$T$ = 20 K, and the pressure was maintained at less than $4\times 10^{-11}$
Torr. The first-principles calculations based on the density functional theory
(DFT) were performed by using the Vienna Ab initio Simulation Package (VASP)
Sun  _et al._ (2003); Hafner (2007), with the generalized gradient
approximation of the Perdew-Burke-Ernzerhof type as the exchange-correlation
functional Takeshi  _et al._ (2004); Jain  _et al._ (2011). The plane-wave
cutoff energy was set to be 420 eV in all first-principles calculations, and
the first Brillouin zone (BZ) was sampled using a $13\times 13\times 5$
Monkhorst-Pack grid. The experimental lattice constants $a$ = $b$ = 3.309 Å
and $c$ = 17.450 Å Chen  _et al._ (2019) were used in the calculations.

##  III Results and discussions

The crystal structure of SnTaS2 is illustrated in Figs. 1(a) and 1(b), and
crystallizes in a layered hexagonal structure with a space group of
$P$63/$mmc$ (No. 194). Isoelectronic PbTaSe2 [$P\bar{6}m_{2}$ (No. 187)] has a
similar layered hexagonal structure. SnTaS2 is a centrosymmetric crystal with
$T$$\cdot$$P$ symmetry, however, noncentrosymmetric PbTaSe2 without
$T$$\cdot$$P$ symmetry has a reflection symmetry caused by the Ta atomic plane
Bian  _et al._ (2016); Ali  _et al._ (2014). The corresponding three-
dimensional (3D) and projected two-dimensional (2D) BZs
($\bar{\Gamma}-\bar{K}-\bar{M}$) are shown in Fig. 1(c), where the $\Gamma$,
$K$, and $M$ points serve as the high-symmetry points of the $k_{z}$ = 0 plane
and the $A$, $H$, and $L$ points serve as the high-symmetry points of the
$k_{z}$ = $\pi$ plane.

To reveal the surface topography and the superconductivity, STM/S measurements
were carried out on the (001) surfaces of SnTaS2. The high-resolution STM
topography taken on the area of the cleaved surface [Fig. 1(d)] shows a flat
TaS2 plane decorated with islands of Sn. The atomic-resolved STM topography
taken on the TaS2 surface [inset of Fig. 1(d)] shows a clear hexagonal atomic
structure with a lattice parameter of $a$ = 3.3 Å. Figure 1(e) displays the
scanning tunneling spectra ($dI/dV$) at $T$ = 0.4 K under a magnetic field of
0 (blue) and 0.013 T (red) perpendicular to the sample surface. To determine
the gap size more precisely, we fit the tunneling spectrum with the Dynes
function. The fitting curve is obtained by the thermal broadened density of
states (DOS), which was calculated from the tunneling current expression Sacks
and Noguera (1991); Sugimoto  _et al._ (2015):

$I(V)\propto\int_{-\infty}^{\infty}\left|\operatorname{Re}\frac{E-i\Gamma}{\sqrt{(E-i\Gamma)^{2}-\Delta^{2}}}\right|[f(E)-f(E+eV)]dE$

where $f(E)$ is the Fermi-Dirac distribution function. The superconducting gap
of about 0.35 meV at $T$ = 0.4 K is estimated based on the fitting curve [blue
curve in Fig. 1(e)]. By applying a magnetic field of 0.013 T perpendicular to
the $ab$ plane, the superconducting gap is suppressed. No vortex state can be
found before the gap is closed, although many attempts have been made in
various regions of different surfaces and samples. Considering the small upper
critical field of $H\parallel c$ [$H_{c2}(0)$ = 203.6 Oe] Chen  _et al._
(2019), the vortex state is hard to find, possibly due to the low vortex
density under such a small applied magnetic field.

We performed orbital-projected DFT calculations to study the low-energy
electronic bands with their orbital characters in SnTaS2. Figure 2(a) shows
total and orbital-projected DOS within $\pm$2 eV at $E\rm_{F}$. The obvious
dip of DOS near $E\rm_{F}$ suggests a semimetallic character. Figures 2(b) and
2(c) show orbital-projected dispersions along the high-symmetry directions
without and with SOC, respectively. The electronic structure near $E\rm_{F}$
is mainly dominated by Ta 5$d$ (red) and Sn 5$p$ (blue) orbitals mixed with
less S 3$p$ (green). The holelike Ta 5$d$ bands and the electronlike Sn 5$p$
bands cross each other, forming several band crossings around the $K$ point
near $E\rm_{F}$, as shown in Fig. 2(b). When SOC is taken into account, band
gaps open at the crossing points, as shown in Fig. 2(c). In order to see the
crossing points clearly, we enlarge the view around the $K$ point near
$E\rm_{F}$, as shown in Figs. 2(d) and 2(e). Without the inclusion of SOC,
there are two crossing points considered as nodes along the $\Gamma$-$K$
direction, namely, N1 and N2. The corresponding crossing point of N2 along the
$\Gamma$-$M$ direction is marked with N2′. With the inclusion of SOC, the
nodes are all gapped out. The sizes of SOC gaps at D1 and D2 (D2′) are about
40 and 80 meV [Fig. 2(e)], respectively, which are comparable with typical
nodal-line materials such as the PbTaSe2, CaAgAs, TiB2, ZrSiS, and CaP3
families Bian  _et al._ (2016); Yamakage  _et al._ (2016); Liu  _et al._
(2018b); Lou  _et al._ (2018); Takane  _et al._ (2016); Topp  _et al._ (2017);
Fu  _et al._ (2019); Xu  _et al._ (2017); Song  _et al._ (2020). To carefully
check whether the band crossings form the nodal lines in the momentum space,
we calculated constant- energy surfaces without SOC at $E\rm_{F}$ and
$E\rm_{F}$ \- 0.39 eV, respectively. The nodal lines (NL1 and NL2) formed by
N1 and N2 (N2′) are indicated by the arrows, as shown in Figs. 2(f) and 2(g).
Because the energy positions of crossing points of NL2 are different
surrounding the $K$ point, NL2 shows various thicknesses at different momentum
positions on the constant-energy surface.

In addition, the drumheadlike surface state is a typical characteristic in
nodal-line semimetals and could bring varieties of unusual intriguing
properties. To study the surface states, we performed the slab calculation
with a 30-unit-thick layer [Figs. 2(h) and 2(i)]. The bands with separation
and the gray shaded regions present the bulk bands, and the others are surface
bands in Figs. 2(h) and 2(i). Figure 2(i) shows the projected spectra for the
Sn-terminated (red) and S-terminated (blue) (001) surfaces of SnTaS2. The
clear surface states connecting the nodes are indicated by the arrows for the
two terminated (001) surfaces. Due to the absence of chiral symmetry in this
material, the drumheadlike surface state is sensitive to surface conditions
and is not flat. However, one can always find the surface bands connecting to
the nodes in the calculation, indicative of the nontrivial topological nature
of the nodal lines.

To directly detect the topological features near $E\rm_{F}$, we performed
ARPES measurements on (001) surfaces of SnTaS2. Since the nodal lines are
located on the $k_{z}$ = 0 plane according to the calculation, we carried out
the photon-energy-dependent ARPES measurement to investigate the detailed
$k_{z}$ dispersions. With an empirical value of the inner potential of
$\sim$11 eV and $c$′ = $c$/2 = 8.725 Å (due to the bilayer in the conventional
unit cell), we found that $hv$ = 60 and 84 eV are close to the $\Gamma$ point
and 48 and 72 eV are close to the $A$ point, according to the free-electron
final-state model Liu  _et al._ (2012). Although the bands near $E\rm_{F}$
show weak $k_{z}$ dispersions in Figs. 3(a) and 3(b), a periodic modulation
along the $k_{z}$ direction can still be found, as shown by the dispersions
marked by the red line in Fig. 3(b). Figures 3(c) and 3(d) show energy-
momentum plots and corresponding second derivative plots along the $A$-$H$
line (48 eV), respectively. The marks on the momentum distribution curve (MDC)
taken at a binding energy of about 0.2 eV refer to the bands crossing
$E\rm_{F}$. The whole band structure looks more like the slab calculation with
Sn-terminated (001) surface states of Fig. 2(i), rather than the bulk states
of Fig. 2(c). Especially, the bands around the BZ center with weak intensities
seem to be stuck together, which is different from the clean bulk-calculation
bands with strong dispersions along the $\Gamma-A$ direction in Fig. 2(c).
This discrepancy is mainly caused by the $k_{z}$ integration and matrix
element effects in the ARPES spectra, which reflect the electronic states
integrated over a certain $k_{z}$ region of the bulk BZ and the states at
$k_{z}$ = 0 and $\pi$ contributed greatly Kumigashira  _et al._ (1998). As
evidenced in Figs. 3(f)-3(h), the Fermi surfaces of the $k_{z}$ $\sim$ 0 and
$\pi$ planes are almost the same. The Fermi surfaces are similar to those of
PbTaSe2, which can be more precisely figured out with the help of calculations
and quantum oscillations Xu  _et al._ (2019). In recent studies on the
topological semimetals Bian  _et al._ (2016); Takane  _et al._ (2016); Topp
_et al._ (2017); Liu  _et al._ (2018b); Lou  _et al._ (2018), it is noticeable
that the states from $k_{z}$ = 0 and $\pi$ can be detected on other $k_{z}$
planes in the ARPES measurements. Thus, ARPES data are usually compared with
the calculation results of the slab and the surface states. The calculated
surface states along $\bar{\Gamma}-\bar{K}$ are shown in Fig. 3(e). The
observed surface band indicated by experimental data [Fig. 3(d)] corresponds
well to the calculated surface states for the Sn-terminated (001) surface, as
indicated in Fig. 3(e). Compared with the observations, the calculated bands
around the nodal-line feature need to be relatively shifted up and down due to
correlation effects of $d$-electron and interband coupling near $E\rm_{F}$ Liu
_et al._ (2012, 2017).

In Fig. 4, we compare in detail the observed dispersions with the calculated
surface states along the high-symmetry lines. Figures 4(a)-4(c) show ARPES
intensity plots along the $\Gamma$-$K$, $K$-$M$, and $\Gamma$-$M$ directions,
respectively. The band crossings are clearly shown in the dashed rectangles in
Figs. 4(a) and 4(b). As discussed above, the observed bands look more like the
slab calculation with a Sn-terminated (001) surface. We thus show DFT-
projected bulk bands and surface bands of the Sn-terminated (001) surface
along the three corresponding directions in Figs. 4(d)-4(f), respectively. The
surface states and the Dirac nodes are indicated by the black arrows and the
transparent dots, respectively. The calculation results are very consistent
with the ARPES measurements including $k_{z}$ integration, except that the
calculated holelike bands near $M$ need to be shifted down and the calculated
nodal-line features near $K$ need to be shifted up for the reasons mentioned
above. Combining the measured and calculated bands along $\Gamma$-$K$ [Figs.
4(a) and 4(d)], one can figure out the surface state (SS1) and the Dirac node
(D1), which can be more clearly seen in the MDC plot in Fig. 4(g). D1 is
located about 0.3 eV below $E\rm_{F}$, and SS1 corresponds to the marked
surface states in Figs. 3(e) and 4(d). Because of the upward-shifting band,
SS2 and D2 should be located above $E\rm_{F}$ and cannot be detected along
$\Gamma$-$K$ and $K$-$M$. However, the consistency between experiment and
calculation provides solid evidence of the existence of SS2 and D2 near
$E\rm_{F}$. Compared with other nodal-line semimetals, the surface state (SS1)
can be more clearly distinguished from the bulk states near $E\rm_{F}$. The
results will supply valuable information for a further study of the Dirac
nodal-line fermions.

##  IV Conclusion

By presenting an investigation of the low-energy electronic structures in both
experiments and calculations, we provided evidence of topological nodal lines
and surface states in centrosymmetric superconducting SnTaS2. STS measurements
indicate that a superconducting gap of about 0.35 meV at $T$ = 0.4 K is
suppressed by a magnetic field of 0.013 T perpendicular to the sample surface.
As typical characteristics in nodal-line semimetals, the surface states
connecting the nodes make SnTaS2 a good platform for exploring the topological
nodal-line fermions. Our findings are also beneficial for studies on TSCs.

##  V ACKNOWLEDGEMENTS

This work was supported by the National Key R&D Program of the MOST of China
(Grants No. 2016YFA0300204, No. 2016YFA0300200, and No. 2017YFA0303004), the
National Science Foundation of China (NSFC; Grants No. 11704394, No. 11804176,
No. 11888101, No. 11790310, No. 11421404, No. 11674165, No. 11834006, and No.
U2032208), and the Fundamental Research Funds for the Central Universities
(Grant No. 020414380149). D.W.S. is supported by the Award for Outstanding
Member in Youth Innovation Promotion Association CAS. The ARPES experiments
were performed at the Dreamline beamline of SSRF and supported by the Ministry
of Science and Technology of China (Grants No. 2016YFA0401002, and No.
2017YFA0403401) and the CAS Pioneer Hundred Talents Program. Part of this
research used the 03U Beamline of the SSRF, which is supported by the ME2
Project (Grant No. 11227902) of NSFC.

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