RAPID COMMUNICATIONSPHYSICAL REVIEW B 92, 100508(R) (2015)

Observation of two-dimensional bulk electronic states in the superconducting topological insulator
heterostructure Cu x (PbSe)5 (Bi2 Se3 )6 : Implications for unconventional superconductivity
K. Nakayama,1 H. Kimizuka,1 Y. Tanaka,1 T. Sato,1 S. Souma,2 T. Takahashi,1,2 Satoshi Sasaki,3,*
Kouji Segawa,3,† and Yoichi Ando3,‡
1

2

Department of Physics, Tohoku University, Sendai 980-8578, JapanWPI Research Center, Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
3
Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan(Received 2 April 2015; published 28 September 2015)

We have performed angle-resolved photoemission spectroscopy (ARPES) on Cux (PbSe)5 (Bi2 Se3 )6 [(CPSBS);x = 1.47], a superconductor derived from a topological insulator heterostructure, to elucidate the electronic states
relevant to the occurrence of possible unconventional superconductivity. Upon Cu intercalation into the parent
compound (PbSe)5 (Bi2 Se3 )6 , we observed a distinct energy shift of the bulk conduction band due to electron
doping. Photon-energy-dependent ARPES measurements of CPSBS revealed that the observed bulk band forms
a cylindrical electronlike Fermi surface at the Brillouin-zone center. The two-dimensional nature of the bulk
electronic states puts strong constraints on the possible topological character of the superconducting state in
CPSBS.
DOI: 10.1103/PhysRevB.92.100508

PACS number(s): 74.78.Fk, 73.20.−r, 75.70.Tj, 79.60.−i

Topological superconductors (TSCs) manifest a novel
quantum state of matter in which the nontrivial topology ofthe bulk state leads to the emergence of gapless Andreev
bound states often consisting of Majorana fermions [1–5].
Owing to the exotic characteristics of Majorana fermions, suchas particle-antiparticle symmetry and potential use for faulttolerant quantum computations, the search for TSCs is one of
the most attractive and emergent topics in condensed-matterphysics. It has been theoretically proposed that topologicalinsulators (TIs), which are characterized by the gapless topological edge or surface states originating from a band inversion
due to strong spin-orbit coupling, can provide a platform torealize topological superconductivity when enough carriers
are introduced [6,7]. A first example of the superconductor
derived from TIs is Cux Bi2 Se3 [8,9] in which point-contact
spectroscopy experiments observed a pronounced zero-bias
conductance peak indicative of unconventional surface Andreev bound states [10]. A subsequent experiment in a doped
topological crystalline insulator (TCI) Sn1−x Inx Te [11] hasalso reported similar Andreev bound states. These results suggest that the carrier doping into the TIs and TCIs is an effective
strategy to search for three-dimensional (3D) TSCs, whereas
the electronic states (in particular, the superconducting pairing
symmetry) and their relationship to possible TSC nature arestill under intensive debate [9–20].
Very recently, it has been suggested that a new TIbased superconductor Cux (PbSe)5 (Bi2 Se3 )6 (called CPSBS
here) is an intriguing candidate of TSC [21]. The parent
compound is the m = 2 phase of lead-based homologous
series (PbSe)5 (Bi2 Se3 )3m (called PSBS) in which Bi2 Se3 with*Present address: School of Physics and Astronomy, University ofLeeds, Leeds LS2 9JT, United Kingdom.
†
Present address: Department of Physics, Kyoto Sangyo University,
Kyoto 603-8555, Japan.
‡
Present address: Institute of Physics II, University of Cologne,
Köln 50937, Germany.
1098-0121/2015/92(10)/100508(5)the thickness of m quintuple layers (QLs) alternates with abilayer PbSe unit, forming a natural multilayer heterostructureof TI (Bi2 Se3 ) and an ordinary insulator (PbSe) [22–26]. Anangle-resolved photoemission spectroscopy (ARPES) study
has revealed two Dirac-cone states hybridized with each other,suggestive of the existence of topological interface states atthe boundaries between 2-QL Bi2 Se3 and PbSe layers [27,28].
Upon Cu intercalation into the van der Waals gap at the
middle of the 2-QL Bi2 Se3 unit, superconductivity withTc ∼ 2.9 K has been realized [21]. An important distinctionof CPSBS from other known TI/TCI-based superconductors
is that an unconventional bulk superconductivity with gap
nodes is inferred from the specific-heat measurement [21].
This is intriguing because the existence of gap nodes in astrongly spin-orbit coupled superconductor would give rise tospin-split Andreev bound states that are nothing but helical
Majorana fermions [10,30]. Since the topological character ofthe superconducting state strongly depends on the topology of
Fermi surface in the normal state, it is of particular importanceto establish the bulk electronic states relevant to possible
unconventional superconductivity in CPSBS.
In this Rapid Communication, we report a high-resolution
ARPES study of PSBS (m = 2) and its Cu-intercalated
counterpart CPSBS. By utilizing a relatively long escape depthof photoelectrons excited by low-energy photons, we havesucceeded in observing a previously unidentified intrinsic bulk
band structure of the 2-QL Bi2 Se3 unit lying deeper beneaththe surface. Our most important finding is that the near-EF
bulk band responsible for the superconductivity shows a
two-dimensional (2D) character, in contrast to the 3D characterof bulk Bi2 Se3 . We discuss the implications of the present
result in relation to the possible topological superconductivity
in CPSBS.
High-quality single crystals of a parent compound PSBS
(m = 2) were grown by a modified Bridgman method using
high-purity elements Pb (99.998%), Bi (99.9999%), and Se(99.999%). The crystal quality has been greatly improved fromthat in the previous ARPES study of PSBS [27]; in particular,

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PHYSICAL REVIEW B 92, 100508(R) (2015)

PSBS
domain 1

domain 2

(c)

CPSBS

(f)

Intensity (arb. units)

(a)

0.8

B. E. (eV)

we obtained single-phase crystals of m = 2 without detectable
inclusions of the m = 1 phase in the x-ray diffraction analysis.
Cu intercalation has been achieved by the electrochemical
technique, and the superconducting transition temperature(Tc ) of Cu-intercalated sample (CPSBS) was estimated to be∼ 2.9 K by a magnetic susceptibility measurement. The Cu
concentration x has been determined to be 1.47 from the masschange and the inductively coupled plasma-atomic emission
spectroscopy analyses. Details of the sample preparations
were described elsewhere [21]. ARPES measurements were
performed with MBS-A1 and VG-Scienta SES2002 electron
analyzers equipped with a xenon-plasma discharge lamp(hν = 8.437 eV) at Tohoku University and with tunable
synchrotron lights of hν = 13–23 eV at the beamline BL-7U
at the Ultra Violet Synchrotron Orbital Radiation Facility(UVSOR) as well as hν = 60 eV at the beamline BL-28A
at Photon Factory, respectively. The energy and angularresolutions were set at 10–30 meV and 0.2◦ –0.3◦ , respectively.
Samples were cleaved in situ along the (111) crystal plane inan ultrahigh vacuum at better than 1 × 10−10 Torr. A shiny
mirrorlike surface was obtained after cleaving the samples,
confirming their high quality. The Fermi level (EF ) of thesamples was referenced to that of a gold film evaporated onto
the sample holder.
First, we present high-resolution ARPES data measured
with hν = 60 eV for pristine PSBS (m = 2) where we revealed
the presence of two domains at the surface (named domains
1 and 2), each of which is selectively probed by scanningthe position of a cleaved surface with a small incident
beam spot (a diameter of ∼ 100 μm). Comparison of theenergy distribution curves (EDCs) and corresponding ARPES
intensity plot around the ¯ point of the Brillouin zone in
Figs. 1(a)–1(d) display a marked difference in the near-EF band
structure between the two surface domains. For domain 1, theobserved band structure resembles the previous ARPES resultfor pristine PSBS (m = 2) whose surface is terminated by a
2-QL Bi2 Se3 unit [27]. Two inner electron pockets indicatedby black dots in Fig. 1(a) and black curves in Fig. 1(b)are ascribed to the quantized bulk conduction band (CB) of
2-QL Bi2 Se3 with the Rashba splitting, whereas a pair ofouter-electron and hole bands (blue dots and blue curves)
represent hybridized surface and interface Dirac-cone states.
On the other hand, the band structure for domain 2 showsa single parabola [Figs. 1(c) and 1(d)], which resembles thequantized CB of the topmost 1-QL Bi2 Se3 layer for PSBS
(m = 1) [27]. These results strongly suggest that domains
1 and 2 correspond to 2-QL and 1-QL Bi2 Se3 terminations,
respectively, as schematically illustrated in Fig. 1(e). Taking
into account the single-phase (m = 2) nature of our crystal,the different surface terminations are naturally understood by
the presence of two cleavage planes for PSBS (m = 2), i.e.:(i) between PbSe and Bi2 Se3 units and (ii) between two QLs
within the Bi2 Se3 unit. We note that our growth technique for
the PSBS system has been greatly improved since our originalreport [27], and large single-phase crystals for m = 2 havebecome regularly available [26].
Next we have performed the ARPES measurements on
the Cu-intercalated counterpart (CPSBS). We found that theobserved spectral feature always shows a simple parabolic
band [Figs. 1(f) and 1(g)] irrespective of the incident beam

EF
0.2
0.4
0.6
0.8

0.4

EF 0.8 0.4 EF 0.8
Binding Energy (eV)

(b)

(d)

0.4

EF

(g)

High

hν = 60 eV-0.2 0.0 0.2

(e)
1-QL
Bi2Se3
PbSe

Low-0.2 0.0 0.2 -0.2 0.0 0.2
Wave Vector ( -1)(h) Cudomain 1
domain 2

FIG. 1. (Color online) (a)–(d) Comparison of near-EF EDCs
around the ¯ point [(a) and (c)] and corresponding ARPES intensity[(b) and (d)]. The data in (a)–(d) were taken on different domains of
the cleaved surface of pristine PSBS. All the data have been obtained
at T = 30 K with 60-eV photons. Black and blue dots in (a) [aswell as black and blue curves in (b)] are a guide to the eyes totrace the dispersions of bulk conduction band and gapped Dirac-conebands, respectively. (e) Schematic of the cleaved surface of PSBS.
(f) and (g) Near-EF EDCs and corresponding ARPES intensity plot,
respectively, for CPSBS measured with hν = 60 eV. (h) Schematicof the cleaved surface of CPSBS.

position on the cleaved surface, unlike the case of PSBS. As
one can immediately recognize from a side-by-side comparison of Figs. 1(d) and 1(g), the observed band dispersion for
CPSBS is very similar to that for domain 2 of PSBS (except
for the small downward energy shift due to electron doping).
This suggests that the CPSBS sample is always cleaved at
the Cu layer as illustrated in Fig. 1(h), presumably due tothe expanded van der Waals gap by Cu intercalation. We thus
conclude that the observed parabolic band is the quantized CBof the topmost 1-QL Bi2 Se3 (hereafter we call CB1st ) partiallycovered with residual Cu atoms on the surface, and it may notreflect genuine bulk electronic properties. The absence of bulk
electronic states (derived from the 2-QL Bi2 Se3 unit which lies
beneath the PbSe layer) at this photon energy (hν = 60 eV)
is reasonable since the photoelectron escape depth (∼ 5 Å) ismuch shorter than the distance between the cleaved surface andthe second Bi2 Se3 unit (∼ 15 Å). Nevertheless, we demonstrate

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RAPID COMMUNICATIONSOBSERVATION OF TWO-DIMENSIONAL BULK . . .

CPSBShν = 21 eV
0.8 0.4 EF
B. E. (eV)Wave Vector kx-0.2 0.0 0.2

(d)
K
M

CPSBS
Γ

(c) CPSBS
High

CB1st
60 eV

Low

-0.2 0.0 0.2
-1)

(e)
A
High

CB1st

E = EF
0.28 eV Low
Wave Vector ky(f) PSBS
EF
0.2
A
0.4
0.6 CB1stB
0.8 21 eV(g) PSBS(h) 21 eV

CB1st

PbSe
Bi2Se3Cux(i) CPSBS(j) CPSBS

EF

60 eV

0.2
0.4
0.6 fresh-0.2 0.0 0.2

aged-0.2 0.0 0.2-1)Intensity (a. u.)(b) CPSBS
EF
0.2
A
0.4
0.6 CB1st
0.8 21 eV

Binding Energy (eV)Intensity (arb. units)

B. E. (eV)(a)

PHYSICAL REVIEW B 92, 100508(R) (2015)
60 eV

Incr. (l) CB1st
time
A

(k)
0.6 0.4 0.2 EF 0.1 0.2
k -1)
B. E. (eV)FIG. 2. (Color online) (a) EDCs around the ¯ point for CPSBS measured at T = 30 K with hν = 21 eV. (b) Second-derivative intensity of(a) plotted as a function of binding energy and wave vector. (c) Same as (b), but measured with hν = 60 eV. (d) and (e) ARPES intensity map
at EF and 0.28-eV binding energy, respectively, for CPSBS plotted as a function of the in-plane wave vector. The intensity was obtained byintegrating the second-derivative EDCs within ±10 meV centered at EF or 0.28 eV. (f) and (g) Second-derivative ARPES intensity around the¯ point for PSBS at hν = 21 and 60 eV, respectively. (h) Schematic of the photoemission process for hν = 21 and 60 eV. (i) and (j) Near-EF
band dispersions obtained soon after cleaving and ∼10 h after cleaving, respectively, at hν = 15 eV. (k) Comparison of the EDCs at the ¯point for fresh and aged surfaces of CPSBS. (l) Momentum distribution curve at EF extracted from the raw ARPES intensity of (j). Arrows
indicate the Fermi wave vectors of CB1st and band A.

in the following that the utilization of lower-energy photons(hν = 8.4–23 eV), which have a relatively long photoelectron
escape depth (10–50 Å), allows us to investigate more intrinsic
bulk electronic states of the 2-QL Bi2 Se3 unit.
Figures 2(a) and 2(b) display the EDCs of CPSBS andcorresponding second-derivative intensity plots, respectively,
measured with hν = 21 eV. In addition to CB1st , one can findanother band at a binding energy of ∼ 0.28 eV (labeled A),
which was not clearly visible with hν = 60 eV [Fig. 2(c)].
This band displays an electronlike dispersion around the ¯point along both ¯ M̄ and ¯ K̄ cuts [see the Supplemental
Material [31] and Fig. 2(i)] and merges to CB1st withapproaching EF (this point will be clarified by the surface aging
measurement as shown later). This can be also recognizedfrom the ARPES intensity mappings as a function of a 2D
wave vector [Figs. 2(d) and 2(e)] where two well-separated
circular intensity profiles at 0.28 eV merge to form a hexagonal
Fermi surface at EF . As shown in Fig. 2(f), we also find asimilar electronlike band with hν = 21 eV at domain 2 of
PSBS together with a holelike band (labeled B), both of which
are not seen in the ARPES data for hν = 60 eV [Fig. 2(g)].
Since the photoelectron escape depth for hν = 21 eV is longer
than that for 60 eV as schematically illustrated in Fig. 2(h),it is natural to ascribe bands A and B to the electronic statesof the 2-QL Bi2 Se3 unit located deeper beneath the surface(note that the appearance of these bands is not due to thephotoionization cross section, final-state effects, kz dispersion,or resolution effects [32]). As shown in Figs. 2(i)–2(k), thesurface aging leads to a clear upward shift of CB1st in contrast
to the stationary nature of band A, indicating that band A is
insensitive to the surface chemical environment as expected
for the bulk electronic states. Thanks to the energy shift ofCB1st , the EF crossing of band A is now clearly visible asmarked by the red arrow in Fig. 2(j) [see also Fig. 2(l) where
the momentum distribution curve at EF contains two peaks

originating from the EF crossings of CB1st and band A]. We
have confirmed the EF crossing of band A with different
experimental conditions (see the Supplemental Material [31]).
All these observations support the second 2-QL Bi2 Se3 originof bands A and B.
Having established the bulk origin of bands A and B, thenext question is their band assignment. In this regard, it isuseful to compare their energy dispersions with those fordomain 1 of PSBS since both originate from the 2-QL Bi2 Se3
unit. As shown in Fig. 2(f), the energy separation between thebottom of the electronlike band A and the top of the holelike
band B is 0.5 eV. This value is in good agreement with anenergy separation between the quantized bulk CB and thelower branch of the gapped Dirac-cone state in the topmost
2-QL Bi2 Se3 of PSBS in Fig. 1(a). Thus, bands A and B
are attributed to the quantized bulk CB and the lower branchof the gapped Dirac-cone state, respectively. It is noted thatbands A and B in Fig. 2(f) are shifted downward by ∼ 0.1 eV
as compared to those in Fig. 1(a). This may be due to theweakened band-bending effect (i.e., weakened out-of-planepotential gradient) at the second Bi2 Se3 unit, which may also
explain the absence of a clear Rashba splitting for band A in
Figs. 2(b) and 2(f). Also, the absence of the upper branch of
the gapped Dirac-cone state is likely due to overlapping by
CB1st with a dominant intensity.
To elucidate the Fermi-surface topology in the 3D momentum space relevant to the possible unconventional superconductivity of CPSBS, we have performed hν-dependent ARPES
measurements by focusing on band A (hereafter we call
CB2nd ). As one can see in the representative second-derivative
intensity plots in Fig. 3(a), we commonly see CB2nd andCB1st over a wide hν range of 8.4–23 eV (note that 8.4-eVphotons are most bulk sensitive). A direct comparison of
the EDC at the ¯ point in Fig. 3(b) measured with various
photon energies reveals that the energy position of CB2nd

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K. NAKAYAMA et al.

EF

CPSBS

CPSBS

CPSBS

CB

2nd
0.2
0.4
CB1st 13 eV
0.6 21 eV-0.2 0.0 0.2 -0.2 0.0 0.2(b)Intensity (arb. units)

CPSBShν (eV)=
8.4
13

CB1st CB2nd

15
17
21
23

0.1

-1)

PSBS

-0.2

1-QL Bi2Se3
Cux
1-QL Bi2Se3
PbSe

CPSBS
CB2nd

0.0

0.2
-1)
ky

kx

kx

CB1st

PbSe
bulk

Low

-0.2 0.0 0.2

CB2nd
hν
8.4
0.2 1315
17
0.3 2123

E

surface 1-QL Bi2Se3

8.4 eVEF

0.6 0.4 0.2 EF
Binding Energy (eV)

(d)

High

(c)

Binding Energy (eV)B. E. (eV)(a)

PHYSICAL REVIEW B 92, 100508(R) (2015)

E CB2nd

kz

kx

kx

ky

FIG. 3. (Color online) (a) Second-derivative ARPES intensity
around the ¯ point for CPSBS at various photon energies plotted
as a function of binding energy and wave vector. CB1st and CB2nd
denote the conduction bands stemming from the topmost 1-QL Bi2 Se3unit and the second Bi2 Se3 unit located beneath the PbSe layer,
respectively. (b) Comparison of the EDC at the ¯ point measured withvarious photon energies. (c) Photon-energy dependence of the band
dispersion for CB2nd in CPSBS extracted from the second-derivative
intensities. Filled circles and triangles represent the results for the
fresh and aged surfaces, respectively. Detailed procedures used
to extract the data points for the aged surface are presented in
Figs. 2(j)–2(l) and the Supplemental Material [31]. The red curve is a
guide for the eyes to trace the band dispersion. The band dispersion for
PSBS measured with hν = 21 eV is also plotted by open diamonds.
(d) Schematic of the band dispersion and the Fermi surface in the
topmost and the second Bi2 Se3 units in CPSBS.

(and CB1st ) is stationary with the hν variation in contrast to
the 3D-like CB of bulk Bi2 Se3 [33]. Indeed, the extracted
band dispersions of CB2nd overlap very well with each otherwithin the experimental uncertainty irrespective of the hν value[Fig. 3(c)], demonstrating the 2D nature of CB2nd . We also
clarified a finite energy shift (∼ 30 meV) of CB2nd upon Cu
intercalation [see Fig. 3(c)], indicating that the intercalated
Cu atoms provide electrons to the Bi2 Se3 layers, in line with
the Hall-coefficient measurements [21]. Based on the present
ARPES results, we show in Fig. 3(d) the schematic of theelectronic structure in CPSBS. Although the topmost 1-QL
Bi2 Se3 produces a single parabolic band (CB1st ) localized at

the surface, the bulk electronic states are confined within the
2-QL Bi2 Se3 unit sandwiched by the insulating PbSe layers
and form a cylindrical Fermi surface at the Brillouin-zone
center [as shown in red in the right bottom panel of Fig. 3(d)],possibly hybridized with the topological interface state (green
dashed curves). It is noted though that the topologicalinterface state is not clearly seen in the present experiment,
presumably due to its fairly weak intensity or a possible
change in the band parity in the 2-QL Bi2 Se3 unit upon Cu
intercalation.
Now we discuss the implications of our results in relation to topological superconductivity. The most important
finding of the present Rapid Communication is that theunconventional superconductivity in CPSBS originates from
2D bulk electronic states. Although the Fermi surface in thenormal state must enclose an odd number of time-reversal
invariant (TRI) momenta to realize TRI 3D topologicalsuperconductivity [6,7], the observed 2D Fermi surface in
CPSBS encloses two TRI momenta ( and Z points). This
excludes the 3D-TSC scenario. The realization of TRI 2D
topological superconductivity on the top surface via thesuperconducting proximity effect [1–5] is also unlikely since
the topological Dirac-cone states do not appear on the cleaved
surface of CPSBS due to termination with trivial 1-QL Bi2 Se3 .
It is noted though that CPSBS still has a chance to be a
topological superconductor because of the possible occurrenceof nodal superconductivity inferred from a recent specific-heatmeasurement [21]. According to the previous theoreticalstudies for Cux Bi2 Se3 with the rhombohedral D3d crystalline
symmetry, the appearance of gap nodes is restricted to the
odd-parity Eu pairing states when the 2D Fermi surface isrealized [34]. Assuming that the theoretical model proposed
for Cux Bi2 Se3 is also applicable to CPSBS, the observed
2D cylindrical Fermi surface would support the odd-parity
Eu pairing in CPSBS. It should be noted, however, that the
crystal structure of CPSBS has the C2h point-group symmetry
due to the presence of the PbSe layer with square symmetry,unlike the D3d symmetry of Cux Bi2 Se3 . The different crystal
symmetries may favor unconventional nodal pairing that was
not discussed in Cux Bi2 Se3 . Indeed, a recent theoretical study
for CPSBS suggested that d-wave superconductivity could
emerge in the case of the 2D Fermi surface [35]. In thisregard, the present result is compatible with the realizationof either the odd-parity Eu pairing or the even-parity dwave pairing. Both states give rise to vertical line nodes
in agreement with the temperature dependence of electronic
specific heat [21]. When vertical line nodes are realizedon the 2D Fermi surface of a strongly spin-orbit coupled
superconductor, spin-split Andreev bound states, which are
the hallmark of topological superconductivity, would emerge
on some side surfaces that are perpendicular to the (111)crystal plane. Therefore, the superconducting state of CPSBS
may provide an intriguing platform to explore Majorana
fermions if nodal superconductivity is indeed realized. To
gain further insight into the topological character, the verification of the nodal superconductivity by phase-sensitive
measurements would be of particular importance. Elucidating
the role of the topological interface state in the unconventional
superconductivity of CPSBS is an important future task worthchallenging.

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PHYSICAL REVIEW B 92, 100508(R) (2015)

In conclusion, we have reported high-resolution ARPES
results on CPSBS to elucidate the band structure relevant tothe unconventional superconductivity with possible nodes in
the gap function. By utilizing tunable low-energy photons, wehave succeeded in separately determining the band dispersionof the 1-QL Bi2 Se3 unit at the topmost surface and thatof the 2-QL Bi2 Se3 unit lying in the bulk. The obtained
bulk band structure gives evidence for the formation of acylindrical Fermi surface in CPSBS. The present result puts
strong constraints on the possible topological character of thesuperconducting state in CPSBS.

We thank T. Toba for his help with the crystal growth.
We also thank T. Shoman, H. Kumigashira, K. Ono, M.
Matsunami, and S. Kimura for their assistance with the ARPES
measurements. This work was supported by JSPS (Grants No.
KAKENHI 23224010, No. 26287071, No. 15H02105, No.
25287079, No. 24540320, and No. 25220708 and Grant-inAid for JSPS Fellows Grant No. 23.4376), MEXT of Japan(Innovative Area “Topological Materials Science”), AFOSR(Grant No. AOARD 124038), Inamori Foundation, the Murata
Science Foundation, KEK-PF (Proposals No. 2012S2-001 andNo. 2015S2-002), and UVSOR (Proposal No. 24-536).

[1] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010).
[2] X.-L. Qi and S.-C. Zhang, Rev. Mod. Phys. 83, 1057 (2011).
[3] Y. Ando, J. Phys. Soc. Jpn. 82, 102001 (2013).
[4] J. Alicea, Rep. Prog. Phys. 75, 076501 (2012).
[5] C. W. J. Beenakker, Annu. Rev. Condens. Matter Phys. 4, 113(2013).
[6] L. Fu and E. Berg, Phys. Rev. Lett. 105, 097001 (2010).
[7] M. Sato, Phys. Rev. B 81, 220504(R) (2010).
[8] Y. S. Hor, A. J. Williams, J. G. Checkelsky, P. Roushan, J. Seo,
Q. Xu, H. W. Zandbergen, A. Yazdani, N. P. Ong, and R. J. Cava,
Phys. Rev. Lett. 104, 057001 (2010).
[9] M. Kriener, K. Segawa, Z. Ren, S. Sasaki, and Y. Ando,Phys. Rev. Lett. 106, 127004 (2011).
[10] S. Sasaki, M. Kriener, K. Segawa, K. Yada, Y. Tanaka, M. Sato,and Y. Ando, Phys. Rev. Lett. 107, 217001 (2011).
[11] S. Sasaki, Z. Ren, A. A. Taskin, K. Segawa, L. Fu, and Y. Ando,Phys. Rev. Lett. 109, 217004 (2012).
[12] T. Kirzhner, E. Lahoud, K. B. Chaska, Z. Salman, and A.
Kanigel, Phys. Rev. B 86, 064517 (2012).
[13] N. Levy, T. Zhang, J. Ha, F. Sharifi, A. A. Talin, Y. Kuk, and J.
A. Stroscio, Phys. Rev. Lett. 110, 117001 (2013).
[14] H. Peng, D. De, B. Lv, F. Wei, and C.-W. Chu, Phys. Rev. B 88,
024515 (2013).
[15] L. A. Wray, S.-Y. Xu, Y. Xia, Y. S. Hor, D. Qian, A. V. Fedorov,
H. Lin, A. Bansil, R. J. Cava, and M. Z. Hasan, Nat. Phys. 6,855 (2010).
[16] E. Lahoud, E. Maniv, M. S. Petrushevsky, M. Naamneh, A.
Ribak, S. Wiedmann, L. Petaccia, Z. Salman, K. B. Chashka, Y.
Dagan, and A. Kanigel, Phys. Rev. B 88, 195107 (2013).
[17] M. Kriener, K. Segawa, S. Sasaki, and Y. Ando, Phys. Rev. B
86, 180505(R) (2012).
[18] T. V. Bay, T. Naka, Y. K. Huang, H. Luigjes, M. S. Golden, andA. de Visser, Phys. Rev. Lett. 108, 057001 (2012).
[19] T. Sato, Y. Tanaka, K. Nakayama, S. Souma, T. Takahashi,
S. Sasaki, Z. Ren, A. A. Taskin, K. Segawa, and Y. Ando,Phys. Rev. Lett. 110, 206804 (2013).
[20] M. Novak, S. Sasaki, M. Kriener, K. Segawa, and Y. Ando,Phys. Rev. B 88, 140502(R) (2013).
[21] S. Sasaki, K. Segawa, and Y. Ando, Phys. Rev. B 90, 220504(R)(2014).
[22] M. G. Kanatzidis, Acc. Chem. Res. 38, 361 (2005).

[23] Y. Zhang, A. P. Wilkinson, P. L. Lee, S. D. Shastri, D. Shu, D.-Y.
Chung, and M. G. Kanatzidis, J. Appl. Cryst. 38, 433 (2005).
[24] L. E. Shelimova, O. G. Karpinskii, and V. S. Zemskov,
Inorg. Mater. 44, 927 (2008).
[25] L. Fang, C. C. Stoumpos, Y. Jia, A. Glatz, D. Y. Chung, H. Claus,
U. Welp, W.-K. Kwok, and M. G. Kanatzidis, Phys. Rev. B 90,
020504(R) (2014).
[26] K. Segawa, A. A. Taskin, and Y. Ando, J. Solid State Chem.
221, 196 (2015).
[27] K. Nakayama, K. Eto, Y. Tanaka, T. Sato, S. Souma, T.
Takahashi, K. Segawa, and Y. Ando, Phys. Rev. Lett. 109,
236804 (2012).
[28] Although the interface sates are effectively encapsulated in thebulk, it can be distinguished from the genuine bulk states by the
topological nature as in the case of ultrathin films [29].
[29] Y. Zhang et al., Nat. Phys. 6, 584 (2010).
[30] M. Sato and S. Fujimoto, Phys. Rev. Lett. 105, 217001 (2010).
[31] See Supplemental Material at http://link.aps.org/supplemental/
10.1103/PhysRevB.92.100508 for details of the azimuth dependence of band dispersions.
[32] The appearance of bands A and B is not due to an increasein the photoionization cross section since: (i) The intensityof these bands is expected to show a similar photon-energy
dependence to that of CB1st because of their similar orbital
characters (Se-4p and/or Bi-6p), and (ii) the cross-sectionalratio of these orbitals is nearly constant in the hν range ofthe present Rapid Communication. Also, the absence of band
A at hν = 60 eV is not due to the absence of available final
states nor the kz dispersion because it is absent over a widehν range of 40–80 eV. Since the energy resolution is similar(∼ 15 meV) between the measurements with 60 and 21 eV, andthe observed flat dispersion around the bottom of CB2nd is much−1
−1
wider (∼ 0.1 Å ) than the momentum resolution (∼ 0.013 Å )at hν = 60 eV, the experimental resolution is not a limiting
factor to observe CB2nd .
[33] Y. Xia, D. Qian, D. Hsieh, L. Wray, A. Pal, H. Lin, A. Bansil,
D. Grauer, Y. S. Hor, R. J. Cava, and M. Z. Hasan, Nat. Phys. 5,398 (2009).
[34] T. Hashimoto, K. Yada, A. Yamakage, M. Sato, and Y. Tanaka,
Supercond. Sci. Technol. 27, 104002 (2014).
[35] L. Fu (private communication).

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