Ultrathin Bismuth Film on High-Temperature Cuprate Superconductor

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Ultrathin Bismuth Film on High-Temperature Cuprate Superconductor BiSrCaCuO as a Candidate of Topological Superconductor 2

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Natsumi Shimamura, Katsuaki Sugawara, Sukrit Sucharitakul, Seigo Souma, Katsuya Iwaya, Kosuke Nakayama, Chi Xuan Trang, Kunihiko Yamauchi, Tamio Oguchi, Kazutaka Kudo, Takashi Noji, Yoji Koike, Takashi Takahashi, Tetsuo Hanaguri, and Takafumi Sato ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b04869 • Publication Date (Web): 17 Oct 2018 Downloaded from http://pubs.acs.org on October 18, 2018

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Ultrathin Bismuth Film on High-Temperature Cuprate Superconductor Bi2Sr2CaCu2O8+ as a Candidate of Topological Superconductor Natsumi Shimamura,1 Katsuaki Sugawara,1,2,3 Sukrit Sucharitakul,4 Seigo Souma,2,3 Katsuya Iwaya,4 Kosuke Nakayama,1 Chi Xuan Trang,1 Kunihiko Yamauchi,5 Tamio Oguchi,5 Kazutaka Kudo,6 Takashi Noji,7 Yoji Koike,7 Takashi Takahashi,1,2,3 Tetsuo Hanaguri,4 & Takafumi Sato1,3,* 1Department

of Physics, Tohoku University, Sendai 980-8578, Japan

2WPI-Advanced

Institute for Materials Research, Tohoku University, Sendai 980-8577,

Japan 3Center

for Spintronics Research Network, Tohoku University, Sendai 980-8577, Japan

4RIKEN

Center for Emergent Matter Science, Wako, Saitama 351-0198, Japan

5Institute

of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-

0047, Japan 6Research

Institute for Interdisciplinary Science, Okayama University, Okayama, 700-

8530, Japan 7Department

of Applied Physics, Tohoku University, Sendai 980-8579, Japan

* Corresponding author: e-mail: [email protected]

ABSTRACT One of key challenges in condensed-matter physics is to establish a topological superconductor which hosts exotic Majorana fermions. Although various heterostructures consisting of conventional BCS (Bardeen-Cooper-Schrieffer) superconductors as well as doped topological insulators were intensively investigated, no conclusive evidence for 1

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Majorana fermions has been provided. This is mainly because of their very low superconducting transition temperatures (Tc) and small superconducting-gap magnitude. Here we report a possible realization of topological superconductivity at very high temperatures in a hybrid of Bi(110) ultrathin film and copper-oxide superconductor Bi2Sr2CaCu2O8+ (Bi2212). Using angle-resolved photoemission spectroscopy and scanning tunneling microscopy, we uncovered that three-bilayer-thick Bi(110) on Bi2212 exhibits a proximity-effect-induced s-wave energy gap as large as 7.5 meV which persists up to Tc of Bi2212 (85 K). The small Fermi energy and strong spin-orbit coupling of Bi(110), together with the large pairing gap and high Tc, make this system a prime candidate for exploring stable Majorana fermions at very high temperatures.

KEYWORDS:

topology,

high-temperature

superconductor,

electronic

structure,

proximity effect, ultrathin film, spin-orbit coupling

Topological superconductors (TSCs) manifest an exotic quantum state of matter, where the nontrivial topology of the bulk leads to the emergence of Majorana zero modes or Majorana bound states (MBSs).1-5 The expected exotic characteristics of Majorana fermions, such as particle-antiparticle symmetry and non-Abelian-statistics nature favorable for fault-tolerant quantum computations, have provoked the intensive investigation to search for TSCs. A promising strategy to realize topological superconductivity is to utilize an odd-parity p-wave superconductor, as initiated by the theoretical study on two-dimensional (2D) p+ip (ref. 6) and 1D p-wave superconductors7 wherein the MBSs are predicted to emerge at the vortices and ends of wire, respectively. However, the p-wave superconductivity is generally difficult to achieve, and only a 2


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limited number of bulk p-wave-superconductor candidates (e.g., Sr2RuO4 [ref. 8] and CuxBi2Se3 [ref. 9]) have been reported. It is more desirable if we can use a conventional s-wave superconductor to realize TSC. Such possibility was theoretically suggested for the s-wave superconducting states of helical Dirac fermions and Rashba-spin-split states with Zeeman field,10,11 igniting intensive experimental investigations on various artificial heterostructures such as semiconducting nanowires,12-14 ferromagnetic atomic chains,15 topological-insulator thin films,16,17 and Rashba metals18 on s-wave superconductors. However, the emergence of Majorana fermions in these superconducting hybrids remains elusive despite the fierce debates.12-15,19-23 This may largely owe to the general characteristics of s-wave superconductors, such as low superconducting transition temperatures (Tc) and inherent fragileness against magnetic field.11-14 Moreover, the MBS can be generated only at very low temperatures, making it difficult to precisely demonstrate the zero-energy behavior because of the very small proximity-induced gap of typically eV scale.12-14 One may naturally think that a more feasible approach is to use copper-oxide (cuprate) superconductors with the highest Tc and largest superconducting gap among known superconductors (at ambient pressure), as it is theoretically suggested that cuprate superconductors host Majorana fermions despite the presence of gap nodes.24-27 So far, a few attempts have been made on cuprate-superconductor hybrids consisting of topological-insulator Bi2Se3 thin film and Bi2Sr2CaCu2O8 (Bi2212) (refs. 28-31). However, the obtained results are inconsistent with each other, and it is still unclear whether the proximity-induced gap, which is a prerequisite for realizing TSC, is induced or not in Bi2Se3 (refs. 28-31). This is probably because the coherence length of Bi2212

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(~10 Å) is much shorter than the thickness of Bi2Se3 film and the proximity effect likely occurs only in the vicinity of interface. To overcome this critical problem and fully utilize the intriguing characteristics of Bi2212, we have deliberately chosen ultrathin Bi because the length of structural unit [3.9 Å for (111) structure] is much shorter than the coherence length of Bi2212 as well as the minimal unit of Bi2Se3 (a quintuple layer, ~ 10 Å). Considering the strong spin-orbit coupling of Bi, we expect that ultrathin Bi is suitable for realizing TSC since the theoretically proposed condition for TSC11 would be satisfied if one can introduce a swave pairing on the Rashba-spin-orbit-coupled states of Bi by the superconducting proximity effect. It is noted here that in this case, topological insulator is not required for the host material unlike the case of Bi2Se3/Bi2212 and CuxBi2Se3. In the present study, we fabricated a heterostructure consisting of high-quality single crystalline Bi ultrathin film and Bi2212, and performed angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy/spectroscopy (STM/STS) to demonstrate that Bi/Bi2212 hybrid is a possible high-Tc topological superconductor (TSC).

RESULTS AND DISCUSSION At first, we explain our control experiment to fabricate Bi/Bi2212 hybrid with a relatively thick Bi film. We prepared a clean surface of Bi2212 by cleaving the crystal under ultrahigh vacuum, as visible from a sharp low-energy electron diffraction (LEED) pattern showing satellite spots arising from the BiO superstructure (Figure 1a). Upon Bi deposition onto Bi2212 at 300 K, the LEED pattern undergoes a drastic change with appearance of 1 × 1 spots from Bi film and disappearance of Bi2212 spots (Figure 1b). The twelve-fold-symmetric LEED pattern suggests formation of two types of (111) 4


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domains (with six-fold-symmetry) rotated by 30° from each other. This is naturally expected from the C4 symmetry (C2 when taking into account the BiO superstructure) of Bi2212 crystal (Figure 1c). Our STM measurements shown in Figure 1d reveal two types of triangular-shaped Bi terraces separated by a domain wall (types I and II). Their (111) orientation is visualized by the atomic image in Figure 1e. The atomic-step height shown in Figure 1f is identical to the thickness of one-bilayer (BL) Bi(111) (3.9 Å). Our ARPES data signify the emergence of several Bi-derived bands upon Bi deposition (Figure 2a) that are totally absent in Bi2212 (Figure 2b). Some of these bands cross the Fermi level (EF) and form a twelve-fold-symmetric intensity pattern at EF (Figure 2c) which originates from three types of pockets; two electron pockets centered at the  and M points, respectively, in the Bi Brillouin-zone (BZ), and an elongated hole pocket located between the  and M points, as in Bi(111)/Si(111) (Figure 2c, bottom). We searched for a proximity-induced gap at several representative Fermi vectors (kF’s) on each pocket (points A-C in Figure 2c), but the recorded energy distribution curves (EDCs) exhibited no discernible leading-edge shift even at low temperatures (Figure 2d; see also the EDCs in the vicinity of EF in Figure 2e which signifies that the intersection of EDCs is always located at EF) in contrast to a sizable anti-nodal gap in Bi2212 (Figure 2f). This is probably because the film is too thick (10BL ~ 40 Å) compared to the coherence length of Bi2212 (~10 Å). The symmetry mismatch between Bi(111) (C3) and Bi2212

(C2)

may

be

also

unfavorable

for

realizing

the

proximity-induced

superconductivity.31 Having established that fabrication of Bi/Bi2212 hybrid is indeed feasible, a next important challenge is to reduce the film thickness down to the scale of coherence length. In this ultrathin regime, the (111) structure is unstable and Bi islands with the C25


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symmetric (110) structure are formed, as evidenced by the LEED pattern in Figure 3a for ~3BL Bi(110) (~20 Å thick) which is distinctly different from that for Bi(111) (Figure 1b). A close inspection of such a film with STM signifies rectangular-shaped 2-4BL islands (Figure 3b) with zigzag atomic image (inset).32 These islands consist of two types of domains with almost square BZ (i.e. with similar a- and b-axis length) rotated by 90° from each other, as illustrated in Figure 3c. The coexistence of multiple BL islands is also seen in the ARPES-derived band dispersion along the X cut, which reasonably agrees with the calculated band structure for 2- and 3-BL Bi(110) slabs, particularly around the  point (Figure 3d). We found no clear signature of the band dispersion of 4BL islands in the ARPES intensity. This may be because the film for ARPES measurements is slightly thinner than that for STM due to a slight difference in the fabrication condition, and as a result the film is composed mainly of 2-3BL islands with negligible amount of 4BL islands. We fabricated the Bi films separately in the STM and ARPES systems to avoid a surface contamination during the transfer of samples between the two systems. Also, we did not clearly observe the band dispersions from the bare substrate (i.e. Bi2212), probably because the total surface area of the substrate is very small. As shown in Figure 3d, the calculated band structure near EF looks very different between 2- and 3-BL Bi(110). This is due to the quantum-size effect, which strongly modifies the band structure near EF, particularly in the case of thin semimetal films, as revealed by the slab calculations.33-35 The Fermi-surface topology of this hybrid is found to be unexpectedly simple, despite the complication from the existence of multiple BL islands. As shown in Figure 4a, one can recognize a single intense spot at the  point in the Fermi-surface mapping. This feature is attributed to the 3A band which originates from the topmost valence band 6


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of 3BL island (Figure 3d). Intriguingly, no other bands seem to cross EF. This indicates that the low-energy excitations responsible for the superconductivity should be attributed only to this single 3A band in the 3BL island. Such simplicity makes this hybrid an ideal platform for further investigation of proximity-induced superconductivity. To see the characteristics of the 3A band in more detail, we have performed ARPES measurements using a 6-eV laser with greatly improved energy and k resolution (2 meV and 0.002 Å-1, respectively). The very low-energy photons also enable us to effectively probe the interface which is hardly accessed by ARPES with higher-energy photons. The results shown in Figure 4b reveal a holelike band crossing EF at 150 K along the X cut (vertical pink line in Figure 4a). Upon decreasing temperature, this band is gradually shifted downward (probably due to the charge transfer from the substrate), accompanied by a systematic shift of the kF position toward the  point (note that the Fermi energy at 12 K is estimated to be ~15 meV). We show in Figure 4c a set of EDCs measured at each kF point at temperatures from 12 K to 150 K, which reveals that a peak located in the vicinity of EF at 12 K is gradually broadened on increasing temperature, which seems to simply obey the temperature dependence of the Fermi-Dirac distribution (FD) function. However, when closely examined, the temperature dependence of EDC exhibits a finite deviation from the simple FD-function-like behavior. As shown in Figure 4d, the intersection of EDCs at 12 K and 150 K is not at EF but at slightly higher binding energy, indicating that the spectral weight is transferred from the near-EF region to the higher binding energy region upon decreasing temperature. Such spectral weight suppression is better visualized in the symmetrized EDCs in Figure 5a which signify a two-peaked structure accompanied with a dip at EF at 12 K. This gap-like feature survives at least up to 60 K and disappears above 90 K, close to Tc 7


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of Bi2212 (85 K). This is supported by the analysis of gap magnitude  as a function of temperature shown in Figure 5b (note that the gap size of ~ 6 meV is much larger than the leading-edge shift of ~1 meV in Figure 4d because of the intrinsic linewidth of peaks in EDC). These experimental results suggest that the observed gap-like feature is induced by the proximity effect from superconducting Bi2212. This conclusion is strongly supported by the absence of any gap-like features in a ~3BL Bi(110) film on nonsuperconducting cuprate (Bi,Pb)2Sr2CuO6 (Figure 5c). It is worth noting that the STM measurement for 3BL Bi(110) on Bi2212 reveals the suppression in the density of states around zero bias (Figure 5d) which may also originate from the superconducting proximity effect. Our ARPES data suggest a s-wave symmetry for the proximity-induced gap in contrast to the d-wave symmetry of Bi2212 substrate. This is recognized from the symmetrized EDC at kF along the M cut (point B in Figure 4a) in Figure 5a that exhibits two-peaked structure with  ~ 7.5 meV comparable to the gap value at point A (~ 6 meV), suggesting a nearly isotropic nature of the gap. It is noted that while it was difficult to measure the EDCs at several kF points because the hole pocket at the  point is extremely small, the demonstration of a similar gap size at two kF points with different Fermi-surface angles rotated by 45° from each other would serve as a simplest proof of the full gap, thanks to the overall C4-like symmetry of the band structure around . It is emphasized here that the observed s-wave gap is essential for the TSC properties since the nodeless superconductivity is a key factor to realize the Majorana zero mode with robust non-Abelian statistics. Also, the present results shed light on the debates on the gap symmetry induced by the proximity effect from the d-wave superconductors.27,36-38

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Through a comprehensive investigation of low-energy excitations in 3BL Bi(110) with ARPES and STM/STS, we found that the overall gap magnitude seen by ARPES (~6-7.5 meV; Figure 5b) is larger than that by STM/STS (~3 meV; Figure 5d). This is likely because the gap gradually decays from the bulk substrate to the surface of Bi film through the interface, as illustrated in Figure 5g. The gap around the topmost surface (~3 meV) is preferentially observed by STS, while the gap around the interface (~ 6-7.5 meV) is also detected by ARPES with 6-eV photons, because the photoelectron escape depth (~20-40 Å) fully covers the thickness of 3BL island (~20 Å). Such argument is also corroborated by the STM observation of a much larger gap of the bare substrate (Figure 5e) compared to that of 3BL Bi(110) (Figure 5d), as well as the observation that the gap feature of Bi(110) becomes obscured for the thicker islands (Figure 5f). These experimental facts, combined with the absence of a proximity-induced gap in a thicker film (~40 Å) (Figure 2d), indicate that the reduction of film thickness down to the coherence-length scale is essential to induce the pairing gap in cuprate-superconductor hybrids. It is remarked here that the Fermi-surface-matching condition between superconductor and overlayer, which was argued to be important for realizing the proximity-induced gap,31 is satisfied in our hybrid when the second BZs of Bi(110) and Bi2212 are taken into account. It should be emphasized that the Bi(110)/Bi2212 hybrid satisfies the key requirements to become a TSC even without using a topological insulator as a host material. The theoretically proposed condition of the Rashba-spin-split states with the swave pairing11 is naturally satisfied because Bi has inherently a strong spin-orbit coupling and the induced gap is s-wave-like. Also, the large g factor of Bi (ref. 39) and the small Fermi energy are well suited to achieve a topological superconducting state under low 9


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magnetic field.11 As shown in Figure 5h, one can thus expect appearance of the MBS at the vortex core of Bi(110) in this hybrid. Importantly, the large proximity-induced gap (7.5 meV) far exceeding the gap value of other s-wave-superconductor hybrids12-18 as well as the large ratio of proximity-induced gap and Fermi energy would be highly useful to distinguish the Majorana zero mode from other quasiparticle states such as low-lying vortex-core bound states.16,40 Furthermore, the survival of pairing gap up to Tc of Bi2212 makes this system an excellent platform to search for long-sought Majorana fermions stable at high temperatures, and could be potentially advantageous for future applications in spintronics and quantum computing.

CONCLUSION In conclusion, we have reported high-resolution ARPES and STM studies of ultrathin Bi film grown epitaxially on high-temperature cuprate superconductor Bi2212. We found that the Bi thin film undergoes the structural transition from (111) to (110) upon reducing the thickness. In three-bilayer-thick Bi(110), we found a superconductingproximity-effect-induced energy gap as large as 7.5 meV which persists up to Tc of Bi2212 (85 K). We also found that the observed gap exhibits an isotropic s-wave character despite the nodal d-wave superconductivity of Bi2212. The present results open an avenue for exploring the topological superconductivity and Majorana fermions at high temperatures.

METHODS High-quality single crystals of Bi2Sr2CaCu2O8+ (Bi2212; slightly overdoped, Tc = 85 K) and (Pb,Bi)2Sr2CuO6+ (Bi2201; overdoped, Tc = 0 K) were grown by the floating10


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zone method. We cleaved Bi2212 and Bi2201 crystals under ultrahigh vacuum to obtain a shiny mirror-like surface, and then deposited Bi on the surface at 300 K and 270 K for Bi(111) and Bi(110) films, respectively, by using the molecular-beam-epitaxy (MBE) technique. The film thickness was controlled by the deposition time at a constant deposition rate. The actual thickness was estimated by a quartz-oscillator thickness monitor, a height profile of the STM image and a comparison of ARPES-derived band dispersions with the band-structure calculations for free-standing multilayer Bi. All these estimations provided the consistent results. ARPES measurements were performed at Tohoku University with the MBS-A1 electron analyzer equipped with a high-intensity He and Xe plasma discharge lamps, together with a 6-eV continuous-wave (CW) laser system consisting of a 820-nm diode laser with two frequency doublers (LEOS solutions). We used the He I (h = 21.218 eV), Xe I (h = 8.437 eV), and 6-eV photons to excite photoelectrons. The energy resolution in ARPES measurements was set at 2-40 meV. We measured the spectrum at the Fermi edge of a gold film deposited on the sample substrate at low temperatures with a very high signal-to-noise ratio and defined the midpoint of the measured Fermi edge as the EF position, as usually employed in ARPES measurements. We have estimated the experimental uncertainty to be within ± 0.1 meV for the measurement with VUV laser. The beam spot size from the He/Xe discharge lamps and the VUV laser are roughly 2 × 2 mm2 and 300 × 300 m2, respectively, both of which far exceed the typical domain size of Bi(110) islands (30 × 10 nm2). Therefore, the present ARPES data reflect the electronic states averaged over all the Bi BL terraces existing at the surface. STM measurements were performed at RIKEN with a commercial 3He-based STM system (UNISOKU USM-1300) modified by ourselves.41 We used electrochemically 11


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etched tungsten tips, which were cleaned and sharpened by field ion microscopy. We applied bias voltages to the sample whereas the tip was virtually grounded at the currentvoltage converter (Femto LCA-1K- 5G). Tunneling spectra were measured by the software-based lock-in detector included in the commercial STM control system (Nanonis). All of the STM/STS data were taken at 1.5 K unless otherwise noted. First-principles band-structure calculations for Bi(111) and Bi(110) slabs were carried out by using a projector augmented wave method implemented in Vienna Ab initio Simulation Package (VASP) code42 with local density approximation (LDA).43 In order to reproduce accurate interlayer distances, long-range van der Waals interaction was included through semi-empirical corrections by DFT-D2 approachs.44 After the crystal structure was fully optimized until forces acting on atoms were less than 1 × 10-3 eV/Å, the spin-orbit coupling was included self-consistently. The k-point mesh was set to be 12 × 12 × 1. The experimental lattice parameter was used in the calculation and the vacuum layer was set to be more than 20 Å.

ACKNOWLEDGEMENTS We thank K. Nomura and G. Bihlmayer for fruitful discussions, and Y. Nakata, K. Nakamura, and H. Oinuma for their assistance in ARPES experiments. We also thank Y. Kohsaka for helping STM experiments. This work was supported by JSPS KAKENHI Grants

(JP25107003,

JP15H05853,

JP15H02105,

JP17H01139,

JP17H04847,

JP18H04227, and JP16K05465), the CREST project [JPMJCR16F2] from Japan Science and Technology Agency (JST), Grant for Basic Science Research Projects from the Sumitomo Foundation, Science Research Projects from Iketani Science and Technology

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Foundation, the Program for Key Interdisciplinary Research, and World Premier International Research Center, Advanced Institute for Materials Research.

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17. Xu, S. -Y.; Alidoust, N.; Belopolski, I.; Richardella, A.; Liu, C.; Neupane, M.; Bian, G.; Huang, S.-H.; Sankar, R.; Fang, C.; Dellabetta, B.; Dai W.; Li, Q.; Gilbert, M. J.; Chou, F.; Samarth, N.; Hasan, M. Z.; Momentum-Space Imaging of Cooper Pairing in a Half-Dirac-Gas Topological Superconductor. Nat. Phys. 2014, 10, 943–950. 18. Sun, H. -H.; Wang, M.-X.; Zhu, F.; Wang, G.-Y.; Ma, H.-Y.; Xu, Z.-A.; Liao, Q.; Lu, Y.; Gao, C.-L.; Li, Y.-Y.; Liu C.; Qian, D.; Guan, D.; Jia, J.-F.; Coexistence of Topological Edge State and Superconductivity in Bismuth Ultrathin Film. Nano Lett. 2017, 17, 3035–3039. 19. Liu, J.; Potter, A. C.; Law, K. T.; Lee, P. A. Zero-Bias Peaks in the Tunneling Conductance of Spin-Orbit-Coupled Superconducting Wires with and without Majorana End-States. Phys. Rev. Lett. 2012, 109, 267002. 20. Rainis, D.; Trifunovic, L.; Klinovaja, J.; Loss, D. Towards a Realistic Transport Modeling in a Superconducting Nanowire with Majorana Fermions. Phys. Rev. B 2013, 87, 024515. 21. Ruby, M.; Pientka, F.; Peng, Y.; von Oppen, F.; Heinrich, B. W.; Franke, K. J. End States and Subgap Structure in Proximity-Coupled Chains of Magnetic Adatoms. Phys. Rev. Lett. 2015, 115, 197204. 22. Sau, J. D.; Brydon, P. M. R. Bound States of a Ferromagnetic Wire in a Superconductor. Phys. Rev. Lett. 2015, 115, 127003. 23. Dumitrescu, E.; Roberts, B.; Tewari, S.; Sau, J. D.; Das Sarma, S. Majorana Fermions in Chiral Topological Ferromagnetic Nanowires. Phys. Rev. B 2015, 91, 094505.

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24. Sato, M.; Fujimoto, S. Existence of Majorana Fermions and Topological Order in Nodal Superconductors with Spin-Orbit Interactions in External Magnetic Fields. Phys. Rev. Lett. 2010, 105, 217001. 25. Linder, J.; Tanaka, Y.; Yokoyama, T.; Sudbø, A.; Nagaosa, N. Unconventional Superconductivity on a Topological Insulator. Phys. Rev. Lett. 2010, 104, 067001. 26. Wong, C. L. M.; Law, K. T. Majorana Kramers Doublets in dx2-y2-wave Superconductors with Rashba Spin-Orbit Coupling. Phys. Rev. B 2012, 86, 184516. 27. Kao, J. T.; Huang, S. M.; Mou, C. Y.; Tsuei, C. C. Tunneling Spectroscopy and Majorana Modes Emergent from Topological Gapless Phases in High- Tc Cuprate Superconductors. Phys. Rev. B 2015, 91, 134501. 28. Zareapour, P.; Hayat, A.; Zhao, S. Y. F.; Kreshchuk, M.; Jain, A.; Kwok, D. C.; Lee, N.; Cheong, S. -W.; Xu, Z.; Yang, A.; Gu, G. D.; Jia, S.; Cava, R. J.; Burch, K. S., Proximity-Induced

High-Temperature

Superconductivity

in

the

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Insulators Bi2Se3 and Bi2Te3. Nat. Commun. 2012, 3, 1056. 29. Wang, E.; Ding, H.; Fedorov, A. V.; Yao, W.; Li, Z.; Lv, Y.-F.; Zhao, K.; Zhang, L. -G.; Xu, Z.; Schneeloch, J.; Zhong, R.; Ji, S.-H.; Wang, L.; He, K; Ma, X.; Gu, G.; Yao, H.; Xue, Q.-K.; Chen, X.; Zhou, S., Fully Gapped Topological Surface States in Bi2Se3 Films Induced by a d-wave High-Temperature Superconductor. Nat. Phys. 2013, 9, 621–625. 30. Yilmaz, T.; Pletikosić, I.; Weber, A. P.; Sadowski, J. T.; Gu, G. D.; Caruso, A. N.; Sinkovic, B.; Valla, T. Absence of a Proximity Effect for a Thin-Films of a Bi2Se3 Topological Insulator Grown on Top of a Bi2Sr2CaCu2O8+δ Cuprate Superconductor. Phys. Rev. Lett. 2014, 113, 067003.

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31. Xu, S. -Y.; Liu, C.; Richardella, A.; Belopolski, I.; Alidoust, N.; Neupane, M.; Bian, G.; Samarth, N.; Hasan, M. Z. Fermi-level Electronic Structure of a TopologicalInsulator/Cuprate-Superconductor Based Heterostructure in the Superconducting Proximity Effect Regime. Phys. Rev. B 2014, 90, 085128. 32. Lu, Y.; Xu, W.; Zeng, M; Yao, G.; Shen, L.; Yang, M.; Luo, Z.; Pan, F.; Wu, K.; Das, T. Topological Properties Determined by Atomic Buckling in Self-Assembled Ultrathin Bi(110). Nano Lett. 2015, 15, 80–87. 33. Koroteev, Y. M.; Bihlmayer, G.; Chulkov, E. V.; Blügel, S. First-principles investigation of structural and electronic properties of ultrathin Bi films. Phys. Rev. B. 2008, 77, 045428. 34. Zubizarreta, X.; Chulkov, E. V.; Chernov, I. P.; Vasenko, A. S.; Aldazabal, I.; Silkin, V. M. Quantum-size effects in the loss function of Pb(111) thin films: An ab initio study. Phys. Rev. B. 2017, 95, 235405. 35. Yunhao, L.; Wentao, X.; Mingang, Z.; Guanggeng, Y.; Lei, S.; Ming, Y.; Ziyu, L.; Feng, P.; Ke, W.; Tanmoy, D.; Pimo, H.; Jianzhong, J.; Jens, M.; Yuan, P. F.; Hsin, L.; Xue-sen, W. Topological Properties Determined by Atomic Buckling in SelfAssembled Ultrathin Bi(110). Nano Lett. 2015, 15, 80-87. 36. Li, Z.-X.; Chan, C.; Yao, H. Realizing Majorana Zero Modes by Proximity Effect Between Topological Insulators and d-wave High-Temperature Superconductors. Phys. Rev. B 2015, 91, 235143. 37. Li, W. J.; Chao, S. P.; Lee, T. K. Theoretical Study of Large Proximity-Induced sWave-Like Pairing from a d-wave Superconductor. Phys. Rev. B 2016, 93, 035140. 38. Black-Schaffer, A. M.; Balatsky, A. V. Proximity-Induced Unconventional Superconductivity in Topological Insulators. Phys. Rev. B 2013, 87, 220506. 17


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39. Cohen, M. H.; Blount, E. I. The g-Factor and de Haas-van Alphen Effect of Electrons in Bismuth. Philosophical Magazine. 1960, 5, 115-126. 40. De Gennes, P. G. Superconductivity of Metals and Alloys, Addison-Wskey, 1989. 41. Hanaguri, T. Development of High-Field STM and its Application to the Study on Magnetically-Tuned Criticality in Sr3Ru2O7. J. Phys. Conf. Ser. 2006, 51, 514–521. 42. Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for ab-initio Total Energy Calculations using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169–11186. 43. Perdew, J. P.; Zunger, A. Self-Interaction Correction to Density-Functional Approximations for Many-Electron Systems. Phys. Rev. B 1981, 23, 5048. 44. Grimme, S. Semiempirical GGA‐Type Density Functional Constructed with a Long Range Dispersion Correction J. Comput. Chem. 2006, 27, 1787.

Author contributions N.S., K.S., S.S., K.N., C.X.T., and T.S. carried out the fabrications of thin films, their characterization, and ARPES measurements. N.S., K.I., S.S., and T.H. performed the STM measurements and analyzed data. K.K. T.N. and Y.K. fabricated single crystals of high-Tc cuprates, and K.Y. and T.O. carried out band-structure calculations. N.S., T.T., T.H., and T.S. finalized the manuscript with input from all of the authors.

Additional information Correspondence and requests for materials should be addressed to T.S (e-mail: 　 [email protected])

Competing financial Interests 18


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We have no competing financial interests.

FIGURE CAPTIONS Figure 1. (a, b) LEED patterns of Bi2212 (Tc = 85 K) and 10BL Bi(111) on Bi2212, respectively, measured at room temperature with primary electron energy of 75 eV. (c) (Top) Brillouin zone (BZ) and (bottom) schematic atomic arrangement for two types of Bi(111) domains and CuO2 plane of Bi2212. (d) Constant-current (100 pA) STM image of the surface area of 175 × 300 nm2 (sample bias voltage = 0.3 V), measured at 1.5 K. (e) High-resolution STM image of the triangular island. (f) Height profile measured along a cut shown by green line in (d).

Figure 2. (a) Plot of ARPES intensity of 10BL Bi(111)/Bi2212 measured at 30 K with the Xe-I line (h = 8.437 eV) along the M cut of Bi(111) BZ. (b) Plot of ARPES intensity of Bi2212 measured at 30 K with the He-I line (h = 21.218 eV) along the M cut of Bi2212 BZ. (c) ARPES-intensity mapping at EF as a function of 2D wave vector for (top) 10BL Bi(111)/Bi2212 and (bottom) 20BL Bi(111)/Si(111). 　 Intensity at EF is obtained by integrating the spectral weight within ± 5 meV of EF. (d) Temperature dependence of EDC for 10BL Bi(111)/Bi2212 measured at three representative k points (points A-C) in (c). (e) Same as (d) but expanded near EF. (f) Temperature dependence of EDC for Bi2212 measured at kF along the MY cut.

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Figure 3. (a) LEED pattern of ~3BL Bi(110)/Bi2212 measured at room temperature with primary electron energy of 70 eV. (b) Constant-current (100 pA) STM image at 1.5 K for ~3BL Bi(110)/Bi2212 (sample bias voltage = 0.3 V). Inset shows the expanded image for the 3BL island. (c) Schematic crystal structure and BZ for two types of Bi(110) domains and CuO2 plane of Bi2212. (d) Second-derivative intensity plot for ~3BL Bi(110)/Bi2212 measured at 30 K along the X1 (X2) cut of Bi(110) BZ, compared with the calculated band structure for free-standing 2BL (blue) and 3BL (red) Bi(110) with the A7 structure. We label prominent energy bands as 2A, 2B (for 2BL) and 3A-3C (for 3BL). Note that some differences in the band dispersion between the experiment and calculation, such as existence of Fermi surface near the X1(X2) point only in the calculation, is likely due to slight mismatch of the lattice parameters between the experiment and calculation.

Figure 4. (a) Fermi-surface mapping at 30 K for ~3BL Bi(110)/Bi2212 obtained by ARPES measurements with the Xe-I line (8.437 eV). (b) Plot of high-resolution ARPES intensity along the X1(X2) cut (vertical pink line in (a)) at 150, 90, and 12 K measured with 6-eV laser. kF points are highlighted by arrows. (c) Temperature dependence of EDC measured at kF along the X1(X2) cut (point A in (a)) for 3BL Bi(110)/Bi2212. (d) Same as (c) but expanded around EF. Arrow indicates the intersection of EDCs at 12 K and 150 K.

Figure 5. (a) Temperature dependence of EDC symmetrized with respect EF, measured at kF along the X1(X2) cut (point A in Fig. 4(a)) for 3BL Bi(110)/Bi2212. Symmetrized EDC at 30 K measured at kF along the M cut (point B in Figure 4(a)) is also shown for comparison at the bottom. (b) Temperature dependence of  at positive (red circles) and 20


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negative (blue triangles) kF’s, overlaid with a guideline assuming a mean-field-like temperature dependence (solid green curve). Vertical dashed line corresponds to Tc of Bi2212. (c) Temperature-dependence of symmetrized EDC for ~3BL Bi(110) on nonsuperconducting (Bi,Pb)2Sr2CuO6 (Bi2201). Symmetrized EDC for Bi2201 measured at 12 K is also shown at the bottom. (d) Typical dI/dV curve at 1.5 K as a function of bias voltage obtained on 3BL island averaged over the region of 7×7 nm2 (data were recorded at two different regions). (e) Typical dI/dV curve at 4.2 K for the bare substrate (Bi2212) without Bi deposition. Even though we intentionally aged the surface at room temperature in ultrahigh vacuum before the measurement to mimic the situation during the Bi deposition, gap feature is clearly seen. (f) Representative dI/dV curves at 1.5 K for 3-6 BL Bi(110). (g) Illustration of the real-space variation of pairing-gap magnitude in Bi(110)/Bi2212 hybrid. (h) Illustration of possible detection of the Majorana zero mode in Bi(110)/Bi2212 hybrid.

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Fig. 1: N. Shimamura et al.

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Binding energy ( eV)

EF

(d)

M Bi(111)/Bi2212

Int ensit y ( arb.units)



(a) 0.2

Bi 6p

0.4 0.6



EF

(e)

M

Int ensit y ( arb.units)

B. E. (eV)

(b)

0.8

0.4 kx( Å -1)

0.0

0.0

0.2 -0.2

Cu 3dx2-y2

0.4 0.6 Bi2212 0.0 -0.4

-0.6

0.8

0.4 kx( Å -1)

0.0

0.4

(c)

0.8

0.2 0.2

A

C

B



Bi(111) /Bi2212

-0.2 -0.2

High High

-0.4

8000

0.2

6000 0.0

4000 2000

-0.2

0.1 0.0 -0.1 -0.2

0.1

-0.6

-0.4

Low -0.2

0.2



0.0

0.4

0.6

0.0

0.0 0.0

0.2

0.4 kx (Å -1) 0.2

0.4

0.6

(f)

Bi(111) /Bi2212 C -50

-3

0

x10

-50.0

-3

0.0

x10

30 K 100 K

60 40 20 EF -20 Binding energy ( meV) -3

x10

10 K 30 K 50 K 70 K 90 K 110 K 130 K

A B -10

0 -3 x10

C 15

10

30 K 100 K

0.0 -3 x10

10

5

0 -3 x10

-5

-10

10 EF -10 Binding energy ( meV)

300

0.8

M

-15

30 K 70 K 80 K 90 K 100 K 110 K 120 K

400

Bi(111)/Si(111)

-0.1

Low

-0.8

0.0

M

B

3

0.0

10 K 30 K 50 K 70 K 90 K 110 K 130 K

x10

0.0

A

Int . ( a.u.)

ky (Å -1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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200

Bi2212

100

100 50 EF -50 -100 Binding energy ( meV) -100

0.8 0.8

0.4

-50

0 -3 x10

50

100


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0.2

Point A

0.1

Intensity (arb.units)

0.2

ky (Å -1)

(c)

M Point B

0.0 0.0



-0.1

X1(X2)

-0.2

High

-0.2

High

-0.3

-0.4 X1(X2) -0.2 0.0

Low

Low

-0.4

-0.2

(b)

-0.1

0.0

0.1

0.2 0.4 kx (Å -1) 0.2

150 K

EF

0.3

0.4

0.5

0.00

12 K

-0.05

-0.05

0.2

3A

100 -0.10

-0.10

0.0

-0.10

0.0

0.1

-0.1

0.0

-0.2

-0.1

0.1

12 K

150 K

-10 10 E0F -10 10 x10 Binding energy (meV) -3

-0.6

-0.4

2000

4000

Low

6000

0.1

(d)

-0.8


-0.2

0.0

0.2

High

8000

-0.1 0.0 0.1 -0.1 0.0 0.1 -0.1 0.0 0.1 High Low ky (Å -1) -0.1

Point A

-3

0.4

50

-0.05

12 30 60 90 120 150

-50 50 E0F 50 -50 x10 Binding energy (meV)

0.7

90 K

0.00

0.00

0.6 0.6

T (K)=


(a)

Binding energy (meV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Graphic table of content T (K)=

EF

0.0

3B

0.6

-0.6

120


2B

0.4

-0.4

90 60 30 Point A

12

900

3C

0.8

30

800

Point B

3

x10

0.1

-0.6

-0.1

Low

0

20

40

0.0 0.2 0.4 0.6 0.0 0.2 0.4 0.6 -1 Low kx (Å ) 0.0

60

100

-0.2 -0.2 High

80

0.2

-0.8

High

25 nm

4BL

2A

0.0

2BL

0.2

-0.2

-0.4

3BL

150

3A

-0.2

20.5 nmnm

Binding energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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700

-40 -20 0 20 40 40 20 0 -20 -40 -3 Energy relative x10 to EF (meV)

600 500

-40

-20

0 -3 x10

20

40

Ultrathin Bismuth Film on High-Temperature Cuprate Superconductor

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