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Coexistence of topological edge state and superconductivity in Bismuth ultrathin film Haohua Sun, Meixiao Wang, Fengfeng Zhu, Guanyong Wang, Haiyang Ma, Zhu-An Xu, Qing Liao, Yunhao Lu, Chunlei Gao, Yao-Yi Li, Canhua Liu, Dong Qian, Dandan Guan, and Jinfeng Jia Nano Lett., Just Accepted Manuscript • Publication Date (Web): 18 Apr 2017 Downloaded from http://pubs.acs.org on April 18, 2017

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Coexistence

of

topological

edge

state

and

superconductivity in Bismuth ultrathin film Hao-Hua Sun,† Mei-Xiao Wang,† Fengfeng Zhu,† Guan-Yong Wang,† Hai-Yang Ma,† Zhu-An ∥

∥

Xu,‡,§ Qing Liao, Yunhao Lu, Chun-Lei Gao,†,§ Yao-Yi Li,†,§ Canhua Liu,†,§ Dong Qian,†,§ Dandan Guan,†,§,* and Jin-Feng Jia†,§

†

Key Laboratory of Artificial Structures and Quantum Control (Ministry of Education),

Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 100140, China ‡

Department of Physics, Zhejiang University, Hangzhou 310027, Zhejiang, China

§

Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China

∥

Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027,

China

Corresponding Author：

*E-mail: [email protected]，Telephone: +86 21 5474 4616

ACS Paragon Plus Environment

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ABSTRACT: Ultrathin freestanding Bismuth film is theoretically predicted to be one kind of two dimensional topological insulators (TIs). Experimentally the topological nature of bismuth strongly depends on the situations of the Bi films. Film thickness and interaction with the substrate often change the topological properties of Bi films. Using angle resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy or spectroscopy (STM/STS) and first-principle calculation (DFT), the properties of Bi(111) ultrathin film grown on the NbSe2 superconducting substrate have been studied. We find the band structures of the ultrathin film is quasi-freestanding, and 1D edge state exists on Bi(111) film as thin as 3 bilayers (BLs). Superconductivity is also detected on different layers of the film and the pairing potential exhibits an exponential decay with the layer thicknesses. Thus, the topological edge state can coexist with superconductivity, which makes the system a promising platform for exploring Majorana fermions.

KEYWORDS: Ultrathin Bi(111), 2D topological insulator, proximity-induced superconductivity, topological edge state, coexistence

Two dimensional (2D) topological insulators (TIs) are 2D materials that possess a pair of topological edge states connecting the bulk band gap

1, 2

. In real space, these chiral edge states

are localized at the edge of the 2D TIs with spin-up electrons and spin-down electrons propagating in the opposite direction. Topological edge states in 2D TIs are protected by time reversal symmetry and are immune to non-magnetic backscattering. The genuine transport properties of 2D TI have promising future in the applications of spintronic devices. Moreover,


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Combining a TI with superconductivity builds a platform for searching the Majorana fermions 18

.

The first experimentally confirmed 2D TI is HgTe/CdTe semiconductor quantum well

9, 10

.

Recently, another semiconductor quantum well InAs/GaSb was also discovered to be a 2D TI 11, 12

. However, fabrication of such quantum well is complex. This may hinder the application of

those materials. Bismuth is a heavy element with strong spin orbit coupling (SOC) effect, and it is a common ingredient of 3D TIs such as Bi2Te3. 2D single-bilayer bismuth was theoretically predicted to be a 2D TI 13. Later, freestanding Bi(111) films up to 8 BLs were also predicted to be 2D TIs

14

. Above 8 BLs, Bi films changed into a topological trivial phase. Even for the top-

most BL of Bi layer, theoretical calculation suggested the band gap closed at Γ when the substrate Bi was thicker than 8 BLs, which makes the topological nature of the edge states on the top-most Bi BL ambiguous

15

. Realizing 2D TIs in this single element material will be great

advantageous for their applications. However, growing freestanding bismuth BL is difficult. Edge states of one BL Bi(111) were first experimentally observed on Bi(111) BL grown on Bi2Te3

16

, and edge states of Bi(111) BLs on Bi2Te2Se and graphene were also reported

17, 18

.

However, the topological properties of these edge states depend on the interactions between the Bi films and the substrates. Recently, edge states of Bi were observed on the top-most layer of bulk Bi crystal

19

and on 96 BLs Bi on Si(111)

20

. The origin of these edge states is due to the

weak interaction between the upper layer in the Bi BL structure and the underneath Bi BL. These results suggest a new direction in searching topologically non-trivial Bi films and motivate other experimental research in similar systems.


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Here we choose to study the electronic properties and edge states of epitaxial Bi(111) ultrathin films on a superconducting substrate 2H-NbSe2 by using ARPES, STM/STS and DFT calculation. First of all, it is possible to find the topological edge state on Bi(111) ultrathin film based on theoretical predictions and former experiments. Second, superconductivity induced by superconducting substrate may be detected on such ultrathin film, and makes the system a new platform to search Majorana fermions.

Bi has a rhombohedra crystal structure as indicated by blue dashed line, or a hexagonal structure indicated by green dashed line in Figure 1a

21

. Each atom and its three covalently bonded

equidistant nearest neighbors, which are coplanar in rhombohedra [111] direction, form a BL structure. The inter-BL bonding is much weaker than the bonding within the BL. Such that bulk Bi crystal has a (111) cleave plane and epitaxial Bi(111) thin films grow in BL mode. Epitaxial Bi thin film was grown on several substrates and exhibited a Bi(110) to Bi(111) transition growth behavior

22-32

. Similarly, Bi forms 2 BLs Bi(110) first on NbSe2 and then Bi(111) BL

films grown above . Figure 1c shows a STM image of Bi on NbSe2 after the transition thickness. Truncated triangle islands are formed on the substrate. In the inset of Figure 1c, an atomic resolution image taken on one of those islands shows a hexagonal shaped lattice. In Figure 1d, a line profile taken along the blue dashed line in Figure 1c shows four Bi layer steps with height of 3.94 Å. Both the atomic resolution image and the step height of the layer agree with the Bi(111) surface crystal structure in Figure 1b.


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The electronic structures of Bi(111)/NbSe2 were studied by ARPES. Figure 2a shows the Fermi surface mapping of the bare NbSe2 substrate before Bi deposition. There are two sets of hole pockets centered at Γ point and also at K point

33

. After growing 3 BLs Bi(111), the Fermi

surface of the sample exhibits two structures as shown in Figure 2b. One is the hole pocket centered at Γ point, which belongs to the Fermi surface of the NbSe2 substrate

34

. Another

Fermi surface inside the hole pocket of NbSe2 belongs to Bi(111), which consists of six hole pockets in the ΓM direction and an electron pocket centered at Γ point 21. The coexistence of two band structures is due to the ultrathin thickness of the Bi(111) on the substrate. 3 BLs Bi(111) film together with 2 BLs Bi(110) have a thickness of 2.66 nm. At such length, the phonon can penetrate through the ultrathin film and interact with the substrate. We note that Bi has a very long mean free path

13

so that the electron from the photo-emission of the substrate

can get through the ultrathin film. The blue hexagon in Figure 2b marks the Brillouin zone of Bi(111) ultrathin film. The hole pocket of NbSe2 shows a hexagonal shape and is at the same direction with its Brillouin zone hexagon (not shown in the figure). The directions of the two Brillouin zones show that the lattice of Bi(111) film is rotated for 30 degrees according to the NbSe2 lattice underneath. To help understand the experimental results, DFT calculations for freestanding 3 BLs Bi(111) were performed. The calculation results (blue dashed line) are plotted on the experimental data along ΓK and ΓM directions, as shown in Figure 2c and 2d respectively. The high symmetry directions are according to the Brillouin zone of Bi(111). Along the ΓK direction the experimental bands fit well with the bands of calculated freestanding Bi (Figure 2c). There are some discrepancies along ΓM direction in the large momentum regime, especially close to M point (Figure 2d). Near the M point, bands originated from the hybridization of pz and py are not consistent with experimental observation. Owing to the


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unsaturated nature of pz orbital, adsorbates (such as hydrogen) can bind on Bi, shifting these two bands away from Fermi Level. The existence of adsorbates in the experiments which is not included in the calculation may lead to the disappearance of the two band. However it does not affect the band features around Γ point.

The band near M point observed in experiment

belongs to the hole pocket feature of NbSe2. The good agreement between the experimental results and the DFT calculations of freestanding Bi indicates that Bi(111) ultrathin films have no strong interaction with NbSe2 substrate or Bi(110) film in between. As mentioned before, Bi(111) BL thin films below 8 BLs have been predicted to be 2D TIs 14, but interactions between Bi and its substrate usually change the bands’ topology. Here the Bi(111) ultrathin films grown on NbSe2 are quasi-freestanding, thus it has great possibility to find the topological edge states on this system and STS measurements are employed to study them.

Figure 3a shows the topography of 3 BLs Bi(111) on the substrate. The hexagonal shape island with 3.94 Å step height corresponds to 1 BL Bi(111). The underlying two layers are also Bi(111) BLs. Three spectra were taken on the island at three typical positions labeled with A, B and C in Figure 3a: position A is on one step edge of the island, position B is on the adjacent step edge and position C is on the island terrace. The two adjacent edges of Bi(111) bulked honeycomb BL have different atomic structures, considering the underlying Bi(111) layer. Both of them have zig-zag structures while one kind of the edges has its out most atoms belong to the upper layer atoms of the BL and the other kind of the edges has its out most atoms belong to the lower layer atoms of the BL. The former kind of edge is defined as type-A edge and the latter one is defined as type-B edge, which follows the definition in the previous work19, 20. In Figure 3b normalized


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dI/dV spectra taken at those three positions manifest different features: At position A, the spectrum (solid (blue) curve) shows a strong peak at STM bias +183 mV. At position B and C (dashed (red) curve for B and dot dashed (black) curve for C), dI/dV maximum do not appear at such position, while broad peaks centered at +240 mV are shown. The +183 mV peak is also detected on the other edge adjacent to position B, which was indicated by a dashed (blue) line in Figure 3a. STS spectra taken along this line are shown in Figure 3c. The existence of the +183 mV peak is consistent along the edge. We also measured the spatial distribution of the +183 mV state, STS spectra (Figure 3d) were taken on 16 equal-distance points on a 13 nm line across the edge, starting from upper Bi(111) BL to the lower Bi(111) BL, indicated by a black arrow in Figure 3a. At first the spectrum shows similar features with that on island terrace: a peak shows up at +240 mV and a dip is near +100 mV. While moving closer to the edge, at point 9, the +183 mV dI/dV peak appears abruptly. Such peak lasts for three spectra points 9, 10 and 11 and then disappears from point 12 to the last point 16. Point 11 is right on the edge and the three points with +183 mV dI/dV peak along the line give a spatial distribution of 1.73 nm. Similar spatial distribution was found on the edge state of Bi(111) on Bi2Te3 system

16

. The consistent

characteristic energy peak at +183 mV and a narrow spatial distribution on the edge are the signatures of a 1D edge state on the hexagonal Bi(111) BL edge. It is worth noting that in the STS spectra taken on all three typical positions shown in Figure 3b, a dip exists at energy around 100 mV. This fact suggests that no significant doping effect exists in the sample that would cause the spectrum peak at 240 mV on the terrace shifting to 183 mV on the type-A edge and disappearing on the type-B edge, since the energy of the dip at the three positions remains the same. We also note that the continuous behavior of the previous spectra peaks shifting in spectra 1 to 6 is clearly different with the consistent peak at 183 mV near the edge in spectra 9 to 11,


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which is an obvious signature of an edge state. This difference can distinguish localized edge states from quantum-confinement caused standing waves. Standing waves usually have their energy moving with their real space position, while the real edge states’ energy position is localized at the edge and irrelevant to the real space position. In Figure 3d, the black dashed line indicates the energy position of the edge states that does not change in three spectra while the gray dashed line indicates moving energy peaks that may be caused by standing wave. This 1D edge state is extremely similar with the previously reported 1D topological edge state identified on one kind of the zig-zag edges of Bi(111) on Bi bulk crystal and on Si(111) 19, 20. Both of them show a +183 mV energy peak and a dependence on edge structures. This result is straight forward since the existence of the 1D edge state is due to the weak interaction between the out most atoms on the edge and the underlying Bi layer, which is “lifted” from the underlying Bi layer due to the buckled honeycomb structure. This weak interaction nature makes the above edge be indifferent to the underlying layer to be epitaxial thin films or a bulk crystal. Thus, the 1D topological edge state of Bi(111) BL can exist at layer thickness as thin as 3 BLs.

After confirming the existence of edge states on the ultrathin film, it is interesting to see the effects of the superconducting substrate on the ultrathin film, since proximity effect is likely to be strong at such thin thickness.

The superconductivity of Bi(111) BLs was studied by cooling down the system to 400 mK. In Figure 4a, dI/dV spectra clearly reveal the superconducting gaps on Bi(111) from 1 BL to 7 BLs. Distance between the coherent peaks which corresponds to the gap size gradually narrows with


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increasing layer thickness. Zero bias conductances are none-zero for all the spectra. The dI/dV spectra are fitted with a BCS s-wave superconducting spectrum function including a broadening factor 35:

dI ∝ dV

∫

+∞

−∞

 Re  

E −i Γ

(E

− i Γ)

2

 ×  2 − ∆  

 exp  ( E + eV ) / kT     k BT 1 + exp  ( E + eV ) / k BT  

{

  dE 2   

(1)

}

Fitted superconducting gap sizes of different BLs are plotted in Figure 4b together with the gap size of NbSe2 substrate. The distance between NbSe2 and 1 BL Bi(111) is an indication of Bi(110) in between. On 1 BL Bi(111), the superconducting size is slightly below 0.80 meV and decays to 0.46 meV on 7 BLs. Gap decay with layer thickness from NbSe2 to Bi(111) can be roughly fitted with an exponential curve. Magnetic field dependence of the superconducting gaps is shown in Figure 4c and 4d for 1 BL and 4 BLs Bi(111) respectively. With magnetic field increasing, superconducting gaps become shallower and eventually vanish at 1.4 T for 1 BL and 0.6 T for 4 BLs. It is worth noting that a BCS-type function can only capture the proximity superconducting gap spectrum inside the coherence peaks, as shown in the inset of Figure 4b. The coherence peak of the proximity superconducting gap is sharper compared with the fitted BCS-type spectrum, which is more broadened outside the gap. This can be explained by a bound state effect between a superconducting phase and a non-superconducting phase 36.

As discussed above, the superconducting gap on Bi(111) thin film are induced from NbSe2 substrate via proximity effect. The decay length of the gap strongly depended on the vertical


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coherence length of NbSe2, Bi(111) BLs and Bi(110) layers in between. Compared with previously reported Bi2Se3/NbSe2 and Bi2Te3/NbSe2 heterostructures, superconducting gap in Bi decay faster with increasing layer thickness than in Bi2Se3 and Bi2Te3, indicating a shorter coherent length in Bi(111) thin film

5, 6

. It is worth noting that the 1D edge state on Bi(111) BL

exists at a very thin layer thickness so that superconductivity from the substrate doesn’t vanish. While for 3 BLs Bi(111) where the 1D edge state is confirmed, the proximity effect still exits, and the superconducting gap is 0.6 meV.

In summary, we have grown Bi(111) ultrathin films on superconducting (SC) substrate NbSe2 for the first time. And Bi(111) BLs can exist on the substrate stably after 2 BLs of Bi(110). This Bi(111)/SC system has two advantages. One is that the interaction between the ultrathin film and the substrate is weak, since the band structure of 3 BLs Bi(111) film shows quasi-freestanding feature. And the 1D edge state also remains the same with that found on the bulk Bi(111) crystal. These indicate the Bi(111) ultrathin film is still topologically non-trivial. Another is that on such ultrathin film, superconducting proximity effect from the substrate is still strong enough to be detected. Therefore, Bi(111)/NbSe2 heterostructure is a good platform to study the topological properties of Bi(111) and even to search Majorana fermions with proximity induced superconductivity.

The ARPES experiment was carried out in a molecular beam epitaxy (MBE)-STM-ARPES joint system. The STM/STS experiment was carried out in a MBE-STM joint system. Samples were grown in the MBE chamber and then measured via different approaches in-situ. The 2H-NbSe2


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substrate was prepared by chemical vapor transport (CVT) method. After degassing at 500 K for more than 4 hours, substrate was cleaved at room temperature in the MBE chamber with base pressure better than 2×10-10 Torr. High purity Bi (99.999%) was then deposited on the substrate using a standard Knudsen cell at substrate temperature 350 K. The ARPES measurements were performed at 90 K using He-Iα light (21.2 eV) in the lab, and some data were taken at the SGM3 beamline of the synchrotron radiation source ASTRID. The energy resolution is better than 15 meV and angular resolution is better than 1% of the Brillouin zone. Measurements for topography and large energy scale STS were performed at 4.2 K. STS measurements of superconductivity were measured at 400 mK, using liquid 3He refrigerator. Electrochemical etched W tips were used for STM and STS measurements. STS spectra were obtained via lock-in technique with modulation frequency at 991 Hz and amplitude at 10 mV. A bias divider was used in the measurement of superconductivity STS. First-principle calculations were performed using the density functional theory (DFT) from Vienna ab initio simulation package with a plane wave basis functional

37

38

. Local density approximation (LDA) was used for the exchange and correlation

, and van der Waals (vdW) corrections to the density functional in the Grimme

implementation

39

were also included. The core electrons were represented by the projector-

augmented-wave (PAW) potential 40. The kinetic energy cutoff is set above 400 eV on a 20 × 20 k-point mesh. The structure optimization process is performed with force convergence criteria at 0.01 eV/Å.

AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected] Telephone: +86 21 5474 4616 Author Contributions H. H. S. conducted the experiments with the help of M. X.W., F. F. Z., G. Y. W. and H. Y. M.. Z. A. X. provided the NbSe2 crystals. Q. L. and Y. H. L carried out the calculations. G. C. L., Y. Y. L, C. H. L., D. Q., D. G. and J. F. J. analyzed the data. H. H. S., D. G. and J. F. J. wrote the manuscript. All the authors discussed the results. Funding Sources The work was supported by the Ministry of Science and Technology (MOST) of China (Grant Nos 2013CB921902, 2016YFA0301003, 2016YFA0300403, 20130073120081), NSFC (Grant Nos 11521404, 11504230, U1632102, 11227404, 11274228, 11174199, 11374206, 11134008, 91421312, 91221302, 11464051, 61574123, 11374009) and Shanghai Committee of Science and Technology, China (Grant Nos 12JC1405300, 13QH1401500, 14XD1401900). Notes The authors declare no competing financial interest.


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(26) Kowalczyk, P. J.; Mahapatra, O.; McCarthy, D. N.; Kozlowski, W.; Klusek, Z.; Brown, S. A. Surf. Sci. 2011, 605, 659-667. (27) Kammler, M.; Horn-von Hoegen, M. Surf. Sci. 2005, 576, 56-60. (28) Nagao, T.; Sadowski, J.; Saito, M.; Yaginuma, S.; Fujikawa, Y.; Kogure, T.; Ohno, T.; Hasegawa, Y.; Hasegawa, S.; Sakurai, T. Phys. Rev. Lett. 2004, 93, 105501. (29) Sharma, H. R.; Fournée, V.; Shimoda, M.; Ross, A. R.; Lograsso, T. A.; Gille, P.; Tsai, A. P. Phys. Rev. B 2008, 78, 155416. (30) Scott, S. A.; Kral, M. V.; Brown, S. A. Phys. Rev. B 2006, 73, 205424. (31) Nagao, T.; Doi, T.; Sekiguchi, T.; Hasegawa, S. Japanese Journal of Appl. Phys. 2000, 39, 4567. (32) Chen, M.; Peng, J.-P.; Zhang, H.-M.; Wang, L.-L.; He, K.; Ma, X.-C.; Xue, Q.-K. Appl. Phys. Lett. 2012, 101, 081603. (33) Kiss, T.; Yokoya, T.; Chainani, A.; Shin, S.; Nohara, M.; Takagi, H. Phys. B 2002, 312, 666-667. (34) Yokoya, T.; Kiss, T.; Chainani, A.; Shin, S.; Nohara, M.; Takagi, H. Science 2001, 294, 2518. (35) Nishio, T.; Ono, M.; Eguchi, T.; Sakata, H.; Hasegawa, Y. Appl. Phys. Lett. 2006, 88, 113115. (36) Truscott, A. D.; Dynes, R. C.; Schneemeyer, L. F. Phys. Rev. Lett. 1999, 83, 1014-1017. (37) Kresse, G.; Hafner, J. Phys. Rev. B 1993, 47, 558-561. (38) Solovyev, I. V.; Dederichs, P. H.; Anisimov, V. I. Phys. Rev. B 1994, 50, 16861-16871. (39) Grimme, S. J. Comput. Chem. 2006, 27, 1787-1799. (40) Blöchl, P. E. Phys. Rev. B 1994, 50, 17953-17979.


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Figure 1. Crystal structure of bulk Bi and epitaxial Bi(111) on NbSe2. (a) Rhombohedra structure (blue solid line) and hexagonal structure (green dashed line) of Bi. (b) Crystal structure of Bi(111) surface. Top layer atoms (red spheres) and bottom layer atoms (blue spheres) form a buckled honeycomb structure. (c) STM image of Bi(111) on NbSe2. Inset: Atomic resolution image taken from the surface in (c). (d) Line profile taken from the dashed (blue) line in (c). The STM image was taken with tunneling current I=0.1 nA and STM bias V=1.0 V. Atomic resolution image was taken with I=0.2 nA and V=100 mV.


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Figure 2. Band structure of NbSe2 and 3 BLs Bi(111) on NbSe2. (a) Fermi surface of NbSe2. (b) Fermi surface of 3 BLs Bi(111)/NbSe2. The blue solid hexagon is the first Brillouin zone of Bi(111). (c) Band structure of 3 BLs Bi(111) )/NbSe2 along the Γ - K high symmetry direction. (d) Band structure of 3BLs Bi(111) )/NbSe2 along the Γ - M high symmetry direction. The DFT calculated bands of freestanding 3 BLs Bi(111) are plotted as blue dashed line in (c) and (d). (a) was taken at photon energy of 70 eV, and the others were taken at photon energy of 21.2 eV.


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Figure 3. dI/dV spectra on Bi(111) BLs. (a) STM image of Bi(111) on NbSe2. The top most hexagonal shape island corresponds to one Bi(111) BL. (b) Three dI/dV spectra taken at three positions on the island labelled by A, B and C in (a). The spectrum (solid (blue) line) taken at edge A shows a +183 mV peak while such peak is missing in the spectra from edge B and the terrace. (c) 16 dI/dV spectra taken along the edge indicated by the dashed (blue) line in (a). The dashed (black) line indicates the consistency of the peak. (d) 16 dI/dV spectra were taken along the arrow in (a), starting (number 1) from the top most Bi(111) BL, crossing the edge and ending (number 16) at the lower Bi(111) BL. The spectra on the terrace, 1 to 8 and 12 to 16, show similar features as spectrum taken at position C in (b). Spectra 9 to 11 were taken near the edge and show a +183 mV peak as in position A, indicated by a dashed (black) line. The peak on the terrace shows an energy shift, indicated by a dash curve (gray). Spectra in (c) and (d) are shifted


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vertically for a clearer view. The STM image (a) was taken at bias 1.0 V and tunneling current 0.1 nA. The STS spectra in (c)-(d) were taken at tunneling current 0.2 nA.

Figure 4. dI/dV spectra on different thickness of Bi(111) ultrathin films with/without magnetic field. (a) dI/dV spectra from Bi(111) 1 BL to Bi(111) 7BLs. Superconducting gaps with strong coherence peaks are shown for all layers. (b) BCS fitted gap sizes of the spectra from the 7 layers. The decay of the gap sizes together with gap size of NbSe2, 1.1 meV, was fitted exponentially against sample thickness (dashed curve). Inset: BCS fitting for the 7 BLs spectrum. (c) and (d) The magnetic field dependence of the superconducting spectra on 1 BL and 4BLs Bi(111) respectively. Spectra in (a), (c) and (d) are shifted vertically for a clearer view. The STS spectra were taken at tunneling current 0.2 nA.


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For TOC Only 72x35mm (300 x 300 DPI)


Coexistence of Topological Edge State and ... - ACS Publications

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