Unusual Electronic States and Superconducting Proximity Effect of Bi

Subscriber access provided by Iowa State University | Library

Article

Unusual Electronic States and Superconducting Proximity Effect of Bi Films Modulated by NbSe2 Substrate Lang Peng, Jingsi Qiao, Jing-Jing Xian, Yuhao Pan, Wei Ji, Wenhao Zhang, and Ying-Shuang Fu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b08051 • Publication Date (Web): 17 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28 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

ACS Nano

Unusual Electronic States and Superconducting Proximity Effect of Bi Films Modulated by NbSe2 Substrate Lang Peng1,#, Jingsi Qiao2,#, Jing-Jing Xian1, Yuhao Pan2, Wei Ji2,†, Wenhao Zhang1,*, YingShuang Fu1,‡ 1.

School of Physics and Wuhan National High Magnetic Field Center, Huazhong University

of Science and Technology, Wuhan 430074, China 2.

Beijing Key Laboratory of Optoelectronic Functional Materials & Micro-Nano Devices,

Department of Physics, Renmin University of China, Beijing 100872, China

KEYWORDS: few-layers bismuth(110), two-dimensional heterostructures, proximity effect, thickness dependence, covalent-like quasi-bonds, scanning tunneling microscopy, density functional theory calculations.

ABSTRACT.

ACS Paragon Plus Environment

1

ACS Nano 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

Page 2 of 28

Heterostructures of two-dimensional layered materials can be functionalized with exotic phenomena that are unpresented with each constituting component. The interface effect plays a key role in determining the electronic properties of the heterostructure, whose characterization requires a correlation with the morphology with atomic-scale precision. Here, we report an investigation on the electronic properties of few-layer Bi(110) films mediated by NbSe2 substrate. By utilizing scanning tunneling microscopy and spectroscopy, we show a significant variation of the density of states at different Bi film thicknesses, resulting in an unusual superconducting proximity effect that deviates from the conventional monotonous decay behavior. Moreover, the electronic states of the Bi films are also prominently modulated by the Moiré pattern spatially. With first-principles calculations, we illuminate these findings as the results of covalent-like quasibonds formed at the Bi/NbSe2 interface, which profoundly alter the charge distributions in the Bi films. Our study indicates a viable way of modulating the electronic properties of ultrathin films by quasi-covalent interfacial couplings beyond conventional van der Waals interactions.

Two-dimensional (2D) layered materials, such as graphene, hexagonal boron nitride (hBN) and transition metal dichalcogenides (TMDs), have attracted considerable interest for exploring a large variety of quantum phenomena that can be tailored through artificial structures.1 In such systems, the coupling strength among the individual interlayers is crucial for determining their compelling physical properties.2,3 For instance, while single-layer NbSe2 exhibits 2D Ising superconductivity with suppressed TC of 3.0 K and strongly enhanced charge density wave (CDW) order at 145 K. Interlayer couplings in multilayer NbSe2 introduce orbital effects and destroy the perfect Ising pairing in the mono-layer (1L), yielding a higher TC (7.2 K) and decreasing CDW temperature (33


2

Page 3 of 28 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

ACS Nano

K) at the bulk limit.4,5 In MoSe2, when the thickness decreases from tri-layer (3L) to 1L, its electronic bandgap increases from 1.3 to 2.2 eV with a concomitant indirect-direct transition of binding energy,6,7 revealing the extreme sensitivity of interlayer coupling in determining the band structure. In the case of black phosphorus (BP), high-mobility transport associated with strong anisotropic electrical, optical and mechanical behaviors emerge in addition to the layer-dependent bandgap with increasing interlayer hybridization.8 Recently, striking layer-dependent ferromagnetism has also been reported in ultrathin CrI3 flakes, where each individual layer is ferromagnetically ordered while the interlayer coupling is antiferromagnetic.9,10 Moreover, the layer stacking of van der Waals (vdW) heterostructures with selective combinations provides even more opportunities for creating interesting functionalities. The spatial alignment and interlayer Moiré potential between vdW layers can be an independent designing parameter to manifest the fractal Hofstadter spectrum and cloning of Dirac fermions in graphene/hBN heterostructures,11,12 giving rise to the possibility of local topological order with periodic modulation.13 The Moiré pattern can also realize superlattice potentials in TMDs, bringing about exotic physics for long-lived topological excitons with the giant spin-orbit coupling (SOC).14,15 Recently, the layer stacking is also illustrated of tuning 2D magnetism in CrI3 bi-layers (2L).16 To date, however, little is known about interlayer-coupling effects in metallic 2D layered hybrid materials, especially their variations of local density of states (LDOS) around the Fermi level (EF), which usually play a vital role in determining electron-related properties of the quantum systems, such as transport, thermal conductivity, chemical reactivity and among the others.17 In this work, we investigate the electronic properties of Bi(110) few-layers grown on NbSe2 substrates by molecular beam epitaxy (MBE). Scanning tunneling microscopy and spectroscopy (STM/STS) reveals that the LDOS of the Bi/NbSe2 heterostructure not only laterally exhibits


3

ACS Nano 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

Page 4 of 28

nanoscale spatial modulation that correlates with the Moiré pattern but also drastically changes as a function of Bi film thickness in the vertical direction. Such layer-dependent modulations lead to an unusual superconducting (SC) proximity effect. Our results of both vertical and lateral modulations are corroborated by density functional theory (DFT) calculations, which explains their physical origins from the stacking order of Bi(110) on NbSe2 and the strong quasi-covalent interlayer interaction. The strong substrate-modulated electronic properties of layered heterostructures presented here emphasize a profound impact on designing tunable properties, with atomic precision, of materials beyond vdW epitaxy. RESULTS AND DISCUSSION Bulk Bi is a semi-metal that crystallizes in a gray-As A7 structure.18 Its thin films with (111) surface orientation have been studied widely on their topological edge states, which can further achieve topological superconductivity when coupled to SC NbSe2 (or Nb).19-23 However, Bi ultrathin films, no thicker than four layers,24,25 adopt a BP-like A17 structure on the NbSe2 substrate as suggested by our DFT calculations (see Figure S1-S3 in the Supporting Information). Inspired by the success of tuning layer-dependent bandgap in semiconducting BP, we investigate the interlayer-coupling effect on the electronic structure of the semimetallic Bi(110) films, sharing a similar crystal structure as BP films. As sketched in Figure 1a, each layer of a Bi(110) film contains two chemically bonded atomic sub-layers with vdW gaps among these layers. Unlike the BP structure, the two Bi atoms in each sub-layer are buckled with a vertical difference of 0.57 Å in 1L, which gradually reduces to 0.35 Å in 3L (Figure S2). Our STM observations affirm that 1L3L Bi films grown on NbSe2 indeed adopt the BP-like structure (Figure 1d). Figure 1e shows a zoomed-in image with atomic resolution of 1L (atomic resolution on 2L can be seen in Figure S4), where a quasi-squared Bi lattice (dark blue rectangle) is clearly resolved with lattice constants of


4

Page 5 of 28 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

ACS Nano

a = 4.7 Å and b = 4.5 Å, consistent with the distorted model shown in Figure 1a. Moreover, a Moiré pattern is evidently observed with a period of roughly 14.0 Å (light blue rhombus), which originates from the relative stacking order between the quasi-squared Bi layer and the triangular NbSe2 substrate.26 Given this Moiré periodicity, we consider several possible atomic configurations of the heterostructure (see Figure S5,S6 for calculation details), then compare their lattice mismatches (Table S1) and total energies. Figure 1b,c illustrate the DFT revealed most stable stacking configuration between a 5×3 Bi(110) mono-layer and a 4√3×4 NbSe2 bi-layer substrate, where the lattice mismatches are -1% and -2% along the armchair (x) and zigzag (y) directions, respectively. Such a stacking order (black dotted rectangle in Figure 1c), in which the shorter axis of the Bi(110) lattice (along y) is oriented parallel to a zigzag direction of NbSe2, is consistent with both the STM image (Figure 1e) and its fast Fourier transform (FFT) image (Figure 1f). The Moiré pattern is irrelevant with the 3×3 superstructure of the CDW order in NbSe2 (Figure S4).27 Similar Bi(110) structure has been reported in the initial growth stage of Bi2Se3 on NbSe2, where a mixture of BiSe is formed at the interface to reduce the lattice mismatch.28 Recently, single monolayer Bi(111) film of triangular lattice is created by epitaxially conforming to the NbSe2 substrate,29 distinctive from our buckled structure. These discrepancies may lie on the very different stacking orientation and interfacial coupling strength when Bi atoms superimposed on NbSe2. To reveal the electronic structure of the Bi films, we perform STS measurements on the surfaces of 1L to 3L Bi(110). All the three surfaces show metallic LDOS as shown in Figure 2a. This is different from those of Bi(110) layers grown on graphene or HOPG substrates in previous studies, where an energy gap of ~0.4 eV is opened around EF due to edge reconstructions or particular atomic buckling.30,31 Strikingly, these spectra show substantially distinctive features for different


5

ACS Nano 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

Page 6 of 28

Bi(110) layers. While the 1L film shows a conductance peak at ~160 mV, there appear a drastically enhanced peak around ~20 mV and a broad peak at ~290 mV on the 2L surface. On the 3L surface, three peaks locate at ~60, 190 and 320 mV, respectively, with less pronounced intensities. Figure 2d,e present 2D conductance plots of the spectra measured along the cyan dashed lines across step edges of different layers in Figure 2b,c, respectively. The spatial distributions of the conductance features are found to be approximately homogeneous throughout the terraces of each layer, whereas exhibit an abrupt change at the step edge (Individual dI/dV curves can been seen in Figure S7). Our DFT calculations reproduce the experimental measurements. The calculated LDOS of the 1L to 3L Bi being supported on a 2L NbSe2 substrate (Figure 2f) are reasonably consistent with the STS spectra in terms of peak positions and their layer-dependent evolution, as labeled by colored arrows in Figure 2a. Plots of LDOS on other Bi sties show similar characteristics of peaks for each thickness of layers, as shown in Figure S8 and S9. We visualize the wavefunctions of all LDOS peaks around EF to get better insights into the origin of these observed peaks. Figure 3a shows that those states of 1L within a range from EF to EF+0.3 eV are mainly comprised of Bi pz orbitals. These pz orbitals are less affected by the substrate above EF in 1L but hybridize at the inter-Bi-layer region in 2L, forming a pair of bonding state and anti-bonding state in ranges of [EF, EF+0.2 eV] (Figure 3b) and [EF+0.2 eV, EF+0.4 eV] (Figure 3c), respectively. Consequently, the LDOS peak residing between 0.1 to 0.2 eV of the 1L (black arrows in Figure 2a and 2f) splits into a lower-energy- and higher-energy-peak (red arrows) in 2L, sitting at EF and 0.3 eV, respectively. The state at EF is an interlayer bonding state of pz orbitals, which vertically spans the whole, including interlayer regions of, Bi thin film and NbSe2 substrate. As a result, a vertical resonance state is formed that essentially enhances the interlayer conductance, giving rise to the observed


6

Page 7 of 28 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

ACS Nano

pronounced conductance peak at EF in STS. This is not the case for anti-bonding states, in which the top Bi surface dominates. On the other hand, the states below EF are largely modulated by the NbSe2 substrate, leading to drastically distorted pz orbitals in 1L, as illustrated by the visualized wavefunctions of states in [EF-0.3 eV, EF] (Figure S10). This may be the reason why STS detects no appreciable states of 1L below EF (Figure 2a). Nevertheless, this distortion quickly decays that those pz orbitals becomes more homogenous from 2L to 3L. Next, we plot the spatial distributions of differential charge densities (DCDs) at the topmost interlayer regions of Bi films (Figure 3e-g) to show their charge redistributions. The DCD for 1L (2L and 3L) thus illustrates the charge redistribution at the Bi-NbSe2 interface (Bi-Bi interlayer regions). Substantial charge variations are found between the bottom Bi layer and the NbSe2 layers underneath, suggesting a strong interfacial interaction at the Bi-NbSe2 interface (Figure 3e), which is comparable with the Bi-Bi interlayer interactions of 2L or 3L (Figure 3f,g). The charge accumulation at the interface and the interlayer overlap of wavefunctions (Figure 3a-c) indicate the interlayer interactions of both Bi-NbSe2 and Bi-Bi possess covalent-like features, which are previously named as covalent-like quasi-bonds instead of pure vdW interactions.16,32 The two interactions compete with each other, which significantly modifies the charge distribution of Bi layers and yields the experimentally observed layer-dependent conductance spectra. In addition to the DCDs, two adjacent Bi atoms on the 1L Bi/NbSe2 surface (marked by the blue dashed rectangles in Figure 3e-g) indicate obvious difference of charge redistribution, as well as their BiBi bond lengths up to 0.05 Å (Figure 3d). Such differences largely reduce on 2L and nearly vanish on 3L, i.e. inappreciable charge variation and 0.02 Å for bond length deviation. Therefore, the layer dependences of visualized wavefunctions, DCDs and Bi-Bi bond lengths suggest that the modulations of both the electronic and geometric structures in Bi overlayers, tuned by the NbSe2


7

ACS Nano 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

Page 8 of 28

substrate, gradually degrade for thicker Bi films, dominated from Bi-NbSe2 to Bi-Bi interlayer interaction. For thinner Bi films, their electronic structure is largely influenced by the NbSe2-Bi interactions, where the interfacial Moiré pattern is evident. Figure 4b,c are two morphological images of Bi(110) mono- and bi-layers, respectively, where a Moiré period of 14.0 Å is observable (Figure S11). The Moiré structure is less explicit on thicker films that reflects the reducing interface effect with thickness. We examine the STS spectra at different sites, with atomic precision, of the Moiré pattern. Figure 4d,e show two STS maps along the lines indicated in Figure 4a for 1L and 2L, respectively. Modulations, agreeing well with the Moiré periodicity, are explicitly found in these maps, especially those peaks near EF of both layers. Similar Moiré modulations are also created in other 2D material hetero-bilayers, e.g., MoS2/WSe2,33 WS2/MoS234 or graphene/hBN,26 where the local electronic structure is also found to be dictated by the interlayer coupling as a function of atomic registry. To quantitatively depict the lateral electronic modulations, we calculated the LDOS of each Bi site on the surfaces of 1L and 2L Bi. The spatial distribution of the visualized electronic wavefunctions norms suggest the electronic states at the surface originate from p orbitals (Figure 3a,b and S10) but their spatial corrugation is rather small. Therefore, the observed Moiré patterns are most likely due to geometric corrugations of the surface Bi atoms, which comes from different local stacking geometries and strong interfacial couplings. This attribution is justified by the good consistency between the calculated height variations of surface Bi atoms and the corrugation of the STM images (Figure S12). Further evidences can be obtained by comparing the LDOS of a number of typical sites within the Moiré pattern between the experiment and the calculations. Point A (B) in Figure 4b stands for the bright (dark) spot of the Moiré pattern on the 1L surface. Its


8

Page 9 of 28 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

ACS Nano

counterpart atom is the Bi-A (Bi-B) atom in Figure 4f, which locates at a representative location of the Moiré supercell. Similar correspondence applies for point C (D) and Bi-C (Bi-D) in Figure 4c,f for the 2L Bi film. As shown in Figure 4g,h, main features of the STS spectra acquired at points A-D (solid curves) are reasonably captured by the calculated LDOS of atoms Bi-A to Bi-D (dashed curves), exhibiting qualitative similarities in variations of peak positions and conductance intensities. Specifically, the LDOS of atoms Bi-A and Bi-B reasonably reproduce the spectroscopic peak around 0.15 eV at points A and B on 1L. For the 2L surface, both the experiment and the theory show a peak sitting around EF and indicate this peak is more pronounced for point C and atom Bi-C. Corresponding calculated LDOS of other Bi atoms of the 1L surface are plotted in Figure S9. The thickness-dependent modulation of LDOS around EF implies that many physical properties, relevant with LDOS around EF, might be also modulated. Here, we demonstrate such modulations using a SC proximity effect from the NbSe2 substrate.35 Figure 5a shows the spatially averaged tunneling spectra taken on the 1L to 3L Bi(110) surfaces at 0.4 K, respectively. Clear SC gaps are observed on all the three layers. By fiiting the SC gaps with the BCS form, we extract the proximity-induced SC gaps of 0.98, 1.08 and 0.96 meV for 1L, 2L and 3L, respectively, as summarized in the inset of Figure 5a (Figure S13). It is exceptional that the 2L has the largest SC gap, which violates the monotonously decreasing behavior previously revealed in the conventional proximity scenario.36 The exceptionally larger gap of 2L is also explicitly seen in the 2D conductance plot across a step between the 1L and 2L Bi surfaces (Figure 4c). By scrutinizing on the spatial distribution of STS spectra in Figure 5c, we find the SC gaps on both 1L and 2L terrace are no longer modulated by the Moiré pattern of 1.4 nm. This may be attributed to the much larger in-plane coherence length of NbSe2 (7.7 nm),37 which well-smear out


9

ACS Nano 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

Page 10 of 28

the Moiré electronic modulation (Figure 4). On the other hand, there exists some spectroscopic inhomogeneity near the step edge of 2L, and the SC gap varies slightly at different locations. Such spatial dependence of SC gap between adjacent Bi films of different thicknesses directly correlates with the film geometries (More details can be found in Figure S13), and have been previously reported as lateral proximity effect in 2D superconducting Pb(111) junctions.38 We also note that a nearly undisturbed SC gap is recently reported on a strongly bonded commensurate Bi(111) layer grown on NbSe2,29 instead of Bi(110) in our case. Therein, a more complicated interface between Bi and NbSe2 is formed with compressive strain. Such strong interaction, possibly due to the formation of ripples and domains by covalently bonding, makes the 1L Bi(111) an integral part of the top NbSe2 tri-layer, thus different from our covalent-like quasi-bonds. We conjecture that value of proximity-induce SC gap on the 1L Bi(111) film very close to the NbSe2 substrate may originate from the even stronger hybridization between Bi and NbSe2, which may favor the overlap of wavefunction and penetration of Cooper pairs in proximity effect. According to the BCS theory, the SC gap is exponentially dependent on the LDOS at EF.39,40 Either 1L or 3L has a moderate LDOS value at EF (Figure 2a), leading to a smaller SC gap of 3L due to the decayed Cooper paring with film thickness. However, the 2L is a special case that it has a pronounced LDOS at EF albeit with a moderate proximity strength. The combination of the both effects results in a much larger SC gap. In light of this, the NbSe2 substrate could even modulate macroscopic quantum properties of Bi few-layers, implying the possibilities of enhancing superconductivity through interfacial couplings. In addition to the SC proximity effect, we infer many other physical properties that are closely related to the LDOS around EF can be tuned. For instance, the Seebeck coefficient can be greatly enhanced when varying the LDOS over a narrow energy range based on the Mahan-Sofo theory, as is implemented in the Tl-doped PbTe system.41,42


10

Page 11 of 28 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

ACS Nano

The large LDOS near EF of Bi films implies a giant Seebeck coefficient, which is most likely valuable for improving the performance of other thermoelectric materials.43 CONCLUSIONS In summary, we have combined STM/STS measurements and DFT calculations to investigate the electronic structure of layered Bi(110) films on NbSe2 substrates, which exhibit intriguing modulations with both the film thickness vertically and the spatial Moiré pattern laterally. Our first-principles calculations indicate that the electronic structure of Bi(110) layers is governed by two competing covalent-like interlayer interactions, namely Bi-Bi and Bi-NbSe2 interlayers, and their competitions are tuned by the Bi film thickness. We also observed an unusual superconducting proxity effect with Bi film thicknesses as a result of their layer-dependent electronic structure. In this sense, our Bi/NbSe2 heterostructures provide a promising platform, which allows atomically precise manipulation of many other physical properties, e.g. thermoelectric effect, of Bi few-layers. The effect of interfacial covalent-like interactions presented here may be generalized to other layered hybrid systmes for exploring more compelling quantum phenomena, which are otherwise difficult to realize for weak-coupled layers or less efficient for the bulk counterparts of strongly interlayer-coupled 2D materials.

METHODS MBE growth. The NbSe2 single crystal is cleaved in ultrahigh vacuum (2×10-10 torr) at 90 K as substrates, onto which high purity Bi (99.999%) is deposited from a standard Knudsen cell at 723 K. The NbSe2 substrate is held at room temperature and the growth rate is ~ 1.5 Å /min. After growth the Bi(110) film is post-annealed at 520 K for 30 minutes to obtain flat surface.


11

ACS Nano 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

Page 12 of 28

STM/STS measurements. The experiments are performed on a commercial Unisoku STM system (1300) at 4.5 K if not specifically noted.22 Normal W tips are cleaned by e-beam heating and checked on Ag islands before the STM and STS measurement. All topographic images are taken in a constant-current mode, and the tunneling dI/dV is recorded by standard lock-in technique, with the feedback loop off. The modulation voltage frequency is 983 Hz with an amplitude (Vmod) of 20 mV. DFT Calculations. Density functional theory calculations are performed using the generalized gradient approximation for the exchange-correlation potential, the projector augmented wave method and a plane-wave basis set as implemented in the Vienna ab-initio simulation package (VASP).44-46 In geometric optimization and electronic properties calculations, van der Waals interactions are considered at the vdW-DF level with the optB86b exchange functional (optB86bvdW),47 which is proved to be accurate in describing the structural properties of layered materials.48 The kinetic energy cut-off for the plane-wave basis set is set to 500 eV and 700 eV for the geometric properties of few-layer Bi(110), NbSe2 unit cell and 3×3 CDW 2L-NbSe2. The kmeshes of 12×15×1, 13×13×1 and 5×5×1 are adapted to sample the first Brillouin zone of them, respectively. The shape and volume of each supercell are fully optimized and all atoms in the supercell are fully relaxed until the residual force per atom is less than 6×10-3 eV·Å-1 (Bi) and 1×10-3 eV·Å-1 (NbSe2). Calculations of lattice mismatch in Bi/NbSe2 heterostructure. The unit cell of Bi/NbSe2 heterostructure is comprised of a 5×3 Bi(110) films and a 4√3×4 NbSe2 substrate. The lattice constants of few-layer Bi(110) and NbSe2 are shown in Table S1. The lattice mismatch between few-layer Bi(110) and NbSe2 is calculated by ε = (LBi - L NbSe2) / LNbSe2 and shows -1.0% ~ 0.0% and -1.6% ~ -1.8% along the armchair (x) and zigzag (y) directions in . In 1L to 3L Bi/NbSe2,


12

Page 13 of 28 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

ACS Nano

respectively. In the geometric and electronic properties of Bi(110)/NbSe2 heterostructures calculations, 400 eV kinetic energy cut-off is set. The k-meshes of 2×4×1 is adapted to sample the first Brillouin zone of Bi(110)/2L-NbSe2. The shape and volume of each supercell are fixed and all atoms in the supercell are fully relaxed until the residual force per atom is less than 2×10-2 eV·Å-1.

Supporting Information: The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Figures of geometric structures for Bi(110) and Bi(111) with different thicknesses on the NbSe2 substrate in DFT calculations; figures of morphology and Bias-dependent STM images of Bi(110) films grown on the NbSe2 substrate; figures of dI/dV spectra crossing the step edge between different thicknesses of Bi(110) films; figures of calculations on the LDOS, Bi-Bi bond lengths of top Bi-layer, wavefunctions and simulations of atomic height from 1L to 3L; figures of SC gaps with various thicknesses of Bi(110) films.

AUTHOR INFORMATION Corresponding Author E-mails: *[email protected], ‡[email protected], †[email protected] ORCID Wenhao Zhang: 0000-0003-2386-0305 Wei Ji: 0000-0001-5249-6624


13

ACS Nano 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

Page 14 of 28

Author Contributions #L.

Peng and J. Qiao contributed equally to this work.

ACKNOWLEDGMENTS This work is funded by the National Key Research and Development Program of China (Grants No. 2017YFA0403501, No. 2016YFA0401003, and No. 2018YFA0307000), the National Science Foundation of China (Grants No. 11774105, No. 11504056, No. 11522431, No. 11474112, No. 11274380, No. 91433103, No. 11622437, No. 61674171, and No. 61761166009), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB30000000), the Fundamental Research Funds for the Central Universities (Grant No. 2017KFXKJC009) and the Research Funds of Renmin University of China (16XNLQ01). Calculations are performed at the Physics Lab of High-Performance Computing of Renmin University of China and the Shanghai Supercomputer Center.

REFERENCES [1] Geim, A. K.; Grigorieva, I. V. Van der Waals Heterostructures. Nature 2013, 499, 419−425. [2] Woods, C. R.; Britnell, L.; Eckmann, A.; Ma, R. S.; Lu, J. C.; Guo, H. M.; Lin, X.; Yu, G. L.; Cao, Y.; Gorbachev, R. V.; Kretinin, A. V.; Park, J.; Ponomarenko, L. A.; Katsnelson, M. I.; Gornostyrev, Y. N.; Watanabe, K.; Taniguchi, T.; Casiraghi, C.; Gao, H.-J.; Geim, A. K.; Novoselov, K. S. Commensurate-Incommensurate Transition in Graphene on Hexagonal Boron Nitride. Nat. Phys. 2014, 10, 451−456.


14

Page 15 of 28 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

ACS Nano

[3] Polini, M.; Guinea, F.; Lewenstein, M.; Manoharan, H. C.; Pellegrini, V. Artificial Honeycomb Lattices for Electrons, Atoms and Photons. Nat. Nanotechnol. 2013, 8, 625−633. [4] Xi, X.; Zhao, L.; Wang, Z.; Berger, H.; Forró, L.; Shan J.; Mak, K. F. Strongly Enhanced Charge-Density-Wave Order in Monolayer NbSe2. Nat. Nanotech. 2015, 10, 765−770. [5] Xi, X.; Wang, Z.; Zhao, W.; Park, J.-H.; Law, K. T.; Berger, H.; Forró, L.; Shan J.; Mak K. F. Ising Pairing in Superconducting NbSe2 Atomic Layers. Nat. Phys. 2016, 12, 139−143. [6] Ugeda, M. M.; Bradley, A. J.; Shi, S.-F.; da Jornada, F. H.; Zhang, Y.; Qiu, D. Y.; Mo, S.-K.; Hussain, Z.; Shen, Z.-X.; Wang, F.; Louie, S. G.; Crommie, M. F. Observation of Giant Bandgap Renormalization and Excitonic Effects in a Monolayer Transition Metal Dichalcogenide Semiconductor. Nat. Mater. 2014, 13, 1091−1095. [7] Bradley, A. J.; Ugeda, M. M.; da Jornada, F. H.; Qiu, D. Y.; Ruan, W.; Zhang, Y.; Wickenburg, S.; Riss, A.; Lu, J.; Mo, S.-K.; Hussain, Z.; Shen, Z.-X.; Louie, S. G.; Crommie, M. F. Probing the Role of Interlayer Coupling and Coulomb Interactions on Electronic Structure in Few-Layer MoSe2 Nanostructures. Nano Lett. 2015, 15, 2594−2599. [8] Qiao, J.; Kong, X.; Hu, Z.-X.; Yang F.; Ji, W. High-Mobility Transport Anisotropy and Linear Dichroism in Few-layer Black Phosphorus. Nat. Commun. 2014, 5, 4475. [9] Huang, B.; Clark, G.; N.-Moratalla, E.; Klein, D. R.; Cheng, R.; Seyler, K. L.; Zhong, D.; Schmidgall, E.; McGuire, M. A.; Cobden, D. H.; Yao, W.; Xiao, D.; J.-Herrero P.; Xu, X. LayerDependent Ferromagnetism in a Van der Waals Crystal Down to the Monolayer Limit. Nature 2017, 546, 270−273.


15

ACS Nano 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

Page 16 of 28

[10] Zhong, D., Seyler, K. L., Linpeng, X., Cheng, R., Sivadas, N., Huang, B., Schmidgall, E., Taniguchi, T., Watanabe, K., McGuire, M. A., Yao, W., Xiao, D.; Fu, K.-M. C.; Xu, X. Van der Waals Engineering of Ferromagnetic Semiconductor Heterostructures for Spin and Valleytronics. Sci. Adv. 2017, 3, e1603113. [11] Dean, C. R.; Wang, L.; Maher, P.; Forsythe, C.; Ghahari, F.; Gao, Y.; Katoch, J.; Ishigami, M.; Moon, P.; Koshino, M.; Taniguchi, T.; Watanabe, K.; Shepard, K. L.; Hone, J.; Kim, P. Hofstadter’s Butterfly and the Fractal Quantum Hall Effect in Moiré Superlattices. Nature 2013, 497, 598−602. [12] Ponomarenko, L. A.; Gorbachev, R. V.; Yu, G. L.; Elias, D. C.; Jalil, R.; Patel, A. A.; Mishchenko, A.; Mayorov, A. S.; Woods, C. R.; Wallbank, J. R.; M.-Kruczynski, M.; Piot, B. A.; Potemski, M.; Grigorieva, I. V.; Novoselov, K. S.; Guinea, F.; Fal’ko, V. I.; Geim, A. K. Cloning of Dirac Fermions in Graphene Superlattices. Nature 2013, 497, 594−597. [13] Tong, Q.; Yu, H.; Zhu, Q.; Wang, Y.; Xu, X.; Yao, W. Topological Mosaics in Moiré Superlattices of Van der Waals Heterobilayers. Nat. Phys. 2017, 13, 356−362. [14] Yu, H.; Liu, G.-B.; Tang, J.; Xu, X.; Yao, W. Moiré excitons: From Programmable Quantum Emitter Arrays to Spin-Orbit-Coupled Artificial Lattices. Sci. Adv. 2017, 3, e1701696. [15] Wu, F.; Lovorn, T.; MacDonald, A. H. Topological Exciton Bands in Moiré Heterojunctions. Phys. Rev. Lett. 2017, 118, 147401. [16] Huang, B.; Clark, G.; Klein, D. R.; MacNeill, D.; Navarro-Moratalla, E.; Seyler, K. L.; Wilson, N.; McGuire, M. A.; Cobden, D. H.; Xiao, D.; Yao, W.; Jarillo-Herrero, P.; Xu, X. Electrical Control of 2D Magnetism in Bilayer CrI3. Nat. Nanotechnol. 2018, 13, 544−548.


16

Page 17 of 28 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

ACS Nano

[17] Keimer, B.; Moore, J. E. The Physics of Quantum Materials. Nat. Phys. 2017, 13, 1045−1055. [18] Hofmann, P. The Surfaces of Bismuth: Structural and Electronic Properties. Prog. Surf. Sci. 2006, 81, 191−245. [19] 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. [20] Du, H.; Sun, X.; Liu, X.; Wu, X.; Wang, J.; Tian, M.; Zhao, A.; Luo, Y.; Yang, J.; Wang B.; Hou, J. G. Surface Landau Levels and Spin States in Bismuth(111) Ultrathin Films. Nat. Commun. 2016, 7, 10814. [21] Liu, X.; Du, H.; Wang, J.; Tian, M.; Sun X.; Wang, B. Resolving the One-Dimensional Singularity Edge States of Bi(111) Thin Films. J. Phys.: Condens. Matter 2017, 29, 185002. [22] Yang, F.; Jandke, J.; Storbeck, T.; Balashov, T.; Aishwarya, A.; Wulfhekel, W. Edge States in Mesoscopic Bi Islands on Superconducting Nb(110). Phys. Rev. B 2017, 96, 235413. [23] Peng, L.; Xian, J.-J.; Tang, P.; Rubio, A.; Zhang, S.-C.; Zhang, W.; Fu, Y.-S. Visualizing Topological Edge States of Single and Double Bilayer Bi Supported on Multibilayer Bi(111) Films. Phys. Rev. B 2018, 98, 245108. [24] Nagao, T.; Sadowski, J.; Saito, M.; Yaginuma, S.; Fujikawa, Y.; Kogure, T.; Ohno, T.; Hasegawa, Y.; Hasegawa, S.; Sakurai, T. Nanofilm Allotrope and Phase Transformation of Ultrathin Bi Film on Si(111)-7×7. Phys. Rev. Lett. 2004, 93, 105501. [25] 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.


17

ACS Nano 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

Page 18 of 28

[26] Xue, J.; S.-Yamagishi, J.; Bulmash, D.; Jacquod, P.; Deshpande, A.; Watanabe, K.; Taniguchi, T.; J.-Herrero, P.; LeRoy, B. J. Scanning Tunnelling Microscopy and Spectroscopy of Ultra-Flat Graphene on Hexagonal Boron Nitride. Nat. Mater. 2011, 10, 282−285. [27] Ugeda, M. M.; Bradley, A. J.; Zhang, Y.; Onishi, S.; Chen, Y.; Ruan, W.; O.-Aristizabal, C.; Ryu, H.; Edmonds, M. T.; Tsai, H.-Z.; Riss, A.; Mo, S.-K.; Lee, D.; Zettl, A.; Hussain, Z.; Shen, Z.-X.; Crommie, M. F. Characterization of Collective Ground States in Single-Layer NbSe2. Nat. Phys. 2016, 12, 92−97. [28] Wang, M.-X.; Li, P.; Xu, J.-P.; Liu, Z.-L.; Ge, J.-F.; Wang, G.-Y.; Yang, X.; Xu, Z.-A.; Ji, S.H.; Gao, C. L.; Qian, D.; Luo, W.; Liu, C.; Jia, J.-F. Interface Structure of a Topological Insulator/Superconductor Heterostructure. New J. Phys. 2014, 16, 123043. [29] Fang, A.; Adamo, C.; Jia, S.; Cava, R. J.; Wu, S.-C.; Felser, C.; Kapitulnik, A. Bursting at the Seams: Rippled Monolayer Bismuth on NbSe2. Sci. Adv. 2018, 4, eaaq0330. [30] Sun, J.-T.; Huang, H.; Wong, S. L.; Gao, H.-J.; Feng, Y. P.; Wee, A. T. S. Energy-Gap Opening in a Bi(110) Nanoribbon Induced by Edge Reconstruction. Phys. Rev. Lett. 2012, 109, 246804. [31] Lu, Y.; Xu, W.; Zeng, M.; Yao, G.; Shen, L.; Yang, M.; Luo, Z.; Pan, F.; Wu, K.; Das, T.; He, P.; Jiang, J.; Martin, J.; Feng, Y. P.; Lin, H.; Wang, X.-S. Topological Properties Determined by Atomic Buckling in Self Assembled Ultrathin Bi(110). Nano Lett. 2015, 15, 80−87. [32] Hu, Z.-X.; Kong, X.; Qiao, J.; Normand, B.; Ji, W. Interlayer Electronic Hybridization Leads to Exceptional Thickness-Dependent Vibrational Properties in Few-Layer Black Phosphorus. Nanoscale, 2016, 8, 2740−2750.


18

Page 19 of 28 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

ACS Nano

[33] Zhang, C.; Chuu, C.-P.; Ren, X.; Li, M.-Y.; Li, L.-J.; Jin, C.; Chou, M.-Y.; Shih, C.-K. Innterlayer Couplings, Moiré Patterns, and 2D Electronic Superlattices in MoS2/WSe2 HeteroBilayers. Sci. Adv. 2017, 3, e1601459. [34] Gong, Y.; Lin, J.; Wang, X.; Shi, G.; Lei, S.; Lin, Z.; Zou, X.; Ye, G.; Vajtai, R.; Yakobson, B. I.; Terrones, H.; Terrones, M.; Tay, B. K.; Lou, J.; Pantelides, S. T.; Liu, Z.; Zhou, W.; Ajayan, P. M. Vertical and In-Plane Heterostructures From WS2/MoS2 Monolayers. Nat. Mater. 2014, 13, 1135−1142. [35] Wang, M. X.; Liu, C. H.; Xu, J. P.; Yang, F.; Yao, M. Y.; Gao, C. L.; Shen, C. Y.; Ma, X. C.; Xu, Z. A.; Liu, Y.; Zhang, S. C.; Qian, D.; Jia, J. F.; Xue Q. K. The Coexistence of Superconductivity and Topological Order in the Bi2Se3 Thin Films. Science 2012, 336, 52−55. [36] Arnold G. B. Theory of Thin Proximity-Effect Sandwiches. Phys. Rev. B 1978, 18, 1076. [37] de Trey, P.; Gygax, S.; Jan, J. P. Anisotropy of the Ginzburg-Landau Parameter κ in NbSe2. J. Low. Temp. Phys. 1973, 11, 421−434. [38] Kim, J.; Chua, V.; Fiete, G. A.; Nam, H.; MacDonald, A. H.; Shih, C.-K. Visualization of Geometric Influences on Proximity Effects in Heterogeneous Superconductor Thin Films. Nature Phys. 2012, 8, 464−469. [39] Guo, Y.; Zhang, Y.-F.; Bao, X.-Y.; Han, T.-Z.; Tang, Z.; Zhang, L.-X.; Zhu, W.-G.; Wang, E. G.; Niu, Q.; Qiu, Z. Q.; Jia, J.-F.; Zhao, Z.-X.; Xue, Q.-K. Superconductivity Modulated by Quantum Size Effects. Science 2004, 306, 1915−1917. [40] Eom, D.; Qin, S.; Chou, M.-Y.; Shih, C. K. Persistent Superconductivity in Ultrathin Pb Films: A Scanning Tunneling Spectroscopy Study. Phys. Rev. Lett. 2006, 96, 027005.


19

ACS Nano 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

Page 20 of 28

[41] Reddy, P.; Jang, S.-Y.; Segalman, R. A.; Majumdar, A. Thermoelectricity in Molecular Junctions. Science 2007, 315, 1568−1571. [42] Heremans, J. P.; Jovovic, V.; Toberer, E. S.; Saramat, A.; Kurosaki, K.; Charoenphakdee, A.; Yamanaka, S.; Snyder, G. J. Enhancement of Thermoelectric Efficiency in PbTe by Distortion of the Electronic Density of States. Science 2008, 321, 554−557. [43] Cheng, L.; Liu, H. J.; Zhang, J.; Wei, J.; Liang, J. H.; Jiang, P. H.; Fan, D. D.; Sun, L.; Shi, J. High Thermoelectric Performance of the Distorted Bismuth(110) Layer. Phys. Chem. Chem. Phys., 2016, 18, 17373−17379. [44] Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953. [45] 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. [46] Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758. [47] Dion, M.; Rydberg, H.; Schröder, E.; Langreth, D. C.; Lundqvist, B. I. Van der Waals Density Functional for General Geometries. Phys. Rev. Lett. 2004, 92, 246401. [48] Zhao, Y.; Qiao, J.; Yu, P.; Hu, Z.; Lin, Z.; Lau, S. P.; Liu, Z.; Ji, W.; Chai, Y. Extraordinarily Strong Interlayer Interaction in 2D Layered PtS2. Adv. Mater. 2016, 28, 2399−2407.

Figures captions


20

Page 21 of 28 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

ACS Nano

Figure 1. (a) Schematics of Bi(110) film in a distorted BP-like A17 structure. (b) Side view of 1L Bi(110) stacked on NbSe2 substrate. (c) Top view of the black dashed rectangle in (b), which is the optimized stacking in Bi/NbSe2 heterostructures. (d) STM morphology of Bi(110) films grown on NbSe2 substrate (constant-current mode with Vbias = +3.0 V and It = 10 pA). (e) Atomically resolved STM image of 1L Bi(110) surface (Vbias = +100 mV, It = 100 pA). The Bi(110) unit-cell is labeled as a dark blue dotted rectangle. The Moiré superstructure is visible as a light blue dashed rhombus. Black dashed rectangle is the same supercell as in (c). (f) 2D FFT of (e). Figure 2. (a) Typical dI/dV spectra taken on 1L to 3L Bi films, respectively, revealing the distinctive peaks indicated by the symbols. The STS are acquired in a constant-height mode with set point: Vbias = +0.6 V, It = 100 pA, and Vmod = 10 mV. (b), (c) STM images of Bi(110) films with single-layer steps (Vbias = +1.0 V, It = 10 pA). (d), (e) dI/dV spectra recorded along the blue dashed lines in (b) and (c), respectively (set point: Vbias = +1.0 V, It = 100 pA, and Vmod = 20 mV). (f) Calculated LDOS for 1L to 3L Bi(110) films on NbSe2 substrate, respectively. The selected Bi atoms for calculations are Bi-A (1L) and Bi-C (2L and 3L) points in Figure 4f, respectively. Figure 3. (a-c) Visualized wavefunctions of Bi(110) films in 1L (a) and 2L (b, c). The energy ranges are [0, 0.3 eV] for (a), [0, 0.2 eV] for (b), and [0.2, 0.4 eV] for (c), respectively. The isosurfaces are 1×10-3 e/Bohr3 (a) and 1.5×10-4 e/Bohr3 (b, c). The Fermi energy level is set as 0. (d) Bi-Bi bond lengths of top Bi-layer (l1~ l10) for 1L to 3L, respectively, as marked in Figure S5(a) and S6(a). (e-g) The calculated interlayer differential charge density for the top two layers in Bi/NbSe2 heterostructures of 1L (e), 2L (f) and 3L (g), respectively (1 ×10-3 e/Bohr3). Red and green isosurface contours correspond to charge accumulation and reduction, respectively.


21

ACS Nano 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

Page 22 of 28

Figure 4. (a) A typical STM image containing both 1L and 2L Bi(110) film (Vbias = +1.0 V, It = 10 pA). (b), (c) STM images acquired on 1L (b) and 2L (c) Bi(110) surfaces, respectively, showing the Moiré patterns (Vbias = +0.2 V , It = 200 pA). (d), (e) 2D plot of tunneling spectra measured along the white (d) and magenta (e) lines in (a), respectively (set point: Vbias = +0.5 V, It = 100 pA, and Vmod = 10 mV). (f) Structural sketch of topmost Bi(110) surfaces with different Bi-sites: BiA, B, C, D. Bi-A and Bi-C are the highest Bi atoms on 1L and 2L surfaces, respectively. (g) Tunneling spectra (solid curves) at different locations on 1L surface [A and B points in (b)], as well as the calculated LDOS (dashed curves) for Bi-A and Bi-B sites in (f). (h) The same as (g) but on 2L Bi(110) surface. Figure 5. (a) Spatially averaged tunneling spectra taken on 1L to 3L Bi films at 0.4 K, respectively (set point: Vbias = +4 mV, It = 200 pA, and Vmod = 20 μV). All spectra are normalized with the conductance at 4 mV. Inset is the thickness evolution of SC gap on Bi(110) film induced by the NbSe2 substrate (1.4 meV). The error bars are extracted from the minimum and maximum values measured for each layer (Figure S13). A magenta dashed line is a guide to the eye. (b) STM image of Bi film containing both 1L and 2L with two step edges (Vbias = 1.0 V, It = 10 pA). (c) 2D plot of tunneling spectra measured along the blue dashed line in (b).


22

Page 23 of 28 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

ACS Nano

For Table of Contents Only

The geometries and electronic states of Bi(110) films are modulated by NbSe2 substrate through the covalent-like interlayer interactions.


23

ACS Nano 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

Figure 1. (a) Schematics of Bi(110) film in a distorted BP-like A17 structure. (b) Side view of 1L Bi(110) stacked on NbSe2 substrate. (c) Top view of the black dashed rectangle in (b), which is the optimized stacking in Bi/NbSe2 heterostructures. (d) STM morphology of Bi(110) films grown on NbSe2 substrate (constant-current mode with Vbias = +3.0 V and It = 10 pA). (e) Atomically resolved STM image of 1L Bi(110) surface (Vbias = +100 mV, It = 100 pA). The Bi(110) unit-cell is labeled as a dark blue dotted rectangle. The Moiré superstructure is visible as a light blue dashed rhombus. Black dashed rectangle is the same supercell as in (c). (f) 2D FFT of (e). 279x168mm (300 x 300 DPI)


Page 24 of 28

Page 25 of 28 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

ACS Nano

Figure 2. (a) Typical dI/dV spectra taken on 1L to 3L Bi films, respectively, revealing the distinctive peaks indicated by the symbols. The STS are acquired in a constant-height mode with set point: Vbias = +0.6 V, It = 100 pA, and Vmod = 10 mV. (b), (c) STM images of Bi(110) films with single-layer steps (Vbias = +1.0 V, It = 10 pA). (d), (e) dI/dV spectra recorded along the blue dashed lines in (b) and (c), respectively (set point: Vbias = +1.0 V, It = 100 pA, and Vmod = 20 mV). (f) Calculated LDOS for 1L to 3L Bi(110) films on NbSe2 substrate, respectively. The selected Bi atoms for calculations are Bi-A (1L) and Bi-C (2L and 3L) points in Figure 4f, respectively.


ACS Nano 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

Figure 3. (a-c) Visualized wavefunctions of Bi(110) films in 1L (a) and 2L (b, c). The energy ranges are [0, 0.3 eV] for (a), [0, 0.2 eV] for (b), and [0.2, 0.4 eV] for (c), respectively. The isosurfaces are 1×10-3 e/Bohr3 (a) and 1.5×10-4 e/Bohr3 (b, c). The Fermi energy level is set as 0. (d) Bi-Bi bond lengths of top Bi-layer (l1~ l10) for 1L to 3L, respectively, as marked in Figure S5(a) and S6(a). (e-g) The calculated interlayer differential charge density for the top two layers in Bi/NbSe2 heterostructures of 1L (e), 2L (f) and 3L (g), respectively (1 ×10-3 e/Bohr3). Red and green isosurface contours correspond to charge accumulation and reduction, respectively.


Page 26 of 28

Page 27 of 28 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

ACS Nano

Figure 4. (a) A typical STM image containing both 1L and 2L Bi(110) film (Vbias = +1.0 V, It = 10 pA). (b), (c) STM images acquired on 1L (b) and 2L (c) Bi(110) surfaces, respectively, showing the Moiré patterns (Vbias = +0.2 V , It = 200 pA). (d), (e) 2D plot of tunneling spectra measured along the white (d) and magenta (e) lines in (a), respectively (set point: Vbias = +0.5 V, It = 100 pA, and Vmod = 10 mV). (f) Structural sketch of topmost Bi(110) surfaces with different Bi-sites: Bi-A, B, C, D. Bi-A and Bi-C are the highest Bi atoms on 1L and 2L surfaces, respectively. (g) Tunneling spectra (solid curves) at different locations on 1L surface [A and B points in (b)], as well as the calculated LDOS (dashed curves) for Bi-A and Bi-B sites in (f). (h) The same as (g) but on 2L Bi(110) surface.


ACS Nano 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

Figure 5. (a) Spatially averaged tunneling spectra taken on 1L to 3L Bi films at 0.4 K, respectively (set point: Vbias = +4 mV, It = 200 pA, and Vmod = 20 μV). All spectra are normalized with the conductance at 4 mV. Inset is the thickness evolution of SC gap on Bi(110) film induced by the NbSe2 substrate (1.4 meV). The error bars are extracted from the minimum and maximum values measured for each layer (Figure S13). A magenta dashed line is a guide to the eye. (b) STM image of Bi film containing both 1L and 2L with two step edges (Vbias = 1.0 V, It = 10 pA). (c) 2D plot of tunneling spectra measured along the blue dashed line in (b). 275x135mm (300 x 300 DPI)


Page 28 of 28

Unusual Electronic States and Superconducting Proximity Effect of Bi

Recommend Documents