Structural Transition and Charge-Density-Wave Proximity Effect

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Ultrathin Bismuth Film on 1T-TaS: Structural Transition and Charge-Density-Wave Proximity Effect Keiko Yamada, Seigo Souma, Kunihiko Yamauchi, Natsumi Shimamura, Katsuaki Sugawara, Chi Xuan Trang, Tamio Oguchi, Keiji Ueno, Takashi Takahashi, and Takafumi Sato Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b01003 • Publication Date (Web): 27 Apr 2018 Downloaded from http://pubs.acs.org on April 27, 2018

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Ultrathin Bismuth Film on 1T-TaS2: Structural Transition and Charge-Density-Wave Proximity Effect Keiko Yamada†, Seigo Souma‡,§, Kunihiko Yamauchi‖, Natsumi Shimamura†, Katsuaki Sugawara‡,§, Chi Xuan Trang†, Tamio Oguchi‖, Keiji Ueno⊥, Takashi Takahashi†,‡,§, and Takafumi Sato*,†,‡ †Department

‡Center

§WPI

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

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

Research Center, Advanced Institute for Materials Research,Tohoku University, Sendai 980-8577, Japan

‖Institute

of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka 567-0047, Japan

⊥Department

of Chemistry, Graduate School of Science and Engineering,Saitama University, Saitama 338-8570, Japan

Corresponding Author *Phone: +81-(0)22-795-6477, E-mail: [email protected].

ACS Paragon Plus Environment

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ABSTRACT

We have fabricated bismuth (Bi) ultrathin films on a charge-density-wave (CDW) compound 1TTaS2, and elucidated electronic states by angle-resolved photoemission spectroscopy and firstprinciples band-structure calculations. We found that the Bi film on 1T-TaS2 undergoes a structural transition from (111) to (110) upon reducing the film thickness, accompanied by a drastic change in the energy band structure. We also revealed that while two-bilayer-thick Bi(110) film on Si(111) is characterized by a dispersive band touching the Fermi level (EF), the energy band of the same film on 1T-TaS2 exhibits holelike dispersion with a finite energy gap at EF. We discuss the origin of such intriguing differences in terms of the CDW proximity effect.

KEYWORDS: Charge density wave, Rashba metal, photoemission spectroscopy, band structure, ultrathin film


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One of key challenges in materials science is the realization of exotic quantum states of matter and new electronic devices by exploiting strong spin-orbit coupling (SOC), as highlighted by the discovery of topological insulators (TIs) with SOC-induced inverted band structure1-3 and the proposals of next-generation spintronics devices such as spin field-effect transistor.4 It is well known that group-V semimetal bismuth (Bi) is often a key element for such realization, e.g. in prototypical three-dimensional (3D) TIs such as Bi2Se3 and Bi2Te3, essentially owing to heavy atomic mass of Bi. Pristine Bi is also an excellent material platform since bulk Bi is a parent material of 3D TI Bi1-xSbx and its surface state is characterized by Rashba-SOC-induced spinsplit energy bands.5-11 When Bi becomes atomically thin and forms one-bilayer (1BL) (111) structure, it is predicted to become a quantum spin Hall insulator with inverted bulk bands hosting nontrivial topological edge states.12 While pristine Bi itself is already an interesting research target, and its electronic states have been intensively investigated so far by various experimental techniques such as angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM), recent studies have suggested that a hybrid (i.e. hetero-junction) of Bi and other materials can provide a fertile ground to realizing more exotic quantum phenomena. For example, when a Bi film is attached onto a superconductor,13 it can host topological superconductivity with Majorana modes, owing to the SOC-induced spin splitting and superconducting proximity effect. Also, a hybrid of Bi(110) film and ferromagnetic Ni(100) thin film was found to become an exotic superconductor with the superconducting transition temperature (Tc) as high as ~ 4 K,14-15 despite nonsuperconducting nature of the host materials. Moreover, a new category of proximity effect, termed topological proximity effect, was discovered in a hybrid of Bi and 3D TI.16-20 in which the Dirac-cone state of the TI migrates into an ultrathin Bi overlayer upon interfacing.20 As


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represented by these examples, it is of essential importance to fabricate a new hybrid system involving Bi in order to explore novel quantum phenomena. In this Letter, we report the first fabrication of ultrathin Bi film on layered transition-metal dichalcognide 1T-TaS2, which is known to exhibit a strong CDW with very high transition temperature (incommensurate CDW transition temperature TICCDW ~ 550 K).21-24 By comprehensively characterizing the film using ARPES, low-energy electron diffraction (LEED), and atomic-force microscopy (AFM), together with the first-principles band-structure calculations, we revealed that Bi thin film with (111) structure is successfully fabricated on a cleaved surface of 1T-TaS2 despite sizable lattice mismatch. Upon reducing the film thickness, the film undergoes a structural transition into the Bi(110) phase. Intriguingly, the 2BL Bi(110) film was found to show apparently different band dispersion depending on the types of substrates (Si(111) or 1T-TaS2). We discuss the origin of such difference in relation to change in lattice parameters, topological phase transition, and CDW proximity effect. High-quality single crystal of 1T-TaS2 was grown by chemical vapor transport method. We cleaved a 1T-TaS2 crystal with scotch tape under ultrahigh vacuum to obtain a shiny mirror-like surface, and then deposited Bi on it at T = 370 K by using molecular-beam-epitaxy (MBE) technique. A Bi(110) film on Si(111), used as a reference, was fabricated at T = 80 K.25 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 as well as by a comparison of ARPES-derived band dispersions with the band-structure calculations for free-standing multilayer Bi.


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ARPES measurements were performed with the MBS-A1 electron analyzer equipped with a high-intensity He and Xe plasma discharge lamps,26 as well as 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 Iα (hν = 21.218 eV), Xe I (hν = 8.437 eV), and 6-eV photons to excite photoelectrons. The energy resolution in ARPES measurements was set at 2-40 meV. The sample was kept at 30 K during the measurements. 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) code27 with local density approximation (LDA).28 In order to reproduce accurate interlayer distances, long-range van der Waals interaction is included through semi-empirical corrections by DFT-D2 approach.29 After the crystal structure was fully optimized until forces acting on atoms were less than 1 × 10−3 eV/Å, the SOC 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. First we present the fabrication and characterization of thicker Bi thin film. Figure 1a shows the low-energy electron diffraction (LEED) pattern of 1T-TaS2 measured at room temperature. Besides 1×1 spots, one can clearly recognize satellite spots with √13×√13 periodicity which arise from nearly commensurate CDW.21-24 Upon Bi deposition onto 1T-TaS2, the LEED pattern undergoes a drastic change accompanied with new 1×1 spots from Bi and disappearance of 1TTaS2 spots (Fig. 1b). Overall six-fold symmetry of the LEED pattern suggests the (111) orientation of Bi thin film (schematically shown in Fig. 1d), as supported by the atomic-forcemicroscopy (AFM) image in Fig. 1c which reveals the existence of triangular-shaped Bi terraces similarly to the case of Bi(111) thin film on Si(111).30-31


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We have traced an evolution of electronic states upon Bi deposition by in-situ ARPES experiment. Figure 2 shows a side-by-side comparison of (a, c) Fermi-surface mapping and (b, d) experimental band dispersions along the ΓM cut in the Brillouin zone for 1T-TaS2 and Bi(111) thin film on it, respectively, measured at T = 30 K with the He-I photons (hν = 21.218 eV). In 1T-TaS2 (Fig. 2b), multiple Ta 5d sub-bands originating from the CDW-induced band folding are observed at EB ~ 0.2, 0.5, and 1.0 eV. A Mott-Hubbard gap opens at EF ,32-35 as also inferred from the featureless intensity pattern in Fig. 2a. Upon Bi deposition, the CDW-related sub-bands disappear and highly dispersive holelike bands emerge at Γ (Fig. 2d). Moreover, a snow-flakelike Fermi surface appears around Γ (Fig. 2c). These features are attributed to the Bi 6p states of Bi(111) thin film. It is noted that the energy band located at ~0.2 eV in Fig 2d is ascribed to the Bi 6p band, but not to the Ta 5d band, since a similar band is observed in Bi(111)/Si(111).30 This is also supported by the fact that the photoelectron escape depth (5-10Å) is much shorter than the film thickness (30 Å). We have estimated the interval between the Γ and M points of Bi thin film from the periodicity of band dispersions to be 0.82 Å-1. This value is much smaller than that of 1T-TaS2 (1.08 Å-1) due to the lattice mismatch, consistent with the LEED patterns in Figs. 1a and 1b. We also found that the epitaxial strain in Bi thin film is rather weak since the in-plane lattice constant of the film estimated from the Γ M interval (a = 4.4±0.1 Å) is almost same as that of bulk (a = 4.53 Å). These results suggest that Bi(111) single-crystal film is fabricated on 1T-TaS2 despite the large lattice mismatch of ~ 35 % (referred to the Ta lattice, see schematics in Fig. 1d), which far exceeds those for other Bi(111) films on Si (18%),31 Bi2Te3 (4 %),36 and TlBiSe2 (7%). 20,37 To better visualize the band dispersions, we have performed ARPES measurements with higher accuracy using the Xe-I photons (hν = 8.437 eV). Obtained ARPES-intensity and


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corresponding second-derivative-intensity plots (Figs. 2e and 2f) signify several Bi-derived quantum-well states with energy dispersions similar to those of Bi/Si(111).38,39 A reasonable agreement is found in the overall band dispersion between ARPES and calculation for a freestanding 8 bilayer (BL) Bi(111) film (Fig. 2f), in good agreement with the film thickness estimated from the thickness monitor. It is remarked that no obvious signature of the 1T-TaS2substrate-indced anomaly was found in the band dispersion, probably because the film is too thick (8 BL; ~ 30 Å). This suggests the necessity to reduce the film thickness. Figure 3a shows the second-derivative ARPES intensity for the ultrathin Bi film (2BL thick) measured along the ΓM cut of 1T-TaS2 Brillouin zone. One can immediately recognize that the band dispersion is drastically different from that of 8BL Bi(111) (Fig. 2f). For instance, the number of bands in the EB range of 1 eV is much reduced due to the difference in the quantization condition, and no bands cross EF. Also, a characteristic “M”-shaped dispersion (dashed curve in Fig. 3a) appears in the vicinity of EF with the top of dispersion at ~ 0.5 Å-1 (marked by arrow). This band is reasonably reproduced by the slab calculation along the ΓX1 cut for 2BL Bi(110) (with Puckered-layer structure P2/m) (Fig. 3c) but not for 2BL Bi(111) (Fig. 3b). A comparison between Fig. 3a and Figs. 3(b, c) suggests that the overall experimental band structure shows a better agreement with the calculation for Bi(110). The experimental results in Figs. 2c-f and 3a-c all together indicate that the structural transition from (111) to (110) takes place upon reducing the film thickness (Fig. 3d). This is also supported by the AFM image for 2BL film (Fig. 3e) composed of several needle-like features which resembles that of Bi(110) on HOPG (highly oriented pyrolytic graphite)40. Since the terrace size was found to depend on the type of substrates (e.g., the terrace size of Bi(110) on 1T-TaS2 is much smaller than that on HOPG40) probably due to difference in the crystal symmetry and lattice parameters, we think that


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the Bi(110) islands are possibly coupled with the 1T-TaS2 substrate. This is different from the situation of thicker Bi(111) films which show similar triangular islands on the surface irrespective of the type of substrates.30 It is noted here that we have calculated the band structure for 2BL Bi(110) with black phosphorus (Pmc21) structure but the results are apparently different from our APRES result due to band splitting arising from inversion-symmetry-broken crystal structure. Therefore, our 2BL Bi(110) likely forms inversion-symmetric Puckered-layer (P2/m) structure showing no band splitting. A careful look at Fig. 3a reveals that the topmost valence band showing the M-shaped dispersion does not reach EF (highlighted by arrow) suggesting the semiconducting nature of the film. We will come back to this point later. A detailed analysis of the LEED pattern on 2BL Bi(110) film (Fig. 3f) suggests that the film is actually composed of three domains rotated by 120˚ from each other [this is naturally expected from the symmetry of Bi(110) overlayer (C2) and 1T-TaS2 substrate (C3)] and each domain further consists of two sub-domains rotated by ~ 6˚ (Fig. 3g). It is thus necessary to take into account multiple domains when interpreting the ARPES data of 2BL Bi(110)/1T-TaS2. As shown in Fig 4a, the second derivative of ARPES intensity at EB = 0.08 eV for 2BL Bi(110)/1TTaS2 measured with the He-Iα line (hν = 21.218 eV) reveals a couple of ring-like intensity contours which originate from the sub-domains rotated by ~ 6˚ (see left panel of Fig. 4a), consistent with the LEED pattern. Here we selected the second Brillouin zone (cut A) to more clearly observe these two sub-domains, since the two domains are better distinguished in the second zone due to a wider k separation than in the first zone, and the ARPES intensity near EF is


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stronger in the second zone with the He-Iα line due to the matrix-element effect of photoemission process. To gain further insight into the spectral feature of the topmost valence band, we performed ARPES measurements with continuous-wave (CW) laser (hν = 6 eV) with improved energy and momentum resolutions, and resolved the band structure of each domain. As shown in Fig. 4b, the ARPES intensity along the cut crossing the rings (cut A’ in Fig. 4a in the first Brillouin zone, which is identical to cut A in the reduced Brillouin-zone scheme and can be accessed even with the 6-eV laser) consists of two Bi-derived holelike bands topped at EB = 45 and 120 meV, respectively, and each band splits along the k direction due to the mixture of two domains. To examine possible influence of substrate to the electronic states, we have fabricated 2BL Bi(110) film on Si(111); this can serve as a good reference since the Bi(110) film on Si(111) is known to be nearly free-standing and Si(111) exhibits no CDW.31 One can immediately recognize from the intensity and second derivative plots in Figs. 4c and 4d, that Bi(110) film on Si(111) shows a Λshaped band dispersion touching EF. We drew a guideline of energy bands in Figs. 4c and 4d by tracing the peak position in the second-derivative intensity of momentum distribution curves (MDCs) since MDC is more reliable and accurate than energy distribution curve (EDC) to experimentally determine the band dispersions when the spectral feature is broad and the intensity suffers a strong suppression near EF. Considering the fact that the velocity of this band (4 eVÅ) matches that of topmost holelike band at higher EB in 2BL Bi(110)/1T-TaS2, these bands would commonly originate from the topmost valence band (note that the origin of second band topped at 120 meV is unclear at present). Thus, the observed marked difference in the nearEF spectral feature between Bi(110)/1T-TaS2 and Bi(110)/Si(111) is an intrinsic property of epitaxial Bi(110) film.


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Now we discuss the origin of observed difference in the topmost valence-band dispersion between Bi(110) on 1T-TaS2 and Si(111). First possibility is the surface reconstruction which takes place only in Bi(110)/1T-TaS2. However, this can be excluded because the LEED pattern does not contain higher-order satellite spots and the ARPES data show no obvious band folding. Second possibility is the change in the lattice parameters. Our LEED analysis indicates that aand b-axis lengths are identical between the two films within our experimental accuracy [a = 4.4±0.1 Å and b = 4.9±0.1 Å for Bi(110)/1T-TaS2, and a = 4.5±0.1 Å and b = 4.9±0.1 Å for Bi(110)/Si(111)], and therefore the variation in a and b may not be responsible for the observed change in the dispersion. Our slab calculation revealed that the energy position of the topmost valence band is sensitive to the buckling parameter h of Bi bilayer (see inset to Fig. 4e), and the h value of 0.4 Å (0.5 Å) with fixing a and b to the experimental values was found to reasonably reproduce the energy location of the topmost valence band for Bi(110)/1T-TaS2 (Bi(110)/Si(111)), as shown in Fig. 4e. Thus, it would be possible to attribute the change in the band dispersion to the change in h, although this is not experimentally justified due to difficulty in determining the actual h value from the experiment. It is noted that the observed Λ-shaped dispersion of the topmost valence band was not reproduced in the calculations, which always show a rounded shape at the top of valence band within a reasonable parameter range. As another plausible scenario to account for the change in the band dispersion, one may point out topological phase transition. A recent first-principles band-structure calculation41 predicted that ultrathin Bi(110) film is located on the verge of topological phase transition. The topological character is dominated by touching/detaching of the conduction-band bottom and the valenceband top at the midway between Γ and X2, as in the case of isostructural black phosphorus,42 and the change in lattice parameters triggers the topological phase transition between 2D ordinary


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insulator and 2D TI through Dirac-semimetal phase.41 In this context, the Λ-shaped band in Bi(110)/Si(111) could be attributed to the lower Dirac-cone band near/at the topological phase boundary. However, such a band touching/detaching has been predicted to occur along the ΓX2 cut, but not the ΓX1 cut which we are discussing here (note that we found no signature of the band touching along the ΓX2 cut in our ARPES data). Also, we have confirmed topologically trivial nature of our 2BL Bi(110) from the analysis of the parity of the calculated valence-band wave functions. Thus, the difference between previous theory41 and the present result would essentially originate from the difference in the lattice parameters a, b, and h. The last, and more exotic interpretation for the observed band modulation in 2BL Bi(110)/1TTaS2 is the CDW proximity effect. Namely, electrons in Bi film feel the effective potential of periodic lattice distortions originating from commensurate CDW in 1T-TaS2 through the interface. Such an influence of CDW may be more prominent in the region closer to EF and would trigger the energy gap opening at EF, like in Fig. 4b (note that we have performed temperature-dependent ARPES experiment and confirmed that the gap persists at least up to 100 K. Above 150 K, it was difficult to resolve the gap opening due to strong thermal broadening of the ARPES spectra). This interpretation is consistent with the experimental fact that no band anomaly is visible in the thicker film; this is natural since the proximity effect would occur only around the interface. If CDW proximity effect indeed takes place in this system, the present study would provide a new concept to modulate band dispersion by utilizing hybrid structure involving CDW materials. Further studies on the electronic states of Bi(110) on various transition-metal dichalcogenide substrates would be necessary to firmly establish this novel CDW proximity effect.


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FIGURES

Figure 1. (a), (b) Low-energy electron diffraction (LEED) patterns of (a) pristine 1T-TaS2 and (b) Bi(111) thin film [thickness d = 8 bilayer (BL)] on 1T-TaS2 measured at room temperature with a primary electron energy of 100 eV. (c) Atomic-force-microscope (AFM) image of 8BL Bi(111)/1T-TaS2. (d) Schematic atomic arrangement of Bi(111) and 1T-TaS2 which highlights a large lattice mismatch between Bi(111) overlayer and 1T-TaS2 substrate.


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Figure 2. (a), (b) Plot of ARPES intensity at EF for pristine 1T-TaS2 as a function of in-plane wave vector around the ΓM line, and ARPES intensity as a function of binding energy (EB) and wave vector along the ΓM cut, respectively, measured with the He-Iα photons (hν = 21.218 eV). (c), (d) Same as (a), (b) but for 8BL Bi(111) on 1T-TaS2. (e), (f) ARPES and its second derivative intensities, respectively, for 8BL Bi(111)/1T-TaS2 as a function of EB and wave vector along the ΓM cut measured with the Xe-I photons (hν = 8.437 eV). Solid curves in (f) are calculated band dispersions for free-standing 8BL Bi(111). The ARPES data were recorded at 30 K.


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Figure 3. (a) Plot of ARPES intensity for 2BL Bi(110) ultrathin film on 1T-TaS2 as a function of EB and wave vector along the ΓX1 cut measured with the Xe-I photons at T = 30 K. (b), (c) Calculated band structures for free-standing 2BL Bi(111) and Bi(110), respectively. (d) Comparison of atomic arrangement between Bi(111) and Bi(110) to highlight the structural transition with film thickness. (e) AFM image of 2BL Bi(110) on 1T-TaS2. (f) LEED pattern of 2BL Bi(110) on 1T-TaS2. White circles and lines highlight the spots and reciprocal lattice of 1TTaS2, respectively, while blue and light blue circles/lines are from sub-domains of Bi(110) film rotated by ~ 6°. (g) Schematics to explain the observed LEED pattern by taking into account three main domains rotated by 120° to each other together with sub-domains rotated by ~ 6°.


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Figure 4. (a) Second derivative of ARPES intensity at EB = 0.08 eV for 2BL Bi(110) on 1T-TaS2 as a function of 2D wave vector around the valence-band top in the second Brillouin zone (BZ) of two sub-domains, measured with the He-Iα line (hν = 21.218 eV). Dashed circles highlight the contribution from two domains. (b) ARPES-intensity plot along cut A’ in the first BZ (shown in (a) and identical to cut A in the second BZ in the reduced BZ scheme) measured with 6-eV laser. Red and blue dashed curves represent the band dispersion originating from two different domains, respectively. (c), (d) ARPES intensity and second-derivative intensity plots of momentum distribution curves, respectively, for 2BL Bi(110) on Si(111). (e) Calculated near-EF band dispersions for 2BL Bi(110) for h = 0.4 and 0.5. In-plane lattice parameters are fixed to the experimental values (a = 4.5 Å and b = 4.95 Å). Inset shows the schematic crystal structure of 2BL Bi(110) with puckered-layer structure with buckling parameter h. The ARPES data were recorded at 30 K.


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AUTHOR INFORMATION Author Contributions The work was planned and proceeded by discussion among K.Y., S.S., K.S., and T.S. and K.Y., N.S., and K.U. carried out the samples’ growth and their characterization. K.Y., S.S., N.S., K.S., and C.T. performed ARPES measurements. K.Y., S.S., T.T., and T.S. finalized the manuscript with inputs from all the authors. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by Grant-in-Aid for Scientific Research on Innovative Areas "Topological Materials Science" (JSPS KAKENHI Grant Number JP15H05853), "Science of Atomic Layers" (JSPS KAKENHI Grant Numbers JP25107003, JP25107004), and Grant-in-Aid for Scientific Research (JSPS KAKENHI Grant Numbers JP17H01139, JP15H02105, JP26287071, and JP25287079).


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Structural Transition and Charge-Density-Wave Proximity Effect

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