Abstract
It is well known that a topologically protected gapless state appears at an interface between a topological insulator and an ordinary insulator; however, the physics of the interface between a topological insulator and a metal has largely been left unexplored. Here we report a novel phenomenon termed topological proximity effect, which occurs between a metallic ultrathin film and a threedimensional topological insulator. We study one bilayer of bismuth metal grown on the threedimensional topological insulator material TlBiSe_{2}, and by using spin and angleresolved photoemission spectroscopy, we found evidence that the topological Diraccone state migrates from the surface of TlBiSe_{2} to the attached onebilayer Bi. We show that such a migration of the topological state occurs as a result of strong spindependent hybridization of the wave functions at the interface, which is also supported by our firstprinciples calculations. This discovery points to a new route to manipulating the topological properties of materials.
Introduction
The topological surface state of threedimensional topological insulators (3D TIs) is protected by timereversal symmetry and characterized by a linearly dispersing Diraccone energy band with helical spin texture^{1,2,3,4,5}. This peculiar surface state is a consequence of interfacing TIs with an ordinary insulator, including vacuum. When a hybrid structure of a 3D TI and a superconductor is fabricated, it has been predicted that superconducting proximity effect occurring at the interface would lead to the appearance of twodimensional (2D) topological superconductivity hosting Majorana fermions^{6}. Also, when a 3D TI is in contact with a ferromagnetic insulator, the resulting magnetic proximity effect at the interface leads to the opening of a gap at the Dirac point^{7,8}, which is prerequisite to realizing topological magnetoelectric effects^{9,10}. On the other hand, no such exotic phenomena have been discovered for the interface between a TI and a metal; in fact, despite the obvious importance of understanding the TI/metal interface for device applications, there have been only a limited number of experimental studies of such an interface^{11,12,13}, while several theoretical predictions have been made on possible proximity effects occurring in TI heterostuctures involving semiconductor, graphene, metal and normalinsulator interfaces^{14,15,16,17,18}. To search for nontrivial effects at the TI/metal interface, it would be better if the energy bands in the metal are spin split, because spindependent phenomena can be more easily studied in such a metal.
In the following, we present spin and angleresolved photoemission spectroscopy (ARPES) measurements of the TI/metal interface, and show a novel phenomenon termed topological proximity effect, in which the topological Diraccone state migrates from the TI surface to an attached metal film. We have chosen ultrathin (only onebilayer (1BL) thick) 2D films of Bi metal where the Rashba splitting of the energy bands is strong. Also, one would expect the interface effect to become stronger if the energy states of a TI hybridize more strongly with those in a metal. This consideration led us to choose the ternary chalcogenide TI material TlBiSe_{2} (ref. 19), in which layers of Tl, Se and Bi are covalently bonded and stacked in the sequence of TlSeBiSe along the [111] direction (Fig. 1a). This material has a nontopological cousin, TlBiS_{2} (refs 20, 21), which is useful for making a control experiment. The covalent nature of the interlayer bonding in TlBiX_{2} (X=S, Se) would leave dangling bonds on the cleaved surface and cause strong hybridization with deposited metals; this is in contrast to Bi_{2}Se_{3} or Bi_{2}Te_{3}, whose surfaces have no dangling bonds and consequently the surface states present just a weak hybridization^{11} and no change in the topology^{12} when interfaced with Bi. It should be noted that 1BL Bi is structurally unstable in a freestanding form and it can only exist on some substrate; it turned out that TlBiS_{2} is a useful noninvasive substrate for 1BL Bi to identify the pristine Biderived bands, because of its nontopological nature and the resulting absence of any surface state^{20,21}. We have traced the variation of the electronic states upon depositing Bi on TlBiX_{2}, and our corelevel analysis suggests that the Bi atoms first passivate the dangling bonds on the cleaved surface and then start to grow layer by layer (Supplementary Figs 1 and 2; Supplementary Notes 1 and 2).
Results
Rashba splitting of 1BL Bi/TlBiS_{2}
Let us start by presenting our control experiment on Bi/TlBiS_{2} hybrid. As one can see in the ARPES data along the cut (the bulk and surface Brillouin zones and their highsymmetry points are depicted in Fig. 1b), the Bi deposition creates a couple of holelike bands centred at the point (dashed curves in Fig. 1c) superimposed on the bulk bands of the pristine TlBiS_{2} (Fig. 1d). These bands are attributed to the quantumwell states (quantized bulk bands) of Bi because of their 2D nature, which can be inferred from the photonenergy invariance of the band location for the HeI (hν=21.218 eV) and XeI (hν=8.437 eV) photons (Fig. 1c,e). Successful fabrication of 1BL Bi films can be judged from the agreement between the observed quantumwell states (called Bi bands hereafter) and the calculated band structure for freestanding 1BL Bi (blue line in Fig. 1e).
When closely examined, the Bi bands present evidence for Rashba spinsplitting. As visualized in the secondderivative ARPES intensity plot of the energy distribution curves (EDCs) along the cut for the 1BL Bi/TlBiS_{2} hybrid in Fig. 2a, a pair of dispersions can be found for each of the large and small holelike bands; we emphasize that the band dispersions determined from the peak positions in the momentum distribution curves (MDCs) by numerical fittings (Supplementary Fig. 3 and Supplementary Note 3), shown with white dots in Fig. 2a, agree with the secondderivative peaks in the EDCs, so the splitting of the bands is a robust observation. Those four bands each form a Fermi surface (labelled LO, LI, SO and SI, standing for ‘large outer’ etc.) as shown in Fig. 2b, in which the two large pockets (LI and LO) exhibit a sizable warping.
We have investigated the inplane spin polarization by performing spinresolved ARPES measurements at three representative k points, A, B and C, shown in Fig. 2a. As one can see in the top part of Fig. 2c, the spinresolved EDCs at point A (where the SO branch is located near E_{F}; see Fig. 2a) demonstrate that the upspin component is dominant, and consequently the spin polarization is positive. Similarly, the upspin component is more prominent at point B (where the SI branch is located at ~0.4 eV). At point C, on the other hand, a sign reversal in the spin polarization is observed near E_{F}, and this is due to the contribution from the nearE_{F} LO band, while the contribution of the LI band (located at ~0.4 eV) causes a positive spin polarization at higher binding energy; the smallness of the spin polarization for the LO band is probably related to the suppression of photoelectron intensity due to the matrixelement effect (although we have performed measurements at other k points around point C, it was difficult to observe the negative spin polarization at energies away from E_{F}, probably because of the strong intensity smearing of the LO and SO bands). These results establish that the four Bi bands are spin nondegenerate, and therefore they are most likely forming Rashbasplit Kramers pairs (the Rashba splitting is also reproduced by firstprinciples calculations, as detailed later); the spin texture inferred from our spinresolved ARPES is shown in Fig. 2a,b. The Rashba splitting is likely triggered by an electrostatic potential normal to the surface plane for vacuum/1BL Bi/TlBiS_{2}, which breaks spaceinversion symmetry.
Topological proximity effect in 1BL Bi/TlBiSe_{2}
Having understood the Bi bands from our control experiment, we now discuss the topological insulator hybrid, 1BL Bi/TlBiSe_{2}. One may expect a simple superposition of the Bi bands and the topological Diraccone states of TlBiSe_{2}, like in 1BL Bi on Bi_{2}Te_{3} (ref. 11). However, stronger hybridization due to the dangling bonds on TlBiX_{2} leads to a highly nontrivial band structure. Figure 3a displays the nearE_{F} ARPES intensity around the point measured with the HeI photons, which basically signifies an Xshaped Diracconelike state inside the two Bi bands, SI and SO (the data for a wider energy and momentum range are shown in Fig. 1f). Obviously, the upper branch of the Xshaped band does not reach E_{F} but exhibits an unusual dispersingback behaviour with the top of the dispersion at around the intersection with the SO band (white arrow); this situation is better visualized in the secondderivative image of the MDCs in Fig. 3b and the extracted band dispersion in Fig. 3f. One can see that the Xshaped band originates from the topological Diraccone state in TlBiSe_{2} because (i) it has a 2D nature as confirmed by the hνinvariance (Fig. 3c), (ii) its dispersion around the point is similar to that observed in pristine TlBiSe_{2} (Fig. 3d) and (iii) the Xshaped band is absent in the 1BL Bi/TlBiS_{2} hybrid (Fig. 3e).
We speculate that the observed modulation of the Xshaped band is caused by the strong hybridization through the interface between the upper branch of the Diraccone surface state of TlBiSe_{2} and the SO band of Bi, as schematically illustrated in Fig. 4a. It is important to recognize that, for such a hybridization to take place, the spin eigenvalue of the two states must agree with each other. In this respect, the upper Dirac cone and the SO band have the same clockwiserotating spin texture, and hence they can hybridize. This causes the resulting Xshaped band to display a dispersingback behaviour and, simultaneously, the portion of the SO band near the point is pushed up into the unoccupied region (see right panel of Fig. 4a). Also, this spin matching argument naturally explains the absence of hybridization between the upper Dirac cone and the SI band seen in Fig. 3b,c. As a result of this spindictated hybridization, two new types of bands emerge as shown in the right panel of Fig. 4a: (i) Rashbasplit bands of hole character, whose Kramers point was the original Dirac point of TlBiSe_{2} and (ii) topological Diraccone band (thick curves) whose Dirac point was the original Kramers point of the Rashbasplit Bi bands (SO and SI) located above E_{F}.
Band calculations of 1BL Bi on TlBiS_{2} and TlBiSe_{2}
Our firstprinciples band structure calculations for model crystals of 1BL Bi on TlBiS_{2} and TlBiSe_{2} (Fig. 4b,c; see Supplementary Fig. 4 and Supplementary Note 4 for details) strongly supports the existence of Rashbasplit Bi bands as well as the band diagram speculated above. As shown in Fig. 4b, the calculated band structure for 1BL Bi/TlBiS_{2} reproduces many important aspects of the experimental data in Fig. 2. First, we identify spinsplit Biderived holelike bands corresponding to the LO, LI, SO and SI bands in the experiment. Remarkably, the spin texture (clockwise or anticlockwise rotation) for all of these bands is identical to the ARPES data on 1BL Bi/TlBiS_{2} (thanks to a small overlap of the Bi bands with the bulkband projection of TlBiS_{2}, the Bi band largely keeps the freestandinglike dispersion in a wide range of the k space). Similar Rashbasplit bands are also recognized in the calculated band structure of 1BL Bi/TlBiSe_{2} (Fig. 4c), although the band structure around the point shows some characteristic differences from the TlBiS_{2} hybrid, owing to the hybridization between the Bi bands and the topological Diraccone states of TlBiSe_{2}. Second, an extremely holedoped nature of the Bi bands (chemical potential shift of ~0.4 eV) is also reproduced by the calculation in both 1BL Bi/TlBiS_{2} and 1BL Bi/TlBiSe_{2}, as a consequence of strong hybridization and resulting band repulsion between the S/Se and Bi states due to covalent bonding (note that the predicted 2D TI nature of 1BL Bi and its semiconducting property^{22} would not have a role in the electronic states near E_{F} owing to a complete mismatch of the band gaps between 1BL Bi and TlBi(S/Se)_{2}, and the physics is essentially that of a metal/insulator heterostructure). Third, in 1BL Bi/TlBiSe_{2} (Fig. 4c), we recognize a new lower Dirac cone whose Dirac point (black arrow) is located at slightly above E_{F} in the calculation, in agreement with the location of the new Dirac cone expected from ARPES; note that this lower Dirac cone and the SO band intersects without strong hybridization (red arrow), confirming the spindictated hybridization. It is noted that while the upper Dirac cone is not visible in this calculation because it is buried in the bulkband projection (the calculation underestimates the bulkband gap (~0.1 eV as opposed to ~0.35 eV in the experiment^{19}) due to the wellknown limitation of the density functional theory), the lower Dirac cone must connect to an upper branch exactly at the point to produce a full Diraccone dispersion, due to its spin nondegenerate nature and the requirement of Kramers degeneracy; in other words, the spinnondegenerate nature of the SI band and the timereversal symmetry guarantees the existence of the upper Dirac cone. The SO band which forms the Kramers pair with the SI band in 1BL Bi/TlBiS_{2} (Fig. 4b) is not responsible for this band connection, since it sinks below E_{F} in 1BL Bi/TlBiSe_{2} (Fig. 4c). For dispersions below E_{F}, by taking into account the calculated spin helical texture of the SO band and the former lower Dirac cone of the TlBiSe_{2} surface state, the existence of the new Kramers point is also assured in 1BL Bi/TlBiSe_{2}, even though it is buried in the bulkband projection in the calculation.
It should be noted that the calculated Dirac cone above E_{F} in 1BL Bi/TlBiSe_{2} has a dominant contribution from the p orbital of topmost and second Bi layers (85% of total weight; nearly equal contribution from these two layers), suggesting that it is of 1BLBi origin and is migrated from the surface of TlBiSe_{2} as a result of strong hybridization; this phenomenon may be called a topological proximity effect. Intriguingly, the calculated surface/interface states have an odd number of band crossings in the bulkbandgap region between two timereversalinvariant momenta and irrespective of the position of the chemical potential, as opposed to an even number of crossings in 1BL Bi/TlBiS_{2}, indicating that the 1BL Bi/TlBiSe_{2} hybrid system maintains the topologically nontrivial nature unlike the TlBiS_{2} hybrid. This can also be confirmed by counting of experimental band crossings in Figs 2a and 4a.
Discussion
We emphasize that a key ingredient for realizing the present topological proximity effect is to contact a TI with a quasi2D metal thin film, rather than a 3D bulk metal; in this respect, it may be conceived as a manipulation of the boundary condition for the TI. Besides, the topological proximity effect would not be peculiar to a strongly spin–orbitcoupled metal like Bi, but it would occur for a wider range of metals as suggested in a pioneering theoretical study of proximity effects in semiconductor/TI heterostructures^{15}. Fabrications of metal thin films on TIs with various film thickness and their characterizations via spinresolved ARPES would be particularly useful for studying the topological proximity effect.
Topological proximity effect may also have some relevance to applications. For example, it may be possible to enhance the performance of a metallic spintronic material (for example, to generate highlyconducting channel^{18}) by giving it the blessed property of topological protection. Also, fabrication of topologically protected spinplasmon devices^{23} by patterning Rashba metals on the surface of TIs may be conceivable. On the fundamental side, the topological proximity effect is the first step towards understanding the peculiarities of the metalfilm/TI interface. Extending the spinresolved ARPES technique to other TI heterostructures would prove useful for testing theoretical proposals of interesting proximity effects in a variety of TI hetersotructures^{14,15,16,17,18}.
Methods
Sample preparation
Highquality single crystals of TlBiSe_{2} and TlBiS_{2} were grown by a modified Bridgman method^{20}. To prepare the Bi film, we first cleaved the TlBiSe_{2} or TlBiS_{2} crystals to obtain a shiny mirrorlike surface, and then deposited Bi atoms at room temperature in ultrahigh vacuum of 1 × 10^{−10} Torr. The 1 × 1 surface structure originating from Bi was confirmed by the lowenergy electron diffraction measurement. The film thickness was controlled by the deposition time at a constant deposition rate. The actual thickness was estimated by a quartzoscillator thickness monitor and also from a comparison of the ARPESderived band dispersion with the bandstructure calculations for freestanding multilayer Bi^{24,25,26}.
ARPES experiments
ARPES measurements were performed with the MBSA1 electron analyser equipped with highintensity He and Xe plasma discharge lamps^{27} without breaking the vacuum after the Bi film was grown. We used the HeIα resonance line (hν=21.218 eV) and one of the XeI lines (hν=8.437 eV) to excite photoelectrons. The energy resolution for the regular and spinresolved ARPES measurements was set at 6–20 and 40 meV, respectively. The sample temperature was kept at 30 K during the measurements. We used the Sherman function value of 0.07 to obtain spinresolved ARPES data.
Additional information
How to cite this article: Shoman, T. et al. Topological proximity effect in a topological insulator hybrid. Nat. Commun. 6:6547 doi: 10.1038/ncomms7547 (2015).
References
 1
Hasan, M. Z. & Kane, C. L. Colloquium: Topological insulators. Rev. Mod. Phys. 82, 3045–3067 (2010) .
 2
Qi, X.L. & Zhang, S.C. Topological insulators and superconductors. Rev. Mod. Phys. 83, 1057–1110 (2011) .
 3
Ando, Y. Topological insulator materials. J. Phys. Soc. Jpn 82, 102001 (2013) .
 4
Xia, Y. et al. Observation of a largegap topologicalinsulator class with a single Dirac cone on the surface. Nat. Phys. 5, 398–402 (2009) .
 5
Souma, S. et al. Direct measurement of the outofplane spin texture in the Diraccone surface state of a topological insulator. Phys. Rev. Lett. 106, 216803 (2011) .
 6
Fu, L. & Kane, C. L. Superconducting proximity effect and Majorana fermions at the surface of a topological insulator. Phys. Rev. Lett. 100, 096407 (2008) .
 7
Chen, Y. L. et al. Massive Dirac fermion on the surface of a magnetically doped topological insulator. Science 329, 659–662 (2010) .
 8
Xu, S.Y. et al. Hedgehog spin texture and Berry’s phase tuning in a magnetic topological insulator. Nat. Phys. 8, 616–622 (2012) .
 9
Qi, X.L., Li, R., Zang, J.D. & Zhang, S.C. Inducing a magnetic monopole with topological surface states. Science 323, 1184–1187 (2009) .
 10
Qi, X.L., Hughes, T. L. & Zhang, S.C. Topological field theory of timereversal invariant insulators. Phys. Rev. B 78, 195424 (2008) .
 11
Hirahara, T. et al. Interfacing 2D and 3D topological insulators: Bi(111) bilayer on Bi2Te3 . Phys. Rev. Lett. 107, 166801 (2011) .
 12
Wang, Z. F. et al. Creation of helical Dirac fermions by interfacing two gapped systems of ordinary fermions. Nat. Commun. 4, 1384 (2013) .
 13
Miao, L. et al. Quasiparticle dynamics in reshaped helical Dirac cone of topological insulators. Proc. Natl Acad. Sci. USA 110, 2758–2762 (2013) .
 14
Wang, X., Bian, G., Miller, T. & Chiang, T.C. Topological spinpolarized electron layer above the surface of Caterminated Bi2Se3 . Phys. Rev. B 87, 035109 (2013) .
 15
Hutasoit, J. A. & Stanescu, T. D. Induced spin texture in semiconductor/topological insulator heterostructures. Phys. Rev. B 84, 085103 (2011) .
 16
Guang, L. X. et al. Proximity effects in topological insulator heterostructures. Chin. Phys. B 22, 097306 (2013) .
 17
Zhang, J., Triola, C. & Rossi, E. Proximity effect in graphenetopologicalinsulator heterostructures. Phys. Rev. Lett. 112, 096802 (2014) .
 18
Essert, S., Krueckl, V. & Richter, K. Using topological insulator proximity to generate perfectly conducting channels in materials without topological protection. New J. Phys. 16, 113058 (2014) .
 19
Sato, T. et al. Direct evidence for the Diraccone topological surface states in the ternary chalcogenide TlBiSe2 . Phys. Rev. Lett. 105, 136802 (2010) .
 20
Sato, T. et al. Unexpected mass acquisition of Dirac fermions at the quantum phase transition of a topological insulator. Nat. Phys. 7, 840–844 (2011) .
 21
Xu, S.Y. et al. Topological phase transition and texture inversion in a tunable topological insulator. Science 332, 560–564 (2011) .
 22
Murakami, S. Quantum spin Hall effect and enhanced magnetic response by spinorbit coupling. Phys. Rev. Lett. 97, 236805 (2006) .
 23
Raghu, S., Chung, S. B., Qi, X.L. & Zhang, S.C. Collective modes of a helical liquid. Phys. Rev. Lett. 104, 116401 (2010) .
 24
Koroteev, Y. M., Bihlmayer, G., Chulkov, E. V. & Blügel, S. Firstprinciples investigation of structural and electronic properties of ultrathin Bi films. Phys. Rev. B 77, 045428 (2008) .
 25
Hirahara, T. et al. Direct observation of spin splitting in bismuth surface states. Phys. Rev. B 76, 153305 (2007) .
 26
Takayama, A. et al. Tunable spin polarization in bismuth ultrathin film on Si(111). Nano Lett. 12, 1776–1779 (2012) .
 27
Souma, S., Sato, T., Takahashi, T. & Baltzer, P. Highintensity xenon plasma discharge lamp for bulksensitive photoemission spectroscopy. Rev. Sci. Instrum. 78, 123104 (2007) .
Acknowledgements
We thank Y. Tanaka, M. Nomura, K. Nakayama, H. Kumigashira, K. Ono and S. Kimura for their assistance in ARPES measurements and K. Eto for his assistance in crystal growth. This work was supported by JSPS (KAKENHI 23224010, 24654096, 25287079 and 25220708), MEXT of Japan (Innovative Area ‘Topological Quantum Phenomena’), AFOSR (AOARD 124038), the Mitsubishi Foundation, UVSOR (Proposal No. 24536) and KEKPF (Proposal No. 2012S2001).
Author information
Affiliations
Contributions
T.Sh., A.T., T.Sa., S.S. and T.T. performed ARPES measurements. T.O. carried out the bandstructure calculations. K.S. and Y.A. carried out the growth of the single crystals and their characterizations. T.Sh., T.Sa., and Y.A. conceived the experiments and wrote the manuscript.
Corresponding authors
Correspondence to T. Sato or Yoichi Ando.
Ethics declarations
Competing interests
The authors declare no competing financial interests.
Supplementary information
Supplementary Information
Supplementary Figures 14, Supplementary Notes 14 and Supplementary References (PDF 462 kb)
Rights and permissions
About this article
Cite this article
Shoman, T., Takayama, A., Sato, T. et al. Topological proximity effect in a topological insulator hybrid. Nat Commun 6, 6547 (2015) doi:10.1038/ncomms7547
Received
Accepted
Published
DOI
Further reading

Iodine Adsorption on Bi2 Se3 Topological Insulator
physica status solidi (RRL)  Rapid Research Letters (2019)

Perfect Andreev reflection due to the Klein paradox in a topological superconducting state
Nature (2019)

Inverse proximity effect in s wave and d wave superconductors coupled to topological insulators
Physical Review B (2019)

Topological proximity effects in a Haldane graphene bilayer system
Physical Review B (2019)

Large Tunable SpintoCharge Conversion Induced by Hybrid Rashba and Dirac Surface States in Topological Insulator Heterostructures
Nano Letters (2019)
Comments
By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.