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A bulk superconductor possessing a topological surface state at the Fermi level is a promising system to realise long-sought topological superconductivity. Although several candidate materials have been proposed, experimental demonstrations concurrently exploring spin textures and superconductivity at the surface have remained elusive. Here we perform spectroscopic-imaging scanning tunnelling microscopy on the centrosymmetric superconductor β-PdBi2 that hosts a topological surface state. By combining first-principles electronic-structure calculations and quasiparticle interference experiments, we determine the spin textures at the surface, and show not only the topological surface state but also all other surface bands exhibit spin polarisations parallel to the surface. We find that the superconducting gap fully opens in all the spin-polarised surface states. This behaviour is consistent with a possible spin-triplet order parameter expected for such in-plane spin textures, but the observed superconducting gap amplitude is comparable to that of the bulk, suggesting that the spin-singlet component is predominant in β-PdBi2.
Superconductivity arising from non-degenerate spin-polarised Fermi surfaces is expected to consist of a mixing of spin-singlet and triplet order parameters as proposed in non-centrosymmetric superconductors1, 2. If superconductivity is induced in the spin-polarised topological surface states (TSSs), such a situation may be realised and the induced spin-triplet order parameter could play a key role for the emergence of Majorana fermions at edge states or in vortex cores, offering potential applications such as topological quantum computing3–6. To accomplish topological superconductivity, various candidate systems including carrier doped topological insulators7–9 and superconductor/topological insulator heterostructures10–14 have been investigated, in addition to other systems such as magnetic15 and semiconducting16 nanowires fabricated on superconductors.
Among these systems, stoichiometric superconductors possessing the TSSs at the Fermi level E F are promising candidates. The non-centrosymmetric superconductor PbTaSe2 17 is one of such materials for which a recent spectroscopic-imaging scanning tunnelling microscopy (SI-STM) study has revealed the existence of both the TSS and a fully opened superconducting (SC) gap18. At the cleaved surface of PbTaSe2, the inversion symmetry is broken not only along the surface normal direction but also along the in-plane direction because of the non-centrosymmetric crystal structure. This causes intricate spin-orbit coupling that gives rise to out-of-plane spin components. The SC order parameter favoured in such a situation is interesting but complicated.
Here we study another stoichiometric superconductor β-PdBi2 with a SC transition temperature Tc~5.4K19. β-PdBi2 crystallises into centrosymmetric tetragonal lattice with space group I4/mmm. Since in-plane inversion symmetry is always maintained, the spin texture induced by spin-orbit coupling is expected to be aligned in the in-plane direction at the surface. Angle-resolved photoemission spectroscopy (ARPES) has revealed the presence of the TSS as well as the topologically trivial surface state at E F 20. The SC gap has been studied by scanning tunnelling spectroscopy experiments21, 22. However, reported tunnelling spectra do not agree with each other; there is only one BCS-like gap at the surface of the bulk single crystal cleaved in the air21, whereas two gaps are identified in the case of the in situ molecular-beam-epitaxy-grown thin film22. Moreover, the simultaneous investigation of the TSS and the SC gap, which is critically important to discuss topological superconductivity, has not yet been done. We have performed SI-STM on clean surfaces of β-PdBi2 prepared by cleaving bulk single crystals in ultra high vacuum to study the SC gap opening in the TSS. Quasiparticle interference (QPI) imaging combined with numerical simulations has been utilised to explore the spin textures at the surface. We find that the observed QPI patterns are qualitatively consistent with the spin textures predicted by the first-principles calculations, which reveal that not only the TSS but also all other states at the surface are spin polarised. This suggests that the surface of β-PdBi2 can potentially harbour the mixed spin-singlet and triplet superconductivity. A fully opened single SC gap Δ~0.8meV has been identified in the tunnelling spectrum without any detectable residual spectral weight near E F, being consistent with the result of air-cleaved sample21. These results indicate that the electronic states detected by SI-STM including the TSS are all spin-polarised and exhibit fully gapped superconductivity. We argue that the spin-triplet order parameter expected from the spin textures is consistent with the observed isotropic gap. The observed gap amplitude, however, is comparable to that of the bulk23, suggesting that the spin-singlet component is dominant. Therefore, the topological nature of the observed superconductivity, if any, may manifest itself at lower temperatures.
We first examine the spin textures at the surface of β-PdBi2 by first-principles calculations within the framework of the density functional theory. The surface exposed after cleaving a single crystal along (001) plane is expected to be composed of Bi atoms since the weakest bond in the crystal structure is the van der Waals bond between adjacent Bi layers (Fig. 1a inset). Therefore, we mainly focus on low-energy electronic states composed of Bi 6p orbitals at the top-most layer predominating the tunnelling current. (Details of the calculation is described in Methods section.)
The calculated band dispersions and the constant energy contours (CECs) at energy E=+30meV are shown in Fig. 1a, b, respectively. Each of the surface and bulk states is labelled in the same manner as the previous report20. The surface states are found in the close vicinity of the bulk bands. The trivial surface state S1 is located near the bulk β band, whereas the TSS S2 is located at the bottom of the bulk γ and δ bands. For comparison with the experimental results, we performed a Wannier transformation as written in Methods section. Figure 1c shows the spectral function at +30meV with the Wannier transformation. The spectral weights of bulk γ and δ bands are very small. For the TSS S2 surrounding points, the weight is negligibly small except for the section parallel to the − direction.
Figure 1d–f show x, y and z components of the spin polarisations weighted by the spectral function. It is clear that the spin polarisations are predominantly aligned in-plane, as expected from the symmetry of the crystal structure. It should be noted that not only the TSS S2 but also S1, and even the bulk bands are all spin polarised. The spin orientations of S1 and S2 are consistent with the results of spin-resolved ARPES20. The spin polarisations of the bulk bands are likely to be induced by local broken inversion symmetry at the Bi site in the bulk as well as at the surface24 (Fig. 1a inset). The spin textures of these bulk bands have never been studied before and play an important role in QPI as we shall show below.
In order to confirm the calculated spin textures experimentally, we utilise QPI imaging. The QPI effect is nothing but the formation of electronic standing waves. In the presence of elastic scatterers, two quantum states on the same CEC with a scattering wave vector q may interfere and generate a standing wave with a wavelength of 2π/|q|. This yields E-dependent spatial oscillations in the local density-of-states distribution that manifest themselves in the tunnelling conductance dI/dV|V=E/e images, where I is the tunnelling current, V is the bias voltage and e is the elementary charge. Fourier analysis allows us to identify various QPI signals appearing at different q vectors. The pre-requisite to have QPI signals at q is that q must connect two states with high enough spectral weights. In addition, the spin polarisations of these two states should not be antiparallel because spin-antiparallel states are orthogonal and thereby cannot interfere with each other. In this way, the QPI imaging can provide information on the spin textures in the k space25, 26.
Here we outline expected QPI patterns from the calculated spin textures at the surface (Fig. 1d–f). In the case of non-magnetic scatterers, any intra-band back-scattering of spin-polarised bands is forbidden because the spin polarisations of the two states relevant for scattering are always antiparallel due to time-reversal symmetry. The inter-band back-scatterings from S1 to -centred S2 and from α to -centred S2 are also forbidden, because the spin orientations of S1, S2 and α are the same in the − direction. The inter-band forward-scatterings from S1 to -centred S2 and from S1 to α are allowed but the expected q vectors are small; it may be difficult to distinguish these QPI signals from extrinsic small q modulations associated with inhomogeneously distributed unavoidable defects in a real material. As a result, three scattering channels may govern the QPI patterns; inter-band forward-scattering from α to -centred S2 (q 1), inter-band back-scatterings from S1 to β band (q 2) and from -centred S2 to β band (q 3) (Fig. 1c).
We start our experiments by checking the quality of the cleaved surface using constant-current STM imaging. An atomically flat area as large as 100×100nm2 is observed as shown in Fig. 2a. In a magnified image (Fig. 2a inset), at least two kinds of defects are observed as a local suppression and a subtle protrusion surrounded by a suppressed area. Although the exact nature of these defects is unknown, they may work as quasiparticle scatterers that cause QPI. Fourier-transformed STM image (Supplementary Fig. 1) exhibits Bragg peaks that are consistent with the in-plane lattice constant a 0=0.337nm19. This allows us to identify the crystallographic directions in real and reciprocal spaces.
QPI imaging has been performed in the same field of view of Fig. 2a. Figure 2b, c shows a dI/dV map at E=+30meV and its Fourier-transformed image, respectively. We identify three distinct line-like QPI signals in Fig. 2c, which are parallel with each other and normal to the − direction. These three QPI signals show hole-like dispersions (d|q|/dV s<0) as seen in Fig. 2d. The line-like shapes and hole-like dispersions are consistent with the square-shaped CECs of S1, S2, α and β bands shown in Fig. 1b and the band dispersions shown in Fig. 1a, respectively. Namely, the observed QPI signals may correspond to q 1, q 2 and q 3.
We confirm this conjecture by comparing experimental data with the numerically simulated QPI patterns calculated by using the results of first-principles calculations with the standard T-matrix formalism. In addition to a full (standard) simulation, we have also performed a spinless simulation. The spin degree of freedom is included in the former and hypothetically suppressed in the latter. The contrast between the two simulations highlights the role of the spin textures. (See Methods section for details.)
The result of the full simulation at E=+30meV is shown in Fig. 3a. Three parallel line-like QPI signals normal to the − direction are identified. These features correspond to q 1, q 2 and q 3 indicated in Fig. 1c and their energy evolutions reasonably agree with the experimental observation (Fig. 3b), indicating that q 1, q 2 and q 3 scatterings indeed dominate the QPI patterns.
The key role of the spin textures becomes evident if we compare the result of full simulation with that of spinless simulation (Fig. 3c), and the experimental data. Figure 3d compares line profiles along the − direction from experimental and simulated QPI patterns. In the spinless simulation, there are three sharp peaks but their |q| values do not agree with the experimental observations. These sharp peaks correspond to the intra-surface-state back-scatterings in S1 (q S1−S1) and in -centred S2 (q S2−S2), and inter-band back-scattering from S1 to -centred S2 (q S1−S2). Because the spin orientations are antiparallel between initial and final states of these scatterings, they are suppressed in the full simulation that well captures the experimental observations.
The above comprehensive approach combining experiments and calculations provide the following important implications. First, our SI-STM data primarily reflect the electronic state of the top-most Bi layer as we assumed. Second, the spin degrees of freedom is crucial for QPI. Finally, the calculated spin textures shown in Fig. 1d–f adequately capture the real spin textures at the surface. Notably, the fourfold QPI pattern is similarly identified at E F in the normal state (T=1.5K, B=12T) as shown in Supplementary Fig. 2. This indicates that the spin textures discussed here indeed exist at E F in the normal state.
Given the surface sensitivity of our measurement evidenced in the QPI patterns, we are able to argue the nature of superconductivity at the surface. Figure 4a shows the temperature T evolution of the tunnelling spectrum in the SC state. At T=0.4K, the SC gap fully opens and there is no residual spectral weight inside the gap, indicating that all of the states at the surface are gapped. Each spectrum can be fitted well with the Dynes function for the single isotropic gap, meaning that the SC gap is k independent. We also investigate QPI patterns near the SC gap energy and find no clear superconductivity-induced QPI signals except at q=0 (Supplementary Fig. 2), which means that the SC gap is spatially uniform. The SC gap amplitude Δ(T) is estimated to be 0.8meV at T=0.4K, being reasonably consistent with the result of air-cleaved sample21. We note that any noticeable spectroscopic features are not observed near E F at step edges (Supplementary Fig. 3). The temperature dependence of the SC gap well follows the BCS behaviour (Fig. 4b).
We next examine the electronic state of vortices that may host Majorana fermions6, 27, 28. Isotropic vortices are clearly imaged in the dI/dV map at E=0meV (Fig. 4c inset), as being similar to the previous result obtained at the surface of the air-cleaved bulk crystal21. The line profile of the vortex core is also reasonably consistent with the previous result29 (Supplementary Fig. 4). To estimate the in-plane coherence length and compare it with that obtained from H c2 measurements, detailed magnetic field dependence of the vortex core size is required as proposed recently29. Considering the good agreement with our result and those in ref. 29, we believe that a similar coherence length as discussed in refs. 21, 29 would be expected in our samples. The spectrum taken in the vortex core is almost flat (Fig. 4c). This result indicates that the core is in the dirty limit (mean free path l<ξ) where vortex bound states as well as the Majorana zero mode are not well defined. Although the zero-energy local density-of-states peak in the vortex core cannot be an evidence of the Majorana zero mode by itself18, it would be possible to investigate the spin structures that are unique to the Majorana state27, 28, for example. We anticipate that β-PdBi2 will be a good touchstone for these theories in future because it is a stoichiometric material that can be made cleaner in principle.
Our concurrent investigations of the surface electronic states and superconductivity reveal that the SC gap fully opens in all of the spin-polarised surface states. Since the mixing of spin-singlet and triplet order parameters is generally expected in the presence of non-degenerate spin-polarised surface states1, 2 and the triplet component can possess nodes, it is intriguing to argue the possible nodal structure of the SC states of β-PdBi2. It has been shown that the spin-triplet order parameter d(k) favours to be aligned along the spin direction when space inversion symmetry is broken30. If this is the case at the surface of β-PdBi2 where the point group symmetry is lowered from D 4h to C 4v, the triplet pairing is of p-wave type with nodes along the out-of-plane direction30. Since the surface states are two dimensional in nature, the out-of-plane nodes would not be active and the SC gap should look like isotropic as observed.
Next we discuss the amplitude of the SC gap. The SC gap amplitude in the presence of the parity mixing is given by |Ψs(k)|±|d(k)| where Ψs denotes s-wave order parameter that represents the bulk gap2, 30–32. Thus the difference between the SC gap amplitudes of bulk and surface gives us an estimate of the amplitude of the p-wave component. The bulk SC gap has been estimated to be about 0.9meV by the specific heat measurement23, which is close to the value of 0.8meV that we observed at the surface. According to the theory32, the difference between the surface SC gap and the bulk counterpart depends on how far the surface state is separated from the bulk band at E F in k space. We estimate the quantity , which is introduced in ref. 32, to be about 0.05 ( and k F denote the Fermi momenta of the TSS and the bulk band, respectively). This is reasonably small to explain the similar SC gap amplitudes between the surface and the bulk. Namely, even though it is allowed, the p-wave component at the surface of β-PdBi2 is small. The absence of edge states shown in Supplementary Fig. 3 might be due to the small p-wave component. In order to detect this small p-wave component, if any, we need to avoid comparing the results of two different experiments, STM and specific heat, as well as to improve the energy resolution. As the bulk bands appear at the surface, too, it would be possible to detect the surface and bulk SC gaps simultaneously by STM alone, provided higher energy resolution could be achieved. To this end, the use of a SC tip at lower temperatures is important.
The β-PdBi2 single crystals were grown by a melt growth method20, and characterised by x-ray diffraction (XRD) and transport measurements. Pd (3N5) and Bi (5N) at a molar ratio of 1:2 were encapsulated in an evacuated quartz tube, pre-reacted at temperature above 1000°C until it completely melted and mixed. Then, it was kept at 900°C for 20h, cooled down at a rate of 3°C/h down to 500°C, and finally rapidly quenched into cold water. PdBi2 has two different crystallographic and superconducting phases: α-phase with the space group C2/m (Tc~1.7K33) and β-phase with I4/mmm (Tc~5.4K). The last quenching procedure is important to selectively obtain the β-phase. Any trace amount of α-phase was not detected by XRD. For STM measurements, we used single crystals with residual resistivity ratio of 15, larger than those of the previous reports19, 21, 23. The crystals were cleaved along the (001) plane at room temperature in ultra high vacuum conditions to obtain clean and flat surfaces needed for STM. A commercial 3He-based STM system (UNISOKU USM-1300) modified by ourselves34 was employed in this study. We used electrochemically etched tungsten tips, which were cleaned and sharpened by field ion microscopy. The tips were subsequently treated and calibrated on clean Au(100) surfaces before used for β-PdBi2. We applied bias voltages to the sample whereas the tip was virtually grounded at the current-voltage converter (Femto LCA-1K-5G). Tunnelling spectra were measured by the software-based lock-in detector included in the commercial STM control system (Nanonis).
To calculate the surface electronic structure and its corresponding QPI pattern, we first performed a DFT calculation using the Perdew−Burke−Ernzerhof exchange-correlation functional35 as implemented in the WIEN2K program36. Relativistic effects including spin-orbit coupling were fully taken into account. For all atoms, the muffin-tin radius R MT was chosen such that its product with the maximum modulus of reciprocal vectors K max become R MT K max=7.0. Considering the tetragonal phase of β-PdBi2, the corresponding Brillouin zone was sampled using a 20×20×5 k-mesh. For the surface calculations, a 100 unit tight-binding supercell was constructed using maximally localised Wannier functions37–39. The 6p orbitals of Bi atoms were chosen as the projection centres.
We simulated QPI patterns by incorporating the eigenvalues and eigenvectors obtained from our tight-binding supercell calculations into the standard T-matrix formalism40. The top-most Bi 6p orbitals were considered in the calculations. For all the simulations, the broadening factor was chosen to be 5meV, and a localised, spin-preserving and orbital-preserving scatterer of strength 0.1eV were employed. For comparison with the experimental results, we performed a basis transformation from the lattice model to the continuum model with the Wannier function41 constructed by projecting the Bi 6p orbitals. We calculated the local density of state at 0.5nm above the top-most Bi atoms. The basis transformation is also applied to the spectral function shown in Fig. 1c to present the electronic structure used for the QPI calculation. For the spinless simulation, the sum of the Green’s functions constructed from the spin-up and spin-down subspaces were used for the T-matrix calculations.
All relevant data are available on request, which should be addressed to K.I.
The authors thank M. Sakano, K. Ishizaka, T. Mizushima, Y. Tanaka and Y. Yanase for valuable discussions and Y. Okada for experimental advices. This work was supported by a CREST project (JPMJCR16F2) from Japan Science and Technology Agency (JST) and JSPS KAKENHI Grant Nos.16K05465, 16H06012, and 16H03847. M.S.B. was supported by CREST, JST (Grant No. JPMJCR16F1).
K.I. carried out the experiments and the data analyses with assistance from Y.K., T.M. and T.H. Y.K., T.S., and M.S.B. carried out the first-principles calculations and numerical simulations. K.O. and T.S. synthesised and characterised the single crystals. T.H. supervised the project. K.I., Y.K. and T.H. wrote the manuscript.
The authors declare no competing financial interest.
A correction to this article is available online at https://doi.org/10.1038/s41467-017-02137-4.
Electronic supplementary material
Supplementary Information accompanies this paper at 10.1038/s41467-017-01209-9.
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