Abstract
A bulk superconductor possessing a topological surface state at the Fermi level is a promising system to realise longsought 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 spectroscopicimaging scanning tunnelling microscopy on the centrosymmetric superconductor βPdBi_{2} that hosts a topological surface state. By combining firstprinciples electronicstructure 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 spinpolarised surface states. This behaviour is consistent with a possible spintriplet order parameter expected for such inplane spin textures, but the observed superconducting gap amplitude is comparable to that of the bulk, suggesting that the spinsinglet component is predominant in βPdBi_{2}.
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Introduction
Superconductivity arising from nondegenerate spinpolarised Fermi surfaces is expected to consist of a mixing of spinsinglet and triplet order parameters as proposed in noncentrosymmetric superconductors^{1, 2}. If superconductivity is induced in the spinpolarised topological surface states (TSSs), such a situation may be realised and the induced spintriplet 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 computing^{3,4,5,6}. To accomplish topological superconductivity, various candidate systems including carrier doped topological insulators^{7,8,9} and superconductor/topological insulator heterostructures^{10,11,12,13,14} have been investigated, in addition to other systems such as magnetic^{15} and semiconducting^{16} nanowires fabricated on superconductors.
Among these systems, stoichiometric superconductors possessing the TSSs at the Fermi level E _{F} are promising candidates. The noncentrosymmetric superconductor PbTaSe_{2} ^{17} is one of such materials for which a recent spectroscopicimaging scanning tunnelling microscopy (SISTM) study has revealed the existence of both the TSS and a fully opened superconducting (SC) gap^{18}. At the cleaved surface of PbTaSe_{2}, the inversion symmetry is broken not only along the surface normal direction but also along the inplane direction because of the noncentrosymmetric crystal structure. This causes intricate spinorbit coupling that gives rise to outofplane spin components. The SC order parameter favoured in such a situation is interesting but complicated.
Here we study another stoichiometric superconductor βPdBi_{2} with a SC transition temperature \({T_{\rm{c}}}\sim 5.4\) K^{19}. βPdBi_{2} crystallises into centrosymmetric tetragonal lattice with space group I4/mmm. Since inplane inversion symmetry is always maintained, the spin texture induced by spinorbit coupling is expected to be aligned in the inplane direction at the surface. Angleresolved 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 experiments^{21, 22}. However, reported tunnelling spectra do not agree with each other; there is only one BCSlike gap at the surface of the bulk single crystal cleaved in the air^{21}, whereas two gaps are identified in the case of the in situ molecularbeamepitaxygrown thin film^{22}. 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 SISTM on clean surfaces of βPdBi_{2} 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 firstprinciples 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 βPdBi_{2} can potentially harbour the mixed spinsinglet and triplet superconductivity. A fully opened single SC gap \({\rm{\Delta }}\sim 0.8\) meV has been identified in the tunnelling spectrum without any detectable residual spectral weight near E _{F}, being consistent with the result of aircleaved sample^{21}. These results indicate that the electronic states detected by SISTM including the TSS are all spinpolarised and exhibit fully gapped superconductivity. We argue that the spintriplet 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 bulk^{23}, suggesting that the spinsinglet component is dominant. Therefore, the topological nature of the observed superconductivity, if any, may manifest itself at lower temperatures.
Results
Calculating band structures and spin textures at the surface
We first examine the spin textures at the surface of βPdBi_{2} by firstprinciples 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 lowenergy electronic states composed of Bi 6p orbitals at the topmost 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 = +30 meV are shown in Fig. 1a, b, respectively. Each of the surface and bulk states is labelled in the same manner as the previous report^{20}. 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 +30 meV with the Wannier transformation. The spectral weights of bulk γ and δ bands are very small. For the TSS S2 surrounding \(\overline {\rm{M}} \) points, the weight is negligibly small except for the section parallel to the \(\overline {\rm{M}} \)−\(\overline {\rm{X}} \) 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 inplane, 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 spinresolved ARPES^{20}. 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 surface^{24} (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.
Spin textures revealed by QPI imaging
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 Edependent spatial oscillations in the local densityofstates 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 prerequisite 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 spinantiparallel 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 space^{25, 26}.
Here we outline expected QPI patterns from the calculated spin textures at the surface (Fig. 1d–f). In the case of nonmagnetic scatterers, any intraband backscattering of spinpolarised bands is forbidden because the spin polarisations of the two states relevant for scattering are always antiparallel due to timereversal symmetry. The interband backscatterings from S1 to \(\overline {\rm{\Gamma }} \)centred S2 and from α to \(\overline {\rm{\Gamma }} \)centred S2 are also forbidden, because the spin orientations of S1, S2 and α are the same in the \(\overline {\rm{\Gamma }} \)−\(\overline {\rm{M}} \) direction. The interband forwardscatterings from S1 to \(\overline {\rm{\Gamma }} \)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; interband forwardscattering from α to \(\overline {\rm{\Gamma }} \)centred S2 (q _{1}), interband backscatterings from S1 to β band (q _{2}) and from \(\overline {\rm{\Gamma }} \)centred S2 to β band (q _{3}) (Fig. 1c).
We start our experiments by checking the quality of the cleaved surface using constantcurrent STM imaging. An atomically flat area as large as 100 × 100 nm^{2} 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. Fouriertransformed STM image (Supplementary Fig. 1) exhibits Bragg peaks that are consistent with the inplane lattice constant a _{0} = 0.337 nm^{19}. 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 = +30 meV and its Fouriertransformed image, respectively. We identify three distinct linelike QPI signals in Fig. 2c, which are parallel with each other and normal to the \(\overline {\rm{\Gamma }} \)−\(\overline {\rm{M}} \) direction. These three QPI signals show holelike dispersions (dq/dV _{s} < 0) as seen in Fig. 2d. The linelike shapes and holelike dispersions are consistent with the squareshaped 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 firstprinciples calculations with the standard Tmatrix 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 = +30 meV is shown in Fig. 3a. Three parallel linelike QPI signals normal to the \(\overline {\rm{\Gamma }} \)−\(\overline {\rm{M}} \) 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 \(\overline {\rm{\Gamma }} \)−\(\overline {\rm{M}} \) 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 intrasurfacestate backscatterings in S1 (q _{S1−S1}) and in \(\overline {\rm{\Gamma }} \)centred S2 (q _{S2−S2}), and interband backscattering from S1 to \(\overline {\rm{\Gamma }} \)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 SISTM data primarily reflect the electronic state of the topmost 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 = 1.5 K, B = 12 T) as shown in Supplementary Fig. 2. This indicates that the spin textures discussed here indeed exist at E _{F} in the normal state.
Superconductivity at the surface
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 = 0.4 K, 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 superconductivityinduced 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 0.8 meV at T = 0.4 K, being reasonably consistent with the result of aircleaved sample^{21}. 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 fermions^{6, 27, 28}. Isotropic vortices are clearly imaged in the dI/dV map at E = 0 meV (Fig. 4c inset), as being similar to the previous result obtained at the surface of the aircleaved bulk crystal^{21}. The line profile of the vortex core is also reasonably consistent with the previous result^{29} (Supplementary Fig. 4). To estimate the inplane 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 recently^{29}. 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 zeroenergy local densityofstates peak in the vortex core cannot be an evidence of the Majorana zero mode by itself^{18}, it would be possible to investigate the spin structures that are unique to the Majorana state^{27, 28}, for example. We anticipate that βPdBi_{2} will be a good touchstone for these theories in future because it is a stoichiometric material that can be made cleaner in principle.
Discussion
Our concurrent investigations of the surface electronic states and superconductivity reveal that the SC gap fully opens in all of the spinpolarised surface states. Since the mixing of spinsinglet and triplet order parameters is generally expected in the presence of nondegenerate spinpolarised surface states^{1, 2} and the triplet component can possess nodes, it is intriguing to argue the possible nodal structure of the SC states of βPdBi_{2}. It has been shown that the spintriplet order parameter d(k) favours to be aligned along the spin direction when space inversion symmetry is broken^{30}. If this is the case at the surface of βPdBi_{2} where the point group symmetry is lowered from D _{4h } to C _{4v }, the triplet pairing is of pwave type with nodes along the outofplane direction^{30}. Since the surface states are two dimensional in nature, the outofplane 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 Ψ_{ s }(k) ± d(k) where Ψ_{ s } denotes swave order parameter that represents the bulk gap^{2, 30,31,32}. Thus the difference between the SC gap amplitudes of bulk and surface gives us an estimate of the amplitude of the pwave component. The bulk SC gap has been estimated to be about 0.9 meV by the specific heat measurement^{23}, which is close to the value of 0.8 meV that we observed at the surface. According to the theory^{32}, 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 \(\delta \equiv \left( {k_{\rm{F}}^{\rm{D}}  {k_{\rm{F}}}} \right){\rm{/}}{k_{\rm{F}}}\), which is introduced in ref. ^{32}, to be about 0.05 (\(k_{\rm{F}}^{\rm{D}}\) 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 pwave component at the surface of βPdBi_{2} is small. The absence of edge states shown in Supplementary Fig. 3 might be due to the small pwave component. In order to detect this small pwave 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.
Methods
Sample preparation and STM measurements
The βPdBi_{2} single crystals were grown by a melt growth method^{20}, and characterised by xray diffraction (XRD) and transport measurements. Pd (3N5) and Bi (5 N) at a molar ratio of 1:2 were encapsulated in an evacuated quartz tube, prereacted at temperature above 1000 °C until it completely melted and mixed. Then, it was kept at 900 °C for 20 h, cooled down at a rate of 3 °C/h down to 500 °C, and finally rapidly quenched into cold water. PdBi_{2} has two different crystallographic and superconducting phases: αphase with the space group C2/m (\({T_{\rm{c}}}\sim 1.7\) K^{33}) and βphase with I4/mmm (\({T_{\rm{c}}}\sim 5.4\) K). The last quenching procedure is important to selectively obtain the βphase. Any trace amount of αphase was not detected by XRD. For STM measurements, we used single crystals with residual resistivity ratio of 15, larger than those of the previous reports^{19, 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 ^{3}Hebased STM system (UNISOKU USM1300) modified by ourselves^{34} 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 βPdBi_{2}. We applied bias voltages to the sample whereas the tip was virtually grounded at the currentvoltage converter (Femto LCA1K5G). Tunnelling spectra were measured by the softwarebased lockin detector included in the commercial STM control system (Nanonis).
Calculation of the electronic state
To calculate the surface electronic structure and its corresponding QPI pattern, we first performed a DFT calculation using the Perdew−Burke−Ernzerhof exchangecorrelation functional^{35} as implemented in the WIEN2K program^{36}. Relativistic effects including spinorbit coupling were fully taken into account. For all atoms, the muffintin 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 βPdBi_{2}, the corresponding Brillouin zone was sampled using a 20 × 20 × 5 kmesh. For the surface calculations, a 100 unit tightbinding supercell was constructed using maximally localised Wannier functions^{37,38,39}. The 6p orbitals of Bi atoms were chosen as the projection centres.
QPI simulation
We simulated QPI patterns by incorporating the eigenvalues and eigenvectors obtained from our tightbinding supercell calculations into the standard Tmatrix formalism^{40}. The topmost Bi 6p orbitals were considered in the calculations. For all the simulations, the broadening factor was chosen to be 5 meV, and a localised, spinpreserving and orbitalpreserving scatterer of strength 0.1 eV were employed. For comparison with the experimental results, we performed a basis transformation from the lattice model to the continuum model with the Wannier function^{41} constructed by projecting the Bi 6p orbitals. We calculated the local density of state at 0.5 nm above the topmost 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 spinup and spindown subspaces were used for the Tmatrix calculations.
Data availability
All relevant data are available on request, which should be addressed to K.I.
Change history
14 December 2017
The original version of this article contained an error in Fig. 3. The calculated patterns of quasiparticle interference in the figure were incorrect due to the wrong Wannier transformation in the calculation. This correction does not affect the discussion or the conclusion of the article.
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Acknowledgements
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).
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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 firstprinciples 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.
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Iwaya, K., Kohsaka, Y., Okawa, K. et al. Fullgap superconductivity in spinpolarised surface states of topological semimetal βPdBi_{2} . Nat Commun 8, 976 (2017). https://doi.org/10.1038/s41467017012099
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DOI: https://doi.org/10.1038/s41467017012099
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