Abstract
The newly discovered topological crystalline insulators feature a complex band structure involving multiple Dirac cones^{1,2,3,4,5,6}, and are potentially highly tunable by external electric field, temperature or strain. Theoretically, it has been predicted that the various Dirac cones, which are offset in energy and momentum, might harbour vastly different orbital character^{7}. However, their orbital texture, which is of immense importance in determining a variety of a material’s properties^{8,9,10} remains elusive. Here, we unveil the orbital texture of Pb_{1−x}Sn_{x}Se, a prototypical topological crystalline insulator. By using Fouriertransform scanning tunnelling spectroscopy we measure the interference patterns produced by the scattering of surfacestate electrons. We discover that the intensity and energy dependences of the Fourier transforms show distinct characteristics, which can be directly attributed to orbital effects. Our experiments reveal a complex band topology involving two Lifshitz transitions^{11} and establish the orbital nature of the Dirac bands, which could provide an alternative pathway towards future quantum applications.
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Main
A counterpart to charge and spin, electron orbitals are of great importance in the underlying physical processes of a variety of systems. The orbital degrees of freedom, for example, play a crucial role in the colossal magnetoresistance effect in manganese oxides, and contribute to the anisotropic electronic and magnetic properties in many other transitionmetal oxide systems^{8}. More recently, orbital ordering within the superconducting FeAs layer has been thought to govern structural phase transitions and ‘stripe’like antiferromagnetism in Febased hightemperature superconductors^{9,10}. Similarly, topological materials host complex orbital arrangements often strongly coupled to other electronic degrees of freedom^{12,13,14,15,16,17,18}. Topological crystalline insulators (TCIs) in particular are predicted to exhibit intricate band, spin and orbital textures, potentially relevant for interactions in the quantum Hall regime. Although previous experiments provided a glimpse into the complex band topology present in TCIs (refs 3, 4, 5, 19, 20), these experimental efforts have not been able to shed light onto its orbital texture. Here we use Fouriertransform (FT) scanning tunnelling spectroscopy (STS) to reveal the distinct orbital nature of the Dirac bands in the TCI, Pb_{1 − x}Sn_{x}Se.
In its stoichiometric state, Pb_{1 − x}Sn_{x}Se with x = 0 is a trivial insulator under the Z_{2} topological classification of materials owing to the absence of band inversion. The process of adding Sn, which substitutes for Pb, leads to band inversion at an even number of timereversal points, and the solutions remain Z_{2} trivial. However, for a particular region of the composition–temperature parameter space, such as x > ∼0.23 and room temperature, or x > ∼0.18 and 4 K, topologically protected surface states emerge owing to the nontrivial band topology classified by crystalline symmetries^{3}. Pb_{1 − x}Sn_{x}Se crystals cleave along the (001) crystal direction based on the square lattice observed in scanning tunelling microscope (STM) topographs (Fig. 1a, b) and 3 Å step heights (Supplementary Section II). We therefore consider the band structure at the (001) face. From theory, the SS band structure consists of two ‘parent’ Dirac cones centred at X and vertically offset in energy^{2,21}. When they intersect, the hybridization between the electron branch of the lower parent Dirac cone and the hole branch of the upper parent Dirac cone opens a gap at all points except along the mirror line, leading to the formation of a pair of ‘child’ Dirac points shifted away in momentum space from the highsymmetry point X (refs 2, 21; Fig. 1c). In addition, the SS band structure exhibits two Lifshitz transitions^{11} at energies E_{VHS+} and E_{VHS−} where the constant energy contours (CECs) in momentum space (kspace) change from two disjointed hole pockets located symmetrically with respect to X, to one large hole pocket and a smaller electron pocket, both centred at X (Fig. 1c, d; refs 2, 21).
Figure 1e shows positiondependent dI/dV spectra for this material. In general, the dI/dV spectra are Vshaped, with two welldefined peaks (Fig. 1e). From our previous studies, the minimum in the density of states of the Vshaped feature denotes the energy of the Dirac point, and the two peaks on either side represent Van Hove singularities (E_{VHS+}/E_{VHS−}) associated with the Lifshitz transition^{22}. Interestingly, despite the disordered nature of our samples, with greater than 30% of the Pb sites replaced by Sn, we find that the Dirac point and the overall spectra remain homogeneous to within a few meV (Fig. 1e and also see Supplementary Section III). Having identified the important energy scales, we proceed to the application of the Fouriertransform quasiparticle interference (FTQPI) of this system.
FTQPI imaging has been successfully applied to extract the band structure of many complex systems, such as hightemperature superconductors^{23}, heavyfermion compounds^{24,25} and Z_{2} topological materials^{26,27,28}, but this technique has yet to be used fully for spin and orbitaltexture mapping of extracted bands. Representative FTs of dI/dV conductance maps acquired over a 1300 Å square region of Pb_{0.63}Sn_{0.37}Se are shown in Fig. 2c, f. To analyse the FTs, as a starting point, the expected QPI pattern can be obtained using a simple autocorrelation of the CECs, not including any matrix element effects (Fig. 2b, e). The pattern produced can be understood by studying the Fermi surface topology shown in Fig. 2a, d. From this, we see that two main sets of scattering wavevectors (Q_{1}/Q_{3} and Q_{2}/Q_{4}) are expected for the (001) SS. Above the Lifshitz transition, for example (Fig. 2d, e), the QPI pattern around the momentum positions Q_{1}/Q_{3} originates from scattering between two (110) mirrorsymmetric pairs of Dirac cones, and is expected to be quasielliptical. In contrast, the pattern at Q_{2}/Q_{4} marks the scattering between two pairs of parent Dirac cones rotated 90 degrees with respect to one other, and should be nearly circular (Supplementary Section IV).
By studying the experimental FTs, our first observation is that there is a marked difference between the QPI patterns above E_{VHS+} (Fig. 3a), where clear, nearly circular dispersing features appear, at E_{VHS+} (Fig. 3b), and below E_{VHS+}, where a (nondispersing) ‘clover’like set of four dots appear (Fig. 3c). This remarkable switch in the QPI pattern is directly connected to the change in Fermi surface topology across the Lifshitz transition (Fig. 3a–f). The simple autocorrelation of the CECs shown in Fig. 2b, e confirms this picture; the disconnected CECs below E_{VHS+} result in a nondispersing clover pattern near Q_{2}/Q_{4}, whereas the continuous ovalshaped CECs above E_{VHS+} result in a quasicircular dispersing pattern.
The main challenge of any QPI study is extracting the band structure in kspace from the observed dispersive modes in qspace. An important piece of information is provided in the set of scattering wavevectors Q_{2} close to the Dirac point where the clover is observed (Figs 2a–c and 3c). As the clover is a product of quasiparticle scattering between four child Dirac cones in the first Brillouin zone, the average distance in qspace between the centres of neighbouring clover leafs allows us to precisely extract the kspace distance between two Dirac cones symmetric around X to be 0.060 ± 0.006 Å^{−1}. Interestingly, as the kspace location of the Dirac cones in TCIs is directly connected to doping^{19}, and nominal doping is often quite different from the actual and can also vary within the sample, this method presents an alternative way to locally determine and compare the doping level in this class of materials.
To extract SS dispersion anisotropy in kspace, we need to use the qspace dispersions of the sets of wavevectors at both Q_{1} and Q_{2}. Remarkably, although Q_{1} is expected to be weak, the high quality of our data allows us to track Q_{1} over a small range of energies (Supplementary Fig. 4). Using the method explained in Supplementary Section IV, we obtain the dispersions along the Γ–X and X–M directions close to the Dirac point (Fig. 3g) and determine the velocities to be v_{x} = 3.49 eV Å and v_{y} = 2.20 eV Å, giving an anisotropy coefficient of 1.59 and the geometric mean of velocities to be (v_{x}v_{y})^{1/2} = 2.77 eV Å. It is important to note that no theoretical models were used to obtain these SS band dispersion velocities. Furthermore, although these velocities have been calculated without any assumptions about the exact shape of the CECs or the positions of the Lifshitz transitions, the position of the upper Lifshitz transition from Fig. 3g matches beautifully with the energy of the corresponding feature in the average dI/dV spectrum obtained on the same sample (Fig. 3h). Furthermore, the arithmetic mean of dispersion velocities (v_{x}v_{y})^{1/2} from Landau levels in Pb_{1 − x}Sn_{x}Se are found to be 2.60 eV Å and 2.70 eV Å for x ∼ 0.33 and x ∼ 0.30 respectively^{22}, and are in excellent agreement with the value of 2.77 eV Å obtained from QPI dispersion in the x ∼ 0.37 sample.
Having determined the SS dispersion, we turn to one of the most striking aspects of the data, namely, the asymmetry in the intensity of the QPI pattern across E_{d}, which can be observed by comparing Fig. 4a, c (Supplementary Section V). As we show in the following discussion, this asymmetry is a direct consequence of the highly nontrivial orbital texture of the Dirac SS bands. The spin texture in this material can be simply stated as exhibiting lefthanded chirality for the electron branches of all parent and child Dirac cones (Fig. 1d). The orbital character is, however, more complex. Whereas the two child Dirac cones are necessarily related by timereversal symmetry and mirror symmetry, the two parent Dirac cones do not have to be similar (each of the parent Dirac cone maps to itself under either the timereversal or mirror operator). In fact, by symmetry arguments aided by firstprinciples band structure computations^{7}, the two parent Dirac cones are expected to have different orbital character and are associated with orbitals with opposite signs of mirror eigenvalues. Let us now consider the two sets of parent/child Dirac cones in kspace, one around X (π/a_{0}, 0) and the other around Y (0, π/a_{0}) (Fig. 1c). Theoretically, whereas the upper branch of the lower Sn parent Dirac cone is associated with p_{z} orbital character for both X and Y momenta, the lower branch of the upper Se parent Dirac cone has a different orbital wavefunction—that is, Se p_{x} around X and Se p_{y} around Y (Fig. 1c). The different orbital textures below E_{d} at (π/a_{0}, 0) and (0, π/a_{0}) positions should suppress scattering between them and therefore result in diminished QPI patterns below E_{d}, exactly as seen in Fig. 4. Our data thus provide direct experimental confirmation of the proposed orbital arrangement of the TCI SS.
To further substantiate this picture and elucidate the origin of the specific orbitalselective scattering processes responsible for the subtle features in the QPI patterns, we use the proposed spin and orbital textures in model simulations of the QPI data. We find that the resulting simulations capture many features of our data, including changes in intensity with angle in the set of scattering wavevectors Q_{2} (Supplementary Section VII) that were clearly lacking in the simple autocorrelations. Take the QPI pattern at E_{d} + 122 meV as an example (Fig. 4d–f). The outermost highintensity ring is due to scattering between the two Sn p_{z} parent cones across X and Y neighbourhood. The inner ring comes from the scattering between the Se parent cone at one Brillouin zone corner neighbourhood and the Sn cone at the other. Without the spin and orbitalmatrix element effect (Fig. 4e), little intensity variation around the rings was found in the autocorrelation map. When the matrix element effect is turned on (Fig. 4f), the inner ring is greatly suppressed because it represents scattering between different kinds of orbitals, once again consistent with the postulated orbital texture. The intensity variation on the outer ring is in part due to the spin texture on the ovalshaped CEC and partly a consequence of the chosen impurity potential for matching the experimental data (Supplementary Section VII). We note that orbitalselective ARPES experiments^{16} might be able to provide further information regarding this orbital arrangement, especially within the narrow energy range between the two Lifshitz transitions (E_{VHS−} < E < E_{VHS+}) where a weaker QPI signature is observed.
Finally, our data show evidence for asymmetry in the dispersion velocities between the hole and electron branches of the outer Dirac cones. The dashed grey lines in Fig. 2g represent linear fits to the experimental data points and show a slight asymmetry in the slope above and below E_{d}. This asymmetry is also noticeable in the Landau level dispersion in the same material^{22} and the positions of the upper and lower Lifshitz transitions with respect to the Dirac point (Supplementary Section VI). This phenomenon could be attributed to the fact that bulk conduction and valence bands are not necessarily particle–hole symmetric^{29}. Our observations provide strong evidence that, in addition to asymmetric nature of the orbital wavefunction, a particle–hole symmetric model with identical parent Dirac cones dispersions may not be enough to completely encompass the underlying physics present in this class of materials and that nonidentical parent Dirac cones need to be taken into account.
Methods
The Pb_{1 − x}Sn_{x}Se single crystals used for QPI imaging were grown by the selfselecting vapour growth method, cleaved at 77 K, and immediately inserted into the STM head. Doping concentration was checked using energydispersive Xray spectroscopy (EDS) and electron microprobe analysis (see Supplementary Section I for more details). All dI/dV measurements were acquired at 6 K using a standard lockin technique with ∼5 meV peaktopeak modulation at a frequency of 1,488 Hz. We use the Lawler–Fujita driftcorrection algorithm^{30} on all acquired data to remove the effects of slow thermal and piezoelectric drift. The quasiparticle interference (QPI) imaging technique uses the interference of elastically scattered quasiparticles (which are just electrons in this case) with different momenta k_{1} and k_{2}, resulting in a standing wave ‘ripples’ of wavevector q = k_{1} − k_{2} which can be detected in STM dI/dV conductance maps. Twodimensional FTs of dI/dV maps (FTQPI; ref. 23) were used to extract all the scattering wavevectors.
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Acknowledgements
V.M. gratefully acknowledges funding from the US Department of Energy, Scanned Probe Division under Award Number DEFG0212ER46880 for the primary support of I.Z. and Y.O. (experiments, data analysis and writing the paper) and NSFECCS1232105 for the partial support of W.Z. and D.W. (data acquisition). Work at Massachusetts Institute of Technology is supported by US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award DESC0010526 (L.F.), and NSF DMR 1104498 (M.S.). H.L. acknowledges the Singapore National Research Foundation for support under NRF Award No. NRFNRFF201303. The work at Northeastern University is supported by the US Department of Energy grant number DEFG0207ER46352, and benefited from Northeastern University’s Advanced Scientific Computation Center (ASCC), theory support at the Advanced Light Source, Berkeley and the allocation of time at the NERSC supercomputing centre through DOE grant number DEAC0205CH11231. WF.T. and CY.H. were supported by the NSC in Taiwan under Grant No. 1022112M110009. WF.T. also thanks C. Fang for useful discussions. Work at Princeton University is supported by the US National Science Foundation Grant, NSFDMR1006492. F.C. acknowledges the support provided by MOSTTaiwan under project number NSC1022119M002004.
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Y.O., I.Z. and V.M. designed the experiments. Samples were obtained from R.S., F.C. and M.Z.H. Theoretical analysis and calculations were done by CY.H., M.S., WF.T., H.L., A.B. and L.F. STM experiments were carried out by Y.O., I.Z., D.W. and W.Z., I.Z., Y.O., H.L., L.F. and V.M. analysed the data and wrote the paper.
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Zeljkovic, I., Okada, Y., Huang, CY. et al. Mapping the unconventional orbital texture in topological crystalline insulators. Nature Phys 10, 572–577 (2014). https://doi.org/10.1038/nphys3012
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DOI: https://doi.org/10.1038/nphys3012
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