Journal articles: 'Topological surface states'

1

Guo, Yuning, Matheus Rosa, and Massimo Ruzzene. "Symmetry-enforced gapless surface states in three-dimensional acoustic gyroid structures." Journal of the Acoustical Society of America 151, no. 4 (April 2022): A97. http://dx.doi.org/10.1121/10.0010772.

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The discovery of topological gapless phases challenges the perception that topological features necessarily require a bandgap, expanding the understanding of topological phases of matter in various realms including electric, photonic, and phononic systems. The progress on 3D topological gapless states in elastic and acoustic systems is still in its early stages of formulation and design. We here investigate 3D acoustic gyroid crystals supporting symmetry-enforced gapless surface states in minimal surface-based structures. The inherent chirality and morphology of gyroid surfaces enable the implementation of 3D acoustic crystals hosting symmetry-enforced Dirac points and topologically gapless surface states. The associated fourfold degeneracy is protected by the nonsymmorphic space group featuring a combination of screw symmetry and glide reflections. The presence of gapless surface arcs relies on band structure calculations conducted using finite element simulations, while preliminary experimental results on additively manufactured samples validate their occurrence in the proposed gyroid surfaces. With the continuous development in additive manufacturing techniques, the presented surface-based framework provides a platform to explore a variety of topological wave physics phenomena in 3D load-bearing, continuum materials of potential engineering relevance, among which superior acoustic absorption may be particularly promising.

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Ovchinnikov, Yu N. "Topological Insulator: Surface Localized States." Journal of Superconductivity and Novel Magnetism 32, no. 5 (August 10, 2018): 1327–31. http://dx.doi.org/10.1007/s10948-018-4827-0.

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Hsu, Chuang-Han, Xiaoting Zhou, Tay-Rong Chang, Qiong Ma, Nuh Gedik, Arun Bansil, Su-Yang Xu, Hsin Lin, and Liang Fu. "Topology on a new facet of bismuth." Proceedings of the National Academy of Sciences 116, no. 27 (June 13, 2019): 13255–59. http://dx.doi.org/10.1073/pnas.1900527116.

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Bismuth-based materials have been instrumental in the development of topological physics, even though bulk bismuth itself has been long thought to be topologically trivial. A recent study has, however, shown that bismuth is in fact a higher-order topological insulator featuring one-dimensional (1D) topological hinge states protected by threefold rotational and inversion symmetries. In this paper, we uncover another hidden facet of the band topology of bismuth by showing that bismuth is also a first-order topological crystalline insulator protected by a twofold rotational symmetry. As a result, its (11¯0) surface exhibits a pair of gapless Dirac surface states. Remarkably, these surface Dirac cones are “unpinned” in the sense that they are not restricted to locate at specific k points in the (11¯0) surface Brillouin zone. These unpinned 2D Dirac surface states could be probed directly via various spectroscopic techniques. Our analysis also reveals the presence of a distinct, previously uncharacterized set of 1D topological hinge states protected by the twofold rotational symmetry. Our study thus provides a comprehensive understanding of the topological band structure of bismuth.

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Galeeva, Alexandra V., Dmitry A. Belov, Aleksei S. Kazakov, Anton V. Ikonnikov, Alexey I. Artamkin, Ludmila I. Ryabova, Valentine V. Volobuev, Gunther Springholz, Sergey N. Danilov, and Dmitry R. Khokhlov. "Photoelectromagnetic Effect Induced by Terahertz Laser Radiation in Topological Crystalline Insulators Pb1−xSnxTe." Nanomaterials 11, no. 12 (November 26, 2021): 3207. http://dx.doi.org/10.3390/nano11123207.

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Topological crystalline insulators form a class of semiconductors for which surface electron states with the Dirac dispersion relation are formed on surfaces with a certain crystallographic orientation. Pb1−xSnxTe alloys belong to the topological crystalline phase when the SnTe content x exceeds 0.35, while they are in the trivial phase at x < 0.35. For the surface crystallographic orientation (111), the appearance of topologically nontrivial surface states is expected. We studied the photoelectromagnetic (PEM) effect induced by laser terahertz radiation in Pb1−xSnxTe films in the composition range x = (0.11–0.44), with the (111) surface crystallographic orientation. It was found that in the trivial phase, the amplitude of the PEM effect is determined by the power of the incident radiation, while in the topological phase, the amplitude is proportional to the flux of laser radiation quanta. A possible mechanism responsible for the effect observed presumes damping of the thermalization rate of photoexcited electrons in the topological phase and, consequently, prevailing of electron diffusion, compared with energy relaxation.

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Kargarian, Mehdi, Mohit Randeria, and Yuan-Ming Lu. "Are the surface Fermi arcs in Dirac semimetals topologically protected?" Proceedings of the National Academy of Sciences 113, no. 31 (July 19, 2016): 8648–52. http://dx.doi.org/10.1073/pnas.1524787113.

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Motivated by recent experiments probing anomalous surface states of Dirac semimetals (DSMs) Na3Bi and Cd3As2, we raise the question posed in the title. We find that, in marked contrast to Weyl semimetals, the gapless surface states of DSMs are not topologically protected in general, except on time-reversal-invariant planes of surface Brillouin zone. We first demonstrate this finding in a minimal four-band model with a pair of Dirac nodes at k=(0,0,±Q), where gapless states on the side surfaces are protected only near kz=0. We then validate our conclusions about the absence of a topological invariant protecting double Fermi arcs in DSMs, using a K-theory analysis for space groups of Na3Bi and Cd3As2. Generically, the arcs deform into a Fermi pocket, similar to the surface states of a topological insulator, and this pocket can merge into the projection of bulk Dirac Fermi surfaces as the chemical potential is varied. We make sharp predictions for the doping dependence of the surface states of a DSM that can be tested by angle-resolved photoemission spectroscopy and quantum oscillation experiments.

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Jost, Andreas, Michel Bendias, Jan Böttcher, Ewelina Hankiewicz, Christoph Brüne, Hartmut Buhmann, Laurens W. Molenkamp, et al. "Electron–hole asymmetry of the topological surface states in strained HgTe." Proceedings of the National Academy of Sciences 114, no. 13 (March 9, 2017): 3381–86. http://dx.doi.org/10.1073/pnas.1611663114.

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Topological insulators are a new class of materials with an insulating bulk and topologically protected metallic surface states. Although it is widely assumed that these surface states display a Dirac-type dispersion that is symmetric above and below the Dirac point, this exact equivalence across the Fermi level has yet to be established experimentally. Here, we present a detailed transport study of the 3D topological insulator-strained HgTe that strongly challenges this prevailing viewpoint. First, we establish the existence of exclusively surface-dominated transport via the observation of an ambipolar surface quantum Hall effect and quantum oscillations in the Seebeck and Nernst effect. Second, we show that, whereas the thermopower is diffusion driven for surface electrons, both diffusion and phonon drag contributions are essential for the hole surface carriers. This distinct behavior in the thermoelectric response is explained by a strong deviation from the linear dispersion relation for the surface states, with a much flatter dispersion for holes compared with electrons. These findings show that the metallic surface states in topological insulators can exhibit both strong electron–hole asymmetry and a strong deviation from a linear dispersion but remain topologically protected.

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7

Shen, Yuanyuan, Shengguo Guan, and Chunyin Qiu. "Topological valley transport of spoof surface acoustic waves." Journal of Applied Physics 133, no. 11 (March 21, 2023): 114305. http://dx.doi.org/10.1063/5.0137591.

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In recent years, topological physics has attracted broad attention in condensed matter systems. Here, we report an experimental study on topological valley transport of spoof surface acoustic waves (SAWs). Specifically, we realize valley pseudospins and a valley Hall phase transition by tuning the structural size of adjacent grooves. In addition to a direct visualization of the vortex chirality-locked beam splitting for the bulk valley states, valley-projected edge states are observed in straight and bent interface channels formed by two topologically distinct valley Hall insulating phases. The experimental data agree well with our numerical predictions. The topological transport of spoof SAWs, encoded with valley information, provides more possibilities in design novel acoustic devices based on the valley-contrasting physics.

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8

Rider, Marie S., Maria Sokolikova, Stephen M. Hanham, Miguel Navarro-Cía, Peter D. Haynes, Derek K. K. Lee, Maddalena Daniele, et al. "Experimental signature of a topological quantum dot." Nanoscale 12, no. 44 (2020): 22817–25. http://dx.doi.org/10.1039/d0nr06523d.

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9

Shtanko, Oles, and Leonid Levitov. "Robustness and universality of surface states in Dirac materials." Proceedings of the National Academy of Sciences 115, no. 23 (May 22, 2018): 5908–13. http://dx.doi.org/10.1073/pnas.1722663115.

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Ballistically propagating topologically protected states harbor exotic transport phenomena of wide interest. Here we describe a nontopological mechanism that produces such states at the surfaces of generic Dirac materials, giving rise to propagating surface modes with energies near the bulk band crossing. The robustness of surface states originates from the unique properties of Dirac–Bloch wavefunctions which exhibit strong coupling to generic boundaries. Surface states, described by Jackiw–Rebbi-type bound states, feature a number of interesting properties. Mode dispersion is gate tunable, exhibiting a wide variety of different regimes, including nondispersing flat bands and linear crossings within the bulk bandgap. The ballistic wavelike character of these states resembles the properties of topologically protected states; however, it requires neither topological restrictions nor additional crystal symmetries. The Dirac surface states are weakly sensitive to surface disorder and can dominate edge transport at the energies near the Dirac point.

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10

Li, Peng, James Kally, Steven S. L. Zhang, Timothy Pillsbury, Jinjun Ding, Gyorgy Csaba, Junjia Ding, et al. "Magnetization switching using topological surface states." Science Advances 5, no. 8 (August 2019): eaaw3415. http://dx.doi.org/10.1126/sciadv.aaw3415.

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Topological surface states (TSSs) in a topological insulator are expected to be able to produce a spin-orbit torque that can switch a neighboring ferromagnet. This effect may be absent if the ferromagnet is conductive because it can completely suppress the TSSs, but it should be present if the ferromagnet is insulating. This study reports TSS-induced switching in a bilayer consisting of a topological insulator Bi2Se3 and an insulating ferromagnet BaFe12O19. A charge current in Bi2Se3 can switch the magnetization in BaFe12O19 up and down. When the magnetization is switched by a field, a current in Bi2Se3 can reduce the switching field by ~4000 Oe. The switching efficiency at 3 K is 300 times higher than at room temperature; it is ~30 times higher than in Pt/BaFe12O19. These strong effects originate from the presence of more pronounced TSSs at low temperatures due to enhanced surface conductivity and reduced bulk conductivity.

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11

Fang, Chen, Ling Lu, Junwei Liu, and Liang Fu. "Topological semimetals with helicoid surface states." Nature Physics 12, no. 10 (June 6, 2016): 936–41. http://dx.doi.org/10.1038/nphys3782.

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12

Pankratov, O. A. "Understanding surface states of topological insulators." Uspekhi Fizicheskih Nauk 188, no. 11 (December 2017): 1226–37. http://dx.doi.org/10.3367/ufnr.2017.12.038307.

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13

Pankratov, O. A. "Understanding surface states of topological insulators." Physics-Uspekhi 61, no. 11 (November 30, 2018): 1116–26. http://dx.doi.org/10.3367/ufne.2017.12.038307.

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14

Dai, C. M., W. Wang, and X. X. Yi. "Quantum light and topological surface states." Journal of Physics B: Atomic, Molecular and Optical Physics 50, no. 23 (November 9, 2017): 235501. http://dx.doi.org/10.1088/1361-6455/aa92e5.

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15

Schnyder, Andreas P., and Philip M. R. Brydon. "Topological surface states in nodal superconductors." Journal of Physics: Condensed Matter 27, no. 24 (May 22, 2015): 243201. http://dx.doi.org/10.1088/0953-8984/27/24/243201.

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16

Seo, Jungpil, Pedram Roushan, Haim Beidenkopf, Y. S. Hor, R. J. Cava, and Ali Yazdani. "Transmission of topological surface states through surface barriers." Nature 466, no. 7304 (July 2010): 343–46. http://dx.doi.org/10.1038/nature09189.

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17

Tyagi, Udai Prakash, Kakoli Bera, and Partha Goswami. "On Strong f-Electron Localization Effect in a Topological Kondo Insulator." Symmetry 13, no. 12 (November 24, 2021): 2245. http://dx.doi.org/10.3390/sym13122245.

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We study a strong f-electron localization effect on the surface state of a generic topological Kondo insulator (TKI) system by performing a mean-field theoretic (MFT) calculation within the framework of the periodic Anderson model (PAM) using the slave boson technique. The surface metallicity, together with bulk insulation, requires this type of localization. A key distinction between surface states in a conventional insulator and a topological insulator is that, along a course joining two time-reversal invariant momenta (TRIM) in the same BZ, there will be an intersection of these surface states, an even/odd number of times, with the Fermi energy inside the spectral gap. For an even (odd) number of surface state crossings, the surface states are topologically trivial (non-trivial). The symmetry consideration and the pictorial representation of the surface band structure obtained here show an odd number of crossings, leading to the conclusion that, at least within the PAM framework, the generic system is a strong topological insulator.

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Hou, Yusheng, Jeongwoo Kim, and Ruqian Wu. "Magnetizing topological surface states of Bi2Se3 with a CrI3 monolayer." Science Advances 5, no. 5 (May 2019): eaaw1874. http://dx.doi.org/10.1126/sciadv.aaw1874.

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To magnetize surfaces of topological insulators without damaging their topological feature is a crucial step for the realization of the quantum anomalous Hall effect (QAHE) and remains as a challenging task. Through density functional calculations, we found that adsorption of a semiconducting two-dimensional van der Waals (2D-vdW) ferromagnetic CrI3 monolayer can create a sizable spin splitting at the Dirac point of the topological surface states of Bi2Se3 films. Furthermore, general rules that connect different quantum and topological parameters are established through model analyses. This work provides a useful guideline for the realization of QAHE at high temperatures in heterostructures of 2D-vdW magnetic monolayers and topological insulators.

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Kim, Minkyung, Wenlong Gao, Dasol Lee, Taewoo Ha, Teun‐Teun Kim, Shuang Zhang, and Junsuk Rho. "Extremely Broadband Topological Surface States in a Photonic Topological Metamaterial." Advanced Optical Materials 7, no. 20 (July 31, 2019): 1900900. http://dx.doi.org/10.1002/adom.201900900.

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Guan, Syu-You, Peng-Jen Chen, Ming-Wen Chu, Raman Sankar, Fangcheng Chou, Horng-Tay Jeng, Chia-Seng Chang, and Tien-Ming Chuang. "Superconducting topological surface states in the noncentrosymmetric bulk superconductor PbTaSe2." Science Advances 2, no. 11 (November 2016): e1600894. http://dx.doi.org/10.1126/sciadv.1600894.

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The search for topological superconductors (TSCs) is one of the most urgent contemporary problems in condensed matter systems. TSCs are characterized by a full superconducting gap in the bulk and topologically protected gapless surface (or edge) states. Within each vortex core of TSCs, there exists the zero-energy Majorana bound states, which are predicted to exhibit non-Abelian statistics and to form the basis of the fault-tolerant quantum computation. To date, no stoichiometric bulk material exhibits the required topological surface states (TSSs) at the Fermi level (EF) combined with fully gapped bulk superconductivity. We report atomic-scale visualization of the TSSs of the noncentrosymmetric fully gapped superconductor PbTaSe2. Using quasi-particle scattering interference imaging, we find two TSSs with a Dirac point atE≅ 1.0 eV, of which the inner TSS and the partial outer TSS crossEF, on the Pb-terminated surface of this fully gapped superconductor. This discovery reveals PbTaSe2as a promising candidate for TSC.

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Chen, Fan W., Luis A. Jauregui, Yaohua Tan, Michael Manfra, Gerhard Klimeck, Yong P. Chen, and Tillmann Kubis. "In-surface confinement of topological insulator nanowire surface states." Applied Physics Letters 107, no. 12 (September 21, 2015): 121605. http://dx.doi.org/10.1063/1.4931975.

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22

Luo, Kaifa, Rui Yu, and Hongming Weng. "Topological Nodal States in Circuit Lattice." Research 2018 (September 2, 2018): 1–10. http://dx.doi.org/10.1155/2018/6793752.

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The search for artificial structure with tunable topological properties is an interesting research direction of today’s topological physics. Here, we introduce a scheme to realize topological nodal states with a three-dimensional periodic inductor-capacitor (LC) circuit lattice, where the topological nodal line state and Weyl state can be achieved by tuning the parameters of inductors and capacitors. A tight-binding-like model is derived to analyze the topological properties of the LC circuit lattice. The key characters of the topological states, such as the drumhead-like surface bands for nodal line state and the Fermi arc-like surface bands for Weyl state, are found in these systems. We also show that the Weyl points are stable with the fabrication errors of electric devices.

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23

Zhao, W., L. Ding, B. Zhou, J. Wu, Y. Bai, Z. Man, and X. Luo. "Phase diagrams of superconducting topological surface states." Condensed Matter Physics 24, no. 4 (2021): 43701. http://dx.doi.org/10.5488/cmp.24.43701.

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In this paper, we present a detailed study on the phase diagrams of superconducting topological surface states, especially, focusing on the interplay between crystalline symmetry and topology of the effective BdG Hamiltonian. We show that for the 4 x 4 kinematic Hamiltonian of the normal state, a mirror symmetry M can be defined, and for the M-odd pairings, the classification of the 8 x 8 BdG Hamiltonian is ℤ⊕ℤ, and the time-reversal symmetry is broken intrinsically. The topological non-trivial phase can support chiral Majorana edge modes, and can be realized in the thin films of iron-based superconductor such as FeSeTe.

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Ghazaryan, Areg, Emilian M. Nica, Onur Erten, and Pouyan Ghaemi. "Shadow surface states in topological Kondo insulators." New Journal of Physics 23, no. 12 (December 1, 2021): 123042. http://dx.doi.org/10.1088/1367-2630/ac4124.

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Abstract The surface states of 3D topological insulators in general have negligible quantum oscillations (QOs) when the chemical potential is tuned to the Dirac points. In contrast, we find that topological Kondo insulators (TKIs) can support surface states with an arbitrarily large Fermi surface (FS) when the chemical potential is pinned to the Dirac point. We illustrate that these FSs give rise to finite-frequency QOs, which can become comparable to the extremal area of the unhybridized bulk bands. We show that this occurs when the crystal symmetry is lowered from cubic to tetragonal in a minimal two-orbital model. We label such surface modes as ‘shadow surface states’. Moreover, we show that the sufficient next-nearest neighbor out-of-plane hybridization leading to shadow surface states can be self-consistently stabilized for tetragonal TKIs. Consequently, shadow surface states provide an important example of high-frequency QOs beyond the context of cubic TKIs.

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Sharma, Prince, Yogesh Kumar, V. P. S. Awana, and Mahesh Kumar. "Temperature-dependent evolution of topological surface states." Solid State Sciences 125 (March 2022): 106829. http://dx.doi.org/10.1016/j.solidstatesciences.2022.106829.

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26

Xiu, Faxian, Liang He, Yong Wang, Lina Cheng, Li-Te Chang, Murong Lang, Guan Huang, et al. "Manipulating surface states in topological insulator nanoribbons." Nature Nanotechnology 6, no. 4 (February 13, 2011): 216–21. http://dx.doi.org/10.1038/nnano.2011.19.

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Cantele, G., D. Ninno, and G. Iadonisi. "Topological surface states in deformed quantum wires." Physical Review B 61, no. 20 (May 15, 2000): 13730–36. http://dx.doi.org/10.1103/physrevb.61.13730.

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28

Mal, Priyanath, Bipul Das, G. Bera, P. Rambabu, G. R. Turpu, C. V. Tomy, and Pradip Das. "Spin splitted topological surface states in PbBi4Te7." Journal of Physics D: Applied Physics 53, no. 48 (September 25, 2020): 484003. http://dx.doi.org/10.1088/1361-6463/abad62.

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29

Wang, Mei-Xiao, Canhua Liu, Jin-Peng Xu, Fang Yang, Lin Miao, Meng-Yu Yao, C. L. Gao, et al. "The Coexistence of Superconductivity and Topological Order in the Bi2Se3 Thin Films." Science 336, no. 6077 (March 15, 2012): 52–55. http://dx.doi.org/10.1126/science.1216466.

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Three-dimensional topological insulators (TIs) are characterized by their nontrivial surface states, in which electrons have their spin locked at a right angle to their momentum under the protection of time-reversal symmetry. The topologically ordered phase in TIs does not break any symmetry. The interplay between topological order and symmetry breaking, such as that observed in superconductivity, can lead to new quantum phenomena and devices. We fabricated a superconducting TI/superconductor heterostructure by growing dibismuth triselenide (Bi2Se3) thin films on superconductor niobium diselenide substrate. Using scanning tunneling microscopy and angle-resolved photoemission spectroscopy, we observed the superconducting gap at the Bi2Se3 surface in the regime of Bi2Se3 film thickness where topological surface states form. This observation lays the groundwork for experimentally realizing Majorana fermions in condensed matter physics.

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Su, Zhaoxian, Wenlong Gao, Bingyi Liu, Lingling Huang, and Yongtian Wang. "Three-dimensional Dirac semimetal metamaterial enabled by negative couplings." New Journal of Physics 24, no. 3 (March 1, 2022): 033025. http://dx.doi.org/10.1088/1367-2630/ac575a.

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Abstract Three-dimensional (3D) semimetals with fourfold degenerate Dirac points are of prominent importance in topological photonics as the parent states to Weyl nodes, line nodes, & etc. The dispersions on all the momentums’ directions are linear, which represents that the Dirac point and topologically protected helicoid surface states may exist. Here, we have demonstrated an acoustic metamaterial with Dirac points by designing the sign of coupling terms, specifically incorporating negative couplings. Tuning the coupling parameter along longitudinal direction, the transition from 3D Dirac point to Weyl points can be obtained. In realistic topological metamaterial designing, the negative coupling is realized by inserting additional off-resonant sites. The simulated band dispersion clearly shows four-band crossing point. The helicoid surface states are also proved. Our study provides a new approach of constructing 3D topological phase and shows the transition between nodal ring and Dirac point. Our results can be the theoretical basement of topological protected devices.

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Wang, Yihua. "Broken-symmetry states in topological insulators." Modern Physics Letters B 29, no. 25 (September 20, 2015): 1530006. http://dx.doi.org/10.1142/s0217984915300069.

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Breaking the time-reversal symmetry (TRS) on the surface of a three-dimensional topological insulator (TI) transforms its metallic surface into a Chern insulator. The TRS-broken surface states are essential for many exotic emergent particles in condensed matter. In this review, I will show broken TRS surface states of TI induced by magnetism and by light imaged with scanning microscopy and photoemission spectroscopy, respectively. Our capability to manipulate mesoscopic magnetic structures as well as to shape ultrafast light pulses makes broken-symmetry states in TI promising platforms to simulate elusive fundamental particles such as magnetic monopoles and Majorana fermions.

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Jiang, Yang, Ya Bai, Zeyi Ye, Na Li, Candong Liu, and Peng Liu. "Distinguishing high-harmonic generation from surface and bulk states in topological insulator Bi2Se3." Chinese Optics Letters 21, no. 4 (2023): 043801. http://dx.doi.org/10.3788/col202321.043801.

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Casado Aguilar, Pablo, Fabian Calleja, Chia-Nung Kuo, Chin Shan Lue, Barun Ghosh, Amit Agarwal, Antonio Politano, et al. "Atomic-scale study of type-II Dirac semimetal PtTe₂ surface." Journal of Physics: Materials 5, no. 4 (September 30, 2022): 044003. http://dx.doi.org/10.1088/2515-7639/ac92a8.

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Abstract Dirac semimetals (DSM) host linear bulk bands and topologically protected surface states, giving rise to exotic and robust properties. Platinum ditelluride (PtTe2) belongs to this interesting group of topological materials. Here, we employ scanning tunneling microscopy (STM) in combination with first-principles calculations to visualize and identify the native defects at the surface of a freshly cleaved PtTe2 crystal. Around these defects, short-wavelength electron density oscillations are observed. Fourier transform analysis of the energy-dependent quasiparticle interference patterns is in good agreement with our calculated joint density of states, demonstrating the singular properties of the surface of this type-II DSM. Our results evidence the power of STM in understanding the surface of topological materials.

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Yakovkin, I. N. "Surface and Interface Bands of the CdTe–HgTe–CdTe Heterostructure: Evidence of Metallicity." Ukrainian Journal of Physics 66, no. 7 (August 4, 2021): 630. http://dx.doi.org/10.15407/ujpe66.7.630.

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Performed full-relativistic DFT calculations have demonstrated that thin HgTe layers are metallic and, with increasing thickness, do not become insulators – either ordinary band insulators or topological insulators. The variations of the potential at the CdTe–HgTe interfaces are found to be negligible in comparison with those at the terminating surfaces of the CdTe–HgTe–CdTe films, so that the interfaces in fact do not form any potential well. It is shown that the interface-related bands of the CdTe–HgTe–CdTe films are situated well below EF, so that a dominant input into the density of states at EF and, therefore, to the conductivity is provided not by the interface states, but by the surface bands of the net layered system. It is reasonable therefore to consider an alternative interpretation of the reported thickness dependence of the conductivity of the system, such as the possible surface segregation of components or unavoidable contaminations, which seems much more realistic than the interpretation based on involving topological insulators and topologically protected surface states.

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Xu, Su-Yang, Ilya Belopolski, Daniel S. Sanchez, Chenglong Zhang, Guoqing Chang, Cheng Guo, Guang Bian, et al. "Experimental discovery of a topological Weyl semimetal state in TaP." Science Advances 1, no. 10 (November 2015): e1501092. http://dx.doi.org/10.1126/sciadv.1501092.

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Weyl semimetals are expected to open up new horizons in physics and materials science because they provide the first realization of Weyl fermions and exhibit protected Fermi arc surface states. However, they had been found to be extremely rare in nature. Recently, a family of compounds, consisting of tantalum arsenide, tantalum phosphide (TaP), niobium arsenide, and niobium phosphide, was predicted as a Weyl semimetal candidates. We experimentally realize a Weyl semimetal state in TaP. Using photoemission spectroscopy, we directly observe the Weyl fermion cones and nodes in the bulk, and the Fermi arcs on the surface. Moreover, we find that the surface states show an unexpectedly rich structure, including both topological Fermi arcs and several topologically trivial closed contours in the vicinity of the Weyl points, which provides a promising platform to study the interplay between topological and trivial surface states on a Weyl semimetal’s surface. We directly demonstrate the bulk-boundary correspondence and establish the topologically nontrivial nature of the Weyl semimetal state in TaP, by resolving the net number of chiral edge modes on a closed path that encloses the Weyl node. This also provides, for the first time, an experimentally practical approach to demonstrating a bulk Weyl fermion from a surface state dispersion measured in photoemission.

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Li, Yanan, Qiangqiang Gu, Chen Chen, Jun Zhang, Qin Liu, Xiyao Hu, Jun Liu, et al. "Nontrivial superconductivity in topological MoTe2−xSx crystals." Proceedings of the National Academy of Sciences 115, no. 38 (August 30, 2018): 9503–8. http://dx.doi.org/10.1073/pnas.1801650115.

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Topological Weyl semimetals (TWSs) with pairs of Weyl points and topologically protected Fermi arc states have broadened the classification of topological phases and provide superior platform for study of topological superconductivity. Here we report the nontrivial superconductivity and topological features of sulfur-doped Td-phase MoTe2 with enhanced Tc compared with type-II TWS MoTe2. It is found that Td-phase S-doped MoTe2 (MoTe2−xSx, x ∼ 0.2) is a two-band s-wave bulk superconductor (∼0.13 meV and 0.26 meV), where the superconducting behavior can be explained by the s+− pairing model. Further, measurements of the quasi-particle interference (QPI) patterns and a comparison with band-structure calculations reveal the existence of Fermi arcs in MoTe2−xSx. More interestingly, a relatively large superconducting gap (∼1.7 meV) is detected by scanning tunneling spectroscopy on the sample surface, showing a hint of topological nontrivial superconductivity based on the pairing of Fermi arc surface states. Our work demonstrates that the Td-phase MoTe2−xSx is not only a promising topological superconductor candidate but also a unique material for study of s+− superconductivity.

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Palacio-Morales, Alexandra, Eric Mascot, Sagen Cocklin, Howon Kim, Stephan Rachel, Dirk K. Morr, and Roland Wiesendanger. "Atomic-scale interface engineering of Majorana edge modes in a 2D magnet-superconductor hybrid system." Science Advances 5, no. 7 (July 2019): eaav6600. http://dx.doi.org/10.1126/sciadv.aav6600.

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Topological superconductors are predicted to harbor exotic boundary states—Majorana zero-energy modes—whose non-Abelian braiding statistics present a new paradigm for the realization of topological quantum computing. Using low-temperature scanning tunneling spectroscopy, here, we report on the direct real-space visualization of chiral Majorana edge states in a monolayer topological superconductor, a prototypical magnet-superconductor hybrid system composed of nanoscale Fe islands of monoatomic height on a Re(0001)-O(2 × 1) surface. In particular, we demonstrate that interface engineering by an atomically thin oxide layer is crucial for driving the hybrid system into a topologically nontrivial state as confirmed by theoretical calculations of the topological invariant, the Chern number.

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Liu, Yizhou, Yong Xu, and Wenhui Duan. "Three-Dimensional Topological States of Phonons with Tunable Pseudospin Physics." Research 2019 (July 31, 2019): 1–8. http://dx.doi.org/10.34133/2019/5173580.

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Efficient control of phonons is crucial to energy-information technology, but limited by the lacking of tunable degrees of freedom like charge or spin. Here we suggest to utilize crystalline symmetry-protected pseudospins as new quantum degrees of freedom to manipulate phonons. Remarkably, we reveal a duality between phonon pseudospins and electron spins by presenting Kramers-like degeneracy and pseudospin counterparts of spin-orbit coupling, which lays the foundation for “pseudospin phononics”. Furthermore, we report two types of three-dimensional phononic topological insulators, which give topologically protected, gapless surface states with linear and quadratic band degeneracies, respectively. These topological surface states display unconventional phonon transport behaviors attributed to the unique pseudospin-momentum locking, which are useful for phononic circuits, transistors, antennas, etc. The emerging pseudospin physics offers new opportunities to develop future phononics.

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Ding, Yuanfeng, Chen Li, Jian Zhou, Hong Lu, and Yan-Feng Chen. "Transport evidence of the spin-polarized topological surface states of α-Sn grown on CdTe by molecular beam epitaxy." Applied Physics Letters 121, no. 9 (August 29, 2022): 093102. http://dx.doi.org/10.1063/5.0098585.

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It is necessary but challenging to verify topological surface states of α-Sn by electrical transport. In this work, we demonstrate conclusive transport evidence on topological properties of an α-Sn film grown on a CdTe substrate by molecular beam epitaxy. A Berry phase determined from Shubnikov–de Haas oscillations is 0.98π. A two-dimensional (2D) Fermi surface is clearly demonstrated by angle-dependent oscillations. We believe the nontrivial topology originates from the 2D Dirac fermions of the topological surface states. In addition, both anisotropic magneto-resistance and planar Hall effect have negative amplitudes at higher fields, which we attribute to the spin-flip backscattering in the topological surface states. We also show that these topological surface states have a long relaxation time of ∼95 fs, making α-Sn a potential candidate for high-efficiency spintronics.

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Mehrpour Bernety, Hossein, and Mark A. Cappelli. "An electromagnetic scattering approach to identifying topological and non-topological unidirectional edge states at gyrotropic plasma interfaces." Journal of Applied Physics 133, no. 10 (March 14, 2023): 104902. http://dx.doi.org/10.1063/5.0124165.

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We present an approach to identifying topological edge states in two dimensional (2D) problems. Such scattering-immune surface waves are typically identified using the Berry concept, which evaluates Chern numbers of bulk transmission bands. Instead, here, we approach the task from an analytical electromagnetic scattering perspective, which enables the study of a wide class of configurations in which waves are confined to propagate at the interface between a topologically non-trivial magnetized plasma and a topologically trivial medium such as air. Although it is of theoretical significance to classify interfacial states by their topological invariants, we believe that this approach enables the engagement of a broader range of researchers, particularly experimentalists who seek to design devices that exploit the ensuing non-reciprocal and scattering-immune properties.

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Ngabonziza, Prosper. "Quantum transport and potential of topological states for thermoelectricity in Bi₂Te₃ thin films." Nanotechnology 33, no. 19 (February 15, 2022): 192001. http://dx.doi.org/10.1088/1361-6528/ac4f17.

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Abstract This paper reviews recent developments in quantum transport and it presents current efforts to explore the contribution of topological insulator boundary states to thermoelectricity in Bi2Te3 thin films. Although Bi2Te3 has been used as a thermoelectric material for many years, it is only recently that thin films of this material have been synthesized as 3D topological insulators with interesting physics and potential applications related to topologically protected surface states. A major bottleneck in Bi2Te3 thin films has been eliminating its bulk conductivity while increasing its crystal quality. The ability to grow epitaxial films with high crystal quality and to fabricate sophisticated Bi2Te3-based devices is attractive for implementing a variety of topological quantum devices and exploring the potential of topological states to improve thermoelectric properties. Special emphasis is laid on preparing low-defect-density Bi2Te3 epitaxial films, gate-tuning of normal-state transport and Josephson supercurrent in topological insulator/superconductor hybrid devices. Prospective quantum transport experiments on Bi2Te3 thin-film devices are discussed as well. Finally, an overview of current progress on the contribution of topological insulator boundary states to thermoelectricity is presented. Future explorations to reveal the potential of topological states for improving thermoelectric properties of Bi2Te3 films and realizing high-performance thermoelectric devices are discussed.

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Qu, Qing, Bin Liu, Hongtao Liu, Jing Liang, Jiannong Wang, Ding Pan, and Iam Keong Sou. "Role of topological surface states and mirror symmetry in topological crystalline insulator SnTe as an efficient electrocatalyst." Nanoscale 13, no. 43 (2021): 18160–72. http://dx.doi.org/10.1039/d1nr05089c.

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The intrinsic activities of SnTe (001) and (111) surfaces with robust topological surface states (TSSs) are superior to that of a (211) surface with fragile or without TSSs, attributing to the enhanced charge transfer between H atoms and TSSs.

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43

Park, Wan Kyu, Lunan Sun, Alexander Noddings, Dae-Jeong Kim, Zachary Fisk, and Laura H. Greene. "Topological surface states interacting with bulk excitations in the Kondo insulator SmB6 revealed via planar tunneling spectroscopy." Proceedings of the National Academy of Sciences 113, no. 24 (May 27, 2016): 6599–604. http://dx.doi.org/10.1073/pnas.1606042113.

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Samarium hexaboride (SmB6), a well-known Kondo insulator in which the insulating bulk arises from strong electron correlations, has recently attracted great attention owing to increasing evidence for its topological nature, thereby harboring protected surface states. However, corroborative spectroscopic evidence is still lacking, unlike in the weakly correlated counterparts, including Bi2Se3. Here, we report results from planar tunneling that unveil the detailed spectroscopic properties of SmB6. The tunneling conductance obtained on the (001) and (011) single crystal surfaces reveals linear density of states as expected for two and one Dirac cone(s), respectively. Quite remarkably, it is found that these topological states are not protected completely within the bulk hybridization gap. A phenomenological model of the tunneling process invoking interaction of the surface states with bulk excitations (spin excitons), as predicted by a recent theory, provides a consistent explanation for all of the observed features. Our spectroscopic study supports and explains the proposed picture of the incompletely protected surface states in this topological Kondo insulator SmB6.

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Koumoulis, Dimitrios, Gerald D. Morris, Liang He, Xufeng Kou, Danny King, Dong Wang, Masrur D. Hossain, et al. "Nanoscale β-nuclear magnetic resonance depth imaging of topological insulators." Proceedings of the National Academy of Sciences 112, no. 28 (June 29, 2015): E3645—E3650. http://dx.doi.org/10.1073/pnas.1502330112.

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Considerable evidence suggests that variations in the properties of topological insulators (TIs) at the nanoscale and at interfaces can strongly affect the physics of topological materials. Therefore, a detailed understanding of surface states and interface coupling is crucial to the search for and applications of new topological phases of matter. Currently, no methods can provide depth profiling near surfaces or at interfaces of topologically inequivalent materials. Such a method could advance the study of interactions. Herein, we present a noninvasive depth-profiling technique based on β-detected NMR (β-NMR) spectroscopy of radioactive 8Li+ ions that can provide “one-dimensional imaging” in films of fixed thickness and generates nanoscale views of the electronic wavefunctions and magnetic order at topological surfaces and interfaces. By mapping the 8Li nuclear resonance near the surface and 10-nm deep into the bulk of pure and Cr-doped bismuth antimony telluride films, we provide signatures related to the TI properties and their topological nontrivial characteristics that affect the electron–nuclear hyperfine field, the metallic shift, and magnetic order. These nanoscale variations in β-NMR parameters reflect the unconventional properties of the topological materials under study, and understanding the role of heterogeneities is expected to lead to the discovery of novel phenomena involving quantum materials.

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Ono, Seishiro, Hoi Chun Po, and Haruki Watanabe. "Refined symmetry indicators for topological superconductors in all space groups." Science Advances 6, no. 18 (May 2020): eaaz8367. http://dx.doi.org/10.1126/sciadv.aaz8367.

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Topological superconductors are exotic phases of matter featuring robust surface states that could be leveraged for topological quantum computation. A useful guiding principle for the search of topological superconductors is to relate the topological invariants with the behavior of the pairing order parameter on the normal-state Fermi surfaces. The existing formulas, however, become inadequate for the prediction of the recently proposed classes of topological crystalline superconductors. In this work, we advance the theory of symmetry indicators for topological (crystalline) superconductors to cover all space groups. Our main result is the exhaustive computation of the indicator groups for superconductors under a variety of symmetry settings. We further illustrate the power of this approach by analyzing fourfold symmetric superconductors with or without inversion symmetry and show that the indicators can diagnose topological superconductors with surface states of different dimensionalities or dictate gaplessness in the bulk excitation spectrum.

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Nagai, Yuki, Hiroki Nakamura, and Masahiko Machida. "Surface and vortex bound states in topological superconductors." Physica C: Superconductivity 494 (November 2013): 17–19. http://dx.doi.org/10.1016/j.physc.2013.04.018.

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Yang, Jiali, Baobing Zheng, Zhongjia Chen, Wangping Xu, Rui Wang, and Hu Xu. "Robust Topological States in Bi2Se3 against Surface Oxidation." Journal of Physical Chemistry C 124, no. 11 (February 25, 2020): 6253–59. http://dx.doi.org/10.1021/acs.jpcc.0c00458.

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48

Yang, Yihao, Jian-ping Xia, Hong-xiang Sun, Yong Ge, Ding Jia, Shou-qi Yuan, Shengyuan A. Yang, Yidong Chong, and Baile Zhang. "Observation of a topological nodal surface and its surface-state arcs in an artificial acoustic crystal." Nature Communications 10, no. 1 (November 15, 2019). http://dx.doi.org/10.1038/s41467-019-13258-3.

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AbstractThree-dimensional (3D) gapless topological phases can be classified by the dimensionality of the band degeneracies, including zero-dimensional (0D) nodal points, one-dimensional (1D) nodal lines, and two-dimensional (2D) nodal surfaces. Both nodal points and nodal lines have been realized recently in photonics and acoustics. However, a nodal surface has never been observed in any classical-wave system. Here, we report on the experimental observation of a twofold symmetry-enforced nodal surface in a 3D chiral acoustic crystal. In particular, the demonstrated nodal surface carries a topological charge of 2, constituting the first realization of a higher-dimensional topologically-charged band degeneracy. Using direct acoustic field measurements, we observe the projected nodal surface and its Fermi-arc-like surface states and demonstrate topologically-induced robustness of the surface states against disorders. This discovery of a higher-dimensional topologically-charged band degeneracy paves the way toward further explorations of the physics and applications of new topological semimetal phases.

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Yang, ZhiLong, and Hai-Jun Zhang. "Evolution of surface states of antiferromagnetic topological insulator MnBi₂Te₄ with tuning the surface magnetization." New Journal of Physics, July 5, 2022. http://dx.doi.org/10.1088/1367-2630/ac7e64.

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Abstract The interplay between magnetism and topologically non-trivial electronic states is an important subject in condensed matter physics. Recently, the stoichiometric intrinsic magnetic material MnBi2Te4 provides an ideal platform to study the magnetic topological phenomena, such as quantum anomalous Hall effect, axion insulator state, topological magnetoelectric effect. However, it is still controversial whether the topological surface state in the (111) plane is gapped or not. Here, we develop an effective method to study different surface magnetizations based on first-principles calculations. Then we investigate the band dispersions, the Fermi surfaces (FSs), the quasiparticle interferences (QPIs) and the spin texture of topological surface states of MnBi2Te4 with tuning the surface magnetization. We find that the surface magnetization has significant effects on the surface states. Our results also indicate that the symmetry breaking of FSs and QPIs may be a useful way to determine the possible surface magnetization of MnBi2Te4 .

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Zhao, Yulin, Feng Liang, Jianfei Han, Deshuang Zhao, and Bing-Zhong Wang. "Extended terahertz valley-locked surface waves in designer surface plasmon crystals." Journal of Physics D: Applied Physics, March 15, 2023. http://dx.doi.org/10.1088/1361-6463/acc46f.

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Abstract Topological valley-locked edge states have been attracting much attention in terahertz (THz) and optical regimes due to their unique unidirectional backscattering-immune feature. However, these one-dimensional (1D) edge transports are essentially not compatible to traditional waveguides or devices. In this work, we propose a THz topological waveguide supporting two dimensional (2D) valley-locked surface waves based on designer surface plasmon crystals (DSPCs). The waveguide is implemented by designing a sandwich-like A|C|B heterostructure with three domains. The central domain C carrying a Dirac cone in the band structure is topologically trivial. The A and B domains consist of two distinct topological structures with opposite valley-Chern numbers. Unlike topological edge states existing only at the interface of conventional A|B domain wall structure, extended topological valley-locked surface states propagating along the whole B domain are observed in our proposed structure. This heterostructure with designable waveguide width is more flexible for interfacing with existing THz devices, and is quite suitable for high-throughput and high-power-capacity applications. Besides, the unique features of momentum-valley locking and immunity against sharp bends are reserved. This work may promote future topological and traditional integrated functional devices in THz and optical regimes.

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Journal articles on the topic 'Topological surface states'

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