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Quasi-one-dimensional TaSe3: A New Topological Superconductor Candidate

2020; Elsevier BV; Volume: 4; Issue: 1 Linguagem: Inglês

10.1016/j.matt.2020.11.020

2590-2393

Yan Liang, Jingyue Wang, Hailin Peng,

Electronic and Structural Properties of Oxides

Topological superconductors are a new type of quantum matter with the combination of superconductivity and nontrivial electronic topology. Such a unique class of material has become one of the hottest research topics in condensed matter physics, due to its rich scientific implications (predicted to host exotic emergent particles such as Majorana bound states) and considerable application potentials in fault-tolerant quantum computation. Up to date, only very few materials are experimentally verified as candidates of topological superconductors. Searching for new material system is always the focus in this intriguing field. Recently, Chen et al., for the first time, observed both nontrivial topological surface states and bulk superconductivity in a quasi-one-dimensional compound TaSe3, suggesting a promising new candidate of topological superconductors. Moreover, unlike other existing candidates, the simultaneously combination of stoichiometric, exfoliable, and air-stable properties in TaSe3 makes it an ideal platform for both fundamental studies and further applications. Topological superconductors are a new type of quantum matter with the combination of superconductivity and nontrivial electronic topology. Such a unique class of material has become one of the hottest research topics in condensed matter physics, due to its rich scientific implications (predicted to host exotic emergent particles such as Majorana bound states) and considerable application potentials in fault-tolerant quantum computation. Up to date, only very few materials are experimentally verified as candidates of topological superconductors. Searching for new material system is always the focus in this intriguing field. Recently, Chen et al., for the first time, observed both nontrivial topological surface states and bulk superconductivity in a quasi-one-dimensional compound TaSe3, suggesting a promising new candidate of topological superconductors. Moreover, unlike other existing candidates, the simultaneously combination of stoichiometric, exfoliable, and air-stable properties in TaSe3 makes it an ideal platform for both fundamental studies and further applications. In a topological superconductor (TSC), the opening of superconducting gap is accompanied by the emergence of zero-energy excitations, which are their own antiparticles.1Sato M. Ando Y. Topological superconductors: a review.Rep. Prog. Phys. 2017; 80: 076501Crossref PubMed Scopus (630) Google Scholar These zero-energy excitations are called Majorana bound states (MBSs), which was first raised by Italian scientist Ettore Majorana in the context of elementary particle physics in the 1930s. MBSs obey non-Abelian statistics and is useful as a qubit for topological quantum computation,2Nayak C. Simon S.H. Stern A. Freedman M. Sarma S.D. Non-Abelian anyons and topological quantum computation.Rev. Mod. Phys. 2008; 80: 1083-1159Crossref Scopus (3974) Google Scholar making it one of the most concerned research directions in the condensed matter field in recent decades. Searching for potential material candidates for TSC is key to realize MBSs. A promising ground for topological superconductivity is the odd-parity pairing state, such as two-dimensional (2D) spinless chiral p-wave superconductor.3Fu L. Berg E. Odd-parity topological superconductors: theory and application to CuxBi2Se3.Phys. Rev. Lett. 2010; 105: 097001Crossref PubMed Scopus (600) Google Scholar However, p-wave superconductor is sensitive to impurities and the experimental confirmation of the p-wave symmetry as well as the corresponding topological edge states is challenging, greatly limiting the research progress. In 2008 and 2009, theorists proposed that the superconducting proximity-effect, existing in heterointerface of s-wave superconductors and topological insulators (or semiconductor with Rashba spin-splitting states),4Fu L. Kane C.L. Superconducting proximity effect and majorana fermions at the surface of a topological insulator.Phys. Rev. Lett. 2008; 100: 096407Crossref PubMed Scopus (3397) Google Scholar,5Sato M. Takahashi Y. Fujimoto S. Non-Abelian topological order in s-wave superfluids of ultracold fermionic atoms.Phys. Rev. Lett. 2009; 103: 020401Crossref PubMed Scopus (581) Google Scholar can induce spin non-degenerate topological surface states (TSSs) which superconducts. Such heterointerface is equivalent to a spinless p-wave superconductor and exhibits great potential to host MBSs. This revolutionary idea ignited an international competition to find MBSs in the artificial p-wave superconductor heterointerfaces. Following this regime, a number of platforms for realizing MBSs have been put forward in the laboratory and unambiguous experimental evidences of MBSs are emerging.1Sato M. Ando Y. Topological superconductors: a review.Rep. Prog. Phys. 2017; 80: 076501Crossref PubMed Scopus (630) Google Scholar However, the requirement of low temperature and complicated heterostructures challenged further exploration and applications of MBSs. Later, superconducting proximity effect was generalized to momentum space, and a series of superconductors with TSSs have been explored, containing FeTe0.55Se0.45 (ref. 6Wang D. Kong L. Fan P. Chen H. Zhu S. Liu W. Cao L. Sun Y. Du S. Schneeloch J. et al.Evidence for Majorana bound states in an iron-based superconductor.Science. 2018; 362: 333-335Crossref PubMed Scopus (329) Google Scholar) and 2M phase WS2 (ref. 7Yuan Y.H. Pan J. Wang X.T. Fang Y.Q. Song C.L. Wang L.L. et al.Evidence of anisotropic Majorana bound states in 2M-WS2.Nat. Phys. 2019; 15: 1046-1051Crossref Scopus (45) Google Scholar) with MBSs been observed. However, these compounds are either non-stoichiometric or air-sensitive. The realization of topological superconductivity in an intrinsically stoichiometric, exfoliable, and air-stable material is still essentially lacking. In 2018, a stable and exfoliable superconductor TaSe3 was theoretically proposed as a three-dimensional (3D) strong topological insulator with TSSs8Nie S. Xing L.Y. Jin R.Y. Xie W.W. Wang Z.J. Prinz F.B. Topological phases in the TaSe3 compound.Phys. Rev. B. 2018; 98: 125143Crossref Scopus (28) Google Scholar (illustrated in Figure 1A), suggesting TaSe3 is a promising candidate for TSC. TaSe3 crystallized into a monoclinic layered structure (Figure 1B), which forms the quasi-one-dimensional (quasi-1D) crystal structure along the b axis. Experimentally, the non-trivial Berry phase of TaSe3 has been obtained from analyzing Shubnikov-de Haas (SdH) quantum oscillation measurement, which provides evidence for the existence of a 3D strong topological state.9Xia W. Shi X. Zhang Y. Su H. Wang Q. Ding L. Chen L. Wang X. Zou Z. Yu N. et al.Bulk Fermi surface of the layered superconductor TaSe3 with three-dimensional strong topological state.Phys. Rev. B. 2020; 101: 155117Crossref Scopus (11) Google Scholar However, the direct experimental observation of TSSs is still absent. Recently, writing in Matter, Yulin Chen and colleagues first closed this question by performing complementary synchrotron- and laser-based angle-resolved photoemission spectroscopy (ARPES) measurements, proving TaSe3 as a promising candidate of TSC with great potential for investigating novel physical phenomena.10Chen C. Liang A. Liu S. Nie S. Huang J.W. Wang M.X. Li Y.W. Pei D. Yang H.F. Zheng H.J. et al.Observation of topological electronic structure in quasi-1D superconductor TaSe3.Matter. 2020; 3: 1-11Abstract Full Text Full Text PDF Scopus (12) Google Scholar TaSe3 single crystal used in this work was characterized by single-crystal X-ray diffraction (XRD) and X-ray photoemission spectroscopy (XPS), showing good crystallinity without any identifiable impurity phases. The electronic properties of TaSe3 single crystal were first characterized by magneto-transport measurements, giving superconducting transition temperature of 2 K and two Fermi surface (FS) pockets with area of 0.008 ± 0.001 A-2 and 0.017 ± 0.001 A-2, in line with recent experimental results.9Xia W. Shi X. Zhang Y. Su H. Wang Q. Ding L. Chen L. Wang X. Zou Z. Yu N. et al.Bulk Fermi surface of the layered superconductor TaSe3 with three-dimensional strong topological state.Phys. Rev. B. 2020; 101: 155117Crossref Scopus (11) Google Scholar The morphology of the cleaved surface by scanning tunneling microscopy (STM) revealed clean cleaved surface along the [101¯] direction and quasi-1D TaSe3 chains along the [010] direction (Figure 1C). Besides, scanning tunneling spectroscopy (STS) was performed on the cleaved TaSe3 surface at 1.2 K, suggesting a Bardeen-Cooper-Schrieffer-type (BCS) superconducting gap (Figure 1D). The atomically well-defined surface and robust superconducting features imply TaSe3 as a model platform for further research of novel quantum phenomena such as MBSs. The researchers — who are mainly based at ShanghaiTech University, University of Oxford, and Tsinghua University — performed synchrotron-based ARPES with a serials of photon energies to detect the overall surface and bulk electronic structure of TaSe3. They found that the bulk electron pocket at X˜ point near the FS was intersected with a hole-like bulk band and thus forms the inverted band gap and in-gap TSSs. In order to reveal the details of the inverted band structure and TSSs, Chen et al. further performed laser-based ARPES (hν = 6.994 eV) measurement with high energy and momentum resolutions. The local band gap formed by band inversion was demonstrated as 20 meV. Moreover, the position and dispersive shape of TSSs have also been comprehensively discussed (Figure 1E and F). The TSSs are more dispersive along one high-symmetry directions (X˜−S˜) but form flatter bands along another direction (X˜−Γ˜), showing good agreement with the calculations. The above discussion based on ARPES results thoroughly proved the topological nature of TaSe3. Combined with the electronic transport and STS results, TaSe3 was experimentally demonstrated as an ideal candidate for TSC, providing an ideal platform for realizing MBSs. In summary, the present study by Chen at al. systematically unveiled the inverted band gap and in-gap TSSs in quasi-1D superconductor TaSe3, holding promise for the study of novel phenomena in TSC and device applications. Future synthesis of single crystal with higher quality is also anticipated and the realizing or even manipulating of MBSs in TaSe3 single crystal is expected. Observation of Topological Electronic Structure in Quasi-1D Superconductor TaSe3Chen et al.MatterOctober 8, 2020In BriefOnly a few intrinsic materials have been found to be candidates as a topological superconductor, which has promising research and application potentials. In this work, following the theoretical prediction, we experimentally proved TaSe3 to be a new candidate by proving both the superconductivity and topological nature of the compound. Full-Text PDF Open Archive