Session R14: Topological Materials - Synthesis

Quantized chiral edge conduction on reconfigurable domain walls of a magnetic topological insulator

The theoretical prediction [1] and experimental discovery [2] of quantum anomalous Hall effect (QAHE) in a magnetic topological insulator (TI) have opened a new playground to study the interaction between quantum Hall (QH) physics and magnetism. In quantum anomalous Hall state, the chiral edge state (CES) is predicted to appear not only at the sample edge but also at the magnetic domain wall (DW) between the up and down domains due to the discontinuous change in the Chern number. Here, we design and fabricate the magnetic domains with the tip of the magnetic force microscope (MFM) on Cr doped (Bi_xSb_1−x)₂Te₃ with stable QAHE [3], and confirmed the existence of the CES along the prescribed DWs by in-situ transport measurement. Furthermore, the proof-of-concept devices based on the reconfigurable CES are demonstrated for multiple-domain configurations [4]. The present discovery, combined with the recent spintronic developments based on the spin-momentum locking at the surface state of TI [5,6], would enable all-electrical control of the mobile DW and the CES, leading to the low power-consumption CES-based logic and memory devices in the future. [1] R. Yu et al, Science 329, 61 (2010). [2] C. Z. Chang et al, Science 340, 167(2013). [3] M. Mogi et al., Appl. Phys. Lett. 107, 182401 (2015). [4] K. Yasuda et al., arXiv:1707.09105 (2017). Accepted to Science. [5] A. R. Mellnik et al., Nature 511, 449 (2014). [6] K. Yasuda et al., Phys. Rev. Lett. 119, 137204 (2017).   

Landau levels induced by pseudo-magnetic field in strained topological crystal insulator SnTe thin films

Topological crystalline insulators (TCI) are a class of new quantum phases with their non-trivial topology arisen from crystalline symmetries. One of unique properties of TCIs is the highly tunability under external strain, which can break the crystalline symmetries and hence manipulate the nontrivial topological boundary states. Typically, strain can be used to realize many novel phenomena, such as the mass generation at Dirac points, as well as topological phase transitions. Here, by mean of molecular beam epitaxy, we successfully grow SnTe thin films on SrTiO₃ substrate to fabricate a strained TCI system. Interestingly, pseudo Landau quantization is observed in this system for the first time, induced by a strong uniaxial strain with inhomogeneity. The extracted pseudo magnetic field is estimated to over 100 Tesla, which is the highest record ever reported in the family of the Dirac-cone like linear dispersed surface states materials. Our findings may support the newly developed research field of ‘strain engineering’.
  

Fermi-level tuning in topological insulator (Bi1-xSbx)2Se3 thin films

Fermi-level (E_F) tuning is an indispensable technique for detection of Dirac features in 3D topological insulators (TIs). The alloy system of (Bi_1-xSb_x)₂Te₃ [1] has been a playground to observe quantum transport phenomena at the Dirac surface states owing to good E_F controllability with the Sb content. In this study, we report a successful E_F tuning in another prototypical 3D TI, (Bi_1-xSb_x)₂Se₃(BSS), thin films with Bi/Sb composition ratio in molecular-beam epitaxy growth and electric field effect. So far, the different crystal structures between rhombohedral Bi₂Se₃ and orthorhombic Sb₂Se₃ have limited thin films research based on BSS due to the low solubility limit of Sb into Bi₂Se₃ below x ~ 0.5. By insertion of Bi₂Se₃ buffer layer, the limit can be expanded to x ~ 0.8 [2]. Angle-resolved photoemission spectroscopy revealed Dirac surface states even at x ~ 0.7. We demonstrated ambipolar operation in the top-gate-type field-effect transistors based on the BSS film with x ~ 0.7, indicating electrical tuning of E_F across the Dirac point [3]. Our results will make a step forward to detailed investigation on the Dirac surface states in BSS-based heterostructures. [1] J. Zhang et al., Nat. Comm. 2, 574 (2011). [2] Y. Satake et al., submitted. [3] Y. Satake et al., in preparation.   

Pressure Induced topological phase transition in LaAs

Rare-earth pnictides have emerged as an important class of topological phase materials, where fundamental phenomena associated with band crossings have been predicted. Here, we use density functional theory with the Heyd, Scuseria, and Ernzerhof (HSE06) screened hybrid functional to study the band structure and the effects of spin-orbit coupling in LaAs. In DFT within the generalized gradient approximation (GGA), we find that LaAs is predicted to display a topological phase without any applied pressure due to the overestimated band overlap. The As-related p bands cross the La-related d bands near the X point. Such crossing does not happen when the band overlap is corrected in the HSE06 functional. However, we find that LaAs undergoes a topological phase transition without breaking any symmetry under the effect of hydrostatic pressure at around 7 GPa. The crystal structure of LaAs remains unchanged under applied pressure up to 21 GPa, which is far above the topological phase transition pressure. These findings are important in exploring the novel topological states and their evolution from the viewpoint of topological phase transition in the family of rare-earth pnictides.
  

LaAs: a Key Material to Understanding the Origin of Extreme Magnetoresistance

The first growth and complete study of the LaAs single crystal are presented. LaAs belongs to the lanthanum monopnictide family, where the two heavy members, LaBi and LaSb, have recently attracted much attention due to their non-trivial topology and extreme magnetoresistance (XMR). The origin of XMR is unclear because both of the possible explanations- non-trivial topology and electron-hole compensation- have been shown to exist in LaBi and LaSb. Therefore, it is impossible to recognize which mechanism is responsible for XMR. By studying LaAs crystals, we observed a lack of Dirac cones from ARPES, and ruled out the role of topology in LaAs. From transport measurements, we established the fermiology of LaAs, confirmed it has the same degree of compensation as LaSb and LaBi, and observed all the features of XMR remain in LaAs while the magnitude was reduced by 10²~10³. As a result, we concluded that electron-hole compensation is the true origin of XMR and the mobility mismatch constrains the magnitude of XMR. We also pointed out the difficulty in capturing the band structure using DFT in semimetals with very small Fermi surfaces such as LaAs.   

Electric Field Tuned Quantum Phase Transition from Conventional to Topological Insulator in Few-Layer Na3Bi

Na₃Bi in bulk form represents a zero-bandgap topological Dirac semimetal (TDS), but when confined to few-layers it is predicted that a non-zero bandgap can be opened that in monolayer Na₃Bi is ~300 meV.¹ Application of an electric field to few-layer Na₃Bi has been predicted to induce a topological phase transition from conventional to topological insulator.² However, opening a bandgap in TDS has proven elusive, as efforts to grow thin films have only succeeded in growing 15-20 nm films that remain zero-bandgap semimetals. Here we demonstrate the growth of epitaxial few-layer Na₃Bi via MBE, and probe its electronic structure and response to an electric field using scanning probe microscopy/spectroscopy and angle-resolved photoelectron spectroscopy. We demonstrate a bandgap >400 meV in ultrathin Na₃Bi. Furthermore, via application of an electric field the bandgap can be tuned to semi-metallic and then re-opened (presumably in the quantum spin Hall phase) to greater than 100 meV. The electric fields required to induce this transition are below the breakdown field of many conventional dielectrics, making the creation of a topological transistor based on a few-layer TDS within reach.
¹ C. Niu, et al., Phys. Rev. B 95, 075404 (2017)
² H. Pan, et al., Scientific Reports 5, 14639 (2015)   

Remotely-Doped Sb Quantum-Well Structures

Antimony (Sb) has topological surface states, but its bulk is semimetallic. Our goal is to study the transport properties of the topological states by suppressing the bulk conductivity through quantum confinement and enhancing the surface conductivity through remote n-type doping. A series of Sb quantum-well (QW) structures were grown by molecular beam epitaxy using GaSb barrier layers and GaSb or GaAs substrates. Transport measurements on undoped Sb QWs, 0.7 to 6 nm thick, show a suppression of the bulk conduction. Interpretation of Hall-effect measurements is complicated by the presence of both electrons and holes. We have begun experiments to populate the topological electron states by n-doping the GaSb barrier with Te atoms. We observed that the Hall coefficient at low temperature decreases with increasing distance between the doped layer and the QW. This indicates that the QW is populated by electrons from the dopants in the barrier layer, but parallel conduction by holes is still significant. We will report on the next series of structures with Al(x)Ga(1-x)Sb barriers and optimized doping densities.

  

Crystal Growth of Weyl Semimetals TaAs Family and HgCr2Se4

Weyl fermions are massless chiral fermions that were suggested in quantum field theory more than eight decades ago. Though having not been observed in high-energy physics, Weyl fermions are expected to exist in low-energy excitations of condensed matter systems. TaAs family and HgCr₂Se₄ are two kind of Weyl semimetals that play as the model systems for broken spatial inversion symmetry and broken time-reversal symmetry, respectively. TaAs has been identified as an ideal Weyl semimetal by a series of experiments, while HgCr₂Se₄ is still to be confirmed. For both cases, large-size high-quality crystals are required for ongoing research and future applications. In this work^[1], we address these issues by reporting the chemical vapor transport method for obtaining TaAs single crystals with size of ~10 mm and HgCr₂Se₄ crystals ~5 mm. The crystals have regular morphology and shiny facets, and show good quality in structural, electrical and magnetic characterizations. Growth thermodynamics and crystal morphology are also investigated.

[1] Z. L. Li, H. X. Chen, S. F. Jin et.al., Cryst. Growth Des.16, 3 (2016).   

Quasi-one Dimensional Topological Insulator -Se wires on Ge (111) surface

Topological insulator studies have been done mainly in 2D or 3D systems. However, topological orders can also be defined and preserved in certain one-dimensional chains. We have found a system, Se wires on a Ge(111) surface, that behaves as a quasi-one dimensional topological insulator. The system was fabricated by molecular beam epitaxy and measured by scanning tunneling microscopy. Within the chain, the gap is large while edge states are present at the ends of wires. This system can be understood by extended Su-Schrieffer-Heeger model that successfully explained the conductivity of ploy-acetylene chains. The origin of the topological edge states in this chain is confirmed by the scanning tunneling microscopy results and the density functional calculations. Moreover, we observed defect induced edge states in the chain which can also be described along with the topological insulator model.   

Observation of quantum transport in protected topological states of SnTe thin films

Synthesizing high quality thin films of topological insulators (TIs) and topological crystalline insulators (TCIs) is critical to understanding their unique properties and incorporating them in functional electronic devices. Here we report on efforts to characterize the topological states of the TCI SnTe. High quality, single-domain SnTe films are grown on SrTiO₃ by molecular beam epitaxy. Measurements of carrier density reveal conduction through both bulk and surface states in SnTe films of various thickness. In the thinnest films (6 nm), bulk conduction is largely suppressed, leaving conduction solely through surface states at the SnTe/SrTiO₃ interface. Magnetotransport measurements of the isolated surface states reveal weak antilocalization (WAL) behavior, a signature of two-dimensional topological transport. The WAL is analyzed to understand the nature of the topological transport through the surface states.   

Exotic artificial forms of low-dimensional silicon

The first realization of the archetype epitaxial silicene phase on Ag(111) in 2012 [1], was quickly followed by the synthesis of heavier elemental counterparts of graphene: germanene and stanene [2-4]. Such exotic forms of Si, Ge and Sn, directly compatible with the Si-based technology, may help prolong Moore’s law [5], and find applications in spintronics and quantum computing, as they are predicted to be robust 2D topological insulators, possibly hosting the quantum spin Hall effect at practical temperatures [6]. In my talk, I will describe the growth and enticing properties of novel artificial Si allotropes, realized on Ag(110) surfaces upon further reducing the dimensionality, i.e., 0D, benzene-like, Si nanodots, and a striking manifold of massively parallel 1D pentasilicene-like nanoribbons [7], which can be individually lifted off with an STM tip [8].
[1] P. Vogt et al., Phys. Rev. Lett., 108, 155501 (2012)
[2] M.E. New Journal of Physics 16, 095002 (2014)
[3] F.-f. Zhu et al., Nature Mater., 14, 1020 (2015)
[4] J. Yuhara et al., in revision
[5] L. Tao et al., Nature Nanotechnol., 10, 227 (2015)
[6] M. Ezawa, J. Phys. Soc. Japan, 84, 121003 (2015)
[7] J.L. Cerdá et al., Nature Comm., 7, 13076 (2016)
[8] R. Hiraoka et al., Beilstein J. Nanotechnol., 8, 1699 (2017)
  

Pressure-induced Dirac nodal-loop semimetal and topological phase transition in a single-component molecular crystal, [Pd(dddt)2]

A single-component molecular crystal, [Pd(dddt)₂] is insulating at ambient pressure, but it becomes conducting under a pressure of 12.6 GPa [1]. By performing structural optimization with first-principles calculations, we have ascribed an unusual conducting state in [Pd(dddt)₂], to the formation of Dirac cones under the pressure of 8 GPa. Based on our tight-binding analysis for the optimized structure at 8 GPa, multi-orbital nature (HOMO-LUMO mixing) is essential for the formation of the Dirac cones. The contact points of Dirac cones form a single nodal-loop in Brillouin zone. By including spin-orbit coupling, we find that the electronic state changes from the nodal-loop semimetal to an insulator with a small band gap. To understand whether it is a topological or normal insulator, we calculate Z₂ topological invariants by two methods, Fu-Kane’s TRIM and Fukui-Hatsugai's methods. The calculated Z₂ indices for the band structure at 8 GPa suggest the system is 3D “strong topological insulator”. On the other hand, Z₂ indices calculated for the ambient-pressure band structure show that the system is a normal insulator. Therefore, our results strongly suggest that pressure can induce topological phase transition in this system. [1] R. Kato et al, J. Am. Chem. Soc. 139, 1770 (2017).   

Topological and thermoelectric potential of pnictogen-based double antiperovskites

In spite of several known classes of topological insulators, the search for new ones continue, primarily because most are not insulating enough in the bulk to allow study of their surface bands, nor to consider the anticipated applications. The band gap without spin-orbit coupling (SOC) must be small enough that band inversion can be driven by SOC, but large enough to inhibit bulk conductivity and enable application at room temperature. We present a broad study of pnictide-based double antiperovskites focusing on X₆AA'B₂ materials where X atoms are alkaline earth elements, and A and B are pnictogen atoms. Density functional calculations show that these semiconducting band gaps can be larger than that of corresponding single perovskites, for which we have provided a survey of 75 possibilities[1]. While useful topological characteristics will be difficult to realize, our results indicate that members in this class may be useful in thermoelectric applications.

[1] W. F. Goh and W. E. Pickett, Survey of the class of isovalent antiperovskite alkaline earth-pnictide compounds, arXiv:1710.10716.