Session X02: Topological Materials -- New Materials I

Observation of topological nodal-loop fermionic state in CaAs3 family

Nontrivial states in topological semimetals are attracting intense interest driven by their numerous novel properties such as high bulk carrier mobility, Fermi arcs, chiral anomaly, large negative magnetoresistance, and high density of states. Here, using high-resolution angle-resolved photoemission spectroscopy (ARPES) along with first-principles calculations, we report the discovery of a topological nodal-loop (TNL) state in CaAs₃ family. Our analysis reveals that surface projections of the bulk nodal-points are connected by surface states. Notably, the observed topological state in CaAs₃ family are well separated from other bands in the vicinity of the Fermi level. CaAs₃ family thus offers a unique materials realization of a prototype nodal-loop semimetal, and a platform for gaining a deeper understanding of the interplay between TNL and topological insulator physics in semimetals.
  

Prediction of a large-gap and switchable Kane-Mele quantum spin Hall insulator from first-principles simulations

Fundamental research and technological applications of topological insulators are hindered by the rarity of materials exhibiting a robust topologically non-trivial phase, especially in two dimensions. Here, by means of extensive first-principles calculations, we propose a novel quantum spin Hall insulator (QSHI) with a sizeable band gap of ∼0.5 eV that is a monolayer of jacutingaite [1, 2], a naturally occurring layered mineral first discovered in 2008 in Brazil [3] and recently synthesised [4]. This system realises the paradigmatic Kane-Mele model for QSHIs in a potentially exfoliable two-dimensional monolayer, with helical edge states that are robust and that can be manipulated exploiting a unique strong interplay between spin-orbit coupling, crystal-symmetry breaking and dielectric response [1]. Finally, we give an update on ongoing experimental efforts in the synthesis and characterisation of bulk and monolayer jacutingaite.

[1] A. Marrazzo et al., Phys. Rev. Lett. 120, 117701 (2018)
[2] N. Mounet et al., Nature Nanotechnology 13, 246 (2018)
[3] A.R. Cabral et al., Terra Nova 20, 32 (2008)
[4] A. Vymazalová et al., Can. Mineral. 50, 431 (2012)   

Clarification of charge-density wave gap symmetry and topology in bulk 1T-TiSe2

We examine the symmetry of the charge density wave (CDW) phase of 1T-TiSe₂ and answer the connection of electronic structures between the normal and the CDW states. Especially, we unravel the consequential irreducible representations (IRs) of electronic states and, more ahead, those of gap functions among bands when the CDW occurs. Normally symmetry-related topology will be achieved directly, so we assert that the theory is valuable and practical for the search of topological CDW insulators.   

Exotic Topological properties in compensated half-metallic inverse-Heusler systems

Recently three-dimensional (3D) topological features mixing with zero-dimensional (0D) band-crossings (Dirac, Weyl, multi-Weyl, and triple-nodal points [TNPs]) have stimulated further interest. An exotic phase, so called nexus fermion, has been expected when a 0D TNP coincides with a 1D nodal line. The nexus fermions are a yet more intricate excitation that have been proposed, but no realistic system has been proposed for such an exotic phase.
In this presentation, we will address a unique topological metallic system displaying simultaneous pure spin WPs, TNPs, nodal loops, and nexus fermions in the absence of (minor) spin-orbit coupling (SOC), in Cr-based inverse-Heusler compensated half-metals with neither inversion nor time-reversal symmetry, using density functional theory-based calculations. This combination gives rise to fully spin polarized nexus fermions, in a system with broken time-reversal symmetry but negligible macroscopic magnetic field. The observed high Curie temperature of 750 K and calculated SOC hybridization mixing of several meV should make these nexus fermions readily measurable.   

Topological Materials

Topological materials host various novel quantum phases of electrons which are characterized by band topology and topologically protected surface/edge states. [1] Despite recent progress, intense world-wide research activity in search of new classes of topological materials is continuing unabated. This interest is driven by the need for materials with greater structural flexibility and tunability to enable viable applications in spintronics and quantum computing. We have used first-principles band theory computations to successfully predict many new classes of 3D topologically interesting materials, including Bi₂Se₃ series, [2] ternary half-Heuslers, [3] TlBiSe₂ family, [4] Li₂AgSb-class, and GeBi₂Te₄ family as well as topological crystalline insulator (TCI) SnTe family [5] and Weyl semimetals TaAs, [6,7] SrSi₂, [8] (Mo,W)Te₂,[9] Ta₃S₂, and LaAlGe family. [10] I will also highlight our recent work on unconventional chiral fermions in RhSi, [11] cubic Dirac points in LiOsO₃, [12], rotaional symmetry protected TCIs [13], and Kramer-Weyl fermions in non-magnetic chiral cyrstals. [14]


[1] A. Bansil, H. Lin, and T. Das, Rev. Mod. Phys. 88, 021004 (2016).
[2] Y. Xia, D. Qian, D. Hsieh, L. Wray, A. Pal, H. Lin, A. Bansil, D. Grauer, Y. S. Hor, R. J. Cava, and M. Z. Hasan, Nat. Phys. 5, 398 (2009).
[3] H. Lin et al., Nat. Mater. 9, 546 (2010).
[4] H. Lin et al., Phys. Rev. Lett. 105, 036404 (2010).
[5] T. H. Hsieh, H. Lin, J. Liu, W. Duan, A. Bansi, and L. Fu, Nat. Commun. 3, 982 (2012).
[6] S.-M. Huang et al., Nat. Commun. 6, 7373 (2015).
[7] S.-Y. Xu et al., Science 349, 613 (2015).
[8] S.-M. Huang et al., Proc. Natl. Acad. Sci. 113, 1180 (2016).
[9] T.-R. Chang et al., Nat. Commun. 7, 10639 (2016).
[10] S.-Y. Xu et al., Sci. Adv. 3, e1603266 (2017).
[11] G. Chang et al., Phys. Rev. Lett. 119, 206401 (2017).
[12] W. C. Yu et al., Phys. Rev. Mater. 2, 051201 (2018).
[13] X. Zhou et al., Phys. Rev. B in press (2018).
[14] G. Chang et al., Nat. Mater. 17, 978 (2018).   

Quantum Oscillations in Trigonal PtBi1.6 Single Crystals

Transition-metal dichalcogenide PtBi₂ is known to form multiple structures with extreme large magnetoresistance (XMR). We have successfully grown high-quality PtBi_1.6 single crystals with the trigonal structure. In addition to the XMR effect, the magnetization exhibits de Haas–van Alphen (dHvA) oscillations at low temperatures. Through Fourier transformation, we identify two oscillation frequencies F_δ = 4 T and F_α = 39 T with the sign of Zeeman splitting. Using Lifshitz–Kosevich (LK) formula to fit experimental data, we obtain the effective masses (m^*_δ = 0.092m₀ and m^*_α = 0.189m₀) and Berry phases (Φ_Bδ = (0.82 + 2δ)π and Φ_Bα = (0.86 + 2δ)π) for both δ and α bands. The results indicate the trigonal PtBi_1.6 has non-trivial topological properties.   

Crystal Structure determination of Ge2Sb2Te5 and Se-doped Ge2Sb2Te5 material

The [GeTe]_n[Sb2Te3]_m layered materials have been widely used as component of data storage devices and electronic memories. Ge₂Sb₂Te₅ is a well-known phase change material that exhibits three phases: an amorphous phase, a rock-salt NaCl-type metastable phase and a hexagonal stable phase. To obtain the amorphous phase in bulk samples, we synthesized Ge₂Sb₂Te_5-xSe_x where we replaced Te with Se. At about 90 % of Se, the system becomes amorphous by quenching. The Se atom function as a stabilizer of the amorphous phase and helps to trigger the local displacements of the Sb and Ge atoms within the unit cell, thus breaking the long-range order and render the system into an amorphous one. The rich physics within this transition will be discussed. Moreover, Ge₂Sb₂Te₅ is associated with naturally occurring stacking faults and a topological phase transition depends sensitively on the distribution of these faults. It has been proposed theoretically that depending on the stacking sequence, GST can undergo a topological phase transition from the low temperature topological insulating phase with the so-called Kooi structure to the high temperature Weyl semi-metal phase with the Ferro structure. We synthesized the Kooi structure and the results from the DFT calculations will be discussed.   

Topological properties are not protected in unstable crystal structures

The ability of DFT to reveal the consequences of assumed crystal symmetries on band inversion, band crossing and degeneracy in solids has led to a plethora of predicted compounds that should be topological insulators(TI’s), Dirac Semimetals(DSM) and unconventional quasiparticles. Before offering predicted compounds for experimental evaluation, it may be wise to use not only the band structures as design filters, but also the DFT total energy (E_tot) of the given crystal. Inspecting E_tot one finds: i) the predicted 3D non-symmorphic DSM band crossing of BiO₂ in assumed SiO₂ structure is dynamically unstable and disappears if its structure is allowed to relax to another polymorph;ii) the predicted 8-fold band degeneracy of CuBi₂O₄ in assumed nonmagnetic configuration disappears once energy-lowering spin polarization is allowed; iii) the predicted oxide TI BaBiO₃ with upshifted Fermi level disappears once allowed to lower its energy in response to n-type doping; iv) the predicted unconventional quasiparticle in Ba₄Bi₃ with downshifted Fermi energy is destabilized by the required p-type doping. Thus, although proven bulk topological characteristics lead to protected surface/edge states, nothing protects bulk states from thermodynamic instability.   

Giant negative magnetoresistance in a topological hourglass candidate

Non-symmorphic materials have been recently predicted to host protected surface fermions displaying hourglass dispersion [1]. Here we report the physical properties of Eu₅In₂Sb₆ single crystals crystallizing in the non-symmorphic space group Pbam (55) [2]. Our results show that Eu₅In₂Sb₆ is a narrow-gap antiferromagnetic insulator that exhibits giant negative magnetoresistance driven by magnetic polarons. We discuss the potential surface states in Eu₅In₂Sb₆, and the role of magnetic correlations on hourglass fermions.

[1] Z. Wang, A. Alexandradinata, R. J. Cava, and B. A. Bernevig, Nature 532, 190 (2016).
[2] Wieder et al., Science 361, 246 (2018).   

Properties of Bi2Se3 and BixSb(2-x)Te3 grown by MBE

We have studied molecular beam epitaxy growth of Bi₂Se₃ and Bi_xSb_(2-x)Te₃. We have demonstrated that by using a two-step growth recipe, very flat films of both materials can be obtained. The film quality was confirmed using RHEED oscillations during growth and, post-growth XRD, AFM, and STEM. Under other growth and processing conditions, we have observed a more complicated film morphology in the growth of Bi_xSb(_2-x)Te₃, which we attribute to compositional phase segregation. Using different substrates strongly impacts the film morphology. By tuning the ratio of Bi to Sb, the carrier type can be tuned from p-type to n-type measured via the Hall effect. The films were also studied by ARPES using a flip-chip cleaving approach in which the film plane at the interface to the substrate is measured after cleaving. The Hall and ARPES results agreed with regard to carrier type.
  

Epitaxial growth of ultrathin Bi2Se3 films as characterized by in situ angle-resolved photoemission spectroscopy

Combining molecular beam epitaxy with time- and angle-resolved photoemission spectroscopy (MBE-trARPES) provides a powerful way to investigate electronic structures of quantum materials. By using our in situ MBE-trARPES system, we measured ultrathin epitaxial films of Bi₂Se₃ grown on sapphire substrate. In its ultrathin limit, i.e. below 6 quintuple layers (QL), topological surface states are gapped due to hybridization and theory predicts the film to become a two-dimensional topological insulator (2D TI) until 3QL. From 3QL to 2QL, topological phase transition to trivial insulator is expected to occur. Previous ARPES experiments explored this phase transition in ultrathin Bi₂Se₃ films grown on bilayer graphene on SiC and Si substrate. In this experiment, we investigated various growth methods for ultrathin (2-6 QL) Bi₂Se₃ film on sapphire substrate: one-step growth, two-step growth, and growth on (Bi_0.5In_0.5)₂Se₃ buffer layer. By measuring ARPES spectra at several positions on the film, we evaluated band structure and uniformity of film. This work provides a guideline for growth of ultrathin Bi₂Se₃ on sapphire substrate for future experiments exploring topological phase transition in ultrathin Bi₂Se₃.   

Crystal phase control in an YBiO3 thin film by using a BaBiO3 buffer layer

Topological insulating materials are very promising for applications in spintronics and quantum computing. The presently confirmed topological insulators are not suitable for room temperature applications. Perovskite oxides are interesting in this respect, since topological insulating phases have theoretically been predicted with large band gaps. YBiO₃ has been predicted to be a topological insulator for the perovskite phase with yttrium and bismuth located at the A-site and B-site, respectively. However, the fluorite phase turns out to be thermodynamically more stable than the perovskite phase. In this work we show that we were able to engineer the perovskite crystal phase of thin film YBiO₃ by using a BaBiO₃ buffer layer and interval deposition. When the YBiO₃ is deposited on top of the BaBiO₃, a single oriented perovskite phase is observed with the expected lattice constants. These findings pave a way towards the fabrication of quantum devices for testing the hypothesized topological insulating phase in YBiO₃.   

Structure-Property relationships and superconductivity in Single Crystals of NbxBi2Ch3, (Ch ≡ S, Se, Te)

Studies of Topological Insulators, possibly a new quantum phase of matter believed to possess a 2-dimensional Fermi surface, have led to a search for other insulators or semi-metals in which topologically non-trivial properties can be tuned using a chemical, structural, or external thermodynamic parameter. Topological insulators such as Bi₂Se₃ and Bi₂Te₃ have attracted much interest due to not just their non-trivial topology, but also due to the appearance of superconductivity upon intercalation or substitution with elements such as Cu, Sr and Nb. Many potentially interesting transition-metal intercalated/doped Bismuth-Chalcogenides have not yet been explored. Here, we discuss our growth and study of Nb_0.2Bi₂Ch₃ crystals (Ch ≡ S, Se, Te). We report resistivity measurements and confirm a superconducting transition near 3K for Nb_0.2Bi₂Se₃. We also report a possible charge density wave (CDW) transition in Nb_0.2Bi₂Te₃. In contrast, Nb_0.2Bi₂S₃ was found to be a good conductor with no phase transitions. Detailed fitting of x-ray diffraction shows that all three families of single crystals have a primary phase of Bi₂Ch₃ and a minor secondary phase of BiNbCh₃. We discuss our results together with Raman Spectroscopy, X-ray Diffraction and Electron Diffraction studies of our crystals.