Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session V14: Topological Materials  SpectroscopyFocus

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Sponsoring Units: DMP Chair: Yulin Chen, University of Oxford Room: LACC 304B 
Thursday, March 8, 2018 2:30PM  3:06PM 
V14.00001: Topological electronic structures in metallic phases Invited Speaker: Yulin Chen Following the discovery of 2D and 3D topological insulators, in the past few years, topological electronic structures in metallic phases were discovered and actively investigated, such as the topological Dirac/Weyl semimetals and topological linenode semimetals. These new topological phases can host interesting exotic particles and unusual physical phenomena (such as Weyl fermions, surface Fermiarcs, negative magnetoresistance, chiral magnetic effects and topological superconductivity, etc.) which are not only interesting in fundamental physics, but also attractive to novel future applications. In this talk, I will discuss how to identify the nontrivial bulk and surface topological electronic structures in these interesting metallic phases by angel resolved photoemission spectroscopy (i.e. ARPES) in our recent works. Furthermore, I will also briefly introduce the recently advances of ARPES (with spatial, spin and time resolution) which can be used in the future investigations on topological electronic structures, even along the edges and in the interfaces of materials. 
Thursday, March 8, 2018 3:06PM  3:42PM 
V14.00002: Novel electronic states of topological nodal semimetals studied by ARPES Invited Speaker: Takafumi Sato Search for new types of nodal fermions, such as Dirac and Weyl fermions, associated with the crystal symmetry is currently a hot topic in topological materials science. ARPES is an excellent experimental technique to verify new topological materials owing to its unique capability to directly visualize the Diraccone energy bands. In this talk, we will present novel electronic states of topological semimetals associated with the mirror and nonsymmorphic symmetry of crystal, such as trigonal tellurium with Weyl nodes protected by screw symmetry [1], CaAgAs with a line node associated with mirror symmetry [2], and linenode semimetal HfSiS showing unexpected Diracnode arc [3]. We will also discuss interplay between nodal fermions and crystal symmetry in comparison with other topological materials [4,5]. 
Thursday, March 8, 2018 3:42PM  3:54PM 
V14.00003: Detection of SpinMomentum Locking in Topological Insulators with SpinPolarized FourProbe STM Saban Hus, Xiaoguang Zhang, Corentin Durand, Wonhee Ko, Yong Chen, AnPing Li The spinmomentum locking in topological insulators (TIs) does not only provide a topological protection against backscattering but also ensures a spinpolarized surface conductance. The electrical detection of such a spinmomentumlocking, however, remains elusive. We developed a spinpolarized fourprobe STM by combining spinpolarized STM and fourprobe STM to achieve spinsensitive multiprobe characterization. Here we report its applications in detecting spin current on pristine TI surfaces. By examining the potential difference between a ferromagnetic probe and a nonmagnetic probe as a function of the spatial separation of them, not only can we differentiate surface and bulk conductance [Nano Lett. 16, 2213 92016)], but also isolate the nonvanishing spindependent electrochemical potential from the Ohmic contribution [Phys. Rev. Lett. 119, 137202 (2017)]. The measured spin chemical potentials directly come from the 2D charge current. In this manner, we achieved a direct and quantitative measurement of spin current generation efficiency, which opens a new avenue to access the intrinsic spin transport associated with pristine topological surface states. 
Thursday, March 8, 2018 3:54PM  4:06PM 
V14.00004: Observation of a Topological Insulator Dirac Cone Reshaped by Nonmagnetic Impurity Resonance Lin Miao, Yishuai Xu, Wenhan Zhang, Daniel Older, Stanley Breitweiser, Haowei He, Takehito Suzuki, Jonathan Denlinger, Rudro Biswas, Joseph Checkelsky, Weida Wu, Lewis Wray The helical Dirac cone states on the surface of topological insulator (TI) are thought to be insensitive to weak, nonmagnetic disorder. Recently, theoretical modeling has shown that, without violating the time reversal symmetry, randomly distributed point defects/impurities of a crystal lattice can give rise to resonance states that greatly change the Dirac surface state dispersions and the physical nature of the lowenergy quasiparticles. In this talk, I will present our linear dichroic angular resolved photoemission spectroscopy (LDARPES) study on disorderenriched Bi_{2}Se_{3}. The LDARPES spectra reveal a large anomalous kinklike feature in the Dirac cone dispersion, which matches the predicted signature of coherent hybridization with an impurity resonance. The dispersion anomaly is found to be associated with a DOS peak, and both features are progressively attenuated when successive low temperature annealing is applied to reduce disorder. The experimental observation of these features, which closely resemble theoretical predictions, clarifies the interplay between a topological Dirac cone and the pointdefect disorder common in applied systems. 
Thursday, March 8, 2018 4:06PM  4:18PM 
V14.00005: Dynamic axion field in a trivial magnetoelectric insulator Junlei Wang, Christian Binek It has been shown that the magnetoelectric susceptibility in the topologically trivial insulator Cr_{2}O_{3} can be decomposed into a pseudoscalar component and a tensor component. The former component has been identified as the axion piece. We use a nonlinear magnetooptical effect to experimentally investigate the axion and tensor components of the magnetoelectric susceptibility. Fluctuations of the axion piece originate from fluctuations of the antiferromagnetic order parameter. The fluctuations of the axion piece contribute to fluctuations in the electric field induced magnetization. Measuring the latter as a function of temperature allows separating pseudoscalar and tensor contributions. 
Thursday, March 8, 2018 4:18PM  4:30PM 
V14.00006: Universal Faraday Effect in 3D Topological Insulator Alexey Shuvaev Topological insulators and related materials have become a hot topic 
Thursday, March 8, 2018 4:30PM  4:42PM 
V14.00007: Scanning Tunneling Spectroscopic Studies of Surface States on Emergent Topological Insulator BiTeI Jianfeng Ge, Cyrus Hirjibehedin, Daniel Larson, Yang He, Zhihuai Zhu, Mohammed Saghir, Geetha Balakrishnan, Jennifer Hoffman We use scanning tunneling microscopy and spectroscopy to investigate the emergent topological insulator BiTeI. We discover three different kinds of surfaces where the Fermi level lies above, below, and inside the band gap. By imaging the real and momentumspace differential conductance, we observe charging effects of acceptortype defects, as well as quasiparticle interference (QPI) up to 1.2 eV above the Fermi level. Furthermore, QPI modes reveal the absence of backscattering, which implies that Rashba spin splitting persists up to this high energy. 
Thursday, March 8, 2018 4:42PM  4:54PM 
V14.00008: PhononMediated Excited State Decay in High Energy Unoccupied Bands of the Topological Insulator Bi_{1.5}Sb_{0.5}Te_{1.7 }Se_{1.3} Daniel Nevola, Omadillo Abdurazakov, Jonathan Boltersdorf, Alexander Bataller, Kenan Gundogdu, Paul Maggard, Alexander Kemper, Daniel Dougherty

Thursday, March 8, 2018 4:54PM  5:06PM 
V14.00009: Scanning tunneling spectroscopy of Dirac electrons in doped SnTe Yuwen Hu, Dillon Wong, Benjamin Feldman, Mallika Randeria, Hao Ding, Satya Kushwaha, Robert Cava, Ali Yazdani SnTe has attracted interest both as a ferroelectric material and because it is a topological crystalline insulator with Dirac surface states. However, due to the propensity for Sn vacancies, the Dirac point is generally well above the Fermi level in asgrown SnTe crystals and therefore difficult to experimentally access. Here we study doped SnTe samples using a scanning tunneling microscope. Quasiparticle interference measurements reveal the surface state dispersion and show that the Dirac point is shifted closer to the Fermi level. We further explore the behavior of these Dirac electrons in the presence of a magnetic field. This regime is particularly interesting in SnTe because it has multiple degenerate valleys, each of which lacks twofold rotational symmetry. Therefore, valley polarized electronic states at odd integer filling factors are anticipated to possess an inplane dipole moment, giving rise to a quantum Hall ferroelectric phase. 
Thursday, March 8, 2018 5:06PM  5:18PM 
V14.00010: Circular photogalvanic effect in magneticallydoped topological insulator thin films Suelen De Castro, Timothy Pillsbury, Anthony Richardella, Nitin Samarth There is growing interest in inducing magnetic order in topological insulators (TIs) to pursue novel spintronic devices in the emerging field of ‘topological spintronics.’ Currently, there are two methods for achieving this: TI films can be doped with magnetic elements or interfaced with a magnetic material. It is important in this context to develop probe of the magnetism of a topological spintronic device using methods that directly involve the helical Dirac surface states. The circularphotogalvanic effect (CPGE) in TI films provides a natural route for achieving this goal. CPGE involves the preferential optical excitation of charge carriers via transitions that depend on the helicity of the incident light. The effect has been thoroughly studied recently in nonmagnetic TIs [Nat. Comm. 8, 1037 (2017)]. Here, we discuss how the CPGE changes with magnetic doping by studying the phenomenon in Crdoped (Bi,Sb)_{2}Te_{3} thin films as a function of temperature, chemical potential, and magnetic field. We compare these measurements to other magnetically dependent photocurrents in these samples. Ultimately, this will offer a new way to probe and control magnetism in future topological spintronic devices. 
Thursday, March 8, 2018 5:18PM  5:30PM 
V14.00011: Topofermiology: revealing the topology of Fermisurface wavefunctions from magnetic oscillations Aris Alexandradinata, Leonid Glazman, Wang Chong In fermiology, the shape of the Fermi surface is deducible from the period of fieldinduced oscillations of the magnetization (dHvA) and resistivity (SdH). Here, we propose that the topology of the Fermisurface wavefunctions are deducible from the phase offset $\lambda$ of these oscillations. $\lambda$ encodes not just the Berry phase, but also the orbital magnetic moment of wavepackets about their center of mass. In some metals and for certain field orientations, the symmetry of the orbit fixes $\lambda$ to integer multiples of $\pi$, i.e., $\lambda$ are the topological invariants of magnetotransport. Our comprehensive symmetry analysis identifies any metal for which $\lambda$ is a topological invariant, as well as identifies the symmetryenforced degeneracy of Landau levels. The analysis is simplified by our formulation of ten (and only ten) symmetry classes for closed, Fermisurface orbits. Case studies are discussed for graphene, transition metal dichalchogenides, crystalline/$\mathbb{Z}_2$ topological insulators, 3D Weyl/Dirac metals. We caution that the phase offset in the fundamental dHvA/SdH harmonic should not be viewed as a smoking gun for 3D Dirac metals. Alternative methods to extract $\lambda$ include the scanningtunneling microscope and planar tunneling junction. 
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