Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session F23: Frontiers of Spectroscopy and Topological Materials: DCMP and IUPAP Prize SssionInvited Prize/Award
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Sponsoring Units: DCMP Chair: Alan MacDonald, University of Texas Room: New Orleans Theater B |
Tuesday, March 14, 2017 11:15AM - 11:51AM |
F23.00001: IUPAP C-10 Award Talk: From Topological Insulators to Quantum Anomalous Hall Effect Invited Speaker: Cui-Zu Chang The quantum anomalous Hall (QAH) effect can be considered as the quantum Hall (QH) effect without external magnetic field, which can be realized by time reversal symmetry breaking in a topologically non-trivial system [1, 2]. A QAH system carries spin-polarized dissipationless chiral edge transport channels without the need for external energy input, hence may have huge impact on future electronic and spintronic device applications for ultralow-power consumption. The many decades quest for the experimental realization of QAH phenomenon became a possibility in 2006 with the discovery of topological insulators (TIs). In 2013, the QAH effect was observed in thin films of Cr-doped TI for the first time [3]. Two years later in a near ideal system, V-doped TI, contrary to the negative prediction from first principle calculations, a high-precision QAH quantization with more robust magnetization and a perfectly dissipationless chiral current flow was demonstrated [4]. In this talk, I will introduce the route to the experimental observation of the QAH effect in above-mentioned two systems [3, 4], and discuss the zero magnetic field dissipationless edge current flow as well as the origin of the dissipative channels in the QAH state [5]. Finally I will talk about our recent progress on the QAH insulator-Anderson insulator quantum phase transition and its scaling behaviors [6]. References [1] F. D. M. Haldane, Phys. Rev. Lett. 61, 2015-2018 (1988). [2] R. Yu et al, Science 329, 61-64 (2010). [3] Cui-Zu Chang et al, Science 340, 167(2013). [4] Cui-Zu Chang et al, Nature Materials 14, 473(2015). [5] Cui-Zu Chang et al, Physics Review Letters 115, 057206 (2015). [6] Cui-Zu Chang et al, Physics Review Letters 117, 126802 (2016).. [Preview Abstract] |
Tuesday, March 14, 2017 11:51AM - 12:27PM |
F23.00002: Weyl and Heusler compounds Invited Speaker: Claudia Felser Topological insulators (TIs), Weyl and Dirac semimetals are new quantum states of matter. Heusler compounds are a remarkable class of materials which exhibit a wide range of multifunctionalities including tunable topological insulators [1]. The required band inversion has already been unambiguously identified by angle-resolved photoemission [2]. Weyl and Dirac semimetals open up new research directions and applications that result from the large Berry phases that they exhibit: these lead to giant anomalous Hall effect (AHE) and spin Hall effects [3]. In the C1b Heusler compounds, the inclusion of rare earth atoms allows the use of magnetic exchange fields to induce Weyl points [4] in magnetic fields, which break time-reversal symmetry. In GdPtBi several signatures of a Weyl semimetal have been observed, ranging from a large longitudinal negative Magnetoresistance, to an AHE and a Seebeck effect [4]. Recently Co$_{\mathrm{2}}$TiSn and other Co$_{\mathrm{2}}$-Heusler compounds were found to be Weyl semimetals [5]: these materials have an energy-gap for one spin orientation and crossing points in the other spin direction. The Berry phase induces a giant AHE in these ferromagnets. However, even antiferromagnetic Heusler compounds can be designed with large Berry phases as a consequence of Weyl points close to the Fermi energy [6]: this has recently been proven via a giant AHE for single crystals of Mn$_{\mathrm{3}}$Sn and Mn$_{\mathrm{3}}$Ge [7]. [1] Chadov, et al., Nat. Mat.. 9,541 (2010), Lin, et al., Nat. Mat. 9, 546 (2010) [2] Liu, et al., N. Nat. Com. 7 12924 (2016) [3] Sun, et al., arXiv:1604.07167 [4] Hirschberger et al. Nat. Mat. (2016) Shekhar, et al. arXiv: 1604.01641 [5] Wang et al., arXiv:1603.00479, K\"{u}bler and Felser, EPL 114, 47005 (2016) [6] K\"{u}bler and Felser, EPL 108 67001 (2014), Zhang, et al., arXiv:1610.04034 [7] Nayak, et al., Science Advances 2 e1501870 (2016) , Nakatsuji, Kiyohara and Higo, Nature 527 212 (2015) [Preview Abstract] |
Tuesday, March 14, 2017 12:27PM - 1:03PM |
F23.00003: Algebra, topology, and the solid state: New perspectives on insulators and semimetals Invited Speaker: Barry Bradlyn The interplay of topology and geometry has been -- and continues to be -- a rich area of study for condensed matter physics. Recently, we have realized that spatial symmetries allow for the stabilization of topological phases much more exotic than those that can be found with time-reversal symmetry alone. Examples include topological crystalline insulators, "hourglass Fermion" phases, and Dirac and double-Weyl semimetals. However, a complete and unified theory of these phases is still missing. In this talk, I will examine topological metals and insulators stabilized by any of the 230 crystal symmetry groups. I will develop a topological band theory that relates the symmetry properties of real space Wannier functions to the global topology of energy bands in momentum space. From this I will derive a predictive classification of topological crystalline phases, well suited for both predictions and ab-initio materials searches. Focusing first on insulating phases, I will show how our topological band theory sheds new light on old topological insulators, before moving on to present a new slew of topological insulators that we have predicted with our method. Additionally, I will show how non-symmorphic crystal symmetries can protect topological insulators with novel surface states, through symmetry constraints on the band structure; this includes a new toplogical phase whose surface spectrum consists of a single four-fold degenerate Dirac fermion. Moving on to topological semimetals, I will show how these same non-symmorphic symmetries require the existence of gapless free-fermion excitations unlike any found in high-energy physics. This includes the first natural generalization of the Weyl fermion, described by a k⋅S Hamiltonian. [Preview Abstract] |
Tuesday, March 14, 2017 1:03PM - 1:39PM |
F23.00004: The Art of Photoelectron Spectroscopy, from Micro to Nano Invited Speaker: Eli Rotenberg Angle-resolved photoemission spectroscopy (ARPES) was developed for the determination of the electronic bandstructure of solids. In the last 20 years, ARPES has become nearly unlimited with respect to instrumental resolution, and therefore able to illuminate more subtle electronic aspects, such as ground-state symmetry breaking and the many-body interactions (MBIs) that characterize ground states such as superconductivity. These MBIs involve exchange of momentum among electrons or with excitations such as phonons, and can therefore couple to nanoscale structures. By controlling the structure at the nanoscale, we can therefore hope to control or enhance the ground state properties of materials through nanoscale engineering. This dream has motivated the development of nanoscale ARPES (nanoARPES) machines that are now coming online worldwide. After a brief overview, I will show the latest results from the new nanoARPES endstation at the MAESTRO facility (Microscopic and Electronic Structure Observatory), a new user beamline commissioned this year at the Advanced Light Source (ALS). We achieved routine operation at spatial resolution around 120 nm, and expect improvement down to 50 nm or better. Examples will include graphene and 2D-metal-chalcogenide heterostructures. I will also discuss the prospects for dramatic improvements expected as new diffraction-limited light sources such as the ALS-U project are realized. [Preview Abstract] |
Tuesday, March 14, 2017 1:39PM - 2:15PM |
F23.00005: Surface states, skyrmions, and synchrotrons Invited Speaker: Stephen Kevan Over the past several decades, soft x-ray techniques have grown from esoteric spectroscopies applied largely to probe model systems to an indispensable suite of scattering, spectroscopy, and microscopy tools used to characterize and understand the structure and emergent properties of materials that lie at many frontiers of modern condensed matter physics. I will offer a brief historical perspective on this evolution in the area of angle-resolved photoemission, then describe recent soft x-ray experiments on magnetic skyrmions and related structures, and end with some thoughts about the very bright future of soft x-ray science in general. [Preview Abstract] |
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