Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session L44: Dirac and Weyl Semimetals: ARPESFocus
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Sponsoring Units: DMP Chair: Dan S. Dessau, University of Colorado, Boulder Room: 391 |
Wednesday, March 15, 2017 11:15AM - 11:51AM |
L44.00001: Observation of Weyl fermions, Fermi arcs and chirality in Weyl semimetals Invited Speaker: Suyang Xu A Weyl semimetal is a novel topological phase that hosts Weyl fermions as emergent quasiparticles and admits a topological classification that protects Fermi arc surface states. We report our latest theoretical and experimental progress on topological semimetals. First, we present a methodology for searching for topological semimetals by sifting through the materials parameter space, using which we show the theoretical identification of a large number of candidate materials with distinct topological properties. Second, we show photoemission spectroscopy measurements. By measuring the bulk and surface band structures, we directly observe the Weyl fermions and the Fermi arcs in both type-I and type-II Weyl semimetals. Third, we report on the mid-infrared optoelectronic response of the Weyl semimetal TaAs. We show that the coupling between Weyl fermions and chiral photons leads to a giant photocurrent, which allows us to detect the chirality of the Weyl fermions in TaAs. Our theory and photoemission experiments discover the first Weyl semimetal in real materials, and our photocurrent results represent an initial attempt to control and manipulate the Weyl fermions and the associated quantum anomalies by electrical and optical means. [Preview Abstract] |
Wednesday, March 15, 2017 11:51AM - 12:03PM |
L44.00002: Discovery of Lorentz-violating type-II Weyl fermions in LaAlGe Daniel S. Sanchez, Su-Yang Xu, Nasser Alidoust, Guoqing Chang, Hong Lu, Bahadur Singh, Ilya Belopolski, Xiao Zhang, Guang Bian, Hao Zheng, Marius-Adrian Husanu, Yi Bian, Shin-Ming Huang, Chuang-Han Hsu, Tay-Rong Chang, Horng-Tay Jeng, Arun Bansil, Titus Neupert, Vladimir N. Strocov, Hsin Lin, Shuang Jia, M. Zahid Hasan Recently, a few crystal candidates were predicted to be type-II Weyl semimetals, but have yet to be conclusively confirmed with conventional ARPES techniques. Here we present the first direct experimental observation of a type-II Weyl semimetal in LaAlGe by using ARPES to resolve its type-II Weyl nodes and cones. Unlike the previously proposed type-II Weyl semimetals, the type-II Weyl nodes in LaAlGe are observed to be located at the Fermi level and, therefore, dominate the low-energy physics. [Preview Abstract] |
Wednesday, March 15, 2017 12:03PM - 12:15PM |
L44.00003: A minimal ``hydrogen atom" version of a time reversal invariant Weyl semimetal Songtian S. Zhang, Ilya Belopolski, Peng Yu, Daniel S. Sanchez, Yukiaki Ishida, Tay-Rong Chang, Su-Yang Xu, Daixiang Mou, Hao Zheng, Guoqing Chang, Guang Bian, Horng-Tay Jeng, Takeshi Kondo, Adam Kaminski, Hsin Lin, Zheng Liu, Shik Shin, Zahid Hasan Typical Weyl semimetals host many Weyl points; for instance, TaAs has 24 Weyl points. However, the minimal nonzero number of Weyl points for a time reversal invariant crystal is four. Finding such a material would provide the simplest ``hydrogen atom" example of a Weyl semimetal. Applications may also be simpler in a system with as few Weyl points as possible. TaIrTe$_4$ is predicted to be a minimal Weyl semimetal. However, the Weyl points are well above the Fermi level, making them inaccessible to conventional ARPES. Here, we use pump-probe ARPES to directly observe Weyl points and topological Fermi arcs in TaIrTe$_4$ above the Fermi level. We find a total of four Weyl points, demonstrating the first minimal Weyl semimetal. [Preview Abstract] |
Wednesday, March 15, 2017 12:15PM - 12:27PM |
L44.00004: Time-resolved ARPES study of the candidate type II Weyl-semimetal WTe2 Elia Razzoli, F. Boschini, M. Michiardi, M. Zonno, P. Nigge, B. Zwartsenberg, G. Levi, B. Yan, V. Suess, C. Felser, A. K. Mills, D. J. Jones, A. Damascelli Since their theoretical prediction in 1929, Weyl fermions have eluded the high-energy physics community. Only very recently quasiparticle behaving like Weyl fermions have been identified first in the semimetal TaAs, then in various compounds such as NdAs, TaP and NdP [1]. Soon after these observations it was realized that, contrary to high energy physics particles, Weyl quasi-particles in solids can break Lorentz invariance. Materials hosting these novel quasiparticles are named type-II Weyl semimetals and WTe2 was proposed as the first realization of such exotic state of matter [2]. In this contribution we will present a pump-probe time-resolved ARPES study to investigate the relaxation dynamics in the candidate type II Weyl-semimetal WTe2. We identify fast and slow components in the electron relaxation dynamics, which display a strong dependence on both the electron momentum k and temperature. The interplay between the temperature evolution of this dynamics, details of the electronic band structure, and putative Weyl quasiparticle behaviour, will be discussed. [1] Z. K. Liu et al., Nature Materials 15, 27--31 (2016). [2] A. A. Soluyanov et al., Nature 527, 495--498 (2015). [Preview Abstract] |
Wednesday, March 15, 2017 12:27PM - 12:39PM |
L44.00005: Elemental topological Dirac semimetal: $\alpha $-Sn on InSb(111) Cai-Zhi Xu, Peng Chen, Xiao-Xiong Wang, Man-Hong Wong, Joseph Hlevyack, Tai-Chang Chiang, Yang-Hao Chan, Mei-Yin Chou, Yige Chen, Hae-Young Kee, Hyejin Ryu, Nobumichi Tamura, Zahid Hussain, Sung-Kwan Mo, Catherine Dejoie Three-dimensional (3D) topological Dirac semimetals (TDSs) feature 3D Dirac fermions associated with bulk states and nontrivial surface states near the Fermi level. With their unique electronic structure, 3D TDSs show a number of exotic electronic properties and provide a versatile platform for realizing other novel electronic phases and exploring topological phase transitions. While previously discovered 3D TDSs have complicated chemical structures, here we report the first realization of such phase in an elemental form, $\alpha $-Sn films epitaxially-grown on InSb(111). Using angle-resolved photoemission spectroscopy (ARPES), we have observed 3D Dirac cones in this system. As the film thickness is reduced the character of the Dirac cone changes from 3D to 2D. The epitaxial strain in our $\alpha $-Sn films is key to the formation of the observed TDS phase as shown by first-principles calculations. Our results suggest ample opportunities for engineering the electronic and topological properties of thin films through thickness and strain control. [Preview Abstract] |
Wednesday, March 15, 2017 12:39PM - 12:51PM |
L44.00006: Trivial and topological Fermi arcs in the type-II Weyl semimetal candidate MoTe2 Anna Tamai, QuanSheng Wu, Irene Cucchi, Flavio Bruno, Celine Barreteau, Enrico Giannini, Alexey Soluyanov, Felix Baumberger Weyl semimetals are commonly identified by detecting their characteristic open surface state Fermi arcs in angle-resolved photoemission (ARPES) experiments. However, in type-II Weyl semimetals the Fermi arcs generally disappear in the bulk carrier pockets before reaching the Weyl points where they terminate - making it harder to unambiguously identify this new electronic state. Using laser-based ARPES, we have resolved multiple distinct Fermi arcs on the inequivalent top and bottom (001) surfaces of the candidate type-II Weyl semimetal MoTe$_{2}$. By comparing our ARPES data with systematic electronic structure calculations simulating different Weyl point arrangements, we show that some of these arcs are “false positives” as they can be explained without Weyl points, while others are only reproduced in scenarios with at least eight Weyl points. Our results thus suggest that MoTe$_{2}$ is the first experimental realisation of a type-II Weyl semimetal. [Preview Abstract] |
Wednesday, March 15, 2017 12:51PM - 1:03PM |
L44.00007: Experimental Observation of Three-Component ‘New Fermions’ in Topological Semimetal MoP B.Q. Lv, Z.-L. Feng, Q.-N. Xu, J.-Z. Ma, L.-Y. Kong, Pierre Richard, Y.-B. Huang, V. N. Strocov, C. Fang, H.-M. Weng, Y.-G. Shi, Tian Qian, Hong DIng Condensed matter systems can host quasiparticle excitations that are analogues to elementary particles such as Majorana, Weyl, and Dirac fermions. Recent advances in band theory have expanded the classification of fermions in crystals, and revealed crystal symmetry-protected electron excitations that have no high-energy counterparts. Here, using angle-resolved photoemission spectroscopy, we demonstrate the existence of a triply degenerate point in the electronic structure of MoP crystal, where the quasiparticle excitations are beyond the Majorana-Weyl-Dirac classification. Furthermore, we observe pairs of Weyl points in the bulk electronic structure coexisting with the ‘new fermions’, thus introducing a platform for studying the interplay between different types of fermions. [Preview Abstract] |
Wednesday, March 15, 2017 1:03PM - 1:15PM |
L44.00008: Weyl semimetals with a boundary at $z=0$- a photoemission study David Schmeltzer We consider a Weyl semimetal Hamiltonian with two nodes and derive the scattering Hamiltonian in the presence of a boundary at $z=0$. We compute the photoemission spectrum and demonstrate the presence of the Fermi arcs which connect the two nodes. We observe the one dimensional chiral anomaly in the presence of an electric field parallel to the scattering surface. [Preview Abstract] |
Wednesday, March 15, 2017 1:15PM - 1:27PM |
L44.00009: Distinct evolutions of Weyl fermion quasiparticles and Fermi arcs with bulk band topology in Weyl semimetals Nan Xu, Gabriel Autes, Christian Matt, Baiqing Lv, Federico Bisti, Vladimir Strocov, Dariusz Gawryluk, Ekaterina Pomjakushina, Kazimierz Conder, Nicholas Plumb, Milan Radovic, Tian Qian, Oleg Yazyev, Joel Mesot, Hong Ding, Ming Shi By performing ARPES and first-principle calculations, we demonstrate that Weyl fermions quasiparticles in bulk and Fermi arc on surface show distinct evolutions with the bulk band topology in transition-metal monophosphides. While Weyl fermion quasiparticles exist only when the chemical potential is located between two saddle points of the Weyl cone features, the Fermi arc states extend in a larger energy scale and are robust across the bulk Lifshitz transitions associated with the recombination of two non-trivial Fermi surfaces enclosing one Weyl point into a single trivial Fermi surface enclosing two Weyl points of opposite chirality. Therefore, in some systems (NbP), Fermi arc states are preserved even if Weyl fermion quasiparticles are absent in the bulk. Our findings not only provide insight into the relationship between the exotic physical phenomena and the intrinsic bulk band topology in Weyl semimetals, but also resolve the apparent puzzle of the different magneto-transport properties observed in TaAs, TaP and NbP, where the Fermi arc states are similar. [Preview Abstract] |
Wednesday, March 15, 2017 1:27PM - 1:39PM |
L44.00010: Surface Fermi arc connectivity in the type-II Weyl semimetal candidate WTe$_2$ Oliver Rader, Jaime S\'anchez-Barriga, Maia G. Vergniory, Daniil Evtushinsky, Irene Aguilera, Andrei Varykhalov, Stefan Bl\"ugel We perform ultrahigh-resolution angle-resolved photoemission experiments at a temperature T = 0.8 K on the type-II Weyl semimetal candidate WTe$_2$. We find a surface Fermi arc connecting the bulk electron and hole ockets on the (001) surface. Our results show that the surface Fermi arc connectivity to the bulk bands is strongly mediated by distinct surface resonances dispersing near the border of the surface-projected bulk band gap. By comparing the experimental results to first-principles calculations, we argue that the coupling to these surface resonances, which are topologically trivial, is compatible with the classification of WTe$_2$ as a type-II Weyl semimetal hosting topological Fermi arcs. We further support our conclusion by a systematic characterization of the bulk and surface character of the different bands and discuss the similarity of our findings to the case of topological insulators. [Preview Abstract] |
Wednesday, March 15, 2017 1:39PM - 1:51PM |
L44.00011: Discovery of a new type of topological Weyl fermion semimetal in Mo$_x$W$_{1-x}$Te$_2$ Ilya Belopolski, Daniel S. Sanchez, Yukiaki Ishida, Xingchen Pan, Peng Yu, Su-Yang Xu, Guoqing Chang, Tay-Rong Chang, Hao Zheng, Nasser Alidoust, Guang Bian, Madhab Neupane, Shin-Ming Huang, You Song, Haijun Bu, Guanghou Wang, Shisheng Li, Goki Eda, Horng-Tay Jeng, Takeshi Kondo, Hsin Lin, Zheng Liu, Fengqi Song, Shik Shin, M. Zahid Hasan The Mo$_x$W$_{1-x}$Te$_2$ series hosts Type II, or strongly Lorentz-violating, Weyl fermions and is promising for novel electronics. However, the Weyl cones and Fermi arcs are predicted to sit above the Fermi level, making them strictly inaccessible to conventional ARPES. Here, we use pump-probe ARPES to image the unoccupied band structure of Mo$_x$W$_{1-x}$Te$_2$. We directly observe topological Fermi arcs. Unexpectedly, we find that certain Weyl points are close to the Fermi level, making Mo$_x$W$_{1-x}$Te$_2$ uniquely suited to transport and device applications of Weyl semimetals. [Preview Abstract] |
Wednesday, March 15, 2017 1:51PM - 2:03PM |
L44.00012: Time- and angle-resolved photoemission studies of ZrTe5 Changmin Lee, Fahad Mahmood, Takehito Suzuki, Joseph Checkelsky, Nuh Gedik With a surge of interest in various topological states of matter, ZrTe$_{\mathrm{5}}$ has been recently reinvestigated due to its unique electronic and optical properties. While there is still an ongoing debate about whether ZrTe$_{\mathrm{5}}$ is a strong/weak topological insulator or a Dirac semimetal, time-resolved studies and information about the conduction band have been lacking. Here, we report time- and angle-resolved photoemission measurements performed on single crystals of ZrTe$_{\mathrm{5\thinspace }}$with various pump laser pulses with energies ranging from near- to mid-infrared. Our results reveal relaxation dynamics across different energies and set a limit on the topological classification of ZrTe$_{\mathrm{5}}$. [Preview Abstract] |
Wednesday, March 15, 2017 2:03PM - 2:15PM |
L44.00013: Angle resolved photoemission study on the electronic structure of potential 3D Dirac semimetal, KMgBi Binbin Fu, Junzhang Ma, Nan Xu, Christian Matt, Tian Qian, Ming Shi, Ding Hong Three dimensional Dirac semimetal is a new topological quantum state with discrete touching points, hosting Dirac fermion, in momentum space. Both type-I and type-II Dirac semimetals have already been predicted and verified by angle resolved photoemission spectroscopy(ARPES). However, experimental evidences of topological phase transitions between various topological semimetal states are still deficient. KMgBi is supposed to be a perfect Dirac semimetal candidate lying on the boundary of type-I and type-II phases. In this talk will present some interesting results of ARPES study on KMgBi, by changing the elements composition and doping level. [Preview Abstract] |
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