Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session U59: Weyl semimetals, ARPES, STM and opticsFocus
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Sponsoring Units: DMP Room: Mile High Ballroom 3C |
Thursday, March 5, 2020 2:30PM - 3:06PM |
U59.00001: Probing the Ultrafast Nonlinear Response in the Transition Metal Monopnictide Family of Weyl Semimetals Invited Speaker: Nicholas Sirica Weyl semimetals have been the focus of intense experimental and theoretical investigation, due to their broad appeal in fundamental science and applied technology alike. Recently, several studies have centered on the nonlinear optical properties of these materials, where it is believed that characteristic features of Weyl physics can be observed. To date, many of these studies have been limited to static or quasi-static measurements, but new and important insight can come about through extending nonlinear optical probes into the time domain. To do so, we use terahertz (THz) emission spectroscopy and time-resolved second harmonic generation (TR-SHG) spectroscopy to provide a contact free measure of ultrafast photocurrents in the transition metal monopnictide family of type-I Weyl semimetals. On the basis of our data, we are able to clearly distinguish between helicity-dependent, transverse photocurrents generated within the ab-plane from polarization-independent photocurrents flowing along the noncentrosymmetric c-axis. By using the photocurrent response as a probe of underlying crystal symmetry, we explore the role that polarization-dependent photoexcitation has on the assignment of point group symmetry, including the possibility of transient symmetry breaking. These findings highlight the robust nonlinear response exhibited by this class of materials, making them promising candidates for next generation sources and detectors in the mid-IR and THz frequency ranges. |
Thursday, March 5, 2020 3:06PM - 3:42PM |
U59.00002: Giant nonlinear optical response in topological semimetals Invited Speaker: Liang Wu The second-order optical nonlinearity has been a focus of basic research and technological development for decades as it is both a probe of inversion symmetry breaking in media and the basis for generating coherent light from far-infrared to ultraviolet wavelengths. Here, we focus on experimental investigations on the relation between band geometry/topology and nonlinear optics. I would like to discuss how do the second-order responses that originates from band geometry and topology in polar and chiral topological semimetals respectively. In both cases, we observed large optical nonlinearity and the possible origins will be discussed. |
Thursday, March 5, 2020 3:42PM - 3:54PM |
U59.00003: Imaging current in semimetals with cryogenic scanning NV magnetometry Uri Vool, Assaf Hamo, Xu Zhou, Johannes Gooth, Claudia Felser, Amir Yacoby Among their many unique features, Weyl semimetals are predicted to enable complex electron-electron and electron-phonon interactions. Such behavior has been predicted, and recently measured, to induce correlated flow of electrons through the semimetal. |
Thursday, March 5, 2020 3:54PM - 4:06PM |
U59.00004: Observation of a Dirac nodal loop in the non-symmorphic semimetal Nb3SiTe6 Ro-Ya Liu, Angus Huang, Chih-Chuan Su, Raman Sankar, Shih-Chang Weng, Meng-Kai Lin, Peng Chen, Joseph Hlevyack, Alexei V Fedorov, Cheng-Maw Cheng, Jonathan Denlinger, Chia-Seng Chang, Horng-Tay Jeng, Tien-Ming Chuang, Tai-Chang Chiang In 3-dimensional non-symmorphic crystals, band crossings protected by translational symmetry can host Dirac nodal loops in a 2-dimensional cut in the momentum surface. These nodal loops, often with an hourglass shape, give rise to unusual bulk and surface properties, including anisotropic electronic transport and chiral anomaly, based on transport or optical response measurements. This talk will present a detailed study of the band structure of Nb3SiTe6 by angle-resolved photoemission spectroscopy. The dispersion relations along the XS direction, which is perpendicular to the sample surface, is mapped out by varying the incident photon energy. The results show that the crossing point (Dirac point) of a Dirac cone at gradually shifts to higher binding in going from X to S. The measurements provide evidence for the presence of a Dirac nodal loop in this system. |
Thursday, March 5, 2020 4:06PM - 4:18PM |
U59.00005: Momentum space signature of Berry curvature monopoles in a Weyl semimetal: from first principles to direct experimental observation Philipp Eck, Maximilian Ünzelmann, Tim Figgemeier, Friedrich Reinert, Hendrik Bentmann, Domenico Di Sante, Giorgio Sangiovanni We investigate the Fermi arcs and the Weyl points in the prototypical non-centrosymmetric time-reversal invariant Weyl semimetal family TaAs/TaP in a joint analysis of state-of-the-art Ab-initio calculations and advanced photoemission spectroscopy methods. The Weyl physics is induced by the broken inversion symmetry giving rise to a predominant orbital magnetization, which results in the interplay with the spin-orbit interaction in the emergence of Weyl points. We characterize the local orbital magnetization in the vicinity of the Weyl points and explore indeed, in agreement with the local Berry curvature, clear features of Berry monopoles at the Weyl points. |
Thursday, March 5, 2020 4:18PM - 4:30PM |
U59.00006: Study of hot electrons in topological nodal-line semimetal ZrSiS using time- and angle-resolved photoemission spectroscopy Yangyang Liu, M. Mofazzel Hosen, Gyanendra Dhakal, Christopher Sims, John E Beetar, Sabin Regmi, Klauss Dimitri, Firoza Kabir, Dariusz Kaczorowski, Michael Chini, Madhab Neupane We have studied the ultrafast hot electron cooling process on topological nodal-line semimetal ZrSiS using an advanced XUV-based time- and angle-resolved photoemission spectroscopy (trARPES) with sub-30 meV energy resolution and 320 fs time resolution. The transient ARPES spectra in the vicinity of the nodal-line bulk states and surface states are individually measured using pump-probe scans employing 1.2 eV pump and 21.8 eV probe pulses. Our data reveals that the relaxation decay of the transient electronic temperature in the nodal-line bulk states (around 700 fs) is longer than the surface states (around 320 fs). Furthermore, the relaxation process of the hot electrons in the nodal-line bulk states exists not only a fast decay of ~700 fs, but also a second slow decay (> 6 ps) process, which indicates the existence of a second filling channel in the nodal-line bulk states. Our studies reveal the unique ultrafast properties of the hot electrons in the nodal-line semimetal ZrSiS. |
Thursday, March 5, 2020 4:30PM - 4:42PM |
U59.00007: Symmetry-Enforced Dirac Fermions in Nonsymmorphic α-Bismuthene Guang Bian According to recent theoretical works, nonsymmorphic crystal symmetries can enforce the formation of Dirac cones, providing a new route to establishing Dirac states in 2D materials. Here we will discuss our recent work on the realization of the symmetry-enforced Dirac fermions in nonsymmorphic α-bismuthene (Bi monolayer). The bismuthene was synthesized by the method of molecular beam epitaxy (MBE). The Dirac band structure was observed by the micro-angle-resolved photoemission (μ-ARPES) experiment. The Dirac points are located at high-symmetry momentum points which are entirely determined by the lattice symmetry. This correspondence of Dirac states to the nonsymmorphic symmetry group can potentially lead to the discovery of a range wide of new 2D Dirac materials. In addition, the Dirac fermions in α-bismuthene is of spin-orbit type in contrast to the spinless Dirac states in graphene. The result will accelerate the search of 2D Dirac materials and extend “graphene” physics into new territory where strong spin-orbit coupling is present. |
Thursday, March 5, 2020 4:42PM - 4:54PM |
U59.00008: Molecular Beam Epitaxy, Band Structure, and Electrical Properties of the Dirac Semimetal ZrTe2 Timothy Pillsbury, Anthony R. Richardella, Run Xiao, Wilson Yanez, Max Stanley, Hemian Yi, Cui-Zu Chang, Nitin Samarth Topological semimetals (TSMs), a class of gapless electronic phases exhibiting topologically stable crossings of bulk energy bands, have attracted tremendous interest in the condensed matter physics community recently. Based on the origin of the band crossing, TSMs can be classified into Dirac semimetals (DSMs), Weyl semimetals (WSMs) and others. Recently, the transition metal dichalcogenide ZrTe2 has been theoretically predicted and experimentally demonstrated to possess a DSM phase. We used molecular beam epitaxy to synthesize ZrTe2 thin films on insulating sapphire (0001) substrates and performed systematic in vacuo angle-resolved photoemission spectroscopy (ARPES) measurements. The observation of the Dirac cone in the ARPES spectra of ZrTe2 thin films confirms the DSM phase. We also measured the temperature dependence of the ex-situ magneto-transport and found that negative magnetoresistance is absent in our films. We are also exploring the quantum phase transition from DSM to WSM by transition metal doping into these ZrTe2 films. |
Thursday, March 5, 2020 4:54PM - 5:06PM |
U59.00009: Observation of linearly dispersive edge modes in a magnetic Weyl semimetal Co3Sn2S2 Sean Howard, Lin Jiao, Zhenyu Wang, Praveen Vir, Chandra Shekhar, Claudia Felser, Taylor Hughes, Vidya Madhavan The physical realization of Chern insulators is of fundamental and practical interest, as they are predicted to host the quantum anomalous Hall effect (QAHE) and chiral edge states which carry dissipationless current. Realization of the QAHE state has however been challenging due to the complex heterostructures and sub-Kelvin temperatures required. Magnetic Weyl semimetals, essentially stacks of Chern insulators with inter-layer coupling, may provide a new platform for the higher temperature realization of robust 2D QAHE edge states. In this work we present a combined scanning tunneling spectroscopy and theoretical investigation of a newly discovered magnetic Weyl semimetal, Co3Sn2S2. Using numerical simulations we find that chiral edge states can be localized on partially exposed Kagome planes on the surface of a Weyl semimetal. As such, our STM dI/dV maps on narrow kagome Co3Sn terraces show linearly dispersing quantum well-like states, which can be attributed to hybridized chiral edge modes. Our results suggest a new paradigm for studying chiral edge modes in time-reversal breaking Weyl semimetals. More importantly, this work leads a practical route for realizing higher temperature QAHE. |
Thursday, March 5, 2020 5:06PM - 5:18PM |
U59.00010: Generation of strain-induced pseudo-magnetic field in a doped type-II Weyl semimetal Suman Kamboj, Partha Sarathi Rana, Anshu Sirohi, Aastha Vasdev, Manasi Mandal, Sourav Marik, R P Singh, Tanmoy Das, Goutam Sheet As per the theory, it is in principle possible to realize a pseudo-magnetic field in strained Weyl semimetals. The basic idea is that strain alters the hopping parameter which may manifest itself as a gauge potential, which in turn may lead to a quantity analogous to the magnetic field. This pseudo-magnetic field may help the formation of quantized Landau levels and thus may become observable in Weyl semimetals. We will show the emergence of a pseudo-magnetic field in a doped and intrinsically strained type-II Weyl semimetal by Scanning Tunneling Spectroscopy (STS) down to 300 mK. In these experiments, clear Landau level oscillations are observed in the absence of an externally applied magnetic field. The system also has a known superconducting phase at very low temperature, which remains unaffected by the pseudo-magnetic field. |
Thursday, March 5, 2020 5:18PM - 5:30PM |
U59.00011: Theoretical Study of Quasiparticle Interference in Weyl Semimetals with ab initio Band Structure Informed Tight-Binding Hamiltonian Zhao Huang, Dzmitry Yarotski, Antoinette Taylor, Jian-Xin Zhu The Weyl semimetal is a group of interesting materials where Weyl fermions emerge around singularity points of Berry curvature in the band structure (so called Weyl nodes). A direct consequence of the Weyl nodes is the anomalous surface state arising from the separation of Weyl nodes with opposite topological charges. The energy bands of the surface states cross the Fermi level and form a Fermi arc which connects the projections of Weyl nodes on the surface. Experimentally, observation of the Fermi arc is taken as a fingerprint of the Weyl semimetals. Here we theoretically study the quasiparticle interference (QPI) due to impurity scattering on surfaces in several Weyl semimetals. The scattering between Fermi arcs can lead to special patterns of QPI as indirect observation of the Fermi arcs. In particular, we adopt the accurate tight-binding multi-orbital Hamiltonian constructed from the first-principle electronic structure calculations for each material. We anticipate that our theoretical results will help interpret STM-based measurements on a more comparable footing, thus providing guidance for the identification of Weyl physics. |
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