Su-Yang Xu

Princeton University

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M. Zahid Hasan

Chinese Academy of Sciences

Hsin Lin

Institute of Physics, Academia Sinica

Tay‐Rong Chang

National Cheng Kung University

Ilya Belopolski

RIKEN Center for Emergent Matter Science

Horng‐Tay Jeng

National Tsing Hua University

Guang Bian

University of Missouri

Arun Bansil

Northeastern University

Nasser Alidoust

Princeton University

Daniel S. Sanchez

HRL Laboratories (United States)

Guoqing Chang

Nanyang Technological University

Cooperative Institutions

Princeton University

Chinese Academy of Sciences

Massachusetts Institute of Technology

Institute of Physics, Academia Sinica

Peking University

Lawrence Berkeley National Laboratory

The University of Tokyo

University of Chinese Academy of Sciences

Paul Scherrer Institute

National University of Singapore

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Theory of quasiparticle interference in mirror-symmetric two-dimensional systems and its application to surface states of topological crystalline insulators

Physical Review B (2013)

Chen Fang Matthew J. Gilbert Su-Yang Xu B. Andrei Bernevig M. Zahid Hasan

We study symmetry-protected features in the quasiparticle interference (QPI) pattern of two-dimensional (2D) systems with mirror symmetries and time-reversal symmetry, around a single static point impurity. We show that, in the Fourier-transformed local density of states (FT-LDOS) $\ensuremath{\rho}(\mathbf{q},\ensuremath{\omega})$, while the position of high-intensity peaks generically depends on the geometric features of the iso-energy contour at energy $\ensuremath{\omega}$, the absence of certain peaks is guaranteed by the opposite mirror eigenvalues of the two Bloch states that are (i) on the mirror-symmetric lines in the Brillouin zone (BZ) and (ii) separated by scattering vector $\mathbf{q}$. We apply the general result to the QPI on the $\ensuremath{\langle}001\ensuremath{\rangle}$ surface of the topological crystalline insulator Pb${}_{1\ensuremath{-}x}$Sn${}_{x}$Te and predict all vanishing peaks in $\ensuremath{\rho}(\mathbf{q},\ensuremath{\omega})$. The model-independent analysis is supported by numerical calculations using an effective four-band model derived from symmetry analysis.

Brillouin zone

Topological insulator

Mirror symmetry

10.1103/physrevb.88.125141

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Citations (35)

Surface versus bulk Dirac state tuning in a three-dimensional topological Dirac semimetal

Physical Review B (2015)

Madhab Neupane Su-Yang Xu Nasser Alidoust R. Sankar Ilya Belopolski

Recently, a crystalline-symmetry-protected three-dimensional (3D) bulk Dirac semimetal phase has been experimentally identified in a stoichiometric high-mobility compound, ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$. The Dirac state observed in ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ has been attributed to originate mostly from the bulk state while calculations show that the bulk and surface states overlap over the entire Dirac dispersion energy range. In this study we unambiguously reveal doping induced evolution of the ground state of surface and bulk electron dynamics in a 3D Dirac semimetal. The technique demonstrated in this study by simultaneously using angle-resolved photoemission spectroscopy (ARPES) and in situ surface deposition isolates the surface and bulk states in ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$. Our experimental results provide a method for tuning the chemical potential as well as to observe surface states degenerate with bulk states, which will be useful for future applications of a 3D Dirac semimetal.

Surface States

10.1103/physrevb.91.241114

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Atomic-Scale Visualization of Quantum Interference on a Weyl Semimetal Surface by Scanning Tunneling Microscopy

ACS Nano (2016)

Hao Zheng Su-Yang Xu Guang Bian Cheng Guo Guoqing Chang

Weyl semimetals may open a new era in condensed matter physics, materials science and nanotech after graphene and topological insulators. We report the first atomic scale view of the surface states of a Weyl semimetal (NbP) using scanning tunneling microscopy/spectroscopy. We observe coherent quantum interference patterns that arise from the scattering of quasiparticles near point defects on the surface. The measurements reveal the surface electronic structure both below and above the chemical potential in both real and reciprocal spaces. Moreover, the interference maps uncover the scattering processes of NbP's exotic surface states. Through comparison between experimental data and theoretical calculations, we further discover that the scattering channels are largely restricted by the orbital and/or spin texture of the surface band. The visualization of the scattering processes can help design novel transport effects and electronics on the topological surface of a Weyl semimetal.

Weyl semimetal

Topological insulator

Surface States

10.1021/acsnano.5b06807

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Drumhead surface states and topological nodal-line fermions inTlTaSe2

Physical review. B./Physical review. B (2016)

Guang Bian Tay‐Rong Chang Hao Zheng Saavanth Velury Su-Yang Xu

A topological nodal-line semimetal is a state of matter with one-dimensional bulk nodal lines and two-dimensional so-called drumhead surface bands. Based on first-principles calculations and an effective $\mathbit{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbit{p}$ model, we theoretically propose the existence of topological nodal-line fermions in the ternary transition-metal chalcogenide ${\mathrm{TlTaSe}}_{2}$. The noncentrosymmetric structure and strong spin-orbit coupling give rise to spinful nodal-line bulk states which are protected by a mirror reflection symmetry of this compound. This is remarkably distinguished from other proposed nodal-line semimetals such as ${\mathrm{Cu}}_{3}\mathrm{NPb}$(Zn) in which the nodal line exists only in the limit of vanishing spin-orbit coupling and thus is not as robust. In addition, we show that the drumhead surface states in ${\mathrm{TlTaSe}}_{2}$, which are associated with the topological nodal lines, exhibit an unconventional chiral spin texture and an exotic Lifshitz transition as a consequence of the linkage among multiple drumhead surface-state pockets.

Surface States

Line (geometry)

Topological insulator

10.1103/physrevb.93.121113

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Citations (300)

Non-Kondo-like Electronic Structure in the Correlated Rare-Earth HexaborideYbB6

Physical Review Letters (2015)

Madhab Neupane Su-Yang Xu Nasser Alidoust Guang Bian D. J. Kim

We present angle-resolved photoemission studies on the rare-earth hexaboride YbB$_6$, which has recently been predicted to be a topological Kondo insulator. Our data do not agree with the prediction and instead show that YbB$_6$ exhibits a novel topological insulator state in the absence of a Kondo mechanism. We find that the Fermi level electronic structure of YbB$_6$ has three 2D Dirac cone like surface states enclosing the Kramers' points, while the f-orbital which would be relevant for the Kondo mechanism is $\sim1$ eV below the Fermi level. Our first-principles calculation shows that the topological state which we observe in YbB$_6$ is due to an inversion between Yb $d$ and B $p$ bands. These experimental and theoretical results provide a new approach for realizing novel correlated topological insulator states in rare-earth materials.

Kondo insulator

Topological insulator

10.1103/physrevlett.114.016403

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Citations (48)

Unconventional topological phase transition in non-symmorphic material KHgX (X = As, Sb, Bi)

npj Computational Materials (2019)

Chin-Shen Kuo Tay‐Rong Chang Su-Yang Xu Horng‐Tay Jeng

Abstract Traditionally topological phase transition describes an evolution from topological trivial to topological nontrivial state. Originated from the non-symmorphic crystalline symmetry, we propose in this work an unconventional topological phase transition scheme between two topological nontrivial insulating states mediated by a Dirac gapless state, differing from the traditional topological phase transition. The KHgX (X = As, Sb, Bi) family is the first experimentally realized topological non-symmorphic crystalline insulator (TNCI), where the topological surface states are characterized by the Mobius-twisted connectivity. Based on first-principles calculations, we present a topological insulator–metal transition from TNCI into a Dirac semimetal (DSM) via applying an external pressure on KHgX. We find an unusual mirror Chern number C m = −3 for the DSM phase of KHgX in the non-symmorphic crystal structure, which is topologically distinct from the traditional DSM such as Na 3 Bi and Cd 3 As 2 . Furthermore, we predict a new TNCI phase in KHgX via symmetry breaking. The topological surface states in this new TNCI phase display zigzag connectivity, different from the unstressed one. Our results offer a comprehensive study for understanding how the topological surface states evolve from a quantum phase transition in non-symmorphic system.

Topological insulator

Surface States

Topological degeneracy

Zigzag

10.1038/s41524-019-0201-4

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Atomic-Scale Visualization of Quasiparticle Interference on a Type-II Weyl Semimetal Surface

Physical Review Letters (2016)

Hao Zheng Guang Bian Guoqing Chang Hong Lu Su-Yang Xu

We combine quasiparticle interference simulation (theory) and atomic resolution scanning tunneling spectromicroscopy (experiment) to visualize the interference patterns on a type-II Weyl semimetal ${\mathrm{Mo}}_{x}{\mathrm{W}}_{1\ensuremath{-}x}{\mathrm{Te}}_{2}$ for the first time. Our simulation based on first-principles band topology theoretically reveals the surface electron scattering behavior. We identify the topological Fermi arc states and reveal the scattering properties of the surface states in ${\mathrm{Mo}}_{0.66}{\mathrm{W}}_{0.34}{\mathrm{Te}}_{2}$. In addition, our result reveals an experimental signature of the topology via the interconnectivity of bulk and surface states, which is essential for understanding the unusual nature of this material.

Weyl semimetal

Surface States

Interconnectivity

10.1103/physrevlett.117.266804

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Citations (65)

Type-II Symmetry-Protected Topological Dirac Semimetals

Physical Review Letters (2017)

Tay‐Rong Chang Su-Yang Xu Daniel S. Sanchez Wei-Feng Tsai Shin-Ming Huang

The recent proposal of the type-II Weyl semimetal state has attracted significant interest. In this Letter, we propose the concept of the three-dimensional type-II Dirac fermion and theoretically identify this new symmetry-protected topological state in the large family of transition-metal icosagenides, MA3 (M=V, Nb, Ta; A=Al, Ga, In). We show that the VAl3 family features a pair of strongly Lorentz-violating type-II Dirac nodes and that each Dirac node can be split into four type-II Weyl nodes with chiral charge ±1 via symmetry breaking. Furthermore, we predict that the Landau level spectrum arising from the type-II Dirac fermions in VAl3 is distinct from that of known Dirac or Weyl semimetals. We also demonstrate a topological phase transition from a type-II Dirac semimetal to a quadratic Weyl semimetal or a topological crystalline insulator via crystalline distortions.Received 20 December 2016DOI:https://doi.org/10.1103/PhysRevLett.119.026404© 2017 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasFirst-principles calculationsSemimetalsSpin-orbit couplingSurface statesTopological materialsTopological phases of matterCondensed Matter, Materials & Applied Physics

Weyl semimetal

Topological insulator

Helical Dirac fermion

10.1103/physrevlett.119.026404

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Citations (165)

Observation of the nonlinear Hall effect under time-reversal-symmetric conditions

Nature (2018)

Qiong Ma Su-Yang Xu Huitao Shen David MacNeill Valla Fatemi

Thermal Hall effect

10.1038/s41586-018-0807-6

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