Discovery of a Weyl Fermion semimetal state in NbAs
We report the discovery of Weyl semimetal NbAs featuring topological Fermi arc surface states.
cond-mat.mes-hall↗
arXiv subjects
Guang Bian
Publications and source records attributed to Guang Bian.
Theoretical Discovery/Prediction: Weyl Semimetal states in the TaAs material (TaAs, NbAs, NbP, TaP) class
The recent discoveries of Dirac fermions in graphene and on the surface of topological insulators have ignited worldwide interest in physics and materials science. A Weyl semimetal is an unusual crystal where electrons also behave as massless quasi-particles but interestingly they are not Dirac fermions. These massless particles, Weyl fermions, were originally considered in massless quantum electrodynamics but have not been observed as a fundamental particle in nature. A Weyl semimetal provides a condensed matter realization of Weyl fermions, leading to unique transport properties with novel device applications. Here, we THEORETICALLY identify the first Weyl semimetal in a class of stoichiometric materials (TaAs, NbAs, NbP, TaP), which break crystalline inversion symmetry, including TaAs, TaP, NbAs and NbP. Our first-principles calculation-based predictions on TaAs reveal the spin-polarized Weyl cones and Fermi arc surface states in this compound. We also observe pairs of Weyl points with the same chiral charge which project onto the same point in the surface Brillouin zone, giving rise to multiple Fermi arcs connecting to a given Weyl point. Our results show that TaAs is the first topological semimetal identified which does not depend on fine-tuning of chemical composition or magnetic order, greatly facilitating an exploration of Weyl physics in real materials. (Note added: This theoretical prediction of November 2014 (see paper in Nature Communications) was the basis for the first experimental discovery of Weyl Fermions and topological Fermi arcs in TaAs recently published in Science (2015) at http://www.sciencemag.org/content/early/2015/07/15/science.aaa9297.full.pdf)
cond-mat.mtrl-sci↗
Discovery of a Weyl Fermion Semimetal and Topological Fermi Arcs
We report discovery of a Weyl Fermion semimetal and Topological Fermi arcs in TaAs
cond-mat.mes-hall↗
Topological Nodal-Line Fermions in the Non-Centrosymmetric Superconductor Compound PbTaSe2
We report the identification of a Topological Nodal-Line Semimetal state in PbTaSe2.
cond-mat.mes-hall↗
Unconventional transformation of spin Dirac phase across a topological quantum phase transition
The topology of a topological material can be encoded in its surface states. These surface states can only be removed by a bulk topological quantum phase transition into a trivial phase. Here we use photoemission spectroscopy to image the formation of protected surface states in a topological insulator as we chemically tune the system through a topological transition. Surprisingly, we discover an exotic spin-momentum locked, gapped surface state in the trivial phase that shares many important properties with the actual topological surface state in anticipation of the change of topology. Using a spin-resolved measurement, we show that apart from a surface band-gap these states develop spin textures similar to the topological surface states well-before the transition. Our results offer a general paradigm for understanding how surface states in topological phases arise and are suggestive for future realizing Weyl arcs, condensed matter supersymmetry and other fascinating phenomena in the vicinity of topological quantum criticality.
cond-mat.mes-hall↗
A new type of Weyl semimetal with quadratic double Weyl fermions in SrSi2
We identify a Weyl semimetal state in an inversion breaking, stoichiometric compound strontium silicide, SrSi$_2$, with many new and novel properties that are distinct from the TaAs family. We theoretically show that SrSi$_2$ is a Weyl semimetal even without spin-orbit coupling and that, after the inclusion of spin-orbit coupling, two Weyl fermions stick together forming an exotic double Weyl fermion with quadratic dispersions and a higher chiral topological charge of 2. Moreover, we find that the Weyl nodes with opposite charges are located at different energies due to the absence of mirror symmetry in SrSi$_2$, leading to a unique topological quantum response that an external magnetic field can induce a dissipationless current. Our systematic results not only identify a much-needed robust Weyl semimetal candidate but also open the door to new topological Weyl physics that is not possible in the TaAs family of materials.
cond-mat.mes-hall↗
Lifshitz transition and van Hove singularity in a Topological Dirac Semimetal
A topological Dirac semimetal is a novel state of quantum matter which has recently attracted much attention as an apparent 3D version of graphene. In this paper, we report critically important results on the electronic structure of the 3D Dirac semimetal Na3Bi at a surface that reveals its nontrivial groundstate. Our studies, for the first time, reveal that the two 3D Dirac cones go through a topological change in the constant energy contour as a function of the binding energy, featuring a Lifshitz point, which is missing in a strict 3D analog of graphene (in other words Na3Bi is not a true 3D analog of graphene). Our results identify the first example of a band saddle point singularity in 3D Dirac materials. This is in contrast to its 2D analogs such as graphene and the helical Dirac surface states of a topological insulator. The observation of multiple Dirac nodes in Na3Bi connecting via a Lifshitz point along its crystalline rotational axis away from the Kramers point serves as a decisive signature for the symmetry-protected nature of the Dirac semimetal's topological groundstate.
cond-mat.mes-hall↗
Observation of the Adler-Bell-Jackiw chiral anomaly in a Weyl semimetal
We report the experimental discovery of Adler-Bell-Jackiw chiral anomaly in a Weyl semimetal crystal.
cond-mat.mes-hall↗
Observation of Fermi Arc Surface States in a Topological Metal: A New Type of 2D Electron Gas beyond Z2 Topological Insulators
In a topological insulator, it is the electrons on the surface or edge that carry the signature of topology. Recently, a novel topological state has been proposed in metals or semimetals (gapless) whose band-structure is similar to that of a three-dimensional analog of graphene. However, to this date the signature of its topology remains an open question. We report the experimental discovery of a pair of polarized Fermi arc surface state modes in the form of a new type of two-dimensional polarized electron gas on the surfaces of Dirac semimetals. These Fermi arc surface states (FASS) are observed to connect across an even number of bulk band gapless nodes and found to have their spin uniquely locked to their momentum. We show that these states are distinctly different from the topological surface states (TSS) seen in all known topological insulators. Our observed exotic two-dimensional states not only uncover the novel topology of gapless Dirac metals (such as sodium tribismuth Na$_3$Bi) but also opens new research frontiers for the utilization of topological Fermi arc electron gases for a wide range of fundamental physics and spintronic studies.
cond-mat.mes-hall↗
Non-Kondo-like Electronic Structure in the Correlated Rare-Earth Hexaboride YbB$_6$
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.
cond-mat.str-el↗
Surface Versus Bulk Dirac States Tuning in a Three-Dimensional Topological Dirac Semimetal
Recently, crystalline-symmetry-protected three-dimensional (3D) bulk Dirac semimetal phase has been experimentally identified in a stoichiometric high-mobility compound, Cd3As2. The Dirac state observed in Cd3As2 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. We develop a systematic technique to isolate the surface and bulk states in Cd3As2, by simultaneously utilizing angle-resolved photoemission spectroscopy (ARPES) and in-situ surface deposition. 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 3D Dirac semimetal.
cond-mat.mes-hall↗
Observation of metallic surface states in the strongly correlated Kitaev-Heisenberg candidate Na2IrO3
We report high-resolution angle-resolved photoemission spectroscopy measurements on the honeycomb iridate Na2IrO3. Our measurements reveal the existence of a metallic surface band feature crossing the Fermi level with nearly linear dispersion and an estimated surface carrier density of 3.2 $\times$ 10$^{13}$ cm$^{-2}$, which has not been theoretically predicted or experimentally observed, and provides the first evidence for metallic behavior on the boundary of this material, whereas the bulk bands exhibit a robust insulating gap. We further show the lack of theoretically predicted Dirac cones at the $\overline{M}$ points of the surface Brillouin zone, which confirms the absence of a stacked quantum spin Hall phase in this material. Our data indicates that the surface ground state of this material is exotic and metallic, unlike as predicted in theory, and establishes Na2IrO3 as a rare example of a strongly correlated spin-orbit insulator with surface metallicity.
cond-mat.str-el↗
Momentum space imaging of Cooper pairing in a half-Dirac-gas topological superconductor (a helical 2D topological superconductor)
Superconductivity in Dirac electrons has recently been proposed as a new platform between novel concepts in high-energy and condensed matter physics. It has been proposed that supersymmetry and exotic quasiparticles, both of which remain elusive in particle physics, may be realized as emergent particles in superconducting Dirac electron systems. Using artificially fabricated topological insulator-superconductor heterostructures, we present direct spectroscopic evidence for the existence of Cooper pairing in a half Dirac gas 2D topological superconductor. Our studies reveal that superconductivity in a helical Dirac gas is distinctly different from that of in an ordinary two-dimensional superconductor while considering the spin degrees of freedom of electrons. We further show that the pairing of Dirac electrons can be suppressed by time-reversal symmetry breaking impurities removing the distinction. Our demonstration and momentum-space imaging of Cooper pairing in a half Dirac gas and its magnetic behavior taken together serve as a critically important 2D topological superconductor platform for future testing of novel fundamental physics predictions such as emergent supersymmetry and quantum criticality in topological systems.
cond-mat.supr-con↗
Observation of monolayer valence band spin-orbit effect and induced quantum well states (QWS) in MoX2
Transition metal dichalcogenides have attracted much attention recently due to their potential applications in spintronics and photonics as a result of the indirect to direct band gap transition and the emergence of the spin-valley coupling phenomenon upon moving from the bulk to monolayer limit. Here, we report high-resolution angle-resolved photoemission spectroscopy on MoSe2 (molybdenum diselenide) single crystals and monolayer films of MoS2 grown on Highly Ordered Pyrolytic Graphite substrate. Our experimental results, for the first time, resolve the two distinct bands at the Brillouin zone corner of the bulk MoSe2, and provide evidence for the critically important spin-orbit split bands of the monolayer MoS2. Moreover, by depositing potassium on cleaved surfaces of these materials, the process through which quantum well states form on the surfaces of transition metal dichalcogenides is systematically imaged. We present a theoretical model to account for the observed spin-orbit splitting and the rich spectrum of the quantum well states observed in our experiments. Our findings taken together provide important insights into future applications of transition metal dichalcogenides in nanoelectronics, spintronics, and photonics devices as they critically depend on the spin-orbit physics of these materials.
cond-mat.mes-hall↗
Observation of Quantum-Tunneling Modulated Spin Texture in Ultrathin Topological Insulator Bi2Se3 Films
Understanding the spin-texture behavior of boundary modes in ultrathin topological insulator films is critically essential for the design and fabrication of functional nano-devices. Here by using spin-resolved photoemission spectroscopy with p-polarized light in topological insulator Bi2Se3 thin films, we report tunneling-dependent evolution of spin configuration in topological insulator thin films across the metal-to-insulator transition. We observe strongly binding energy- and wavevector-dependent spin polarization for the topological surface electrons in the ultra-thin gapped-Dirac-cone limit. The polarization decreases significantly with enhanced tunneling realized systematically in thin insulating films, whereas magnitude of the polarization saturates to the bulk limit faster at larger wavevectors in thicker metallic films. We present a theoretical model which captures this delicate relationship between quantum tunneling and Fermi surface spin polarization. Our high-resolution spin-based spectroscopic results suggest that the polarization current can be tuned to zero in thin insulating films forming the basis for a future spin-switch nano-device.
cond-mat.mes-hall↗
Search for superconducting proximity effect in a topological insulator and high temperature superconductor heterostructure Bi2Se3/Bi2Sr2CaCu2O8+
Understanding the superconducting proximity effect on the surface of a topological insulator is of critical importance in realizing topological superconductivity and Majorana fermions in solid state settings. We fabricate delicate heterostructure samples between topological insulator (TI) Bi2Se3 thin film and high temperature superconductor Bi2Sr2CaCu2O8+ (Tc ~ 91 K). Using angle-resolved photoemission spectroscopy, we probe the electronic structure and the possible existence of superconducting gap on the top surface of Bi2Se3 thin films. Our systematic data show no significant proximity-induced superconducting gap in the topological surface states with sample temperature down to 10 K (<<91 K) and with a confidence level of sub 5 meV, much smaller than what have been claimed previously, which indicates the near absence of the proximity-induced superconductivity on the TI surface. Our momentum space imaging provides evidence for the coexistence of two crystalline phases in Bi2Se3/Bi2Sr2CaCu2O8+ samples, which we argue to be due to the strong mismatch of lattice crystalline symmetries. Our data identify the major contributors in reducing the proximity-induced superconducting gap below the meV range, including the lack of momentum space overlap between the Bi2Se3 and Bi2Sr2CaCu2O8+ Fermi surfaces, the strong mismatch of lattice crystalline symmetries and superconducting pairing symmetries, as well as the very short superconducting coherence length in Bi2Sr2CaCu2O8+. Our ARPES studies not only provide critical momentum space insights into the Bi2Se3/Bi2Sr2CaCu2O8+ heterostructure, but also set an upper bound on the proximity induced gap for realizing a Majorana fermion condition in this system, which may be further destabilized by the d-wave nodes dominated in a cuprate superconductor.
cond-mat.str-el↗
Observation of a bulk 3D Dirac multiplet, Lifshitz transition, and nestled spin states in Na3Bi
Symmetry or topology protected Dirac fermion states in two and three dimensions constitute novel quantum systems that exhibit exotic physical phenomena. However, none of the studied spin-orbit materials are suitable for realizing bulk multiplet Dirac states for the exploration of interacting Dirac physics. Here we present experimental evidence, for the first time, that the compound Na3Bi hosts a bulk spin-orbit Dirac multiplet and their interaction or overlap leads to a Lifshitz transition in momentum space - a condition for realizing interactions involving Dirac states. By carefully preparing the samples at a non-natural-cleavage (100) crystalline surface, we uncover many novel electronic and spin properties in Na3Bi by utilizing high resolution angle- and spin-resolved photoemission spectroscopy measurements. We observe two bulk 3D Dirac nodes that locate on the opposite sides of the bulk zone center point $Γ$, which exhibit a Fermi surface Lifshitz transition and a saddle point singularity. Furthermore, our data shows evidence for the possible existence of theoretically predicted weak 2D nontrivial spin-orbit surface state with helical spin polarization that are nestled between the two bulk Dirac cones, consistent with the theoretically calculated (100) surface-arc-modes. Our main experimental observation of a rich multiplet of Dirac structure and the Lifshitz transition opens the door for inducing electronic instabilities and correlated physical phenomena in Na3Bi, and paves the way for the engineering of novel topological states using Na3Bi predicted in recent theory.
cond-mat.mes-hall↗
Wedgebox analysis of four-lepton events from neutralino pair production at the LHC
`Wedgebox' plots constructed by plotting the di-electron invariant mass versus the di-muon invariant mass from pp -> e^+e^- mu^+ mu^- + missing energy signature LHC events. Data sets of such events are obtained across the MSSM input parameter space in event-generator simulations, including cuts designed to remove SM backgrounds. Their study reveals several general features: (1)Regions in the MSSM input parameter space where a sufficient number of events are expected so as to be able to construct a clear wedgebox plot are delineated. (2)The presence of box shapes on a wedgebox plot either indicates the presence of heavy Higgs bosons decays or restricts the location to a quite small region of low μand M_2 values \lsim 200 GeV, a region denoted as the `lower island'. In this region, wedgebox plots can be quite complicated and change in pattern rather quickly as one moves around in the (μ, M_2) plane. (3)Direct neutralino pair production from an intermediate Z^{0*} may only produce a wedge-shape since only \widetildeχ_2^0\widetildeχ_3^0 decays can contribute significantly. (4)A double-wedge or wedge-protruding-from-a-box pattern on a wedgebox plot, which results from combining a variety of MSSM production processes, yields three distinct observed endpoints, almost always attributable to \widetildeχ_{2,3,4}^0 \to \widetildeχ_1^0 \ell^+\ell^- decays, which can be utilized to determine a great deal of information about the neutralino and slepton mass spectra and related MSSM input parameters. Wedge and double-wedge patterns are seen in wedgebox plots in another region of higher μand M_2 values, denoted as the`upper island.' Here the pattern is simpler and more stable as one moves across the (μ, M_2) input parameter space.
hep-ph↗