Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Q10: Emergent Dirac and Weyl Semimetals in Three Dimensions |
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Sponsoring Units: DCMP Chair: Arijit Kundu, University of Indiana at Bloomington Room: 007A |
Wednesday, March 4, 2015 2:30PM - 2:42PM |
Q10.00001: Building topologic device through emerging robust helical surface states Zibo Wang, Hua Jiang, Xincheng Xie In a 3D $Cd_3As_2$ ribbon with proper sizes, the system can exhibit a unique finite-size effect. Namely, if magnetic impurities are doped on one side, the surface states on the other side can be altered according to the strengths of these magnetic impurities. As a result, the conductance of the system will also be changed. This finding can be explained by the backscattering between the hybridized surface states due to finite size confinement and the normal surface states. Moreover, this phenomenon can be used to build new topologic devices. [Preview Abstract] |
Wednesday, March 4, 2015 2:42PM - 2:54PM |
Q10.00002: Photoemission studies on the 3D Dirac semiemtal state in Na$_3$Bi Suyang Xu, Liu Chang, Satya K. Kushwaha, Raman Sankar, Jason W. Krizan, Ilya Belopolski, Madhab Neupane, Guang Bian, Nasser Alidoust, Tay-Rong Chang, Horng-Tay Jeng, Cheng-Yi Huang, Wei-Feng Tsai, Hsin Lin, Pavel P. Shibayev, Fangcheng Chou, Robert J. Cava, M. Zahid Hasan A three-dimensional Dirac semimetal is a novel state of matter that has recently attracted interest in condensed matter physics and materials science. We present electronic structure measurements on the (100) surface of a recently discovered Dirac semimetal material Na$_3$Bi. Our experimental data, for the first time, reveal a Lifshitz transition between the two bulk Dirac cones in the bulk band structure of Na$_3$Bi. These results identify the first example of a band structure singularity in 3D Dirac materials. This is in contrast to its 2D analogs such as in twisted bilayer graphene or the surface states of topological crystalline insulators, which have been studied extensively. The observation of multiple bulk Dirac nodes along the rotational crystal axis away from the Kramers point also serve as a signature for the symmetry-protected nature of the Dirac semimetal state in Na$_3$Bi as elaborated in recent theories. [Preview Abstract] |
Wednesday, March 4, 2015 2:54PM - 3:06PM |
Q10.00003: Linear magnetoresistance caused by mobility fluctuations in a bulk three-dimensional Dirac semi-metal, Cd$_{3}$As$_{2}$ Arjun Narayanan, Matthew Watson, Samuel Blake, Yulin Chen, Dharmalingam Prabhakaran, Binghai Yan, Nicolas Bruyant, Lois Drigo, Igor Mazin, Claudia Felser, Tai Kong, Paul Canfield, Amalia Coldea Cd$_{3}$As$_{2}$ is a three-dimensional Dirac semi-metal with a variety of exciting transport properties such as exceedingly high mobility and an unusual non-saturating linear magnetoresistance. Given the fundamental interest in bulk band structure and potential for applications based on transport properties, understanding the transport~properties is crucial. We report magnetotransport and tunnel diode oscillation measurements on Cd$_{3}$As$_{2}$, in pulsed magnetic fields up to 65 T and temperatures between 1.5 K and 300 K. We study the unconventional non-saturating linear magnetoresistance up to 65 T and conclude that it is linked to disorder effects. Analysis of quantum oscillations in the magnetoresistance data allow us to further characterize the electronic state and discuss the bulk Fermi surface of Cd3As2. [Preview Abstract] |
Wednesday, March 4, 2015 3:06PM - 3:18PM |
Q10.00004: Thermopower and Nernst effect in the Dirac semimetal Cd$_3$As$_2$ Tian Liang, Quinn Gibson, Mazhar Ali, Minhao Liu, Robert Cava, Nai Phuan Ong Dirac semimetals and Weyl semimetals are 3D analogues of graphene in which crystalline symmetry protects the nodes against gap formation. Cd$_3$As$_2$ was predicted to be Dirac semimetal [1], and recently confirmed to be so by photoemission [2,3]. Here we report an interesting property in Cd$_3$As$_2$ that was unpredicted, namely a remarkable protection mechanism that strongly suppresses backscattering in zero H. In single crystals, the protection results in ultrahigh mobility $\sim 9\times 10^6$ cm$^2$ V$^{-1}$ s$^{-1}$ [4] at 5 K. Suppression of backscattering results in a transport lifetime 10$^4$ times longer than the quantum lifetime. The lifting of this protection by H leads to a very large magnetoresistance. Quantum oscillations in resistivity, Seebeck and Nernst, show beating effect.We discuss how this may relate to changes to the Fermi surface induced by H. [1] Wang, Z. J. et al. , Phys. Rev. B $\mathbf{88}$, 125427 (2013). [2] Sergey, B. et al. , Phys. Rev. Lett. $\mathbf{113}$, 027603 (2014). [3] Neupane, M. et al. , Nature Commun. $\mathbf{5}$, 3786 (2014). [4] Tian Liang et al. , Nature Materials, in press. [Preview Abstract] |
Wednesday, March 4, 2015 3:18PM - 3:30PM |
Q10.00005: Optical gyrotropy as a test for dynamic chiral magnetic effect of Weyl semi-metals Sumanta Tewari, Pallab Goswami, Girish Sharma We identify the dynamic chiral magnetic effect and the optical gyrotropy as the manifestations of the same physical phenomenon, namely the dynamic magnetoelectric effect. We show that the measurement of natural optical activity for the transmitted light or the rotary power provides a direct confirmation of the existence of the dynamic chiral magnetic effect. We derive a general formula for the gyrotropic conductivity of a noncentrosymmetric metal in the high frequency limit, and apply our results to the special cases of inversion symmetry breaking Weyl semimetals and noncentrosymmetric cubic metals. [Preview Abstract] |
Wednesday, March 4, 2015 3:30PM - 3:42PM |
Q10.00006: Interacting Dirac liquid in three-dimensional semimetals Johannes Hofmann, Edwin Barnes We study theoretically the properties of the interacting Dirac liquid, a novel three-dimensional many-body system which was recently experimentally realized and in which the electrons have a chiral linear relativistic dispersion and a mutual Coulomb interaction. We find that the ``intrinsic'' Dirac liquid, where the Fermi energy lies exactly at the nodes of the band dispersion, displays unusual Fermi liquid properties, whereas the ``extrinsic'' system with finite detuning or doping behaves as a standard Landau Fermi liquid. We present analytical and numerical results for the self-energy and spectral function based on both Hartree-Fock and the random phase approximation (RPA) theories and compute the quasiparticle lifetime, residue, and renormalized Fermi velocity of the extrinsic Dirac liquid. A full numerical calculation of the extrinsic RPA spectral function indicates that the Fermi liquid description breaks down for large-energy excitations. Furthermore, we find an additional plasmaron quasiparticle sideband in the spectral function which is discontinuous around the Fermi energy. Our predictions should be observable in ARPES and STM measurements. [Preview Abstract] |
Wednesday, March 4, 2015 3:42PM - 3:54PM |
Q10.00007: Axial anomaly and negative longitudinal magnetoresistance: theory vs. experiment Pallab Goswami, Jed Pixley Traditionally axial anomaly is associated with quantum mechanical violation of U(1) axial symmetry of relativistic Dirac/Weyl fermions in odd spatial dimensions, in the presence of electromagnetic gauge fields. Recently there has been considerable interest in both condensed matter and high energy physics communities in Nielsen and Ninomiya's original proposal that the axial anomaly can lead to a negative longitudinal magnetoresistance for condensed matter realization of Weyl fermions. In this talk I will show that the axial anomaly~can arise in any generic three dimensional material placed under parallel electric and quantizing~ magnetic fields. However, the emergence of negative magnetoresistance depends crucially on the forward scattering nature of the underlying relaxation mechanism. Therefore, sufficiently clean and dilute three dimensional materials without magnetism or magnetic impurities can be promising candidates for observing this phenomenon. I will briefly discuss concrete experimental evidence of this enigmatic effect in quasi-2D layered materials. [Preview Abstract] |
Wednesday, March 4, 2015 3:54PM - 4:06PM |
Q10.00008: Molecular beam epitaxial growth of a three-dimensional topological Dirac semimetal Na$_3$Bi Sung-Kwan Mo, Yi Zhang, Yeongkwan Kim, Zahid Hussain, Zhongkai Liu, Zhi-Xun Shen, Bo Zhou, Yulin Chen Three-dimensional topological Dirac semimetals (3D TDS) represent an unusual state of quantum matter that can be viewed as a bulk analogue of graphene. The realization of 3D TDS is recently verified experimentally in Na$_3$Bi and Cd$_3$As$_2$. We report a molecular beam epitaxial growth of Na$_3$Bi on bilayer graphene and Si(111). Our in-situ angle-resolved photoemission data reveal the 3-dimensional Dirac-cone band structure in such thin films even down to 12 unit cell thickness. Our approach of growing Na$_3$Bi thin film provides a potential route for fabricating it into practical devices while preserving unique properties of 3D TDS. *Yi Zhang et al. Appl. Phys. Lett. 105, 031901 (2014). [Preview Abstract] |
Wednesday, March 4, 2015 4:06PM - 4:18PM |
Q10.00009: Magnetic Catalysis and Axionic Charge-Density-Wave order in Weyl semimetals Bitan Roy, Jay Deep Sau Three-dimensional Weyl and Dirac semiemtals are extremely robust against weak electronic correlations. However, when placed in strong magnetic fields, they can support a chiral-symmetry-breaking charge-density-wave order even for sufficiently weak electron-electron repulsion. Such novel phenomena stems from the existence of one-dimensional chiral zeroth Landau levels which can hybridize for arbitrarily weak interactions. In the former systems, due to the momentum space separation of Weyl nodes the ordered phase lacks translational symmetry and represents a dynamic axionic phase of matter. I will demonstrate the scaling behavior of the spectral gap for a wide range of subcritical (weak) interactions as well as that of diamagnetic susceptibility with magnetic field. I will argue that similar mechanism for charge-density-wave ordering remains operative in double-Weyl semimetals, where dispersion is linear (quadratic) for the z (planar) component(s) of momentum. Role of topological defects, e.g., axion strings, existence of 1-dimensional gapless dispersive modes along the core of such defects, anomaly cancellation through Callan-Hervey mechanism will also be discussed. Pertinence of our study in recently observed Dirac semimetals, such as Cd$_3$As$_2$, Na$_3$Bi, will be addressed. [Preview Abstract] |
Wednesday, March 4, 2015 4:18PM - 4:30PM |
Q10.00010: Dirac and Weyl Superconductors in Three Dimensions Shengyuan Yang, Hui Pan, Fan Zhang Inspired by the recent discovery of Dirac and Weyl semimetals, we introduce the concept of 3D Dirac (Weyl) superconductors (SC), which have protected bulk four(two)-fold nodal points and surface Andreev arcs at zero energy. We provide a sufficient criterion for realizing them in centrosymmetric SCs with odd-parity pairing and mirror symmetry. Pairs of Dirac nodes appear in a mirror-invariant plane when the mirror winding number is nontrivial. Breaking mirror symmetry may gap Dirac nodes producing a topological SC. Each Dirac node evolves to a nodal ring when inversion-gauge symmetry is broken, whereas it splits into a pair of Weyl nodes only when time-reversal symmetry is broken. Our physics might be realized in the nodal phase of Cu-doped Bi2Se3 or UPt3. [Preview Abstract] |
Wednesday, March 4, 2015 4:30PM - 4:42PM |
Q10.00011: Magnetization-dynamics-induced current in Weyl semimetals Daichi Kurebayashi, Kentaro Nomura Weyl semimetals recently gather attention because of its novel transport properties, the anomalous Hall effect (AHE) and the chiral magnetic effect (CME), originated from the chiral anomaly. In contrast to the AHE which has been understood after intensive theoretical and experimental studies in condensed matters, the existence of the CME in static cases is still in debate. We propose a magnetically induced current with temporal varying chiral vector potential, which is new transport phenomenon and essentially different from the CME. We consider the magnetically doped Weyl semimetals, and theoretically study the dynamical effect of the magnetization. As a model, we use the Dirac Hamiltonian coupled to the magnetizations with the exchange interaction. We apply field theoretical methods to derive the expression of the current, and find the magnetic-field-driven current induced by the dynamics of magnetic collective excitations. Furthermore, we also conduct the numerical calculation for the lattice model, and obtain the results which agree with the analytical result. Our result, therefore, is not an artifact of taking continuous model, but experimentally observable in solids. [Preview Abstract] |
Wednesday, March 4, 2015 4:42PM - 4:54PM |
Q10.00012: Quantum transport in three-dimensional Weyl electron system -- in the presence of charged impurity scattering Yuya Ominato, Mikito Koshino Quantum transport in 3D Wey electron system with the charged impurity is studied theoretically using a self-consistent Born approximation (SCBA). The scattering strength is characterized by the effective fine structure constant which depends on the dielectric constant and the Fermi velocity of the linear band. The Boltzmann transport theory works well in a condition that the level broadening is much smaller that the Fermi energy, but it fails near the Weyl point. At the Weyl point, the conductivity takes a nearly constant value which is almost independent of the effective fine structure constant, even though the density of states linearly increases with the effective fine structure constant. The qualitative behavior is significantly different from the case of the Gaussian impurities, where the conductivity exhibits critical behavior and vanishes at the Weyl point below a certain critical disorder strength. [Preview Abstract] |
Wednesday, March 4, 2015 4:54PM - 5:06PM |
Q10.00013: Anomalous Conductivity Tensor and Quantum Oscillations in the Dirac Semimetal Na$_{3}$Bi Jun Xiong, Satya Kushwaha, Jason Krizan, Tian Liang, Robert J. Cava, Nai Phuan Ong Na$_{3}$Bi is a 3D Dirac semimetal with protected nodes. Angle-resolved photoemission experiments have observed these massless Dirac fermions in the bulk band, but transport experiments have been hampered by the extreme air sensitivity of Na$_{3}$Bi crystals. Transport experiments can potentially address interesting issues such as charge pumping between the separated Weyl nodes when the time-reversal symmetry is broken by a strong magnetic field. Here we report a transport measurement that reveals robust anomalies in both the conductivity and resistivity tensors. The resistivity $\rho_{xx}$ is B-linear up to 35 T, while the Hall angle exhibits an unusual profile approaching a step-function. In addition, we have also observed a prominent beating pattern in the Shubnikov de Haas (SdH) oscillations indicating the existence of two nearly equal SdH frequencies when the Fermi energy falls inside the non-trivial gap-inverted regime. [Preview Abstract] |
Wednesday, March 4, 2015 5:06PM - 5:18PM |
Q10.00014: Unconventional localisation transition in high-dimensional semiconductors and Weyl semimetals Sergey Syzranov, Victor Gurarie, Leo Radzihovsky We study a class of non-interacting electron systems with a power-law quasiparticle dispersion $\xi_{\bf k}\propto k^\alpha$ and a random short-correlated potential. We show that, unlike the case of lower dimensions, for $d>2\alpha$ there exists a critical disorder strength (set by the band width), at which the system exhibits a disorder-driven quantum phase transition at the bottom of the band, that lies in a universality class distinct from the Anderson transition. In contrast to the conventional wisdom, it manifests itself in, e.g., the disorder-averaged density of states. For systems in symmetry classes that permit localisation, the striking signature is a non-analytic behaviour of the mobility edge, that is pinned to the bottom of the band for subcritical disorder and grows for disorder exceeding a critical strength. Focusing on the density of states, we calculate the critical behaviour (exponents and scaling functions) at this novel transition, using a renormalisation group, controlled by an $\varepsilon=d-2\alpha$ expansion. We also apply our analysis to Dirac materials, e.g., Weyl semimetal, where this transition takes place in physically interesting three dimensions. [Preview Abstract] |
Wednesday, March 4, 2015 5:18PM - 5:30PM |
Q10.00015: Density of states scaling at the semimetal to metal transition in three dimensional topological insulators Igor Herbut, Ken Imura, Tomi Ohtsuki, Koji Kobayashi The quantum phase transition between the three dimensional Dirac semimetal and the diffusive metal can be induced by increasing disorder. Taking the system of disordered Z2 topological insulator as an important example, we compute the single particle density of states by the kernel polynomial method. We focus on three regions: the Dirac semimetal at the phase boundary between two topologically distinct phases, the tricritical point of the two topological insulator phases and the diffusive metal, and the diffusive metal lying at strong disorder. The density of states obeys a novel single parameter scaling, collapsing onto two branches of a universal scaling function, which correspond to the Dirac semimetal and the diffusive metal. The diverging length scale critical exponent and the dynamical critical exponent are estimated, and found to differ significantly from those for the conventional Anderson transition. Critical behavior of experimentally observable quantities near and at the tricritical point is also discussed. (K. Kobayashi et al, Phys. Rev. Lett. vol. 112, 016402 (2014)) [Preview Abstract] |
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