Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session C44: Topological Protection in Correlated Electron Systems 1 |
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Sponsoring Units: DCMP Chair: Laurel Winter, Los Alamos National Laboratory Room: LACC 504 |
Monday, March 5, 2018 2:30PM - 2:42PM |
C44.00001: Electrical Transport in Non-symmorphic Weyl Semimetals Takehito Suzuki , Lucile Savary , Jianpeng Liu , Jeffrey Lynn , Leon Balents , Joseph Checkelsky Weyl semimetals support unusual transport properties including the chiral anomaly and significant band-structure driven anomalous Hall effect. We study here the effect of nonsymmorphic symmetries on the transport response in Weyl semimetals, focusing on the relationship between magnetoresistance and symmetry. For magnetic systems we find the temperature and field modulation of the magnetic symmetries strongly affect the transport response. We discuss the relationship of this response to the expected Weyl points in the system and compare to that observed in thermodynamic and spectroscopic probes. |
Monday, March 5, 2018 2:42PM - 2:54PM |
C44.00002: Transport properties of interacting Weyl semi-metals Shaheen Acheche , Reza Nourafkan , A.-M. Tremblay Predicted theoretically in 2011 and experimentally discovered a few years later, Weyl semi-metals have become a vast and dynamic field in condensed matter. This interest is due to rich physics linked to exceptional topological properties of these metals, in particular the chirality of their nodes. The non-trivial topology influences the transport properties of the Weyl semimetals, leading to negative magneto-resistance and anomalous Hall effect, for instance. |
Monday, March 5, 2018 2:54PM - 3:06PM |
C44.00003: Effects of topological quantum phase transitions on chiral transport properties of Weyl fermions Pallab Goswami Topological Weyl semimetals are separated from trivial metals or insulators by a quantum critical point with anisotropic dispersion relations. We discuss the exotic quantum critical properties associated with such phase transitions. We particularly emphasize how such a quantum critical point governs the scaling laws for chiral transport properties arising due to the underlying Berry curvature in a time reversal symmetry breaking system. We also discuss the relevance of our results for time reversal symmetry breaking magnetic Weyl materials. |
Monday, March 5, 2018 3:06PM - 3:18PM |
C44.00004: Electronic transport in strain Weyl semimetals Enrique Munoz , Rodrigo Soto Garrido We consider the electronic transport in Weyl semimetals (WSM) due to the superposition of an external magnetic field and mechanical strain. We find exact analytical solutions for single-particle states inside and outside of the strained region. We obtain analytical expressions for the scattering cross-section in this setup, that allows us to explicitly calculate analytically the current-voltage characteristics, as well as the differential conductance through the strained region within the Landauer approximation [1]. We study the dependence of the conductance as a function of temperature, magnetic field and mechanical strain. |
Monday, March 5, 2018 3:18PM - 3:30PM |
C44.00005: Dissolution of topological Fermi arcs in a dirty Weyl semimetal Robert-Jan Slager , Bitan Roy , Vladimir Juricic Weyl semimetals (WSMs) have attracted much attention recently as they provide for a condensed matter realization of a chiral anomaly, feature topologically protected Fermi arc surface states, and sustain sharp chiral Weyl quasiparticles up to a critical disorder at which a continuous quantum phase transition (QPT) drives the system into a metallic phase. We present numerical results that demonstrate that the Fermi arc gradually loses its sharpness upon increasing disorder, resulting in the dissolution of the arc into the metallic bath of the bulk close to the WSM-metal QPT [1]. These results verify the predicted topological nature of the WSM-metal QPT and the corresponding bulk-boundary correspondence across the transition. Moreover, by following the continuous deformation of the Fermi arcs with increasing disorder in discovered Weyl materials, this effect should be directly discernible in angle-resolved photoemission spectroscopy (ARPES) and Fourier transformed scanning tunneling microscopy (STM) measurements. |
Monday, March 5, 2018 3:30PM - 3:42PM |
C44.00006: A generic phase between disordered Weyl semimetal and diffusive metal Xiang Rong Wang , Ying Su , X. S. Wang Quantum phase transitions of Weyl semimetals (WSMs) subject to on-site disorder are investigated. Contrary to a previous belief that a direct transition from a WSM to a diffusive metal (DM) occurs, an intermediate phase of Chern insulator (CI) between the two distinct metallic phases should exist. The critical exponent of localization length is 1.3 for both the WSM-CI and CI-DM transitions, in the same universality class of the 3D Gaussian unitary ensemble of Anderson localization transition. The CI phase was confirmed by quantized nonzero Hall conductance in the bulk insulating phase established by localization length calculations. The disorder-induced various plateau-plateau transitions in both WSM and CI phases were observed and explained by the self-consistent Born approximation. Furthermore, we clarify that the occurrence of zero density of states at Weyl nodes is not a good criterion for the disordered WSM, and there is no fundamental principle to support the hypothesis of divergence of localization length at the WSM-DM transition. |
Monday, March 5, 2018 3:42PM - 3:54PM |
C44.00007: Ballistic Weyl semimetal to diffusive metal phase transitions driven by quasiperiodicity Jed Pixley , Justin Wilson , David Huse , Sarang Gopalakrishnan We discuss our recent study on the effects of a quasiperiodic potential and the absence of strong randomness on the stability of the semimetal phase in three-dimensional Dirac and Weyl semimetals. We find that the semimetal phase at the Weyl node energy is stable in the presence of three-dimensional quasiperiodic potentials and tuning the strength of the potential leads to a series of Weyl semimetal to metal quantum phase transitions. We demonstrate that the metallic phase at the Weyl node energy has a non-zero density of states and is diffusive as determined through level statistics that are consistent with random matrix theory and wave packet dynamics. We will also present the critical properties at the bonafide semimetal to metal quantum phase transition. |
Monday, March 5, 2018 3:54PM - 4:06PM |
C44.00008: Optical conductivity of an interacting Weyl liquid in the collisionless regime Vladimir Juricic , Bitan Roy Optical conductivity can serve as an indispensable probe of correlation effects in a wide range of materials, from high-Tc superconductors, heavy fermion systems, Fe-based superconductors to graphene, as well as Weyl and Dirac semimetals. As we will show, the long range Coulomb interaction yields a universal enhancement of the zero-temperature optical conductivity that depends solely on the number of Weyl points at the Fermi level [1]. To this end, we use dimensional regularization about three spatial dimensions, since this regularization scheme explicitly preserves gauge invariance. The scaling of optical conductivity is a remarkable consequence of an interplay between the quantum-critical nature of an interacting Weyl liquid, marginal irrelevance of the long-range Coulomb interaction and the violation of hyperscaling in three spatial dimensions. Experimental consequences of this effect in recently discovered Weyl and Dirac materials will be outlined. |
Monday, March 5, 2018 4:06PM - 4:18PM |
C44.00009: Interacting line-node semimetal: Proximity effect and spontaneous symmetry breaking Bitan Roy Effects of short-range electronic interactions in a three-dimensional line-node semimetal that supports linearly dispersing quasiparticles around an isolated loop in the Brillouin zone will be presented. I will argue that due to the vanishing density of states ( ρ(E) ~ |E|) various orderings in the bulk of the system, such as the antiferromagnet and charge-density-wave, set in for sufficiently strong onsite (U) and nearest-neighbor (V) repulsions, respectively. While onset of these two orderings from the semimetallic phase takes place through continuous quantum phase transitions, a first-order transition separates two ordered phases. I will demosntrate that topologically protected drumhead shaped surface states, on the other hand, can undergo charge or spin orderings, depending on relative strength of U and V, even when they are sufficiently weak. Such surface orderings as well as weak long-range Coulomb interaction can be conducive to spontaneous symmetry breaking in the bulk for weaker interactions. I will present numerical results supporting the scenario of proximity effect driven spontaneous symmetry breaking in the bulk for subcritical strength of interactions due to flat surface band. |
Monday, March 5, 2018 4:18PM - 4:30PM |
C44.00010: Symmetric-Gapped Surface States of Fractional Topological Insulators Gil Young Cho , Jeffrey Teo , Eduardo Fradkin We construct the symmetric-gapped surface states of a fractional topological insulator with electromagnetic θ-angle θ_{em} = π/3 and a discrete Z_{3} gauge field. They are the proper generalizations of the T-pfaffian state and pfaffian/anti-semion state and feature an extended periodicity compared with their of “integer” topological band insulators counterparts. We demonstrate that the surface states have the correct anomalies associated with time-reversal symmetry and charge conservation. |
Monday, March 5, 2018 4:30PM - 4:42PM |
C44.00011: Anomalies of discrete symmetries in theories of Weyl fermions Chang-Tse Hsieh , Shinsei Ryu Weyl fermions, which are massless fermions with a definite chirality, are thought of as a building block for fermions in quantum field theory. They are not only proposed as fundamental particles in nature (though not observed yet) but can be realized as emergent quasiparticles in a low-energy condensed matter system, such as Weyl semimetals and Weyl superconductors. As Weyl fermions are chiral, symmetries acting on them in a chiral way might be anomalous. While anomalies of Weyl fermions with continuous symmetries like U(1) have been well understood, the cases of discrete symmetries have been studied much less. In this talk, I will discuss anomalies of discrete symmetries in theories of Weyl fermions, and address some possible applications in condensed matter systems. |
Monday, March 5, 2018 4:42PM - 4:54PM |
C44.00012: Surface Magnetism in Topological Crystalline Insulators Sahinur Reja , Herbert Fertig , Luis Brey , Shixiong Zhang Topological crystalline insulators (TCI's) are a class of materials in which the energy bands can host non-trivial topology protected by a crystalline symmetry rather than by time reversal symmetry as in the case of usual topological insulators. By deriving an analytic surface theory and numerical simulations, we study TCT's doped with magnetic impurities, in which ferromagnetism at the surface lowers the electronic energy by spontaneous breaking of a crystalline symmetry. The number of energetically equivalent groundstates is sensitive to the crystalline symmetry of the surface, as well as the precise density of electrons at the surface. We show that for a SnTe model in the topological state, magnetic states can have two-fold, six-fold symmetry, or eight-fold degenerate minima. We compute spin stiffnesses within the model to demonstrate the stability of ferromagnetic states, and consider their ramifications for thermal disordering. Possible experimental consequences of the surface magnetism are discussed. |
Monday, March 5, 2018 4:54PM - 5:06PM |
C44.00013: Topological semimetals induced by magnetic control of the cluster multipoles in pyrochlore iridates Taekoo Oh , Bohm-Jung Yang Pyrochlore iridate antiferromagnet is the first material in which Weyl fermions are predicted to exist in condensed matter. However, the accumulated reports show that Weyl semimetal (WSM) phase in the material can be realized in a narrow region in the parameter space, so the smoking-gun evidence for WSM is still missing. As unique transport phenomena under magnetic field, which can appear by Weyl fermions, have been reported, in this work, we theoretically propose that magnetic field is a promising way to realize WSM phase, by expanding the WSM range and creating novel WSM phases through additional band inversions. Here, the central role is played by the quadratic band crossing (QBC) at Γ point with fourfold degeneracy in the paramagnet state. Due to the large degeneracy of QBC and strong spin-orbit coupling, Luttinger q-term or anisotropic Zeeman Effect can be included, as well as usual Zeeman Effect. Moreover, the relative magnitude of two Zeeman terms can be controlled by varying the orientation of four spins in the unit cell, which, in turn, manipulates the topological property of iridium band structure. The intriguing behavior occurs beacuse the unit cell comprises a cluster of four spins in a tetrahedron whose multipole moments can be tuned by the spin orientation. |
Monday, March 5, 2018 5:06PM - 5:18PM |
C44.00014: Dielectric anomalies and interactions in the 3D quadratic band touching Luttinger semimetal Pr$_2$Ir$_2$O$_7$ Bing Cheng , Takumi Ohtsuki , Dipanjan Chaudhuri , Satoru Nakatsuji , Mikk Lippmaa , Peter Armitage Dirac and Weyl semimetals with linearly crossing bands are the focus of much recent interest in condensed matter physics. Although they host fascinating phenomena, their physics can be understood in terms of weakly interacting electrons. In contrast, more than 40 years ago, Abrikosov pointed out that quadratic band touchings are generically strongly interacting. We have performed terahertz spectroscopy on films of the conducting pyrochlore Pr$_2$Ir$_2$O$_7$, which has been shown to host a quadratic band touching. A dielectric constant as large as $ \tilde{\varepsilon }/\epsilon_0 \sim 180$ is observed at low temperatures. In such systems the dieletric constant is a measure of the relative scale of interactions, which are therefore in our material almost two orders of magnitude larger than the kinetic energy. Despite this, the scattering rate exhibits a $T^2$ dependence, which shows that for finite doping a Fermi liquid state survives, however with a scattering rate close to the maximal value allowed. |
Monday, March 5, 2018 5:18PM - 5:30PM |
C44.00015: A First Principles Plus Many-Body Approach to the Correlated Spin Dynamics in SmB_{6} Christopher Singh , Wei-Cheng Lee The emergent dynamical processes in heavy fermion materials like SmB_{6} are highly dependent on the momentum space electronic structure. Here we present a microscopic model to capture these effects by imposing many-body correlations on a high quality single particle description produced by density functional theory. We calculate the low energy spin excitation spectrum within the random phase approximation, and find good agreement with magnetic neutron scattering experiments. We also calculate the mean field magnetic phase diagram, and show the ground state is highly quantum critical with several competing phases. We further find novel interplay between the magnetic ordering and the orbital ordering in the f-levels, which could drive non-trivial topological phase transitions at finite temperatures. Implications of our results in light of recent experiments will be discussed. This work emphasizes the importance of a high quality momentum space description for describing correlation effects in heavy fermion systems. |
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