Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session V44: Dirac and Weyl Semimetals: Theory V |
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Sponsoring Units: DMP Chair: Madhab Neupane, University of Central Florida Room: 391 |
Thursday, March 16, 2017 2:30PM - 2:42PM |
V44.00001: Inverse Perovskites - A New Platform For 3D Dirac Electron Physics A.W. Rost, J. Kim, S. Shota, K. Hayama, V. Abdolazimi, J.A.N. Bruin, C. Muehle, A. Schnyder, A.N. Yaresko, J. Nuss, H. Takagi 3D Dirac semimetals show a wealth of phenomena including ultrahigh mobility, extreme transverse magnetoresistance and potential for negative longitudinal magnetoresistance. Furthermore, by introducing a gap these are often found to be topological crystalline insulators. Here, I will introduce our experiments on a new family of 3D Dirac materials -- the inverse perovskites A$_{\mathrm{3}}$BO (A$=$Ca,Sr,Eu/B$=$Pb,Sn). These open up the possibility to chemically control the properties of Dirac electrons including (i) the anisotropy of the Dirac dispersion, (ii) role of spin orbit coupling, and (iii) magnetism. Our physical property measurements show all (Ca/Sr)$_{\mathrm{3}}$(Pb/Sn)O compounds host Dirac electrons at the Fermi energy with no other bands crossing $E_{\mathrm{F}}$. Quantum oscillations unveil small Fermi surfaces (frequencies \textless 5 T) and light carriers (\textless 0.02 $m_{\mathrm{e}})$ only consistent with Dirac electrons. With the successful synthesis of Sr$_{\mathrm{3}}$Pb$_{\mathrm{0.5}}$Sn$_{\mathrm{0.5}}$O this group of materials therefore offers a unique chemical control over the physical properties of 3D Dirac electrons. Crucially, Eu$_{\mathrm{3}}$(Pb/Sn)O compounds allow for the introduction of magnetism. I will discuss the implications of this in particular with respect to surface states in these topological crystalline insulators. [Preview Abstract] |
Thursday, March 16, 2017 2:42PM - 2:54PM |
V44.00002: Strain engineering of topological phase transition in elemental gray tin: Dirac semimetal phase in the missing half of strain spectrum Huaqing Huang, Feng Liu Gray tin was previously found to be a strong topological insulator under compressive uniaxial strain. Here, based on effective $k.p$ analysis and first-principles calculations, we discover that gray tin becomes a Dirac semimetal in the other missing half of strain spectrum, under tensile uniaxial strain. In this newly found Dirac semimetal state, two Dirac points which are tunable by tensile [001] strains, lie in the $k_z$ axis and Fermi arcs appear in the (100) surface. A large negative magnetoresistance is anticipated in this half of strain spectrum, which shows as a strong signature of the chiral anomaly effect. Comparing to other Dirac semimetal materials, the proposed Dirac semimetal state in the nontoxic elemental gray tin can be more easily manipulated and accurately controlled. We envision that gray tin provides a perfect platform for strain engineering of topological phase transitions by sweeping through the strain spectrum from positive to negative and vice versa. [Preview Abstract] |
Thursday, March 16, 2017 2:54PM - 3:06PM |
V44.00003: Magnetic field control of Weyl node population in Pb$_{1-x}$Sn$_x$Te Nicholas Kioussis, Jinwoong Kim, Tian Liang, N. P. Ong For materials possessing topological phase transition, the Weyl semimetal phase can be induced by breaking either the time-reversal or inversion symmetry. The topological crystalline insulator, Pb$_{1-x}$Sn$_x$Te exhibits topological phase transition upon the band inversion strength which can be tailored by the substitutional mixing ratio, strain, thermal expansion, ferroelectric displacement, and/or material thickness via quantum confinement effect. The SnTe building block of the compound is also known to exhibit a ferroelectric transition at low temperatures which leads to inversion symmetry breakdown. Therefore one can expect that Pb$_{1-x}$Sn$_x$Te exhibits diverse topological phases including a Weyl semimetal phase. In this study, using $\it{ab}$-$\it{initio}$-tight-binding calculations we have explored the parameter space associated with both band inversion and ferroelectric displacement. The calculated topological phase diagram shows the emergence of a Weyl semimetal phase. We will also present results of the evolution of Weyl nodes with magnetic field. [Preview Abstract] |
Thursday, March 16, 2017 3:06PM - 3:18PM |
V44.00004: First-principles study of band-gap inverted bulk black phosphorus Han-gyu Kim, Hyoung Joon Choi We performed first-principles calculations to study effects of spin-orbit coupling on the electronic band structure of bulk black phosphorus (BP) when the conduction band minimum and the valence band maximum are inverted. Without the spin-orbit coupling, our calculation shows that the crossing points of the valence and conduction bands in the band-gap inverted bulk BP form a closed loop in the momentum space. When we include the spin-orbit coupling in our calculations, the crossing of the valence and conduction bands is lifted at every point in the loop with k-dependent splitting size. We discuss the effects of spin-orbit coupling in band-gap inverted bulk BP by analyzing the symmetry of the atomic structure and the orbital character of the wave functions. This work was supported by NRF of Korea (Grant No. 2011-0018306) and KISTI supercomputing center (Project No. KSC-2016-C3-0052). [Preview Abstract] |
Thursday, March 16, 2017 3:18PM - 3:30PM |
V44.00005: Chemically-induced Large-Gap Quantum Anomalous Hall Insulator States in III-Bi Honeycombs Christian P. Crisostomo, Zhi-Quan Huang, Chia-Hsiu Hsu, Feng-Chuan Chuang, Hsin Lin, Arun Bansil The search for new novel materials has increased due to their interesting properties and promising applications. Quantum anomalous Hall (QAH) effect was recently realized in magnetic topological insulators (TIs) but only observable at extremely low temperatures. In this letter, we predict large-gap QAH insulating phases in chemically functionalized III-Bi honeycombs using first-principles electronic structure calculations. QAH insulator phases were found in functionalized AlBi and TlBi, while GaBi and InBi were identified as semimetals with non-zero Chern number. Remarkably, TlBi exhibits robust and large-gap QAH insulator phases with band gap as large as 466 meV for one-sidedly functionalized and buckled honeycomb. Furthermore, the electronic band spectrum of functionalized TlBi nanoribbon with zigzag edge is shown to possess only one chiral edge band crossing the Fermi level within the band gap. With the recent progress of III-V materials, our results suggest that III-Bi honeycomb would provide a new platform for developing novel spintronics devices based on the QAH phase. [Preview Abstract] |
Thursday, March 16, 2017 3:30PM - 3:42PM |
V44.00006: $Z_2$ topological insulator in alkaline earth - pnictide antiperovskites Wen Fong Goh, Warren Pickett Compounds with antiperovskite structure have been suggested to be potential topological insulators, due to their small band gap or gapless electronic characteristics. Using first principles calculations, we survey the entire class of $3\times5\times5$ cubic alkaline earth - pnictide antiperovskite compounds, viz. $\mathrm{Ae_3Pn_APn_B}$, where $\mathrm{Ae=Ca,Sr,Ba}$ and $\mathrm{Pn_A,Pn_B=N,P,As,Sb,Bi}$, and have classified these compounds into trivial insulator or topological semimetal. For the trivial insulators, strain can invert the band ordering to produce topological insulators, while for the topological semimetals, where the band ordering has been inverted by spin-orbit coupling but leaves a gapless bulk state, strain can open up a gap while maintaining the inverted band ordering. Among the topological semimetals, the narrow gap semiconductor $\mathrm{Ca_3BiP}$, is used as an example to illustrate the role played by the spin-orbit coupling and strain in the topological insulator to Dirac semimetal phase transition. Results show that it can be driven into a topological insulating phase under uniaxial compression, and a Dirac semimetallic state under uniaxial expansion. The topological surface states and Fermi arc will be presented and discussed. [Preview Abstract] |
Thursday, March 16, 2017 3:42PM - 3:54PM |
V44.00007: The Electronic Structure of Rare-Earth Pnictides. shoaib khalid, Anderson Janotti The structural parameters and electronic band structure of rare-earth pnictides are investigated using density functional theory (DFT) with the Heyd, Scuseria, and Ernzerhof (HSE06) screened hybrid functional. The rare-earth pnictides which are the focus of this study include RE-V compounds, where RE$=$La, Nd, Sm, Gd, Tb, Er, Lu, and V$=$As, Sb, Bi, in the rock-salt crystal structure. All the calculations include spin-orbit interaction, and we pay special attention to the effects of including the RE $f$ electrons in the valence. The results of HSE06 calculations are compared with DFT within the generalized gradient approximation (GGA) and other previous calculations. Finally, we also investigate the effect of hydrostatic and biaxial strain on the band structure, in special the role of epitaxial strain in lifting degeneracies of the bands near the Fermi level. [Preview Abstract] |
Thursday, March 16, 2017 3:54PM - 4:06PM |
V44.00008: Ab initio study of topological phases of lithium at high pressures Stephanie Mack, Sinead Griffin, Jeffrey Neaton Solid lithium under high but experimentally achievable pressures is known to exhibit a rich landscape of phases beyond what is expected for a ``simple" metal. Notably, lithium adopts increasingly lower-symmetry phases with increasing density [1] rather than retaining close-packed structures, possibly even culminating in a paired phase. As the overlap between neighboring lithium core states increases, Pauli exclusion and orthogonality force valence electrons into the interstitial regions, driving phase transitions to lower symmetry structures [2,3] and the onset of semi-metallic behavior [3,4]. Using first-principles calculations, we show that the electronic structure of theoretically predicted phases exhibit massless Dirac fermions at the Fermi energy. We explore the possibility of nontrivial topological properties in other lithium phases and extend our study to high-pressure phases of other alkali metals. [1] Guillaume et al., Nat. Phys. 7, 211 (2011) [2] Neaton and Ashcroft, Nature 400, 141 (1999) [3] Marqu\'{e}s et al., PRL 106, 095502 (2011) [4] Matsuoka et al., PRB 89, 144103 (2014) [Preview Abstract] |
Thursday, March 16, 2017 4:06PM - 4:18PM |
V44.00009: Tuning topological phases in the \textit{X}MnSb$_2$ system via chemical substitution from first principles Sinead M. Griffin, Jeffrey B. Neaton New Dirac materials are sought for their interesting fundamental physics and for their potential technological applications. Protected symmetries offer a route to potential zero mass Dirac and Weyl fermions, and can lead unique transport properties and spectroscopic signatures. In this work, we use first-principles calculations to study the \textit{X}MnSb$_2$ family of materials and show how varying \textit{X} changes the nature of bulk protected topological features in their electronic structure. We further discuss new design rules for predicting new topological materials suggested by our calculations. [Preview Abstract] |
Thursday, March 16, 2017 4:18PM - 4:30PM |
V44.00010: Dirac semimetal phase in the hexagonal LiZnBi Wendong Cao, Peizhe Tang, Yong Xu, Jian Wu, Bing-Lin Gu, Wenhui Duan Based on first-principles calculations, we find that LiZnBi, a metallic hexagonal $ABC$ compound, can be driven into a topologically nontrivial Dirac semimetal by strain. The nontrivial topological nature of the strained LiZnBi is directly demonstrated by calculating its $Z_2$ index at $k_z=0$. We show that there are two Dirac points located at the rotation axis, protected by $C_{6v}$ symmetry and time-reversal symmetry. In the calculated surface states, the Fermi arcs connecting the projections of these two Dirac points are found. We also present how the low-energy states as well as topological properties change under different strain configurations. The finding of Dirac semimetal phase in LiZnBi may intrigue further researches on the topological properties of hexagonal $ABC$ materials and promote new practical applications. [Preview Abstract] |
Thursday, March 16, 2017 4:30PM - 4:42PM |
V44.00011: Weyl Ferroelectric Semimetal Hongming Weng, Ronghan Liang, Yuanfeng Xu, Jiangang He, Sami Ullah, Jiangxiu Li, Junming Liu, Dianzhong Li, Cesare Franchini, Xing-Qiu Chen The recent discoveries of ferroelectric metal and Weyl semimetal (WSM) have stimulated a natural question: whether these two exotic states of matter can coexist in a single material or not. These two discoveries ensure us that physically it is possible since both of them share the same necessary condition, the broken inversion symmetry. By using first-principles calculations, we demonstrate that the experimentally synthesized nonmagnetic HgPbO$_3$ is such a hybrid "Weyl ferroelectric semimetalâ€ť. Its centrosymmetric $R$\bar{3}c$ phase will undergo a ferroelectric phase transition to the ferroelectric $R3c$ structure. Both phases are metallic. Most importantly, it also harbors six pairs of chiral Weyl nodes around the Fermi level to be an oxide WSM. The coexistence of ferroelectricity and Weyl nodes in HgPbO$_3$ is an ideal platform for exploring multiphase interaction and mutual control. The Weyl nodes can be tuned by temperature or external pulse electric field, which is promising for potential applications. [Preview Abstract] |
Thursday, March 16, 2017 4:42PM - 4:54PM |
V44.00012: First-Principles Studies of TiAsRh, a new Dirac nodal line system Sophie Weber, Ru Chen, Qimin Yan, Jeffrey Neaton A recent development in condensed matter physics is the discovery of topological semimetals which have features in the band structure protected by an interplay of symmetry and topology. Dirac Nodal Line (DNL) systems are one such class of materials in which the conduction and valence bands touch in a closed loop in momentum space. Using density functional theory (DFT) calculations, we propose that TiAsRh is a DNL system and study the symmetry leading to the DNL. We find that a mirror plane protects the crossings without spin-orbit coupling (SOC), and demonstrate that SOC modestly gaps out the NL. We perform our study with standard semi-local and hybrid density functionals and show that the nodal line is robust with respect to functional. Implications for experiments on this compound, which has been previously synthesized, are discussed. [Preview Abstract] |
Thursday, March 16, 2017 4:54PM - 5:06PM |
V44.00013: Topological Nodal Line Semimetals in CaP3 family of materials Qiunan Xu, Rui Yu, Zhong Fang, Xi Dai, Hongming Weng By using first-principle calculations and k$\cdot$p model analysis, we propose that the three-dimensional (3D) topological nodal line semimetal state can be realized in CaP$_3$ family of materials, which include CaP$_3$, CaAs$_3$, SrP$_3$, SrAs$_3$ and BaAs$_3$, when spin-orbit coupling (SOC) is ignored. The closed topological nodal line near the Fermi energy is protected by time-reversal symmetry and spatial inversion symmetry. Moreover, a drumhead-like two-dimensional surface states are also obtained on the c-direction surface of these materials. When SOC is included, the nodal line will open a gap and becomes a strong topological insulator with (1;100) Z$_2$ indices. [Preview Abstract] |
Thursday, March 16, 2017 5:06PM - 5:18PM |
V44.00014: Electron interactions, spin-orbit coupling, intersite correlations in pyrochlore iridates: a comparison of single-site and cluster calculations Runzhi Wang, Ara Go, Andrew Millis Pyrochlore iridates ($R_2Ir_2O_7$) are studied using density functional theory plus single-site and cluster dynamical mean-field theory (DFT+DMFT). The calculations include spin-orbit coupling. Significant differences between the single-site and cluster calculations are found. The single-site approximation fails to account for the properties of the paramagnetic insulator phase, in particular predicting a larger gap than found in experiments, while cluster calculations yield gaps consistent with transport data. A ground-state phase diagram is computed. Paramagnetic metal, metallic all-in/all-out (AIAO) and insulating AIAO phases are found. Tilted Weyl cones are observed in the AIAO metallic phase for a relatively wide range of interaction strength. Our paramagnetic calculations predict almost identical behaviors for the Y and Eu compound, conflicting with the strong material dependence reported in experiments. Inclusion of magnetic order restores the material difference. The physical origin of the difference is discussed. The results indicate that intersite effects, most likely of antiferromagnetic origin, play an important role in studying the physics of pyrochlore iridates. [Preview Abstract] |
Thursday, March 16, 2017 5:18PM - 5:30PM |
V44.00015: Identification and properties of the non-cubic phase of Mg$_{\mathrm{2}}$Pb. Yuwei Li, Guang Bian, David Singh Mg$_{\mathrm{2}}$Pb is a superconducting semimetal that occurs in a cubic \textit{Fm}-\textit{3m} structure. However, Eldridge and co-workers reported a lower symmetry structure with slight off stoichiometry, but were not able to refine. Here we identify this phase and report its properties, based on first principles calculations and structure predicting methods. We find a metallic tetragonal ($P$4/\textit{nmm}) compound with interesting anisotropy. First principles total energy calculations indicate the enthalpy of $P$4/\textit{nmm} structure is only 2 meV/atom higher than that of \textit{Fm}-3$m$ structure. [Preview Abstract] |
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