Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session T7: Focus Session: Dirac, Weyl Semimetals, Kondo Insulators 
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Sponsoring Units: DMP DCMP Chair: Kai Sun, University of Michigan Room: 006B 
Thursday, March 5, 2015 11:15AM  11:27AM 
T7.00001: First Principles Prediction of Topological Phases in Thin Films of Pyrochlore Iridates Xiang Hu, Zhicheng Zhong, Gregory A. Fiete We make materialsspecific predictions for topological phases using density functional theory combined with HartreeFock theory that includes the full orbital structure of the relevant iridum $d$orbitals and the strong but finite spinorbit coupling strength. We find Y$_2$Ir$_2$O$_7$ bilayer and trilayer films grown along the [111] direction can respectively support a Z$_2$ topological metallic phase and a Chern metallic phase with a direct gap of up to 0.02 eV. These results could potentially bring transition metal oxides to the fore as a new class of topological materials with potential applications in oxide electronics. [Preview Abstract] 
Thursday, March 5, 2015 11:27AM  11:39AM 
T7.00002: ARPES study on a stable threedimensional topological Dirac semimetal Cd3As2 Zhongkai Liu, Juan Jiang, Bo Zhou, Zhijun Wang, Yi Zhang, Hongming Weng, Dharmalingam Prabhakaran, SungKwan Mo, Han Peng, Pavel Dubin, Timur Kim, Moritz Hoesch, Zhong Fang, Xi Dai, ZhiXun Shen, Donglai Feng, Zahid Hussain, Yulin Chen Threedimensional topological Dirac semimetal (TDS) is a new state of quantum matter recently proposed and has attracted increasing attention in physics and material science. A 3D TDS not only is a bulk analogue of graphene, but also shows nontrivial topology in its electronic structure. A TDS can also be driven into other novel states with symmetry breaking, making it a unique ``mother compound'' for the study of these states. By performing angleresolved photoemission spectroscopy, we directly observed a pair of 3D Dirac fermions in Cd3As2, proving that it is a model 3D TDS. Comparing to other 3D TDSs, Cd3As2 is stable and have much higher Fermi velocities. Furthermore, by insitu doping, we were able to tune its Fermienergy, making it a flexible platform for exploring exotic physical phenomena and application potentials. [Preview Abstract] 
Thursday, March 5, 2015 11:39AM  11:51AM 
T7.00003: Landau level splitting in Cd$_{3}$As$_{2}$ under high magnetic fields Cheng Zhang, Junzhi Cao, Sihang Liang, Zhengcai Xia, Liang Li, Faxian Xiu Threedimensional (3D) topological Dirac semimetals (TDSs) are a new kind of Dirac materials that adopt nontrivial topology in band structure and possess degenerated massless Dirac fermions in the bulk. It has been proposed that TDSs can be driven to other exotic phases like Weyl semimetals, topological insulators and topological superconductors by breaking certain symmetries. Here we report the first transport evidence of Landau level splitting in TDS Cd$_{3}$As$_{2}$ single crystals under high magnetic fields, suggesting the removal of spin degeneracy by breaking time reversal symmetry (TRS). The observed Landau level splitting is originated from the joint contributions of orbit and Zeeman splitting in Cd$_{3}$As$_{2}$. In addition, the detected Berry phase is found to vary from nontrivial to trivial at different field directions, revealing a fierce competition between the orbitcoupled field strength and the fieldgenerated mass term. Our results demonstrate a feasible path to generate a Weyl semimetal phase based on the TDSs by breaking TRS. [Preview Abstract] 
Thursday, March 5, 2015 11:51AM  12:27PM 
T7.00004: Magnetic and thermodynamic properties of the harmonic honeycomb iridates Invited Speaker: James Analytis Spin and orbital quantum numbers play a key role in the physics of Mott insulators, but in most systems they are connected only indirectlyvia the Pauli exclusion principle and the Coulomb interaction. Iridiumbased oxides (iridates) introduce strong spinorbit coupling directly, such that these numbers become entwined together and the Mott physics attains a strong orbital character. In the layered honeycomb iridates this is thought to generate highly spinanisotropic magnetic interactions, coupling the spin to a given spatial direction of exchange and leading to strongly frustrated magnetism. Here we report a new iridate structure that has the same local connectivity as the layered honeycomb and exhibits striking evidence for highly spinanisotropic exchange. The basic structural units of this material suggest that a new family of threedimensional structures could exist, the `harmonic honeycomb' iridates, of which the present compound is the first example. [Preview Abstract] 
Thursday, March 5, 2015 12:27PM  12:39PM 
T7.00005: Quantum oscillations in a minimal model for pyrochlore iridates Jun Won Rhim, Yong Baek Kim Motivated by recent experiments on Pr$_2$Ir$_2$O$_7$, we provide a theory of quantum oscillations in a minimal model for pyrochlore iridates. Focusing on the conduction electron degrees of freedom with strong spinorbit coupling and considering the electronic structure near the Fermi level, we compute quantum oscillation signals in the paramagnetic state of the model. We compare our theoretical results with existing experimental data on Pr$_2$Ir$_2$O$_7$ and discuss implication to future experiments. [Preview Abstract] 

T7.00006: ABSTRACT WITHDRAWN 
Thursday, March 5, 2015 12:51PM  1:03PM 
T7.00007: Breakdown of Threedimensional Dirac Semimetal State in pressurized Cd$_{3}$As$_{2}$ Liling Sun, Shan Zhang, Qi Wu, Leslie Schoop, Mazhar Ali, Youguo Shi, Ni Ni, Quinn Gibson, Shan Jiang, Vladimir Sidorov, Xi Dai, Zhong Fang, Robert J. Cava, Zhongxian Zhao Theory predicts that threedimensional Dirac semimetals (3DDSMs) can be turned into a variety of novel quantum phases by tuning control parameters to break either time reversal symmetry or inversion symmetry. Pressure is one such parameter, and can manipulate electronic and crystal structures without changing the chemistry. Here, we report the first observation of a pressureinduced breakdown of the 3DDSM state in Cd$_{3}$As$_{2}$. \textit{Insitu} synchrotron Xray and single crystal resistance measurements find that Cd$_{3}$As$_{2}$ undergoes a structural phase transition from a metallic tetragonal phase to a semiconducting high pressure phase at 2.57 GPa; the phase transition breaks the semimetal state. Applying pressure around the phase transition, we observe unusual physical phenomena, including dramatic changes in mobility, Hall resistance and magnetoresistance in addition to the gap opening, which demonstrate the breakdown of the 3DDSM state. [Preview Abstract] 
Thursday, March 5, 2015 1:03PM  1:15PM 
T7.00008: Magnetoresistance and Hall Effect studies on SmB$_{6}$ films Jie Yong, Yeping Jiang, Xiaohang Zhang, Jongmoon Shin, Brian Kirby, Ichiro Takeuchi, Richard Greene We report magnetoresistance and Hall effect measurements on cosputtered nanocrystalline SmB$_{6}$ films up to 9T. The magnetoresistance (MR) at 2K is linearly positive at low field and becomes negative at higher field. The unusual positive MR is similar to observations in other noncorrelated topological insulators. The Hall coefficient R$_{\mathrm{H}}$ shows a sign change from negative to positive around 50K upon cooling. R$_{\mathrm{H}}$ peaks and becomes nonlinear in field around 10K then tend to saturate in value below 10K. Two samples with different geometries (thickness and lateral dimensions) show contrasting behaviors below and above 50K, which indicates a surface origin of the low temperature carriers. We will also present XPS and PNR (polarized neutron reflectometry) results in an attempt to better understand the mysterious surface magnetism that gives hysteretic behavior in some low temperature transport measurements. [Preview Abstract] 
Thursday, March 5, 2015 1:15PM  1:27PM 
T7.00009: Lowtemperature (1 K) angleresolved photoemission investigation of the predicted topological Kondo insulator behavior of SmB$_6$ Oliver Rader, Peter Hlawenka, Emile Rienks, Konrad Siemensmeyer, Eugen Weschke, Andrei Varykhalov, Natalya Shitsevalova, Slavomir Gabani, Karol Flachbart The system SmB$_6$ is known for its unusual resistivity which increases exponentially with decreasing temperature and saturates below 3 K [1]. This has recently been attributed to topologicalKondoinsulator behavior where a topological surface state is created by Sm $4f$$5d$ hybridization and is responsible for the transport [2]. Localdensityapproximation + Gutzwiller calculations of the (100) surface predict the appearance of three Dirac cones in the surface Brillouin zone [3]. We perform angleresolved photoemission at temperatures below 1 K and reveal surface states at $\overline{\Gamma}$ and $\overline{\rm X}$. Bulk conduction band states near $\overline{\rm X}$ appear at higher temperature. These findings will be discussed in detail vis\'avis the theoretical and experimental literature. [1] J. C. Cooley, M. C. Aronson, Z. Fisk, P. C. Canfield, Phys. Rev. Lett. 74, 1629 (1995). [2] M. Dzero, Kai Sun, V. Galitski, P. Coleman, Phys. Rev. Lett. 104, 106408 (2010). [3] F. Lu, J. Zhao, H. Weng, Z. Fang, Xi Dai, Phys. Rev. Lett. 110, 096401 (2013). [Preview Abstract] 
Thursday, March 5, 2015 1:27PM  1:39PM 
T7.00010: Quantum oscillations in magnetically doped SmB$_{6}$ Gang Li, Colin Tinsman, Benjamin Lawson, Fan Yu, Tomaya Asaba, Xiangfeng Wang, Johnpierre Paglione, Lu Li Quantum oscillation study on pure SmB$_{6}$ has revealed twodimensional Fermi Surfaces in both crystalline (001) and (101) surface planes. On the (101) surface plane, only one area of Fermi surface is observed, consistent with theoretical prediction of surface state protected by timereversal symmetry. It has been further predicted that for the system there is not only time reversal symmetry reservation but also crystalline mirror symmetry reservation that could give rise to protected surface states. By introducing magnetic impurities such as 1\% Fe or 1\% Ni into SmB$_{6}$, the magnetic ground state is altered and no longer has the time reversal symmetry. However, quantum oscillations are still observed for both Fe$_{x}$SmB$_{6}$ and Ni$_{x}$SmB$_{6}$ at higher field, the dispersive angular dependence of frequency will be discussed with comparison to SmB$_{6}$. [Preview Abstract] 
Thursday, March 5, 2015 1:39PM  1:51PM 
T7.00011: Electronic gap in SmB$_6$ studied by Raman spectroscopy Michael Valentine, Seyed Koohpayeh, Xiangfeng Wang, Yasuyuki Nakajima, Johnpierre Paglione, Collin Broholm, William Phelan, Tyrel McQueen, Natalia Drichko SmB$_6$ is a mixed valence compound which is suggested to be a topological Kondo insulator. Studies of insulating vs metallic properties of the bulk and the nature of the surface conduction are still ongoing. Using Raman scattering, we follow the opening of an electronic gap in samples of pure SmB$_6$ and SmB$_6$ with Al and C impurities. In all of the samples we observe an electronic gap in the range of 5065 meV, confirming insulating state in the bulk. The gap appears in the A1g+Eg spectra as a suppression of lowfrequency electronic scattering and a shift of the spectral weight to frequencies above the gap below approximately 100 K. The size of the gap and presence of electronic states in the gap depend on the method of growth (floating zone vs Al flux), and the chemical composition of the sample. We discuss a dependence of these parameters on the impurities and Sm valence. [Preview Abstract] 
Thursday, March 5, 2015 1:51PM  2:03PM 
T7.00012: Torque magnetometry study of Fe and Ni doped SmB6 Colin Tinsman, Gang Li, Benjamin Lawson, Fan Yu, Tomoya Asaba, Xiangfeng Wang, Johnpierre Paglione, Lu Li There has been renewed interest in the past few years regarding Samarium Hexaboride, a promising candidate to be a topological Kondo insulator. Work on this material represents an extension of the categorization of materials by the topology of their electronic band structure into systems with strong correlation effects. It is known that by introducing magnetic impurities, such as Iron, Nickel, and Europium, the magnetic ground state of SmB$_{6}$ could be greatly altered. In this study we will present our torque magnetometry data of Fe and Ni doped SmB$_{6}$, down to 20 mK, and up to 45 Tesla. It is found that the overall symmetry of the angular dependence of torque with respect to magnetic field changed for both Fedoped SmB$_{6}$ and Nidoped SmB$_{6}$. For pure SmB$_{6}$, the angular dependence is proportional to $\sin(2\theta)$, as expected for a paramagnetic material. By contrast, for Fedoped SmB$_{6}$ and Nidoped SmB$_{6}$, the torque vs. tilt angle profile becomes $\sin(4\theta)$. Furthermore, for Fe$_{x}$SmB$_{6}$ the field dependence of torque shows a sharp bend feature around 9 Tesla, which softens with elevating temperature, and could be related to magnetic moment realignment. [Preview Abstract] 
Thursday, March 5, 2015 2:03PM  2:15PM 
T7.00013: Effect of magnetic and nonmagnetic substitution in topological Kondo insulator SmB6 Xiangfeng Wang, Yasuyuki Nakajima, Shanta Saha, Johnpierre Paglione, Jeff Sonier The topological Kondo Insulator SmB6 is believed be the first realization of a topological insulator with true bulk insulating properties that coexist with a robust conducting surface state that mounting evidence suggests is nontrivial. Here we report of a systemic study of the magnetic and nonmagnetic doping effect TKI SmB6, in particular the effect of iron inclusion as well as both magnetic and nonmagnetic rare earth substitution in Sm1xRxB6. We will present our careful investigations of crystal structure, transport properties and magnetization, as well as muon spin rotation studies that together suggest a very profound effect on both the bulk and surface state properties. [Preview Abstract] 
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