Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session R47: Kondo Insulator and Semimetal |
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Sponsoring Units: DCMP Room: 312 |
Thursday, March 17, 2016 8:00AM - 8:12AM |
R47.00001: Non-universal weak antilocalization effect in cubic topological Kondo insulators Maxim Dzero, Maxim Vavilov, Kostyantin Kechedzhi, Victor Galitski In this talk we present the results of our study of the quantum correction to conductivity on the surface of cubic topological Kondo insulators with multiple Dirac bands. We considered the model of time-reversal invariant disorder which induces the scattering of the electrons within the Dirac bands as well as between the bands. When only intraband scattering is present we found three long-range diffusion modes leading to weak antilocalization correction to conductivity which remains independent of the microscopic details such as Fermi velocities and relaxation times. Interband scattering gaps out two diffusion modes leaving only one long-range mode. Depending on the value of the phase coherence time, either three or only one long-range diffusion modes contribute to weak localization correction rendering the quantum correction to conductivity non-universal. [Preview Abstract] |
Thursday, March 17, 2016 8:12AM - 8:24AM |
R47.00002: The tunable chirality and circular dichroism of topological Kondo insulator SmB6 with C$_{\mathrm{2v}}$ symmetry as a function of Rashba and Dresselhaus parameters Parijat Sengupta, Enrico Bellotti A manifestation of optical chirality is circular dichroism (CD) due to a differential absorption of left- and right-circularly polarized light. This effect is an enabler for the design of meta-materials used in polarization sensitive imaging devices and display technologies. Concurrently, topological insulators with helical surface states offer an active control over chiral handedness that can be observed through a varying degree of polarization-dependent absorption. We show that in a band gap open topological Kondo insulator SmB$_{\mathrm{6}}$ with C$_{\mathrm{2v}}$ symmetry at the X point of the surface Brillouin zone, CD can be smoothly varied without any microscopic reconfiguration of the surface. We also show that CD, measured by the degree of circular polarization, can assume both positive and negative values. These findings suggest that left- and right- circularly polarized light can be selectively absorbed in the vicinity of the Dirac point by an adjustment of the Rashba- and Dresselhaus-like parameters that describe the Hamiltonian at the X point. The CD is an experimentally measurable quantity and related to Berry curvature which is an outcome of the parameter-dependent Hamiltonian. We calculate the Berry curvature and establish a pathway to alter CD through the Hamiltonian parameters. [Preview Abstract] |
Thursday, March 17, 2016 8:24AM - 8:36AM |
R47.00003: Nonsymmorphic crystalline Kondo semimetals Po-Yao Chang, Onur Erten, Piers Coleman Kondo semimetals, such as CeNiSn and CeRhSb are a class of "failed" heavy fermion insulator that appear to develop line-nodes in the hybridization between the localized f-states and mobile conduction electrons. The classic theory[1,2] for node formation depends on angular momentum blocking in a continuum, and provides no insight into why the node should be stable in a crystalline environment. Here we examine how the semimetallic phase emerges in orthorhombic Kondo systems with nonsymmorphic symmetries. Using a periodic Anderson model that incorporates the key crystallographic symmetries of CeNiSn, we show that hybridization nodal lines are naturally protected within mirror symmetry planes. The shape of the Fermi surface around the nodes in our model agrees with observations in Shubnikov-de Haas oscillations.\\\\ \hbox{[1]} H. Ikeda and K. Miyake, J. Phys. Soc. Jpn. 65, 1769 (1996). \\ \hbox{[2]} J. Moreno and P. Coleman, Phys. Rev. Lett. 84, 342 (2000). [Preview Abstract] |
Thursday, March 17, 2016 8:36AM - 8:48AM |
R47.00004: Spin fluctuations in the anisotropic Kondo insulator CeRu4Sn6 Wesley T. Fuhrman, J. Haenel, J. Rodriguez, S. Paschen, C. L. Broholm We report and model anisotropic quasi-elastic magnetic neutron scattering from single crystalline CeRu$_4$Sn$_6$. For $T\approx 2$K the magnetic neutron scattering is broad in momentum ($\bf Q$) with a persistent $1/ \hbar \omega$ spectrum throughout the Brillouin zone. This indicates a lack of spatial coherence and no characteristic energy scale beyond the 0.2 meV resolution of the measurement. We find the Q-dependence of the scattering can be modeled by a Kondo-Heisenberg Hamiltonian that describes residual carriers and incompletely compensated localized electrons. These findings support the interpretation of tetragonal CeRu4Sn6 as an anisotropic or nodal Kondo insulator, markedly different from typical cubic Kondo insulators. We further discuss potential topological implications. [Preview Abstract] |
Thursday, March 17, 2016 8:48AM - 9:00AM |
R47.00005: Nanoscale Conducting and Insulating Domains on YbB$_6$ Jennifer Hoffman, Zhihuai Zhu, Yang He, Dae-Jeong Kim, Zachary Fisk Recent photoemission studies on YbB$_6$ reported a metallic surface but without f-states pinned at the Fermi level, in contradiction to the theoretical prediction of YbB$_6$ as a topological Kondo insulator. Thus the topological nature of YbB$_6$ remains unclear and requires a study that can distinguish trivial surface structure and non-trivial topological effects derived from the bulk. We use scanning tunneling microscopy and spectroscopy (STM/STS) to provide a real-space microscopic picture of the surface electronic structure in YbB$_6$. We observe coexisting nanoscale metallic and insulating surface terminations. The surface conductivity of each termination reflects the degree of downward or upward band bending that is determined by the surface polarity. In addition to demonstrating that surface metallically in YbB$_6$ stems from band bending at the polar surface, our study suggests the utility of YbB$_6$ for creating spin-polarized p-n junctions at the atomic scale. [Preview Abstract] |
Thursday, March 17, 2016 9:00AM - 9:12AM |
R47.00006: Quasiparticle Interference Imaging on SmB$_{6}$ Harris Pirie, Yang He, Mohammad Hamidian, Michael Yee, Dae-Jeong Kim, Zachary Fisk, Jennifer Hoffman Theoretical interest in SmB$_{6}$ as a possible topological Kondo insulator with spin-textured Dirac surface states spanning the bulk hybridization gap has been well supported by recent transport, quantum oscillation, and spin-resolved ARPES experiments. However, the influence of surface reconstruction and polarization on the observed dispersion remains unclear. Scanning tunneling microscopy (STM) and spectroscopy (STS) enable simultaneous measurement of local real- and momentum-space structure through quasiparticle interference (QPI) imaging. We use QPI imaging to detect and measure the dispersion of states near the hybridization gap on a non-polar, $2\times 1$ reconstructed surface of SmB$_{6}$. We compare these results with recent theoretical predictions to gain insight into the low energy excitations of SmB$_{6}$. [Preview Abstract] |
Thursday, March 17, 2016 9:12AM - 9:24AM |
R47.00007: Topological surface states in Kondo insulator SmB$_{6}$ via planar tunneling spectroscopy$^{\ast }$ Wan Kyu Park, Lunan Sun, Alex Noddings, Laura Greene, Dae-Jeong Kim, Zachary Fisk Samarium hexaboride (SmB$_{6})$ belongs to a class of materials called Kondo insulators in which the hybridization between itinerant electrons and local moments leads to an emergent state of matter. With inherently large spin-orbit coupling along with strong correlation, SmB$_{6}$ has been recently predicted to be topological meaning that topologically robust conducting states should exist at its surfaces. Although extensive investigations have provided growing evidence for the existence of such states, corroborative spectroscopic evidences are still lacking unlike in the weakly correlated counterparts. We adopt planar tunneling spectroscopy to unveil their detailed nature and behavior utilizing its inherently high energy resolution. Measurements of tunneling conductance on two different crystal surfaces (001) and (011) reveal the expected linear density of states for two and one Dirac cones, respectively. Moreover, it is found that these topological states cease to be protected well before they merge into the bulk states at the gap edges. Microscopic modeling of the tunneling processes accounting for the interaction with spin excitons as predicted by a recent theory [1] provide consistent explanations for all the observed features, corroborating the proposed picture on the incompletely protected surface states in SmB$_{6}$. [1] Kapilevich \textit{et al}., Phys. Rev. B \textbf{92}, 085133 (2015). $^{\ast }$The work at UIUC is supported by the NSF DMR 12-06766. [Preview Abstract] |
Thursday, March 17, 2016 9:24AM - 9:36AM |
R47.00008: Finite temperature topological phase transitions and emergence of Dirac semi-metallic phases in a Kondo lattice Po-Hao Chou, Liang-Jun Zhai, Chung-Hou Chung, Ting-Kuo Lee, Chung-Yu Mou The energy gap in Dirac materials controls the topology and critical behaviors of the quantum phase transition associated with the critical point when the gap vanishes. However, it is often difficult to access the critical point as it requires tunablity of electronic structures. Here by exploiting the many-body screening interaction of localized spins and conduction electrons in a Kondo lattice, we demonstrate that the electronic band structures in a Kondo lattice are tunable in temperature. When spin-orbit interactions are included, we find that below the Kondo temperature, the Kondo lattice is a strong topological insulator at low temperature and undergoes a topological transition to a weak topological insulator at a higher temperature $T_D$. At $T_D$, Dirac points emerge and the Kondo lattice becomes a Dirac semimetal. Our results indicate that the topological phase transition though a Dirac semi-metallic phase at finite temperatures also manifests profound physics and results in critical-like behavior both in magnetic and transport properties near $T_D$. [Preview Abstract] |
Thursday, March 17, 2016 9:36AM - 9:48AM |
R47.00009: Pressure-induced exotic states in mixed-valence rear earth hexaborides Liling Sun, Yazhou Zhou, Qi Wu, Yi-feng Yang, Zhongxian Zhao, Dae-Jeong Kim, Priscila Rosa, Zachary Fisk, Rong Yu, Qimao Si, Joe Thompson Recently, a unique type of well-known compounds, mixed-valence rear earth hexaborides RB$_{\mathrm{6\thinspace }}$(R$=$ Sm and Yb), receive new interests due to the discovery of the coexistence of metallic surface state and insulating bulk state in SmB$_{\mathrm{6}}$. This encourages people to revisit the RB$_{\mathrm{6\thinspace }}$with an attempt to establish a new physics that links the correlated electron systems and topological insulators. Pressure is a way to help understanding the underlying mechanism, therefore it is specially needed in establishing this link because the valence state of RB$_{\mathrm{6}}$ is sensitive to pressure. In this talk, we will report some progress of high pressure studies on the RB$_{\mathrm{6}}$, mainly focusing on the phenomena of pressure-induced exotic states and corresponding quantum phase transitions. The connections between the related phenomena and the valence states are revealed. [Preview Abstract] |
Thursday, March 17, 2016 9:48AM - 10:00AM |
R47.00010: Tunable Kondo effect and spin textures on topological insulators surfaces Ilya Vekhter, Gerardo Ortiz, Leonid Isaev We consider screening of a spin-$1 / 2$ impurity at the surface of a topological insulator, and show that the very existence of Kondo screening strongly depend on details of the bulk material and surface preparation whose details are encoded in time-reversal preserving boundary conditions for electronic wavefunctions. We investigate in detail the formation of the Kondo resonance by studying the "orbital-flip" processes that screen the impurity spin in the resulting strongly spin-orbit coupled system. This mechanism gives rise to spin textures that can be used to experimentally probe signatures of a Kondo resonance in topological insulators, and we give examples relevant to specific materials. [Preview Abstract] |
Thursday, March 17, 2016 10:00AM - 10:12AM |
R47.00011: Incomplete Protection of the Surface Weyl Cones of Kondo Insulators: Spin Exciton Scattering Peter Riseborough, Gary A> Kapilevich, Alex Gray, Miklos Gulacsi, Tomasz Durakiewicz, James L. Smith The material SmB$_6$ is a Kondo Insulator, where the lowest-energy bulk electronic excitations are spin excitons. The material also has surface states which are subjected to strong spin-orbit coupling. It has been suggested that SmB$_6$ is also a topological insulator. Here we show that, despite the absence of time-reversal symmetry breaking and the presence of strong spin-orbit coupling, the chiral spin texture of the Weyl cone is not completely protected. In particular, we show that the spin-exciton mediated scattering produces features in the surface electronic spectrum at energies separated from the surface Fermi-energy by the spin-exciton energy. Despite the features being far removed from the surface Fermi-energy, the features are extremely temperature dependent. The temperature variation occurs over a characteristic scale determined by the dispersion of the spin exciton. The structures may be observed by electron spectroscopy at low temperatures. [Preview Abstract] |
Thursday, March 17, 2016 10:12AM - 10:24AM |
R47.00012: Magnetotransport properties of topological surface states in the presence of ferromagnetic order Kunal Tiwari, William Coish, Tami Pereg-Barnea The surface of a 3D topological insulator hosts a two dimensional Dirac cone which is robust to weak, non-magnetic perturbation. Its presence will dominate low energy transport since the bulk is gapped. However, once magnetic impurities are introduced to the surface they may gap the Dirac dispersion, suppressing or modifying the system’s surface transport properties. In particular, in the presence of uniform ferromagnetic order, the Dirac cone becomes massive and should not conduct for energies near the Dirac point. On the other hand, if the ferromagnetic order has domains with different magnetization directions, current may be carried on the domain walls where the Dirac mass vanishes. Our research aims to elucidate the transport properties of topological insulators in the presence of magnetic domain structures. Our work may be relevant to recent studies on the Kondo topological insulator SmB$_6$. [Preview Abstract] |
Thursday, March 17, 2016 10:24AM - 10:36AM |
R47.00013: Universal edge information from wavefunction deformation Wen Wei Ho, Lukasz Cincio, Heidar Moradi, Guifre Vidal It is well known that the bulk physics of a topological phase constrains its possible edge physics through the bulk-edge correspondence. Therefore, the different types of edge theories that a topological phase can host is a universal piece of data which can be used to characterize topological order. Here, we argue that beginning from only the fixed point wavefunction (FPW) of a nonchiral topological phase and by locally deforming it, all possible edge theories can be extracted from its entanglement Hamiltonian (EH). We illustrate our claim by deforming the FPW of the Wen-plaquette model, the quantum double of $\mathbb{Z}_2 $. We show that the possible EHs of the deformed FPWs reflect the known possible types of edge theories, which are generically gapped, but gapless if translational symmetry is preserved. We stress that our results do not require an underlying Hamiltonian -- thus, this lends support to the notion that a topological phase is indeed characterized by only a set of quantum states and can be studied through its FPWs. [Preview Abstract] |
Thursday, March 17, 2016 10:36AM - 10:48AM |
R47.00014: Landau quantization and spin-momentum locking in topological Kondo insulators Pedro Schlottmann SmB$_6$ has been predicted to be a strong topological Kondo insulator and experimentally it has been confirmed that at low temperatures the electrical conductivity only takes place at the surfaces of the crystal. Quantum oscillations and ARPES measurements revealed several Dirac cones on the (001) and (101) surfaces of the crystal. We considered three types of surface Dirac cones with an additional parabolic dispersion and studied their Landau quantization and the expectation value of the spin of the electrons. The Landau quantization is quite similar in all three cases and would give rise to very similar de Haas-van Alphen oscillations [1]. The spin-momentum locking, on the other hand, differs dramatically. Without the additional parabolic dispersion the spins are locked in the plane of the surface. The parabolic dispersion, however, produces a gradual canting of the spins out of the surface plane. \vskip 0.05in \par\noindent [1] P. Schlottmann, Phys. Rev. B {\bf 90}, 165127 (2014). [Preview Abstract] |
Thursday, March 17, 2016 10:48AM - 11:00AM |
R47.00015: Strong correlations in Kondo topological insulators: Two-dimensional heavy fermions, and beyond Predrag Nikolic Samarium hexaboride (SmB6) is a candidate topological insulator with strong electron correlations. Empowered by the time-reversal (TR) symmetry and topology, the low-energy surface states of hybridized samarium's d and f orbitals can exhibit a rich two-dimensional heavy-fermion phenomenology. This talk will survey several interesting possibilities for correlated surface states, which depend on microscopic surface conditions. A pronounced participation of the f orbitals is expected to create a heavy-fermion Dirac metal, possibly unstable to spin density waves, superconductivity, or exotic Mott insulators (e.g. algebraic and non-Abelian spin liquids). The opposite limit of “localized magnetic moments” can produce a non-Fermi liquid of d electrons that exhibits two-dimensional quantum electrodynamics. Ultrathin films made from topological Kondo insulators can host lattices of SU(2) vortices, which need not break the TR symmetry. Landau-Ginzburg theory and numerical model calculations reveal the nature and stability of such vortex lattices, while field theory arguments predict that their quantum melting could yield novel incompressible quantum liquids with non-Abelian fractional excitations. [Preview Abstract] |
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