Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Q7: Topological Insulators: Theory and Experiment |
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Sponsoring Units: DCMP Chair: Ken Burch, Boston College Room: 006B |
Wednesday, March 4, 2015 2:30PM - 2:42PM |
Q7.00001: Magnetic ordering on edges of topological insulator Yea-Lee Lee, Hee Chul Park, Jisoon Ihm, Young-Woo Son Based on first-principles calculations, we show that the Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ crystal, which is a prototypical topological insulator, with different crystal-face orientation surfaces generates a built-in electric field around the facet edges due to the work function differences. For a given broken time reversal symmetry in the crystal, the electric field, in turn, induces a net magnetic ordering along the edges by the topological magnetoelectric coupling. The predicted magnetic ordering depending on the work function differences between facets would be a unique manifestation of the axion electrodynamics in real solids and suggests a route to reveal novel properties of macroscopic topological edge states. [Preview Abstract] |
Wednesday, March 4, 2015 2:42PM - 2:54PM |
Q7.00002: Exotic quantum phase transitions of 2+1d Dirac fermions, and connections to 2d and 3d topological insulators Kevin Slagle, Yi-Zhuang You, Cenke Xu Using determinant quantum Monte Carlo simulations, we demonstrate that an extended Hubbard model on a bilayer honeycomb lattice has two novel quantum phase transitions, each with connections to symmetry protected topological states. 1) The first is a continuous phase transition between the weakly interacting gapless Dirac fermion phase and a strongly interacting fully gapped and symmetric trivial phase. Because there is no spontaneous symmetry breaking, this transition cannot be described by the standard Gross-Neveu model. We argue that this phase transition is related to the $Z_{16}$ classification of the topological superconductor $^3$He-B phase with interactions. 2) The second is a quantum critical point between a quantum spin Hall insulator with spin $S^z$ conservation and the previously mentioned strongly interacting gapped phase. At the critical point the single particle excitations remain gapped, while spin and charge gaps close. We argue that this transition is described by a bosonic O(4) nonlinear sigma model field theory with a topological $\Theta$-term. [Preview Abstract] |
Wednesday, March 4, 2015 2:54PM - 3:06PM |
Q7.00003: Phase diagrams of disordered 3D topological insulators and superconductors Tomi Ohtsuki, Koji Kobayashi, Ken-Ichiro Imura, Ken Nomura A global phase diagram of disordered weak and strong topological insulators belonging to the class AII is obtained by numerically calculating the conductance, the Lyapunov exponents and the density of states. The location of the phase boundaries, i.e., the mass parameter, is renormalized by disorder, a feature recognized in the study of topological Anderson insulator. We report quantized conductance on the phase boundaries between topologically distinct phases, which is interpreted as the robustness of conductance against disorder. This robustness is also confirmed by the large-scale numerical calculation of the density of states, which remains parabolic up to certain strength of disorder with renormalized Dirac electron velocity. From the size dependence of the conductance, we also point out that the surface states of weak topological insulator are either robust or ''defeated''. The nature of the two distinct types of behavior is further revealed by studying the Lyapunov exponents. (K. Kobayashi et al., Phys. Rev. Lett. vol. 110, 236803 (2013)). We also obtain the phase diagram of disordered topological superconductors belonging to the class DIII. Similar renormalization of mass and velocity due to disorder is found. [Preview Abstract] |
Wednesday, March 4, 2015 3:06PM - 3:18PM |
Q7.00004: Surface-to-bulk scattering in topological insulator films Kush Saha, Ion Garate We present a quantitative microscopic theory of the disorder- and phonon-induced coupling between surface and bulk states in topological insulator (TI) films. We find a simple mathematical structure for the surface-to-bulk scattering matrix elements and confirm the importance of bulk-surface coupling in transport and photoemission experiments, assessing its dependence on temperature, carrier density, film thickness and particle-hole asymmetry. [Preview Abstract] |
Wednesday, March 4, 2015 3:18PM - 3:30PM |
Q7.00005: Sum rule constraints on the surface state conductance of topological insulators K.W. Post, B.C. Chapler, M.K. Liu, H.T. Stinson, M.D. Goldflam, A.R. Richardella, J.S. Lee, A.A. Reijnders, K.S. Burch, N. Samarth, D.N. Basov We report the Drude oscillator strength (D) and the magnitude of the bulk band gap of the epitaxial topological insulator alloy (Bi,Sb)$_2$Te$_3$. The bulk band gap is used in conjunction with f-sum rules to establish an upper bound for the D expected in a typical Dirac like system composed of linear bands. We expand our result from the linear band model to include both hexagonal warping and electron-hole asymmetry, as is typical in topological insulator systems. The corresponding maximum value of D arising from Dirac bands in this more complex system is also determined. The observed D is found to be close to this upper bound, demonstrating the effectiveness of alloying in eliminating bulk charge carriers. Moreover, Hall effect parameters and the weak anti-localization observed in transport on the same sample support assignment of the low-energy conduction to topological surface states. [Preview Abstract] |
Wednesday, March 4, 2015 3:30PM - 3:42PM |
Q7.00006: Many-Body Effects in 2D Topological Kondo Insulators Jason Iaconis, Leon Balents Recently there has been an immense amount of interest in studying the effect of interactions on systems with a nontrivial topology. Perhaps the simplest manifestation of this phenomena, and that which is most closely related to experimental materials, can be found in the so called topological Kondo insulators. In our work we study a model of graphene which is doped with localized partially filled d-shell electron moments. We model this system using a simple Hamiltonian in which the Kondo interaction can lead to the formation of a topological insulator phase with many-body correlations. I will discuss a mean field treatment of this model where we map out the different possible interacting phases of the system. It is of particular interest to study the effect of interactions on the symmetry protected gapless edge states, as such edge states are perhaps the most dramatic consequence of having a band structure with a nontrivial topology. We discuss the possibility that the Kondo interaction leads to edge states which spontaneously break time-reversal symmetry, while preserving the symmetry within the bulk. [Preview Abstract] |
Wednesday, March 4, 2015 3:42PM - 3:54PM |
Q7.00007: Quantum criticality in Kondo quantum dot coupled to helical edge states of interacting 2D topological insulators Chung-Hou Chung, Salman Silotri We investigate theoretically the quantum phase transition (QPT) between the one-channel Kondo (1CK) and two-channel Kondo (2CK) fixed points in a quantum dot coupled to helical edge states of interacting 2D topological insulators (2DTI) by tuning Kondo couplings at a fixed Luttinger parameter $K<1$. The system can be mapped onto an anisotropic two-channel Kondo Hamiltonian, and the 2CK fixed point was argued to be stable for infinitesimally weak tunnelings between dot and the 2DTI. We re-examine this model beyond the bare scaling dimension analysis via a controlled perturbative renormalization group (RG) approach combined with bosonization and re-fermionization techniques near weak-coupling and strong-coupling (2CK) fixed points. For $K$ close to but less than $1$, we find the 2CK fixed point can be unstable towards the 1CK fixed point and the system is expected to undergo a quantum phase transition between 1CK and 2CK fixed points. Our system serves as the first example of the 1CK-2CK QPT that is accessible by the controlled RG approach though this transition has been known to exist in Kondo dot coupled to two conventional Luttinger liquid leads with a critical value $K_c=1/2$. We extract quantum critical and crossover behaviors from various observables near criticality. [Preview Abstract] |
Wednesday, March 4, 2015 3:54PM - 4:06PM |
Q7.00008: Electrical Detection of Spin-Polarized Surface States Conduction in (Bi$_{0.53}$Sb$_{0.47})_{2}$Te$_{3}$ Topological Insulator Jianshi Tang, Li-Te Chang, Xufeng Kou, Koichi Murata, Murong Lang, Yabin Fan, Mohammad Montazeri, Wanjun Jiang, Liang He, Kang L. Wang, Eun Sang Choi, Ying Jiang, Yong Wang Strong spin-orbit interaction and time-reversal symmetry in topological insulators enable the spin-momentum locking for the helical surface states. Here we report the electrical detection of spin-polarized surface states conduction using a Co/Al$_{2}$O$_{3}$ ferromagnetic tunneling contact, in which the compound topological insulator (Bi$_{0.53}$Sb$_{0.47})_{2}$Te$_{3}$ was used to achieve low bulk carrier density [1]. Resistance (voltage) hysteresis was observed when sweeping the magnetic field to change the relative orientation between the Co electrode magnetization and the spin polarization of surface states. The two resistance states were reversible by changing the electric current direction, affirming the spin-momentum locking in the topological surface states. Angle-dependent measurement was also performed to further confirm that the abrupt change in the voltage (resistance) was associated with the magnetization switching of the Co electrode. Our results show a direct evidence of spin polarization in the topological surface states conduction. It might open up great opportunities to explore energy-efficient spintronic devices based on topological insulators. [Preview Abstract] |
Wednesday, March 4, 2015 4:06PM - 4:18PM |
Q7.00009: Tuning Topological Edge States of Bi(111) Bilayer Film by Edge Adsorption Zhengfei Wang, Li Chen, Feng Liu Based on first-principles and tight-binding calculations, we report that the topological edge states of zigzag Bi(111) nanoribbon can be significantly tuned by H edge adsorption. The Fermi velocity is increased by 1 order of magnitude, as the Dirac point is moved from the Brillouin zone boundary to the Brillouin zone center, and the real-space distribution of Dirac states are made twice more delocalized. These intriguing changes are explained by an orbital filtering effect of edge H atoms, which pushes certain components of the p orbital of edge Bi atoms out of the band gap regime that reshapes the topological edge states. In addition, the spin texture of the Dirac states is also modified, which is described by introducing an effective Hamiltonian. Our findings not only are of fundamental interest but also have practical implications in potential applications of topological insulators. This work is supported by DOE-BES (Grant No. DE-FG02-04ER46148) and NSF-MRSEC (Grant No.DMR-1121252) . [Preview Abstract] |
Wednesday, March 4, 2015 4:18PM - 4:30PM |
Q7.00010: Crossover between Weak Antilocalization and Weak Localization of Bulk States in Ultrathin Bi$_{2}$Se$_{3}$ Films Huichao Wang, Haiwen Liu, Yanfei Zhao, Yi Sun, X.C. Xie, Jian Wang, Cuizu Chang, Ke He, Xucun Ma, Qi-Kun Xue, Huakun Zuo, Zhengcai Xia We report studies on the 5 nm thick Bi2Se3 topological insulator films which are grown via molecular beam epitaxy technique. The angle-resolved photoemission spectroscopy data show that the Fermi level of the system lies in the bulk conduction band above the Dirac point, suggesting important contribution of bulk states to the transport results. In particular, the crossover from weak antilocalization to weak localization in the bulk states is observed in the parallel magnetic field measurements up to 50 Tesla. The measured magneto-resistance exhibits interesting anisotropy with respect to the orientation of parallel magnetic field B// and the current I, signifying intrinsic spin-orbit coupling in the Bi2Se3 films. Our work directly shows the crossover of quantum interference effect in the bulk states from weak antilocalization to weak localization. It presents an important step toward a better understanding of the existing three-dimensional topological insulators and the potential applications of nano-scale topological insulator devices. [Preview Abstract] |
Wednesday, March 4, 2015 4:30PM - 4:42PM |
Q7.00011: Crossover from 3D to 2D Quantum Transport in Bi2Se3/In2Se3 Superlattices Zhao Yanfei, Liu Haiwen, Guo Xin, Jiang Ying, Sun Yi, Wang Huichao, Wang Yong, Li Handong, Xie Maohai, Xie Xincheng, Wang Jian The topological insulator/normal insulator (TI/NI) superlattices (SLs) with multiple Dirac channels are predicted to offer great opportunity to design novel materials and investigate new quantum phenomena. Here, we report first transport studies on the SLs composed of TI Bi2Se3 layers sandwiched by NI In2Se3 layers artificially grown by molecular beam epitaxy (MBE). The transport properties of two kinds of SL samples show convincing evidence that the transport dimensionality changes from three-dimensional (3D) to two-dimensional (2D) when decreasing the thickness of building block Bi2Se3 layers, corresponding to the crossover from coherent TI transport to separated TI channels. Our findings provide the possibility to realizing 3D surface states in TI/NI SLs. [Preview Abstract] |
Wednesday, March 4, 2015 4:42PM - 4:54PM |
Q7.00012: Surface state of Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$ investigated by terahertz emission and visible second-harmonic generation techniques Soon-Hee Park, S.Y. Hamh, J.S. Lee, Joonbum Park, Jun Sung Kim One of the key issues in three-dimensional topological insulators is to separate a response of the surface from the bulk to exploit novel spin-momentum-locked Dirac fermionic surface state, and Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$ is one of such materials having a negligible contribution of the bulk conduction. We investigate the surface state of Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$ single crystals by using terahertz emission spectroscopy and second harmonic generation techniques. We observed a clear distinction in the phases of emitted terahertz electric field and azimuth-dependent second harmonic intensity in different pieces of the sample, and found their clear correlation with carrier types which can be attributed to upward or downward band bending. We examined variations of such optical responses after the cleavage of the samples under different atmospheric environments, and discussed the time-evolution of the surface state particularly in comparison with previous results on n-type Bi$_{2}$Se$_{3}$. [Preview Abstract] |
Wednesday, March 4, 2015 4:54PM - 5:06PM |
Q7.00013: Parity blocking in quenching dynamics of Majorana wires Suraj Hegde, Vasudha Shivamoggi, Smitha Vishveshwara, Diptiman Sen We study the non-equilibrium dynamics of the Kitaev Hamiltonian for a one-dimensional p-wave superconductor, which in its topological phase harbors Majorana fermions at the edges. We vary one of the parameters of the Hamiltonian linearly in time within the topological phase and across the quantum critical point into the topologically trivial phase. By starting in the ground state of the initial Hamiltonian, we find that the time evolved state fails to track the instantaneous ground state and flips back and forth between the ground state and an excited state, even when the quenching is very slow. Termed as ``parity-blocking,'' this happens when the fermion parity of the ground state is changing with time. This change in parity is related to the oscillation of near zero energy mid-gap states of the end Majorana modes. Using exact Majorana wave functions and transfer matrix techniques, we track the points where the ground state changes parity. We develop a real-space formalism for calculating the dynamics of quantities such as the many-body wavefunction overlap and residual energy. Using these techniques for numerical calculations, we demonstrate parity blocking and other scaling effects with the quench rate and length of the chain. [Preview Abstract] |
Wednesday, March 4, 2015 5:06PM - 5:18PM |
Q7.00014: Chiral electromagnetic waves in Weyl semimetals Alexander Zyuzin, Vladimir Zyuzin We show that Weyl semimetals with broken time-reversal symmetry is an optically gyrotropic media and can host chiral electromagnetic waves. The magnetization in the system that results in a momentum space separation of a pair of opposite chirality Weyl nodes is also responsible for the non-zero gyration vector in the system. We show that in the region where the magnetization flips its directions (magnetic domain wall) there exist a chiral electromagnetic field localized at the domain wall and propagating along it. The direction of propagation is determined by the sign of the gyrotropy factor. Such magnetic domain walls might appear naturally in the Weyl semimetal materials, or, for example, they can be created with a help of a ferromagnetic material placed in proximity. The chiral electromagnetic wave propagating at the domain walls is an analog of quantum Hall state for photons. [Preview Abstract] |
Wednesday, March 4, 2015 5:18PM - 5:30PM |
Q7.00015: Direct Observation of Chiral Topological Solitons in 1D Charge-Density Waves Tae-Hwan Kim, Sangmo Cheon, Sung-Hoon Lee, Han Woong Yeom Macroscopic and classical solitons are easily and ubiquitously found, from tsunami to blood pressure pulses, but those in microscopic scale are hard to observe. While the existence of such topological solitons were predicted theoretically and evidenced indirectly by the transport and infrared spectroscopy measurements, the direct observation has been hampered by their high mobility and small dimension. In this talk, we show direct observation of topological solitons in the quasi-1D charge-density wave (CDW) ground state of indium atomic wires, which are consisting of interacting double Peierls chains. Such solitons exhibit a characteristic spatial variation of the CDW amplitudes as expected from the electronic structure. Furthermore, these solitons have an exotic hidden topology originated by topologically different 4-fold degenerate CDW ground states. Their exotic topology leads to the chirality of 1D topological solitons through interaction between two solitons in the double Peierls chains. Detailed scanning tunneling microscopy and spectroscopy reveal their chiral nature at the atomic scale. This work paves the avenue toward the microscopic exploitation of the peculiar properties of nanoscale chiral solitons. [Preview Abstract] |
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