Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session P08: Topological Insulators: General Theory and Nanostructures |
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Sponsoring Units: DCMP Chair: Ming Xie, Univ of Texas, Austin Room: LACC 153C |
Wednesday, March 7, 2018 2:30PM - 2:42PM |
P08.00001: Piecewise-Terminated Spherical Topological Insulator as a Virtual Breadboard for Majorana Circuitry Adam Durst, Sriram Ganeshan We consider the surface states of a spherical topological insulator piecewise-terminated by superconductivity or ferromagnetism over various regions of the spherical surface. Such terminations gap the surface states by breaking U(1) particle-number symmetry or time-reversal symmetry, respectively. Interfaces and trijunctions between differently terminated surface regions can host propagating and bound Majorana modes, and the finite size of the spherical system makes it easily amenable to numerical analysis via exact diagonalization of the Bogoliubov-de Gennes Hamiltonian within a truncated Hilbert space. Creative termination patterning therefore allows one to prototype a variety of Majorana circuits, calculating energy spectra and plotting eigenfunctions over the spherical surface. We develop the computational framework for this approach, establishing a virtual breadboard for Majorana circuitry, and apply it to circuits of interest, including the Majorana analog of a Mach-Zehnder interferometer. |
Wednesday, March 7, 2018 2:42PM - 2:54PM |
P08.00002: Three-dimensional Fractional Topological Insulators in Coupled Rashba Layers Yanick Volpez, Daniel Loss, Jelena Klinovaja We propose a model of three-dimensional topological insulators consisting of weakly coupled |
Wednesday, March 7, 2018 2:54PM - 3:06PM |
P08.00003: Magnetotransport in 3D Topological Insulator Nanowires Raphael Kozlovsky, Cosimo Gorini, Klaus Richter We investigate the transport characteristics of 3D topological insulator (3D TI) nanowires in external electric and magnetic fields. The wires host topologically nontrivial surface states wrapped around an insulating bulk and are modelled by surface effective Hamiltonians. A magnetic field along the wire axis leads to Aharonov-Bohm type oscillations of the conductance. Such oscillations have been observed in numerous systems and signal surface transport, though alone cannot prove its topological nature. Furthermore, it is not known how they are affected by the wire specific geometry which is never perfectly tubular as assumed in theoretical models up to now. |
Wednesday, March 7, 2018 3:06PM - 3:18PM |
P08.00004: Topological Phases of Thin Films of Topological Insulators with and without Magnetic Fields. Mahmoud Asmar, Daniel Sheehy, Ilya Vekhter The surface states residing at opposite surfaces of a topological insulator (TI) can hybridize if the thickness of the TI becomes comparable to their decay lengths, leading to a spectral gap. We investigate the conditions under which the resulting system has topological character, and study the topological properties of a free standing TI thin film. To this end, we develop a tunneling formalism for such TI-thin film-based heterostructures, and adapt it to the free standing situation. We find that accounting for the dependence of the decay lengths on the in-plane momentum is critical for accurately identifying the non-trivial topology. As a function of thickness, the film exhibits a sequence of transitions between gapped topological and trivial phases separated by states with a linear semi-metallic dispersion. In its topological phase, the thin film has edge states that carry a spin current with a spin component normal to the edge. In addition we explore the effect of in plane magnetic fields on these edge states. |
Wednesday, March 7, 2018 3:18PM - 3:30PM |
P08.00005: Magnetic Island Induced Electron Scattering on the Surface of 3D Topological Insulators Mark Hirsbrunner, Matthew Gilbert We investigate theoretically the scattering of electrons impinging on a circular magnetically gapped region on the surface of a three-dimensional time-reversal-invariant topological insulator (3D TI). This work is motivated by the recent surge in theoretical and experimental interest in the interplay between topology and magnetism, as it provides insight on electron transport in TI surface states with local time-reversal symmetry breaking. We work in the limit of a massless exterior region (no magnetization), to understand the interface between a gapless region and a Chern number ½ region. Based on calculations of current flow in this system, we predict that an anomalous Hall effect can be generated on the surface of a 3D TI decorated by magnetic islands. |
Wednesday, March 7, 2018 3:30PM - 3:42PM |
P08.00006: Equations of Motion of Bloch Electrons Beyond the Semiclassical Approach Troy Stedman, Lilia Woods A quantum mechanical description based on a Hamiltonian approach for the equations of motion of electrons in periodic systems subjected to perturbing electromagnetic fields is presented. By assuming a well localized wave packet in a Brillouin zone of reciprocal space, projected onto a subset of bands, the quantum mechanical equations of motion for the gauge invariant crystal momentum and wave packet position are obtained. These are found to contain Berry curvature effects and coupling between the bands, which can also have interesting consequences for the associated Boltzmann transport description of currents. These situations are examined in more detail in a two-band model. With this approach, we show the ease with which the equations of motion arise from quantum mechanical principles. |
Wednesday, March 7, 2018 3:42PM - 3:54PM |
P08.00007: Homotopy approach to the classification of band-structure nodes Tomas Bzdusek, Manfred Sigrist While most band-structure nodes are protected by a single topological charge, it was shown using homotopy theory that nodal lines protected by time-reversal and spatial inversion in the absence of spin-orbit coupling carry a pair of independent Z2 charges, which enhances their stability. We generalize the homotopy arguments, and show that in 3D the global symmetries together with spatial inversion are able to protect doubly charged nodes in four out of the ten Atland-Zirnbauer classes. These include nodal lines in classes AI and CI, and nodal surfaces in classes D and BDI. In all cases, the additional invariant leads to enhanced robustness of the nodes, when they cannot be gapped out individually but only by a pairwise annihilation, thus leading to an analogue of the Nielsen-Ninomiya doubling. These exotic nodes are relevant for various semimetallic as well as superconducting phases, including the recently discovered Bogoliubov-Fermi surfaces. We further generalize the homotopy approach to nodes protected by crystalline symmetries. In this way, we are able to understand the topological protection of most common species of nodes in a unified language. |
Wednesday, March 7, 2018 3:54PM - 4:06PM |
P08.00008: Two-dimensional lattice model for the surface states of topological insulators Yan-Feng Zhou, Hua Jiang, Xincheng Xie, Qing-Feng Sun The surface states in three-dimensional (3D) topological insulators can be described by a two-dimensional (2D) continuous Dirac Hamiltonian. However, there exists the fermion doubling problem when putting the continuous 2D Dirac equation into a lattice model. In this paper, we introduce a Wilson term with a zero bare mass into the 2D lattice model to overcome the difficulty. By comparing with a 3D Hamiltonian, we show that the modified 2D lattice model can faithfully describe the low-energy electrical and transport properties of surface states of 3D topological insulators. So this 2D lattice model provides a simple and cheap way to numerically simulate the surface states of 3D topological-insulator nanostructures. Based on the 2D lattice model, we also establish the wormhole effect in a topological-insulator nanowire by a magnetic field along the wire and show the surface states being robust against disorder. The proposed 2D lattice model can be extensively applied to study the various properties and effects, such as the transport properties, Hall effect, universal conductance fluctuations, localization effect, etc. So, it paves a way to study the surface states of the 3D topological insulators. |
Wednesday, March 7, 2018 4:06PM - 4:18PM |
P08.00009: Aharonov-Bohm oscillations in (Bi_{1-x}Sb_{x})_{2}Se_{3 }topological insulator nanoribbon Hong-Seok Kim, Nam-Hee Kim, Yiming Yang, Xingyue Peng, Dong Yu, Yong-Joo Doh The 0- and π-phase Aharonov-Bohm (AB) oscillations have been theoretically predicted in one-dimensional (1D) topological insulators (TIs). Here, we report the experimental observations of highly coherent AB oscillations in (Bi_{1-x}Sb_{x})_{2}Se_{3}_{ }TI nanoribbon with varying temperature, gate voltage and channel length. Temperature dependence of the AB oscillations can be understood by a thermal broadening effect of 1D subbands of the topological surface state. When the channel length increases, the observed AB oscillations resemble a disorder-induced dephasing effect of the DOS. Our experimental results provide a direct evidence that the topologically-protected surface states are quantized along the TI nanoribbon circumference and form 1D subbands to exhibit a unique quantum electronic transport properties. |
Wednesday, March 7, 2018 4:18PM - 4:30PM |
P08.00010: Superconducting quantum interference devices of (Bi_{1-x}Sb_{x})_{2}Se_{3} topological insulator nanoribbons Nam-Hee Kim, Hong-Seok Kim, Yiming Yang, Xingyue Peng, Dong Yu, Yong-Joo Doh Topological insulator (TI) nanoribbons in contact with conventional superconducting electrodes can be a useful platform to generate a Majoarana bound state. As an evidence of the Majorana state, anomalous 4π current-phase relation is theoretically expected in a superconducting proximity junction of topological insulator. Here, we report the fabrication and measurement of a superconducting quantum interference device (SQUID) made of (Bi_{1-x}Sb_{x})_{2}Se_{3} TI nanoribbon. When we applied a magnetic field perpendicular to the substrate, we observed the periodic oscillations of supercurrent flow and the periodicity coincides with a magnetic flux quantum of h/2e. |
Wednesday, March 7, 2018 4:30PM - 4:42PM |
P08.00011: Predicting alloy disorder effects on the Effective Band Structures (EBS) of Topological and other solid solutions. Zhi Wang, Alex Zunger Often interesting physics is found in system with a controlled loss of translational symmetry, most notably in substitutionally disordered alloys, defected crystals, quantum wells, or magnetically disordered spins, or finite nanostructures. Such absence of long-range order prevents in principle plotting and interpreting band structures in the primitive Brillouin zone (BZ), thereby losing a central tool in interpreting electronic structure. Instead, the community has been using large supercells—an efficient tool to include deviations from translational symmetry, yet producing, for energy levels, a complex spaghetti of bands that is difficult to communicate or interpret with the familiar tools of electronic structure theory of crystals. We use the Effective Band Structure (EBS) method [1,2] to map the spectral functions of supercells into primitive BZ, thus restoring the interpretive tools we are all familiar with. In this report, we show how DFT–based EBS helps us understand the tolerances of topological effects to different levels of disorder in HgTe-CdTe and PbSe-SnSe alloys. |
Wednesday, March 7, 2018 4:42PM - 4:54PM |
P08.00012: Characterization of the Dielectric Properties of Polar Metal-Organic Frameworks Containing “Gyroscopic” Ligands Erik Lamb, Salvador Perez-Estrada, Yue-Shun Su, Tim Chung, Trevor Chang, Miguel Garcia-Garibay, Stuart Brown The metal-organic framework (MOF) Zn-F2BODCA-dabco occupies a tetragonal space group and is comprised of polar ligands in the ab plane, and interplanar non-polar ligands. Each ligand, or “rotor,” behaves as a molecular analogue to a gyroscope with a rotation axis in the intraplanar bond directions. Dielectric spectroscopy measurements on powder samples indicate an order-disorder phase transition involving the electric dipoles at 100K, followed by a Debye-like crossover at lower temperature. Analyzing the data as a ferroelectric transition close to 100K reveals a dipole moment on the order of the nominal dipole value of 2 Debye. Nuclear Magnetic Resonance (NMR) measurements suggests an Arrhenius freeze-out of rotational motion of the non-polar interplanar ligands, occurring within a small temperature range of the Debye-like crossover, suggestive of a correlation between the two dynamical effects. We present these results, as well as modeling with Monte Carlo simulations. |
Wednesday, March 7, 2018 4:54PM - 5:06PM |
P08.00013: Cleavage Energies of Layered Materials: Bi_{14}Rh_{3}I_{9}, Bi_{2}TeI, β-Bi_{4}I_{4} and 2H-MX_{2} Madhav Ghimire, Jeroen Van den Brink, Manuel Richter In recent years weakly bonded layered systems have become important for the manufacturing of two-dimensional materials. Precise knowledge of the interlayer bonding allows to understand in detail the exfoliation process in these compounds. Cleavage energies are crucial in this respect. Here we report the cleavage energies and electronic properties of the weak topological insulators (TIs) Bi_{14}Rh_{3}I_{9}, Bi_{2}TeI and β-Bi_{4}I_{4}, as well as of 2H-transition metal dichalcogenides (MX_{2} where M=Mo, W and X=S, Se, Te) determined by means of density functional theory calculations. Our calculations reproduce the experimentally measured value of cleavage energy of graphite, E_{c} (graphite) = 0.37 Jm^{−2}, which we use as a benchmark. Based on this, we calculate the cleavage energies of the three weak TIs and 2H-MX_{2} systems. We find that all energies are smaller than 2×E_{c} of graphite. The obtained values suggest the possibility of exfoliation of individual layers in these materials. |
Wednesday, March 7, 2018 5:06PM - 5:18PM |
P08.00014: Determining the Structural Properties of HfO_{2} Utilizing Reactive Force Fields Yasaman Ghadar, Alvaro Vazquez-Mayagoitia, Leighanne Gallington, John Low, Marius Stan, Chris Benmore The miniaturization of the MOS transistor and associated infrastructure on chip create the demand for ever thinner gate oxides.Hafnia, HfO_{2}, satisfies most of the requirements. This work focuses on structural properties of different phases of hafnia utilizing recently developed EAM+QEQ interatomic potentials. Here a combination of classical MD simulations, fist principle MD, high-energy x-ray and neutron diffraction to benchmark the interatomic potentials in the high temperature stable liquid and low-density amorphous solid states of hafnia. Results obtained from computational simulations revealed that an average Hf-O coordination number of about 7 exists in both the liquid and amorphous nanoparticle. The liquid shows a broad distribution of Hf-Hf interactions, while the formation of low-density amorphous nanoclusters can reproduce the sharp split peak in the Hf-Hf partial pair distribution function observed in experiment. The agglomeration of amorphous nanoparticles is associated with the formation of both edge-sharing and corner-sharing of HfO_{6,7 }which resembles of that observed in the monoclinic phase. |
Wednesday, March 7, 2018 5:18PM - 5:30PM |
P08.00015: Controlling and quantifying two-level systems via growth parameters in vapor deposited amorphous silicon thin films Manel Molina Ruiz, Hilary Jacks, David Castells-Graells, Daniel Queen, Mahat Sushant, David Cahill, Jason Maldonis, Paul Voyles, Matthew Abernathy, Thomas Metcalf, Xiao Liu, Marc Weber, Frances Hellman The structure of electron beam evaporated amorphous silicon is shown to depend strongly upon deposition temperature (from 25 to 425 °C) and total film thickness (from 10 to 300 nm), as well as deposition rate (from 0.05 to 2.5 Å/s). Previous work has hypothesized that the occurrence of structural defects is directly related to the increase of two-level systems. Structural qualities are measured by Rutherford BackScattering, Raman spectroscopy, Doppler Broadening Spectroscopy, and Fluctuation Electron Microscopy, whereas two-level systems are determined by specific heat and internal friction measurements. We show that defects are tuned via growth parameters and intrinsically related to two-level systems. Thicker and higher growth temperature films yields lower defects and thus, a lower density of two-level systems. Correlation between structural and energetic qualities suggests the structural motifs where two-level systems are likely to be formed. |
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