Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session A2: Magnetism in Topological Insulators 
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Sponsoring Units: DCMP GMAG Chair: WangKong Tse, University of Alabama Room: 261 
Monday, March 13, 2017 8:00AM  8:12AM 
A2.00001: Codopingenhanced magnetism in 4f transition metal doped Bi2Se3 Junyi Zhu, Bei Deng, Yiou Zhang, Shengbai Zhang, Yayu Wang, Ke He Despite the great success in the realization of the quantum anomalous Hall effect (QAHE), it was only observed at extremely low temperatures due to the low ferromagnetic Curie temperature and the tiny magnetically induced gap. To fully understand the mechanism of the ferromagnetic ordering, thereby improving the ferromagnetism, we investigated 4f transition metal doped Bi2Se3, using density functional theory approaches. We predict that Eu and Sm can introduce stable longrange ferromagnetic states in Bi2Se3, with large magnetic moments and low impurity disorders. Additionally, codoping is proposed to tune the Fermi level into the gap, which simultaneously improves the magnetic moment and the incorporation of magnetic ions. Our findings, thus, offer a new novel strategy in facilitating the realization of QAHE in TI systems. [Preview Abstract] 
Monday, March 13, 2017 8:12AM  8:24AM 
A2.00002: Giant spin Hall angle from topological insulator Bi$_{\mathrm{x}}$Se$_{\mathrm{(1x)}}$ thin films Mahendra DC, Mahdi Jamali, Junyang Chen, Danielle Hickey, Delin Zhang, Zhengyang Zhao, Hongshi Li, Patrick Quarterman, Yang Lv, Andre Mkhyon, JianPing Wang Investigation on the spinorbit torque (SOT) from large spinorbit coupling materials has been attracting interest because of its low power switching of the magnetization and ultrafast driving of the domain wall motion that can be used in future spin based memory and logic devices. We investigated SOT from~topological insulator Bi$_{\mathrm{x}}$Se$_{\mathrm{(1x)}}$~thin film in Bi$_{\mathrm{x}}$Se$_{\mathrm{(1x)}}$~/CoFeB heterostructure by using the dc planar Hall method, where Bi$_{\mathrm{x}}$Se$_{\mathrm{(1x)}}$~thin films were prepared by a unique industrycompatible deposition process.~The angle dependent Hall resistance was measured in the presence of a rotating external inplane magnetic field at bipolar currents. The spin Hall angle (SHA) from this Bi$_{\mathrm{x}}$Se$_{\mathrm{(1x)}}$~thin film was found to be as large as 22.41, which is the largest ever reported at room temperature (RT).~~The giant SHA and large spin Hall conductivity (SHC) make this Bi$_{\mathrm{x}}$Se$_{\mathrm{(1x)}}$~thin film a very strong candidate as an SOT generator in SOT based memory and logic devices. [Preview Abstract] 
Monday, March 13, 2017 8:24AM  8:36AM 
A2.00003: Hole doping problem in Bi$_{2}$Se$_{3}$ thin films and its solution: Role of interfacial vs bulk defects Jisoo Moon, Nikesh Koirala, Maryam Salehi, Seongshik Oh Bi$_{2}$Se$_{3}$, one of the most widely studied topological insulators, is naturally electron (ntype) doped. In bulk crystals and thick films, ntype Bi$_{2}$Se$_{3}$ can be converted into ptype through compensation doping. However, such a compensation doping scheme has so far failed to achieve ptype Bi$_{2}$Se$_{3}$ thin films. Here, we show that there exists a thicknessdependent pdoping problem in Bi$_{2}$Se$_{3}$ thin films, which originates from the high density of interfacial defects compared with that of the bulk, and provide a solution to this long standing puzzle. Reducing the interfacial defects by employing an effective buffer layer on the bottom and a capping layer on the top, we facilitate the compensation doping scheme and achieved dominant ptype carriers in Bi$_{2}$Se$_{3}$ thin films down to the thinnest topological regime. Availability of ptype Bi$_{2}$Se$_{3}$ thin films will open new opportunities in this active field of topological materials. [Preview Abstract] 
Monday, March 13, 2017 8:36AM  8:48AM 
A2.00004: Optical and magnetooptical study of topological insulators Bi$_2$Te$_3$ and Sb$_2$Te$_3$ S.V. Dordevic, H. Lei, C. Petrovic, J. Ludwig, D. Smirnov We have studied electrodynamical properties of topological insulators Bi$_2$Te$_3$ and Sb$_2$Te$_3$ with the goal of elucidating their electronic structure. Optical and magnetooptical properties of bulk samples of Bi$_2$Te$_3$ and Sb$_2$Te$_3$ will be reported over a broad range of frequencies (from farinfrared to near ultraviolet), temperatures (from room temperature to 4.2 K) and magnetic fields (from zero to 18 Tesla). The spectra reveal strong magnetooptical activity in both Bi$_2$Te$_3$ and Sb$_2$Te$_3$, especially around the plasma minimum in reflectance. From the data we extract some important parameters of charge dynamics, such as carrier mobility and effective mass. The results will be compared and contrasted with similar results on Bi$_2$Se$_3$. [Preview Abstract] 
Monday, March 13, 2017 8:48AM  9:00AM 
A2.00005: Interplay of quantum oscillations and ferromagnetism in magnetic topological insulators D. Nandi, K. Shain, G.H. Lee, CuiZu Chang, K. Huang, J. Ward, J.S. Moodera, P. Kim, A. Yacoby Ferromagnetic topological insulators are of interest because of the experimental realization of quantized anomalous Hall effect. Here we report measurements on ebeam lithographically contacted devices of Vanadium doped (Bi,Sb)$_2$Te$_3$. These devices exhibit unconventional Shuvnikovde Haas type oscillations at zero bias that have a strong dependence on applied bias voltage. These magnetooscillations have been observed with both superconducting and normal metal contacts. Intriguingly, these oscillations are observed to be most prominent in submicron devices and weaken significantly in wider junctions. Logarithmic dependence of the longitudinal resistance and anomalous Hall resistance is observed on temperature and applied bias voltage. A phenomenological model is presented to explain the various experimental observations. [Preview Abstract] 
Monday, March 13, 2017 9:00AM  9:12AM 
A2.00006: Logarithmic voltage bias dependence in ferromagnetic twodimensional topological insulators Jonathan Ward, K. Shain, D. Nandi, G.H. Lee, CuiZu Chang, K. Huang, J.S. Moodera, P. Kim, A. Yacoby The quantum anomalous Hall effect has recently been demonstrated in thin films of $(\text{BiSb})_2\text{Te}_3$ with Vanadium doping. We report the first ebeam lithographically defined devices from this ferromagnetic twodimensional topological insulator. Transport measurements show that, when the bulk is gated into a conducting state, longitudinal resistance and Hall resistance have logarithmic dependence on sourcedrain voltage bias and temperature. As this system may be a suitable platform for Majorana fermions, it is critical to understand the logarithmic dependence as it occurs around zero bias. We present a model to explain the observed logarithmic dependence near zero bias and address other sources of logarithmic dependence that are not present in the device. [Preview Abstract] 
Monday, March 13, 2017 9:12AM  9:24AM 
A2.00007: Band gap modulation in magnetically doped lowdefect thin films of (Bi$_{1x}$Sb$_{x})_{2}$Te$_{3}$ with minimized bulk carrier concentration Yulia Maximenko, Kane Scipioni, Zhenyu Wang, Ferhat Katmis, Charles Steiner, Adam Weis, Dale Van Harlingen, Vidya Madhavan Topological insulators Bi$_{2}$Te$_{3}$ and Sb$_{2}$Te$_{3}$ are promising materials for electronics, but both are naturally prone to vacancies and antisite defects that move the Fermi energy onto the bulk bands. Fabricating (Bi$_{1x}$Sb$_{x})_{2}$Te$_{3}$ (BST) with the tuned $x$ minimizes point defects and unmasks topological surface states by reducing bulk carriers. BST thin films have shown topological surface states and quantum anomalous Hall effect. However, different studies reported variable Sb:Bi ratios used to grow an undoped BST film. Here, we develop a reliable way to grow defectfree subnanometerflat BST thin films having the Fermi energy tuned to the Dirac point. Highresolution scanning tunneling microscopy (STM) and Landau level spectroscopy prove the importance of crystallinity and surface roughnessnot only Sb:Bi ratiofor the final bulk carrier concentration. The BST thin films were doped with Cr and studied with STM with atomic resolution. Counterintuitively, Cr density is anticorrelated with the local band gap due to Cr's antiferromagnetic order. We analyze the correlations and report the relevant band gap values. Predictably, high external magnetic field compromises antiferromagnetic order, and the local band gap increases. [Preview Abstract] 
Monday, March 13, 2017 9:24AM  9:36AM 
A2.00008: Magnetotransport Studies of TypeII InAs/GaSb Quantum Wells in Inverted/Normal Regime with Dualgating and Magnetic Doping Di Xiao, Nitin Samarth TypeII InAs/GaSb quantum wells are of great current interest as quantum spin Hall (QSH) insulators due to the unique inverted band structures in the topological regime. However, recent studies have shown that conductance plateaus and nonlocal signals of edge currents can apparently be observed in the gatecontrolled QSH phase diagram for \textit{both} topological and trivial phases. Here, we report a thorough transport study of InAs/GaSb quantum wells in both the inverted and normal regimes by varying the thickness of the InAs channel layer. By using dual gating, we study the phase diagrams of macroscopic devices, demonstrating the gradual change of the band structures with different layer parameters. Nonlocal measurements on mesoscopic samples confirm the existence of edge currents in both regimes. Finally, motivated by a recent theoretical prediction [Wang et al., PRL, 113, 147201 (2014)] of achieving a quantum anomalous Hall state in these systems by magnetic doping, we describe our ongoing studies of gated InAs/GaSb quantum wells that are Mndoped. Funded by ONR. [Preview Abstract] 
Monday, March 13, 2017 9:36AM  9:48AM 
A2.00009: Topological Charge/spin density Wave in InAs/GaSb Quantum Wells under an Inplane Magnetic Field LunHui Hu, ChihChieh Chen, Yi Zhou, FuChun Zhang, ChaoXing Liu The 2D quantum spin hall (QSH) system, InAs/GaSb quantum wells (QWs), is treated as a bilayer electronhole system. In this work, we begin with the noninteracting BHZ model under inplane magnetic field. For dilute carrier density system, we shall study the effect of Coulomb interaction. Phase diagram can be figured out by mean field theory. One possible phase is the indirect swave type exciton condensation (EC) phase which opens a bulk gap directly, which makes the system a trivial exciton insulator without edge states. On the other hand, we also find that charge/spin density wave (CDW/SDW) opens a minigap through hybridization effect induced by Coulomb interaction. We then derive the low energy effective Hamiltonian to understand each phase with/without helical edge states, and we find the effective Hamiltonian is similar to the noninteracting BZH model with the renormalization of some parameters (inversion gap M, effective hybridization A and inplane gfactor g). Moreover, an intuitive dispersion with helical edge states is provided as a further confirmation. The relevance to experiments is also discussed. [Preview Abstract] 
Monday, March 13, 2017 9:48AM  10:00AM 
A2.00010: Spinorbit torque in two dimensional antiferromagnetic topological insulators Sumit Ghosh, Aurelien Manchon Topological insulators (TI) have been found to be a source of huge spinorbit torque (SOT) \footnote{Y. Fan et. al., Nat. Nanotechnol. 11, 352 (2016).} that originates from their surface states. However, the proximity of a ferromagnetic layer can destroy the surface states which makes the exact nature of the SOT quite argumentative. Recently it has been found that in presence of an antiferromagnetic magnetization, a TI can preserve its gapless states \footnote{R. S. K. Mong, A. M. Essin, and J. E. Moore, Phys. Rev. B 81, 245209 (2010).}. We conduct a systematic study \footnote{S. Ghosh and A. Manchon, arXiv:1609.01174.} on twodimensional antiferromagnetic TI and find that they are more robust compared to a ferromagnetic TI against impurity scattering. It can facilitate a field like SOT due to the intrinsic spinorbit coupling and an antidamping SOT via scattering by scalar impurity. Interestingly, a moderate amount of impurity enhances the staggered spin density at the edges resulting in a uniform antidamping torque with the conductance remaining finite. It is, therefore, possible to manipulate the magnetization either by using a pulse \footnote{T. Jungwirth et. al., Nat. Nanotechnol. 11, 231 (2016).} via field like SOT or by a dc current via antidamping SOT. [Preview Abstract] 
(Author Not Attending)

A2.00011: Emergent Topological order from SpinOrbit Density wave Gaurav Gupta, Tanmoy Das We study the emergence of a Z2 type topological order because of Landau type symmetry breaking order parameter. When two Rashba type SOC bands of different chirality become nested by a magic wavevector [(0,$\backslash $pi) or ($\backslash $pi,0)], it introduces the inversion of chirality between different lattice sites. Such a density wave state is known as spinorbit density wave[1,2]. The resulting quantum order is associated with the topological order which is classified by a Z2 invariant. So, this system can simultaneously be classified by both a symmetry breaking order parameter and the associated Z2 topological invariant. This order parameter can be realized or engineered in two or quasitwodimensional fermionic lattices, quantum wires, with tunable RSOC and correlation strength. [1] T. Das, PRL 109, 246406 (2012).[2] C. Brand, et al Nature Commun. 6, 8118 (2015). [Preview Abstract] 
Monday, March 13, 2017 10:12AM  10:24AM 
A2.00012: Quantum Anomalous Hall Insulator in Asymmetrically Functionalized Germanene ChiaHsiu Hsu, ZhiQuan Huang, Christian P. Crisostomo, YuMing Gu, YiMei Fang, Shunqing Wu, ZiZhong Zhu, Li Huang, FengChuan Chuang, Hsin Lin, Arun Bansil We study the atomic structures and electronic properties of honeycomb germanene passivated asymmetrically with hydrogen and nitrogen (GeHN) atoms using firstprinciples electronic structure calculations. There are three atomic structures considered: planar, buckled, and inversely buckled honeycomb. We found that the inversely buckled structure is in nonmagnetic insulator phase; while the buckled structure is metallic. Using spinpolarized calculations with spinorbit coupling, GeHN in planar structure was identified to exhibit a quantum anomalous Hall (QAH) insulator phase. In order to confirm the topology of the material, we further calculated the Chern number as well as the edge states. A band connecting the conduction and the valence band was found at the edge state of zigzag GeHN nanoribbon. Our results suggest that functionalized germanene might be used in the highly sought spintronic applications based on QAH effect. [Preview Abstract] 
Monday, March 13, 2017 10:24AM  10:36AM 
A2.00013: Smearing of the quantum anomalous Hall effect due to statistical fluctuations of magnetic dopants Mikhail Raikh, Zhang Yue Quantum anomalous Hall effect (QAH) is induced by substitution of a certain portion, $x$, of Bi atoms in a BiTebased insulating parent compound by magnetic ions (Cr or V). We find the density of ingap states, $N(E)$, emerging as a result of statistical fluctuations of the composition, $x$, in the vicinity of the transition point, where the {\em average} gap, ${\overline E}_g$, passes through zero. Local gap follows the fluctuations of $x$. Using the instanton approach, we show that, near the gap edges, the tails are exponential, $\ln{N(E)} \propto  \big({\overline E}_gE\big)$, and the tail states are due to small local gap reduction. Our main finding is that, even when the smearing magnitude exceeds the gapwidth, there exists a semihard gap around zero energy, where $\ln {N(E)} \propto \frac{{\overline E}_g}{E}\ln \Big(\frac{{\overline E}_g}{E}\Big)$. The states responsible for $N(E)$ originate from local gap reversals within narrow rings. The consequence of semihard gap is the Arrhenius, rather than variablerange hopping, temperature dependence of the diagonal conductivity at low temperatures. [Preview Abstract] 
Monday, March 13, 2017 10:36AM  10:48AM 
A2.00014: Dynamical Quantum Anomalous Hall Effect in the Intense Optical Field Regime WooRam Lee, WangKong Tse Topological insulators are characterized by the quantum anomalous Hall effect on the topological surface states under timereversal symmetry breaking. While this effect has been recently observed in a magnetooptical setup upon illumination of weak linearly polarized light, the influence of intense optical field remains largely unexplored. Using the KeldyshFloquet Green's function formalism, we develop a theory for the dynamical Hall conductivity for arbitrary incident optical frequency in the intense optical field regime. We apply our general theory to the adiabatic, lowfrequency regime, and study the breakdown of the onehalf Hall quantization under intense optical field. Our results reveal a strong nonlinear dependence of the dynamical Hall conductivity on the incident optical field, which is triggered by the formation of Floquet subbands and the transitions between them. [Preview Abstract] 
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