Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session L3: Experiments on Quantum Anomalous Hall Effects and 3D Topological Insulators |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Fan Zhang, University of Texas, Dallas Room: 262 |
Wednesday, March 15, 2017 11:15AM - 11:27AM |
L3.00001: Vanadium impurity state in the doped Sb$_{2}$Te$_{3}$ quantum anomalous Hall system Pengcheng Chen, Yu Liu, Yau Chuen Yam, Yang He, Christopher Eckberg, Joshua Samuel, Johnpierre Paglione, Mohammad Hamidian, Jennifer Hoffman The quantum anomalous Hall (QAH) effect has recently been reported in the ferromagnetic topological insulator V-doped (Sb,Bi)$_{2}$Te$_{3}$. However, the microscopic origin of the insulating ferromagnetic ground state is unclear. We employed scanning tunnelling microscopy and spectroscopy on (Sb$_{1-x}$V$_{x}$)$_{2}$Te$_{3}$, and identified two types of V substitutions, in the first and second Sb layers beneath the surface. We found that, second-layer V substitutions induce a peak within the bulk gap, which may form an impurity band at high impurity concentration. However, first-layer V substitutions suppress the impurity state and locally induce a gap in the surface state. Our results clarified the contribution of the V impurity state to the electronic structure of this QAH system. [Preview Abstract] |
Wednesday, March 15, 2017 11:27AM - 11:39AM |
L3.00002: Metrologically precise measurements of Hall quantization and dissipation in the quantum anomalous Hall effect Eli Fox, Ilan Rosen, David Goldhaber-Gordon, Yanfei Yang, George Jones, Randolph Elmquist, Xufeng Kou, Lei Pan, Kang Wang In the quantum anomalous Hall effect, chiral edge states should ideally produce quantized Hall conductance and dissipationless transport. However, early experimental realizations of this effect in magnetic topological insulator films found significant longitudinal resistivity and measurable deviations from quantization of the Hall resistance. Recent progress in the growth and gating of these films has resulted in greatly reduced dissipation and improved Hall quantization, requiring careful metrological techniques to accurately measure. Here, we report the most precise quantization measurements to date in Cr-doped (Bi,Sb)$_2$Te$_3$, and assess competing models for dissipation as a function of current, temperature, and chemical potential. [Preview Abstract] |
Wednesday, March 15, 2017 11:39AM - 11:51AM |
L3.00003: Emergent Momentum-Space Skyrmion Texture on the Surface of Topological Insulators Narayan Mohanta, Arno P. Kampf, Thilo Kopp The quantum anomalous Hall effect has been theoretically predicted and experimentally verified in magnetic topological insulators. In addition, the surface states of these materials exhibit a hedgehog-like ``spin'' texture in momentum space. Here, we apply the previously formulated low-energy model for Bi$_2$Se$_3$, a parent compound for magnetic topological insulators, to a slab geometry in which an exchange field acts only within one of the surface layers. In this sample set up, the hedgehog transforms into a skyrmion texture beyond a critical exchange field. This critical field marks a transition between two topologically distinct phases. The topological phase transition takes place without energy gap closing at the Fermi level and leaves the transverse Hall conductance unchanged and quantized to $e^2/2h$. The momentum-space skyrmion texture persists in a finite field range. It may find its realization in hybrid heterostructures with an interface between a three-dimensional topological insulator and a ferromagnetic insulator. [Preview Abstract] |
Wednesday, March 15, 2017 11:51AM - 12:03PM |
L3.00004: Tuning the Quantum Anomalous Hall Effect in Magnetic Topological Insulators Yang Feng, Gaoyuan Jiang, Weixiong Wu, Yunbo Ou, Ke He, Xucun Ma, Qikun Xue, Yayu Wang Quantum anomalous Hall effect (QAHE) has been experimentally realized recently in molecular beam epitaxy (MBE)-grown thin films of Cr or V doped (Bi,Sb)$_{\mathrm{2}}$Te$_{\mathrm{3}}$ topological insulators in the absence of external magnetic field. However, so far the QAHE state can only be observed at very low temperature in thin films with accurately controlled chemical compositions. In order to find better QAHE materials and facilitate its applications in low energy consumption electronic and spintronic devices, it is crucial to understand the various parameters that affect the QAHE. In this talk we report recent progress on the tuning and understanding of QAHE in magnetic topological insulators. In particular, we have carried out systematic experimental studies on the effect of magnetic doping level, film thickness, and electrical gate configurations on the QAHE. We show that the competition between surface and bulk transport, as well as that between magnetism and topology, are the crucial factors that determine the parameter space for the optimization of the QAHE. These results shed new lights on searching for better QAHE materials and novel device applications. [Preview Abstract] |
Wednesday, March 15, 2017 12:03PM - 12:15PM |
L3.00005: Interface induced ferromagnetism in topological insulator above room temperature Chi Tang, Cui-Zu Chang, Yawen Liu, Tingyong Chen, Jagadeesh Moodera, Jing Shi The quantum anomalous Hall effect (QAHE) observed in magnetic topological insulators (TI), an outcome of time reversal symmetry broken surface states, exhibits many exotic properties. However, a major obstacle towards high temperature QAHE is the low Curie temperature in the disordered magnetically doped TI systems. Here we report a study on heterostructures of TI and magnetic insulator in which the magnetic insulator, namely thulium iron garnet or TIG, has perpendicular magnetic anisotropy. At the TIG/TI interface, TIG magnetizes the surface states of the TI film by exchange coupling, as revealed by the anomalous Hall effect (AHE). We demonstrate that squared AHE hysteresis loops persist well above room temperature. The interface proximity induced high-temperature ferromagnetism in topological insulators opens up new possibilities for the realization of QAHE at high temperatures. [Preview Abstract] |
Wednesday, March 15, 2017 12:15PM - 12:27PM |
L3.00006: Ferromagnetism of vanadium doped Bi$_2$Se$_3$ thin films Liguo Zhang, Dapeng Zhao, Yunyi Zang, Yonghao Yuan, Gaoyuan Jiang, Ke He, Xucun Ma, Qikun Xue Bi$_2$Se$_3$ is a representative three-dimensional topological insulator with a bulk band gap of about 300 meV. The quantum anomalous Hall effect (QAHE) has never been realized in Bi$_2$Se$_3$-based magnetic topological insulators due to the difficulties in introducing ferromagnetism in them. With molecular beam epitaxy (MBE), we have grown vanadium-doped Bi$_2$Se$_3$ films with decent crystalline quality and homogeneous distribution of V impurities. The films are all electron-doped and show square-shaped hysteresis loops of Hall resistance with coercivity up to 0.2T at 2K, indicating ferromagnetism with perpendicular magnetic anisotropy in them. Both the ferromagnetism and anomalous Hall resistance are enhanced by decreasing electron density. We have systematically studied the magneto-transport properties of the films with varying V concentration, film thickness, and carrier density and discussed the mechanism of ferromagnetic coupling. The study demonstrates that V-doped Bi$_2$Se$_3$ films are candidate QAHE materials if their electron density can be further reduced. [Preview Abstract] |
Wednesday, March 15, 2017 12:27PM - 12:39PM |
L3.00007: Trends of electronic and magnetic properties of transition-metal impurities in Sb$_{\mathrm{2}}$Te$_{\mathrm{3}}$ topological insulator. Carlo M. Canali, Anna Pertsova, Fhokrul Islam, Alexander Balatsky The interaction between topological insulator (TI) surface states and magnetic order is crucial for future applications in spintronics and for the realization of novel quantum phenomena such as the quantum anomalous Hall effect (QAHE). Despite experimental progress in measuring the signatures of the QAHE in thin films of magnetically-doped TIs, the fate of the topological surface states, i.e. the presence or absence of a gap upon magnetic doping [1] as well as the nature of magnetic interactions in these systems [2] are under debate. Motivated by on-going experiments, we have systematically investigated the electronic structure and the magnetic anisotropy of transition-metal doped Sb$_{\mathrm{2}}$Te$_{\mathrm{3}}$ for both bulk and surface doping, using first-principles calculations based on density functional theory. In addition to magnetization-dependent gap, magnetic dopants introduce impurity states in the gap, whose detailed electronic and magnetic structure depends on the specific character of the dopant. We use these results to identify the conditions for the realization of a robust QAHE. [1] Sessi et al., Nat. Comm. 7, 12027 (2016); [2] Grauer et al., Phys. Rev. B 92, 201304 (2015). [Preview Abstract] |
Wednesday, March 15, 2017 12:39PM - 12:51PM |
L3.00008: Field driven insulator-semimetal crossover in ultrathin 3D topological insulator $BiSbTeSe_2$ Yang Xu, Ireneusz Miotkowski, Yong Chen Three dimensional topological insulators (3D TIs) are characterized by an insulating bulk while maintaining gapless surface states that are topologically robust against nonmagnetic impurities. However a hybridization gap would be opened in an ultrathin film of 3D TIs due to the quantum tunneling between the top and bottom surfaces. Experimentally, we observed such gap opening in very thin flakes (<~10 nm) of an intrinsic 3D TI material $BiSbTeSe_2$ by transport measurements. When the gap size small (less than a few millievectronvolts), it is controllable with applying magnetic field or out-of-plane electric field, and can even be closed, indicating an insulator-semimetal crossover. Our work paves the way to explore the application of topological electronics. [Preview Abstract] |
Wednesday, March 15, 2017 12:51PM - 1:03PM |
L3.00009: Measurement of the topological surface state optical conductance in bulk-insulating Sn-doped Bi$_{1.1}$Sb$_{0.9}$Te$_2$S single crystals Bing Cheng, Liang Wu, Satya Kushwaha, Robert Cava, Peter Armitage Topological surface states have been extensively observed via optics in thin films of topological insulators. However, in typical thick single crystals of these materials, bulk states are dominant and it is difficult for optics to verify the existence of topological surface states definitively. In this work, we studied the charge dynamics of the newly formulated bulk-insulating Sn-doped Bi$_{1.1}$Sb$_{0.9}$Te$_2$S crystal by using time-domain terahertz spectroscopy. This compound shows much better insulating behavior than any other bulk-insulating topological insulators reported previously. The transmission can be enhanced an amount which is 5$\%$ of the zero-field transmission by applying magnetic field to 7 T, an effect which we believe is due to the suppression of topological surface states. This suppression is essentially independent of the thicknesses of the samples, showing the two-dimensional nature of the transport. The suppression of surface states in field allows us to use the crystal slab itself as a reference sample to extract the surface conductance, mobility, charge density and scattering rate. Our measurements set the stage for the investigation of phenomena out of the semi-classical regime, such as the topological magneto-electric effect. [Preview Abstract] |
Wednesday, March 15, 2017 1:03PM - 1:15PM |
L3.00010: Observation of long-lived persistent spin polarization in a topological insulator. Jifa Tian, Seokmin Hong, Ireneusz Miotkowski, Supriyo Datta, Yong P. Chen 3D Topological insulators (TI), featuring helically spin-momentum-locked topological surface states (TSS), are considered promising for spintronics applications. Several recent experiments in TIs have demonstrated a current induced electronic spin polarization that may be used for all electrical spin generation and injection. Here, we report spin potentiometric measurements in TIs that have revealed a long-lived persistent electron spin polarization even at zero current. Unaffected by a small bias current and persisting for several days at low temperature, the spin polarization can be induced and reversed by a large ``writing'' current applied for an extended time. Such an electrically controlled persistent spin polarization with unprecedented long lifetime could enable a rechargeable spin battery and rewritable spin memory for potential applications in spintronics and quantum information. [Preview Abstract] |
Wednesday, March 15, 2017 1:15PM - 1:27PM |
L3.00011: Novel Planar Hall Effect in the Surface of Topological Insulators Alexey Taskin, Henry Legg, Fan Yang, Satoshi Sasaki, Yasushi Kanai, Kazuhiko Matsumoto, Achim Rosch, Yoichi Ando The progress in the study of topological materials depends on the ability to measure their surface properties. Recent advances in MBE growth allowed us to obtain suitable topological insulators (TIs). Here we report a magneto-transport study of high-quality bulk-insulating Bi$_{2-x}$Sb$_{x}$Te$_{3}$ thin films, which were fabricated into devices with electrostatic gates on both bottom and top surfaces. For magnetic fields applied parallel to the surface of a TI, we found a clear anisotropy in magnetoresistance (MR) and related planar Hall effect. This anisotropy is a consequence of two fundamental facts: 1) the time-reversal symmetry is broken by the magnetic field, lifting the topological protection of spin-momentum locked Dirac electrons against backscattering from impurities; 2) the in-plane magnetic field does not open the gap in the surface state, preserving the Dirac physics. As a result the back scattering protection can still be maintained for electrons with spins parallel/antiparallel to the direction of the magnetic field, giving rise to the scattering-rate anisotropy. The key signature of anisotropic MR is a strong dependence on the gate voltage with a characteristic two-peak structure near the Dirac point, which was observed by employing the dual-gating technique. [Preview Abstract] |
Wednesday, March 15, 2017 1:27PM - 1:39PM |
L3.00012: Spin-polarized surface resonances accompanying topological surface state formation Kenneth Gotlieb, Chris Jozwiak, Jonathan Sobota, Alexander Kemper, Costel Rotundu, Robert Birgenau, Zahid Hussain, Dung-Hai Lee, Zhi-Xun Shen, Alessandra Lanzara Strong spin-orbit coupling can drive a band inversion that makes a material topologically non-trivial. Using spin, time, and angle-resolved photoemission spectroscopy, we study the unoccupied bandstructure in a topological insulator and find a spin-polarized surface resonance near the topological surface state. This new state is a remnant of Rashba bands on the trivial side of the topological phase transition. From this, we learn how the into the topological surface state emerges upon band inversion. [Preview Abstract] |
Wednesday, March 15, 2017 1:39PM - 1:51PM |
L3.00013: Field effect study of QHE in low carrier density films of Bi2Se3 Nikesh Koirala, Maryam Salehi, Jisoo Moon, Seongshik Oh The novel quantum Hall effect (QHE) arising from topological surface states (TSS) of TIs have been difficult to observe in binary chalcogenide compounds such as Bi2Se3 due to high level of doping in these materials. By growing Bi2Se3 thin films with low starting carrier density, we have not only observed ambipolar transport but also the 1/2 quantized QHE (per surface) associated with Dirac dispersion of the TSSs as indicated by (inverse) odd integered nature of Hall resistance. Furthermore, we will also discuss how QHE signature varies as a function of gate voltage. [Preview Abstract] |
Wednesday, March 15, 2017 1:51PM - 2:03PM |
L3.00014: THz spectroscopy of electric field modulated topological insulator, Bi$_{\mathrm{\mathbf{2}}}$\textbf{Se}$_{\mathrm{\mathbf{3.}}}$ Mintu Mondal, Maryam Salehi, Cheng Wan, Dipanjan Chaudhuri, Nicholas Laurita, Bing Cheng, Michael Quintero, Jisoo Moon, Deepti Jain, Pavel Shibayev, Tyrel McQueen, Seongshik Oh, Peter Armitage Topological Insulators (TIs) belong to a class of materials that exhibit topologically protected conducting surface states with a Dirac like dispersion, with an insulating bulk. This novel class of materials shows a variety of interesting phenomena including a quantized magneto-electric effect and novel spin textures at the surface. They may have enormous potential for applications. However, in real topological insulators, the bulk is fairly conducting and the chemical potential lies inside the conduction band, which gives great difficulties to study their properties. In this talk, I will present the THz response of Dirac surface states of the topological insulator, Bi$_{\mathrm{2}}$Se$_{\mathrm{3\thinspace }}$modulated by gate voltage$_{\mathrm{.\thinspace }}$To tune the chemical potential$_{\mathrm{,\thinspace }}$we have used well known ionic liquid gel techniques as top gate. As a function of gate voltage, we have observed significant change in conductance which allows us to study the magneto-electric effect through measurement of Faraday and Kerr rotation close to Dirac point. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2020 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
1 Research Road, Ridge, NY 11961-2701
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700