Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session W42: Focus Session: Magnetic Topological Insulators |
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Sponsoring Units: DMP Chair: Don Heiman, Northeastern University Room: Mile High Ballroom 4A |
Thursday, March 6, 2014 2:30PM - 3:06PM |
W42.00001: Magnetism in Magnetically Doped Topological Insulators Invited Speaker: Ke He A topological insulator (TI) has topologically non-trivial electronic property induced by spin-orbit coupling (SOC) and protected by time reversal symmetry (TRS). Breaking the TRS of a three-dimensional (3D) TI with ferromagnetism can gap the Dirac surface states and induce novel quantum phenomena. Magnetic doping is a convenient approach to introduce magnetism into a TI. A crucial issue is if long-range ferromagnetic order could be built in magnetically doped TIs in the insulating regime. Combining angle-resolved photoemission spectroscopy, scanning tunneling microscopy, transport measurement, and first principles calculation, we have systematically studied the surface band structure, magnetism and transport properties of molecular beam epitaxy-grown Cr-doped 3D TIs: Bi$_{2}$Se$_{3}$, Bi$_{2}$Te$_{3}$, and Sb$_{2}$Te$_{3}$. We have found that Cr-doped Bi$_{2}$Te$_{3}$, Sb$_{2}$Te$_{3\, }$ and their alloys show long-range ferromagnetic order robust against variation in charge carriers. The ferromagnetism is likely mediated by the strong van Vleck susceptibility of the host materials due to the SOC-induced inverted band structure. Cr-doped Bi$_{2}$Se$_{3}$, on the other hand, could not show long-range ferromagnetic order, but exhibit gap-opening at the Dirac surface states. The absence of long-range ferromagnetic order and the observed gapped surface states are partly due to the superparamagnetic multimers formed by Cr dopants, and partly due to significant reduction of the SOC of Bi$_{2}$Se$_{3}$ by Cr substitution for Bi, which turns off the van Vleck mechanism. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W42.00002: Quantum Anomalous Hall Effect with Higher Plateaus Jing Wang, Biao Lian, Haijun Zhang, Yong Xu, Shou-Cheng Zhang The quantum anomalous Hall (QAH) effect in magnetic topological insulators is driven by the combination of spontaneous magnetic moments and spin-orbit coupling. Its recent experimental discovery raises the question if higher plateaus can also be realized. Here, we present a general theory for a QAH effect with higher Chern numbers and show by first-principles calculations that a thin film magnetic topological insulator of Cr-doped Bi2(Se,Te)3 is a candidate for the C$=$ 2 QAH insulator. Remarkably, whereas a higher magnetic field leads to lower Hall conductance plateaus in the integer quantum Hall effect, a higher magnetic moment leads to higher Hall conductance plateaus in the QAH effect. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W42.00003: Measurement of optical rotation in the magnetic topological insulator (Bi,Sb)2Te3 Shreyas Patankar, Daniel Golubchik, Joseph Orenstein, Eli Fox, David Goldhaber-Gordon, Xiao Feng, Ke He, Yayu Wang, Qi-kun Xue Topological insulators with surface states that break time reversal symmetry have been predicted to have exotic topological quantum properties. One way of realizing these is through topological insulators that also have magnetic ordering. Recently, measurements of quantum anomalous hall effect were reported in thin films of chromium doped (Bi,Sb)2Te3, which gave evidence for the presence of spontaneous magnetic order in this topological insulator. We report measurements of magneto-optic Kerr effect in this material, which provides an alternative quantification of magnetization. Kerr rotation was measured as a function of applied magnetic field and of temperature. Preliminary data suggests a transition to a ferromagnetic state at 12K and a coercive field of 30mT. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W42.00004: Magneto-optical Kerr effect in Cr-doped (Bi,Sb)$_{2}$Te$_{3}$ Thin Films Yu Pan, Bing Yao, Anthony Richardella, Abhinav Kandala, Robert Fraleigh, Joon Sue Lee, Nitin Samarth, Andrew Yeats, David D. Awschalom When a three-dimensional (3D) topological insulator (TI) is interfaced with magnetism, the breaking of time reversal symmetry results in new phenomena such as the recently observed quantum anomalous Hall effect [C.-Z. Zhang \textit{et al., Science} \textbf{340}, 167 (2013)]. Thus motivated, we use the polar-mode magneto-optical Kerr effect (MOKE) to probe the temperature- and field-dependent magnetization in molecular beam epitaxy grown Cr-doped thin films of the 3D TI (Bi,Sb)$_{2}$Te$_{3}$. Square MOKE hysteresis loops observed at low temperatures indicate robust ferromagnetism with a perpendicular magnetic anisotropy and Curie temperature that varies from $\sim$ 5 K to $\sim$ 150 K, depending on sample details. A key question is the nature of the ferromagnetism: is it a carrier-mediated mechanism, Van Vleck mechanism or due to extrinsic clusters? We address this issue by varying the magnetic ion concentration and carrier density via sample composition as well as by varying the chemical potential by back gating. Finally, we use spatially-resolved MOKE to image the magnetization in these samples. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W42.00005: Proximity effect induced magnetoresistance hysteresis loops in a topological insulator/YIG heterostructure Mohammad Montazeri, Murong Lang, Mehmet C. Onbasli, Xufeng Kou, Liang He, Caroline A. Ross, Kang L. Wang We experimentally demonstrate the proximity effect induced hysteretic magnetoresistance in an 8 quintuple layers of Bi2Se3 films grown on Gallium Gadolinium Garnet (GGG) (111) substrates with a 50 nm Yttrium Iron Garnet (YIG) buffer layer by molecular beam epitaxy. With in-plane and out-of-plane magnetic field, square wave shaped and butterfly shaped resistance hysteresis loops can be observed up to 25 K, respectively. The relationship between the hysteretic MR curves and the magnetic switching of the YIG will be discussed in the context of a proximity effect between the YIG and the TI. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W42.00006: Correlation of Lattice-Symmetry, Electronic Anisotropy and Transport in Topological Insulators Thin Films and Heterostructures Ferhat Katmis, Valla Fatemi, Peng Wei, Hadar Steinberg, John Freeland, Pablo Jarillo-Herrero, Jagadeesh Moodera To explore the intrinsic features of topological insulators (TIs) thin films and thus to correlate structure with the exotic electronic properties as well as interaction with other material systems careful structural studies are needed. Molecular beam epitaxy ideally allows us to engineer the required system for observing the intrinsic properties of TI thin films and heterostructures, thereby accessing the optimum Dirac surface states. In well-defined films and heterostructures, we elucidated the role of imperfections e.g. vacancies, line defects, twinning etc., on the symmetry of the material that leads to internal atomic ordering by the decoration of the defects. Charge transport is seen to relate with film growth induced strain and relaxation, as well as exhibit strong directional dependence on the defect geometry. In TI with ferromagnetic insulator (FI), the observation of symmetry breaking strongly depends on the interface coupling between FI and TI, where the exchange interaction occurs defined by the hybridization at interface. Synchrotron based GIXRD, XAFS, and XAS/XMCD helped reveal strain, hybridization, and magnetic interaction between FI and TI thin films show strong structural and magnetic interactions. MIT MRSEC through NSF Grant No. DMR-0819762. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W42.00007: Heterostructure of a Topological Insulator and a Ferromagnetic Insulator bi-layer Valeria Lauter, Ferhat Katmis, Badih Assaf, Jagadeesh Moodera The short-range nature of magnetic proximity coupling with a ferromagnetic insulator (FI) induces ferromagnetic interactions in the TI surface state with the symmetry breaking right at the interface. Here we investigate Bi$_{2}$Se$_{3}$/EuS heterostructures and the mechanism to induce ferromagnetic order at the surface of Bi$_{2}$Se$_{3}$ thin films by using the FI EuS. SQUID measurements demonstrated excessive magnetic moment for the EuS film alone, thus indicating that EuS might induce a magnetization in Bi$_{2}$Se$_{3}$. Using PNR we reveal that EuS induces a significant magnetic moment in the Bi$_{2}$Se$_{3}$ films. Thus, it creates broken time-reversal symmetry and should appear as magnetic signatures in electrical transport. The ferromagnetism of EuS and the coupling between EuS and Bi$_{2}$Se$_{3}$ have to be strong to signify surface magnetization effect. These findings may be used to investigate interesting emergent phenomena, because the local time-reversal symmetry breaking is essential for inducing a quantized topological magnetoelectric response. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W42.00008: Gate-Tunable Quantum Corrections in Topological Insulator/ Insulating Ferromagnet Heterostructures Joon Sue Lee, Anthony Richardella, Robbie Fraleigh, Chao-xing Liu, Nitin Samarth Heterostructures that interface a topological insulator (TI) and an insulating ferromagnet (IFM) are of current interest for potential spintronics applications, as well as for fundamental explorations of quantum phenomena resulting from broken time reversal symmetry (TRS). Since angle resolved photoemission spectroscopy cannot directly probe the modified topological surface state at the buried interface between a TI and an IFM, we use the quantum corrections to the magneto-conductance (MC) as a possible probe of broken TRS. We report systematic studies of the quantum corrections by varying temperature and chemical potential in (Bi,Sb)$_{2}$Te$_{2}$Se/ Ga$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As/ InP (111)A heterostructures grown by molecular beam epitaxy. We select the Ga$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As composition to yield a conductivity of orders of magnitude lower than the TI, with a ferromagnetic Curie temperature of 16 $\sim$ 40 K. At fixed chemical potential, we observe a crossover from negative MC (weak anti localization) to positive MC (weak localization) as the temperature is lowered. A similar crossover is observed when the chemical potential is electrically tuned using a top gate. The results are interpreted in terms of the opening of a gap at the Dirac point. Funded by ONR and DARPA. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W42.00009: Magnetotransport in Gate Tunable Insulating Ferromagnet -Topological Insulator Heterostructure Devices Abhinav Kandala, Anthony Richardella, Robert Fraleigh, Nitin Samarth Magnetic perturbations to topological insulator (TI) surface states are predicted to result in a host of exotic effects that are of great fundamental and applied interest. Complementary to magnetic doping, interfacing TI's with an insulating ferromagnets (IF) enables transport studies of magnetism solely at the surface state. In a previous study [A. Kandala et al., Appl. Phys. Lett. 103, 202409 (2013)], we used IF-TI heterostructure devices wherein the chemical potential in the TI was fixed in the bulk conduction band. By using Sb doping and growth on SrTiO$_3$ substrates, we have now fabricated IF-TI devices where the surface states are accessed by back-gate tuning of the chemical potential. Our unique device geometry enables a direct comparison of magneto-conductance in bare (control) and magnetically capped TI channels as the chemical potential is swept through the Dirac point. Analysis of the low-temperature magneto-conductance provides insights into the influence of the magnetic overlayer on quantum corrections to the diffusive transport. Supported by DARPA and ONR. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W42.00010: Characterization of Chemical Trends in Magnetically Doped, Electrically Gated Topological Insulator Thin Films Anthony Richardella, Abhinav Kandala, Joon Sue Lee, Robbie Fraleigh, Nitin Samarth, Minhao Liu, Nai Phuan Ong, Jing Tao Interfacing topological insulators (TIs) with magnetism breaks time reversal symmetry and opens a gap in the surface states at the Dirac point. This results in novel phenomena, such as the recently reported quantized conductance at zero applied external magnetic field due to the quantum anomalous Hall effect (QAHE) in Cr doped (Bi$_{\mathrm{x}}$Sb$_{\mathrm{1-x}})_{\mathrm{2}}$Te$_{\mathrm{3}}$ [C-Z. Chang, et al., Science 340, 167 (2013)]. We have studied magnetically doped (Bi$_{\mathrm{x}}$Sb$_{\mathrm{1-x}})_{\mathrm{2}}$Te$_{\mathrm{3}}$ thin films grown by MBE on SrTiO3(111) (STO) substrates using Cr, Fe and Mn as magnetic dopants and as a function of the Bi and Sb composition. These films are carefully characterized by XRD, AFM, SQUID magnetometry and TEM. The chemical composition is determined using SIMS, RBS and XRF. Low temperature transport shows a large gate-tunable Hall effect in Cr doped samples and systematically varying longitudinal magneto-conductance as the Fermi energy is tuned through the Dirac point. The origin of ferromagnetism and its dependence on the chemical potential, chemical composition and sample thickness is discussed. Funded by DARPA and ARO-MURI. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W42.00011: The anomalous Hall effect in ultrathin films of gate-tuned Cr-doped (Bi,Sb)2Te3 Minhao Liu, Anthony Richardella, Abhinav Kandala, Nitin Samarth, N. P. Ong The surface states of topological insulator are predicted to display exotic properties in a magnetization field \textbf{M}. Quantized anomalous Hall effect was recently observed in Cr-doped (Bi,Sb)2Te3 ultrathin films, in which the surfaces states hybridize with each other. We investigate the situation when surface states are not coupled. SrTiO3 (111) crystals are chosen as the substrate due to its large electric polarization at low temperatures. Large anomalous Hall effect is observed for 8QL-thick films. The anomalous Hall conductance shows a maximum slightly lower than half conductance quanta (0.46e2/h) at 300mK when the Fermi level is tuned close to the neutral point by electric gating. It decreases monotonically as the hole carrier density becomes higher by negative gate voltage. On the electron doping side, the anomalous Hall conductance decreases and then saturates at a value of 0.4e2/h when the gate voltage is higher than 5V. The electron density is pinned at about 3e12/cm2 in the whole positive gate voltage range, only slightly increased (10{\%}) by a gate voltage of 150V. These results shed light on the robustness of the quantized Hall response of magnetically gapped Dirac surface state. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W42.00012: Engineering thin films of magnetically doped topological insulators for quantum anomalous Hall effect Xiao Feng, Yang Feng, Yunbo Ou, Kang Li, Jinsong Zhang, Minghua Guo, Zuocheng Zhang, Xintong Li, Liguo Zhang, Chang Liu, Zhenqi Hao, Yayu Wang, Shuaihua Ji, Xi Chen, Lili Wang, Ke He, Xucun Ma, Qikun Xue Quantum anomalous Hall effect (QAHE) is a kind of quantum Hall effect that can occur in zero external magnetic field. Recently, QAHE has been experimentally observed in thin films of Cr-doped (Bi,Sb)$_{2}$Te$_{3}$ topological insulator grown on SrTiO$_{3}$ (111) substrate by molecular beam epitaxy. The QAHE in Cr-doped (Bi,Sb)$_{2}$Te$_{3}$ films is found to be easily destroyed by slight variations in sample chemical composition, film thickness, substrate condition and capping layer. We have systematically investigated the influence of these parameters on the magnetism and anomalous Hall effect of Cr-doped (Bi,Sb)$_{2}$Te$_{3}$ films. The crucial factors preventing QAHE are discussed based on the results. This work is helpful for a detailed understanding of QAHE and for routine preparation of QAHE samples for further studies. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W42.00013: High-Temperature Quantum Anomalous-Hall Effect in a $n$-$p$ Codoped Topological Insulator Shifei Qi, Zhenhua Qiao, Hua Chen, Xiaohong Xu, Zhenyu Zhang Quantized anomalous-Hall effect (QAHE) has been theoretically predicted for over twenty years. Recently, it has been experimentally observed in magnetic topological insulators (TI). However, the extremely small band gap severely limits its potential application in novel nanoelectronics. In the present work, we use density functional theory calculations to establish a new materials design approach, compensated $n-p$ codoping, to predict a long-range ferromagnetic and insulating topological insulator with a relatively larger intrinsic band gap. By analyzing the band gap evolution as a function of the sample thickness and further calculating the corresponding Berry curvature, we show that the surface of the n-p codoped topological insulator supports a quantum anomalous Hall state at much higher temperatures than previously observed or predicted systems. [Preview Abstract] |
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