Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session K28: Quantum Anomalous Hall Effect IIFocus

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Sponsoring Units: DMP Chair: Anthony Richardella, Pennsylvania State University Room: 327 
Wednesday, March 16, 2016 8:00AM  8:36AM 
K28.00001: Searching for better magnetic topological insulator materials for the quantum anomalous Hall effect Invited Speaker: Ke He The recent experimental observation of the quantum anomalous Hall (QAH) effect in thin films of magnetic topological insulators (TIs) paves the ways for practical applications of dissipationless quantum Hall edge states and for realizations of the novel quantum phenomena such as chiral topological superconductivity and axion magnetoelectric effect. Further studies in these directions require magnetic TI materials that are able to show the QAH effect at higher temperature and with conduction channels of lower dissipation. We have performed systematic study on the QAH effect in magnetically doped TI films with different thicknesses, magnetic dopants and compositions. The results clarify the relations between the QAH effect and energy band structure, electronic localization and ferromagnetism, which not only give a comprehensive understanding on the nature of the QAH effect but also provide insights into designing and fabrication of superior QAH materials. [Preview Abstract] 
Wednesday, March 16, 2016 8:36AM  9:12AM 
K28.00002: Precise Quantization of Anomalous Hall Effect Invited Speaker: Andrew Bestwick In the quantum anomalous Hall effect, electron transport in a magneticallydoped topological insulator takes place through chiral, dissipationless edge channels. In this talk, we discuss the behavior of a nearly ideal implementations of the effect in which the Hall resistance is within a part per 10,000 of its quantized value and the longitudinal resistivity can reach below 1 $\Omega$ per square. Nearly all Crdoped topological insulator samples demonstrate extreme temperature dependence that is wellmodeled by a small effective gap, allowing control over quantization with an unexpected magnetocaloric effect. We also discuss measurements of new device geometries and nonlocal resistances that identify the sources of dissipation that limit the effect. [Preview Abstract] 
Wednesday, March 16, 2016 9:12AM  9:24AM 
K28.00003: Distinct Effect of Cr Bulk and Surface Doping on the Local Environment and Electronic Structure of Bi2Se3 Turgut Yilmaz, Ivo Pletikosic, Tonica Valla, Boris Sinkovic We report on studies of Cr doping of Bi2Se3 by comparing surface doped with bulk doped Bi2Se3 films and their electronic and local structures studied by insitu ARPES and corelevel photoemission spectroscopies, respectively. In the case of surface doping we see the evidence for Cr substituting the Bi by observation of the extra feature in the Bi 5d photoemission spectra that increases with doping. On the other hand the Cr 3p spectra show two distinct chemical states indicating that there are two different Cr locations with different local electronic configuration. However, unlike theoretical expectations, the electronic structure measured at 15 K shows that surface states preserve gapless feature with well defined Dirac cone and presence of quantum well states, induced by doping. In contrast, the bulk Cr doped Bi2Se3 films show gapped surface states with gap energy as large as 100 meV even at room temperature, which is far above the reported ferromagnetic transition temperature. [Preview Abstract] 
Wednesday, March 16, 2016 9:24AM  9:36AM 
K28.00004: Tuning the electronic structure in~nearly gapless HgCdTe with temperature: infrared magnetospectroscopy study\textbf{~} Seongphill Moon, M. Marcinkiewicz, C. Consejo, S. Ruffenach, W. Knap, F. Teppe, J. Ludwig, K. Thirunavukkuarasu, D. Smirnov, S. Krishtopenko, V. I. Gavrilenko, S. A. Dvoretskii, N. N. Mikhailov Replace this text with your abstract body. Recently,~a temperatureinduced transition from a conventional twodimensional semiconductor to a topological insulator has been demonstrated through magneto transport studies on HgTe/CdHgTe quantum~wells [Wiedmann, S. et al. Phys. Rev. B 91, 205311 (2015)]. Here we report on a temperaturedriven semiconductortosemimetal transition in 3dimensional CdxHg1xTe (x$=$0.15) revealed by infrared magnetospectroscopy. ~We show that changing the temperature from~4K to 120K enables~continuous~tuning of the band structure from inverted to normal alignment through a critical gapless state realized at \textasciitilde 80K, where the interLandau level transitions exhibit a characteristic~~sqrt(B) dependence intersecting at zero energy. Using an effective Dirac model, we determine the effective mass and the Fermi velocity for the studied temperature range. ~ [Preview Abstract] 
Wednesday, March 16, 2016 9:36AM  9:48AM 
K28.00005: Distorted weak antilocalization effects in Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$/La$_{\mathrm{0.70}}$Sr$_{\mathrm{0.30}}$MnO$_{\mathrm{3}}$ (TI/FM) heterostructures grown by pulsed laser deposition Frank Hunte, Raj Kumar, YiFang Lee, Sandhyarani Punugupati, Justin Schwartz, Jay Narayan Topological insulator/ferromagnet (TI/FM) heterostructures with broken time reversal symmetry by interfaceinduced magnetism are the potential platforms for the observation of novel quantum transport phenomena, magnetic monopoles and exotic quantum magnetoelectric effects. TI/FM heterostructures with low Curie temperature ferromagnets i. e. GdN, EuS have been fabricated and studied. One of the challenges encountered with these heterostructures is their low Curie temperatures which limits their potential for applications in spintronic devices at room temperature. To address this issue, we have grown Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$/\textbf{ }La$_{\mathrm{0.70}}$Sr$_{\mathrm{0.30}}$MnO$_{\mathrm{3}}$ (TI/FM) heterostructures by the method of pulsed laser deposition. La$_{\mathrm{0.70}}$Sr$_{\mathrm{0.30}}$MnO$_{\mathrm{3\thinspace }}$(LSMO) is a strong ferromagnetic material with Tc \textasciitilde 350 K and Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ is the most studied topological insulator. XRD and phi scan results show that epitaxial thin films of Bi$_{\mathrm{2}}$Se$_{\mathrm{3\thinspace }}$are grown on the LSMO template. Strong inplane magnetization is confirmed by magnetometry measurements of the Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$/LSMO heterostructure. Magnetotransport measurements show a distorted weak antilocalization effect with hysteretic behavior due to interface induced ferromagnetism in the Bi$_{\mathrm{2}}$Se$_{\mathrm{3\thinspace }}$TI films. This work was supported, in part, by National Science Foundation ECCS1306400. [Preview Abstract] 
Wednesday, March 16, 2016 9:48AM  10:00AM 
K28.00006: Growth and Characterization of Mn Doped InAs/GaSb Quantum Wells: Toward a 2D Quantum Anomalous Hall Insulator Di Xiao, Susan Kempinger, Nitin Samarth The new discovery of a class of quantum spin Hall insulators (QSHIs), namely the typeII broken gap InAs/GaSb quantum wells (QWs), has drawn much attention in the condensed matter community. Counterpropagating helical states protected by time reversal symmetry exist at the edge, giving a quantized Hall conductance of 2e$^2$/h, while the bulk remains insulating in this 2D topological insulator (TI). Compared to other TI systems, InAs/GaSb QW has a great advantage that the band structure, with a small hybridization gap, can be continuously tuned through electric fields, allowing the topological phase transition between trivial and topological phases. A recent theoretical proposal [PRL, 113.14(2014)] indicates that it is possible to keep only one chiral edge state without external magnetic field, i.e. the quantum anomalous Hall (QAH) state, by introducing longrange ferromagnetic order into this QW system. (In,Mn)As and (Ga,Mn)Sb have been well studied as diluted magnetic semiconductors, making Mndoping a reasonable choice. Here, we present preliminary results on the MBE growth and characterization of electricallygated Mn doped InAs /GaSb QWs. We will present a discussion of magnetization and magnetotransport measurements. Funded by ONR. [Preview Abstract] 
Wednesday, March 16, 2016 10:00AM  10:12AM 
K28.00007: Interface driven states in ferromagnetic topological insulator heterostructures. Valeria Lauter, Ferhat Katmis, Jagadeesh Moodera The broken time reversal symmetry (TRS) states can be introduced into a topological insulator (TI) material by ferromagnetic ordering at the interface. Recently [1] we demonstrated a fundamental step towards realization of high temperature magnetization in Bi$_{2}$Se$_{3}$EuS TIFMI heterostructures through observation of magnetic proximityinduced symmetry breaking on the Bi$_{2}$Se$_{3}$ surface via the exchange interaction by depositing EuS film on the top of the Bi$_{2}$Se$_{3}$ surface.\textbf{ }Here we show that we can independently break the TRS on both surfaces of a TI, which brings the longrange induced magnetism on either or both surfaces of a TI in a controlled way. We provide a depthsensitive data on details of magnetic proximity effect in hidden interfaces by Polarized Neutron Reflectometry. The proximity coupling strength and penetration depth of magnetism into TI are extracted as functions of temperature, magnetic field and magnetic history. The large neutron absorption of Eu atoms serves as the element sensitivity and enables us to identify such magnetism in TI as proximity magnetism. This provides a next step to realization of complex heterostructures of TI and FMI leading to wide applications in TIbased next generation spintronic devices. [1] F. Katmis, V. Lauter et al, submitted. [Preview Abstract] 
Wednesday, March 16, 2016 10:12AM  10:24AM 
K28.00008: Stepwise switching of anomalous Hall effect in a topological insulator Lukas Zhao, Zhiyi Chen, Inna Korzhovska, Shihua Zhao, Lia KrusinElbaum, Marcin Konczykowski Surfaces of threedimensional (3D) topological insulators (TIs) have emerged as one of the most remarkable states of condensed quantum matter where exotic charge and spin phases of Dirac particles could arise. The main challenge to finding these phases comes from a nonvanishing conductivity of the bulk. Recently we have demonstrated that we can access 2D surface transport and reach the charge neutrality point (CNP) by compensating intrinsically \textit{p}type TIs using high energy electron beams, and increase bulk resistivity by orders of magnitude. Here we report a discovery of anomalous Hall signal (AHE) at the CNP in Bi$_2$Te$_3$ of unprecedented appearance; it shows regions of plateaus on sweeping the temperature, where Hall resistivity is flat in temperature, and has sharp (nearly discontinuous) `steps' inbetween the plateaus. The height of the steps increases on cooling, consistently following the ratio of 1:3 with each step. We will show by electrostatically tuning gated structures how this macroscopic switching of spins evolves in the vicinity of CNP and discuss the phenomenon of stepwise AHE in the context of charge inhomogeneities (puddles) and correlations between the localized bulk spins and Dirac spins. [Preview Abstract] 
Wednesday, March 16, 2016 10:24AM  10:36AM 
K28.00009: Prediction of Quantum Anomalous Hall Insulator in Functionalized GaBi Honeycomb Christian Crisostomo, SungPing Chen, ZhiQuan Huang, ChiaHsiu Hsu, FengChuan Chuang, Hsin Lin, Arun Bansil Using firstprinciples electronic calculations, we predict functionalized GaBi honeycomb under tensile strain to harbor quantum anomalous hall (QAH) insulating phase. A single band inversion at $\Gamma $ point was found in spinpolarized band structure of halffluorinated planar strained GaBi. In order to confirm the topological properties, we evaluated the Chern number (C) and found that C $=$ 1, indicating the presence of QAH phase. Additionally, the same value was also obtained by using hydrogen atoms, instead of fluorine atoms, as the adsorbate in both planar and buckled GaBi. Moreover, the electronic spectrum of a halffluorinated GaBi nanoribbon with armchair or zigzag edges possess only one edge band crossing the Fermi level within the band gap. Finally, a suitable substrate which could induce the similar effect of halfhydrogenation or halffluorination on the GaBi honeycomb could be used for spintronic devices. [Preview Abstract] 
Wednesday, March 16, 2016 10:36AM  10:48AM 
K28.00010: Metaltoinsulator switching in quantum anomalous Hall states Lei Pan, Xufeng Kou, Jing Wang, yabin fan, Eun Sang Choi, Qiming Shao, Shou Cheng Zhang, Kang Lung Wang Quantum anomalous Hall effect (QAHE) was recently achieved in magnetic topological insulator films as a form of dissipationless transport without external magnetic field. However, the universal phase diagram of QAHE and its relation with quantum Hall effect (QHE) remain to be investigated. Here, we report the experimental observation of the giant longitudinal resistance peak and zero Hall conductance plateau at the coercive field in the six quintuplelayer (Cr$_{0.12}$Bi$_{0.26}$Sb$_{0.62})_{2}$Te$_{3}$ film, and demonstrate the metaltoinsulator switching between two opposite QAHE plateau states up to 0.3 K. The universal QAHE phase diagram is further confirmed through the angledependent measurements. Our results address that the quantum phase transitions in both QAHE and QHE regimes are in the same universality class, yet the microscopic details are different. [Preview Abstract] 
Wednesday, March 16, 2016 10:48AM  11:00AM 
K28.00011: Investigating dissipation in the quantum anomalous Hall effect Eli Fox, Andrew Bestwick, David GoldhaberGordon, Yang Feng, Yunbo Ou, Ke He, Yayu Wang, Qikun Xue, Xufeng Kou, Lei Pan, Kang Wang In the quantum anomalous Hall effect, a magnetic exchange gap in a 3D topological insulator gives rise to dissipationless chiral edge states. Though the effect has recently been realized in a family of ferromagneticallydoped (Bi,Sb)$_2$Te$_3$ topological insulator thin films, experiments to date have found nonvanishing longitudinal resistance, contrary to initial theoretical expectations. Proposed sources of this dissipation include extra gapless or activated quasihelical edge states, thermally activated 2D conduction, and variablerange hopping. Here, we discuss transport measurements of Corbino disk and nonlocal geometries to identify the mechanism of nonideal behavior. [Preview Abstract] 
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