Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session E45: Topological Materials: Synthesis and Characterization  Magnetic Thin FilmFocus

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Sponsoring Units: DMP Chair: Sergy Medevedev, Max Planck Institute Room: 392 
Tuesday, March 14, 2017 8:00AM  8:36AM 
E45.00001: Local optical control of ferromagnetism and chemical potential in a topological insulator Invited Speaker: Andrew L. Yeats The symmetryprotected surface and edge states of topological insulators (TIs) are attractive for applications in spintronics and quantum computing. Many proposed experiments involving TI materials require spatial control over timereversal symmetry and chemical potential. We demonstrate rewritable micronscale optical control of both magnetization and chemical potential in ferromagnetic thin films of Cr(Bi,Sb)$_2$Te$_3$ grown on SrTiO$_3$ [1]. By optically modulating the coercivity of the films, we write and erase arbitrary patterns in their remnant magnetization, which we then image with Kerr microscopy. Additionally, by optically manipulating a space charge layer in the underlying SrTiO$_3$ substrates, we control the local chemical potential of the films. This optical gating effect allows us to write and erase $\it{p}$$\it{n}$ junctions in the films, which we study with photocurrent microscopy [2]. Both effects persist for $>$ 16 hours and may be applied simultaneously. We will present systematic Kerr microscopy, photocurrent microscopy, and electrical transport studies of these materials, as well as various electronic and magnetic structures patterned on them. We will discuss the prospects for using these optical phenomena to study and control the unique physics of TIs, such as 1D edgestate transport in the quantum anomalous Hall regime. $\\$ [1] A. L. Yeats, P. J. Mintun, Y. Pan, A. Richardella, N. Samarth, D. D. Awschalom, in prep., (2016). $\\$ [2] A. L. Yeats, Y. Pan, A. Richardella, P. J. Mintun, N. Samarth, D. D. Awschalom, Science Advances 1, e1500640 (2015). [Preview Abstract] 
Tuesday, March 14, 2017 8:36AM  8:48AM 
E45.00002: Detection of Current Induced Spin Polarization in Topological Insulators via FourProbe Spectroscopy Saban Hus, Xiaoguang Zhang, Giang Nguyen, Yong Chen, AnPing Li Charge currents carried by the nontrivial surface states (SS) of topological insulators (TIs) exhibit a net spin polarization due to spinmomentum locking. Electrical detection of this spin polarization usually relies on measurements with lithographically defined contacts. However, this method has several drawbacks. First, it is difficult to detect the ratio of current carried by SS versus the coexisting bulk states in TIs using fixed contacts. Second, the exsitu lithographic processes used during the preparation of contacts may significantly affect the topological SS. Here we report insitu, spin sensitive fourprobe spectroscopy measurements which address both drawbacks. In a multiprobe scanning tunneling microscopy system a ferromagnetic probe detects the net spin accumulation on the SS of TI single crystals, while a set of fourprobe spectroscopy measurement is used for a quantitative separation of 2D and 3D conduction. Using this method we measured the density and the net spin polarization of the current carried by the SS of pristine, as cleaved Bi$_{\mathrm{2}}$Te$_{\mathrm{2}}$Se single crystals. This research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. [Preview Abstract] 
Tuesday, March 14, 2017 8:48AM  9:00AM 
E45.00003: Magnetically doped topological materials tuned by electron irradiation Zhiyi Chen, Haiming Deng, Shihua Zhao, Inna Korzhovska, Marcin Konczykowski, Lia KrusinElbaum 
Tuesday, March 14, 2017 9:00AM  9:12AM 
E45.00004: Abstract Withdrawn In this work we consider a topological insulator (TI)  ferromagnet (FM) bilayer and we study the transmission properties of the TI surface states. More specifically we impose a magnetization m(x,y) on the FM which induces via proximity effect an exchange field which interacts with the spinorbit coupled travelling electrons on the TI surface. Due to the spinorbit coupling of the surface state, scattering is expected due to the presence of m(x,y). The transfer matrix method has been used to solve the similar quasi  1D problem. However, when the magnetization is a function of both x and y, it is a purely 2D problem and such a method can no longer be used, at least directly. For this reason, we adapt to the Dirac equation problem the quantum transmitting boundary method (QTBM) for general magnetization profiles. These can include magnetization steps, domain walls and skyrmions. 
Tuesday, March 14, 2017 9:12AM  9:24AM 
E45.00005: Lowtemperature magnetotransport in topological insulatorferromagnetic insulator heterostructures James Kally, Tao Liu, Hailong Wang, Danielle Reifsnyder Hickey, K. Andre Mkhoyan, Mingzhong Wu, Anthony Richardella, Nitin Samarth The spin polarized surface states of a topological insulator (TI) have potential for topological spintronics applications wherein the surface states are used for electrically detecting and manipulating the magnetization of a ferromagnetic (FM) material. Heterostructures that interface a TI with a FM insulator are ideal in this context since they isolate the charge current to the topological insulator, thus allowing a clean probe of any phenomena related to spincharge conversion between the TI surface states and the FM material. We use molecular beam epitaxy to deposit crystalline Bi$_{2}$Se$_{3}$ films on highquality yttrium iron garnet (YIG) thin films [Wang \textit{et al}., Phys. Rev. Lett. \textbf{117}, 076601 (2016)] and report on the magnetotransport properties of these heterostructures at low temperature (400 mK \textless $T$ \textless 4.2 K). Our measurements show evidence for a magnetic coupling between the FM insulator and the TI thin film. [Preview Abstract] 
Tuesday, March 14, 2017 9:24AM  9:36AM 
E45.00006: Observation of the QuantumAnomalousHall Insulator to Anderson Insulator Quantum Phase Transition in Magnetic Topological Insulators CuiZu Chang, WeiWei Zhao, Jian Li, J.K. Jain, Chaoxing Liu, Jagadeesh S. Moodera, Moses H. W. Chan The quantum anomalous Hall (QAH) effect can be considered as the quantum Hall (QH) effect without external magnetic field, which can be realized by time reversal symmetry breaking in a topologically nontrivial system [1,2], and in thin films of magneticallydoped TI [3]. A QAH system carries spinpolarized dissipationless chiral edge transport channels without the need for external energy input, hence may have huge impact on future electronic and spintronic device applications for ultralowpower consumption. The observation of QAH effect has opened up exciting new physics and thus understanding the physical nature of this novel topological quantum state, can lead to a rapid development of this field. In this talk, we will report our recent progress about the experimental observation of a quantum phase transition from a quantumanomalousHall (QAH) insulator to an Anderson insulator by tuning the chemical potential, and finally discuss the existence of scaling behavior for this quantum phase transition. References [1] F. D. M. Haldane, Phys. Rev. Lett. 61, 20152018 (1988). [2] R. Yu et al, Science 329, 6164 (2010). [3] C. Z. Chang et al, Science 340, 167(2013); Nature Materials 14, 473(2015). [Preview Abstract] 
Tuesday, March 14, 2017 9:36AM  9:48AM 
E45.00007: Dimensional Crossover Induced Topological Hall Effect in a Magnetic Topological Insulator Chang Liu, Yunyi Zang, Wei Ruan, Yan Gong, Ke He, Xucun Ma, Qikun Xue, Yayu Wang The realization of quantum anomalous Hall effect in magnetic topological insulators (TIs) unambiguously proves the intrinsic mechanism of anomalous Hall effect (AHE) associated with the Berry curvature in momentum space. The real space configuration of the local moments in magnetic TIs may also have unique topological properties. The entanglement of momentum and real space topology in magnetic TI can lead to novel quantum phenomena, such as the emergence of skyrmions. Here we report experimental investigations of Mndoped Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$ TI films with accurately controlled thickness grown by molecular beam epitaxy. We found that films thicker than 5 quintuplelayer (QL) exhibit the usual AHE as commonly observed in magnetic TIs. When the thickness is reduced to 4 QL, however, characteristic features associated with the topological Hall effect (THE) emerge. More surprisingly, the THE vanishes again when the film thickness is further reduced to 3 QL. Theoretical calculations demonstrate that the coupling between the top and bottom surface states at the dimensional crossover regime leads to magnetic skyrmion structure that is responsible for the THE. [Preview Abstract] 
Tuesday, March 14, 2017 9:48AM  10:00AM 
E45.00008: Topological transitions induced by antiferromagnetism in a thinfilm topological insulator Gen Yin, Qinglin He, Luyan Yu, Lei Pan, Kang Wang Ferromagnetism introduced in topological insulators (TIs) opens a nontrivial exchange band gap, providing an exciting platform to control the topological order through an external magnetic field. The magnetization induces a topological transition that breaks timereversal symmetry, resulting in anomalous Hall effects. Recently, it was experimentally shown that the surface of an antiferromagnetic (AFM) thin film can magnetize the surface Dirac fermions in a TI thin film similar to the case induced by ferromagnetism. Here, we show that when a TI thin film is sandwiched between two antiferromagnetic layers, an unsynchronized magnetic reversal introduces two intermediate spin configurations during the scan of the external field, resulting in a new topological phase with second Chern numbers. This topological phase introduces two counterpropagating chiral edge modes inside the exchange gap, changing the total number of transport channels drastically when the fermi level is close to the Dirac point. Induced by this change, the magnetoresistance of the channel presents an antisymmetric feature during the field scan. With the the help of the high ordering temperature of AFM layers, this transport signature of the phase transition persists up to 90K experimentally. [Preview Abstract] 
Tuesday, March 14, 2017 10:00AM  10:12AM 
E45.00009: Distorted weak antilocalization in pulsed laserdeposited Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$/La$_{\mathrm{0.70}}$Sr$_{\mathrm{0.30}}$MnO$_{\mathrm{3}}$ heterostructures 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 potential platforms for the observation of novel quantum transport phenomena, magnetic monopoles and exotic quantum magnetoelectric effects. One of the challenges encountered with TI/FM heterostructures is the low Curie temperatures of the ferromagnets studied so far which limits their potential for applications in spintronic devices at room temperature. In order to address this issue, we have grown Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$/La$_{\mathrm{0.70}}$Sr$_{\mathrm{0.30}}$MnO$_{\mathrm{3}}$\textbf{ }heterostructures by the method of pulsed laser deposition. La$_{\mathrm{0.70}}$Sr$_{\mathrm{0.30}}$MnO$_{\mathrm{3\thinspace }}$is a strong ferromagnetic material with T$_{\mathrm{c\thinspace }}$\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 interfaceinduced 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] 
Tuesday, March 14, 2017 10:12AM  10:24AM 
E45.00010: Microscopic evidence of enhanced ferromagnetism in Cr/V doped (Bi,Sb)2Te3 thin films Wenbo Wang, Yunbo Ou, Chang Liu, Yayu Wang, Ke He, QiKun Xue, Weida Wu Quantum anomalous Hall effect (QAHE) was experimentally realized in a ferromagnetic topological insulator Crdoped (Bi,Sb)$_2$Te$_3$ (BST) thin film for the first time[1]. Recently, a more robust QAHE has been observed in Vdoped BST thin films, which has a much larger coercive field and higher Curie temperature at the same doping level[2]. However, the quantization of Hall resistance in Vdoped BST still occurs at extremely low temperature (< 150 mK). In order to further enhance the QAHE temperature, we codoped BST thin films by both Cr and V. The codoped samples show enhanced QAHE temperature, comparing to the end members. Our magnetic force microscopy (MFM) results reveal an improved ferromagnetism in the codoped samples. This is consistent with the insitu Hall measurements, which show a steeper magnetization reversal with enhanced Hall resistance. [1] C.Z. Chang et al., Science 340, 167 (2013). [2] C.Z. Chang et al., Nature Materials 14, 473–477(2015). [Preview Abstract] 
Tuesday, March 14, 2017 10:24AM  10:36AM 
E45.00011: Visualizing local electronic properties of defects in magnetic topological insulators Wenhan Zhang, Damien West, Y. Qiu, Y.S. Hor, S.B. Zhang, Weida Wu Quantum anomalous Hall effect (QAHE) manifests as a quantized dissipationless Hall conduction due to chiral edge state circulating along the edge of 2D electron systems without external magnetic field. This effect relies on strong spinorbit coupling and ferromagnetism. QAHE has been experimentally realized in both Crdoped and Vdoped (Sb$_{1−x}$Bi$_x$)$_2$Te$_3$, which are magnetic topological insulators (TIs). Vdoped (Sb$_{1−x}$Bi$_x$)$_2$Te$_3$ exhibits higher Curie temperature and much less zerofield longitudinal resistance. Despite several earlier studies on these materials, it is still unclear why V is better than Cr for QAH effect. Here we present scanning tunneling microscopy and spectroscopy (STM/STS) studies in conjunction with first principle calculations on the local electronic properties in both Cr$_x$Sb$_2$Te$_3$ and V$_x$Sb$_2$Te$_3$ single crystals. Preliminary STM/STS results indicate the local properties of defects strongly influence the magnetic ordering in doped TIs, which is crucial for robust QAHE at elevated temperature. [Preview Abstract] 
Tuesday, March 14, 2017 10:36AM  10:48AM 
E45.00012: Strong Quantum Spin Hall Effect and Topological Phase Transition in TwoDimensional Materials with Dirac Cones Yongzheng Luo, Lei Shen, Yuan Ping Feng Spinorbit coupling (SOC) can open a band gap in the 2D Dirac semimetals for the application of the quantum spin hall effect (QSH), i.e., the twodimensional (2D) topological insulators (TIs). The edge current of 2D TIs is topologically protected from backscattering, and thus hold great potential for applications in spintronics and quantum information. To find strong QSH states, the way of combining the effects of SOC and fundamental symmetries has drawn much more attention. Simultaneously, inspired by the development of graphene, seeking new 2D materials with Dirac cones as hosts of possible 2D TIs becomes more fashionable. Via the firstprinciples calculations with maximally localized Wannier functions, here, we propose a general way to produce 2D TIs with strong QSH states and demonstrate some nontrivial 2D quantum spin hall insulators by calculating the Z2 invariants and Berry curvature. Furthermore, a topological quantum phase transition between a nontrivial QSH phase and a trivial insulating/metallic phase can be realized by strain, and also, the SOC gap can be enhanced by strain. Thus our theoretical analysis can help searching large band gap 2D TIs. [Preview Abstract] 
Tuesday, March 14, 2017 10:48AM  11:00AM 
E45.00013: Coupled spin1/2 ladders as microscopic models for nonAbelian chiral spin liquids PoHao Huang, JyongHao Chen, Adrain Feiguin, Claudio Chamon, Christopher Mudry We construct a twodimensional (2D) lattice model that is argued to realize a gapped chiral spin liquid with (Ising) nonAbelian topological order. The building blocks are spin1/2 twoleg ladders with $SU(2)$symmetric spinspin interactions. The twoleg ladders are then arranged on rows and coupled through $SU(2)$symmetric interactions between consecutive ladders. The intraladder interactions are tuned so as to realize the $c=1/2$ Ising conformal field theory, a fact that we establish numerically via Density Matrix Renormalization Group (DMRG) studies. Timereversal breaking interladder interactions are tuned so as to open a bulk gap in the 2D lattice system. This 2D system supports gapless chiral edge modes with Ising nonAbelian excitations but no charge excitations, in contrast to the Pfaffian nonAbelian fractional quantum Hall state. [Preview Abstract] 
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