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 symmetry-protected 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 time-reversal symmetry and chemical potential. We demonstrate rewritable micron-scale 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 edge-state 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 Four-Probe Spectroscopy Saban Hus, Xiaoguang Zhang, Giang Nguyen, Yong Chen, An-Ping Li Charge currents carried by the nontrivial surface states (SS) of topological insulators (TIs) exhibit a net spin polarization due to spin-momentum 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 ex-situ lithographic processes used during the preparation of contacts may significantly affect the topological SS. Here we report in-situ, spin sensitive four-probe spectroscopy measurements which address both drawbacks. In a multi-probe scanning tunneling microscopy system a ferromagnetic probe detects the net spin accumulation on the SS of TI single crystals, while a set of four-probe 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 Krusin-Elbaum We have recently demonstrated that irradiation of topological materials, such as Bi$_2$Te$_3$ and Ca:Bi$_2$Se$_3$, with high energy (2.5 MeV) electron beams can sweep Fermi level E$_F$ across the bulk gap to charge neutrality point (CNP) [1]. Here we show that such irradiation technique applied to magnetically doped topological insulators (TIs) can be used to tune anomalous Hall effect (AHE). We performed irradiation experiments on a series of Mn-doped Bi$_2$Te$_3$ crystals, (Bi$_{1_x}$Mn$_x$)$_2$Te$_3$, using both \textit{p}- and \textit{n}-type starting materials. All materials, displayed magnetic hysteresis loops consistent with ferromagnetic order present. However, in transport the two conductivity types were found to be surprisingly different. While the \textit{p}-type crystals did convert to \textit{n}-type across CNP, no trace of AHE was detected. In contrast, \textit{n}-type materials showed pronounced hysteretic anomalous Hall resistance, consistent with magnetization. In the latter case, charge density has decreased and the zero-field Hall signal increased after irradiation. We will discuss how AHE in irradiated magnetically doped TIs can be fine tuned by electrostatic gating in the vicinity of CNP. [1]L. Zhao \textit{et al}, \textit{Nat. Comm.} \textbf{7}, 10957 (2016) [Preview Abstract] |
Tuesday, March 14, 2017 9:00AM - 9:12AM |
E45.00004: Abstract Withdrawn
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Tuesday, March 14, 2017 9:12AM - 9:24AM |
E45.00005: Low-temperature magneto-transport in topological insulator-ferromagnetic 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 spin-charge conversion between the TI surface states and the FM material. We use molecular beam epitaxy to deposit crystalline Bi$_{2}$Se$_{3}$ films on high-quality yttrium iron garnet (YIG) thin films [Wang \textit{et al}., Phys. Rev. Lett. \textbf{117}, 076601 (2016)] and report on the magneto-transport 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 Quantum-Anomalous-Hall Insulator to Anderson Insulator Quantum Phase Transition in Magnetic Topological Insulators Cui-Zu Chang, Wei-Wei 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 non-trivial system [1,2], and in thin films of magnetically-doped TI [3]. A QAH system carries spin-polarized 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 ultralow-power 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 quantum-anomalous-Hall (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, 2015-2018 (1988). [2] R. Yu et al, Science 329, 61-64 (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 Mn-doped Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$ TI films with accurately controlled thickness grown by molecular beam epitaxy. We found that films thicker than 5 quintuple-layer (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 thin-film topological insulator Gen Yin, Qinglin He, Luyan Yu, Lei Pan, Kang Wang Ferromagnetism introduced in topological insulators (TIs) opens a non-trivial 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 time-reversal 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 counter-propagating 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 anti-localization in pulsed laser-deposited Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$/La$_{\mathrm{0.70}}$Sr$_{\mathrm{0.30}}$MnO$_{\mathrm{3}}$ heterostructures Frank Hunte, Raj Kumar, Yi-Fang Lee, Sandhyarani Punugupati, Justin Schwartz, Jay Narayan Topological insulator/ferromagnet (TI/FM) heterostructures with broken time reversal symmetry by interface-induced magnetism are potential platforms for the observation of novel quantum transport phenomena, magnetic monopoles and exotic quantum magneto-electric 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 in-plane magnetization is confirmed by magnetometry measurements of the Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$/LSMO heterostructure. Magneto-transport measurements show a distorted weak anti-localization 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 ECCS-1306400. [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, Qi-Kun Xue, Weida Wu Quantum anomalous Hall effect (QAHE) was experimentally realized in a ferromagnetic topological insulator Cr-doped (Bi,Sb)$_2$Te$_3$ (BST) thin film for the first time[1]. Recently, a more robust QAHE has been observed in V-doped 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 V-doped BST still occurs at extremely low temperature (< 150 mK). In order to further enhance the QAHE temperature, we co-doped BST thin films by both Cr and V. The co-doped samples show enhanced QAHE temperature, comparing to the end members. Our magnetic force microscopy (MFM) results reveal an improved ferromagnetism in the co-doped samples. This is consistent with the in-situ 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 spin-orbit coupling and ferromagnetism. QAHE has been experimentally realized in both Cr-doped and V-doped (Sb$_{1−x}$Bi$_x$)$_2$Te$_3$, which are magnetic topological insulators (TIs). V-doped (Sb$_{1−x}$Bi$_x$)$_2$Te$_3$ exhibits higher Curie temperature and much less zero-field 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 Two-Dimensional Materials with Dirac Cones Yongzheng Luo, Lei Shen, Yuan Ping Feng Spin-orbit coupling (SOC) can open a band gap in the 2D Dirac semi-metals for the application of the quantum spin hall effect (QSH), i.e., the two-dimensional (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 first-principles calculations with maximally localized Wannier functions, here, we propose a general way to produce 2D TIs with strong QSH states and demonstrate some non-trivial 2D quantum spin hall insulators by calculating the Z2 invariants and Berry curvature. Furthermore, a topological quantum phase transition between a non-trivial 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 spin-1/2 ladders as microscopic models for non-Abelian chiral spin liquids Po-Hao Huang, Jyong-Hao Chen, Adrain Feiguin, Claudio Chamon, Christopher Mudry We construct a two-dimensional (2D) lattice model that is argued to realize a gapped chiral spin liquid with (Ising) non-Abelian topological order. The building blocks are spin-1/2 two-leg ladders with $SU(2)$-symmetric spin-spin interactions. The two-leg ladders are then arranged on rows and coupled through $SU(2)$-symmetric interactions between consecutive ladders. The intra-ladder 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. Time-reversal breaking inter-ladder 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 non-Abelian excitations but no charge excitations, in contrast to the Pfaffian non-Abelian fractional quantum Hall state. [Preview Abstract] |
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