Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session F10: Magnetism and Topological Insulators |
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Sponsoring Units: DCMP Chair: Jim Eckstein, University of Illinois-Urbana Room: 007A |
Tuesday, March 3, 2015 8:00AM - 8:12AM |
F10.00001: Room Temperature Ferromagnetism on the Topological Insulators Surface by Proximity Effect Ferhat Katmis, Valeria Lauter, Vahid Sazgari, Ismet I. Kaya, Donald Heiman, Jagadeesh Moodera Generating exchange-induced ferromagnetism on the surface of a topological insulator (TI) with a ferromagnetic layer (FM) provides a cleaner approach for realizing a ferromagnetic TI that may lead to exhibiting other quantum functionality.$^{\mathrm{1}}$ Here we demonstrate further that room temperature magnetic state may be reached in the TI and FM heterostructures through magnetic proximity-induced time reversal symmetry breaking on the TI surface. Using different magnetic characterization methods we provide evidence of this enhanced proximity-induced magnetism in TI. We show that such effects persist up to room temperature, far above the Curie temperature of the FM, signifying a significantly different behavior in TI. The project supported by grants NSF (DMR-1207469), MIT MRSEC through the MRSEC Program of the NSF (DMR-0819762) and NSF (ECCS-1402738). 1. ``Exchange-Coupling-Induced Symmetry Breaking in Topological Insulators'', Peng Wei, et al, PRL. 110, 186807 (2013). [Preview Abstract] |
Tuesday, March 3, 2015 8:12AM - 8:24AM |
F10.00002: A first-principles study of magnetic phase transitions in Fe-doped Bi$_{2}$Se$_{3}$ Jeongwoo Kim, Seung-Hoon Jhi Magnetic impurities perturb helical surface states of topological insulators because they act as the spin-flipping scattering centers. Counter-intuitively, the energy gap of the surface states, presumably opened by such scattering, decreases with Fe concentration in Fe-doped topological insulating Bi2Te3 [1]. In addition, the ground magnetic phase of the compound is changed as Fe concentration is increased, which is atypical in dilute magnetic semiconductors. We study the magnetic phase transition and the behavior of surface states in Fe-doped Bi2Se3 using first-principles calculations. We find that the localized spin states of Fe atoms are aligned via hybridization with conduction electrons at dilute doping regimes (\textless 1.7 {\%}) but, at dense impurity levels ( \textgreater 1.7{\%}), are ordered mainly via the super-exchange interaction. We show that topological surfaces states are sensitive to the type of magnetic ordering of adjacent Fe impurities and that the ground magnetic phase barely perturbs the linear band dispersion and the helical nature of the surface states. This finding explains the observation of the band gap of the surface-states in the presence of magnetic impurities.\\[4pt] [1] H.-J. Kim et al., Phys. Rev. Lett. 110, 136601 (2013). [Preview Abstract] |
Tuesday, March 3, 2015 8:24AM - 8:36AM |
F10.00003: Metal-organic coordination networks on top of the topological insulator surface Mikhail Otrokov, Evgueni Chulkov, Andres Arnau To realize the ferromagnetic ordering in the system of adatoms at the topological insulator surface one has to organize their ordered arrays [1] and prevent them from the diffusion [2] (intercalation [3]) inside the bulk (van der Waals gaps) of topological substrate. In principle, this could be achieved by growing the metal-organic coordination network on top of the topological insulator surface. Using ab initio density functional theory calculations, we demonstrate that a network formed by TCNE-like strong acceptor molecules and Co atoms, acting as magnetic centers, shows ferromagnetic coupling between Co centers when deposited on top of the topological insulator. We discuss the issue of the magnetic anisotropy and its influence on the surface electronic spectrum of such a system. [1] L. Chotorlishvili, A. Ernst, V.K. Dugaev, et al., Phys. Rev. B 89, 075103 (2014) [2] T. Schlenk, M. Bianchi, M. Koleini, et al., Phys. Rev. Lett. 110, 126804 (2013). [3] M.M. Otrokov, S.D. Borisovaa, V. Chis, et al., JETP Lett. 96, 714 (2013). [Preview Abstract] |
Tuesday, March 3, 2015 8:36AM - 8:48AM |
F10.00004: Signatures of Dirac fermion-mediated magnetic order Paolo Sessi, Felix Reis, Thomas Bathon, Konstantin Kokh, Oleg Tereshchenko, Matthias Bode The spin-momentum locking of topological states offers an ideal platform to explore novel magneto-electric effects. These intimately depend on the ability to manipulate the spin texture in a controlled way. Here, we combine low-temperature scanning tunneling microscopy with single-adatom deposition technique to directly map the evolution of the electronic properties of topological states under the influence of different magnetic perturbations. By analyzing energy-resolved quasi-particle interference maps, we reveal signatures of Dirac fermion-mediated surface magnetic order for extremely dilute adatom concentrations. By using different magnetic elements and coverages, we find that this striking observation crucially depends on two parameters: single adatoms magnetic anisotropy direction and energy-level alignment. [Preview Abstract] |
Tuesday, March 3, 2015 8:48AM - 9:00AM |
F10.00005: Magnetization Dynamics of a Ferromagnet Attached to the Surface State of a Topological Insulator: A Time Dependent Keldysh Green Function Approach Farzad Mahfouzi, Nicholas Kioussis Motivated by the recent experiments on the Spin Orbit Torque (SOT) generated by the Topological Insulators (TI) we investigate the conditions under which the SOT due to the in-plane current flowing through the surface state can switch the magnetic orientation of a ferromagnet attached to the TI. Using the Keldysh Green function approach, we developed the theoretical formalism for a classical system coupled to an electronic system out of equilibrium due to both bias voltage and the adiabatic variation of the classical degree of freedom. In this approach the quantities of interest that are calculated have the form of the generalized Fisher-Lee formula describing the electronic current and spin accumulation in terms of the Green functions in a unified approach. We show that due to the Edlestein effect the direction of the easy axis changes with the applied voltage which makes it difficult to separate the SOT into field like and anti-damping like components. [Preview Abstract] |
Tuesday, March 3, 2015 9:00AM - 9:12AM |
F10.00006: Magnetization switching of a nanomagnet by spin polarized surface states of a topological insulator Urmimala Roy, Rik Dey, Tanmoy Pramanik, Bahniman Ghosh, Leonard F. Register, Sanjay K. Banerjee Due to the spin-momentum helical locking, a charge current supported by the topological insulator (TI) surface states leads to a spin accumulation at the TI surface. In this theoretical study, we consider a thermally-stable, conducting nanomagnet subject to spin-polarized current injection from TI surface states, in order to evaluate possible non-volatile memory applications such as in spin-transfer-torque random access memory. We simulate parallel transport in the TI and the ferromagnetic metal, and evaluate the efficiency of magnetization switching for varying ease of transport between the TI and the ferromagnetic metal. With the assumed parameters, transport in the TI beneath the ferromagnetic metal is diffusive in nature at room temperature and is modeled by drift-diffusion simulation, which we believe to be sufficient for this purpose, and allows for ready interpretation. We use self-consistent transport and magnetization dynamics calculation to predict switching time and energy spent per write operation. Based on our simulation, we believe that a large in-plane resistivity of the ferromagnetic layer--perhaps not a simple ferromagnetic metal layer--along with an interface with the TI that is transparent to charge transport, will lead to minimum switching time and write energy. [Preview Abstract] |
Tuesday, March 3, 2015 9:12AM - 9:24AM |
F10.00007: The influence of proximity induced ferromagnetism, superconductivity and Fermi-velocity on evolution of Berry phase in Bi$_{2}$Se$_{3}$ topological insulator Parijat Sengupta Bi$_{2}$Se$_{3}$ is a well-known 3D-topological insulators (TI) with a non-trivial Berry phase of (2n$+$1) $\pi $ attributed to the topology of the band structure. The Berry phase shows non-topological deviations from (2n$+$1) $\pi $ in presence of a perturbation that destroys time reversal symmetry and gives rise to a quantum system with massive Dirac fermions and finite band gap. Such a band gap opening is achieved on account of the exchange field of a ferromagnet or the intrinsic energy gap of a superconductor that influences the topological insulator surface states by virtue of the proximity effect. The Berry phase of such gapped systems with massive Dirac fermions is considered. Additionally, it is shown that the Berry phase for such a system also depends on the Fermi-velocity of the surface states which can be tuned as a function of the TI film thickness. The role of higher order warping terms in the surface state Hamiltonian which influences deviations to the Berry phase is evaluated. Finally, a connection between Berry phase and circular dichroism is examined through explicit calculation of the optical matrix elements. [Preview Abstract] |
Tuesday, March 3, 2015 9:24AM - 9:36AM |
F10.00008: Quantum Anomalous Hall Effect in Magnetic Insulator Heterostructure Gang Xu, Jing Wang, Claudia Felser, Xiaoliang Qi, Shoucheng Zhang Based on \emph{ab initio} calculations, we predict that a monolayer of Cr-doped (Bi,Sb)$_2$Te$_3$ and GdI$_2$ heterostructure is a quantum anomalous Hall insulator with a non-trivial band gap up to 38~meV. The principle behind our prediction is that the band inversion between two topologically trivial ferromagnetic insulators can result in a non-zero Chern number, which offers a better way to realize the quantum anomalous Hall state without random magnetic doping. According to our study, the working temperature of QAH effect will be enormously increased. Moreover, we predict that 3D quantum anomalous Hall insulator could be realized in (Bi$_{2/3}$Cr$_{1/3} $)$_2$Te$_3$ /GdI$_2$ superlattice. [Preview Abstract] |
Tuesday, March 3, 2015 9:36AM - 9:48AM |
F10.00009: Persistent ferromagnetism and topological phase transition at the interface of a superconductor and a topological insulator Wei Qin, Zhenyu Zhang At the interface of an $s$-wave superconductor and a three-dimensional topological insulator, Majorana zero modes and Majorana helical states have been proposed to exist respectively around magnetic vortices and geometrical edges. Here we first show that randomly distributed magnetic impurities at such an interface will induce bound states that broaden into impurity bands inside (but near the edges of) the superconducting gap, which remains open unless the impurity concentration is too high. Next we find that an increase in the superconducting gap suppresses both the oscillation magnitude and period of the RKKY interaction between two magnetic impurities. Within a mean field approximation, the ferromagnetic Curie temperature is found to be essentially independent of the superconducting gap, an intriguing phenomenon due to a compensation effect between the short-range ferromagnetic and long-range anti-ferromagnetic interactions. The existence of robust superconductivity and persistent ferromagnetism at the interface allows realization of a novel topological phase transition from a non-chiral to a chiral superconducting state at sufficiently low temperatures, providing a new platform for topological quantum computation. [Preview Abstract] |
Tuesday, March 3, 2015 9:48AM - 10:00AM |
F10.00010: Revealing dissipationless chiral edge channel in magnetic topological insulator via non-local transport measurement Wei-Li Lee, Xufeng Kou, Shih-Ting Guo, Yabin Fan, Lei Pan, Murong Lang, Ying Jiang, Qiming Shao, Tianxiao Nie, Koichi Murata, Jianshi Tang, Yong Wang, Liang He, Ting-Kuo Lee, Kang L. Wang We observed quantum anomalous Hall effect (QAHE) in our 10-quintuple layer Cr-doped (BiSb)$_2$Te$_3$ film grown by MBE technique. The Hall resistance $R_{xy}$ attains quantized value of h/e$^2$ (25.8 k$\Omega$) as temperature drops below 85 mK. Unlike previous report in a thinner Cr-doped (BiSb)$_2$Te$_3$ film, a finite longitudinal resistance is found in the QAHE regime and remains non-zero up to 15 Tesla suggesting the coexistence of the chiral edge channel and certain dissipative conduction channel. From macroscopic non-local transport measurements with leads separated by few millimeters, we further identify the dissipationless nature of the chiral edge channel associated with the QAHE. Detailed T-dependence and field-dependence of the non-local signals will be presented and discussed. [Preview Abstract] |
Tuesday, March 3, 2015 10:00AM - 10:12AM |
F10.00011: Inducing magnetism onto the surface of a topological crystalline insulator Badih A. Assaf, Ferhat Katmis, Peng Wei, Biswarup Satpati, Jagadeesh S. Moodera, Don Heiman Magnetically-doped topological crystalline insulators (TCI) have been predicted to host a quantum anomalous Hall state characterized by a Chern number, as large a C$=$4 [1]. An alternative way to achieve this quantum state is by inducing magnetism onto the surface via magnetic proximity with a ferromagnetic insulator such as EuS. Similar to the proximity effect achieved in EuS/Bi2Se3 bilayers [2], we have induced magnetism onto the TCI SnTe in an MBE-grown SnTe/EuS/SnTe trilayer. Transport measurements at T$=$2K exhibit an anomalous Hall effect that is induced at the SnTe surfaces by the insulating ferromagnet EuS. The in-plane magnetoresistance (MR) exhibits a pronounced hysteresis that is isotropic with the direction of the applied magnetic field. Unlike the case of ferromagnetic semiconductors and metals, where the in-plane MR is highly anisotropic as a result of spin-scattering, the present MR is evidence of additional conduction inside the domain-walls at the EuS-SnTe interfaces. Further MR measurements in the minor loop regime confirm this effect. This work is a significant step to realizing exotic quantum states in TCI thin films. [1] C. Fang, et al. Phys. Rev. Lett. \textbf{112}, 046801 (2014). [2] P. Wei et al. Phys. Rev. Lett. \textbf{18,} 186807 (2013). [Preview Abstract] |
Tuesday, March 3, 2015 10:12AM - 10:24AM |
F10.00012: Electrically Tunable Magnetism in Magnetic Topological Insulators Shou-Cheng Zhang, Jing Wang, Biao Lian The external controllability of the magnetic properties in topological insulators would be important both for fundamental and practical interests. Here we predict the electric-field control of ferromagnetism in a thin film of insulating magnetic topological insulators. The decrease of band~inversion by the application of electric fields results in a reduction of magnetic susceptibility, and hence in the modication of magnetism. Remarkably, the electric field could even induce the magnetic quantum phase transition from ferromagnetism to paramagnetism. We further propose a topological transistor device in which the dissipationless charge transport of chiral edge states is controlled by an electric field. The simultaneous electrical control of magnetic order and chiral edge transport in such a device may lead to electronic and spintronic applications for topological insulators. [Preview Abstract] |
Tuesday, March 3, 2015 10:24AM - 10:36AM |
F10.00013: Imaging Dirac-Mass Disorder from Magnetic Dopant-Atoms in the Ferromagnetic Topological Insulator Cr$_{\mathrm{x}}$(Bi$_{0.1}$Sb$_{0.9})_{\mathrm{2-x}}$Te$_{3}$ -- Part I Chung Koo Kim, Inhee Lee, Jinho Lee, Simon Billinge, Ruidan Zhong, John Schneeloch, Tiansheng Liu, John Tranquada, Genda Gu, J. C. Seamus Davis Topological insulators (TI) have a gapless surface state of Dirac fermions protected by the time reversal symmetry (TRS). However, TRS can be broken in the ferromagnetic state induced by magnetic doping. This leads to the opening of ``mass gap'' at the Dirac point. Such a gap is predicted to involve many exotic phenomena for which understanding the microscopic role of magnetic dopants is critical. But it is unknown how the spatial arrangements of the magnetic dopant atoms influence the Dirac-mass gap at the atomic scale. Here we image the locations of the magnetic (Cr) dopant atoms in the ferromagnetic TI Cr$_{0.08}$(Bi$_{0.1}$Sb$_{0.9})_{1.92}$Te$_{3}$. Simultaneous visualization of the Dirac-mass gap $\Delta $(r) reveals its intense disorder, which we demonstrate directly is related to fluctuations in n(r), the areal Cr atom density at the surface. The relationship of the surface-state Fermi wavevectors to both the correlation length and anisotropic structure of $\Delta $(r) are found consistent with predictions for ferromagnetism mediated by the surface states. [Preview Abstract] |
Tuesday, March 3, 2015 10:36AM - 10:48AM |
F10.00014: Imaging Dirac-Mass Disorder from Magnetic Dopant-Atoms in the Ferromagnetic Topological Insulator Cr$_{x}$(Bi$_{0.1}$Sb$_{0.9}$)$_{2-x}$Te$_{3}$ - Part II Inhee Lee, Chung Koo Kim, Jinho Lee, Simon Billinge, Ruidan Zhong, John Schneeloch, Tiansheng Liu, John Tranquada, Genda Gu, J. C. Davis We present Part II of the spectroscopic imaging - scanning tunneling microscopy (SI-STM) study of ferromagnetic Cr$_{x}$(Bi$_{0.1}$Sb$_{0.9}$)$_{2-x}$Te$_{3}$ single crystals measured at 4.5 K. As Part II we show how both spectroscopic analysis in real and momentum space demonstrate the coincident Dirac mass gap identified. Distribution of gap width, gap center, and gap anisotropy will be discussed. The anticipated relationship $\Delta(r) \propto n(r)$ is confirmed throughout, and exhibits an electron-dopant interaction energy $J^{*}=$145 meV$\cdot$nm$^{2}$. These observations reveal how magnetic dopant atoms actually generate the TI mass gap and that, to achieve the novel physics expected of time-reversal-symmetry breaking TI materials, control of the resulting Dirac-mass gap disorder will be essential. [Preview Abstract] |
Tuesday, March 3, 2015 10:48AM - 11:00AM |
F10.00015: Spin-based Mach-Zehnder interferometry in topological insulator p-n junctions Roni Ilan, Fernando de Juan, Joel Moore A p-n junction, an interface between two regions of a material populated with carriers of opposite charge, is a basic building block of solid state electronic devices. From the fundamental physics perspective, it often serves as a tool to reveal the unconventional transport behavior of novel materials. We show that a p-n junction made from a three dimensional topological insulator in a magnetic field realizes an electronic Mach-Zehnder interferometer with virtually perfect visibility. This is owed to the confinement of the topological Dirac fermion state to a closed two-dimensional surface, which offers the unprecedented possibility of utilizing external fields to design networks of chiral modes wrapping around the bulk in closed trajectories, without the need of complex constrictions or etching. Remarkably, this junction also acts as a spin filter, where the path of the particle is tied to the direction of spin propagation. It therefore constitutes a novel and highly tunable spintronic device where spin polarized input and output currents are naturally formed and could be accessed and manipulated separately. [Preview Abstract] |
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