Session C28: Quantum Anomalous Hall Effect I

Achieving High-Temperature Ferromagnetic Topological Insulator

Topological insulators (TIs) are insulating materials that display conducting surface states protected by time-reversal symmetry, wherein electron spins are locked to their momentum. This unique property opens new opportunities for creating next-generation electronic and spintronic devices, including TI-based quantum computation. Introducing ferromagnetic order into a TI system without compromising its distinctive quantum coherent features could lead to a realization of several predicted novel physical phenomena. In particular, achieving robust long-range magnetic order at the TI surface at specific locations without introducing spin scattering centers could open up new possibilities for devices. Here, we demonstrate topologically enhanced interface magnetism by coupling a ferromagnetic insulator (FMI) to a TI (Bi2Se3); this interfacial ferromagnetism persists up to room temperature, even though the FMI (EuS) is known to order ferromagnetically only at low temperatures (\textless 17 K). The induced magnetism at the interface resulting from the large spin-orbit interaction and spin-momentum locking feature of the TI surface is found to greatly enhance the magnetic ordering (Curie) temperature of the TI/FMI bilayer system. Due to the short range nature of the ferromagnetic exchange interaction, the time-reversal symmetry is broken only near the surface of a TI, while leaving its bulk states unaffected. The topological magneto-electric response originating in such an engineered TI could allow for an efficient manipulation of the magnetization dynamics by an electric field, providing an energy efficient topological control mechanism for future spin-based technologies. Work supported by MIT MRSEC through the MRSEC Program of NSF under award number DMR-0819762, NSF Grant DMR-1207469, the ONR Grant N00014-13-1-0301, and the STC Center for Integrated Quantum Materials under NSF grant DMR-1231319.   

Probing the Spin Transfer Efficiency at Topological Insulator/Ferromagnetic Insulator Interfaces

The development of next-generation spintronics devices has driven extensive studies of spin-charge conversion through measurement of the inverse spin Hall effect (ISHE) and ferromagnetic resonance (FMR) driven spin pumping of pure spin currents in ferromagnet/non-magnet bilayers. Topological insulators (TIs) such as the Bi-chalcogenides are naturally relevant in this context because the inherent spin-momentum “locking” in their surface states promises very efficient spin-charge conversion, although the first experimental studies have involved ferromagnetic metals that provide a shunting current path [e.g. Nature, 511,449 (2014)]. To circumvent the current shunting problem, we are growing and characterizing bilayers of TIs and the ferrimagnetic insulator Y$_3$Fe$_5$O$_{12}$ (YIG). Here, we report measurements of FMR-driven spin pumping in TI/YIG bilayers, showing robust spin pumping signals at room temperature. Analysis of the ISHE voltages and FMR linewidth broadening show that, as in other studies of spin pumping into TIs [Nano Lett., 15 (10) (2015)], the interface condition presents a critical challenge for enhancing the spin conversion efficiency in these devices.   

Spin manipulation at the interface of a topological insulator/GaAs heterostructure

One primary goal of spintronics is to discover materials and devices, which enable efficient electrical control of spins. The emerging field of topological insulator (TI) provides intriguing opportunities for spin generation and manipulation, owing to its strong spin-orbit character. Here we report that spins can be driven from a topological insulator thin film (Bi$_{\mathrm{0.5}}$Sb$_{\mathrm{0.5}})_{\mathrm{2}}$Te$_{\mathrm{3}}$ into an adjacent semiconductor GaAs at room temperature. In a TI/GaAs heterostructure, a photo-induced spin current flows across the interface and induces an electrical current via the inverse spin Hall effect, which converts the spin current into a charge current. We find that the magnitude and direction of the helicity-dependent photocurrent can be controlled by gate-voltage, indicative of electric tuning of the spin configuration.   

\textbf{Spin Pumping into Topological Insulator Bi}$_{\mathrm{\mathbf{2}}}$\textbf{Te}$_{\mathrm{\mathbf{3}}}$

A spin chemical potential bias can induce a spin polarized current by the exchange interaction of a ferromagnet with the spin-momentum locking surface states of the topological insulators. We carried out our ferromagnetic resonance experiment in a NiFe/Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$ heterostructure. Apart from the enhanced Gilbert damping constant, we observed strong enhancement of the effective magnetic field at low temperatures. The enhanced field decreased exponentially with increasing temperature at an energy scale of 2.5 meV, representing the strength of the exchange coupling. We attribute the enhanced field to the induced spin polarized current in the surface states of Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$.   

Magnetic proximity effect in a topological insulator-magnetic insulator heterostructure

Ferromagnetic topological insulators (TIs) have become one of the most actively pursued materials in condensed matter physics due to their unique properties, where several exotic phenomena have been predicted and observed, such as the quantum anomalous Hall effect and the topological magneto-electric effect. In this talk, I will introduce the fabrication and characterization of a heterostructure consisting of a thin film of the topological insulator Bi$_{2}$Se$_{3}$ and the magnetic insulator Y$_{3}$Fe$_{5}$O$_{12}$ (YIG), and study the low temperature transport properties. Compared to non-magnetic Bi$_{2}$Se$_{3}$, the magnetoresistance (MR) of Bi$_{2}$Se$_{3}$-YIG deviates from the typical weak antilocalization behavior in low perpendicular magnetic fields. In parallel fields, we observe unusual negative MR and sharp MR jumps when single domains nucleate and annihilate. Furthermore, magnetization measurements reveal that this unusual MR correlates to domain wall configurations of the YIG layer. These results can be explained due to the appearance of a perpendicular magnetic exchange field at the interface. The understanding of the interfacial interaction is valuable to further reveal unique physics in TI based magnetic heterostructures.   

Epitaxial Growth and Characterization of Iron Chalcogenide/Bismuth Chalcogenide Heterostructures

Heterostructures consisting of topological insulators (TIs) interfaced with superconductors and with ferromagnets have been predicted to give rise to phenomena of both fundamental and applied interest. With superconductors, the region of proximity-induced superconductivity should have $p_x + ip_y$ symmetry, and vortices in this region have been predicted to host Majorana modes, which may be useful as quantum bits. With ferromagnets, such phenomena as the topological magnetoelectric effect have been predicted. Iron chalcogenides, such as iron selenide and iron telluride, are ideal candidates for combining with TIs, since, with only minor changes to growth conditions, they can be superconducting, ferromagnetic, or antiferromagnetic. We describe the growth and characterization of heterostructures that combine thin films of the iron and bismuth chalcogenides, focusing on low temperature magnetoresistance measurements. Our measurements reveal a transient hysteretic magnetoresistance with surprisingly long relaxation times (minutes). This phenomenon appears to be a generic characteristic of all heterostructures that interface TIs with magnetic spins, albeit with structure-specific relaxation times. We discuss possible origins of this unusual phenomenon. Funded by ARO/MURI.   

Epitaxial Growth of Two-Dimensional Stanene

Ultrathin semiconductors present various novel electronic properties. The first experimental realized two-dimensional (2D) material is graphene. Searching 2D materials with heavy elements bring the attention to Si, Ge and Sn. 2D buckled Si-based silicene was realized by molecular beam epitaxy (MBE) growth. Ge-based germanene was realized by mechanical exfoliation. Sn-based stanene has its unique properties. Stanene and its derivatives can be 2D topological insulators (TI) with a very large band gap as proposed by first-principles calculations, or can support enhanced thermoelectric performance, topological superconductivity and the near-room-temperature quantum anomalous Hall (QAH) effect. For the first time, in this work, we report a successful fabrication of 2D stanene by MBE. The atomic and electronic structures were determined by scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES) in combination with first-principles calculations. This work will stimulate the experimental study and exploring the future application of stanene.   

Topological Surface States in Sb Quantum Wells on GaSb(111)A Substrates

A topoelectronic transition is predicted as a function of Sb quantum-well (QW) thickness. Bulk Sb is a semimetal with a negative bandgap, with neither the conduction band minimum nor the valence band maximum at the $\Gamma $ point. Our goal is to measure the topological surface states by suppression of the bulk conductivity through quantum confinement and enhancement of the surface conductivity through remote n-type doping at the $\Gamma $ point. Conductivity measurements on undoped QWs (0.7 to 6 nm thick) show a suppression of the bulk states, such that the surface conductivity is about 15{\%} for a 3.6 nm QW. Hall-effect measurements, which nominally indicate p-type conduction, are complicated by the presence of both electrons and holes. We have begun experiments to populate the topological electron states by doping the GaSb barrier with Te atoms, creating donor states at the $\Gamma $ point. At the $\Gamma $ point of the QW, the topological electron states have a lower energy than the bulk conduction band minimum. We observe n-type conduction for a remotely-doped Sb QW with a 94 nm spacer between the doped GaSb layer and the Sb QW. We plan to make high-field magneto-transport measurements to verify that the Sb surface states are populated.   

First-principles study of topological surface states in Bi$_2$Se$_3$/ZnSe superlattices

Topological insulators (TIs) are interesting due to robustness of surface states within a bulk band gap in the presence of time reversal symmetry. Various TI heterostructures are based on the robustness of the topological surface states. Thus, it is crucial to understand how the topological surface states are influenced by interfaces. Recently Bi$_2$Se$_3$/Zn$_x$Cd$_{1-x}$Se superlattices grown by molecular beam epitaxy showed interesting magneto-transport properties such as a single two-dimensional conducting channel per TI layer with the Berry phase of $\pi$. Intrigued by this experiment, we investigate topological surface states of the Bi$_2$Se$_3$/ZnSe superlattice by using density-functional theory. Based on the stoichiometry and the charge balance of the ZnSe layer, when one side of the ZnSe layer is terminated with Zn in the superlattice, the other side must be terminated with Se. Using the superlattice model and two slab models with either a Zn-terminated or Se-terminated interface, we calculate the effect of the inherent asymmetry of the ZnSe layer on the topological surface states of Bi$_2$Se$_3$, and compare our result to the experiment.   

Experimental preparation of lateral Heterojunction Sb$_{\mathrm{2}}$Te$_{\mathrm{3}}$/Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$ Nanoplates

For the first time, lateral heterojunction of Sb$_{\mathrm{2}}$Te$_{\mathrm{3}}$-Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$ was successfully realized using a two-step solvothermal method. The two crystalline components were separated well by a sharp lattice-matched interface when the optimized procedure was used. Inspecting the heterojunction using high-resolution transmission electron microscopy showed that epitaxial growth occurred along the horizontal plane. The semiconducting temperature-resistance curve and crossjunction rectification were observed, which reveal a staggered-gap lateral heterojunction with a small junction voltage. Quantum correction from the weak antilocalization reveals the well-maintained transport of the topological surface state. This is appealing for a platform for spin filters and one-dimensional topological interface states. The relevant works on materials optimization and fabrication of spin devices are already under way. (Nanoletters 2015, 15, 5905$-$5911))   

Nonlinear optical probe of interface ferromagnetism of EuS-Bi$_{2}$Se$_{3}$ heterostructures

EuS-Bi$_{2}$Se$_{3}$ heterostructure is a novel magnetic topological insulator system with canted ferromagnetism induced at the interface between EuS and Bi$_{2}$Se$_{3.}$ Here we use magnetic second harmonic generation (MSHG) to probe interface ferromagnetism of EuS-Bi$_{2}$Se$_{3}$ heterostructures. MSHG is a powerful nonlinear optical technique that selectively probes magnetism at the surfaces and interfaces of a centrosymmetric material. In order to study how the thickness of the magnetic EuS layer affects interface ferromagnetism, we have grown EuS-Bi$_{2}$Se$_{3}$ heterostructures with varying EuS thicknesses. We have also grown heterostructures in which the EuS thickness increases linearly across a single sample. We discuss how the magnetic layer thickness affects the strength and canting angle of interface magnetism.   

Van Vleck Nature of Carrier-Free Ferromagnetic Order in Vanadium-Doped Three-Dimensional Topological Insulators

We experimentally demonstrate that the long-range ferromagnetic (FM) order in vanadium (V)-doped topological insulator Sb$_{\mathrm{2}}$Te$_{\mathrm{3}}$ has the nature of van Vleck-type ferromagnetism, using the state-of-art low-temperature Electron Energy Loss Spectroscopy (EELS). Contrary to the temperature-independent Te M$_{\mathrm{4,5\thinspace }}$peak, there is an unusual redshift of the V L$_{\mathrm{3}}$ and L$_{\mathrm{2}}$ peak positions and unambiguous change of the L$_{\mathrm{3}}$:L$_{\mathrm{2}}$ peak ratio at T$=$10K. Further high-order Green's function's EELS simulation and magnetotransport show that the shift of the peak position and change of the L$_{\mathrm{3}}$:L$_{\mathrm{2}}$ ratio are originated from the development of the core-level FM order, indicating that in V-doped Sb$_{\mathrm{2}}$Te$_{\mathrm{3}}$, partially filled core states will also contribute to FM order. Since van Vleck magnetism is a result of summing over all states, this magnetization of core level verifies the van Vleck-type ferromagnetism in a direct manner.   

Imaging Spatially Varying Magnetic Order in Proximity Induced Magnetic Topological Insulator

Broken time-reversal symmetry on the surface states of a three dimensional topological insulator, such as Bi$_2$Se$_3$, results in quantized anomalous Hall conductance and is predicted to exhibit topological magneto-electric effects. We plan investigate how Dirac fermions interact with magnetism by imaging the magnetization of a topological insulator (Bi$_2$Se$_3$) sandwiched between two ferromagnetic insulator layers (EuS) with a scanning SQUID microscope. Cooling in an in-plane field leads to a magnetization that varies spatially on a micron scale, with 6-fold rotational symmetry. Understanding the origin of this magnetism may shed light on the exchange interaction and electronic properties of topological insulators.