Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session C08: Topological Insulator Experiment |
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Sponsoring Units: DCMP Chair: Yang-hao Chan, Institute of Physics Academia, Sinica Room: LACC 153C |
Monday, March 5, 2018 2:30PM - 2:42PM |
C08.00001: Two-Stage Proximity-Induced Gap-Opening in Topological Insulator–Insulating Ferromagnet (Bi_{x}Sb_{1-x})_{2}Te_{3}–EuS Bilayers Qi Yang, Aharon Kapitulnik To further investigate the interplay between ferromagnetism and topological insulators, the low-carrier topological insulator (Bi_{x}Sb_{1-x})_{2}Te_{3} thin films were deposited on the insulating ferromagnet EuS (100) in situ. AC susceptibility indicates magnetic anomalies at T ≈ 30 K and T ≈ 60 K, above the Curie temperature T_{C} ≈ 15 K. When the Fermi level is close to the Dirac point and the surface state dominates the electric conduction, sharp increases in resistance were observed at the same temperature range. Together with the positive-negative magnetoresistance crossover at the Curie temperature, a two-stage gap-opening process due to magnetic proximity is proposed. |
Monday, March 5, 2018 2:42PM - 2:54PM |
C08.00002: Carrier tuned magnetism in topological insulator Mn:Bi_{2}Te_{3} Haiming Deng, Zhiyi Chen, Shihua Zhao, John Villanova, Kyungwha Park, Marcin Konczykowski, Agnieszka Wolos, Lia Krusin-Elbaum Here we report a study of Mn-doped Bi_{2}Te_{3} where we found magnetism and Anomalous Hall Effect (AHE) strongly dependent on conduction type. In n-type Mn:Bi_{2}Te_{3}, we observe a pronounced AHE proportional to magnetization M arising from magnetic moments of Mn ions, measured independently using Hall sensor arrays. In p-type, while direct M measurements show the system to be weakly ferromagnetic and AHE is NOT observed. In our experiment we lower electron density using high energy (~2.5 MeV) electron beams. We find that for n-type, AHE decreases with decreasing electron density. By way of contrast, in p-type crystals the irradiation produces conversion from p- to n-type and no AHE is observed. Our DFT calculations show that the magnetic moment on the Mn atom increases from n- to p-doping in the intercalated and interstitial cases; the opposite is true in the substitutional case. We demonstrate how AHE can be tuned by changing carrier density either by anneals or electrostatic gating. We discuss how this system with band inversion differs from trivial dilute magnetic semiconductors where moments couple via itinerant hole carriers by RKKY interaction. |
Monday, March 5, 2018 2:54PM - 3:06PM |
C08.00003: Tuning the magnetism in bilayers of magnetic topological insulators (TIs) with a back gate and circularly polarized light Chien-Chang Chen, Adrian Llanos, Ben Seddon, Marcus Teague, Nai-Chang Yeh, Lei Pan, Koichi Murata, Kang Wang Studies of MBE-grown magnetic bilayers revealed the appearance of spatially inhomogeneous surface state (SS) gap below T_{c}^{2D} (>~210 K) by scanning tunneling spectroscopy (STS) and a bulk Curie temperature below T_{c}^{3D}~35 K by anonymous Hall effect (AHE) measurements. The bilayer samples consisted of 1-QL of pure TI on top of 6-QL of Cr-doped TI, where TI was either Bi_{2}Se_{3} or (Bi,Sb)_{2}Te_{3}. Strong magnetic fields normal to the TI bilayers were found to enhance the homogeneity of the SS gap distribution. On the other hand, topological spin textures associated with isolated magnetic impurities were observed along the boundary of gapped and gapless SS in zero fields, which disappeared under high magnetic fields. To better elucidate the microscopic mechanism of proximity-induced magnetism in the bilayer TIs, we investigate the interplay of circularly polarized light and external magnetic fields on the SS gap homogeneity and the values of T_{c}^{2D} and T_{c}^{3D} by both STS and AHE measurements. We further evaluate the effect of circularly polarized light on the two-level spectral resonances of isolated SS magnetic impurities and examine the effect of tuning the Fermi level via a back gate on inducing correlations among magnetic impurities. |
Monday, March 5, 2018 3:06PM - 3:18PM |
C08.00004: Two-component anomalous Hall effect in a magnetically doped topological insulator Nan Liu, JIng Teng, Yongqing Li We report on observation of the anomalous Hall (AH) effect that has been elusive in Mn-doped Bi_{2}Se_{3} thin films. As the Mn concentration is increased, the sign of AH resistances can be changed from positive to negative, and these two types of AH resistances are found to coexist in the crossover regime. Such a two-component AH effect and the sign reversal can also be obtained by lowering the chemical potential in the samples with low Mn-doping levels. Based on their different dependences on the gate voltage and magnetic field, the positive and negative AH components are assigned to the bulk and surface states, respectively. Our results provide a missing experimental piece for understanding the puzzling interplay between the surface states and the magnetic doping effects in this prototype three-dimensional topological insulator (TI). The knowledge gained in this work will be valuable for optimizing magnetically doped TIs and hybrid structures, which have become a promising platform for observing novel quantum phenomena and realizing spintronic applications. |
Monday, March 5, 2018 3:18PM - 3:30PM |
C08.00005: Time Reversal Symmetry Breaking in Topological Insulator/Magnetic Insulator Heterostructures Revealed by the Negative Magnetoresistance Shang Rong Yang, Ko-Hsuan Chen, C. C. Chen, Chi-Nan Wu, C. C. Tseng, C. K. Cheng, Minghwei Hong, Jueinai Kwo Topological insulator (TI) is one of emergent quantum materials exhibiting topological surface states (TSSs) with spin momentum locking. Breaking time reversal symmetry (TRS) of TSSs leads to novel phenomena such as quantum anomalous Hall effect (QAHE). In this work, we utilized magnetic insulator yttrium iron garnet (YIG) to break TRS through the magnetic proximity effect (MPE) that enables uniform magnetization without introducing crystal defects, as opposed to magnetic dopings of Cr into TI. High quality YIG thin films were obtained by sputtering followed by high temperature anneals, and Bi_{2}Se_{3} thin films were deposited at 280 ^{○}C by MBE. Carrier concentrations and mobilities of Bi_{2}Se_{3}/YIG were found to be comparable to those of Bi_{2}Se_{3}/Sapphire. It was observed that, in contrast to Bi_{2}Se_{3}/Sapphire, the distinctive negative magnetoresistance showed in Bi_{2}Se_{3}/YIG as the temperature decreased. Magnetoconductances can be well described by incorporating a weak localization term into the fitting function, which indicates the gap opening of TSSs according to the theory developed by Hai-Zhou et al. The systematic dependence of the WL and WAL components on Bi_{2}Se_{3} thickness varying from 7 to 40 nm will be reported. Our study may pave the way to realize QAHE at higher temperatures. |
Monday, March 5, 2018 3:30PM - 3:42PM |
C08.00006: Non-local Influence of the local ferromagnetic impurities on electronic structure of the topological insulator Cr_{0.08}(Bi_{0.1}Sb_{0.9})_{1.92}Te_{3} Jung Hoon Yoo, Sang Hyun Joo, Min Seok Park, Sung Jun Park, Kyoung Seok Lee, Jae-joon Kim, Genda Gu, Jinho Lee Magnetic impurities break a time-reversal symmetry of topological insulator(TI) and generate a back scattering between the surface states. As a result, a gap opens at the time-reversal invariant momentum and the electronic properties of TI’s undergo a drastic change. We investigated the local density of states(LDOS) of Cr doped TI (Cr_{0.08}(Bi_{0.1}Sb_{0.9})_{1.92}Te_{3}) using the spectroscopic imaging scanning tunneling microscopy(SI-STM) and found that there are two types of impurities: one kind from the 1^{st} Bi-Sb layer and another kind from the 2^{nd} Bi-Sb layer. We found that the local Dirac gap magnitude are more correlated to the position of impurities from the 2^{nd} layer and the quasi-particle interference shows a trace of the back scattering which is prohibited in the undoped TI’s. We will also present a Spectroscopic feature near the Fermi energy and its possible implication. |
Monday, March 5, 2018 3:42PM - 3:54PM |
C08.00007: Superparamagnetism-induced Mesoscopic Electron Focusing in Topological Insulators Philipp Rüßmann, Phivos Mavropoulos, Stefan Bluegel Breaking time-reversal symmetry by the presence of magnetic dopants re-opens the backscattering channel in the surface state of strong topological insulators. Recent quasiparticle interference measurements at the surface of Mn-doped Bi_{2}Te_{3} at 4K reveal directionally focused and undamped interference patterns [1]. |
Monday, March 5, 2018 3:54PM - 4:06PM |
C08.00008: Enhanced electron dephasing in three-dimensional topological insulators Jian Liao, Yunbo Ou, Haiwen Liu, Ke He, Xucun Ma, Qikun Xue, Yongqing Li Study of the dephasing in electronic systems is not only important for probing the nature of their ground states, but also crucial to harnessing the quantum coherence for information processing. In contrast to well-studied conventional metals and semiconductors, it remains unclear which mechanism is mainly responsible for electron dephasing in three-dimensional topological insulators (TIs). Here, we report on using weak antilocalization effect to measure the dephasing rates in highly tunable (Bi,Sb)_{2}Te_{3} thin films. As the transport is varied from a bulk-conducting regime to surface-dominant transport, the dephasing rate is observed to evolve from a linear temperature dependence to a sublinear power-law dependence. Although the former is consistent with the Nyquist electron-electron interactions commonly seen in ordinary 2D systems, the latter leads to enhanced electron dephasing at low temperatures and is attributed to the coupling between the surface states and the localized charge puddles in the bulk of 3D TIs. |
Monday, March 5, 2018 4:06PM - 4:18PM |
C08.00009: Bulk-Induced Correlated Mobility-Number Density Fluctuations in Molecular Beam Epitaxy Grown Topological Insulators Saurav Islam, Semonti Bhattacharyya, Abhinav Kandala, Anthony Richardella, Nitin Samarth, Arindam Ghosh We present a detailed study of 1/f noise in large-area molecular beam epitaxy grown thin (~10 nm) films of the topological insulator (Bi,Sb)_{2}Te_{3} on strontium titanate (STO) substrate as a function of temperature (T), gate voltage and magnetic field [1]. The temperature dependence of 1/f noise displays a sharp cusp at T = 50 K which can be attributed to generation-recombination processes in the impurity band in the bulk band gap. The gate voltage dependence of noise reveals that both bulk and surface states are influenced by correlated mobility-number density fluctuations caused by defects in the bulk with a density D_{it} = 3.2×10^{17} cm^{-2}eV^{-1}. In the presence of magnetic field, the 1/f noise follows a parabolic dependence which is qualitatively similar to mobility and charge density fluctuation noise in nondegenerate semiconductors. Our studies reveal that even in thin films of (Bi,Sb)_{2}Te_{3} with thickness as low as 10 nm, the bulk defects are the dominant source of noise. (1) S. Islam et al. APL 111, 062107 (2017) |
Monday, March 5, 2018 4:18PM - 4:30PM |
C08.00010: Effects of Nitrogen Exposure on the Bismuth Selenide Density of States Michael Gottschalk, Eric Goodwin, Mal-Soon Lee, Ian Dayton, Thomas Chasapis, S Mahanti, Mercouri Kanatzidis, Stuart Tessmer Bi_{2}Se_{3} is an intensely studied topological insulator; the crystals typically exhibit a Dirac point about 300 meV below the Fermi level, behavior attributed to charged selenium vacancies which act as electron donors. We will present measurements and theoretical calculations of shifting of the Dirac point in response to exposing the surface of the crystals to different gas environments including nitrogen, helium and air. The experiments were conducted on crystals cleaved in a glove-box over-pressured with the desired gas and transferred directly into a chamber pumped to ~10^{-6} torr. Tunneling conductance spectra were obtained using scanning tunneling microscopy (STM) at 77K in this vacuum. We find substantial shifts in the Dirac point depending on the gas. To understand experimental observations, we calculated geometric and electronic structural properties of gas-adsorbed systems with and without defects by employing density-functional-theory based method. We will also discuss band structures, projected density of states, and hence the effect of gas adsorption and defects on the Dirac point shift. |
Monday, March 5, 2018 4:30PM - 4:42PM |
C08.00011: Scanning Tunneling Microscopy Study of 1D Modes Trapped by Topological Defects in Pb_{1−}_{x}Sn_{x}Se Davide Iaia, Sean Howard, Daniel Walkup, Ilija Zeljkovic, Zhenyu Wang, Vidya Madhavan Defects such as step edges and dislocations in topological crystalline insulators (TCIs), can host topological protected states which connect Dirac points in momentum space. Features like high spin polarization, robustness against magnetic perturbation and temperature, make these topological states suitable for spintronics applications. Here we present a scanning tunneling microscopy (STM) study of 1D modes trapped by two different edge defects in Pb_{1−}_{x}Sn_{x}Se: screw dislocations and step edges. The differential conductance (dI/dV) maps show an enhancement of the density of states at the Dirac point energy. Previous data on step edges showed no dependence on magnetic field. Interestingly, in our work we find that while some step edges show peaks that are independent of magnetic field, other edges show magnetic field dependence. We will discuss the implications of our findings on the topologically protected 1D states expected in these materials. |
Monday, March 5, 2018 4:42PM - 4:54PM |
C08.00012: Imaging the Tunable Topological Surface State in GeBi_{2-x }Sb_{x}Te_{4} Sean Howard, Davide Iaia, Zhenyu Wang, Raman Sankar, Fangchang Chou, Hsin Lin, Vidya Madhavan 3D Topological insulators (TI) host robust, spin-momentum locked, linearly dispersing surface states which have lead to the successful observation of quantum anomalous hall effect and chiral Majorana modes and also promise interesting applications. Isolation of these topological surface states is often hindered by large contributions by bulk bands, which can be mediated by a Dirac point in the band gap near the fermi energy. Here we investigate a new TI system, GeBi_{2-x }Sb_{x}Te_{4} ,_{ }using low temperature scanning tunneling microscopy. We track the surface state band structure with quasiparticle interference as Bi is substituted with Sb. Interestingly, we find Sb substitution effectively p dopes the sample, shifting the Dirac point from approximately -280 meV below the Fermi energy for GeBi_{2}Te_{4} to close to the Fermi energy for intermediate substitutions. As the Dirac point is located within the band gap, tuning with substitution could provide enhanced isolation of topological surface states. |
Monday, March 5, 2018 4:54PM - 5:06PM |
C08.00013: Tuning the Topological Phases of ZrTe5 Single Crystals by Uniaxial Stress Joshua Mutch, Wei-Chih Chen, Cheng-Chien Chen, Jiun-Haw Chu ZrTe5 has been theoretically predicted to be a quantum spin Hall insulator in single layer form. In bulk form, the calculation suggested that ZrTe5 resides near a phase transition between a strong and weak topological insulator (TI), and the exact location sensitively depends on the lattice constants. Here, we report a systematic investigation that combines electrical transport measurements and DFT calculations on the effect of uniaxial stress along the crystalline a-axis. We have observed a non-monotonic, temperature dependent resistance-strain relationship. The DFT calculations also reveal a closing and opening of the energy gap as a function of uniaxial strain. The implications of this observation with respect to the existence of a strain induced topological phase transition will be discussed. |
Monday, March 5, 2018 5:06PM - 5:18PM |
C08.00014: Magnetic heterostructures of topological insulators for gigantic magnetoresistance and axion insulator Masataka Mogi, Minoru Kawamura, Atsushi Tsukazaki, Ryutaro Yoshimi, Kei Takahashi, Masashi Kawasaki, Yoshinori Tokura The quantum anomalous Hall (QAH) effect observed in magnetic topological insulators (TIs) draws much attention to possible application for low-energy consumption electronic devices and a platform for the axion electrodynamics as fundamental physics. Here, we designed a magnetic TI with a trilayer structure in which a nonmagnetic layer of (Bi, Sb)2Te3 is sandwiched by a soft-ferromagnetic Cr-doped (Bi, Sb)2Te3 and a hard-ferromagnetic V-doped (Bi, Sb)2Te3. Accompanied by the QAH effect, we observed wide zero Hall conductivity plateaus via the magnetization reversal. Two-terminal resistance during the zero-Hall plateau state reaches as high as giga-ohms, leading to gigantic magnetoresistance upon the transition from the QAH state (~ 26 kilo-ohms). Furthermore, the high resistance state is a new topological phase termed as axion insulator state, promising realization of topological magnetoelectric effect. |
Monday, March 5, 2018 5:18PM - 5:30PM |
C08.00015: Engineering Topological Superlattices: Prospects and Limitations Pavel Shibayev, Maryam Salehi, Jisoo Moon, Seongshik Oh We developed a new artificial topological insulator (TI) system in heterostructure films consisting of alternating Bi_{2}Se_{3} (topological) and In_{2}Se_{3} (trivial) layers grown via molecular beam epitaxy (MBE). Building upon our past work on similar superlattices, here we systematically explore both the prospects and limitations of this model. Due to weak interlayer bonding energies in these van der Waals systems, we find that this topological superlattice structure tends to develop non-ideal, wavy interfaces as the number of superlattices grows. However, despite such structural imperfections, we discover that some of the electronic transport properties such as the weak antilocalization (WAL) effect robustly represent the well-defined quantized values of superlattice unit cells. The particularly remarkable robustness of the WAL effect allows us to map out a phase diagram for this artificial topological material as a function of intra- and inter- coupling strengths between the topological and trivial unit layers. |
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