Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session P3: Topological Insulators: Nanostructures and Heterostructures |
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Sponsoring Units: DCMP Chair: H, Kang L. Wang, University of California, Los Angeles Room: 262 |
Wednesday, March 15, 2017 2:30PM - 2:42PM |
P3.00001: Proximity-induced unconventional superconductivity in hybrid superconductor-topological insulator devices Erik Huemiller, Can Zhang, David Hamilton, Gilbert Arias, Martin Stehno, Dale Van Harlingen Topological insulators in proximity to an s-wave superconductor or doped into a bulk superconducting state have been predicted to exhibit signatures of unconventional superconductivity. These system have been predicted to show signatures of a p-wave like superconducting order parameter, but the pairing symmetry has not been definitively confirmed by experiment. We will present ongoing work on Josephson interferometry measurements to probe the superconducting order parameter in Nb-Bi2Se3 interfaces and in NbxBi2Se3 crystals. We also investigate the transport properties in these materials by measuring local and non-local resistance using two and three terminal devices. We see signatures of a p-wave superconducting order parameter in the conductance of both systems and an unusual asymmetrical non local response that may result from the topological nature of the system. [Preview Abstract] |
Wednesday, March 15, 2017 2:42PM - 2:54PM |
P3.00002: Ferromagnetic resonance study of interlayer exchange coupling in topological insulator/ferrimagnetic insulator heterostructures Y. T. Fanchiang, H. Y. Lin, C. C. Tseng, K. S. Chen, C. N. Wu, C. K. Cheng, S. F. Lee, J. G. Lin, M. Hong, J. Kwo Introducing magnetic order in topological insulators via magnetic proximity coupling when in contact with a ferrimagnet is a promising way to realize novel topological physics such as quantum anomalous Hall effect. We have performed ferromagnetic resonance (FMR) of heterostructures made of yttrium iron garnet (YIG) films of varying thickness (15-30 nm) and Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ films (25 QL) from 300 K down to 5 K. Frequency- and angle-dependent FMR show sizable Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$-induced effective field in bilayer samples at room temperature, which is manifested as enhanced in-plane magnetic anisotropy over that of single layer YIG. The origin of such magnetic anisotropy occurring at interface is clarified by the YIG thickness dependence study. As the temperature decreases, the exchange effective field builds up pronouncedly and the FMR remains detectable. Specifically, for the bilayer sample of thin YIG (15 nm), the exchange effective field can induce FMR at microwave frequency of at least 3.5 GHz in absence of applied field below 50 K. Our study reveals the role of strong interlayer exchange coupling between Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ and YIG in magnetization dynamics, leading to potential field-free spintronics application. [Preview Abstract] |
Wednesday, March 15, 2017 2:54PM - 3:06PM |
P3.00003: Magnetic Proximity Effect in a Transferred Topological Insulator Thin Film on a Magnetic Insulator Xiaoyu Che, Koichi Murata, Lei Pan, Qinglin He, Gen Yin, Yabin Fan, Lei Bi, Kang Lung Wang Exotic physical phenomena such as the quantum anomalous Hall effect (QAHE) arise by breaking the time-reversal symmetry (TRS) in topological insulators. However, substantial efforts have been made in improving the temperature for realizing the QAHE via magnetically doping, while the proximity coupling is another approach to develop the magnetic order without the introduction of additional carriers or the presence of local Fermi level fluctuation. Here we demonstrate the experimental signature of magnetic proximity effect in a molecular beam epitaxy-grown TI thin film of Bi2Se3 transferred to a magnetic substrate of yttrium iron garnet using a wet transfer technique. Comparing to the TI/GaAs control sample, the magnetic order is manifested by the anomalous Hall effect in magneto-transport characterization. Furthermore, due to TRS breaking by the proximity effect we observed a constituent weak localization component accompanied with the weak antilocalization behavior. The present work takes a step further toward realizing QAHE at higher temperature and opens up a new path in TI device designs for applications. [Preview Abstract] |
Wednesday, March 15, 2017 3:06PM - 3:18PM |
P3.00004: Stepwise Quantized surface state and Hall plateaus in Co cluster decorated topological insulator BiSbTeSe2 Shuai Zhang, Fengqi Song Topological surface state (TSS) of a three-dimensional (3D) topological insulator (TI) usually hosts special and intriguing transport phenomena. Compared with two-dimensional electron gas, Hall conductivities of a single TSS quantize to a half-integer quantum Hall (QH) state in a magnetic field. However, there are two TSS (top surface and bottom surface) of 3D TI, which usually leads to an integer QH effect. Here, we observe a stepwise quantization of the two TSSs by depositing Co clusters on the top surface of TI. The renormalization group flow was plotted, which exhibits a unique way to converge to the points of integer QH state. We can see the bottom surface is quantized earlier than the top surface, which with Co cluster decorating, with increasing the magnetic field from the flow. In higher magnetic field, more Hall plateaus are exhibited. Also, the Co clusters can induce to dissipative channels. [Preview Abstract] |
Wednesday, March 15, 2017 3:18PM - 3:30PM |
P3.00005: Understanding of the giant enhancement of the exchange interaction in Bi2Se3-EuS heterostructure. Jeongwoo Kim, Kyoung-Whan Kim, Hui Wang, Jairo Sinova, Ruqian Wu We study the unusual magnetic behaviors of EuS-Bi2Se3, such as the enhancement of magnetic ordering, change of magnetic anisotropy and induced magnetic moments, based on the model Hamiltonian and the first-principles calculations. The interplay between topological surfaces states and the magnetic proximity of EuS strengthens the interfacial magnetic ordering and produces large magnetic anisotropy at the interface. Moreover, we find that the magnetic moments at the interfacial EuS and Bi2Se3 layers by the proximity effect are not as dramatically enhanced as reported before. Our results provide the understanding of the unresolved interfacial magnetism and should be useful for the design of novel magnetic materials that involve topological materials. Work was supported as part of the SHINES, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Grant No. SC0012670. Calculations were performed on parallel computers at NERSC supercomputer centers. [Preview Abstract] |
Wednesday, March 15, 2017 3:30PM - 3:42PM |
P3.00006: Thin Film of Topological Insulator-Semiconductor Heterostructures Mahmoud M. Asmar, Daniel E. Sheehy, Ilya Vekhter We consider a thin film topological insulator deposited on a substrate or sandwiched between two semiconductors.~ Motivated by recent work [1] showing that the properties of the interface states in a bulk topological insulator-semiconductor heterostructures depend on the residual symmetries of the interface potential, we analyze the spectral and transport properties of the thin film structures. We show that the reduced symmetry of the interface is reflected in the nature of the hybridization of the topological states at the opposite sides of the film. Consequently, the energy spectrum as well as the spin and charge properties of the hybridized states critically depend on the specific choice of the materials and the surface preparation. Our results have important implications for the systems utilizing hybrids of topological insulating thin films and semiconductors [2-3]. [1] M. M. Asmar, D. E. Sheehy and I. Vekhter. Unpublished. [2] A. A. Burkov and L. Balents. Phys. Rev. Lett~\textbf{107}, 127205 (2011). [3] G.Y. Cho and J. E. Moore. Phys Rev B~\textbf{84}, 165101 (2011).~ [4] A. A. Zyuzin, M. D. Hook, and A. A. Burkov. Phys. Rev. B~\textbf{83}, 245428 (2011). [Preview Abstract] |
Wednesday, March 15, 2017 3:42PM - 3:54PM |
P3.00007: Tuning the vertical location of topological surface states in ZnSe/Bi$_2$Se$_3$ heterostructures via the ZnSe overlayer thickness Leiqiang Li, Guangfen Wu, Jiang Zeng, Wei Qin, Ping Cui, Zhenyu Zhang The robust metallic topological surface states (TSSs) of topological insulators (TI) have attracted intensive research interest both fundamentally and for their potential applications. The precise location of the TSSs defines the boundary between the TI and topologically inequivalent system such as a conventional insulator (CI).\footnote{G. Wu \textit{et al.}, Sci. Rep. \textbf{3}, 1233 (2013)} Here, by employing first-principles density functional theory calculations, we study the prototype systems of ZnSe/Bi$_2$Se$_3$ heterostructures to reveal accurate tunability of the vertical location of the TSSs. We show that the TSSs float to the top of the ZnSe overlayer when its thickness is 1 or 2 monolayers, but are pushed down into the TI when the overlayer is thicker. We further investigate how the atop TSSs may serve as an electron bath in enhancing the catalytic activity of the ZnSe overlayer, using CO oxidation as a test case and paying particular attention to the effects of spin-orbit coupling.\footnote{H. Chen \textit{et al.}, Phys. Rev. Lett. \textbf{107}, 056804 (2011)} [Preview Abstract] |
Wednesday, March 15, 2017 3:54PM - 4:06PM |
P3.00008: Visualizing ferromagnetism in Fe-doped Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ topological insulator individual nanowires Wei Niu, Xuefeng Wang Time-reversal symmetry is broken by magnetic doping into the topological insulators, and an energy gap is opened thus generating massive surface carriers. Magnetically-doped Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ nanowires have been prepared by chemical vapor deposition technique with a high Curie temperature. The magnetic force microscope images still reveal the obvious ferromagnetic contrast signal of the Fe-doped Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ nanowires even at 40 K, indicating the direct evidence of visulizing ferromagnetism in magnetically-doped topological insulator nanowires. Through systematic magnetoresistance measurements on undoped and Fe-doped Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ nanowires, we observe the weak anti-localization effect in Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ nanowires indicating a strong spin-orbital coupling effect. Furthermore, we also observed a transition from weak anti-localization to weak localization behavior when magnetically-doping into topological insulator nanowires. Our results pave a potential way for future novel magnetoelectronic device applications. [Preview Abstract] |
Wednesday, March 15, 2017 4:06PM - 4:18PM |
P3.00009: Surface state-dominated photoconduction and THz-generation in topological Bi$_{2}$Te$_{2}$Se-nanowires Paul Seifert, Kristina Vaklinova, Klaus Kern, Marko Burghard, Alexander Holleitner Topological insulators constitute a fascinating class of quantum materials with non-trivial, gapless states on the surface and trivial, insulating bulk states. In revealing the optoelectronic dynamics in the whole range from femto- to microseconds, we demonstrate that the long surface lifetime of Bi$_{2}$Te$_{2}$Se-nanowires allows to access the surface states by a pulsed photoconduction scheme and that there is a prevailing bolometric response of the surface states. The interplay of the surface state dynamics on the different timescales gives rise to a surprising physical property of Bi$_{2}$Te$_{2}$Se-nanowires: their pulsed photoconductance changes polarity as a function of laser power. Moreover, we show that single Bi$_{2}$Te$_{2}$Se-nanowires can be used as THz-generators for on-chip high-frequency circuits at room temperature. Our results open the avenue for single Bi$_{2}$Te$_{2}$Se-nanowires as active modules in optoelectronic high-frequency and THz-circuits. [Preview Abstract] |
Wednesday, March 15, 2017 4:18PM - 4:30PM |
P3.00010: Edge states and topological insulating phases generated by curving a nanowire with Rashba spin-orbit coupling Carmine Ortix, Paola Gentile, Sudhakar Pandey, Mario Cuoco In this talk, I will discuss how curvature effects in low-dimensional nanomaterials can promote the generation of topological states of matter considering the paradigmatic example of quantum wires with Rashba spin-orbit coupling, which are bent in a nanoscale periodic serpentine structure. The effect of the periodic curvature generally results in the appearance of insulating phases with a corresponding novel butterfly spectrum characterized by the formation of finite measure complex regions of forbidden energies. When the Fermi energy lies in the gaps, the system displays localized end states protected by topology. These results suggest that the local curvature and the topology of the electronic states are inextricably intertwined in geometrically deformed nanomaterials. [Preview Abstract] |
Wednesday, March 15, 2017 4:30PM - 4:42PM |
P3.00011: Magnetotransport in 3D Topological Insulator Nanowires Raphael Kozlovsky, Cosimo Gorini, Klaus Richter We investigate the transport characteristics of nanowires in external electric and magnetic fields. In particular, we are interested in systems consisting of three-dimensional topological insulator (3D TI) materials, which we model by bulk and surface Hamiltonians. In such 3D TI nanowires, a magnetic field along the wire leads to prominent Aharonov-Bohm oscillations that indicate the surface nature of the Dirac states forming due to a non-trivial topological invariant. We investigate their transport properties with specific focus on wires with a non-constant radius along the wire direction giving rise to a spatial variation of the enclosed magnetic flux and implying novel quantum transport phenomena. [Preview Abstract] |
Wednesday, March 15, 2017 4:42PM - 4:54PM |
P3.00012: Transport signatures of conduction along domain walls in a quantum anomalous Hall insulator Ilan Rosen, Eli Fox, David Goldhaber-Gordon, Xufeng Kou, Lei Pan, Kang Wang The quantum anomalous hall system, which hosts robust chiral edge modes, has been realized in ferromagnetic topological insulators. Edge modes carry current along the boundary of a magnetic topological insulator film, and are expected to do the same along magnetic domain walls if the film has more than one domain. Magnetic domains are formed in a magnetic topological insulator as its magnetization is reversed by an external field, and may be tailored more intentionally by applying a spatially inhomogeneous field. We use transport measurements to study conduction along magnetic domain walls in Cr-(Bi,Sb)$_2$Te$_3$, motivated by the prospect of development of reconfigurable electrical connections in topological materials. [Preview Abstract] |
Wednesday, March 15, 2017 4:54PM - 5:06PM |
P3.00013: Topological Material-Based Spin Devices Minhao Zhang, Xuefeng Wang Three-dimensional topological insulators have insulating bulk and gapless helical surface states. One of the most fascinating properties of the metallic surface states is the spin-momentum helical locking. The giant current-driven torques on the magnetic layer have been discovered in TI/ferromagnet bilayers originating from the spin-momentum helical locking, enabling the efficient magnetization switching with a low current density.$^{\mathrm{1}}$. We demonstrated the current-direction dependent on-off state in TIs-based spin valve devices for memory and logic applications.$^{\mathrm{2}}$\textbf{.} Further, we demonstrated the Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ system will go from a topologically nontrivial state to a topologically trivial state when Bi atoms are replaced by lighter In atoms. Here, topologically trivial metal (Bi$_{\mathrm{x}}$In$_{\mathrm{y}})_{\mathrm{2}}$Se$_{\mathrm{3\thinspace }}$with high mobility also facilitates the realization of its application in multifunctional spintronic devices. .$^{\mathrm{1 }}$A. Mellnik, et al, Nature \textbf{511} (7510), 449 (2014). $^{\mathrm{2 }}$M. Zhang, et al, IEEE Electron Device Letters \textbf{37} (9), 1231 (2016). [Preview Abstract] |
Wednesday, March 15, 2017 5:06PM - 5:18PM |
P3.00014: Interplay between Spin-Orbit Torque and Unidirectional Magnetoresistance in Modulation-Doped Topological Insulators Yabin Fan, Qiming Shao, Lei Pan, Xiaoyu Che, Qinglin He, Gen Yin, Kang L. Wang We study the current-induced spin-orbit torque (SOT) and unidirectional magnetoresistance (UMR) in modulation-doped topological insulators (TIs). In the bilayer structures composed of TI and Cr-doped TI (Cr-TI, for short), both the SOT and UMR are very large with the values orders of magnitudes greater than those reported in traditional materials, such as the heavy metal-based magnetic structures or the doped diluted magnetic semiconductors. Furthermore, both the SOT and UMR are consistent with the current-induced spin polarization on the TI surface arising from the spin-momentum locking feature of the topological surface states. On the other hand, in the TI/Cr-TI/TI trilayer structures where the Cr-TI layer is in the middle, both the SOT and UMR values become significantly smaller. Through modulation doping and structure engineering, we reveal that the giant SOT and UMR in TI/Cr-TI bilayers originate in the topological spin-momentum locked Dirac Fermions on the TI surface, and they exhibit a strong correlation to each other. [Preview Abstract] |
Wednesday, March 15, 2017 5:18PM - 5:30PM |
P3.00015: Designing topological states, spin textures and spin interferometers by shape deformations Mario Cuoco, Zujian Ying, Paola Gentile, Carmine Ortix Low-dimensional semiconducting nanomaterials play a relevant role in the area of topological states of matter. However, apart from these conventional material geometries the most recent advances in nanotechnology have made it possible to have at hand an entirely novel family of low-dimensional nanostructures: flexible semiconductor nanomaterials which are bent into curved, deformable objects ranging from semiconductor nanotubes, to nanohelices. Motivated by the excitement in both topological states of matter and novel shape deformed nanostructures, we have theoretically considered the possible interplay between curvature effects on the electronic properties and the topological properties of the quantum states in low-dimensional nanomaterials. We present the intricate twist between spin texture and spin transport in shape deformed nanostructures. We show that non-uniform Rashba spin-orbit coupling in shape deformed nanowires leads to spin textures with a tunable topological character. These topologically non trivial spin patterns affect the electron spin interference in the deformed quantum ring, thereby resulting in different geometry-driven electronic transport behavior [1]. [1] Z.-J. Ying, P. Gentile, C. Ortix, and M. Cuoco, Phys. Rev. B 94, 081406(R) (2016). [Preview Abstract] |
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