Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session K48: Spin Transport in Topological InsulatorsFocus

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Sponsoring Units: GMAG DMP DCOMP FIAP Chair: Nitin Samarth, Pennsylvania State University Room: 395 
Wednesday, March 15, 2017 8:00AM  8:12AM 
K48.00001: Multiterminal potentiometric spin signal measurement on channels with spinmomentum locking Shehrin Sayed, Seokmin Hong, Supriyo Datta We will discuss multiterminal potentiometric measurements on channels with spinmomentum locking (SML) e.g. topological insulator, Rashba interface, heavy metals etc. Using these results in conjunction with the Onsager reciprocity relation [1], we argue that multiterminal spin valves on such channels [2] should show two distinct values of antiparallel resistance $R_{AP}^{(1)}$ and $R_{AP}^{(2)}$ such that $R_{AP}^{(1)}>R_P>R_{AP}^{(2)}$ depending on the direction of spin flow relative to SML ($R_P$: parallel resistance). This remarkable signature originating from SML can only be observed in multiterminal measurements and has been experimentally observed recently on heavy metals [3]. We argue from Onsager reciprocity that 2terminal measurement will only show the usual result $R_{AP}^{(1)}=R_{AP}^{(2)}>R_P$. We present numerical results using a detailed semiclassical model which uses four electrochemical potentials depending on group velocity ($+$ or $$) and spin polarization (up or down) of the channel electronic states. Finally, we propose novel spintronic applications based on the potentiometric measurement. [1] Jacquod et al., Phys. Rev. B 86, 155118, 2012. [2] Sayed et al., Sci. Rep. 6, 35658, 2016. [3] Pham et al., Nano Lett., DOI:10.1021/acs.nanolett.6b02334, 2016. [Preview Abstract] 
Wednesday, March 15, 2017 8:12AM  8:24AM 
K48.00002: Spindependent photocurrent in topological insulator/magnetic insulator heterostructures Yu Pan, Timothy Pillsbury, Yunqiu (Kelly) Luo, James Kally, Hailong Wang, Anthony Richardella, Tao Liu, Mingzhong Wu, Roland Kawakami, Nitin Samarth The emerging field of 'topological spintronics' relies on interfacing the helical Dirac surface states of topological insulators (TIs) with magnetism. Heterostructures that combine TIs with insulating magnetic~materials are particularly relevant within this context. Here, we describe the discovery of a spindependent photocurrent~(PC) in heterostructures of~yttrium iron garnet (YIG) and 3D topological insulators. We find that the magnetic fielddependent PC maps out the magnetization state of the YIG layer, as confirmed~by a direct comparison with magnetooptical Kerr effect measurements. We gain insight into the phenomenon by studying the spindependent PC as a function of the chemical potential of the TI film, as well as by examining its variation with the temperature and the wavelength of the optical excitation. [Preview Abstract] 
Wednesday, March 15, 2017 8:24AM  8:36AM 
K48.00003: Unidirectional spin Hall magnetoresistance in topological insulator/ferromagnetic layer heterostructures. James Kally, Yang Lv, Delin Zhang, Joon Sue Lee, Nitin Samarth, JianPing Wang The surface states of topological insulators offer a potentially very efficient way to generate spins and spinorbit torques to magnetic moments in proximity. The switching by spinorbit torque itself only requires two terminals so that a charge current can be applied. However, a third terminal with additional magnetic tunneling junction structure is needed to sense the magnetization state if such devices are used for memory and logic applications. The recent discovery of unidirectional spin Hall magnetoresistance in heavy metal/ferromagnetic and topological insulator/magnetically doped topological insulator systems offers an alternative way to sense magnetization while still keeping the number of terminals to minimal two. The unidirectional spin Hall magnetoresistance in topological insulator/strong ferromagnetic layer heterostructure system has yet not been reported. In this work, we report our experimental observations of such magnetoresistance. It is found to be present and comparable to the best result of the previous reported Ta/Co systems in terms of magnetoresistance per current density per total resistance. [Preview Abstract] 
Wednesday, March 15, 2017 8:36AM  8:48AM 
K48.00004: Magnetoanisotropic tunneling transport in topological insulators Alex Matos Abiague, Benedikt Scharf, Jong E. Han, Igor Zutic We investigate the anisotropy of the tunneling transport with respect to the magnetization orientation of a magnetic barrier on a topological insulator surface. The spinmomentum locking of the topological surface states lead to large changes in the magnetoresistance (MR) when the magnetization is rotated in the plane of the surface. In contrast to the small tunneling anisotropic MR (TAMR) expected for topologically trivial Rashba states [1], the large values of TAMR predicted here suggest that the Edelstein effect [2] leads to a highly efficient spin polarization of the topological states [3]. The Hall voltage resulting from the tunneling planar Hall effect [4] also exhibits a strong magnetoanisotropy. Due to resonant effects inherent to Klein tunneling, the Hall voltage changes sign not only under magnetization reversal, but also when the magnetization orientation is slightly shifted around certain directions.\newline [1] T. Leeney, C. Shen, A. MatosAbiague, B. Scharf, J. E. Han, and I. Zutic (unpublished). [2] A. G. Aronov and Y. LyandaGeller, JETP Lett. 50, 431 (1989); V. Edelstein, Solid State Commun. 73, 233 (1990). [3] C. H. Li et al., arXiv:1605.07155 (2016). [4] B. Scharf, A. MatosAbiague, J. E. Han, E. M. Hankiewicz, and I. Zutic, PRL 117, 166806 (2016). [Preview Abstract] 
Wednesday, March 15, 2017 8:48AM  9:00AM 
K48.00005: MOKE measurements of spin polarization in topological insulators Berend Jonker, Olaf van 't Erve, S. Rajput, Lian Li, Connie Li The Dirac surface states of a topological insulator (TI) exhibit spinmomentum locking, where an unpolarized charge current creates a net spin polarization whose amplitude and orientation are controlled by the charge current [13]. This polarization has been detected electrically using a magnetic contact. Here we use the magneto optic Kerr effect (MOKE) to detect this spin polarization optically. We deposit a 10nm layer of Al on a Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ TI film and measure the Kerr rotation produced by an unpolarized bias current. Aluminum was chosen because it is a good spin diffusion layer and is optically opaque, making the MOKE measurement more sensitive to the spins generated near the top surface of the TI film. Modulating the charge current through the TI produces a corresponding response in the MOKE signal. We also show that spin diffusion from the TI into an Fe surface contact can rotate the Fe magnetization by spin transfer torque [4]. We apply a constant bias field of 32 Oe and modulate the inplane bias current while measuring the Kerr rotation of the Fe contact. A clear correlation between charge current and Kerr rotation is observed that cannot be explained by simple Oersted fields arising from the charge current, indicating that the spin transfer torque from Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ into Fe is responsible. [1] C.H. Li et al, \textit{Nature Nanotech}. 9, 218 (2014) [2] J.S. Lee et al, \textit{Phys. Rev. B} 92, 155312 (2015) [3] C.H. Li et al, arXiv:1605.07155v1; \textit{Nature Commun}. (Nov. 2016) [4] A. R. Mellnik, et al. \textit{Nature} 511, 449 (2014) [Preview Abstract] 
Wednesday, March 15, 2017 9:00AM  9:36AM 
K48.00006: Spincharge conversion at magnetic insulator/topological insulator interfaces Invited Speaker: Hailong Wang The development of nextgeneration spintronic devices has driven extensive studies of spincharge conversion through measurements of the inverse spin Hall effect and/or the inverse Rashba–Edelstein effect in both threedimensional and twodimensional material systems. Topological insulators such as the Bichalcogenides are naturally relevant in this context due to the expected large spinorbit coupling strength and the inherent spinmomentum “locking” in their surface states. We report the observation of robust ferromagnetic resonancedriven spin pumping signals in threedimensional topological insulator thin films  Bi$_2$Se$_3$ and (Bi, Sb)$_2$Te$_3$  deposited by molecular beam epitaxy on the ferrimagnetic insulator Y$_3$Fe$_5$O$_{12}$. By systematically varying the Bi$_2$Se$_3$ film thickness, we show that the spincharge conversion efficiency, characterized by the inverse Rashba–Edelstein effect length, increases dramatically as the film thickness is increased from two quintuple layers, saturating above six quintuple layers [Phys. Rev. Lett. 117, 076601 (2016)]. For bulk insulating (Bi, Sb)$_2$Te$_3$ thin films, by electrical gating and varying the chemical compositions, we demonstrate that the spincharge conversion efficiency follows a constant value when the Fermi level lies within the bulk band gap and shows opposite variation trends when Fermi level enters the conduction and valance bands. Our results uncover the spincharge conversion mechanism in topological insulators and suggest the dominant role played by spinmomentum “locking” and spinorbit coupling for surface and bulk states respectively. This work was carried out in collaboration with James Kally, Joon Sue Lee, Tao Liu, Houchen Chang, Danielle Reifsnyder Hickey, K. Andre Mkhoyan, Mingzhong Wu, Anthony Richardella, and Nitin Samarth. We acknowledge support from the Center for Spintronic Materials, Interfaces, and Novel Architectures (CSPIN), a funded center of STARnet, a Semiconductor Research Corporation (SRC) program sponsored by MARCO and DARPA. [Preview Abstract] 
Wednesday, March 15, 2017 9:36AM  9:48AM 
K48.00007: Direct comparison of currentinduced spin polarization in topological insulators and InAs Rashba states. Connie Li, Olaf van 't Erve, Shivani Rajput, Lian Li, Berry Jonker 3D topological insulators (TIs) exhibit timereversal symmetry protected, linearly dispersing Dirac surface states. Band bending at the TI surface may also lead to coexisting trivial twodimensional electron gas (2DEG) states with parabolic energy dispersion that exist as spinsplit pairs due to Rashba spinorbit coupling (SOC). A bias current is expected to generate spin polarization in both systems arising from their helical spinmomentum locking, but with different magnitude and sign.$^{1}$ Here, we compare spin potentiometric measurements of bias currentgenerated spin in Bi$_{2}$Se$_{3}$(111) films where Dirac surface states coexist with trivial 2DEG states, and InAs(001) where only trivial 2DEG states are present.$^{2,3}$ We observe spin polarization in both cases, with opposite signs of the spin voltage for the TI and InAs. We present a model based on spin dependent electrochemical potentials to directly derive the signs expected for the TI surface states, and show that the currentgenerated spin measured in TI is dominated by Dirac surface states. This direct electrical access of the helical spin texture of Dirac and Rashba 2DEG states is an enabling step towards the electrical manipulation of spins in next generation TI and SOC based quantum devices. 1. S. Hong et al., PRB \textbf{86}, 085131 (2012). 2. C. H. Li et al., \textit{Nature Nanotech}. \textbf{9, }218 (2014). 3. C. H. Li et al., \textit{Nat. Commun.}, \textit{in press} (2016). [Preview Abstract] 
Wednesday, March 15, 2017 9:48AM  10:00AM 
K48.00008: Tunneling planar Hall effect induced by Rashba states Timothy Leeney, Chenghao Shen, Alex MatosAbiague, Benedikt Scharf, Jong E. Han, Igor Zutic We investigate the effects of Rashba spinorbit coupling (SOC) on tunneling across a magnetic barrier deposited on top of a twodimensional electron gas. By performing numerical simulations of both longitudinal and transverse transport in tunneling fourterminal devices we show that the interplay between magnetism and SOC results in a sizable tunneling anisotropic magnetoresistance. The numerical calculations reveal that although considerable smaller, the recently proposed tunneling planar Hall effect [1] is not exclusive to topological insulators but can also emerge from topologically trivial Rashba states. Complementary model calculations are performed for a better physical understanding of the main trend observed in the numerical results. \newline [1] B. Scharf, A. MatosAbiague, J. E. Han, E. M. Hankiewicz, and I. Zutic, Phys. Rev. Lett. 117, 166806 (2016). [Preview Abstract] 
Wednesday, March 15, 2017 10:00AM  10:12AM 
K48.00009: Electronic properties of giant ferroelectric Rashba semiconductor BiSb: A firstprinciple study of bulk and monolayer Sobhit Singh, A. C. GarciaCastro, I. Valencia Jaime, W. IbarraHernandez, A. H. Romero, F. Munoz We investigate the electronic properties of layered BiSb compound in bulk and twodimensions. Our firstprinciple calculations reveal that BiSb is a ferroelectric Rashba semiconductor that inherits large Rashba effect due to the presence of strong spinorbit interactions and broken inversionsymmetry of the crystal. The theoretical maximum value of the Rashba energy (E$_{R})$ and Rashba constant ($\alpha_{R})$ in bulk BiSb is 147.3 meV and 10.43 eV{\AA}, respectively [1]. We notice that the strength of the Rashba spinsplitting can be effectively tuned by applying an external stress or biaxial strain. Interestingly, a novel Weyl semimetallic phase emerges in the bulk BiSb when the external applied pressure is in the 4.06.0 GPa range [2]. This Weyl semimetallic phase can be efficiently harnessed by gaining control over the ferroelectric polarization of the bulk BiSb [2]. We further study the electronic and vibrational properties of the BiSb monolayer and BiSb/BN heterostructure. Our calculations suggest that BiSb monolayer and BiSb/BN heterostructure systems are thermodynamically stable and exhibit intriguing electronic properties. [1] Sobhit Singh et al., \textit{Phys. Chem. Chem. Phys. }18, 2977129785 (2016) [2] Sobhit Singh et al., \textit{Phys. Rev. B} 94, 161116(R) (2016) [Preview Abstract] 
Wednesday, March 15, 2017 10:12AM  10:24AM 
K48.00010: Nonreciprocal electrical transport phenomena in Rashba system Keita Hamamoto, Toshiya Ideue, Shota Koshikawa, Motohiko Ezawa, Sunao Shimizu, Yoshio Kaneko, Yoshinori Tokura, Naoto Nagaosa, Yoshinori Iwasa Nonreciprocal response is a consequence of the inversion symmetry breaking where lots of physical responses have directivity. This is essentially a nonlinear response like a circular dichroism and second harmonic generation in nonlinear optics. The electrical resistivity, which is the most fundamental physical property of materials, also shows the nonreciprocity; the resistivity depends on the direction of the current. In this study, we have investigated the nonreciprocal electrical transport in polar semiconductor BiTeBr which has simple Rashbatype band structure. The measured nonreciprocity for this material is quantitatively reproduced by simple model; single relaxation time Boltzmann equation for Rashba Hamiltonian with inplane Zeeman field. In this presentation, we explain mainly about the theoretical model and the analysis of the nonreciprocal electrical transport. [Preview Abstract] 
Wednesday, March 15, 2017 10:24AM  10:36AM 
K48.00011: Observation of nonreciprocal electric transport in bulk Rashba system Koshikawa Shota, Toshiya Ideue, Keita Hamamoto, Motohiko Ezawa, Sunao Shimizu, Yoshio Kaneko, Yoshinori Tokura, Naoto Nagaosa, Yoshihiro Iwasa BiTeBr is a bulk polar semiconductor, in which Rashbatype band structure confirmed by the angle resolved photoemission spectroscopy and quantum oscillations reflecting the split Fermi surface have been reported. However, characteristic transport originating from the spinpolarization of electronic band or polarity of the crystal has been elusive except for the photocurrent experiments. Here, we report the nonreciprocal electric transport, one of the manifestations of spin polarization in Rashbatype band structure. Observed nonreciprocal resistance can be quantitatively explained by the theoretical calculation of nonlinear electric response considering the giant Rashba spinorbit coupling, offering a simple electrical means to estimate the spinorbit parameter in noncentrosymmetric systems. [Preview Abstract] 
Wednesday, March 15, 2017 10:36AM  10:48AM 
K48.00012: Skyrmion lattices and topological insulators in ordinary noninteracting 2DEGs Jiyong Fu, Poliana Penteado, Marco O. Hachiya, J. Carlos Egues, Daniel Loss Electrons in twosubband quantum wells are subject to an intersubband spinorbit coupling [1] that can lead to interesting physical phenomena such as a giant intrinsic spin Hall effect [2] and topological insulator behavior [3]. When the competing Rashba and Dresselhaus couplings are considered, we find that skyrmionic excitations are possible in these ordinary noninteracting electron systems [4]. These excitations can be probed/imaged via transient spin grating experiments and Kerr rotation spectroscopy with available experimental techniques [5]. Here we will discuss how topological spin textures and topological insulating behavior can occur in ordinary IIIV quantum wells. This opens up the unique possibility to investigate topological phenomena such as the skyrmion Hall effect in gardenvariety type IIIV system. [1] Bernardes et al. Phys Rev. Lett. \textbf{99}, 076603 (2007). [2] Khaetskii and Egues, arXiv:1602.00026. [3] Erlingsson and Egues Phys. Rev. B \textbf{91}, 035312 (2015). [4] Fu, Penteado, Hachiya, Loss, and Egues, Phys. Rev. Lett., in press. [5] Koralek et al., Nature \textbf{458}, 610 (2009); Walser et al., Nat. Phys. \textbf{8}, 757 (2012). [Preview Abstract] 
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