Session K48: Spin Transport in Topological Insulators

Multi-terminal potentiometric spin signal measurement on channels with spin-momentum locking

We will discuss multi-terminal potentiometric measurements on channels with spin-momentum 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 multi-terminal spin valves on such channels [2] should show two distinct values of anti-parallel 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 multi-terminal measurements and has been experimentally observed recently on heavy metals [3]. We argue from Onsager reciprocity that 2-terminal 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.   

Spin-dependent photocurrent in topological insulator/magnetic insulator heterostructures

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 spin-dependent photocurrent~(PC) in heterostructures of~yttrium iron garnet (YIG) and 3D topological insulators. We find that the magnetic field-dependent PC maps out the magnetization state of the YIG layer, as confirmed~by a direct comparison with magneto-optical Kerr effect measurements. We gain insight into the phenomenon by studying the spin-dependent 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.   

Unidirectional spin Hall magnetoresistance in topological insulator/ferromagnetic layer heterostructures.

The surface states of topological insulators offer a potentially very efficient way to generate spins and spin-orbit torques to magnetic moments in proximity. The switching by spin-orbit 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.   

Magnetoanisotropic tunneling transport in topological insulators

We investigate the anisotropy of the tunneling transport with respect to the magnetization orientation of a magnetic barrier on a topological insulator surface. The spin-momentum 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. Matos-Abiague, B. Scharf, J. E. Han, and I. Zutic (unpublished). [2] A. G. Aronov and Y. Lyanda-Geller, 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. Matos-Abiague, J. E. Han, E. M. Hankiewicz, and I. Zutic, PRL 117, 166806 (2016).   

MOKE measurements of spin polarization in topological insulators

The Dirac surface states of a topological insulator (TI) exhibit spin-momentum locking, where an unpolarized charge current creates a net spin polarization whose amplitude and orientation are controlled by the charge current [1-3]. 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 in-plane 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)   

Spin-charge conversion at magnetic insulator/topological insulator interfaces

The development of next-generation spintronic devices has driven extensive studies of spin-charge conversion through measurements of the inverse spin Hall effect and/or the inverse Rashba–Edelstein effect in both three-dimensional and two-dimensional material systems. Topological insulators such as the Bi-chalcogenides are naturally relevant in this context due to the expected large spin-orbit coupling strength and the inherent spin-momentum “locking” in their surface states. We report the observation of robust ferromagnetic resonance-driven spin pumping signals in three-dimensional 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 spin-charge 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 spin-charge 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 spin-charge conversion mechanism in topological insulators and suggest the dominant role played by spin-momentum “locking” and spin-orbit 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 (C-SPIN), a funded center of STARnet, a Semiconductor Research Corporation (SRC) program sponsored by MARCO and DARPA.   

Direct comparison of current-induced spin polarization in topological insulators and InAs Rashba states.

3D topological insulators (TIs) exhibit time-reversal symmetry protected, linearly dispersing Dirac surface states. Band bending at the TI surface may also lead to coexisting trivial two-dimensional electron gas (2DEG) states with parabolic energy dispersion that exist as spin-split pairs due to Rashba spin-orbit coupling (SOC). A bias current is expected to generate spin polarization in both systems arising from their helical spin-momentum locking, but with different magnitude and sign.$^{1}$ Here, we compare spin potentiometric measurements of bias current-generated 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 current-generated 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).   

Tunneling planar Hall effect induced by Rashba states

We investigate the effects of Rashba spin-orbit coupling (SOC) on tunneling across a magnetic barrier deposited on top of a two-dimensional electron gas. By performing numerical simulations of both longitudinal and transverse transport in tunneling four-terminal 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. Matos-Abiague, J. E. Han, E. M. Hankiewicz, and I. Zutic, Phys. Rev. Lett. 117, 166806 (2016).   

Electronic properties of giant ferroelectric Rashba semiconductor BiSb: A first-principle study of bulk and monolayer

We investigate the electronic properties of layered BiSb compound in bulk and two-dimensions. Our first-principle calculations reveal that BiSb is a ferroelectric Rashba semiconductor that inherits large Rashba effect due to the presence of strong spin-orbit interactions and broken inversion-symmetry 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 spin-splitting can be effectively tuned by applying an external stress or bi-axial strain. Interestingly, a novel Weyl semimetallic phase emerges in the bulk BiSb when the external applied pressure is in the 4.0-6.0 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, 29771-29785 (2016) [2] Sobhit Singh et al., \textit{Phys. Rev. B} 94, 161116(R) (2016)   

Nonreciprocal electrical transport phenomena in Rashba system

Nonreciprocal response is a consequence of the inversion symmetry breaking where lots of physical responses have directivity. This is essentially a non-linear response like a circular dichroism and second harmonic generation in non-linear 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 Rashba-type band structure. The measured nonreciprocity for this material is quantitatively reproduced by simple model; single relaxation time Boltzmann equation for Rashba Hamiltonian with in-plane Zeeman field. In this presentation, we explain mainly about the theoretical model and the analysis of the nonreciprocal electrical transport.   

Observation of nonreciprocal electric transport in bulk Rashba system

BiTeBr is a bulk polar semiconductor, in which Rashba-type 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 spin-polarization 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 Rashba-type band structure. Observed nonreciprocal resistance can be quantitatively explained by the theoretical calculation of nonlinear electric response considering the giant Rashba spin-orbit coupling, offering a simple electrical means to estimate the spin-orbit parameter in noncentrosymmetric systems.   

Skyrmion lattices and topological insulators in ordinary noninteracting 2DEGs

Electrons in two-subband quantum wells are subject to an intersubband spin-orbit 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 non-interacting 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 III-V quantum wells. This opens up the unique possibility to investigate topological phenomena such as the skyrmion Hall effect in garden-variety type III-V 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).