Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session T8: Focus Session: Spin-Dependent Phenomena in Semiconductors: Spin Hall Effect and Topological Insulators |
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Sponsoring Units: GMAG DMP FIAP Chair: Enrique Cobas, Naval Research Laboratory Room: 104 |
Thursday, March 6, 2014 11:15AM - 11:51AM |
T8.00001: Optical inverse spin-Hall effect in semiconductors and metal/semiconductor junctions Invited Speaker: Federico Bottegoni III-V and group-IV semiconductors lie at the cutting edge of spintronics due to their large spin-orbit interaction (SOI) and electron spin lifetimes. The spin transport and dynamics in metal/semiconductor junctions can be deeply investigated through the inverse spin-Hall effect, where a spin current, injected into the semiconductor, is converted into a transverse electromotive field at the edges of the non-magnetic metal layer. In this context, we studied the properties of optically injected spin currents in a Pt/Ge and a Pt/GaAs junction under diffusive regime at room temperature, as a function of the initial electron spin polarization, generation depth and doping of the structures. Moreover, considering heavily Si-doped bulk GaAs, we exploited the optical extrinsic inverse spin-Hall effect to experimentally evaluate the spin-Hall conductivity of the system at room temperature. [Preview Abstract] |
Thursday, March 6, 2014 11:51AM - 12:03PM |
T8.00002: Spin pumping and inverse spin Hall effect in germanium Juan-Carlos Rojas Sanchez, C. Vergnaud, L. Vila, J-P Attane, A. Marty, Henri Jaffres, Matthieu Jamet, Jean-Marie George We have measured the inverse spin Hall effect (ISHE) in n-Ge at room temperature. The spin current in germanium was generated by spin pumping from a CoFeB/MgO magnetic tunnel junction in order to prevent the impedance mismatch issue. A clear electromotive force was measured in Ge at the ferromagnetic resonance of CFB. The same study was then carried out on several test samples, in particular, we have investigated the influence of the MgO tunnel barrier and sample annealing on the ISHE signal. The reference CFB/MgO bilayer grown on SiO$_2$ exhibits a clear electromotive force due to anisotropic magnetoresistance and anomalous Hall effect, which is dominated by an asymmetric contribution with respect to the resonance field. We also found that the MgO tunnel barrier is essential to observe ISHE in Ge and that sample annealing systematically leads to an increase of the signal. We propose a theoretical model based on the presence of localized states at the interface to account for these observations. Finally, all of our results are fully consistent with the observation of ISHE in heavily doped n-Ge with a spin Hall angle around 0.001.\\[4pt] Rojas S\'{a}nchez et al. Phys. Rev. B 88, 064403 (2013) [Preview Abstract] |
Thursday, March 6, 2014 12:03PM - 12:15PM |
T8.00003: Hybrid Spin Noise Spectroscopy and the Spin Hall Effect Valeriy Slipko, Nikolai Sinitsyn, Yuriy Pershin The spin noise spectroscopy (SNS) is an emergent experimental technique that has been used to measure spin-related parameters of different materials and systems. In a typical semiconductor material, such as GaAs, the spin Hall effect is relatively weak. It cannot appreciably modify spin fluctuations observed by the standard optical SNS setup. In order to overcome this difficulty, we suggest a method of hybrid spin noise spectroscopy, which is based on a simultaneous analysis of spin and transverse voltage fluctuations [1]. By using this method, one can experimentally determine spin-transverse voltage and transverse voltage-transverse voltage correlation functions which are sensitive to the spin Hall coefficient. Opposite to the conventional Hall effect, the spin Hall effect in homogeneous systems is not accompanied by any transverse voltage on average. However, as we demonstrated, in the spin Hall regime the spin fluctuations are dressed by charge dipoles that are responsible for the transverse voltage fluctuations. Therefore, the transverse voltage fluctuations correlate with the spin fluctuations and their strength is proportional to the spin Hall coefficient. We anticipate that the proposed method find applications in studies of spin-charge coupling in semiconductors and other materials. \\[4pt] [1] V. A. Slipko,~N. A. Sinitsyn, and~Y. V. Pershin, Phys. Rev. B 88, 201102(R) (2013). [Preview Abstract] |
Thursday, March 6, 2014 12:15PM - 12:27PM |
T8.00004: Collective Spin-Hall Effect for Electron-Hole Gratings Giovanni Vignale, Ka Shen We study the coupled spin-density transport in a periodically modulated electron gas in a GaAs quantum well. We show that an electric field parallel to the wavefronts of an electron-hole grating generates, via the electronic spin Hall effect, a spin grating of the same wave vector. We refer to this phenomenon as ``collective spin Hall effect". In our calculation, we include not only the intrinsic but also the extrinsic spin Hall mechanisms. In the extrinsic mechanism we include both skew scattering and side jump. A detailed study of the coupled-spin charge dynamics for quantum wells grown in different directions reveals rich features in the time evolution of the induced spin density. For example, in the symmetric (110) quantum well the amplitude of the induced spin density is controlled solely by skew scattering and can be as large as 1\% of that of the initial density modulation.Similarly, the collective spin Hall effect in (001) QWs with identical Rashba and Dresselhaus SOC strengths is also entirely controlled by skew scattering. In this case, the skew scattering generates a spiral spin density wave when the wave vector of the initial grating matches the wave vector of the spin-orbit coupling. Ref: Ka Shen and G. Vignale, PRL 111, 136602 (2013). [Preview Abstract] |
Thursday, March 6, 2014 12:27PM - 12:39PM |
T8.00005: Spin Hall effect in spin-valley coupled monolayers of transition metal dichalcogenides Wenyu Shan, Haizhou Lu, Di Xiao We study both the intrinsic and extrinsic spin Hall effect in spin-valley coupled monolayers of transition metal dichalcogenides. We find that whereas the skew-scattering contribution is suppressed by the large band gap, the side-jump contribution is comparable to the intrinsic one with opposite sign in the presence of scalar and magnetic scattering. Intervalley scattering tends to suppress the side-jump contribution due to the loss of coherence. By tuning the ratio of intra- to intervalley scattering, the spin Hall conductivity shows a sign change in hole-doped samples. The multiband effect in other doping regimes is considered, and it is found that the sign change exists in the heavily hole-doped regime, but not in the electron-doped regime. [Preview Abstract] |
Thursday, March 6, 2014 12:39PM - 12:51PM |
T8.00006: Large Intrinsic Spin Hall Conductivity in Bismuth, Antimony and Bi$_{1-x}$Sb$_{x}$ Alloys Cuneyt Sahin, Michael E. Flatt\'e Bismuth and antimony, which are building blocks of 3 dimensional topological insulators, are expected to exhibit a large spin Hall conductivity due to their large spin-orbit couplings. Furthermore the semimetal characteristics of these materials that originate from slightly overlapping conduction and valence bands can be altered by opening a gap through alloying them up to certain concentration. This so called semi-metal semiconductor transition also allows Bi$_{1-x}$Sb$_x$ alloy to exhibit topologically protected states [1]. In this work we use a low-energy effective spin-orbit Hamiltonian within a tight-binding approach for Bi and Sb as well as Bi$_{1-x}$Sb$_x$ alloys. Beginning with this low-energy Hamiltonian and band structure we calculate the intrinsic spin Hall conductivity using a Berry's curvature technique in the clean static limit. We have also investigated the behavior of the Berry's curvature in a full zone picture and observed that several symmetry points contribute largely to the SHC due to extreme curvature. Robust spin-orbit couplings and Berry curvatures in bulk Bi, Sb and Bi$_{1-x}$Sb$_x$ alloys result in SHC which is comparable to platinum and considerably larger than conventional semiconductors and metals.\\[4pt] [1] Zhang et al., Nature Physics 5, 438, (2009) [Preview Abstract] |
Thursday, March 6, 2014 12:51PM - 1:03PM |
T8.00007: Investigation of Spin Pumping in Fe$_{3}$Si/GaAs and Fe$_{3}$Si/Bi$_{2}$Se$_{3}$ Bilayer Structure Hung-Yi Hung, Hsiao-Yu Lin, Jueinai Kwo, Tsung-Hung Chiang, Jauyn G. Lin, Shang Fan Lee, Bei Zhen Syu, Minghwei Hong Spin pumping, a dynamical spin-injection method to generate a pure spin current by magnetization precession, can be used to conduct the spin injection into a wide range of materials. Here we report the spin pumping experiment by utilizing epitaxial ferromagnetic Fe$_{3}$Si thin films interfaced with GaAs for spin injection into semiconductor, and interfaced with Bi$_{2}$Se$_{3}$ for exploitation of topological insulator (TI) edge or surface states at the TI/ferromagnet (FM) interfaces. An inverse spin Hall effect voltage as large as 49 $\mu$V, and 19 $\mu$V was detected in Fe$_{3}$Si/p-GaAs, and Fe$_{3}$Si/n-GaAs structures, respectively, under a microwave power of 100 mW. Our analysis showed that the spin injection efficiency is affected by the Schottky barrier height of Fe$_{3}$Si/(n- or p-) GaAs interface, and so is the spin mixing conductance. As for the TI/FM structure, an out of plane spin transfer torque is generated due to current driven spin accumulations. Spin pumping in Fe$_{3}$Si/Bi$_{2}$Se$_{3}$ structure using Pt electrodes has been carried out to elucidate spins diffusion process from Fe$_{3}$Si via Bi$_{2}$Se$_{3}$ into Pt, and will be reported. [Preview Abstract] |
Thursday, March 6, 2014 1:03PM - 1:15PM |
T8.00008: Optical spectroscopy and Fermi surface studies of BiTeCl and BiTeBr Catalin Martin, A.V. Suslov, S. Buvaev, A.F. Hebard, Philippe Bugnon, Helmuth Berger, Arnaud Magrez, D.B. Tanner The observation of a large bulk Rashba effect in the non-centrosymmetric semiconductors BiTeX(X=Cl, Br, I) has stimulated the interest in these sys- tems, as promising candidates for studying spin related phenomena and for the realization of spin devices. Here we present a comparative study of the electronic properties of BiTeCl and BiTeBr, determined from temperature dependent infrared spectroscopy and Shubnikov-de Haas oscillations. In par- ticular, we compare the angle dependence of quantum oscillations between the two compounds and discuss possible differences between the topology of their Fermi surfaces. [Preview Abstract] |
Thursday, March 6, 2014 1:15PM - 1:27PM |
T8.00009: Large bulk Rashba-type spin splitting in copper-doped noncentrosymmetric BiTeI Wei-Li Lee, Chang-Ran Wang, Jen-Chuan Tung, R. Sankar, Chia-Tso Hsieh, Yung-Yu Chien, Guang-Yu Guo, F.C. Chou BiTeI exhibits large Rashba spin splitting due to its noncentrosymmetric crystal structure. The study of the chemical doping effect is important in order to either tune the Fermi level or refine the crystal quality. Here, we report the magnetotransport measurement in high quality BiTeI single crystals with different copper dopings. We found that a small amount of copper doping improves the crystal quality significantly, which is supported by the transport data showing higher Hall mobility and larger amplitude in Shubnikov-de Haas oscillation at low temperature. Two distinct frequencies in Shubnikov-de Haas oscillation were observed, giving extremal Fermi surface areas of $A_S$ = 9.1 $\times$ 10$^{12}$ cm$^{-2}$ and $A_L$ = 3.47 $\times$ 10$^{14}$ cm$^{-2}$ with corresponding cyclotron masses $m^*_s$= 0.0353 $m_e$ and $m^*_L$= 0.178 $m_e$, respectively. Those results are further compared with relativistic band structure calculations using three reported Te and I positions. Our analysis infers the crucial role of Bi-Te bond length in the observed large bulk Rashba-type spin-splitting effect in BiTeI. [Preview Abstract] |
Thursday, March 6, 2014 1:27PM - 1:39PM |
T8.00010: Transport properties of polar semiconductor BiTeI under pressure Toshiya Ideue, Joseph Checkelsky, Hiroshi Murakawa, Saeed Bahramy, Yoshio Kaneko, Naoto Nagaosa, Yoshinori Tokura BiTeI is a polar semiconductor in which a atrong Rashba type spin orbit interaction causes spin splitting of the electronic band. Recently, emergent transport properties arising from this band structure have been theoretically predicted and experimentally explored. We have studied transport properties of BiTeI under the application of hydrostatic pressure. Magnetoresistivity shows Shubnikov-de Haas oscillations with two different periods, reflecting the inner Fermi surface and outer Fermi surface of the Rashba type band structure. With the application of pressure, both oscillation periods change, while the Hall effect and associated carrier number remain unchanged. As the period of SdH oscillations corresponds to the area of Fermi surface, we interpret this in terms of a pressure induced band deformation that alters the relative position of Fermi level and Dirac point of the Rashba type band structure. We will also report a comparative study of the Hall and Nernst effect in BiTeI. The Nernst effect exhibits a sign change around the Dirac point and is extremely sensitive to the Fermi level, whereas the Hall effect is electron-like and linear in magnetic field in all samples. We discuss possible mechanisms of the anomalous behavior of the Nernst effect. [Preview Abstract] |
Thursday, March 6, 2014 1:39PM - 1:51PM |
T8.00011: Field-Effect Birefringent Spin Lens in Ultrathin Film of Magnetically Doped Topological Insulators Peizhe Tang, Lu Zhao, Bing-Lin Gu, Wenhui Duan We investigate the low-energy electron dynamics in two-dimensional ultrathin film of magnetically doped topological insulators in the context of gate-tuned coherent spin manipulation. Our first-principles calculations for such film unambiguously identify its spin-resolved topological band structure arising from spin-orbit coupling and time-reversal symmetry breaking. Exploiting this characteristic, we predict a negative birefraction for chiral electron tunneling through a gate-controlled p-n interface in the film, analogous to optical birefringence. By fine-tuning the gate voltage, a series of unusual phenomena, including electron double focusing, spatial modulation of spin polarizations, and quantum-interferenceinduced beating patterns, could be efficiently implemented, offering a powerful platform to establish spin-resolved electron optics by all-electrical means. L. Z. and P. T. contributed equally to this work. We acknowledge support from the National Natural Science Foundation of China (Grants No. 11204154 and No. 11074139) and the Ministry of Science and Technology of China (Grants No. 2011CB606405 and No. 2011CB921901).\\[4pt] [1] L. Zhao; P. Tang, B.-L. Gu, W. Duan; PRL 111, 116601 (2013). [Preview Abstract] |
Thursday, March 6, 2014 1:51PM - 2:03PM |
T8.00012: Tunable exotic Kondo effect on topological insulator surfaces Leonid Isaev, Gerardo Ortiz, Ilya Vekhter We study the fate of a spin-$1/2$ impurity on the surface of a $3D$ topological insulator (TI). Within a simple model, we derive an effective Hamiltonian which governs coupling of surface states to the impurity and show that Kondo screening of the local moment strongly depends on details of the bulk band structure and on specific surface properties. The Kondo exchange interaction has an $XXZ$ form whose anisotropy can be tuned by changing parameters in the boundary conditions for the electron wavefunction at the TI surface. We determine the phase diagram of the resulting pseudogap Kondo impurity model as a function of these parameters. Our conclusions can be tested in the recently discovered TIs ${\rm Pb}_{1-x}{\rm Sn}_x({\rm Se,Te})$. Moreover, we argue that magnetic impurities can be used as an experimental probe to discriminate between topological and band insulators, and that TIs serve as a ``lab'' to realize exotic Kondo physics. [Preview Abstract] |
Thursday, March 6, 2014 2:03PM - 2:15PM |
T8.00013: Implementing Majorana fermions in quantum wires with periodic Zeeman fields Xiaoyu Zhu, Wei Chen, Rui Shen, Dingyu Xing We introduce a category of periodic Zeeman field and apply it to 1-D quantum wire placed on an s-wave superconductor substrate. By decomposing the field into two counter-propagating spiral fields, we argue that each spiral component corresponds to a separate topological non-trivial region where Majorana fermions emerge. As a result, Majoranas exhibit reentrance behavior with the increase of chemical potential. The position of non-trivial regions in phase diagram can be adjusted through modulating Rashba amplitude and periods of Zeeman fields. Furthermore, we find that different non-trivial regions determined by the two spiral components begin to overlap when Zeeman fields increase to a certain point, with the overlapping area supporting fractional fermions instead of Majoranas. In the end, we study the spin texture of Majorana zero mode bound states and demonstrate that local spin polarization depends strongly on phases of Zeeman fields as well as on chemical potential, suggesting a feasible way to modulate Majorana spin. [Preview Abstract] |
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