Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Z31: Focus Session: Spin-Dependent Phenomena in Semiconductors: Topological Insulators and Spin Hall Effect |
Hide Abstracts |
Sponsoring Units: GMAG DMP FIAP Chair: Pengke Li, University of Maryland, College Park Room: 207A |
Friday, March 6, 2015 11:15AM - 11:51AM |
Z31.00001: Tuning Dirac states at grain boundaries in the topological insulator Bi2Se3 Invited Speaker: Lian Li Symmetry protected Dirac states have been experimentally observed in topological insulator (TI) bismuth chalcogenides. Recently, we have further demonstrated direct electrical generation and detection of spin accumulation induced by spin-momentum locking of Dirac surface states in Bi2Se3, a critical step forward towards future electronic and spintronic applications. In this talk, I will give an overview of the opportunities and challenges in the epitaxial growth of these layered TIs that exhibit a strong (covalent) intra-layer bonding and weak (van der Waals) inter-layer bonding. Using Bi2Se3 as an example, I will show that this characteristic anisotropic bonding facilitates a spiral growth mode on virtually any substrates by molecular beam epitaxy [2]. The coalescence of these spirals results in a high density of grain boundaries (GBs) [3,4]. Using scanning tunneling and transmission electron microscopies, and density functional theory calculations, I will further show that near the zero-angle GBs (i.e., anti-phase domain boundaries), caused by vertical shifts of a fraction of a Bi2Se3 quintuple layer, the Dirac states are robust against scattering by these extended structural defects. However, electrostatic fields on the order of 108 V/m are found, which locally charge the Dirac state, shifting the Dirac point by up to 120 meV [3]. On the other hand, low-angle (\textless 15o) GBs are found to be of the tilt variant, consisting of alternating edge dislocation pairs [4], resulting in periodic in-plane stretching and compression. Scanning tunneling spectroscopy reveals that in-plane stretching reduces the van der Waals gap, enhancing the Dirac states; while in-plane compression expands the inter-quintuple separation, therefore destroying the Dirac states and opens a gap in the local density of states. These findings demonstrate the tunability of Dirac states by electric field and strain at the atomic scale, and also highlight the inherent formation of GBs during vapor phase epitaxy of layered TIs. Finally, I will discuss methods to possibly control the density and types of GBs to minimize their impact on carrier transport. [1] C. H. Li, O. M. J. van`t Erve, J. T. Robinson, Y. Liu, L. Li, and B. T. Jonker, Nat. Nanotechnol. 9, 218 (2014). [2] Y. Liu, M. Weinert, and L. Li, Phys. Rev. Lett. 108, 115501 (2012). [3] Y. Liu, Y. Y. Li, D. Gilks, V. K. Lazarov, M. Weinert, and L. Li, Phys. Rev. Lett. 110, 186804 (2013). [4] Y. Liu, Y. Y. Li, S. Rajput, D. Gilks, L. Lari, P. L. Galindo, M. Weinert, V. K. Lazarov, and L. Li, Nat. Phys. 10, 294 (2014). [Preview Abstract] |
Friday, March 6, 2015 11:51AM - 12:03PM |
Z31.00002: Magnetic Correlations in the Quasi-2D Semiconducting Ferromagnet CrSiTe$_3$ Travis Williams, Adam Aczel, Mark Lumsden, Steve Nagler, Matt Stone, Jianqiang Yan, David Mandrus The quasi-two-dimensional, semiconducting ferromagnet CrSiTe$_3$ is a particularly attractive candidate for spintronics applications due its relatively accessible transition temperature and large magnetic moment. In this study, we use neutron scattering to measure the static and dynamic magnetic properties. Neutron diffraction shows 3D ordering below T$_C$=33K, but two dimensional static correlations persist up to at least 300K. The inelastic neutron scattering data shows two distinct spin wave bands, which are nearly dispersionless along the c-axis. The exchange constants extracted from the data suggest that the spins are very nearly Heisenberg, but only weakly coupled perpendicular to the 2D planes. Above the Curie temperature, the spin wave intensity decreases drastically but, like the static correlations, these dynamic magnetic correlations persist within the 2D planes up to room temperature. [Preview Abstract] |
Friday, March 6, 2015 12:03PM - 12:15PM |
Z31.00003: Infrared spectroscopy of Cr and V doped Sb$_2$Te$_3$: dilute magnetic semiconductors David Crandles, Jason Manson, Anthony Madubuonu, Ctirad Uher, Petr Lostak Temperature dependent optical reflectance measurements on well characterized samples of non-intentionally doped, Cr-doped and V-doped Sb$_2$Te$_3$ show that both the parent compound and the Cr-doped version are narrow-gap semiconductors (E$_g\approx 0.25$ eV) with a conventional Drude free carrier absorption. The carrier density increases slightly with decreasing temperature while the scattering rate increases quadratically with temperature which is a sign of optical phonon scattering. Vanadium doping introduces a change in the temperature dependence of the scattering rate as well as higher electrical resistivity than Cr-doped Sb$_2$Te$_3$. An analysis of the literature values of the saturation magnetization for $H\parallel c$ suggests that V is in a mixed valence state V$^{3+}$/V$^{4+}$. [Preview Abstract] |
Friday, March 6, 2015 12:15PM - 12:27PM |
Z31.00004: Ferromagnetic and Semiconducting CoTe Nanostructures Bishnu R. Dahal, Keshab R. Sapkota, Rajendra P. Dulal, Ian L. Pegg, John Philip One dimensional Cobalt-Telluride, CoTe nanostructures were synthesized with modified hydrothermal route. Morphology of the nanostructures was analyzed by using scanning electron microscope and found that the length of the nanowires varied from 20-80 $\mu $m while diameter varied from 50-300 nm. The crystal structure of as grown nanostructures was studied by using X-ray diffractometer and found that they exhibit the hexagonal crystal structure with the space group, p63/mmc and having the lattice parameters a$=$3.893 A, c$=$5.375 A. Magnetic characteristics were studied using the vibrating sample magnetometer and found that the as grown nanostructures were ferromagnetic at room temperature for Co and Te ratio 1:1. I-V characteristics were analyzed from single NW devices, fabricated using e-beam lithography, and found that they exhibit semiconducting behavior. [Preview Abstract] |
Friday, March 6, 2015 12:27PM - 12:39PM |
Z31.00005: Electrical detection of spin-momentum locking in topological insulators Connie Li, Olaf van 't Erve, Jeremy Robinson, Yaoyi Li, Lian Li, Berry Jonker One of the most striking properties of topological insulators (TIs) is that of spin-momentum locking -- the spin of the TI surface state lies in-plane, and is locked at right angle to the carrier momentum. While anticipated by theory, direct electrical access to this spin system in a simple transport structure had been challenging, due to that the bulk is typically unintentionally doped and contributes to transport. Using a ferromagnet/tunnel barrier detector contact that preferentially probes surface/interface spins, we have demonstrated the first direct electrical detection of spin-momentum locking in the TI surface states in MBE-grown Bi$_2$Se$_3$ [1]. However, as the bulk carrier concentration for Bi$_2$Se$_3$ is typically in the 10$^{19}$/cm$^3$ range, the Fermi level is well within the conduction band, where a significant portion of the current is shunted through the bulk. Moving the Fermi level to within the gap is desirable to eliminate current shunting, as well as contribution from Rashba 2DEG states that may dilute the signal [2]. These results, as well as how they affect the spin signal measured will be discussed at the meeting. [1] C. H. Li, et. al., Nat. Nanotech. 9, 218 (2014). [2] S. Hong et. al., PRB 86, 085131 (2012). [Preview Abstract] |
Friday, March 6, 2015 12:39PM - 12:51PM |
Z31.00006: The effect of low-symmetry defects in semiconductors on spin Hall conductivity Matthew D. Mower, Michael E. Flatt\'{e} We study the effect of low-symmetry defects in semiconductors on the spin Hall conductivity of carriers. It has previously been shown that these defects, e.g. DX centers in direct-gap III-V semiconductors, couple to carriers via a rather large, novel spin-orbit interaction. Compared to translational- or bulk-asymmetry based spin-orbit interactions, this spin-orbit interaction considerably enhances the carrier spin relaxation rate. However, we find that it does not make appreciable contributions to transverse spin currents. At the level of the 1st and 2nd Born approximations, there is neither side-jump nor skew scattering from these defects. Thus, we imagine a scenario where shifting impurities between substitutional and interstitial (low symmetry) positions quickly relaxes a spin system with negligible effects on existing transverse spin currents. [Preview Abstract] |
Friday, March 6, 2015 12:51PM - 1:03PM |
Z31.00007: Intraband and interband spin-orbit torques in non-centrosymmetric ferromagnets H. Li, K. Vyborny, T. Jungwirth, H. Gao, J. Sinova, A. Manchon Experimental observations of the spin-orbit torque in non-centrosymmetric ferromagnets such as multilayered ferromagnetic metals and dilute magnetic semiconductors, have recently been reported [1]. Two scenarios have been invoked to explain the origin of these current-driven torques. In the first one, the spin orbit coupling generates an in-plane non-equilibrium spin density and exerts a field-like torque on the magnetization. In the second one, the torque originates from the spin Hall effect occurring in the normal metal placed below the ferromagnet. Recently, a large (anti-)damping-like torque has been observed in a single magnetic GaMnAs layer [3]. Obviously, the torque cannot be attributed to the spin Hall effect owing to the absence of the adjacent heavy metal. Such a torque might be attributed to the interband contribution to the non-equilibrium spin polarization in the linear-response Kubo formula. This intrinsic (scattering-independent) mechanism is related to (a specific type of) the Berry curvature and our calculations corroborate its link to actual experiments. Our numerical results show the parametric dependences of the different torque components, they exhibit similarities to the analytical results for the Rashba two-dimensional electron gas in the weak disorder limit and open new perspectives in the development of current-driven spin-orbit torques by structural design.[1] A. Chernyshov, et.al., Nat. Phys. 5, 656(2009). L. Liu et al. Science 336, 555 (2012) [2] H. Kurebayashi,et.al., Nat. Nano. 9, 211 (2014). [Preview Abstract] |
Friday, March 6, 2015 1:03PM - 1:15PM |
Z31.00008: Current induced spin orbit torques in antiferromagnets Huawei Gao, Jakub Zelezny, T. Jungwirth, Jairo Sinova In magnetic material with bulk inversion asymmetry or structure inversion asymmetry, unpolarized electric current can induce non equilibrium spin polarization due to spin orbit coupling. This non equilibrium spin polarization is exchange coupled with the magnetic orders and act on them as torques. These torques are called spin orbit torques(SOT) which can be used to manipulate the magnetic orders. In ferromagnets, SOT effects have been observed experimentally. We extend the study of SOT to antiferromagnetic systems. Besides similar effects as in ferromagnets, we show staggered SOT in antiferromagnets following exactly the antiferromagnetic lattice which couple with the Neel order directly. We'll report the study in both a 2D collinear antiferromagnetic model and a non-collinear antiferromagnet IrMn3. [Preview Abstract] |
Friday, March 6, 2015 1:15PM - 1:27PM |
Z31.00009: Spin Hall Effect and Irreversible Thermodynamics; Center-to-Edge Transverse Current-Induced Voltage Wayne Saslow For the first time the Dyakonov and Perel theory of the Spin Hall Effect (SHE) is examined from the viewpoint of irreversible thermodynamics, which is significantly more constraining than the symmetry arguments of pure phenomenology. As thermodynamic driving forces we include the thermal gradient, the gradient of the electrochemical potential (rather than the potential gradient and density gradient separately), and the ``internal'' magnetic field that is thermodynamically conjugate to the magnetization. In turn, we obtain the form of bulk transport coefficients relating the fluxes to the thermodynamic forces. Relative to Dyakonov and Perel, in addition to the new terms due to thermal gradients, the Onsager relations require three new (non-linear) terms in the current density, and minor revisions in the current density and spin current density. For a longitudinal current along a strip, the center-to-edge transverse voltage difference, due both to the $-\beta\vec{P}\times\vec{E}$ term of the number current density $\vec{q}$ and to one of the new current density terms, is determined. An ac capacitative probe likely would not significantly disturb this effect. We estimate a $\Delta V_{\perp}$ as large as $10^{-4}$ V for GaAs, but only $10^{-8}$ V for Pt. [Preview Abstract] |
Friday, March 6, 2015 1:27PM - 1:39PM |
Z31.00010: Disentangling the spin torques in a ferromagnet/semiconductor bilayer Timothy D. Skinner, Kamil Olejnik, Lucy K. Cunningham, Hidekazu Kurebayashi, Richard P. Campion, Bryan L. Gallagher, Tomas Jungwirth, Andrew J. Ferguson Current-induced spin torques measured in ferromagnet/paramagnetic metal bilayers can originate from the spin-Hall effect (SHE) and inverse spin galvanic effect (ISGE). Distinguishing the two effects has proved difficult as they can both possess the same symmetries, but it is essential for our basic physical understanding of the spin torques at the ferromagnet/paramagnet interface to experimentally disentangle the SHE and ISGE contributions. In our approach, we look to zinc-blende crystals (such as III-V semiconductors), where the ISGE has a symmetry which depends on the crystal orientation. The field-like [1] and antidamping [2] torques, arising from the ISGE in the magnetic III-V semiconductor (Ga,Mn)As, are well understood because of low-temperature spin-torque ferromagnetic resonance (ST-FMR) measurements. Through new ST-FMR measurements, we show that in a room-temperature ferromagnetic metal/paramagnetic semiconductor bilayer, the SHE and ISGE co-exist and can be unambiguously separated and quantified by their symmetries.\\[4pt] [1] D. Fang et al., Nature Nanotechnol. 6, 413–417 (2011\\[0pt] [2] H. Kurebayashi, Jairo Sinova, D. Fang, A.C. Irvine, T.D. Skinner et al., Nature Nanotechnol. 9, 211-217 (2014) [Preview Abstract] |
Friday, March 6, 2015 1:39PM - 1:51PM |
Z31.00011: ABSTRACT WITHDRAWN |
Friday, March 6, 2015 1:51PM - 2:03PM |
Z31.00012: Inverse spin-Hall effect in NiFe\textbar $p$-type diamond Naoki Fukui, Hiroki Morishita, Satoshi Kobayashi, Shinji Miwa, Norikazu Mizuochi, Yoshishige Suzuki We present DC electromotive forces (EMFs) experiments in NiFe\textbar $p-$type diamond under a ferromagnetic resonance (FMR) of the NiFe. We measured the DC EMFs as a function of a resonance-microwave field and a rotational symmetry of ones. Their measurements suggest that one of the DC EMFs has same symmetry with the inverse spin-Hall effect. The sign of the observed inverse spin-Hall EMFs are opposite to that in the NiFe layer. Our results therefore show the presence of the inverse spin-Hall effect in the NiFe\textbar $p$-type diamond. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700