Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session K12: Topological Insulators: Towards Applications |
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Sponsoring Units: DCMP Room: 308 |
Wednesday, March 16, 2016 8:00AM - 8:12AM |
K12.00001: Plasmon-enhanced electron-phonon coupling in Dirac surface states of the thin-film topological insulator Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ Yuri D. Glinka, Sercan Babakiray, David Lederman Raman measurements of a Fano-type surface phonon mode associated with Dirac surface states (SS) in Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ topological insulator thin films allowed an unambiguous determination of the electron-phonon coupling strength in Dirac SS as a function of film thickness ranging from 2 to 40 nm. A non-monotonic enhancement of the electron-phonon coupling strength with maximum for the 8-10 nm thick films was observed. The non-monotonicity is suggested to originate from plasmon-phonon coupling which enhances electron-phonon coupling when free carrier density in Dirac SS increases with decreasing film thickness and becomes suppressed for thinnest films when anharmonic coupling between in-plane and out-of-plane phonon modes occurs. The observed about four-fold enhancement of electron-phonon coupling in Dirac SS of the 8-10 nm thick Bi$_{\mathrm{2}}$Se$_{\mathrm{3\thinspace }}$films with respect to the bulk samples may provide new insights into the origin of superconductivity in this-type materials and their applications. [Preview Abstract] |
Wednesday, March 16, 2016 8:12AM - 8:24AM |
K12.00002: Inverse spin-galvanic effect in topological-insulator graphene heterostructures. Martin Rodriguez-Vega, Georg Schwiete, Jairo Sinova, Enrico Rossi We study theoretically the inverse spin-galvanic effect in heterostructures formed by a layer of a three dimensional strong topological insulator (TI) and a graphenic layer (single layer graphene, and bilayer graphene). We also consider trilayer structures in which a ferromagnetic thin film is added on top of the graphenic layer. % We consider the cases of coherent, and random tunneling between states in the TI and the graphenic layer. % We obtain the strength of the inverse spin-galvanic effect, taking into account both intraband and interband contributions, as a function of the system's parameters both for the case in which the disorder is short-range and for the case in which the disorder is long-range as when charge impurities are the dominant source of disorder. % We find that for a large range of system's parameters the presence of the graphenic layer enhances the strength of the inverse spin-galvanic effect. Finally, we discuss the relevance of our results for recent experiments. [Preview Abstract] |
Wednesday, March 16, 2016 8:24AM - 8:36AM |
K12.00003: Non-local magnetoelectric effects via Coulomb interaction in TI-FMI heterostructures Stefan Rex, Flavio S. Nogueira, Asle Sudb\O Magnetic order on the surface of a $3D$ topological insulator (TI) has been predicted to evoke a topological magnetoelectric effect (TME) by the breaking of time-reversal invariance. In the TME, an electric field leads to a magnetic polarization in the same direction as the field and vice versa. Here, we consider heterostructures of TI and ferromagnetic insulator (FMI) layers. We show that in the presence of long-range Coulomb interactions the magnetization couples non-locally to the fluctuating electric field (non-local TME) by performing a field-theoretic calculation of the vacuum polarization. In addition, we obtain a Landau-Lifshitz equation for the magnetization dynamics, and find that charged magnetic textures lead to a net magnetization even at a large distance. Such textures can be induced by an external electric field with nonzero in-plane divergence. We apply this effect to a FMI-TI-FMI trilayer heterostructure with two parallel interfaces being well-separated by the bulk TI, where we propose to non-locally control the magnetic texture at one interface by proper gating of the other interface. A preprint can be found at arXiv:1510.04285 [Preview Abstract] |
Wednesday, March 16, 2016 8:36AM - 8:48AM |
K12.00004: Electric field induced quantum anomalous Hall effect in two-dimensional antiferromagnetic triphenyl-lead lattice Hyun-Jung Kim, Chaokai Li, Ji Feng, Zhenyu Zhang, Jun-Hyung Cho The tuning of topological states is of significant fundamental and practical importance in contemporary condensed matter physics, for which the extension to two-dimensional (2D) organometallic systems is particularly attractive.[1] Using first-principles calculations, we find that a 2D hexagonal triphenyl-lead lattice composed of only main group elements is susceptible to a magnetic instability, characterized by a antiferromagnetic (AFM) insulating state with a renormalized valley gaps with gap difference of 24 meV due to the spin and valley coupling. This AFM state will be subject to a anomalous valley Hall effect under the action of Berry curvature-induced spin and valley currents via, for example, injection of circularly polarized light.[2] Furthermore, such a AFM band insulator can be tuned into a topologically nontrivial quantum anomalous Hall state with a Chern number of one by the application of an out-of-plane electric field. These findings further enrich our understanding of 2D hexagonal organometallic lattices for potential applications in spintronics and valleytronics. [1] M Z. F. Wang, Z. Liu, and F. Liu, Nat. Comm. 4, 1471 (2013) [2] X. Li, T. Cao, Q. Niu, J. Shi, and J. Feng, Proc. Natl. Acad. Sci. 110, 2738 (2012) [Preview Abstract] |
Wednesday, March 16, 2016 8:48AM - 9:00AM |
K12.00005: A generalized spin diffusion equation with four electrochemical potentials for channels with spin-orbit coupling Shehrin Sayed, Seokmin Hong, Supriyo Datta We will present a general semiclassical theory for an arbitrary channel with spin-orbit coupling (SOC) [1], that uses four electrochemical potential ($U+$, $D+$, $U-$, and $D-$) depending on the sign of $z$-component of the spin (up $(U)$, down $(D)$) and the sign of the $x$-component of the group velocity $(+,-)$. This can be considered as an extension of the standard spin diffusion equation [2] that uses two electrochemical potentials for up and down spin states, allowing us to take into account the unique coupling between charge and spin degrees of freedom in channels with SOC. We will describe applications of this model to answer a number of interesting questions in this field such as: (1) whether topological insulators can switch magnets, (2) how the charge to spin conversion is influenced by the channel resistivity, and (3) how device structures can be designed to enhance spin injection. $[1]$ S. Hong, S. Sayed, and S. Datta, ``Spin Circuit Model for 2D Channels with Spin-Orbit Coupling". (Under Review). $[2]$ T. Valet, and A. Fert, ``Theory of the perpendicular magnetoresistance in magnetic mutilayers", Phys. Rev. B, 48, 7099, 1993. [Preview Abstract] |
Wednesday, March 16, 2016 9:00AM - 9:12AM |
K12.00006: Measurements of current-induced spin polarizations in topological insulators Bi$_{\mathrm{2}}$Te$_{\mathrm{2}}$Se and Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ thin flakes. Jifa Tian, Ireneusz Miotkowski, Seokmin Hong, Supriyo Datta, Yong Chen Topological insulators (TIs) possess nontrivial spin-momentum-locked topological surface states (TSS). Real TI can also host trivial surface 2DEG with strong Rashba spin-orbit coupling derived from the bulk states. Both TSS and Rashba 2DEG can generate current induced spin polarization, although the dominant helicities of their spin-momentum locking (SML) are expected to be opposite. Here, we report spin potentiometric measurements in exfoliated bulk-insulating Bi$_{\mathrm{2}}$Te$_{\mathrm{2}}$Se and bulk-metallic Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ thin flakes. In both materials, the voltage measured by a FM electrode shows a hysteretic step-like change when the FM magnetization is switched by an in-plane magnetic field. The trend of the voltage change can be reversed by reversing the direction of the dc current, and the amplitude of the spin signal increases linearly with increasing bias current. Such a spin signal is consistent with a current induced spin polarization arising from a helical SML. However, the observed trend of the voltage change is opposite between Bi$_{\mathrm{2}}$Te$_{\mathrm{2}}$Se and Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$, suggesting opposite signs of dominant spin helicity that we attribute to TSS and Rashba 2DEG respectively. [Preview Abstract] |
Wednesday, March 16, 2016 9:12AM - 9:24AM |
K12.00007: Spintronics device made of topological materials Jiansheng Wu, Zhangsheng Shi, Maoji Wang Topological Materials is a new state of matter of which the bulk states are gapped insulator or superconductor while the surface states are gapless metallic states. Such surface states are robust against local disorder and impurities due to its nontrivial topology. It induces unusual transport properties and shows nontrivial topological spin texture in real space. We have made use of these two exotic properties to make application in spintronics. For example, we propose to make spin-filter transistor using of 1D or 2D quantum anomalous Hall insulator or 2D topological Weyl semimetal, we also propose a device to measure the spin-polarization of current, a device to generate entangled entangled electron pairs. [Preview Abstract] |
Wednesday, March 16, 2016 9:24AM - 9:36AM |
K12.00008: Topological Insulator and Thermoelectric Effects Yong Xu The recent discovery of topological insulator (TI) offers new opportunities for the development of thermoelectricity, because many TIs (like Bi$_2$Te$_3$) are excellent thermoelectric materials. In this talk, I will first introduce our theoretical predictions of anomalous Seebeck effect and strong size effect in TI [PRL 112, 226801 (2014)]. Then I will report our recent proof experiments, which find in TI thin films that (i) the hole-type Seebeck effect and the electron-type Hall effect coexist in the same TI sample for all the measured temperatures (up to 300 K), and (ii) the thermoelectric properties depend sensitively on the film thickness. The unconventional phenomena are revealed to be closely related to the topological nature of the material. These findings may inspire new ideas for designing TI-based high-efficiency thermoelectric devices. [Preview Abstract] |
Wednesday, March 16, 2016 9:36AM - 9:48AM |
K12.00009: Electric-field control of spin-orbit torque in magnetically doped topological insulators Yabin Fan, Qiming Shao, Xufeng Kou, Pramey Upadhyaya, Kang Wang Recent advances of spin-orbit torques (SOTs) generated by topological insulators (TIs) [1-2] have drawn increasing interest to the spin-momentum locking feature of TIs' surface states, which can potentially provide a very efficient means to generate SOTs for spintronic applications. In this presentation, we will show the magnetization switching through current-induced giant SOT in both TI/Cr-doped TI bilayer [1] and uniformly Cr-doped TI films [3] In particular, we show that the current-induced SOT has significant contribution from the spin-momentum locked surface states of TIs. We find that the spin torque efficiency is in general three orders of magnitude larger than those reported in heavy metal/ferromagnetic heterostructures. In the second part, we will present the electric-field control of the giant SOT in magnetically doped TIs [3], which suggests promising gate-controlled spin-torque device applications. The giant SOT and efficient current-induced magnetization switching exhibited by the magnetic TIs may lead to innovative spintronic applications such as ultralow power dissipation memory and logic devices. [1] Y. Fan, \textit{et al.}, \textit{Nature Mater. }\textbf{13}, 699-704 (2014). [2] A. R. Mellnik, \textit{et al.}, \textit{Nature} \textbf{511}, 449-451 (2014). [3] Y. Fan, \textit{et al}., \textit{under} \textit{preparation}. [Preview Abstract] |
Wednesday, March 16, 2016 9:48AM - 10:00AM |
K12.00010: Optical Control of Ferromagnetism in a Magnetically-Doped Topological Insulator Andrew L. Yeats, Peter J. Mintun, Yu Pan, Anthony Richardella, Nitin Samarth, David D. Awschalom Many proposed experiments involving topological insulators (TIs) require spatial control over time-reversal symmetry and chemical potential. We demonstrate micron-scale optical control of both magnetization and chemical potential in thin films of Cr-doped (Bi,Sb)$_{2}$Te$_{3}$. By optically modulating the coercivity of the films, we write and erase arbitrary spatial configurations of their magnetization, which we then image with Kerr microscopy. Additionally, by optically manipulating a space charge layer in the underlying SrTiO$_{3}$ substrates, we can control the local chemical potential of the films. This allows us to write and erase $\it{p}$-$\it{n}$ junctions in the films, which we image with photocurrent microscopy\footnote{A. L. Yeats, Y. Pan, A. Richardella, P. J. Mintun, N. Samarth, and D. D. Awschalom, Science Advances 1, e1500640 (2015).}. Both effects persist for $>$ 16 hours. We will present systematic Kerr microscopy, photocurrent microscopy, and electrical transport studies of these materials and various electronic and magnetic structures patterned on them. We will discuss the prospects for using these optical phenomena to study and control the unique physics of TIs, such as edge-state transport in the quantum anomalous Hall regime. [Preview Abstract] |
Wednesday, March 16, 2016 10:00AM - 10:12AM |
K12.00011: Edge channel transport in a HgTe waveguide modulated by two magnetic barriers Feng Zhai, Xuanping Jin, JUNQIANG LU we investigate the effects of a magnetic double barrier on the ballistic transport properties of edge-states in a HgTe waveguide with inverted band structures. When its energy is in the bulk gap and close to the bulk conduction band of leads, the electron incident from a quantum spin Hall (QSH) state can be almost totally reflected as the two magnetic barriers are in the antiparallel configuration. For the magnetic double barrier in the parallel configuration, the same electron can transmit nearly perfectly for a proper spin orientation. In the antiparallel configuration, the spin polarization of the output current vanishes. This distinction in the transmission indicates that the proposed edge-state device has two functions: magnetic switching and spin filtering. Our calculations also indicate that nonuniform magnetic fields can break the QSH states more effectively than uniform ones. [Preview Abstract] |
Wednesday, March 16, 2016 10:12AM - 10:24AM |
K12.00012: Dynamical electron compressibility in the 3D topological insulator Bi$_2$Se$_3$ Andreas Inhofer, Badih Assaf, Quentin Wilmart, Louis Veyrat, Christian Nowka, Joseph Dufouleur, Romain Giraud, Silke Hampel, Bernd Buechner, Gwendal Fève, Jean-Marc Berroir, Bernard Placais Measurements of the quantum capacitance $c_q$, related to the electron compressibility $\chi=c_q/e^2$ is a sensitive tool to probe the density of states. In a topological insulator (TI) the situation is enriched by the coexistence and the interplay of topologically protected surface states and massive bulk carriers. We investigate top-gate metal-oxyde-TI capacitors using Bi$_2$Se$_3$ thin crystals at GHz frequencies. These measurements provide insight into the compressibillity of such a two electron-fluid system. Furthermore, the dynamical response yields information about electron scattering properties in TIs. More specifically, in our measurements we track simultaneously the conductivity $\sigma$ and the compressibility as a function of a DC-gate voltage. Using the Einstein relation $\sigma = c_q D$, we have access to the gate dependence of the electron diffusion constant $D(V_g)$, a signature of the peculiar scattering mechanisms in TIs. [Preview Abstract] |
Wednesday, March 16, 2016 10:24AM - 10:36AM |
K12.00013: Observation of Majorana fermion states in rf-SQUIDs constructed on Pb-Bi$_2$Te$_3$ surface Li Lu, Yuan Pang, Jie Shen, Fanming Qu, Zhaozheng Lyu, Junhua Wang, Junya Feng, Jie Fan, Guangtong Liu, Zhongqing Ji, Xiunian Jing, Changli Yang, Qingfeng Sun, X. C. Xie, Liang Fu Recently, much attention has been paid to search for Majorana fermions in solid-state systems. Among various proposals there is one based on radio-frequency superconducting quantum interference devices (rf-SQUIDs), in which a 4$\pi$-perioded current-phase relation is expected if Majorana fermion states exist. In this talk we report observations of truncated 4$\pi$-perioded (i.e., 2$\pi$-perioded but fully skewed) oscillatory patterns of contact resistance, on rf-SQUIDs constructed on the surface of three-dimensional topological insulator Bi$_2$Te$_3$. The results reflect the existence of Majorana fermion states in the devices. [Preview Abstract] |
Wednesday, March 16, 2016 10:36AM - 10:48AM |
K12.00014: Quantum well states in Rashba semiconductor BiTeI Yang He, Zhihuai Zhu, Mohammad Hamidian, Pengcheng Chen, Yau Chuen Yam, Jennifer Hoffman BiTeI displays large Rashba-type spin splitting in both valence and conduction bands. In this work, we use scanning tunneling microscopy to reveal the bipolar nature of BiTeI, confirming the previously observed p-n junction electronic structure. We also discover two-dimensional quantum well states both below and above the semiconducting gap on the Te-terminated surface. This work sheds light on the origin of the giant Rashba splitting in the system.~ [Preview Abstract] |
Wednesday, March 16, 2016 10:48AM - 11:00AM |
K12.00015: Kondo and Majorana doublet interactions in quantum dots Younghyun Kim, Dong E. Liu, Erikas Gaidamauskas, Jens Paaske, Karsten Flensberg, Roman Lutchyn We study the properties of a quantum dot coupled to a normal lead and a time-reversal topological superconductor with Majorana Kramers pair at the end. We explore the phase diagram of the system as a function of Kondo and Majorana-induced coupling strengths using perturbative renormalization group study and slave-boson mean-field theory. We find that, in the presence of coupling between a quantum dot and a Majorana doublet, the system flows to a new fixed point controlled by the Majorana doublet, rather than the Kondo coupling, which is characterized by correlations between a localized spin and the fermion parity of each spin sector of the topological superconductor. We find that this fixed point is stable with respect to Gaussian fluctuations. We also investigate the effect of spin-spin interaction between a quantum dot and Majorana doublet and compare the result with a case where a normal lead is directly coupled to Majorana doublet. [Preview Abstract] |
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