Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session J7: Focus Session: Spin Transport & Optical Phenomena in Topological Insulators 
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Sponsoring Units: DMP DCMP Chair: Nitin Samarth, Pennylvania State University Room: 006B 
Tuesday, March 3, 2015 2:30PM  2:42PM 
J7.00001: SpinPolarized Tunneling Study on SpinMomentum Locking in the Topological Insulator Bismuth Selenide ChingTzu Chen, Luqiao Liu, Anthony Richardella, Ion Garate, Yu Zhu, Nitin Samarth In this talk, we will demonstrate that the helical spin texture on topological insulator (TI) surfaces can be electrically detected using fourterminal tunnel junction devices with ferromagnetic top electrodes. Consistent results are obtained in both the Edelstein and spingalvanic effect configurations, allowing a quantitative determination of the chargespin conversion efficiency in bismuth selenide. By applying finite DC biases at the junction, we further extract the energy dependence of the effective spin polarization in bismuth selenide. The observed temperature stability up to 200K suggests that TIs can be highly promising for roomtemperature spintronics applications [Preview Abstract] 
Tuesday, March 3, 2015 2:42PM  2:54PM 
J7.00002: Tunable Intrinsic Spin Hall Conductivities in Bi$_2$(Se,Te)$_3$ Topological Insulators C\"{u}neyt \c{S}ahin, Michael E. Flatt\'e It has been recently shown by spintransfer torque measurements that Bi$_2$Se$_3$ exhibits a very large spin Hall conductivity (SHC)[1]. It is expected that Bi$_2$Te$_3$, a topological insulator with similar crystal and band structures as well as large spinorbit coupling, would also exhibit a giant SHC. In this study we have calculated intrinsic spin Hall conductivities of Bi$_2$Se$_3$ and Bi$_2$Te$_3$ topological insulators from a tightbinding Hamiltonian including two nearestneighbor interactions. We have calculated the Berry curvature, used the Kubo formula in the static, clean limit and shown that both materials exhibit giant spin Hall conductivities, consistent with the results of Ref. 1 and larger than previously reported Bi$_{1x}$Sb$_x$ alloys[2]. The density of Berry curvature has also been computed from the full Brillouin zone in order to compute the dependence of the SHC in these materials on the Fermi energy. Finally we report the intrinsic SHC for Bi$_2$(Se,Te)$_3$ topological insulators, which changes dramatically with doping or gate voltage. [1] A.R. Mellnik et al., Nature 511, 449  451 (2014) [2] C.\c{S}ahin and M.E.Flatt\'e, arXiv:1410.7319 [Preview Abstract] 
Tuesday, March 3, 2015 2:54PM  3:06PM 
J7.00003: Inverse spin galvanic effect in topologicalinsulator based heterostructures Martin RodriguezVega, Georg Schwiete, Jairo Sinova, Enrico Rossi We study the inverse spin galvanic effect in heterostructures formed by a layer of a three dimensional strong topological insulator (TI) and a magnetic material. We consider different configurations for the heterostructure and for the contacts. We carefully treat the effect on the TI bands of the proximity of a magnetic material and take into account both intraband and interband contributions to the currentinduced spin polarization of the TI surface states. Finally, we discuss the relevance of our results for recent experiments. [Preview Abstract] 
Tuesday, March 3, 2015 3:06PM  3:42PM 
J7.00004: Low energy electrodynamics of topological insulator thin films Invited Speaker: N. Peter Armitage Topological insulators are recently discovered states of matter with robust metallic surface states protected by the topological properties of their bulk wavefunctions. A quantum phase transition from a topological insulator to a conventional insulator and a change in topological class can occur only when the bulk bandgap closes. In this work, we have used timedomain terahertz spectroscopy to investigate the lowfrequency conductance in (Bi$_{1x}$ In$_x$)$_2$Se$_3$ as we tune through this transition by In substitution. Above certain substitution levels we observe a collapse in the transport lifetime that indicates the destruction of the topological phase. We associate this effect with the threshold where states from opposite surfaces hybridize. The substitution level of the threshold is thickness dependent and only asymptotically approaches the bulk limit $x \sim 0.06$ where a maximum in the midinfrared absorption is exhibited. This absorption can be identified with the bulk bandgap closing and change in topological class. The correlation ength associated with the quantum phase transition appears as the evanescent length of the surface states. The observation of the thickness dependent collapse of the transport lifetime shows the unusual role that finitesize effects play in this topological quantum phase transition. In even more recent work on bulk insulating films, we have identified a cyclotron resonance feature. At high magnetic fields we identify a anomalous increase of the scattering rate. I will discuss the reasons for this increase and put it in the context of existing theories for charge transport in topological insulators. [Preview Abstract] 
(Author Not Attending)

J7.00005: Evidence for surfacegenerated photocurrent in (Bi,Sb)$_{2}$Se$_{3}$ and (Bi,Sb)$_{2}$Te$_{3}$ thin films Yu Pan, Anthony Richardella, Bing Yao, Joon Sue Lee, Thomas Flanagan, Abhinav Kandala, Nitin Samarth, Andrew Yeats, Peter Mintun, David Awschalom Illumination with circularly polarized light is known produce a helicitydependent photocurrent in topological insulators such as Bi$_{2}$Se$_{3}$ [e.g. Nature Nanotech. \textbf{7}, 96 (2012)]. However, the exact origin of this effect is still unclear since it is observed with photons well above the bulk band gap. We report measurements of the polarizationdependent photocurrent in a series of (Bi,Sb)$_{2}$Se$_{3}$ thin films with different carrier concentrations and find that the photocurrent is enhanced as we increase the population of the surface states. This finding is supported by a study of helicitydependent photocurrents in backgated (Bi,Sb)$_{2}$Te$_{3}$ thin films, where the chemical potential is varied electrostatically. By illuminating our samples at different wavelengths, we show that the helicitydependent photocurrent is enhanced when the photon energy approaches the energy difference between the lowest and first excited (unoccupied) topological surface states. This leads us to attribute the helicitydependent photocurrent in topological insulators to optical excitations between these two spintextured surface states. We will also discuss experiments imaging the spatial variation of these helicitydependent photocurrents. [Preview Abstract] 
Tuesday, March 3, 2015 3:54PM  4:06PM 
J7.00006: Quantum Spin Hall Effect in TwoDimensional Transition Metal Dichalcogenides Xiaofeng Qian, Junwei Liu, Liang Fu, Ju Li Quantum spin Hall (QSH) effect materials have an insulating bulk but conducting edge states that are topologically protected from backscattering by time reversal symmetry. However, the small band gap and the lack of efficient on/off switching in materials that have been identified as QSH insulators limit their applications. Here using firstprinciples calculations we predict a class of largegap QSH insulators in twodimensional transition metal dichalcogenides with 1T' structure, namely, 1T'MX$_2$ with M=(W, Mo) and X=(Te, Se, S)\footnote{Xiaofeng Qian, Junwei Liu, Liang Fu, and Ju Li, arXiv:1406.2749 (2014)}. A structural distortion causes an intrinsic band inversion between chalcogenide$p$ and metal$d$ bands. Additionally, spinorbit coupling opens a gap that is tunable by vertical electric field and strain. We propose a topological field effect transistor made of van der Waals heterostructures of 1T'MX$_2$ and 2D dielectric layers that can be rapidly switched off by electric field through topological phase transition instead of carrier depletion. [Preview Abstract] 
Tuesday, March 3, 2015 4:06PM  4:18PM 
J7.00007: Observing the Zeeman effect of topological surface state Yingshuang Fu, Tetsuo Hanaguri, Shuhei Yamamoto, Kyushiro Igarashi, Minoru Kawamura, Yuhki Kohsaka, Katsuya Iwaya, Hidenori Takagi, Takao Sasagawa Dirac fermions in the topological surface state (TSS) have helical spin textures. This is different from those in graphene, which are both valley and spin degenerated. The spin degeneracy can be lifted by Zeeman effect, which is manifested as a spinsplitting of Landau levels (LLs). In the case of TSS, LLs instead should exhibit monotonic shift with magnetic field since the spin degeneracy is lacking. While the Zeeman splitting of LLs in graphene has been successfully observed, the expected features in TSS still lack experimental proof. With scanning tunneling microscopy and spectroscopy, we observed the Zeeman shifting of zeroth LL in the TSS of Bi2Se3 and Sb2Te2Se unambiguously. Moreover, we exclude the extrinsic influence on LL shifting from potential variations and the nonideal band dispersions of TSS in actual materials by modeling. The surface g factor in Sb2Te2Se and Bi2Se3 is estimated as 10 and 16, respectively. This observation indicates that the g factor of TSS is significantly material dependent, which may be related to the atomic orbital character of the compound. [Preview Abstract] 
Tuesday, March 3, 2015 4:18PM  4:30PM 
J7.00008: Optical transitions to second Dirac point in a ferromagnetic topological insulator Shreyas Patankar, Joel Griesmar, Andrew Bestwick, Eli Fox, Xufeng Kou, Yabin Fan, Kang Wang, David GoldhaberGordon, Joseph Orenstein It is well known that the bismuth telluride family of 3D topological insulators has a surface state in the bulk gap and that doping with magnetic impurities opens a gap in its Diraclike dispersion. Recent twophoton ARPES measurements [Sobota et al., PRL, 2013] have revealed the existence of a second Dirac band about 1.8eV above the energy of the surface state node. To probe transitions from the first to second Dirac bands we studied the spectral response of thin films of the magnetic topological insulator Cr:(Bi,Sb)$_2$Te$_3$ using magnetooptic Kerr (MOKE) spectroscopy at visible and nearinfrared frequencies. We observe a strong resonant enhancement of the Kerr response near 1.8eV. The enhancement enables measurement of Kerr rotation in ultrathin films, allowing us to explore the predicted [Lu et al., PRB, 2010] transitions between topologically trivial and nontrivial states as a function film thickness. [Preview Abstract] 
Tuesday, March 3, 2015 4:30PM  4:42PM 
J7.00009: Observation of Current induced Optical Kerr Rotation in Topological Insulators Nirajan Mandal, Irek Mitkowski, Yong Chen Topological insulators (TIs) are an unusual phase of quantum matter with an insulating bulk gap and gapless spinmomentum locked Dirac surface states (SS), showing exotic topological quantum properties. However, optical identification of the spinmomentum locked SS is still challenging. Here, we report roomtemperature, current induced magnetooptical Kerr rotation effect observed from various TI bulk crystals grown by the Bridgman method. The Kerr rotation was measured while a square wave bias current was applied across the samples. We find that the Kerr angle increases linearly with the applied bias current and reverses its sign as the polarity of the current is reversed. Such an observation is consistent with the spinmomentum locking of the surface states. The largest Kerr rotation we measured is 4x106 radians/(A/cm2)) from Bi2Te2Se1. [Preview Abstract] 
Tuesday, March 3, 2015 4:42PM  4:54PM 
J7.00010: Electrical Probing of Inherent Spin Polarization in a Topological Insulator with Electrical Gating Joon Sue Lee, Anthony Richardella, Nitin Samarth The hallmark of a timereversal symmetry protected threedimensional topological insulator is the helically spintextured surface state. Although electrical detection of spin polarization in topological insulators has been demonstrated very recently, there have not been any electrical measurements to demonstrate the entire mapping of the spin polarization throughout the surface state. We report the electrical probing of the spinpolarized surface state using a magnetic tunnel junction as a spin detector while the chemical potential of a topological insulator (Bi,Sb)$_{2}$Te$_{3}$ is tuned by back gating. Hysteretic spin signals were observed as the magnetization of the detector ferromagnet (permalloy) switches with inplane magnetic field. Changing the direction of bias current through the topological insulator channel flips the direction of the spin polarization, resulting in the reverse of sign of the detected spin signals. We demonstrate the control of the Fermi energy, which has importance not only in further understanding of the spinmomentum locking in the surface state but also in possible electrical tuning of the spin polarization for potential spinbased devices. [Preview Abstract] 
Tuesday, March 3, 2015 4:54PM  5:06PM 
J7.00011: Micrometric electronic patterning of a topological band structure using a photon beam Mark Golden, Emmanouil Frantzeskakis, Nick de Jong, Yingkai Huang, Dong Wu, Yu Pan, Anne de Visser, Erik van Heumen, Tran Van Bay, Berend Zwartsenberg, Pieter Pronk, Shyama Varier Ramankutty, Alona Tytarenko, Nan Xu, Nick Plumb, Ming Shi, Milan Radovic, Andrei Varkhalov The only states crossing E$_F$ in ideal, 3D TIs are topological surface states. Single crystals of Bi$_2$Se$_3$ and Bi$_2$Te$_3$ are too defective to exhibit bulkinsulating behaviour, and ARPES shows topologically trivial 2DEGs at E$_F$ in the surface region due to downward band bending. Ternary \& quaternary alloys of Bi$\big /$Te$\big /$Se$\big /$Sb hold promise for obtaining bulkinsulating crystals. Here we report ARPES data from quaternary, bulkinsulating, Bibased TIs. Shortly after cleavage in UHV, downward band bending pulls the bulk conduction band below E$_F$, once again frustrating the ``topological only'' ambition for the Fermi surface. However, there is light at the end of the tunnel: we show that a superbandgap photon beam generates a surface photovoltage sufficient to flatten the bands, thereby recovering the ideal, ``topological only'' situation. In our bulkinsulating quaternary TIs, this effect is local in nature, and permits the writing of arbitrary, micronsized patterns in the topological energy landscape at the surface. [Preview Abstract] 
Tuesday, March 3, 2015 5:06PM  5:18PM 
J7.00012: Quantum Spin Hall Effect in Ultrasonic Metamaterials S. Hossein Mousavi, Alexander B. Khanikaev, Zheng Wang The discovery of topological order without breaking timereversal symmetry, such as that in Quantum Spin Hall (QSH) effect and Topological Insulators, is one of the most groundbreaking advancements of recent years in condensed matters physics. The approach to topological order without breaking timereversal symmetry is particularly important in elastics because no natural elastic materials show linear nonreciprocal response. Here we illustrate the first elasticwave system emulating QSH effect and demonstrate existence of topologically protected elastic edge states. The system represents an elastic metamaterialbased phononic crystal. In this crystal, we achieved degenerate linear dispersion for two sets of modes, classified by one of the system's symmetries. Then, by relaxing and removing that symmetry by deliberately engineering a gauge field emulating a strong spinorbit coupling of QSH, we observe opening a complete topological bandgap. Finally, the hallmark of the topological order, namely the presence of oneway chiral edge waves insensitive to nonmagnetic defects and disorders, is demonstrated in such elastic metacrystals. We illustrate the unique property of these elastic edge waves to flow around sharp corners without backreflection or localization. [Preview Abstract] 
Tuesday, March 3, 2015 5:18PM  5:30PM 
J7.00013: Thickness tunable quantum interference between surface phonon and Dirac plasmon states in thinfilms of the topological insulator Bi$_{2}$Se$_{3}$ Yuri Glinka, Sercan Babakiray, Trent Johnson, David Lederman Raman scattering has been applied to study thin films of the topological insulator Bi$_{2}$Se$_{3}$. We observed a more than 100fold enhancement of Raman responses~if laser photon energy switches from 2.33 eV (532 nm) to 1.58 eV (785 nm), which is due to direct optical coupling to Dirac surface states (SS) at the resonance energy of about 1.5 eV (a thicknessindependent enhancement) and due to nonlinearly excited Dirac plasmon (a thicknessdependent enhancement). Owing to the direct optical coupling, we were able to monitor an inplane phonon mode of hexagonally arranged Seatoms associated with a continuous network of Dirac SS. This mode revealed a Fano lineshape for films below 15 nm thick, resulting from quantum interference between surface phonon and Dirac plasmon states. [Preview Abstract] 
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