Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session L29: TwoDimensional Topological Insulators: Heterostructures and Beyond 
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Sponsoring Units: DCMP DMP Chair: Fan Zhang, University of Texas, Dallas Room: 328 
Wednesday, March 16, 2016 11:15AM  11:27AM 
L29.00001: Topological Phases in Perovskite Oxide Heterostructures Rokyeon Kim, Jaejun Yu, Hosub Jin Quantum spin Hall (QSH) insulator is a new state of matter characterized by gapless edge states and insulating bulk states. Because the edge states are topologically protected and therefore robust against nonmagnetic perturbations, it has a potential to be utilized in spintronics devices. Quauntum vally Hall (QVH) phase, on the other hand, is another class of topological state exhibiting valleycontrasting Berry curvature and spin splitting, which could yield novel transport properties, such as valley Hall effect and valley spin Hall effect. We propose a new kind of perovskite (111) heterostructures which can host both QSH and QVH phases with appropriate choices of composing elements. By carrying out firstprinciples calculations, we demonstrate that a Dirac cone emerges in a particular choice of heterostructure, and a sizable spinorbit coupling turns the system into the QSH phase. In addition, the QVH phase with different Berry phases and spin textures in each valley is shown to be realized in the heterostructure with broken inversion symmetry. We porpose that these perovskite heterostructures can provide a feasible platform for spintronics, valleytronics, and topological engineering of the twodimensional electron system. [Preview Abstract] 
Wednesday, March 16, 2016 11:27AM  11:39AM 
L29.00002: Structure of the Surface States at Topological InsulatorSemiconductor Interfaces Mahmoud M. Asmar, Daniel Sheehy, Ilya Vekhter Topologicallyprotected surface states of threedimensional topological insulators (TIs) are characterized by spin and momentum locking. In the simplest picture the emergent two dimensional semimetal displays opposite helicities around the point of degeneracy. Possible applications of TIs rely on forming interfaces with other materials, such as semiconductors or superconductors. In such heterostructures, the dispersion and quantum numbers of the surface states become not only dependent on bulk properties but also on the specifics of the boundaries between the TI and the material in contact. Making use of the three dimensional $k\cdot p$ Hamiltonian describing TIs, and taking in to account surface potentials compatible with the symmetries of the TI and the semiconducting material, we find the effects of the latter on the energymomentum dispersion and spin structure of the surface state and explore the consequences this may have on physical observables. [Preview Abstract] 
Wednesday, March 16, 2016 11:39AM  11:51AM 
L29.00003: Interplay of Dirac surface states and magnetic fluctuations in topological insulator heterostructures Hilary M. Hurst, Dmitry K. Efimkin, Victor Galitski We consider the proximity effect between Dirac states at the surface of a topological insulator and a ferromagnet with easy plane anisotropy, which is described by the XYmodel and undergoes a BerezinskiiKosterlitzThouless (BKT) phase transition. Classical magnetic fluctuations interacting with the surface states of a topological insulator can be described by an effective gauge field. This model can be mapped onto the problem of Dirac fermions in a random magnetic field, however this analogy is only partial in the presence of electronhole asymmetry or warping of the Dirac dispersion which results in screening of magnetic fluctuations. We show that this proximity coupling leads to anomalous transport behavior of the surface states near the BKT transition temperature. [Preview Abstract] 
Wednesday, March 16, 2016 11:51AM  12:03PM 
L29.00004: Transforming common IIIV/IIVI insulating building blocks into topological heterostructure via the intrinsic electric polarization Alex Zunger, Xiuwen Zhang, Leonardo Abdalla, Qihang Liu Currently known topological insulators (TIs) are limited to narrow gap compounds incorporating heavy elements, thus severely limiting the material pool available for such applications. We show how a heterovalent superlattice made of common semiconductor building blocks can transform its nonTI components into a topological heterostructure. The heterovalent nature of such interfaces sets up, in the absence of interfacial atomic exchange, a natural internal electric field that along with the quantum confinement leads to band inversion, transforming these semiconductors into a topological phase while also forming a giant Rashba spin splitting. We demonstrate this paradigm of designing TIs from ordinary semiconductors via firstprinciple calculations on IIIV/IIVI superlattice InSb/CdTe. We illustrate the relationship between the interfacial stability and the topological transition, finding a ``window of opportunity'' where both conditions can be optimized. This work illustrates the general principles of coevaluation of TI functionality with thermodynamic stability as a route of identifying realistic combination of common insulators that could produce topological heterostructures. [Preview Abstract] 
Wednesday, March 16, 2016 12:03PM  12:15PM 
L29.00005: Emergence of quantum spin Hall and ``halftopological'' states at Graphene/TMDC heterostructures Denis Kochan, Martin Gmitra, Petra H\"{o}gl, Jaroslav Fabian We discuss orbital and spinorbital proximity effects emerging in graphene deposited on a monolayer transitionmetal dichalcogenides (TMDCs: MoS2, MoSe2, WS2, WSe2) and analyze the impact on spin transport in such graphene/TMDC heterostructures. Firstprinciples investigations show that graphene on MoS2, MoSe2, and WS2 has a topologically trivial band structure, while graphene on WSe2 exhibits inverted bands. The essential low energy physics can be well described by a symmetry inspired realistic tightbinding Hamiltonian. We predict topologically protected helical edge states for graphene zigzag nanoribbons on WSe2, demonstrating the emergence of the quantum spin Hall effect. Our model also features "halftopological states", which are protected against timereversal disorder on one edge only. Unlike in pristine graphene, the proximity spinorbit coupling in graphene on TMDCs is significant (orders of meV), making the predicted effect testable experimentally. References: M. Gmitra, D. Kochan, P. H\"{o}gl, J. Fabian; Trivial and inverted Dirac bands, and emergence of quantum spin Hall states in graphene on transitionmetal dichalcogenides, arXiv:1510.00166 [Preview Abstract] 
Wednesday, March 16, 2016 12:15PM  12:27PM 
L29.00006: Tunneling spectroscopy in engineered SrTiO$_{\mathrm{3}}$ heterostructures Adrian Swartz, Hisashi Inoue, Tyler Merz, Yasuyuki Hikita, Harold Hwang Despite decades of intense research on the topic, superconductivity in the dilute high$k$ semiconductor SrTiO$_{\mathrm{3}}$ (STO) has posed a longstanding problem. In light of the recent reports of unconventional 2D superconductivity in STObased heterostructures, this problem deserves renewed attention in the bulk limit. Tunneling spectroscopy, which directly measures the electronic density of states, is a powerful tool to investigate the superconducting ground state as well as the relevant electronphonon couplings. A limiting obstacle for employing this technique is the long depletion lengths formed when semiconducting SrTiO$_{\mathrm{3}}$ is contacted with a metal in Schottky junctions, which obstructs access to the intrinsic bulk electronic properties. Here, using band alignment engineered planar tunneling junctions to minimize these long depletion lengths, we experimentally reexamine canonical tunneling experiments in Nbdoped STO. We discuss our results on the extraction of the electronphonon coupling in SrTiO$_{\mathrm{3}}$ and it's relevance to the superconducting condensate. [Preview Abstract] 
(Author Not Attending)

L29.00007: Origins of conducting channel at LaAlO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ heterointerface investigated by \textit{in situ} ARPES Hyangkeun Yoo, Luca Moreschini, Aaron Bostwick, Andrew Walter, Tae Won Noh, Young Jun Chang, Eli Rotenberg The highmobility conducting interface (CI) between LaAlO$_{\mathrm{3}}$ (LAO) and SrTiO$_{\mathrm{3}}$ (STO) has revealed many fascinating phenomena. But the formation mechanism of the CI has not been conclusively explained. Here, we investigated the CI formation between LAO and STO by \textit{in situ} angleresolved photoemission spectroscopy. By directly imaging the LAO polarityinduced builtin potential (V$_{\mathrm{i}})$ at each step of the LAO growth, we demonstrated that the V$_{\mathrm{i}}$ is proportional to the LAO thickness and the conducting interface is appeared above 3 unit cells of LAO. However, we found that the V$_{\mathrm{i}}$ and the critical thickness are strongly dependent on the amount of the surface oxygen vacancies controlled by the synchrotron ultravioletirradiation and the oxygen gas exposure. This indicates that the only polar catastrophe, theorized to explain the CI formation above a critical thickness, is not adequate. Instead, our results point to a decisive role played by the oxygen vacancy, and explain why the V$_{\mathrm{i}}$ and the critical thickness as reported in several works show some variation, rather than being a universal quantity. [Preview Abstract] 
(Author Not Attending)

L29.00008: Electronic structure study of UV photodoping evolution on the TiO2 terminated SrTiO3 Chaofan Zhang, Zhongkai Liu, Zhuoyu Chen, chunjing Jia, Yao Wang, Yanwu Xie, Wei Li, James. J Lee, Tao Jia, Slavko Rebec, Eric Yue Ma, Sungkwan Mo, Brian Moritz, Robert Moorer, Ruihua He, T.P Devereaux, Worawat Meevasana, Zhixun Shen The metallic two dimensional electron gas (2DEG) has been observed on the UV light irradiated bare SrTiO3 surface of various terminations ((001),(110),(111)) using angular resolved photoemission spectroscopy (ARPES). The study of electronic structure of 2DEG opens a window to study the complex physical properties on the bare SrTiO3 surface, such as the superconductivity, the high mobility and ferromagnetism. In this talk, we will show the clear polaron band that due to the electron phonon coupling formed at low carrier density gradually screening out and vanishing as the photodoping increases, instead of that, the quantum well states start appearing at higher doping level. Besides that, the upshifting of both the incoherent ingap and deep valence states towards the Fermi level suggests a huge gap shrinking, which we believe to be the negative electronic compressibility on the 2DEG surface on SrTiO3. All the properties mentioned above were observed at all the three terminations. We also would like to compare their behavior at similar carrier density range. [Preview Abstract] 
Wednesday, March 16, 2016 12:51PM  1:03PM 
L29.00009: Quasi 2D electronic states with high spinpolarization in centrosymmetric $MoS_2$ bulk crystals L Plucinski, M. Gehlmann, G. Bihlmayer, I. Aguilera, E. Mlynczak, M. Eschbach, S. D\"oring, P. Gospodaric, B. Kardynal, S. Bl\"ugel, C. M. Schneider Time reversal dictates that nonmagnetic, centrosymmetric crystals cannot be spinpolarized. However, it has been recently shown that the electronic structure in these crystals can in fact show a high spinpolarization, as long as it is probed locally in real and in reciprocal space [1]. We present the first observation of this type of compensated polarization in $MoS_2$ bulk crystals. Using spin and angleresolved photoemission spectroscopy we directly observed a spinpolarization of more than 65$\%$ for distinct valleys in the electronic band structure. By additionally evaluating the probing depth of our method we find that these valence band states at the K point in the Brillouin zone are close to fully polarized for the individual atomic trilayers of $MoS_2$, which is confirmed by our density functional theory calculations. Furthermore, we show that these states are almost completely confined within two dimensions. Our findings prove that these highly desired properties of $MoS_2$ can be accessed without thinning it down to the monolayer limit. Our results are accessible at the preprint server: M. Gehlmann et al., arXiv:1510.04101 (2015). [1] X. Zhang, Q. Liu, J.W. Luo, A. J. Freeman, and A. Zunger, Nat. Phys. 10, 381 (2014). [Preview Abstract] 
Wednesday, March 16, 2016 1:03PM  1:15PM 
L29.00010: Dirac State in Giant Magnetoresistive Materials Y. Wu, N.H. Jo, M. Ochi, L. Huang, D. Mou, T. Kong, E. Mun, L. Wang, Y. Lee, S. L. Bud'ko, P. C. Canfield, N. Trivedi, R. Arito, A. Kaminski We use ultrahigh resolution, tunable, vacuum ultraviolet laserbased angleresolved photoemission spectroscopy (ARPES) to study the electronic properties of materials that recently were discovered to display titanic magnetoresistance. We find that that several of these materials have Diraclike features in their band structure. In some materials those features are ``ordinary" Dirac cones, while in others the linear Dirac dispersion of two crossing bands forms a linear object in 3D momentum space. Our observation poses an important question about the role of Dirac dispersion in the unusually high, nonsaturating magnetoresistance of these materials. [Preview Abstract] 
Wednesday, March 16, 2016 1:15PM  1:27PM 
L29.00011: ABSTRACT WITHDRAWN 
Wednesday, March 16, 2016 1:27PM  1:39PM 
L29.00012: Topological protection from random Rashba spinorbit backscattering: Ballistic transport in a helical Luttinger liquid HongYi Xie, Heqiu Li, YangZhi Chou, Matthew Foster Rashba spinorbit coupling enables irrelevant backscattering in a timereversal symmetric helical Luttinger liquid (HLL). We study the Landauer conductance $G$ of a HLL in the presence of random Rashba coupling as well as the densitydensity (Luttinger) interaction. We prove that the transport is purely ballistic ($G = e^2/h$) at any temperature due to the topology. The solution involves a unitary transformation that corresponds to a spin1/2 in a random, twocomponent timedependent magnetic field that preserves the projection of the spin along one fluctuating component (integrable dynamics). Our result is exact for a fixed realization of disorder, and avoids difficulties that arise in disorderaveraged perturbative calculations such as bosonization. We compare the HLL with random Rashba coupling to the Dyson model describing an ordinary spinless quantum wire with particlehole symmetry, which exhibits nonballistic transport even at zero temperature. [Preview Abstract] 
Wednesday, March 16, 2016 1:39PM  1:51PM 
L29.00013: Detection of topological states in twodimensional Dirac systems by the dynamic spin susceptibility Masaaki Nakamura, Akiyuki Tokuno We discuss dynamic spin susceptibility (DSS) in twodimensional (2D) Dirac electrons with spinorbit interactions to characterize topological insulators. The imaginary part of the DSS appears as an absorption rate in response to a transverse AC magnetic field, just like an electron spin resonance experiment for localized spin systems. We found that when the system is in a static magnetic field, the topological state can be identified by an anomalous resonant peak of the imaginary part of the DSS as a function of the frequency of the transverse magnetic field $\omega$. This anomalous peak is related to a transition between two Landau levels close to the Fermi level, which is not allowed in the trivial state. In the absence of the static magnetic field, the imaginary part of the DSS becomes a continuous function of $\omega$ with a threshold frequency $\omega_{\rm c}$. In this case, the topological and the trivial phases can also be distinguished by the values of $\omega_{\rm c}$ and by the line shapes. Thus the DSS is an essential and an experimentally observable physical quantity to characterize the topological insulators. [Preview Abstract] 
Wednesday, March 16, 2016 1:51PM  2:03PM 
L29.00014: Magnetic moment coupled to a helical edge can make weak correlations seem strong Jukka Vayrynen, Florian Geissler, Leonid Glazman We study the effect of a localized magnetic moment on the helical edge electron transport. The spin flips caused by the moment can be effective in the electron backscattering. We evaluate the resulting differential conductance as a function of temperature $T$ and applied bias $V$ for any value of $V/T$. At temperatures $T$ above the Kondo temperature, the deviation of the conductance from its quantized value displays a powerlaw temperature dependence, $\delta G \propto T^{\alpha}$. We show that the Luttinger liquid effects with $3/4 < K < 1$ may lead to a small exponent $\alpha < 1/2$. Values of $K$ close to 1 correspond to weaklycorrelated electrons. Our results provide an alternative interpretation of the recent experiment by Li et al. [1] where a powerlaw behavior of the conductance was attributed to strong correlation effects with the value of $K$ finetuned close to $1/4$. \\ \\ {\bf References} \\ [1] Li et al. Phys. Rev. Lett. {\bf 115}, 136804 (2015) [Preview Abstract] 
Wednesday, March 16, 2016 2:03PM  2:15PM 
L29.00015: Chiral exciton in the topological insulator Bi$_2$Se$_3$ HsiangHsi Kung, Maryam Salehi, Xueyun Wang, Nikesh Koirala, Matthew Brahlek, Alexander Lee, SangWook Cheong, Seongshik Oh, Girsh Blumberg Materials with novel band structures can host ``chiral excitons'', where the exciton emission preserves the helicity of the excitation photon, as recently demonstrated in transition metal dichalcogenide monolayers~\footnote{H. Zeng, J. Dai, W. Yao, D. Xiao and X. Cui \textit{Nature Nanotech.} \textbf{7} 490} \footnote{K.F. Mak, K. He, J. Shan and T.F. Heinz \textit{Nature Nanotech.} \textbf{7} 494}. Here, we report the observation of a highly polarized photoluminescence peak, which is due to chiral exciton emission in the topological insulator Bi$_2$Se$_3$. Surprisingly, the energy of the emission is centered at 2.26\,eV, much higher than the 0.3\,eV bulk band gap of Bi$_2$Se$_3$. The excitation profile shows maximum polarization around 2.60\,eV excitation, suggesting the chiral exciton is due to interband transition between the topological surface states and a bulk band. We demonstrate that the polarization of the exciton emission is insensitive to temperature and Bi$_2$Se$_3$ film thickness, providing a convenient and robust platform for optoelectronic applications. [Preview Abstract] 
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