Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session G7: Focus Session: Novel Topological Materials |
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Sponsoring Units: DMP DCMP Chair: Yong Chen, Purdue University Room: 006B |
Tuesday, March 3, 2015 11:15AM - 11:27AM |
G7.00001: Possible Chern insulators based on novel ferromagnetic substrates Jianpeng Liu, David Vanderbilt Previous work\footnote{K. F. Garrity and D. Vanderbilt, Phys. Rev. Lett. \textbf{110}, 116802 (2013).} has opened the possibility that one can obtain non-zero Chern numbers from the surface bands of atomic layers with strong spin-orbit coupling deposited on insulating ferromagnetic thin films. Following this idea, we carry out a theoretical search for Chern (i.e., quantum anomalous Hall) insulators formed by depositing heavy adatoms on top of stable insulating ferromagnetic substrates such as CrSiTe$_3$ and CrGeTe$_3$. These materials have a layered structure with weak van der Waals interlayer coupling, so it seems likely that thin films with clean surfaces will be experimentally stable. Moreover, the insulating character of these materials, with magnetization normal to the surface, are promising for realizing Chern-insulator states when adatoms are added. By searching over a series of heavy elements, we found non-zero Chern numbers in Bi and Tl-deposited CrSiTe$_3$ thin films, although with slightly negative indirect energy gaps. We expect that a global energy gap could be opened up by some further engineering, such as by applying epitaxial strain or additional atomic substitution. [Preview Abstract] |
Tuesday, March 3, 2015 11:27AM - 11:39AM |
G7.00002: Tunable topological electronic structure of silicene on semiconducting Bi/Si(111)-root3xroot3 substrate: a first-principles Chia-Hsiu Hsu, Zhi-Quan Huang, Bo-Hung Chou, Feng-Chuan Chuang, Hsin Lin, Arun Bansil Using first-principles calculations to obtain the crystal and electronic structures, we show that the 1x1 phase of silicene is energetically more favorable than the root3xroot3 silicene superstructure on a semiconducting Bi/Si(111)- root3xroot3 substrate. The band gap of the system is found to be influenced strongly through the participation of Bi-orbitals, which possess a larger spin-orbit coupling strength compared to Si. In particular, the non-trivial (topological) band gap of a few meV in freestanding 1x1 silicene enlarges to 150 meV and becomes trivial in the presence of the substrate. We further show how an out-of-the-plane external electric field can be used to tune the band gap and restore the non-trivial topological phase. [Preview Abstract] |
Tuesday, March 3, 2015 11:39AM - 11:51AM |
G7.00003: Amplified topological characteristics of MBE-grown Bi2Se3/II-VI semiconductor superlattices Zhiyi Chen, Lukas Zhao, Inna Korzhovska, Maria Tamargo, Lia Krusin Access to charge transport in Dirac surface states of topological insulators (TIs) such as Bi$_2$Se$_3$ is faced with two big challenges: one is significant bulk conduction and another is intermixing of topological states with nontopological 2DEG quantum well states formed by bending of bulk electronic bands near the surface. The latter effect is thought to arise via charge transfer from surface adatoms and, therefore, the choice of layers abutting topological surfaces can be critical. Here we report a successful molecular beam epitaxy growth of Bi$_2$Se$_3$/Zn$_{x}$Cd$_{1-x}$Se superlattices that improve topological characteristics of individual 8 - 10 nm thick TI layers. We show that in these superlattices the two-dimensional (2D) weak antilocalization quantum correction to classical magnetoresistance, associated with topological Berry phase, scales with the number of TI layers, with one quantum channel per layer. The Berry phase of $\pi$ obtained independently from Shubnikov de Haas quantum oscillations demonstrates robust topological interfaces in the multilayer structure. [Preview Abstract] |
Tuesday, March 3, 2015 11:51AM - 12:27PM |
G7.00004: Quantum Anomalous Hall Effect in Magnetic Semiconductors Invited Speaker: Chaoxing Liu In the quantum anomalous Hall effect, dissipationless charge currents are carried by chiral edge states and the Hall conductance is quantized, similar to the quantum Hall effect. Different from the conventional quantum Hall effect that requires strong magnetic fields, the quantum anomalous Hall effect is induced by strong exchange coupling between electron spin and magnetic moments in magnetic materials, so it can be realized at zero magnetic field, enabling the potential application of electronic devices with low energy consumption. Recent experiments on Cr or V doped BiSbTe thin films has observed the quantized Hall conductance at zero magnetic field and confirmed this novel effect. In this talk, I would like to discuss our recent work on the quantum anomalous Hall effect in magnetic semiconductors. I will first introduce two key ingredients, inverted band structures and ferromagnetic insulators, for the quantum anomalous Hall effect in realistic magnetic materials. Then, based on these two ingredients, I will discuss different classes of materials for the quantum anomalous Hall effect, focusing on magnetically doped InAs/GaSb quantum wells and magnetically doped LaOSbSe2 films. For magnetically doped InAs/GaSb quantum wells, we will show how band edge singularity can enhance spin susceptibility and lead to the quantum anomalous Hall state at a relatively high critical temperature. For magnetically doped LaOSbSe2 films, we find the quantum anomalous Hall effect can be tuned electrically by a gate voltage and identify layer dependent spin texture as the underlying physical reason. Finally, we will also discuss disordered transport and anisotropic magnetoresistance in the quantum anomalous Hall regime.\\[4pt] [1] Quantum Anomalous Hall Effect in Hg$_{\mathrm{1-y}}$Mn$_{\mathrm{y}}$Te Quantum Wells, Chao-Xing Liu, Xiao-Liang Qi, Xi Dai, Zhong Fang, Shou-Cheng Zhang, Phys. Rev. Lett. 101, 146802 (2008). \\[0pt] [2] In-plane Magnetization Induced Quantum Anomalous Hall Effect, Xin Liu, Hsiu-Chuan Hsu, Chao-Xing Liu, Phys. Rev. Lett. 111, 086802 (2013). \\[0pt] [3] Quantum Anomalous Hall Effect in Magnetically Doped InAs/GaSb Quantum Wells, Qingze Wang, Xin Liu, Hai-Jun Zhang, Nitin Samarth, Shou-Cheng Zhang, Chao-Xing Liu, Phys. Rev. Lett. 113, 147201 (2014). \\[0pt] [4] Electrically tunable multiple Dirac cones in thin films of (LaO)$_{2}$(SbSe$_{2})_{2}$ family of materials, Xiao-Yu Dong, Jian-Feng Wang, Rui-Xing Zhang, Wen-Hui Duan, Bang-Fen Zhu, Jorge Sofo and Chao-Xing Liu, arXiv:cond-mat/1409.3641 (2014). [Preview Abstract] |
Tuesday, March 3, 2015 12:27PM - 12:39PM |
G7.00005: ABSTRACT WITHDRAWN |
Tuesday, March 3, 2015 12:39PM - 12:51PM |
G7.00006: Electrically tunable multiple Dirac cones in thin films of (LaO)2(SbSe2)2 family of materials Xiaoyu Dong, Jianfeng Wang, Ruixing Zhang, Wenhui Duan, Bangfen Zhu, Jorge Sofo, Chaoxing Liu Two-dimensional Dirac physics has aroused great interests in condensed matter physics due to its importance in both fundamental physics and device applications. The ability to control the properties of Dirac cones is essential for the occurrence of various new phenomena and the development of next-generation electronic devices. Based on first-principles calculations and an analytical effective model, we propose a new Dirac system with eight Dirac cones in thin films of the (LaO)$_2$(SbSe$_2$)$_2$ family of materials with an external gate voltage. The advantage of this system lies in its tunability: the existence of gapless Dirac cones, their positions, Fermi velocities and anisotropy all can be controlled by an experimentally feasible gate voltage. We identify the layer dependent spin texture induced by spin-orbit coupling as the underlying physical reason for the tunability of Dirac cones in this system. As a consequence, we show that the electrically tunable quantum anomalous Hall effect with a high Chern number can be induced by introducing magnetization into this system. [Preview Abstract] |
Tuesday, March 3, 2015 12:51PM - 1:03PM |
G7.00007: An Efficient numerical method to calculate the conductivity tensor for disordered topological matter Jose H. Garcia, Lucian Covaci, Tatiana G. Rappoport We propose a new efficient numerical approach to calculate the conductivity tensor in solids. We use a real-space implementation of the Kubo formalism where both diagonal and off-diagonal conductivities are treated in the same footing. We adopt a formulation of the Kubo theory that is known as Bastin formula and expand the Green's functions involved in terms of Chebyshev polynomials using the kernel polynomial method. Within this method, all the computational effort is on the calculation of the expansion coefficients. It also has the advantage of obtaining both conductivities in a single calculation step and for various values of temperature and chemical potential, capturing the topology of the band-structure. Our numerical technique is very general and is suitable for the calculation of transport properties of disordered systems. We analyze how the method's accuracy varies with the number of moments used in the expansion and illustrate our approach by calculating the transverse conductivity of different topological systems. [Preview Abstract] |
Tuesday, March 3, 2015 1:03PM - 1:15PM |
G7.00008: Topological phases in rocksalt rare-earth binary compounds Hsin Lin, Minggang Zeng, Guoqing Chang, Yu-An Chen, Timothy Hsieh, Arun Bansil, Liang Fu Using first-principles calculations, we have investigated the electronic properties of bulk lanthanum monopnictides (LaX, X = N, P, As, Sb, Bi) and their thin films. We predict that lanthanum nitride (LaN) harbors a three-dimensional (3D) Dirac semimetal phase, and undergoes a phase transition driven by Coulomb interaction. Other lanthanum monopnictides are found to be semimetals with a nontrivial Z2 band topology. Furthermore, we find both two-dimensional (2D) Dirac semimetal and topological insulator phases in thin films of lanthanum monopnictides. The gapless Dirac states in bulk lanthanum monopnictides and their thin films can be attributed to the crystalline symmetry protection. [Preview Abstract] |
Tuesday, March 3, 2015 1:15PM - 1:27PM |
G7.00009: Giant magnetoresistance and band structure of topological semimetal RhSb3 Kefeng Wang, Limin Wang, Y. Nakajima, Renxiong Wang, Jie Yong, J. Paglione Recently materials with skutterudite structure such as CoSb3 were predicted to provide a promising platform for the realization of new topological materials such as topological insulators and Dirac-Weyl semimetals. Here we report a detailed study of the electronic structure and magnetotransport properties of high quality RhSb3 single crystals. First-principles electronic structure calculations reveal a highly dispersive band with Sb-p and Rh-3d weight that shows apparent band inversion. Inclusion of spin-orbit coupling leaves the Fermi level pinned to a doublet, indicating a topological semimetal. Our synthesized high-quality single crystals show typical metallic behavior but with very small residual resistivity ratio, a sign of semimetal behavior, in zero field. We will present magnetotrasport data that exhibits a very large magnetoresistance that hints of a very sensitive evolution of electronic properties and Dirac-like spectrum. [Preview Abstract] |
Tuesday, March 3, 2015 1:27PM - 1:39PM |
G7.00010: SmO thin films: a flexible route to correlated flat bands with nontrivial topology Deepa Kasinathan, Klaus Koepernik, Liu Hao Tjeng, Maurits Haverkort Using density functional theory based calculations, we show that the correlated mixed-valent compound SmO is a 3D strongly topological semi-metal as a result of a 4$f$-5$d$ band inversion at the X point. We also show that the topological non-triviality in SmO is very robust and prevails for a wide range of lattice parameters, making it an ideal candidate to investigate topological nontrivial correlated flat bands in thin-film form. Moreover, the electron filling is tunable by strain. In addition, we find conditions for which the inversion is of the 4$f$-6$s$ type, making SmO to be a rather unique system. The similarities of the crystal symmetry and the lattice constant of SmO to the well studied ferromagnetic semiconductor EuO, makes SmO/EuO thin film interfaces an excellent contender towards realizing the quantum anomalous Hall effect in a strongly correlated electron system. [Preview Abstract] |
Tuesday, March 3, 2015 1:39PM - 1:51PM |
G7.00011: Topological transport and atomic tunneling-clustering dynamics for aged Cu-doped Bi$_{2}$Te$_{3}$ crystals Fengqi Song, Taishi Chen, Wenkai Huang, Zhaoguo Li, Xuefeng Wang Here we report the suppression of the bulk conductance of the material by four orders of magnitude by intense aging in (Cu$_{0.1}$Bi$_{0.9})_{2}$Te$_{3.06}$ crystals. The weak antilocalization analysis, Shubnikov de Haas oscillations and scanning tunneling spectroscopy corroborate the transport of the topological surface states. The aging method therefore leads to an optimized band-insulating TI crystal and appeals to a free-of-IB crystal. STM visualizes the novel defect features of Cu dopants and their dynamics during the aging process, based on which the details of the aging process are further revealed by ab initio calculations. These calculations suggest that there exists a diffusion barrier at the interface of the Bi$_{2}$Te$_{3}$ QLs. During the aging process, Cu atoms freely migrate inside the QLs and frequently hit the barrier. The dopant atoms will also form clusters in between the QLs, leaving disorder within the QLs. This leads to a pronounced mobility suppression of the bulk electrons, finally allowing the observation of the TSS-related electron transport in bulk crystal samples. An atomic tunneling-clustering picture across a diffusion barrier of 0.57eV is proposed. (Nature Commun. 5, 5022(2014)) [Preview Abstract] |
Tuesday, March 3, 2015 1:51PM - 2:03PM |
G7.00012: Cu doping in topological insulator Bi$_{2}$Se$_{3}$ Yaoyi Li, Shivani Rajput, Michael Weinert, Lian Li Three-dimensional topological insulators Bi-chalcogenides exhibit spin-momentum locked surface Dirac states that can be tuned by doping. In this work, we grow Bi$_{2}$Se$_{3}$ films by molecular beam epitaxy, and tune the Dirac state by Cu doping. Using scanning tunneling microscopy, we find that in addition to substitutional incorporation, Cu also intercalates within the van der Waals gap between adjacent quintuple layers. Using scanning tunneling spectroscopy, we further find that Cu doping on the Bi$_{2}$Se$_{3}$ surface allows the controlled modification of the Dirac point by as much as 200 meV. These results and comparison with first-principles calculations will be discussed at the meeting. [Preview Abstract] |
Tuesday, March 3, 2015 2:03PM - 2:15PM |
G7.00013: Surface Shubnikov-de Haas oscillations of the topological hole conduction in the bulk insulator Tl$_{\mathrm{1-x}}$Bi$_{\mathrm{1+x}}$Se$_{2}$ Gaku Eguchi, Kenta Kuroda, Kaito Shirai, Akio Kimura, Masashi Shiraishi Three-dimensional (3D) topological insulator is a new series of matters which exhibit the surface-metallic state of Dirac fermions. The surface state has several unique characteristics such as lifted spin degeneracy, and novel electronic and spin transport is expected. However, most of the 3D topological insulators also involve bulk-metallic conduction, making it difficult to separate the surface-metallic conduction [1]. Recently, Fermi-level tuning and bulk-insulating behavior are reported in the self-doped Tl$_{\mathrm{1-x}}$Bi$_{\mathrm{1+x}}$Se$_{2}$ [2]. We report the electric transport properties of the bulk insulator Tl$_{\mathrm{1-x}}$Bi$_{\mathrm{1+x}}$Se$_{2}$. The electron-hole inversion by the doping was revealed by the Hall resistivity measurements. The two-dimensional Shubnikov-de Haas oscillations and the $\pi$ Berry phases, arising from the surface Dirac hole state, were also observed [3]. We compare the results with those reported in other 3D topological insulators, and discuss the surface transport properties.\\[4pt] [1] Y. Ando, Phys. J. Phys. Soc. Jpn. 82, 102001 (2013).\\[0pt] [2] K. Kuroda et al., arXiv~:1308.5521 (2013).\\[0pt] [3] G. Eguchi et al., Phys. Rev. B, in press (2014). [Preview Abstract] |
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