Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session P35: Topological Insulators: Magnetotransport |
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Sponsoring Units: DCMP Chair: Jens Bardarson, University of California, Berkeley Room: C140 |
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P35.00001: Transition from n-type to p-type topological insulator thin films of Bi$_{2}$Te$_{3}$ Jin-Feng Jia, Guang Wang, Xie-Gang Zhu, Yao-Yi Li, Tong Zhang, Jing Wen, Xi Chen, Ke He, Li-Li Wang, Xu-Cun Ma, Yi-Yang Sun, Sheng-Bai Zhang, Qi-Kun Xue By using angle-resolved photoemission spectroscopy, we have investigated the electronic structure of the Bi$_{2}$Te$_{3}$ films on Si(111) prepared by molecular beam epitaxy. It is found that the Bi$_{2}$Te$_{3}$ films change from n-type to p-type topological insulator when the growth mode changes layer-by-layer to step-flow, for a given beam flux ratio of Te$_{2(4)}$/Bi. In situ scanning tunneling microscopy/spectroscopy (STM/STS) measurements reveal formation of different defects, i. e., Te$_{Bi}$ and Bi$_{Te}$ antisite defects, which are responsible for the n- and p-type conductivity transition. A mechanism for the transition is proposed based on the STM experiment and first-principles calculations. The work suggests a simple way to regulating the chemical potential and Dirac fermion density on the surface of a topological insulator without external doping. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P35.00002: Magneto-Transport in Epitaxial Thin Film $Sb_{2}Te_{3}$ E.C. Gingrich, N.O. Birge, G. Wang, C. Uher We report magneto-transport measurements in epitaxially grown thin films of $Sb_{2}Te_{3}$. $Sb_{2}Te_{3}$ is a topological insulator candidate expected to possess a single Dirac cone on its surface.\footnote{H. Zhang et al., Nat. Phys. 5, 438 (2009).} Both semiconducting and metallic samples were measured, identified by the temperature response of their resistivities. Shubnikov-deHaas (SdH) oscillations were found in measurements with fields up to 9T in metallic samples at liquid helium temperature, but have yet to be observed in semiconducting samples. Measurements will be presented along with plans for further research. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P35.00003: Anomalous galvanomagnetism, cyclotron resonance and microwave spectroscopy of topological insulators Ewelina Hankiewicz, Grigory Tkachov The surface quantum Hall state, magneto-electric phenomena and their connection to axion electrodynamics have been studied intensively for topological insulators. One of the obstacles for observing such effects comes from nonzero conductivity of the bulk. To overcome this obstacle we propose to use an external magnetic field to suppress the conductivity of the bulk carriers. The magnetic field dependence of galvanomagnetic and electromagnetic responses of the whole system shows anomalies due to broken time-reversal symmetry of the surface quantum Hall state, which can be used for its detection. In particular, we find [1] linear bulk dc magnetoresistivity and a quadratic field dependence of the Hall angle, shifted rf cyclotron resonance, nonanalytic microwave transmission coefficient and saturation of the Faraday rotation angle with increasing magnetic field or wave frequency. \\[4pt] [1] G. Tkachov and E. M. Hankiewicz arXiv:1011.2756 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P35.00004: Experimental evidence on a diffusive metallic surface state by the magneto-resistance oscillation in the topological insulating Bi$_{2}$Te$_{3}$ Fengqi Song, Yuyuan Qin, Zhaoguo Li, Qianghua Wang, Jianguo Wan, Guanghou Wang, Zhe Qu The spin helicity seems bound to the generation of the surface state (SS) in a three-dimensional strong topological insulating system. Here we demonstrate a diffusive metallic SS by measuring the magneto-resistance oscillations in the Bi$_{2}$Te$_{3}$ nanoflakes. The products of the oscillating periods and the cross sections of the flakes result in the values of the flux quantum (h/e) and the half quantum (h/2e). The first observation of the h/2e oscillation shows the violence of the spin helicity of the SS The h/e oscillation persists during increasing the magnetic field, while the h/2e oscillation fails, indicating the diffusive origin of the SS and its weak antilocalization. The diffusive SS is found robust against the increases of the circumference till 10 micrometers and the temperatures till 64K, when the h/e oscillation disappears. The diffusive SS can be further activated by the spin-polarized scattering formed by the deposited Co islands. All the evidence agrees to recent simulations on a weak-localized SS upon the presence of strong disorder centers. This research was supported by the National Key Projects for Basic Research of China (Grant numbers: 2009CB930501, 2010CB923401). [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P35.00005: Two-dimensional surface state revealed in the quantum limit of a topological insulator R. McDonad, J. Analytis, J.-H. Chu, S. Riggs, I. Fisher, G. Boebinger Topological insulators possess a metallic surface state of massless particles, known as Dirac fermions whose spin is coupled to their momentum. The realization of this in Bi2Se3 has sparked considerable interest owing both to the potential for spintronic devices and in the investigation of the fundamental nature of topologically non-trivial quantum matter. However, the conductivity of these compounds tends to be dominated by the bulk of the material owing to chemical imperfection, making the transport properties of the surface nearly impossible to measure. We have systematically reduced the number of bulk carriers in the material Bi2Se3 to the point where a magnetic field can collapse them to their lowest Landau level. Beyond this field, known as the three-dimensional (3D)`quantum limi', the signature of the 2D surface state can be seen. At still higher fields, we reach the 2D quantum limit of the surface Dirac fermions. In this limit we observe an altered phase of the oscillations, which is related to the peculiar nature of the Landau quantization of topological insulators at high field. Furthermore, we observe quantum oscillations corresponding to fractions of the Landau integers, suggesting that correlation effects can be observed in this new state of matter. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P35.00006: The surface-state of the topological insulator Bi2Se3 revealed by cyclotron resonance Oscar Ayala-Valenzuela, James G. Analytis, Jiun-Haw Chu, Moaz-M. Altarawneh, Ian R. Fisher, Ross D. McDonald Recently, a large number of surface-sensitive probes have reported the existence of Dirac quasiparticles, similar to those reported in graphene, on the surface of single crystals of Bi2Se3 and related compounds. To date transport measurements of TIs have been dominated by the conductivity of the bulk, leading to substantial difficulties in resolving the properties of the surface. To this end, we use high magnetic field, rf- and microwave-spectroscopy to selectively couple to the surface conductivity of Bi2Se3 at high frequency. In the frequency range of a few GHz we observe a crossover from quantum oscillations indicative of a small 3D Fermi surface, to cyclotron resonance indicative of a 2D surface state. The frequency-magnetic field scaling of this resonance is inconsistent with the bulk effective mass, but more consistent with the dispersion and band filling of a Dirac-like surface state as observed by ARPES, with substantial many-body renormalization. Measurements as a function sample thickness aid in separating bulk and surface contributions and indicate that the band filling of the Dirac cone varies from cleave to cleave. [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P35.00007: Surface Hall response of 3D topological insulators Dimitrie Culcer, Sankar Das Sarma We determine the Hall conductivity due to the surface states of 3D topological insulators in the presence of a weak perpendicular magnetic field and/or magnetization. We consider electron doping and calculate all known contributions to the Hall current, including the intrinsic, skew scattering and side jump terms. Skew scattering contributes to in the Born approximation, as well as giving the usual contribution of third order in the scattering potential. We identify a side-jump scattering term together with an intrinsic side-jump term, which give contributions of a similar magnitude. The dominant term by several orders of magnitude is of the order of the conductivity quantum, and includes a topological contribution and a renormalization due to scattering. The result is independent of the Rashba spin-orbit constant, as well as of the impurity concentration. It has different signs depending on whether the principal source of scattering is charged impurities or short-range interface roughness. We expect our results to help disentangle surface transport from bulk transport in these materials [1]. \\[0pt] [1] D. Culcer, E. H. Hwang, T. D. Stanescu, and S. Das Sarma, Phys. Rev. B 82, 155457 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P35.00008: In Search of the Quantum Anomalous Hall Effect in Ferromagnetic Cr-Bi$_{2}$Se$_{3}$ Topological Insulator Thin Films Pascal P.J. Haazen, J.-B. Laloe, D. Heiman, P. Jarillo-Herrero, J.S. Moodera A recent prediction that the topological insulator $Bi_{2}Se_{3}$ can become magnetically ordered upon doping with Cr or Fe opens up the possibility of observing the quantum anomalous Hall effect, in the absence of an external magnetic field [1]. We report on molecular beam epitaxy-grown Cr-Bi$_{2}$Se$_{3}$ thin-films with a Cr content of $0\sim10$ at.\%, and their properties. Our films show highly oriented crystallinity up to a Cr content of 8\%, as required for ferromagnetic ordering. Films with Cr were ferromagnetic; the measured saturation magnetic moment per Cr atom is $1\mu_{B}$ for the crystalline films, with a $T_{C}$ of up to $\sim25 K$. Currently we are investigating the transport characteristics. Varying the electro-chemical potential level in our quantum anomalous Hall insulators by gating should give rise to plateaus of Hall conductance as the Fermi level passes through the energy gap, due to the breaking of time-reversal symmetry caused by the magnetic ions. \\[4pt] [1] R. Yu et al. Science 329, 61 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P35.00009: Quantum Oscillations in a topological insulator Bi$_2$SeTe$_2$ with large bulk resistivity Jun Xiong, DongXia Qu, Robert Cava, N. Phuan Ong To date, transport experiments on Topological Insulators are seriously hampered by bulk conductance $G_b$ arising from impurity bands or band bending. Because of the large $G_b$, the surface currents carried by the massless Dirac surface states have been very difficult to resolve. We report measurements on the new topological insulator Bi$_2$SeTe$_2$ which has an unusually highly bulk resistivity $\rho$ (6 $\Omega$cm at 4 K, or 1,000 times higher than in Bi2Te3). Despite the large $\rho$, Shubnikov-de Haas (SdH) oscillations are clearly resolved in the Hall conductance up to 38 K, which implies a very high surface mobility. In a field $B$ of 14 T, Landau Levels (LLs) n = 4-9 are well resolved. We will describe the value of the Onsager phase $\gamma$ fixed by the index plot of the LLs. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P35.00010: Thermoelectric Properties of Non-Metallic Topological Insulator Bi$_{2}$Te$_{3}$ at High Magnetic Fields Dong-Xia Qu, Yew San Hor, Robert J. Cava, N. Phuan Ong Three-dimensional topological insulators are a new class of electronic systems characterized by a bulk insulating state and conducting surface states with Dirac-like energy-momentum dispersion [1, 2]. One of the interesting aspects of this material is how the surface states affect thermoelectric properties of the whole electronic system, given that the bismuth based topological insulators are also excellent thermoelectric materials. We studied the low-temperature thermoelectric transport properties of high-mobility bulk topological insulator Bi$_{2}$Te$_{3}$ at high magnetic fields up to 35 T. We found remarkably large quantum oscillations in the thermopower of the surface states over a field range of 14 to 35 T. The existence of a non-zero Berry's phase in surface electrons is confirmed from the magneto-oscillations of both thermopower and magnetoresistance.\\[4pt] [1] L. Fu, C. L. Kane, and E. J. Mele, Phys. Rev. Lett. \textbf{98}, 106803 (2007)\\[0pt] [2] Y. Xia \textit{et al}., Nat. Phys. \textbf{5}, 398 (2009). [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P35.00011: Large thermoelectric figure of merit for three-dimensional topological Anderson insulators via line dislocation engineering Oleg Tretiakov, Artem Abanov, Shuichi Murakami, Jairo Sinova We study the thermoelectric properties of three-dimensional topological Anderson insulators with line dislocations. We show that at high densities of dislocations the thermoelectric figure of merit $ZT$ can be dominated by one-dimensional topologically protected conducting states channeled through the lattice screw dislocations in the topological insulator materials with a nonzero time-reversal-invariant momentum such as Bi$_{0.9}$Sb$_{0.1}$. When the chemical potential does not exceed much the mobility edge the $ZT$ at room temperatures can reach large values, much higher than unity for reasonable parameters, hence making this system a strong candidate for applications in heat management of nanodevices. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P35.00012: Quantum Oscillations and Quantum Hall Effect in Topological Insulator Material Bi$_{2}$Se$_{3 }$ Helin Cao, Ireneusz Miotkowski, Tian Shen, Yong Chen Bi$_{2}$Se$_{3}$ has attracted strong attention recently as a prototype topological insulator material. We have measured magneto-transport in metallic Bi$_{2}$Se$_{3}$ crystals. At high magnetic field (B), the longitudinal resistance (R$_{xx})$ displays characteristic Shubnikov--de Haas (SdH) oscillations (periodic in 1/B). The measurements in tilted magnetic field show the SdH oscillations are only controlled by the perpendicular component of B, indicating 2D nature of charge carriers. We also observed quantized plateaus in Hall resistance (R$_{xy})$ concomitant with the minima in R$_{xx}$. From the temperature dependence of the SdH oscillations, we extract a Fermi velocity $\sim $5.9*10$^{5}$m/s, and an effective mass $\sim $0.14m$_{e}$ (m$_{e}$ is the electron mass). We discuss possible relations of our observations to topological surface states, as well as contributions from individual 2D quintuple layers of Bi$_{2}$Se$_{3}$. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P35.00013: High field magnetotransport in high purity crystals of topological insulator Bi$_2$Se$_3$ Nicholas Butch, Paul Syers, Johnpierre Paglione We have synthesized crystals of high purity undoped Bi$_2$Se$_3$ with carrier density less than 10$^{17}$ cm$^{-3}$ and high bulk mobility. Thorough characterizations of transport and optical properties in fields up to 14 T suggest a high surface scattering rate [1-3]. In a search for quantum oscillations from the surface carriers, we performed measurements of longitudinal and Hall resistance in pulsed magnetic fields of up to 60 T. Due to the as-yet poorly characterized environmental sensitivity of the Bi$_2$Se$_3$ surfaces, we performed on-site cleaving and application of leads under dry flowing nitrogen. The Hall measurements and temperature- and angle-dependence of the longitudinal magnetoresistance will be discussed. \\[4pt] [1] N. P. Butch, et al., Phys. Rev. B 81, 241301 (2010) \\[0pt] [2] A. B. Sushkov, et al., Phys. Rev. B 82, 125110 (2010) \\[0pt] [3] G. S. Jenkins, et al., Phys. Rev. B 82, 125120 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P35.00014: Anomalous Aharonov-Bohm Conductance Oscillations from Topological Insulator Surface States Yi Zhang, Ashvin Vishwanath We study Aharonov-Bohm (AB) conductance oscillations arising from the surface states of a topological insulator nanowire, when a magnetic field is applied along its length. With strong surface disorder, these oscillations are predicted to have a component with anomalous period $\Phi_{0}=hc/e$, twice the conventional period. The conductance maxima are achieved at odd multiples of $\Phi_{0}/2$, implying that a $\pi$ AB phase for electrons strengthens the metallic nature of surface states. This effect is special to topological insulators, and serves as a defining transport property. A key ingredient, the surface curvature induced Berry phase, is emphasized here. We discuss similarities and differences from recent experiments on $Bi_{2}Se_{3}$ nanoribbons, and optimal conditions for observing this effect. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P35.00015: First-principles calculation of the full orbital magnetoelectric response Andrei Malashevich, Ivo Souza, Sinisa Coh, David Vanderbilt The possibility of a quantized magnetoelectric (ME) effect in $Z_2$ topological insulators suggests that the orbital part of the ME response can, at least in principle, be comparable in magnitude to the total response of known ME materials.\footnote{S. Coh {\it et al.}, arXiv:1010.6071.} A band theory of the orbital ME response of generic insulators was recently developed,\footnote{A. Malashevich {\it et al.}, New J. Phys. {\bf 12}, 053032 (2010)} paving the way for first-principles calculations. Two types of terms contribute to the response. The Chern-Simons term, which only depends on the unperturbed valence Bloch states, was the subject of a recent Wannier-based calculation.$^1$ The Kubo terms require a knowledge of the Bloch states at first order in the electric field and can be calculated by finite differences from the change in orbital magnetization induced by small electric fields.$^2$ We present preliminary results of such a calculation for Cr$_2$O$_3$, using the Berry-phase approach to calculate the electronic structure under a finite electrical bias. By monitoring the field-induced changes in orbital and spin magnetization and comparing the results obtained with and without structural relaxation, all contributions to the ME coupling can be computed. [Preview Abstract] |
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