Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session B42: Focus Session: Experiments on Samarium Hexaboride |
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Sponsoring Units: DMP Chair: Vidya Madhavan, Boston College Room: Mile High Ballroom 4A |
Monday, March 3, 2014 11:15AM - 11:27AM |
B42.00001: High Field Magnetoresistance Measurements on the Surface States of Samarium Hexaboride using Corbino Structures Steven Wolgast, Yun Suk Eo, Gang Li, Ziji Xiang, Colin Tinsman, Tomoya Asaba, Benjamin Lawson, Fan Yu, J.W. Allen, Kai Sun, Lu Li, Cagliyan Kurdak, Dae-Jeong Kim, Zachary Fisk The recent conjecture of a topologically-protected surface state in SmB$_{6}$ and the verification of robust surface conduction below 4 K have led to a large effort to understand the surface states. Extracting carrier density and charge mobility of these states via Hall measurements is complicated because current can flow on all surfaces of a topological insulator, each of which can have different transport characteristics. We study magnetotransport of SmB$_{6}$ surfaces up to 45 T using a Corbino geometry that is sensitive to individual surfaces. The Corbino allows us to measure conductivity, $\sigma_{xx}$, in both parallel and perpendicular magnetic fields. In the parallel geometry both (110) and (100) samples show a strong negative magnetoresistance. We extracted information about the carrier mobility from the ratio of the perpendicular and parallel magnetoresistance traces. The (110) surface had the highest carrier mobility of 122 cm$^{2}$/Vs with a carrier density of $2.5\times10^{13}$ cm$^{-2}$. The conduction on both polar (100) and non-polar (110) surfaces strongly indicates that the conduction must have a non-polarity-driven origin. [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B42.00002: Low Field Magnetoresistance Measurements on the Surface States of Samarium Hexaboride using Corbino Structures Yun Suk Eo, Steven Wolgast, Cagliyan Kurdak, Gang Li, Ziji Xiang, Colin Tinsman, Tomoya Asaba, Benjamin Lawson, Fan Yu, Lu Li, Kai Sun, James Allen, Dae-Jeong Kim, Zachary Fisk Recently, SmB$_{6}$ attracted great attention by numerous reports suggesting it to be an ideal strong 3D topological insulator. By spin-momentum locking, the quantum correction of conductivity of a topological surface state of this material is expected to result in weak anti-localization (WAL). To study this effect, we have performed low field magnetorestance measurements on (100) and (110) Corbino samples at temperatures down to 60 mK. Many of the Corbino samples that we have studied so far have a dip in the magnetoresistance trace that resembles the WAL feature. The size and temperature dependence of this feature are in general consistent with those expected from a quantum interference correction. However, after careful investigation we found the features shrink in amplitude with slower magnetic field sweep rates. Also, the traces have a hysteretic signal of an unknown origin. The potential coupling between a magnetic oxide layer forming on the surfaces of SmB$_{6}$ and the topological surface states will be discussed. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B42.00003: Surface transport in Topological Kondo Insulator SmB6 Jing Xia, Sean Thomas, Dae-jeong Kim, Fisk Zach, Jing Xia In this talk we will discuss the existence of topological order in a 3D strongly correlated material SmB6, which has recently been proposed theoretically as a topological Kondo insulator. We will present transport evidence for a highly conductive surface state surrounding a truly insulating bulk: At low temperature We found that the Hall resistance scales with the surface, and is independent of the thickness. Using non-local transport measurements, we demonstrate that the electric conducting is mostly along the surface at low temperatures and in zero magnetic field. We demonstrate that the surface dominated conduction is destroyed by small amounts of magnetic doping but survives non-magnetic doping. At even lower temperatures, we demonstrate the weak localization effect, which is consistent with a surface state with spin momentum locking. The Kondo-effect-like resistance saturation will also be discussed. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B42.00004: Surface electronic structure of topological Kondo insulator candidate SmB6: a high-resolution ARPES study Nasser Alidoust, Madhab Neupane, Su-Yang Xu, Chang Liu, Ilya Belopolski, Guang Bian, M. Zahid Hasan, T. Kondo, S. Shin, T.-R. Chang, H.-T. Jeng, L. Balicas, T. Durakiewicz, H. Lin, A. Bansil, D.-J. Kim, Z. Fisk The Kondo insulator SmB6 has been known to exhibit low temperature transport anomaly and has recently attracted attention as a new topological insulator candidate. By combining low temperature and high resolution of the laser-based ARPES, for the first time, we probe the surface electronic structure of the anomalous conductivity regime. We observe that the bulk bands exhibit a Kondo gap of 14 meV and identify in-gap low-lying states within 4 meV of the Fermi level on the surface of this material. The low-lying states are found to form electron-like Fermi surface pockets that enclose the X and the $\Gamma$ points of the surface BZ. These states disappear as temperature is raised in correspondence with the complete disappearance of the 2D conductivity channels. While the topological nature of the in-gap metallic states cannot be ascertained, our measurements are consistent with the predicted first-principle topological Kondo insulator phase in this material. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B42.00005: Growth and Intrinsic Physical Properties of the Kondo Insulator SmB$_{6}$ William Phelan, Seyed Koohpayeh, Patrick Cottingham, Leslie Schoop, Robert Cava, Collin Broholm, Tyrel McQueen SmB$_{6}$ is a long-studied Kondo Insulator that has come back into focus recently following theoretical predictions that it may harbor topologically protected surface states. Materials containing such surface states are referred to as topological insulators, and may impact technologically important areas such as quantum computing and spintronics. We report the preparation of single crystals of SmB$_{6}$ \textit{via} the floating zone technique, and the impact of growth conditions on the physical properties, including low temperature electrical transport. These results provide insights into the nature of the anomalous low temperature state of SmB$_{6}$. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B42.00006: Terahertz transmission studies of the topological Kondo insulator candidate SmB$_6$ Nicholas J. Laurita, Christopher M. Morris, Seyed Koopayeh, Patrick Cottingham, W. Adam Phelan, Leslie Schoop, Tyrel M. McQueen, N. Peter Armitage The Kondo insulator SmB$_6$ has long been known to display anomalous transport behavior at low temperatures (T$<10$ K) and high pressures. At low temperatures, a plateau is observed in the resistivity, contrary to the divergence expected for a normal Kondo insulator. Recent theoretical calculations suggest that SmB$_6$ may be the first topological Kondo insulator, a material with a Kondo insulating bulk, but topologically protected metallic surface states.\footnote{M. Dzero \textit{et al.}, Phys. Rev. Lett. \textbf{104}, 106408 (2010)} Here, time domain terahertz spectroscopy (TDTS) is used to investigate the temperature dependent low frequency optical conductivity of single crystals of SmB$_6$. We find evidence for a substantial bulk conductivity at a frequency of a few hundred GHz, which challenges the notion of this material as having a clean gap. The evidence for topological surface states and their properties will be discussed. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B42.00007: Magnetic Excitations of the Kondo Insulator SmB$_{6}$ Wesley Fuhrman, Jonathan Leiner, Garrett Granroth, Mark Lumsden, Pavel Alekseev, Jean-Michel Mignot, Seyed Koohpayeh, Patrick Cottingham, William Phelan, Leslie Schoop, Robert Cava, Tyrel McQueen, Collin Broholm Research within the past year indicates the long-studied Kondo insulator SmB$_{6}$ may be a topological insulator, with an insulating bulk at low temperatures and topologically protected metallic surface states. Because electron-electron interactions give rise to the insulating state, there is intense interest in SmB$_{6}$. Using time-of-flight inelastic neutron scattering, we have probed magnetic excitations over a wide range of energy and momentum transfer. Consistent with previous work there is a resonant mode near 14 meV and a broad spectrum of excitations centered near 30 meV. This data set provides a comprehensive map of the Q-dependence of the excitations throughout the Brillouin zone allowing for comparison to theoretical models describing the anomalous insulting state. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B42.00008: Two dimensional Fermi surfaces in Kondo insulator SmB$_{6}$ Gang Li, Ziji Xiang, Fan Yu, Tomoya Asaba, Benjamin Lawson, Peng Cai, Colin Tinsman, Adam Berkley, Steven Wolgast, Yun Suk Eo, Dae-Jeong Kim, Cagliyan Kurdak, James Allen, Kai Sun, Xianhui Chen, Yayu Wang, Zachary Fisk, Lu Li Samarium hexaboride SmB$_{6}$ belongs to a class of strongly correlated heavy Fermion semiconductors, in which hybridization between itinerant electrons and localized orbitals lead to opening of a charge gap at low temperature. However, the resistivity of SmB$_{6}$ does not diverge but saturates below $\sim$ 2 Kelvin. Former studies suggested that this residual conductance is contributed by intragap states with various origins. Recent theoretical developments suggest that the particular symmetry of energy bands of SmB$_{6}$ may host a topologically non-trivial surface state, i.e., a topological Kondo insulator. To probe the Fermiology of the possible metallic surface state, we use highly sensitive torque magnetometry to detect the de Haas van Alphen (dHvA) effect due to Landau level quantization. Our detailed angular and temperature data suggest two-dimensional Fermi Surfaces lie in both crystalline (001) and (101) surface planes of SmB$_6$. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B42.00009: High Field Torque Magnetometry of SmB$_6$ Colin Tinsman, Gang Li, Ziji Xiang, Fan Yu, Tomoya Asaba, Benjamin Lawson, Peng Cai, Adam Berkley, Steven Wolgast, Yun Suk Eo, Dae-Jeong Kim, Cagliyan Kurdak, James Allen, Kai Sun, Xianhui Chen, Yayu Wang, Zachary Fisk, Lu Li The Kondo Insulator SmB$_6$ has been observed to have a small, residual surface conductance apparent below 5 K. Torque Magnetometry was employed to find quantum oscillations in the magnetization -- the de Haas-van Alphen effect. Using magnetic fields up to 45 T, we were able to resolve 3 different pieces of Fermi Surface, at frequencies of 35 T, 300 T, and 400 T. Angular dependence of these oscillation frequencies indicate that they are two dimensional in nature. Additionally, Landau Level indexing analysis for the 35 T pocket gives a $-1/2$ intercept in the infinite field limit, a Berry phase contribution consistent with Dirac electronic system such as graphene. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B42.00010: Scanning Tunneling Spectroscopy and Imaging of Topological Kondo Insulators Michael Yee, Yang He, Anjan Soumyanarayanan, Dae-Jeong Kim, Zachary Fisk, Jennifer E. Hoffman Topological insulators host spin-polarized surface states which robustly span the band gap and hold promise for novel applications. Recent theoretical predictions have suggested that topologically protected surface states may similarly span the hybridization gap in some strongly correlated heavy fermion materials, particularly $SmB_6$. Scanning tunneling spectroscopy (STS) is a powerful tool for studying topological materials because it is directly sensitive to the surface states of interest, and their scattering processes. Here we present the first atomic resolution spectroscopic study of the cleaved surface of $SmB_6$ [1]. Using a combination of real space imaging and filled and empty state spectroscopy, we reveal a robust hybridization gap that universally spans the Fermi level on four distinct surface morphologies despite shifts in the f band energy. Using a cotunneling model, we separate the density of states of the hybridized bands from which the predicted topological surface states must be disentangled. Our technique lays the groundwork for understanding the first strongly correlated topological insulator, and implements a generally applicable method to quantitatively understand a wider class of Kondo insulators.\\[4pt] [1] arXiv:1308.1085 [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B42.00011: Emergence of a coherent in-gap state in SmB$_{6}$ Kondo insulator revealed by scanning tunneling spectroscopy Wei Ruan, Cun Ye, Minghua Guo, Fei Chen, Xianhui Chen, Guangming Zhang, Yayu Wang SmB$_{6}$ is a Kondo insulator that exhibits transport anomalies at temperatures below 5 K, where resistivity saturates instead of diverging. It has long been ascribed to in-gap states which become coherent at low temperatures. Recently, a host of theoretical and experimental studies suggest that SmB$_{6}$ may be a topological Kondo insulator with topological protected metallic surface states. In this talk we present STM studies of the (001) surface of cleaved SmB$_{6}$ single crystal. We have observed four different kinds of surface morphologies with similar \textit{dI/dV} spectra. Variable temperature \textit{dI/dV} spectroscopy up to 60 K reveals a gap-like density of state suppression around the Fermi level, which is due to the hybridization between the itinerant Sm 5$d$ band and localized Sm 4$f$ band. At temperatures below 40 K, a sharp coherence peak emerges within the hybridization gap near the lower gap edge. We propose that the in-gap resonance state is due to a collective excitation in magnetic origin that is specific to the mixed valent Kondo insulator. Implications of these results to the electronic structure evolution and transport anomaly in SmB$_{6}$ will be discussed. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B42.00012: Valence and moment of Sm in SmB$_6$ under pressure Nicholas Butch, Jason Jeffries, Paul Syers, Johnpierre Paglione, Yuming Xiao, Paul Chow We studied the valence and magnetic moment of Sm ions in the mixed-valent compound SmB$_6$ via x-ray spectroscopy. Using diamond anvil cells, the pressure dependence of these properties was measured to at least 20 GPa. We will discuss pressure-induced magnetic order in this compound, as well as ramifications for the sought-after ambient-pressure topological state. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B42.00013: Combinational growth and physical properties of possible topological Kondo insulator SmxB1-x films Jie Yong, Richard Ruchoski, Sean Fackler, Ichiro Takeuchi, Richard Greene Kondo insulator Samarium hexboride (SmB6) has caused great interest due to its possible topological surface state and its interplay with correlated physics. Thin films of SmB6 are highly desirable for surface sensitive measurements and novel device fabrications. Since both PLD and sputtering yield highly boron deficient films, we explored the thin film growth through combinational sputtering of a stoichiometrically SmB6 target and Boron target. Thin SmxB1-x films are fabricated with x continuously varies from 0 to 1. We found that when x\textgreater 0.14, resistivity measurements show upturns around 50K and saturations below 10K, consistent with the bulk results. Resistance ratios between 300K and 4K are around 1.5, which is consistent with a much larger surface-to-bulk ratio. The films with x \textless 0.14 are more insulating at room temperature and show insulating behavior. Other details of the characterization of these films will also be presented. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B42.00014: Polarity-driven surface metallicity in SmB$_6$ Andrea Damascelli, Z.-H. Zhu, A. Nicolaou, G. Levy, N.P. Butch, P. Syers, X.F. Wang, J. Paglione, G.A. Sawatzky, I.S. Elfimov By a combined angle-resolved photoemission spectroscopy and density functional theory study, we discover that the surface metallicity is polarity driven in SmB$_6$. Two surface states, not accounted for by the bulk band structure, are reproduced by slab calculations for coexisting B$_6$ and Sm surface terminations. Our analysis reveals that a metallic surface state stems from an unusual property, generic to the (001) termination of all hexaborides: the presence of boron 2$p$ dangling bonds, on a polar surface [1]. The discovery of polarity-driven surface metallicity sheds new light on the 40-year old conundrum of the low temperature residual conductivity of SmB$_6$, and raises a fundamental question in the field of topological Kondo insulators regarding the interplay between polarity and nontrivial topological properties. \\[4pt] [1] Z.-H. Zhu et al., Phys. Rev. Lett. to appear (arXiv:1309.2945) [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B42.00015: Effect of magnetic impurity substitution in topological Kondo insulator SmB6 Xiangfeng Wang, Yasuyuki Nakajima, Yeping Jiang, Richard Greene, Johnierre Paglione A topological insulator is a material with topologically protected metallic boundaries and an insulating bulk. The strongly-correlated Kondo system SmB6 has recently been widely investigated owing to its promise of being the first realized topological Kondo insulator. Many results have confirmed the existence of metallic surface states and provided evidence of their non-trival topological nature. Here we report a study of the effect of magnetic transition metal impurity substitution in SmB6 on transport and thermodynamic properties, providing an important insight into the nature of the surface states. [Preview Abstract] |
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