Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session S3: Experiments on Three-Dimensional Topological Insulators, including SmB6 |
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Sponsoring Units: DCMP Chair: Yang Xu, Purdue University Room: 262 |
Thursday, March 16, 2017 11:15AM - 11:27AM |
S3.00001: Interplay between surface and bulk states in the Topological Kondo Insulator SmB$_6$ Sangram Biswas, Monica Ciomaga Hatnean, Geetha Balakrishnan, Aveek Bid Kondo insulator SmB$_6$ is predicted to have topologically protected conducting surface states(TSS). We have studied electrical transport through surface states(SS) at ultra-low temperatures in single crystals of SmB$_6$ using local-nonlocal transport scheme and found a large nonlocal signal at temperatures lower than bulk Kondo gap scale. Using resistance fluctuation spectroscopy, we probed the local and nonlocal transport channels and showed that at low temperatures, transport in this system takes place only through SS. The measured noise in this temperature range arises due to Universal Conductance Fluctuations whose statistics was found to be consistent with theoretical predictions for that of 2D systems in the Symplectic symmetry class. We studied the temperature dependence of noise and found that, unlike the topological insulators of the dichalcogenide family, the noise in surface and bulk conduction channels in SmB$_6$ are uncorrelated - at sufficiently low temperatures, the bulk has no discernible contribution to electrical transport in SmB$_6$ making it an ideal platform for probing the physics of TSS. [Preview Abstract] |
Thursday, March 16, 2017 11:27AM - 11:39AM |
S3.00002: Neutron scattering on the Topological Kondo insulator SmB6 Wesley T. Fuhrman, J. Leiner, P. Nikolic, G. E. Granroth, M. B. Stone, M. D. Lumsden, L. DeBeer-Schmitt, P. A. Alekseev, J.-M. Mignot, S. M. Koohpayeh, P. Cottingham, W. Adam Phelan, L. Schoop, J. Chamorro, Yang Zhao, Thomas Keller, Jeff Lynn, T.M. McQueen, Collin Broholm We present recent neutron scattering experiments on SmB6 involving the 14meV spin-exciton with emphasis on it's relation to the low-energy DOS, which has had renewed interest in light of anomalous quantum oscillations. The exciton is evidence of strong correlations driving the novel insulating state in SmB6 and can be described by a perturbative slave boson treatment of a hybridized 2 species (d/f) band structure within the Anderson model. [Preview Abstract] |
Thursday, March 16, 2017 11:39AM - 11:51AM |
S3.00003: Comparison of the properties of flux and floating zone grown crystals of the Topological Kondo Insulator SmB6 Geetha Balakrishnan, Talha Ahmad, Monica Ciomaga Hatnean, Martin R. Lees Since the theoretical predictions that the well known Kondo Insulator SmB$_{6}$ exhibits topological behaviour, rapid progress has been made in the study of this new class of Topological Kondo Insulators (TKIs). Experimentalists have taken up the challenge to develop experiments designed to observe the topological surface states in SmB$_{6}$, while working closely with theoreticians in order to understand the topological behaviour of this material. Despite the tremendous developments in the field, to this day, SmB$_{6}$ continues to puzzle researchers. Recent reports place particular emphasis on sample quality and the compositions of single crystals of SmB$_{6}$ synthesized using different methods and suggest that the physical properties they exhibit are greatly affected by any small variations in the composition. We have undertaken a detailed comparative study of the physical properties of flux and floating zone single crystals of SmB$_{6}$ through resistivity, magnetoresistance and Hall effect measurements. We present the correlation of these properties with the chemical composition as well as their surface chemistry. [Preview Abstract] |
Thursday, March 16, 2017 11:51AM - 12:03PM |
S3.00004: Muon spin relaxation investigation of anomalous magnetism in pure and Fe-doped SmB6 Kola Akintola, Anand Pal, Matt Potma, Shanta Saha, Xiangfeng Wang, Johnpierre Paglione, Jeff Sonier The intermediate-valence compound SmB$_6$ is a well-known Kondo insulator, in which hybridization of itinerant conduction electrons with localized \textit{f}-electrons leads to a transition from metallic to insulating behaviour at low temperatures. In recent years SmB$_6$ has attracted considerable attention with studies suggesting that a topological insulating state arises at low temperatures. We have carried out muon spin rotation/relaxation ($\mu$SR) measurements on both pure and 0.5$\%$ Fe-doped SmB$_6$ single crystals. In zero field (ZF) we observe a saturated relaxation rate at low temperatures, indicative of slow fluctuating magnetic moments. The saturated relaxation rate occurs in the same temperature region where the resistance saturates. This is surprising given that the low-temperature conduction arises from two-dimensional surface states that occur in the hybridization gap, whereas the $\mu$SR measurements exclusively probe the bulk. Previously the saturated ZF relaxation rate was attributed to in-gap magnetic states. However, measurements in a magnetic field suggest that this is a consequence of uncompensated Sm moments in a truly insulating bulk state. [Preview Abstract] |
Thursday, March 16, 2017 12:03PM - 12:15PM |
S3.00005: Heat transport by delocalized spin excitations in SmB$_6$ Michael Sutherland, Will Toews, Xiaoye Chen, M. Ciomaga Hatnean, G Balakrishnan, Y.T. Hsu, Suchitra Sebastian, Rob Hill Measurements of thermal conductivity at low temperatures may offer insights into the low-energy landscape of excitations in the bulk of a material. Here we report thermal transport data in the canonical Kondo insulator, SmB$_6$. In zero magnetic field, we find that the heat transport below T=1 K can be entirely accounted for by phonons, consistent with an insulating state with no long-range magnetic order. With the application of a magnetic field however, new heat carriers are introduced which are neither electrons nor phonons. We discuss these results within the framework of delocalized spin excitations, supporting a gapping of the charge, but not the spin degrees of freedom in the bulk of SmB$_6$. [Preview Abstract] |
Thursday, March 16, 2017 12:15PM - 12:27PM |
S3.00006: A study on the effect of Sn concentration on the transport properties of epitaxial Bi$_{\mathrm{2-\delta }}$Sn$_{\mathrm{\delta }}$Te$_{\mathrm{3}}$ thin films Suyoun Lee, Seong Won Cho, Kwang-Chon Kim, Seong Keun Kim, Byung-ki Cheong, Jin-Sang Kim We have performed a systematic study on the transport properties of epitaxial Bi$_{\mathrm{2-\delta }}$Sn$_{\mathrm{\delta }}$Te$_{\mathrm{3}}$ thin films with varying $\delta $ from the underdoped to the overdoped region. By compensating unintentional carriers doped by defects in Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$, the bulk conductivity has been found to be highly suppressed with a very small amount of Sn resulting in the insulating behavior of resistivity at high temperatures. Furthermore, from the temperature- and magnetic field-dependence of the longitudinal resistance and the Hall resistance, we have separately characterized the surface state showing that the phase-relaxation length of the surface state of Sn-doped Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$ films increases up to \textasciitilde 250 nm at 1.8 K, about three times of that of the undoped Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$ film. And, finally, the magnetoresistance ratio (MR) of the Sn-doped Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$ films has been found to have a peak at a certain temperature, which is attributed to the carrier compensation similar to the semimetals. [Preview Abstract] |
Thursday, March 16, 2017 12:27PM - 12:39PM |
S3.00007: Spin-split Surface States and Superconductivity at Twin Boundaries of Non-centrosymmetric BiPd Chi Ming Yim, Christopher Trainer, Ana Maldonado, Darren C. Peets, Peter Wahl In non-magnetic bulk materials lacking a center of inversion symmetry, spin-orbit interactions can lift the spin degeneracy, resulting in Rashba metals whose Fermi surfaces exhibit an intricate spin texture. Combined with superconductivity, this can lead to an admixture of both singlet and triplet components of the superconducting pairing. Using scanning tunneling spectroscopy we study the surface electronic structure in the superconducting state of BiPd, which has previously been reported to exhibit a Dirac-like surface state with a non-trivial spin texture. Topographic images reveal domains of [0$\overline{1}$0] and [010] terminations corresponding to opposing faces of the crystal structure, separated by twin boundaries. From differential conductance spectra obtained on the two terminations we can characterize the surface electronic structure of the two non-equivalent surfaces. The signature of the surface state within domains of the two terminations are located at $\sim$0.4 eV above the Fermi level with only small differences. Intriguingly, we find an additional bound state localized at the twin boundary, the precise energy of which depends on the orientation of the twin boundary. Superconductivity between the two surface terminations and at the twin boundaries is discussed. [Preview Abstract] |
Thursday, March 16, 2017 12:39PM - 12:51PM |
S3.00008: Absence of Magnetic Fluctuation Induced Raman Scattering in EuS/Bi$_{2}$Se$_{3}$ Heterostructures Gavin Osterhoudt, Ryan Carelli, Ferhat Katmis, Jagadeesh Moodera, Kenneth Burch We present our recent Raman scattering results from EuS/Bi$_{2}$2Se$_{3}$ heterostructures which, surprisingly, reveal an absence of magnetic fluctuations in the EuS. Previous experiments have indicated that such heterostructures may indeed produce room temperature ferromagnetism in EuS. We will discuss the possible mechanisms which could explain the absence of magnetic fluctuation induced Raman scattering in our thin films. Furthermore, through the same Raman measurements we are able to investigate the role of interfacial strain in these heterostructures. [Preview Abstract] |
Thursday, March 16, 2017 12:51PM - 1:03PM |
S3.00009: Andreev Reflection Spectroscopy Study of Single-Crystal Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$ Topological Insulators C. R. Granstrom, I. Fridman, H.-C. Lei, C. Petrovic, J. Y. T. Wei To understand the superconducting proximity effect that occurs across the \textit{c}-axis of non-ideal three-dimensional topological insulators, we perform point-contact Andreev reflection (AR) spectroscopy on Bi$_{2}$X$_{3}$ (X=Se,Te) single crystals with Nb tips at 4.2 K. Robust AR spectra are observed, and analyzed with the Blonder-Tinkham-Klapwijk (BTK) theory. Non-BTK behavior is seen at low junction impedance and can be attributed to tip-induced Rashba spin-orbit coupling. However, the subgap enhancement seen in all the spectra is not consistent with the Fermi-surface mismatch between Nb and the topological surface states of Bi$_{2}$X$_{3}$. Rather, bulk band states with substantial $k_{z}$-dispersion are needed to explain the AR data, consistent with the Fermi level lying outside the bulk band gap as seen by scanning tunneling spectroscopy. We discuss the implications of our results on \textit{c}-axis proximity effect experiments. [Preview Abstract] |
Thursday, March 16, 2017 1:03PM - 1:15PM |
S3.00010: Magnetic Doping Effects on the Surface State (SS) of Topological Insulators (TIs): Comparison between MBE-grown Bi$_{2}$Se$_{3}$ and (Bi,Sb)$_{2}$Te$_{3}$ bilayer samples Chien-Chang Chen, Ankit Kumar, Nai-Chang Yeh, Lei Pan, Koichi Murata, Kang L. Wang We report studies of MBE-grown bilayer structures with one quintuple layer (QL) of pure TI on top of 6-QL of Cr-doped TI. Here TI is either Bi$_{2}$Se$_{3}$ or (Bi,Sb)$_{2}$Te$_{3}$. For 10{\%} Cr-doping, both systems exhibited a similar bulk Curie temperature $T_{c}^{3D} \sim $35 K from the onset of anonymous Hall effect (AHE). However, the AHE was much weaker in the Bi$_{2}$Se$_{3}$ system. Scanning tunneling spectroscopic (STS) studies revealed a gap opening in the SS below $T_{c}^{2D} (>\sim $210 K) for both systems, but the gap in the Bi$_{2}$Se$_{3}$ system was much more inhomogeneous, probably due to larger lattice strain variations from Cr doping. Given that the SS gap only opens under out-of-plane long-range magnetization, strain variations could induce varying orientation of magnetic domains, thus the appearance of gap inhomogeneity. Double spectral resonances associated with topological spin textures around isolated magnetic impurities were also observed along the boundaries of gapped and gapless regions only in the Bi$_{2}$Se$_{3}$ system. Theoretically finite coupling among these isolated topological spin textures could be induced by tuning the Fermi level of the TI away from the Dirac point. We shall verify this prediction by studying the spectral evolution of the double resonances in the Bi$_{2}$Se$_{3}$ system with a backgate voltage. [Preview Abstract] |
Thursday, March 16, 2017 1:15PM - 1:27PM |
S3.00011: Faraday rotation due to surface states in the topological insulator $(Bi,Sb)_{2}Te_{3}$ Yinming Shao, Kirk Post, Jhih-Sheng Wu, Siyuan Dai, Alex Frenzel, Anthony Richardella, Joon Sue Lee, Michael Fogler, Nitin Samarth, Alexander Balatsky, Dmitri Kharzeev, Dimitri Basov Using magneto-infrared spectroscopy, we have explored the charge dynamics of $(Bi,Sb)_{2}Te_{3}$ thin films on InP substrates. From the magneto-transmission data we extracted three distinct cyclotron resonance (CR) energies that are all apparent in the broad band Faraday rotation (FR) spectra. This comprehensive FR-CR data set has allowed us to isolate the response of the bulk states from the intrinsic surface states associated with both the top and bottom surfaces of the film. The FR data uncovered that the electron- and hole-type Dirac fermions reside on opposite surfaces of our films, which paves the way for observing many exotic quantum phenomena in topological insulators. [Preview Abstract] |
Thursday, March 16, 2017 1:27PM - 1:39PM |
S3.00012: Topological Ladders in Transition-Metal Dichalcogenides O. J. Clark, M. S. Bahramy, J. Feng, L. Bawden, J. M. Riley, V. Sunko, I. Markovi\'{c}, F. Mazzola, D. Biswas, K. Okawa, T. Sasagawa, G. Balakrishnan, T. Eknapakul, W. Meevasana, P. D. C. King Transition metal dichalcogenides (TMDs) host a rich variety of material properties, from spin-orbit coupled semiconductors to charge density-wave systems and superconductors [1,2]. The stabilisation of their varied ground states is largely thought to be driven by d-orbital physics of their transition-metal derived bands. Here, by combining spin- and angle-resolved photoemission spectroscopy (ARPES) with first-principles calculations, we show that band inversions within the chalcogen p-orbital manifold alone can cause TMDs to also host topological phenomena. First, we uncover a 3D tilted Dirac fermion and multiple topologically non-trivial surface states within the model system 1T-PdTe$_2$ where the p- and d-orbital bands are well separated in energy. We show, however, that the same topological signatures persist even within the more well-studied, d-band dominated members of the TMD classification. Through this, we firmly establish non-trivial band topologies as a generic feature of transition-metal dichalcogenides. [1] X. Xu, Nature Phys. 10 (2014) 343; [2] M. Chhowalla, Nature Chem. 5 (2013) 263. [Preview Abstract] |
Thursday, March 16, 2017 1:39PM - 1:51PM |
S3.00013: Determination of the gap of bulk Bi$_{2}$Se$_{3}$ at low temperatures by photo-luminescence and optical transmission measurements. Gerard Martinez, B Piot, M Hakl, M Orlita, M Potemski, Y.S. Hor, A Materna, G Strzelecka, A Hruban A series of different Bi$_{2}$Se$_{3}$ samples have been investigated at low temperatures by transport, photo-luminescence and optical transmission measurements. A complete analysis of the dielectric function of the material allows to extract the value of the gap Eg which is found to be equal to 0.219 $+$/- 0.001 eV at temperatures below 4.2 K. [Preview Abstract] |
Thursday, March 16, 2017 1:51PM - 2:03PM |
S3.00014: Resonant electronic Raman excitations with $A_{2g}$ symmetry in the topological insulator Bi$_2$Se$_3$ Hsiang-Hsi Kung, Xueyun Wang, Sang-Wook Cheong, Girsh Blumberg Inelastic light scattering from Dirac fermions can lead to electronic excitations odd in time reversal symmetry~\footnote{E. Riccardi et al., PRL 116, 066805}$^,$\footnote{O. Kashuba and V.I. Fal'ko, PRB 80, 241404(R)}. Here, we report the observation of a sharp collective mode with energy of 150\,meV, below the edge of a gapped continuum, in bulk Bi$_2$Se$_3$. Both the gapped continuum and the collective mode have pseudovector-like symmetry of the $A_{2g}$ representation of the $D_{3d}$ group, and are only present when excitation energy is in resonance with interband transitions between the occupied and unoccupied Dirac surface states. The gapped continuum can be understood as the electron-hole excitations between the lower and upper Dirac cones near the Fermi surface, whereas the slightly below gap edge collective mode is the excitonic bound states of the electron-hole pairs. Such inter-Dirac-cone excitation flips the in-plane electron spin, but preserves the out-of-plane angular momentum, resulting in singlet excitonic states. Such exciton is dark in photoluminescence spectra, as opposed to the doublet states which can be populated with circularly polarized light. This is a demonstration of novel collective excitation from the surface states of 3D topological insulators. [Preview Abstract] |
Thursday, March 16, 2017 2:03PM - 2:15PM |
S3.00015: Visualization of surface states in the putative weak and crystalline topological insulator Bi$_{\mathrm{2}}$TeI Nurit Avraham, Andrew Norris, Lin Pan, Shu-Chun Wu, Claudia Felser, Binghai Yan, Haim Beidenkopf Weak topological insulators (TI) are layered materials that possess surfaces with an even number of Dirac cones and surfaces that are fully gapped. This anisotropy provides them with unique properties such as sensitivity to the parity of the number of layers and absence of localization of their surface states. We use scanning tunneling microscopy to study the topological properties of stacked Bi$_{\mathrm{2}}$TeI, a promising candidate for weak TI. We find 1D edge states at the boundaries of terraces perpendicular to the stacking direction, which are consistent with topological surface states on the side surfaces. However on top of the terraces, where the density of states (DOS) is expected to vanish, we observe non-zero DOS showing that the surface is in fact gapless. These findings are supported by recent theoretical calculations showing that this surface hosts topological surface states that are protected by Mirror symmetry, making the weak TI also a crystalline TI. The rich structure of quasi 2D terraces and Islands obtained on such cleaved Bi$_{\mathrm{2}}$TeI surfaces provides an excellent playground to explore some of the most fundamental concepts of TIs. [Preview Abstract] |
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