Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session J10: Topological Kondo Insulators |
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Sponsoring Units: DCMP Chair: Nicholas Butch, National Institute of Standards and Technology Room: 007A |
Tuesday, March 3, 2015 2:30PM - 2:42PM |
J10.00001: A new theoretical approach to SmB$_6$ and related mixed valence compounds without Kondo physics Alfred Cheung, Mona Berciu, Ilya Elfimov, George Sawatzky Currently, samarium hexaboride (SmB$_6$) is widely thought of as a Kondo insulator with the formation of its hybridization gap being attributed to Kondo physics. This is in spite of the fact that Sm is strongly mixed valent and is hence incompatible with the Kondo scenario of fixed spins undergoing spin fluctuations. In this talk, we summarize arguments for why Kondo physics is inappropriate for the case of SmB$_6$. We then present a new model in which the low energy scale atomic multiplet structure of SmB$_6$ and the strong Sm 4f valence fluctuations are properly taken into account, replacing Kondo lattice physics as the principal player in SmB$_6$. [Preview Abstract] |
Tuesday, March 3, 2015 2:42PM - 2:54PM |
J10.00002: 30 MHz Self Oscillator using Topological Kondo Insulator SmB6 Alexander Stern, Daejeong Kim, Sean Thomas, Zachary Fisk, Jing Xia We are studying electrical properties of SmB6 (or Samarium Hexaboride), which is a topological insulator material. By applying a DC current to a tiny crystal of SmB6 and a capacitor, we were able to generate 30 MHz oscillation voltage across the crystal. The frequency range is set by the crystal size and quality and the frequency can be fine-tuned by the amount of DC current. With varying crystal size, we have achieved the frequencies from 20Hz to 30MHz with amplitudes up to 50mV. The DC current driven oscillation is induced by thermal oscillation between topological surface and the bulk states. We will present a theoretical model for this intriguing behavior. This work is supported by DARPA/Air force grant FA 8650-14-1-7407. [Preview Abstract] |
Tuesday, March 3, 2015 2:54PM - 3:06PM |
J10.00003: Emergent photovoltage on SmB6 surface upon bulk-gap evolution revealed by pump-and-probe photoemission spectroscopy Yukiaki Ishida, Toshio Otsu, Tomoko Shimada, Mario Okawa, Yohei Kobayashi, Fumitoshi Iga, Toshiro Takabatake, Shik Shin Recent studies suggest that an exemplary Kondo insulator SmB6 belongs to a new class of topological insulators (TIs), in which non-trivial spin-polarized metallic states emerge on surface upon the formation of Kondo hybridization gap in the bulk. Remarkably, the bulk resistivity reaches more than 20 $\Omega $ cm at 4 K, making SmB6 a candidate for a so-called bulk-insulating TI. We here investigate optical-pulse responses of SmB6 by pump-and-probe photoemission spectroscopy. Surface photovoltage effect is observed below $\sim$ 90 K. This indicates that an optically-active band bending region develops beneath the novel metallic surface upon the bulk-gap evolution. The photovoltaic effect persists for \textgreater 200 $\mu $s, which is long enough to be detected by electronics devices, and could be utilized for optical gating of the novel metallic surface. [Preview Abstract] |
Tuesday, March 3, 2015 3:06PM - 3:18PM |
J10.00004: Surface damage of SmB$_{6}$ through ion-irradiation Nicholas Wakeham, Yongqiang Wang, Zachary Fisk, Filip Ronning, Joe Thompson SmB$_6$ is a Kondo insulator, but there is strong evidence for an intrinsic conductive surface state at low temperatures. Theoretical work indicates that SmB$_6$ may be a topological Kondo insulator with a topologically protected surface state that is robust against time-reversal invariant perturbations. To investigate this robustness, we have used non-magnetic ion-irradiation to damage the (001) surfaces of SmB$_6$ single crystals to varying depths, and have measured the resistivity as a function of temperature for each depth of damage. We observe a reduction in the residual resistivity with increasing depth of damage. Our data are consistent with a model in which the surface state is not destroyed by the ion-irradiation, but instead the damaged layer is poorly conducting and the initial surface state is reconstructed below the damage. This behavior is consistent with a surface state that is topologically protected. Investigations of time-reversal symmetry breaking perturbations of the surface layer, with magnetic ion-irradiation, will also be discussed. [Preview Abstract] |
Tuesday, March 3, 2015 3:18PM - 3:30PM |
J10.00005: Ultrafast quasiparticle dynamics of Kondo insulator SmB$_{6}$ using THz spectroscopy Jingdi Zhang, Jie Yong, Ichiro Takeuchi, Richard Greene, Richard Averitt Samarium Hexboride (SmB$_{6})$ is a prototype Kondo insulator with a hybridization gap of 19meV at low temperatures. It has been theoretically predicted to be the first topological insulator involving electron-electron correlations that is truly insulating in bulk. Recent progress in fabricating thin film SmB$_{6}$ [1] enables using terahertz spectroscopic methods to investigate topological surface states and carriers adjacent to the hybridization gap, potentially distinguishing these distinct contributions to the optical conductivity. We report on the photo-excited quasi-particle (QP) dynamics of the Kondo insulator SmB6, using ultrafast terahertz spectroscopy. The amplitude of the transient change in transmission increases with decreasing temperature, exhibiting a single-exponential decay that significantly decreases below the Kondo temperature. This phonon bottleneck originates from competition between QP recombination and re-excitation of QP across the hybridization gap by phonons.\\[4pt] [1] Yong, Jie, et al.~arXiv preprint arXiv:1408.5413~(2014). [Preview Abstract] |
Tuesday, March 3, 2015 3:30PM - 3:42PM |
J10.00006: Terahertz transmission and reflection studies of the topological Kondo insulator candidate SmB$_6$ Christopher M. Morris, N.J. Laurita, S. Koopayeh, P. Cottingham, W.A. Phelan, L. Schoop, T.M. McQueen, N.P. 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 logarithmic 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 optical conductivity of single crystals of SmB$_6$. A saturation of the transmission is observed associated with the resistance plateau as the bulk becomes insulating. A secondary bulk conduction mechanism remains down to the lowest measured temperature, 1.6 K. Additionally, FTIR measurements have been performed that show the Kondo gap of SmB$_6$ opening at low temperatures. [Preview Abstract] |
Tuesday, March 3, 2015 3:42PM - 3:54PM |
J10.00007: SmB$_{6}$ thin films synthesis and transport properties Yufan Li, Sunxiang Huang, C.-L. Chien SmB$_{6}$, the candidate of the newly proposed topological Kondo insulator, has attracted great research interest in its physical properties. Evidences supporting the presence of the topologically protected surface state were gathered from various experimental studies, however mainly focusing on bulk specimens. In this work, (100) textured SmB$_{6}$~thin films with dominant epitaxy were synthesized by DC magnetron sputtering. The resistivity saturates below 10 K and forms a plateau, while at higher temperatures the insulating behavior manifests a band gap $\sim$3 meV consistent with the reported value of bulk samples. A sign change of the normal Hall coefficient may indicate the formation of the hybridization energy gap. [Preview Abstract] |
Tuesday, March 3, 2015 3:54PM - 4:06PM |
J10.00008: Planar tunneling spectroscopy of the candidate topological Kondo insulator SmB$_{6}^{\ast }$ W. K. Park, L. Sun, L. H. Greene, D. J. Kim, Z. Fisk Topological insulators are a new class of materials harboring topologically protected surface states. SmB$_{6}$, a well-known Kondo insulator, has attracted much interest recently due to its possibility to be topological as a system with strong interaction. Despite intensive investigations in recent years, the nature of the surface states in SmB$_{6}$ still remains intriguing. We adopt planar tunneling spectroscopy to study the electronic density of states in topological insulators and superconductors. As a surface-sensitive technique, it should be able to detect the surface states in SmB$_{6}$. Planar tunnel junctions are made on both (100) and (110) surfaces of high-quality single crystals. Crystal surfaces are prepared by polishing (with sub-nm scale smoothness) and ion-beam cleaning/etching. AlO$_{x}$ tunnel barrier of varying thickness is formed by sputter deposition of Al and subsequent oxidation. Differential conductance is measured as a function of temperature down to 1.7 K and magnetic field up to 9 T. Our tunneling conductance spectra show asymmetric gap-like features, reminiscent of a Fano resonance in a Kondo lattice, up to 40$-$50 K, close to the temperature below which the band renormalization and hybridization is known to occur. We'll discuss how the contributions from the bulk and the surface states can be identified in our conductance data. $^{\ast }$The work at UIUC is supported by the NSF DMR 12-06766. [Preview Abstract] |
Tuesday, March 3, 2015 4:06PM - 4:18PM |
J10.00009: Effects of Surface Morphology on the 3D Topological Insulator Samarium Hexaboride Steven Wolgast, Yun Suk Eo, Cagliyan Kurdak, Dae-Jeong Kim, Zachary Fisk The recent verification of a topologically-protected surface state in SmB$_6$ at low temperatures has led to several transport studies of the surface states. This task is complicated because current can flow on all surfaces of a topological insulator, each of which can have different transport characteristics. Our own measurements using a Corbino disc geometry overcome this difficulty, limiting the conduction to individual surfaces. However, the sheet conductivities of our samples counter-intuitively decrease with finer surface polishing. We therefore investigate surface and sub-surface morphology as a factor affecting the surface conductivity. Specifically, surface cracks may themselves harbor surface states and contribute to the total electrical conduction, yielding a higher measured sheet conductivity than that of a flat surface. This situation may contribute to the (sometimes unphysically) large surface conductivities already observed in SmB$_6$. [Preview Abstract] |
Tuesday, March 3, 2015 4:18PM - 4:30PM |
J10.00010: Ultrafast carrier dynamics of SmB$_{6}$ Sanjay Adhikari, Yanjun Ma, Chang-Beom Eom, Jing Xia, Cheng Cen SmB$_{6}$, a Kondo insulator, has generated lots of interest recently due to the proposed topological insulator state at low temperature. Ultrafast spectroscopy can produce valuable information on carriers/lattice dynamics and their interactions. Here we report femtosecond pump-probe experiment performed in SmB$_{6}$ single crystals and epitaxial thin films at variable temperatures. The observed carrier relaxations as well as different collective oscillation modes in GHz - THz range allow us to study the hybridization gap and the carrier coherence in different samples. [Preview Abstract] |
Tuesday, March 3, 2015 4:30PM - 4:42PM |
J10.00011: ARPES investigation on the surface vs bulk electronic structures of correlated topological insulators YbB6 and other rare earth hexaborides Nan Xu, C.E. Matt, E. Pomjakushina, J.H. Dil, G. Landolt, J.Z. Ma, X. Shi, R.S. Dhaka, N.C. Plumb, M. Radovic, V. Rogalev, V. Strocov, T.K. Kim, M. Hoesch, K. Conder, J. Mesot, H. Ding, Ming Shi Using ARPES performed in wide photon energy range we systematically studied the bulk and surface electronic structures of a topological mixed- valence insulator candidate, YbB6. The bulk B-2p states are probed with bulk-sensitive soft X-ray ARPES, exhibiting strong three-dimensionality with the band top locating 80 meV below the EF at the X point. The measured bulk Yb-4f states are located at 1 and 2.3 eV below EF, which hybridize with the dispersive B-2p states. The bulk band structures obtained by experiments are substantially different from the first principle calculations, but it can be better described by adding a correlation parameter U $=$ 7 eV, indicating YbB6 is a correlated system. Using surface-sensitive VUV ARPES, we revealed two-dimensional surface states which form three electron-like FSs with Dirac-cone-like dispersions. The odd number of surface FSs gives the first indication that the surface states are topological non-trivial. The spin-resolved ARPES measurements provide further evidence that these surface states are spin polarized with spin locked to the crystal momentum. Recent results on the TI phase in other rare earth hexaborides will also be shown. [Preview Abstract] |
Tuesday, March 3, 2015 4:42PM - 4:54PM |
J10.00012: Observation of a non-Kondo-like topological insulator state in the correlated rare-earth hexaboride YbB$_6$ Madhab Neupane, S.-Y. Xu, N. Alidoust, G. Bian, C. Liu, I. Belopolski, T.-R. Chang, H.T. Jeng, T. Durakiewicz, H. Lin, A. Bansil, D.J. Kim, Z. Fisk, M.Z. Hasan We present angle-resolved photoemission studies on the rare-earth hexaboride YbB$_{6}$, which has recently been predicted to be a topological Kondo insulator. Our data do not agree with the prediction and instead show that YbB$_6$ exhibits a novel topological insulator state in the absence of a Kondo mechanism. We find that the Fermi level electronic structure of YbB$_6$ has three 2D Dirac cone surface states enclosing the Kramers' points, while the $f-$orbital which would be relevant for the Kondo mechanism is about 1 eV below the Fermi level. Our first-principles calculation shows that the topological state which we observe in YbB$_6$ is due to an inversion between Yb $d$ and B $p$ bands. I will also present some of our recent results on other member of hexaborides. These experimental and theoretical results provide a new approach for realizing novel correlated topological insulator states in rare-earth materials. [Preview Abstract] |
Tuesday, March 3, 2015 4:54PM - 5:06PM |
J10.00013: Non-topological 2DEG at the surface of YbB$_{6}$ and Divalent Hexaborides J.D. Denlinger, C.H. Min, F. Reinert, Boyoun Kang, D.J. Kim, Z. Fisk, K. Gotlieb, A. Lanzara, C.-J. Kang, B.I. Min, J.W. Allen A recent theoretical prediction of YbB$_{6}$ being an f-d band-inverted mixed-valent topological insulator very similar to SmB$_{6}$ [1] and subsequent angle-resolved photoemission topological interpretations of V-shaped electron pockets [2] are at odds with the previous experimental classification of the material as a divalent small p-d band gap semiconductor. Our angle-resolved photoemission of the (001) surface of YbB$_{6}$ confirms the nearly pure divalency of Yb and demonstrates that in-gap surface electron pockets, with slightly non-parabolic dispersion indicative of a small p-d gap, originate from quantum well states confined to the inversion layer of n-type surface regions with cation termination. Spatial- and time-dependent variations of the surface Fermi-level pinning are shown to be universal polar-surface-driven features of the semiconducting divalent hexaborides including CaB$_{6}$, SrB$_{6}$ and EuB$_{6}$ Also DFT$+$U$+$SO$+$mBJ theoretical band calculations are able to reproduce the experimental energy ordering of a p-d gap existing at E$_{\mathrm{F}}$ above the Yb 4f states with no f-d or p-d band inversions.\\[4pt] [1] H. Weng, et al., PRL 112, 16403 (2014).\\[0pt] [2] M. Xia, et al. arXiv:1404.6217; M. Neupane, et al., arXiv:1404.6814; N. Xu, et al., arXiv:1405.0165. [Preview Abstract] |
Tuesday, March 3, 2015 5:06PM - 5:18PM |
J10.00014: Scanning Tunneling Microscopy Studies of the Topological Insulator Candidate YbB$_6$ Zhihuai Zhu, Y. He, D.-J. Kim, Z. Fisk, J. E. Hoffman We report scanning tunneling microscopy studies of YbB$_6$, a proposed topological insulator candidate with moderate correlation. The in-situ cleaved sample surface has two dominant morphologies: atomic square lattices and disordered rows, which likely correspond to Yb and B terminations, respectively. Spatially resolved $dI/dV$ maps show enhanced tunneling due to the local perturbation of the tip-induced band bending. The $dI/dV$ spectra reveal a bulk gap with distinct in-gap features near the Fermi level on different terminations. Our study presents nanoscale evidence for the interplay between surface structure, correlation and topological properties. [Preview Abstract] |
Tuesday, March 3, 2015 5:18PM - 5:30PM |
J10.00015: Is the black phase of SmS a topological Kondo insulator? Eric Bauer, N. J. Ghimire, F. Ronning, C. Batista, D. Byler, J. D. Thompson, A. Rahmanisisan, Z. Fisk SmS is a prototypical Kondo insulator where electronic correlations drive a system insulating that would otherwise be metallic. Whether or not such a system is also a topological insulator that hosts a protected metallic surface state, depends on the parity of the wavefunction of the occupied states. However, unlike weakly correlated materials, it is unclear whether state-of-the-art electronic structure calculations accurately predict the parity of the occupied wavefunctions of correlated insulators. Nevertheless, Dzero and collaborators suggest that Kondo insulators such as SmB6 can be topological. Like SmB6, Cubic SmS is a non-magnetic semiconductor with an insulating behavior at ambient pressure and low temperatures driven by hybridization with the Sm f-electrons. At 6 kbar, SmS undergoes a phase transition into a valance fluctuating phase accompanied by a visible color change from black to gold. It then undergoes a second phase transition at about 20 kbar to an antiferromagnetic order at low temperatures. We will discuss whether electronic structure calculations indicate a topological state of SmS at P$=$0. We will also discuss whether or not the magnetic, thermal and transport properties of the black phase of SmS are consistent with the existence of a topological protected surface state. [Preview Abstract] |
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