The Kondo insulator SmB6 has long been known to exhibit low temperature (T <
10K) transport anomaly and has recently attracted attention as a new
topological insulator candidate. By combining low-temperature and high
energy-momentum resolution of the laser-based ARPES technique, 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 a 4 meV window of the Fermi level on
the (001)-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 Brillouin zone. These states disappear as temperature is raised
above 15K in correspondence with the complete disappearance of the 2D
conductivity channels in SmB6. While the topological nature of the in-gap
metallic states cannot be ascertained without spin (spin-texture) measurements
our bulk and surface measurements carried out in the
transport-anomaly-temperature regime (T < 10K) are consistent with the
first-principle predicted Fermi surface behavior of a topological Kondo
insulator phase in this material.Comment: 4 Figures, 6 Page

Alidoust, N.

Balicas, L.

Bansil, A.

Belopolski, I.

Chang, T. -R.

Durakiewicz, T.

Fisk, Z.

Hasan, M. Z.

Jeng, H. -T.

Kim, D. -J.

Kondo, T.

Lin, H.

Liu, Chang

Neupane, M.

Shin, S.

Xu, S. -Y.

English

arXiv

The Kondo insulator SmB6 has long been known to exhibit low temperature (T &lt;
10K) transport anomaly and has recently attracted attention as a new
topological insulator candidate. By combining low-temperature and high
energy-momentum resolution of the laser-based ARPES technique, 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 a 4 meV window of the Fermi level on
the (001)-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 Brillouin zone. These states disappear as temperature is raised
above 15K in correspondence with the complete disappearance of the 2D
conductivity channels in SmB6. While the topological nature of the in-gap
metallic states cannot be ascertained without spin (spin-texture) measurements
our bulk and surface measurements carried out in the
transport-anomaly-temperature regime (T &lt; 10K) are consistent with the
first-principle predicted Fermi surface behavior of a topological Kondo
insulator phase in this material

Neupane, M

Alidoust, N

Xu, S-Y

Kondo, T

Kim, D-J

Belopolski, I

Chang, T-R

Jeng, H-T

Durakiewicz, T

Balicas, L

Lin, H

Bansil, A

Shin, S

Fisk, Z

Hasan, MZ

eScholarship - University of California

UC IrvineUC Irvine Previously Published WorksTitleSurface electronic structure of a topological Kondo insulator candidate SmB6: insights from high-resolution ARPESPermalinkhttps://escholarship.org/uc/item/7764s889AuthorsNeupane, MAlidoust, NXu, S-Yet al.Publication Date2013-06-19Copyright InformationThis work is made available under the terms of a Creative Commons Attribution License, availalbe at https://creativecommons.org/licenses/by/4.0/ Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaSurface electronic structure of a topological Kondo insulator candidate SmB6: insightsfrom high-resolution ARPESM. Neupane,1 N. Alidoust,1 S.-Y. Xu,1 T. Kondo,2 D.-J. Kim,3 Chang Liu,1 I. Belopolski,1 T.-R. Chang,4H.-T. Jeng,4, 5 T. Durakiewicz,6 L. Balicas,7 H. Lin,8 A. Bansil,8 S. Shin,2 Z. Fisk,3 and M. Z. Hasan11Joseph Henry Laboratory and Department of Physics,Princeton University, Princeton, New Jersey 08544, USA2ISSP, University of Tokyo, Kashiwa, Chiba 277-8581, Japan3Department of Physics and Astronomy, University of California at Irvine, Irvine, CA 92697, USA4Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan5Institute of Physics, Academia Sinica, Taipei 11529, Taiwan6Condensed Matter and Magnet Science Group, Los Alamos National Laboratory, Los Alamos, NM 87545, USA7National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, USA8Department of Physics, Northeastern University, Boston, Massachusetts 02115, USA(Dated: June 20, 2013)The Kondo insulator SmB6 has long been known to exhibit low temperature (T< 10K) transport anomaly and has recently attracted attention as a new topologicalinsulator candidate. By combining low-temperature and high energy-momentum res-olution of the laser-based ARPES technique, for the first time, we probe the surfaceelectronic structure of the anomalous conductivity regime. We observe that the bulkbands exhibit a Kondo gap of 14 meV and identify in-gap low-lying states within a 4meV window of the Fermi level on the surface of this material. The low-lying states arefound to form electron-like Fermi surface pockets that enclose the X and the Γ points ofthe surface Brillouin zone. These states disappear as temperature is raised above 15Kin correspondence with the complete disappearance of the 2D conductivity channels inSmB6. While the topological nature of the in-gap metallic states cannot be ascertainedwithout spin (spin-texture) measurements our bulk and surface measurements carriedout in the transport-anomaly-temperature regime (T < 10K) are consistent with thefirst-principle predicted Fermi surface behavior of a topological Kondo insulator phasein this material.PACS numbers:Materials with strong electron correlations often ex-hibit exotic ground states such as the heavy fermionstate, Mott insulator, Kondo insulator and related su-perconductors. Kondo insulators are mostly realized inthe rare-earth materials featuring f -electron degrees offreedom, which behave like a correlated metal at hightemperature, with a bulk band gap opening at low tem-perature [1–3]. The energy gap is attributed to hybridiza-tion of the nearly localized, flat 4f bands located near theFermi level with the dispersive conduction band. Withthe advent of 3D topological insulators [4–8] the com-pound SmB6, sometimes categorized as heavy-fermionsemiconductor [1–3], attracted much attention due to thetheoretical proposal that it may possibly host the topo-logically protected states within its Kondo bandgap [9–11]. This compound is metallic at room temperature, butthe resistivity goes up at low temperatures and saturatesto a finite value below 6K [12–14]. The anomalous resid-ual conductivity present in SmB6 at low temperature isbelieved to be associated with the states that lie withinthe Kondo bandgap, indirectly evidenced in various ex-periments [15–21].From a topological insulator perspective, the low tem-perature resistivity anomaly was attributed to topolog-ical surface states [9–11]. Several surface-related trans-port phenomena have recently been revealed in SmB6[23–25] providing further insights into the materials 2Dconduction below 10K. However, a direct experimen-tal evidence of temperature dependent evolution of sur-face states and their momentum resolved structure andanisotropy in this transport regime has not been achievedto this date. Previous ARPES studies have been limitedto the temperature regime of 15-20K or higher [15, 16]with limited resolution or perhaps sample quality. Weperform high-resolution and low temperature ARPESmeasurements to illuminate on the nature of residualconductivity anomaly below 10K from a band structureevolution perspective, and to test the proposal of thepresence of conducting surface states within the Kondobandgap of SmB6. Additionally, systematic tempera-ture dependence is carried out throughout the Kondotransport regime to determine the size of the Kondo gapand the d-f hybridization process and their relation tothe transport anomaly. Experimentally, at low temper-atures we identify low-lying states near the Fermi levelwhich vanish above 15K. We observe clear signatures ofbulk band hybridization at low temperatures featuring aKondo gap of about 14 meV, as well as the in-gap stateswithin 4meV from the Fermi level. These states, as wellas the transport surface conductivity or the 2D anomalydisappear above 15K. The detailed work presented herelays the foundation to understanding the residual con-arXiv:1306.4634v1  [cond-mat.str-el]  19 Jun 20132duction behavior and transport anomaly of SmB6 uni-versally observed at low temperatures (T < 10K).Single crystalline samples of SmB6 used in our mea-surements were grown by Al-flux method in Fisk Lab atUniversity of California (Irvine) which is detailed else-where [23, 24]. Synchrotron-based ARPES measure-ments of the low-energy electronic structure were per-formed at the Synchrotron Radiation Center (SRC), Wis-consin, equipped with high efficiency R4000 electron an-alyzer using PGM beamline. Separate, high-resolutionand low temperature ARPES measurements were per-formed using a Scienta R4000 hemispherical analyzerwith an ultraviolet laser (hν = 6.994eV) at the Institutefor Solid State Physics (ISSP) at Tokyo. The energy res-olution was set to be 4 meV and 20 meV for the measure-ments with the laser source or the synchrotron beamline,respectively, and angular resolution was set to be bet-ter than 0.1o for all measurements. The first-principlescalculations were based on the generalized gradient ap-proximation (GGA) [28] using the projector augmented-wave method [29] as implemented in the VASP package[30]. The experimental crystal structure was used [31] forcalculations. The spin-orbit coupling was included self-consistently in the electronic structure calculations with12×12×12 Monkhorst-Pack k−mesh.We start out presenting crystal symmetry and the Bril-louin zone structure of SmB6 materials, which crystallizein the CsCl-type structure with Sm ions and B6 octahe-dron being located at the corner and body center of thecubic lattice, respectively (see Fig. 1a). The bulk andprojected Brillioun Zone (BZ) of (001) surface for SmB6are shown in Fig. 1(b). The resistivity of our sampleshows a sharp increase at T< 30K and start to saturateat T< 7K (Fig. 1c) in agreement with previous reports[12, 14]. The resistivity versus temperature is plotted inFig. 1c for the sample characterization. In Figs. 1d and1e, we present ARPES intensity plots measured alongM−X−M and X−Γ−X directions over the wide bind-ing energy scale measured with the synchrotron-basedARPES system with photon energy of 26 eV at temper-ature of 12 K. The non-dispersive Sm 4f states near theFermi level are observed which are also marked in the in-tegrated energy distribution curves (EDCs) plotted nextto the intensity plots. The position of the flat bands areestimated to be at binding energy (BE) of 15 meV, 150meV and 1 eV. These states are assigned as 6H5/2,6H7/2and 6F, which are multiples of the Sm2+ (f6 to f5) fi-nal states, in agreement with previous report [15]. Moreimportantly, we observe a dispersive parabolic band cen-tered at X̄ point, which come from Sm 5d band. Thehybridization between the Sm 5d band and Sm 4f flatband is clearly visible especially at binding energy of 150meV in Figs. 1d and 1e.In order to search for the temperature dependent in-gap states, we employ the high-resolution low tempera-ture laser-based ARPES system. Figs. 2a and 2b show aschematic scenario of temperature dependent hybridiza-tion and its integrated density of states. The density ofstates measured below hybridization temperature showsa small dispersive peak crossing the Fermi level whichvanishes above the hybridization temperature. Guidedby recent first-principle calculations predicting in-gapsurface states around Γ and X points [10, 11], we fo-cus our detailed temperature dependent measurementaround these high-symmetry points. In Fig. 2c, weshow the integrated EDCs measured at various temper-atures at the band centered around the X point. Asthe temperature rises, the gap between hybridized statesdecreases and finally the two states merge into a singlestate whose traced cannot be observed at 30K. At lowtemperature, the gap value between hybridized states isestimated about 14 meV. We observe that the f -d hy-bridization evolves with temperature, thus providing thetemperature-dependent hybridization evidenced in elec-tronic structure. It is also important to note that at 7eVphotoionization cross section for the f orbitals is negligi-ble, so the measured states are significantly correspond-ing to the partial d-orbital character of the hybridizedband.Surprisingly, a similar temperature-dependent behav-ior is observed around Γ point where no direct hybridiza-tion between d and f bands is seen or expected (Fig. 2d).These states are clearly the in-gap states, states insidethe Kondo bandgap, and they cannot be traced in thebulk band calculations. However, they are allowed to ap-pear due to the bulk band inversion at Γ point (driven bystrong spin-orbit interaction strength of df -hybrid bands)which is the hallmark of topological non-triviality [9, 11].Fig. 3a shows a Fermi surface map of SmB6 measuredat temperature of 7K with laser energy of 7eV (see sup-plementary information for detailed data). A large oval-shaped and a small nearly circular-shaped Fermi surfaces(FS) are observed at X and Γ point, respectively, whichis in a qualitative agreement with the FS calculated fromfirst-principles [11]. Here we note that the bulk electronicdispersion is not expected to give rise to such a FS. Weconclude that the observed FS is most probably comingfrom the hybridized surface states due to their correspon-dence with surface state calculations and their absence inbulk band calculations. The key signatures of topologicalKondo insulators, such as the existence of a metallic sur-face state characterized by Dirac cones with helical spin-textures, are the same as those of other 3D topologicalinsulators [4, 5, 26, 27]. While the laser light source usedhere provides ultra-high energy resolution, it is limited bysingle photon energy and (lack of) spin resolution whilealso maintaining the same low temperature and ultra-high resolution conditions simultaneously. Therefore, atpresent such studies are not feasible. Present study thusfocuses on testing the proposal of in-gap states and theircorrelation with low-temperature transport regime be-low 15K and comparison of the k-space data with first-principle calculations of topological surface states.We plot ARPES intensity spectra for Γ and X band inFig. 3b and 3c. In both cases, the gap of about 13 or 14meV is observed, which is clearly observed in EDCs and32nd derivative plots. The comparison of the integratedEDCs for Γ and X bands is shown in Fig. 3d, which fur-ther confirms the appearance of small density of statesnear the Fermi level at both points. The observation ofgap at X point is attributed to the hybridization between4f and 5d bands, vanishing clearly by reaching 30K (Fig.2c). The detailed interpretation of the gap observed atΓ point is not straightforward, despite our best effortsof utilizing 4 meV resolution working at 6K tempera-ture, since without the observation of clear connectionof these states with bulk valence and conduction band,and establishing the time-reversal invariant behavior ofit, these states cannot yet be called topological based onexperimental data alone. Fig. 4 shows the calculatedbulk band structure of SmB6, which qualitatively agreeswith the experimental observations. The GGA calcula-tions give an insulating ground state with a small energygap of 15 meV (Fig.4b). From the orbital-decomposedband structure (Fig.4c), we find that the flat Sm 4fbands are located around EF from -0.5 eV. An itinerantSm 5d band with larger dispersion hybridizes with 4fbands, forming an anti-crossing band shape. The agree-ment between low-temperature ARPES data includingthe k-space maps of Fermi surfaces and Kondogap value(15 meV) with our GGA calculations and previous first-principle calculations provide important insights into thesurface electronic structure of SmB6.In conclusion, the topological Kondo insulator candi-date SmB6 has long been known to exhibit anomalous lowtemperature (10K) conductivity anomaly. By combininglow-temperature capability and high energy resolutionof laser-based ARPES technique, for the first time weaccessed this transport regime with spectroscopy. The4f − 5d hybridized states are observed with a Kondobandgap of about 14 meV. Importantly, the hybridizedstates around at X point vanish at high temperatures.Furthermore, the similarly temperature-dependent low-lying states are observed at Γ point. The simultaneouspresence of these states at low temperature both aroundX and Γ points with a 4 meV window of the Fermi levelis responsible for the residual conducting behavior ofSmB6. The in-gap states are found to enclose the Xand the Γ points of the surface Brillouin zone k-space.While the topological nature of the in-gap states can-not be ascertained without Spin-ARPES measurementsour low-temperature ARPES data taken in the trans-port anomaly regime in SmB6 help understand the long-standing puzzle of residual conduction in SmB6.[1] G. Aeppli, Z. Fisk, Comm. Condens. Matter Phys. 16,155 (1992) and references therein.[2] P. Riseborough, Adv. Phys. 49, 257 (2000).[3] P. Coleman, Handbook of Magnetism and AdvancedMagnetic Materials 1, 95 (2007).[4] M. Z. Hasan & C. L. Kane, Rev. Mod. Phys. 82, 3045(2010).[5] M. Z. Hasan & J. E. Moore, Ann. Rev. of Cond. Mat.Phys. 2, 78 (2011).[6] X.-L. Qi & S. -C. Zhang, Rev. Mod. Phys. 83, 1057(2011).[7] D. Hsieh, et al., Nature 452, 970 (2008).[8] Y. Xia et al. Nature Phys. 5, 398 (2009).[9] M. Dzero et al., Phys. Rev. Lett. 104, 106408 (2010).[10] T. Takimoto, J. Phys. Soc. Jpn. 80, 123710 (2011).[11] F. Lu et al., Phys. Rev. Lett. 110, 096401 (2010).[12] A. Menth et al., Phys. Rev. Lett. 22, 295 (1969).[13] J. W. Allen et al., Phys. Rev. B 20, 4807 (1979).[14] J. C. Cooley et al., Phys. Rev. Lett. 74, 1629 (1995).[15] H. Miyazaki et al., Phys. Rev. B 86, 075105 (2012).[16] J. D. Denlinger et al., Physica B 281, 716 (2000).[17] S. Kimura et al., Phys. Rev. B 50, 1406 (1994).[18] T. Nanba et al., Physica B 186, 440 (1993).[19] P. Nyhus et al., Phys. Rev. B 55, 12488 (1997).[20] P. A. Alekseev et al., Physica B 186, 384 (1993).[21] K. Flachbart et al., Phy. Rev. B 64, 085104 (1993).[22] M. Dzero et al., Phys. Rev. B 85, 045130 (2012).[23] S. Wolgast et al., arXiv:1211.5104 (2013).[24] J. Botimer et al., arXiv:1211.6769 (2013).[25] X. Zhang et al., Phys. Rev. X 3, 011011 (2013).[26] D. Hsieh, et al., Science 323, 919 (2009).[27] D. Hsieh, et al., Nature 460, 1101 (2009).[28] P. Perdew, Phys. Rev. Lett. 77, 3865 (1996).[29] P. E. Blochl, Phys. Rev. B 50, 17953 (1994); G. Kresseand J. Joubert, Phys. Rev. B 59, 1758 (1999).[30] G. Kress and J. Hafner, Phys. Rev. B 48, 13115 (1993);G. Kress and J. Furthmuller, Comput. Mater. Sci. 6, 15(1996); Phys. Rev. B. 54, 11169 (1996).[31] A. L. Malyshev et al. Mater. Sci. Forum 62/64, 69(1990).Correspondence and requests for materials should beaddressed to M.Z.H. (Email: mzhasan@princeton.edu).4FIG. 1: Crystal structure and sample characterization of SmB6. (a) Crystal structure of SmB6. Sm ions and B6octahedron are located at the corner and centre of the cubic lattice structure. (b) The bulk and surface Brillouin zones ofSmB6. High-symmetry points are marked. (c) Resistivity versus temperature for SmB6. (d) and (e) Synchrotron-based ARPESexperimental results: Dispersion mapping along M − X − M cut in (d) and X − Γ − X cut in (e). Dispersive Sm 5d band andnon-dispersive flat Sm 4f bands are observed. The integrated energy distribution curves (EDC) are also plotted to highlightthe flat bands of Sm 4f .5FIG. 2: Temperature dependent hybridized states. (a) Schematic view of Sm 4f (flat) and Sm 5d (dispersive parabolic)bands above the hybridization temperature. The small black dashed rectangle represents the region of hybridization shownat T< TH . The big black dashed rectangle shows the bands after hybridization at T< TH . The solid black rectangle showsthae approximate measurement window and black dash lines represent the Fermi level (EF ). (b) The integrated density ofstates measured above and below the hybridization temperature clearly showing the hybridized states and hybridization gap.(c) Temperature dependent measurements: the energy distribution curve measured at X band indicated in Fig (a). Measuredtemperatures are also noted on the plot. (d) Same as in (c) for Γ point. The thermally recycled EDC is also plotted whichconfirms that these hybridization states are robust at low temperature and reproducible even after thermal cycling of thesample.6FIG. 3: Fermi surface and dispersion maps of SmB6. (a) Fermi surface plot of SmB6 measured by 7 eV LASER sourceat temperature of 7 K. A small Γ pocket and a large X pocket are observed. A large elliptical and a small circular shaped blackdash lines around X and Γ points are guides to the eye. Inset shows a schematic plot of Fermi surface in the first Brillouinzone. (b) Electronic dispersion map (left) and its energy distribution curves (EDCs) for Γ pocket. (c) same as (b) for X band.(d) Comparison of integrated EDC for Γ and X band. A gap value of about 15 meV is observed in both cases.FIG. 4: Calculated bulk band structure of SmB6. (a) The calculated band structure of SmB6 by GGA. (b) Zoom in of(a) near Fermi level along Γ − X. The band gap of about 15 meV was obtained in the calculation. (c) Orbital decomposedband structure near Fermi level along Γ−X. The size of blue and yellow sphere is proportional to the weight of Sm 5d and 4forbital, respectively.

Surface electronic structure of a topological Kondo insulator candidate SmB6: insights from high-resolution ARPES

Surface electronic structure of a topological Kondo insulator candidate
  SmB6: insights from high-resolution ARPES

Surface electronic structure of a topological Kondo insulator candidate SmB6: insights from high-resolution ARPES

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