⇦Chinese Physics Letters, 2019, Vol. 36, No. 7, Article code 077401Express Letter Ionic-Liquid-Gating Induced Protonation and Superconductivity in FeSe, FeSe$_{0.93}$S$_{0.07}$, ZrNCl, 1$T$-TaS$_2$ and Bi$_2$Se$_3$ * Yi Cui (崔祎)1,2†, Ze Hu (胡泽)2†, Jin-Shan Zhang (张金珊)1**, Wen-Long Ma (马文龙)3, Ming-Wei Ma (马明伟)3, Zhen Ma (马祯)4, Cong Wang (王聪)2, Jia-Qiang Yan (阎加强)5, Jian-Ping Sun (孙建平)6, Jin-Guang Cheng (程金光)6, Shuang Jia (贾爽)3,7**, Yuan Li (李源)3,7, Jin-Sheng Wen (温锦生)4,8, He-Chang Lei (雷和畅)2, Pu Yu (于浦)9,7, Wei Ji (季威)2, Wei-Qiang Yu (于伟强)2** Affiliations 1School of Mathematics and Physics, North China Electric Power University, Beijing 102206 2Department of Physics, and Beijing Key Laboratory of Opto-electronic Functional Materials & Micro-nano Devices, Renmin University, Beijing 100872 3International Center for Quantum Materials, School of Physics, Peking University, Beijing 100871 4National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093 5Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA 6Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190 7Collaborative Innovation Center of Quantum Matter, Beijing 100871 8Innovative Center for Advanced Microstructures, Nanjing University, Nanjing 210093 9State Key Laboratory of Low Dimensional Quantum Physics and Department of Physics, Tsinghua University, Beijing 100084 Received 25 May 2019, online 29 May 2019 *Work at RUC was supported by the National Natural Science Foundation of China under Grant Nos 51872328, 11622437, 11574394, 11774423 and 11822412, the Strategic Priority Research Program of Chinese Academy of Sciences under Grant No XDB30000000, the Ministry of Science and Technology of China under Grant No 2016YFA0300504, the Fundamental Research Funds for the Central Universities, and the Research Funds of Renmin University of China (RUC) (15XNLQ07, 18XNLG14, 19XNLG17). SJ was supported by the National Natural Science Foundation of China under Grant Nos 11774007 and U1832214. YC was supported by the Outstanding Innovative Talents Cultivation Funded Programs 2018 of Renmin University of China. JQY was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Division of Materials Sciences and Engineering.
†Yi Cui and Ze Hu contributed equally to this study.
**Corresponding authors. Email: zhangjs@ncepu.edu.cn; gwljiashuang@pku.edu.cn; wqyu_phy@ruc.edu.cn Citation Text: Cui Y, Hu Z, Zhang J S, Ma W L and Ma M W et al 2019 Chin. Phys. Lett. 36 077401    Abstract We report protonation in several compounds by an ionic-liquid-gating method, under optimized gating conditions. This leads to single superconducting phases for several compounds. Non-volatility of protons allows post-gating magnetization and transport measurements. The superconducting transition temperature $T_{\rm c}$ is enhanced to 43.5 K for FeSe$_{0.93}$S$_{0.07}$, and 41 K for FeSe after protonation. Superconducting transitions with $T_{\rm c} \sim 15$ K for ZrNCl, $\sim$7.2 K for 1$T$-TaS$_2$, and $\sim$3.8 K for Bi$_2$Se$_3$ are induced after protonation. Electric transport in protonated FeSe$_{0.93}$S$_{0.07}$ confirms high-temperature superconductivity. Our $^{1}$H nuclear magnetic resonance (NMR) measurements on protonated FeSe$_{1-x}$S$_{x}$ reveal enhanced spin-lattice relaxation rate $1/^{1}T_1$ with increasing $x$, which is consistent with the LDA calculations that H$^{+}$ is located in the interstitial sites close to the anions. DOI:10.1088/0256-307X/36/7/077401 PACS:74.70.-b, 74.62.Dh, 78.30.cd, 74.25.nj © 2019 Chinese Physics Society Article Text Carrier doping is an effective method for tuning metal-insulator transitions and superconductivity. In addition to chemical substitution, electric gating also emerged as an efficient method for tuning carrier density in thin films.[1–3] With the development of various room-temperature ionic liquids, the transistor-like gating method[4–9] was found to induce a large carrier density for thin films or crystal flakes, through charge polarization. Lithium doping by ionic solid gating was also found to enhance the superconducting transition temperature of thin flakes of FeSe.[10] Recently, tuning of proton or oxygen concentration, using ionic-liquid-gating as a medium, was introduced to modify the lattice structure and magnetism of SrCoO$_{2.5}$.[11] This H$^+$ implantation method was later applied in iron-based superconductors, to induce superconductivity or enhance the superconducting transition temperature in bulk crystals due to an electron doping effect.[12] It is important to note that H$^+$ originates from water contamination in the ionic liquid by this method.[11] The advantages of this technique are that H$^+$ is nonvolatile and the gating is performed near the ambient conditions, which allow various post-gating measurements. However, multiple superconducting phases in protonated FeSe$_{1-x}$S$_x$ emerge, indicating that proton concentration is inhomogeneous across the bulk crystals. The magnetization data in the protonated FeSe$_{0.93}$S$_{0.07}$ show that the volume fraction of the superconducting phase is very low under the reported gating conditions.[12] In this Letter, we report our optimized protonation conditions with this ionic-liquid-gating method, to improve the superconducting volume ratio and the doping homogeneity. The best protonation temperature is found to be 350 K (higher than the room temperature), with a gating period of 12 days. For FeSe$_{0.93}$S$_{0.07}$, the superconducting volume ratio is largely enhanced compared to the room temperature gating, as determined by the magnetization measurement. Transport measurement is also succeeded to confirm superconductivity. We also apply the optimized protonation on various layered compounds, including FeSe, insulating ZrNCl, 1$T$-TaS$_2$, and Bi$_2$Se$_3$, where protonation either induces superconductivity or enhances the $T_{\rm c}$ largely. In particular for 1$T$-TaS$_2$, we achieve a $T_{\rm c}$ higher than the regular gating method.
cpl-36-7-077401-fig1.png
Fig. 1. (a) An illustration of the protonation setup. Platinum electrodes are placed in a container filled with the ionic liquid. The gating voltage is set to be about 3.0 V. The ionic liquid is heated up to 350 K by a hot plate. (b) A picture of the positive and negative platinum electrodes, with the sample attached on the negative electrode.
In our experiment, pristine FeSe and FeSe$_{0.93}$S$_{0.07}$ single crystals were made by the vapor transport method.[13,14] ZrNCl powders were made by the high-pressure synthesis.[15] The 1$T$-TaS$_2$ single crystal was grown by the chemical vapor transport method.[16] Bi$_2$Se$_3$ was grown by the flux method.[17] FeS single crystal was made by the hydrothermal method.[18] We employ the protonation technique as illustrated in Fig. 1. As shown in Fig. 1, samples are attached to the negative electrodes, and a voltage of 3.0 V is applied as the gating voltage. The ionic liquid EMIM-BF4 is used. The gating temperature is optimized to be 350 K, which improves proton diffusion efficiency in the crystal. Typical gating period is 12 days when water is nearly fully electrolyzed. The dc magnetization is measured with a magnetic property measurement system (MPMS), and the transport is measured with a physical property measurement system (PPMS). These measurements were successfully performed after gating was removed at the room temperature, which indicates nonvolatile protons are inserted, in contrast to conventional ionic-liquid gating where gating cannot be removed during measurements. The proton NMR is performed by the spin-echo method, and the spin-lattice relaxation rate $1/^1T_1$ is measured by the inversion recovering method. In the following, we present protonation measurements on these compounds. FeSe: Recently, FeSe has attracted a great deal of research attention because of its highly tunable superconductivity. Its $T_{\rm c}$ is enhanced from 8.5 K to above 40 K under high pressure,[19,20] by chemical intercalation,[21–24] by ionic-liquid/solid gating,[25,10] or by dimensional reduction into a single-layer phase.[26]
cpl-36-7-077401-fig2.png
Fig. 2. The dc susceptibility of a protonated FeSe single crystal (5 mm$\times$5 mm$\times$1 mm) measured under the field-cooled (FC) and zero-field-cooled (ZFC) conditions with a magnetic field of 10 Oe. The arrows are located at the superconducting transition. Inset: an enlarged view of the susceptibility data close to $T_{\rm c}$.
Figure 2 shows the dc susceptibility $\chi (T)$ of a protonated FeSe single crystal. A rapid drop of $\chi$ at 41 K is clearly seen, indicating the onset of superconductivity. Therefore, the $T_{\rm c}$ of FeSe is also largely enhanced by the protonation technique.
cpl-36-7-077401-fig3.png
Fig. 3. (a) The dc susceptibility of a H$_y$-FeSe$_{0.93}$S$_{0.07}$ single crystal as a function of temperature, measured under ZFC and FC conditions. (b) The resistance of the crystal as a function of temperature. The arrows mark the onset temperature of superconductivity.
FeSe$_{0.93}$S$_{0.07}$: FeSe$_{1-x}$S$_x$ is a series of compounds, whose $T_{\rm c}$ ranges between 8 K and 13 K for $x < 0.12$.[27] Previously, two superconducting transitions at 25 K and 42.5 K were reported in the protonated samples. Here we show that with increased protonation temperature at 350 K, a single high-$T_{\rm c}$ phase is realized. Figure 3 shows the dc susceptibility $\chi (T)$ and the resistance data $R(T)$ of a protonated FeSe$_{0.93}$S$_{0.07}$ single crystal. The susceptibility data shows $T_{\rm c} \approx 41$ K, seen by the drop of $\chi$ (Fig. 3(a)). By contrast, the resistance data show a higher onset $T_{\rm c}$ of 43.5 K as indicted in Fig. 3(b), by a sudden drop of resistance upon cooling. ZrNCl: ZrNCl is a layered material with electric gating or lithium doping. Superconductivity can be induced in ZrNCl by electric gating or lithium doping.[3,4,28] We pressed ZrNCl powders into thin pellets and then doped H$^+$ with the current ionic-liquid-gating method. The samples turn from blue into black upon proton doping. Figure 4 shows the dc susceptibility of the proton-doped ZrNCl. The sharp drop of $\chi$ below 15 K shows the onset of superconductivity, with field up to 1000 Oe. The volume ratio of the superconducting phase, estimated from the ZFC data at 10 Oe field, is about 12$\%$. This suggests that proton doping is very efficient. We note that an ionic-liquid gating on ZrNCl at low temperatures is also reported, which proposes that the depletion of Cl$^{-}$ concentration causes superconductivity.[29]
cpl-36-7-077401-fig4.png
Fig. 4. The dc susceptibility $\chi$ of H$_y$-ZrNCl pellets, measured under FC and ZFC conditions with different fields. The arrow marks the onset temperature of superconductivity.
cpl-36-7-077401-fig5.png
Fig. 5. The dc susceptibility $\chi$ of a protonated 1T-TaS$_2$ single crystal, measured under FC and ZFC condition, at various fields. The arrow marks the onset of the superconducting transition.
1$T$-TaS$_2$: 1$T$-TaS$_2$ is a layered compound with a triangular lattice. It goes through a series of charge-density-wave (CDW) transitions upon cooling.[30,31] Superconductivity can be achieved by chemical doping, where the highest $T_{\rm c}$ is achieved at 3.5 K.[32] Here we performed protonation on 1$T$-TaS$_2$ single crystals. The dc magnetization of a protonated sample is shown in Fig. 5, measured under FC and ZFC conditions at different fields. The superconducting transition temperature $T_{\rm c}$ is found to be $\sim$7.2 K under 10 Oe field. We note that this transition temperature is higher than that achieved by the chemical doping. With an applied field of 500 Oe, the superconducting transition is still observed.
cpl-36-7-077401-fig6.png
Fig. 6. The dc susceptibility $\chi$ of a protonated Bi$_2$Se$_3$ single crystal, measured under FC and ZFC condition at different fields. The arrow marks the onset of superconducting transition at $T_{\rm c}$.
Bi$_2$Se$_3$: As a topological insulator, Bi$_2$Se$_3$ has caused a lot of research interests.[33,34] Superconductivity can be achieved upon Cu or Sr doping into this material.[35,36] Here we find that by protonation, superconductivity can also be achieved. As seen in Fig. 6, the superconducting transition temperature $T_{\rm c}$ is found to be 3.8 K, which is close to that reported by the chemical doping. With an applied field of 500 Oe, superconductivity is highly suppressed. Since protonation does not induce chemical substitution, our study indicates that chemical doping in the interstitial sites is important for the occurrence of superconductivity. Further studies on the protonation of induced superconductivity in this compound, regarding to possible topological superconductivity, are demanded.
cpl-36-7-077401-fig7.png
Fig. 7. The $^1$H spin-lattice relaxation rate divided by temperature $1/^1T_1T$ of H$_y$FeSe$_{1-x}$S$_x$ single crystals, measured with a magnetic field of 5 T along the $c$-axis. Inset: the proton position in the lattice obtained by the LDA calculations.
$^1$H NMR studies on H$_y$FeSe$_{1-x}$S$_{x}$: In H$_y$FeSe$_{1-x}$S$_{x}$ compounds, intrinsic $^1$H NMR spectra was observed. Figure 7 shows the $^1$H spin-lattice relaxation rates divided by temperatures, versus temperature for protonated FeSe$_{1-x}$S$_x$ with $x$=0, 0.07 and 1. Above 50 K, $1/^1T_1T$ stays nearly constant but varies with $x$, which indicate that doped protons are detected by the current measurements. Indeed, the increase of $1/^{1}T_1$ with increasing $x$ is consistent with the LDA calculations that H$^+$ is inserted in the interstitial sites as discussed below. Since the $c$-axis lattice parameter is reduced with increasing $x$,[27] the hyperfine coupling between $^1$H and the FeSe plane increases with increasing S$^{2-} $ concentration. Our LDA calculations indicate that H$^+$ is located in the interstitial sites close to the anion Se$^{2-}$/S$^{2-}$, as shown by the schematic drawing in the inset of Fig. 7. This can be understood as an effect of coulomb attraction between H$^{+}$ and Se$^{2-}$/S$^{2-}$. So far, we have found that this doping method is efficient in layered compounds, which indicates that H$^+$ is most likely doped between the layers as in H$_y$FeSe$_{1-x}$S$_{x}$. Discussions and summary: Our XRD measurement did not resolve the change of lattice structure after protonation, which suggests that the chemical pressure effect of proton insertion is possibly very small. As a result, an electron-doping should be primarily responsible for the change of $T_{\rm c}$.
Table 1. $T_{\rm c}$ of the materials before and after protonation.
Compound FeSe FeSe$_{0.93}$S$_{0.07}$ ZrNCl 1$T$-TaS$_2$ Bi$_2$Se$_3$
$T_{\rm c,before}$ 9 K 8 K 0 0 0
$T_{\rm c,after}$ 41 K 43.5 K 15 K 7.2 K 3.8 K
In Table 1, we summarize all the $T_{\rm c}$ of different compounds, before and after protonation under the current optimized conditions. The optimization at 350 K suggests that the efficiency of proton doping is caused by a balance between proton diffusion into the sample and the evasion out of the sample, both of which increase with temperature. The current method supplies a universal electron doping method, which could be widely used in tuning and searching for superconductivity and metal-insulator transitions in the layered compounds. References Electrostatic modification of novel materialsElectric-field-induced superconductivity in an insulatorMetallic ground state in an ion-gated two-dimensional superconductorLiquid-gated interface superconductivity on an atomically flat filmSuperconductor–insulator transition in La2 − xSr x CuO4 at the pair quantum resistanceSuperconducting Dome in a Gate-Tuned Band InsulatorControlling many-body states by the electric-field effect in a two-dimensional materialEvidence for two-dimensional Ising superconductivity in gated MoS2Enhancement of superconducting transition temperature in FeSe electric-double-layer transistor with multivalent ionic liquidsTuning phase transitions in FeSe thin flakes by field-effect transistor with solid ion conductor as the gate dielectricElectric-field control of tri-state phase transformation with a selective dual-ion switchProtonation induced high- T c phases in iron-based superconductors evidenced by NMR and magnetization measurementsLack of coupling between superconductivity and orthorhombic distortion in stoichiometric single-crystalline FeSeOrbital-driven nematicity in FeSeHigh-pressure synthesis and crystal structures of - M N X ( M Zr, Hf; X Cl, Br, I)Direct Observation of the Superstructure of the Nearly Commensurate Phase in 1TTaS2by High Resolution Electron MicroscopyElectrical, Thermal and Spectroscopic Characterization of Bulk Bi2Se3 Topological InsulatorStrong anisotropy in nearly ideal tetrahedral superconducting FeS single crystalsElectronic and magnetic phase diagram of β-Fe1.01Se with superconductivity at 36.7 K under pressureDome-shaped magnetic order competing with high-temperature superconductivity at high pressures in FeSeSuperconductivity in the iron selenide K x Fe 2 Se 2 ( 0 ≤ x ≤ 1.0 ) New Li-Ethylenediamine-Intercalated Superconductor Li x (C 2 H 8 N 2 ) y Fe 2- z Se 2 with T c = 45 KPhase Diagram of (Li 1– x Fe x )OHFeSe: A Bridge between Iron Selenide and Arsenide SuperconductorsCoexistence of superconductivity and antiferromagnetism in (Li0.8Fe0.2)OHFeSeEvolution of High-Temperature Superconductivity from a Low- T c Phase Tuned by Carrier Concentration in FeSe Thin FlakesInterface-Induced High-Temperature Superconductivity in Single Unit-Cell FeSe Films on SrTiO 3Substitution Effects on FeSe SuperconductorIncrease in T c upon Reduction of Doping in Li x ZrNCl SuperconductorsElectric Field Induced Permanent Superconductivity in Layered Metal Nitride Chlorides HfNCl and ZrNClCharge-density waves and superlattices in the metallic layered transition metal dichalcogenidesScanning tunneling microscopy of the charge-density-wave structure in 1 T - TaS 2 Superconductivity induced by Se-doping in layered charge-density-wave system 1 T -TaS 2− x Se x Topological insulators in Bi2Se3, Bi2Te3 and Sb2Te3 with a single Dirac cone on the surfaceObservation of a large-gap topological-insulator class with a single Dirac cone on the surfaceSuperconductivity in Cu x Bi 2 Se 3 and its Implications for Pairing in the Undoped Topological InsulatorSuperconductivity with Topological Surface State in Sr x Bi 2 Se 3
[1] Ahn C H, Bhattacharya A, Ventra M D, Eckstein J N, Frisbie C D, Gershenson M E, Goldman A M, Inoue I H, Mannhart J, Millis A J, Morpurgo A F, Natelson D and Triscone J M 2006 Rev. Mod. Phys. 78 1185
[2] Ueno K, Nakamura S, Shimotani H, Ohtomo A, Kimura N, Nojima T, Aoki H, Iwasa Y and Kawasaki M 2008 Nat. Mater. 7 855
[3] Saito Y, Kasahara Y, Ye J, Iwasa Y and Nojima T 2015 Science 350 409
[4] Ye J T, Inoue S, Kobayashi K, Kasahara Y, Yuan H T, Shimotani H and Iwasa Y 2010 Nat. Mater. 9 125
[5] Bollinger A T, Dubuis G, Yoon J, Pavuna D, Misewich J and Bozǒvić I 2011 Nature 472 458
[6] Ye J T, Zhang Y J, Akashi R, Bahramy M S, Arita R and Iwasa Y 2012 Science 338 1193
[7] Li L J, O'Farrell1 E C T, Loh K P, Eda G, Özyilmaz B and Castroneto A H 2016 Nature 529 185
[8] Lu J M, Zheliuk O, Leermakers I, Yuan N F Q, Zeitler U, Law K T and Ye J T 2015 Science 350 1353
[9] Miyakawa T, Shiogai J, Shimizu S, Matsumoto M, Ito Y, Harada T, Fujiwara K, Nojima T, Itoh Y, Aida T, Iwasa Y and Tsukazaki A 2018 Phys. Rev. Mater. 2 031801
[10] Lei B, Wang N Z, Shang C, Meng F B, Ma L K, Luo X G, Wu T, Sun Z, Wang Y, Jiang Z, Mao B H, Liu Z, Yu Y J, Zhang Y B and Chen X H 2017 Phys. Rev. B 95 020503
[11] Lu N, Zhang P, Zhang Q, Qiao R, He Q, Li H B , Wang Y, Guo J, Zhang D, Duan Z, Li Z, Wang M, Yang S, Yan M, Arenholz E, Zhou S, Yang W, Gu L, Nan C W , Wu J, Tokura Y and Yu P 2017 Nature 546 124
[12] Cui Y, Zhang G, Li H, Lin H, Zhu X, Wen H H , Wang G, Sun J, Ma M, Li Y, Gong D, Xie T, Gu Y, Li S, Luo H, Yu P and Yu W 2018 Sci. Bull. 63 11
[13] Böhmer A E, Hardy F, Eilers F, Ernst D, Adelmann P, Schweiss P, Wolf T and Meingast C 2013 Phys. Rev. B 87 180505
[14] Hosoi S, Matsuura K, Ishida K, Wang H, Mizukami Y, Watashige T, Kasahara S, Matsuda Y and Shibauchi T 2016 Proc. Natl. Acad. Sci. USA 113 8139
[15] Chen X, Koiwasaki T and Yamanaka S 2002 J. Phys.: Condens. Matter 14 11209
[16] Kuwabara M, Tomita M, Hashimoto H and Endoh H 1986 Phys. Status Solidi A 96 39
[17] Sultana R, Awana G, Pal B, Maheshwari P K, Mishra M, Gupta G, Gupta A, Thirupathaiah S and Awana V P S 2017 J. Supercond. Novel Magn. 30 2031
[18] Borg C K H, Zhou X, Eckberg C, Campbell D J, Saha S R, Paglione J and Rodriguez E E 2016 Phys. Rev. B 93 094522
[19] Medvedev S, McQueen T M, Troyan I A, Palasyuk T, Eremets M I, Cava R J, Naghavi S, Casper F, Ksenofontov V, Wortmann G and Felser C 2009 Nat. Mater. 8 630
[20] Sun J P, Matsuura K, Ye G Z, Mizukami Y, Shimozawa M, Matsubayashi K, Yamashita M, Watashige T, Kasahara S, Matsuda Y, Yan J Q , Sales B C, Uwatoko Y, Cheng J G and Shibauchi T 2016 Nat. Commun. 7 12146
[21] Guo J, Jin S, Wang G, Wang S, Zhu K, Zhou T, He M and Chen X 2010 Phys. Rev. B 82 180520
[22] Hatakeda T, Noji T, Kawamata T, Kato M and Koike Y 2013 J. Phys. Soc. Jpn. 82 123705
[23] Dong X, Zhou H, Yang H, Yuan J, Jin K, Zhou F, Yuan D, Wei L, Li J, Wang X, Zhang G and Zhao Z 2015 J. Am. Chem. Soc. 137 66
[24] Lu X F, Wang N Z, Wu H, Wu Y P, Zhao D, Zeng X Z, Luo X G, Wu T, Bao W, Zhang G H, Huang F Q, Huang Q Z and Chen X H 2015 Nat. Mater. 14 325
[25] Lei B, Cui J H, Xiang Z J, Shang C, Wang N Z, Ye G J, Luo X G, Wu T, Sun Z and Chen X H 2016 Phys. Rev. Lett. 116 077002
[26] Wang Q Y , Li Z, Zhang W H , Zhang Z C , Zhang J S , Li W, Ding H, Ou Y B , Deng P, Chang K, Wen J, Song C L , He K, Jia J F , Ji S H , Wang Y Y , Wang L L , Chen X, Ma X C and Xue Q K 2012 Chin. Phys. Lett. 29 037402
[27] Mizuguchi Y, Tomioka F, Tsuda S, Yamaguchi T and Takano Y 2009 J. Phys. Soc. Jpn. 78 074712
[28] Taguchi Y, Kitora A and Iwasa Y 2006 Phys. Rev. Lett. 97 107001
[29] Zhang S, Gao M R , Fu H Y , Wang X M , Ren Z A and Chen G F 2018 Chin. Phys. Lett. 35 097401
[30] Wilson J A, Di Salvo F J and Mahajan S 1975 Adv. Phys. 24 117
[31] Thomson R E, Burk B, Zettl A and Clarke J 1994 Phys. Rev. B 49 16899
[32] Liu Y, Ang R, Lu W J, Song W H, Li L J and Sun Y P 2013 Appl. Phys. Lett. 102 192602
[33] Zhang H, Liu C X, Qi X L, Dai X, Fang Z and Zhang S C 2009 Nat. Phys. 5 438
[34] Xia Y, Qian D, Hsieh D, Wray L, Pal A, Lin H, Bansil A, Grauer D, Hor Y S, Cava R J and Hasan M Z 2009 Nat. Phys. 5 398
[35] Hor Y S, Williams A J, Checkelsky J G, Roushan P, Seo J, Xu Q, Zandbergen H W, Yazdani A, Ong N P and Cava R J 2010 Phys. Rev. Lett. 104 057001
[36] Liu Z, Yao X, Shao J, Zuo M, Pi L, Tan S, Zhang C and Zhang Y 2015 J. Am. Chem. Soc. 137 10512