Research Activities– Off-line Experiments –18AG029

Laser-ARPES

Study of ARPES, Magnetic and Magneto-transport
Properties of Dy-doped Bi2Te3 Topological Insulator
Shiv Kumara, Vinod K. Gangwarb, Yufeng Zhangc, Prashant Shahid, Swapnil
Patilb, Eike F. Schwiera, Kenya Shimadaa, Yoshiya Uwatokoc, and Sandip
Chatterjeeb
aHiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima City, 739-0046 Japanb
Department of Physics, Indian Institute of Technology (BHU) Varanasi 221005c
Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, JapandDepartment of Physics, D.D.U. Gorakhpur University, Gorakhpur-273009

Keywords: ARPES and Magneto-transport

Topological insulators are characterized by a gapped bulk state and gapless surface or edge states that is
protected by time-reversal symmetry (TRS). TRS can be broken by magnetic impurities. To investigate thecorrelation between topological state and magnetism it is important to introduce magnetic elements into
Topological insulators. In this study, we have synthesized the single crystals of Bi2-xDyxTe3 (x =0.06, 0.10,0.16) and reported the eﬀ ects of Dy-substitution on the topological properties in Bi2-xDyxTe3. XRD data
indicate very good single crystallinity of Bi2-xDyxTe3 without any sign of secondary phase. We havemeasured the magnetization M(H) at diﬀ erent temperature and found that at low temperature it shows weakferromagnetic ordering. From the ARPES and thermoelectric studies we found n-type nature of Dy-doped
Bi2Te3. Figure shows the HR-ARPES spectra measured in s- and p-polarization geometry which revealed
the existence of a Dirac-cone like surface states in Dy-doped Bi2Te3 samples.

Fig.1 Bi2Te3:Dy5_S-polarization

Fig.2 Bi2Te3:Dy5_P-polarization

REFERENCES
1. M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010).
2. S E Harrison, L J Collins-McIntyre, S-L Zhang, A A Baker, A I Figueroa, A J Kellock, A Pushp, S S P Parkin, J S Harris,
G van der Laan and T Hesjedal, J. Phys.: Condens. Matter 27 245602 (2015).
3. Hideaki Iwasawa, Eike F. Schwier, Masashi Arita, Akihiro Ino, Hirofumi Namatame, Masaki Taniguchi, Yoshihiro Aiura,Kenya Shimada, Ultramicroscopy 182, 85–91 (2017).
4. L. B. Duffy, N.-J. Steinke, J. A. Krieger, A. I. Figueroa, K. Kummer, T. Lancaster, S. R. Giblin, F. L. Pratt, S. J. Blundell,T. Prokscha, A. Suter, S. Langridge, V. N. Strocov, Z. Salman, G. van der Laan, and T. Hesjedal, Phys. Rev. B 97, 174427(2018).

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Proposal No.: 18AU018

Nodal gap in electron-doped J=1/2 Mott insulators
Yong Hua, Zhiyuan Weia and Junfeng Hea
aUniversity of Science and Technology of China, Hefei, Anhui 230026, China.

Keywords: Iridates, Mott insulators, Nodal gap

Many exotic states are induced by doping Mott insulators. In the cuprates high-temperature
superconductors, many unconventional phenomena emerge when carriers are doped into the parent Mottstate [1]. One of the most intriguing phenomena in hole-doped cuprates is the existence of an energy gap at
the momentum location where the d-wave superconducting gap node is expected [2]. Continuous efforts
have been made to understand its origin. Besides many interesting theoretical proposals, an intriguingquestion from experimental side is whether the observed nodal gap represents an intrinsic physics of doped
Mott insulators.
Perovskite strontium iridates Sr2IrO4 is an effective spin-1/2 Mott insulator, in which pseudogap [3,4]and d-wave instability [5] have been observed, offering an unprecedented system to test the universality ofthe emergent phenomena in cuprates. To this end, we proposed the first systematic ARPES study on nodal
gap in electron-doped Sr2IrO4.
ARPES measurements were carried out at HiSOR Laser-based μ-ARPES system on the La-doped Sr2IrO4samples. However, we did not obtain clear band structure on our La-doped Sr2IrO4 samples, possibly due tothe matrix element effects in ARPES (the photon energy may not be good for Sr2IrO4). But these initial tests
provided us some very useful information for the follow-up beam time in other places.

ab

FIGURE 1. Fermi surface and band structure of an UD80 K Bi2212 sample. (a) Fermi surface of the UD80 K Bi2212 sample.
(b) Band structure along the nodal direction [(0, 0) - (π, π)]. These data were obtained at 90 K by using μ-ARPES system.

For this beam time, we carried out the back-up plan and performed ARPES measurements on Bi2212
samples using the Laser-based μ-ARPES system. As shown in FIGURE 1, high quality data was obtainedon an UD80 K Bi2212 sample. Detailed temperature dependent measurements were also performed on thissample (not shown here). Systematic data analysis is in process.

- 130 -REFERENCES
1.
2.
3.
4.
5.

Lee, P. A. et al., Rev. Mod. Phys. 78, 17-85 (2006).
Peng, Y.-Y. et al., Nat. Commun. 4, 2459 (2013).
Kim, Y.-K. et al., Science 345, 187-190 (2014).
de la Torre, A. et al., Phys. Rev. Lett. 115, 176402 (2015).
Kim, Y. K. et al., Nat. Phys. 12, 37-41 (2015).

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Laser ARPES

ARPES Studies of Ultrathin Ferromagnetic Films on
Topological Insulators for Spintronic Applications
Andrey K. Kaveev,a Oleg E. Tereshchenko,b Vladimir A. Golyashovb and Eike F.
Schwierc
aIoffe Institute, 194021, 26 Polytechnicheskaya str., Saint-Petersburg, Russia
Rzhanov Institute of Semiconductor Physics, 630090, 13 Ac. Lavrentev av., Novosibirsk, Russia
cHiroshima University, 739-8527, Higashi Hiroshima, Japanb

Keywords: Topological insulators, PbSnTe, BiSbTeSe2, Dirac cone, ferromagnetic metal, interface.

Recently, expressed interest in the field of spintronics has been related to topological insulators,
characterized by a unique electronic structure their surface states [1, 2]. Topological insulators arecharacterized as conventional insulators in the bulk and as objects with the Dirac cone of conducting
spin-polarized states at their surface. The study of a ferromagnetic-topological insulator interface is ofinterest due to the effect of magnetic proximity acting as a factor which breaks the symmetry, with respect
to the time inversion of topological surface states [3].

FIGURE 1. LARPES measured band dispersions of the Pb0.71Sn0.29Te (111) surface states after the treatment in the HCl-iPA
solution and annealing in vacuum at 300°C (a) and subsequent deposition of 0.3 Å (b), 1.1 Å and 3 Å of Co at 300 °C. Γ-Mdirection, hν = 6.3 eV, T = 20 K.

Mono-crystalline epitaxial films of ferromagnetic Co on the Pb0.71Sn0.29Te topological insulator
substrates were grown by the MBE method. The epitaxial relations for the film and the substrate wererevealed using 3D reciprocal space mapping based on RHEED measurements. For the Co/ Pb0.71Sn0.29Te
system, a technology for cleaning the film surface by a thermal annealing in vacuum after a chemicaltreatment was developed. The successful cleaning is demonstrated by the presence of a clear topological- 132 -state in the Laser ARPES measurement. One of the key results of this study is the observation of surface
states with Dirac-like dispersion, occurring at the Г position in the surface Brillouin zone. Fig. 1 (a)demonstrates the dispersion characteristic for the clean Pb0.71Sn0.29Te (111) surface (see [4]) after achemical treatment and a subsequent vacuum annealing. The valence band structure associated with the Γ point position on the Fermi level was shown. The Co deposition leads to the noticeable upward bandbending, with an electron doping induced shift of the electronic state of about 20 meV after deposition of~1.1 Å of Co (Fig. 1 (b, c)). With thick Co coatings (~ 3 Å and more, Fig. 1 (d)), as expected, the metallic
electronic states of Co start to dominate the dispersion. Thus, the stable topological state of PbSnTe wasobserved right up to no less than the monolayer thick Co coverage, including an upward band bending ofaround 20 meV.
This work was supported by the Russian Foundation for Basic Research (grants № 17-02-00729) andby the Russian Science Foundation (Project No.17-12-01047).

REFERENCES
1J. Sánchez-Barriga, A. Varykhalov, G. Springholz, H. Steiner, R. Kirchschlager, G. Bauer, O. Caha, E. Schierle, E.
Weschke, A. A. Ünal, S. Valencia, M. Dunst, J. Braun, H. Ebert, J. Minár, E. Golias, L. V. Yashina, A. Ney, V. Holýand O. Rader, Nat. Comm., 7, 10559 (2016).
2 C. Kane, Nature, 4, 348–349, (2008).
3 P. Sessi, R. R. Biswas, T. Bathon, O. Storz, S. Wilfert, A. Barla, K. A. Kokh, O. E. Tereshchenko, K. Fauth, M. Bode &A. V. Balatsky, Nat. Comm., 7, 12027 (2016).
4. S.-Y. Xu, C. Liu, N. Alidoust, M. Neupane, D. Qian, I. Belopolski, J.D. Denlinger, Y.J. Wang, H. Lin, L.A. Wray, G.
Landolt, B. Slomski, J.H. Dil, A. Marcinkova, E. Morosan, Q. Gibson, R. Sankar, F.C. Chou, R.J. Cava8, A. Bansil,
M.Z. Hasan, Nat. Commun., 3, 1192 (2012).

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