Photon Factory Activity Report 2019 #37 (2020)

BL-28A/2018S2-008

Bulk and surface electronic states of axion insulator candidate EuIn2As2Takafumi Sato,1,2,3* Zhiwei Wang,4 Daichi Takane,1 Seigo Souma,1,2,3 Chaoxi Cui,4 Yongkai Li,4 KosukeNakayama,1,5 Tappei Kawakami,1 Yuya Kubota,1 Yugui Yao,4 and Takashi Takahashi1,2,3
1Department of Physics, Tohoku University, Sendai 980-8578, Japan
2WPI-AIMR, Tohoku University, Sendai 980-8577, Japan
3Center for Spintronics Research Network, Tohoku University, Sendai 980-8577, Japan4Beijing Institute of Technology, 100081 Beijing, China
5
PRESTO-JST, Tokyo 102-0076, Japan
1 Introduction
Interplay between magnetism and topology is now
becoming an exciting topic in condensed-matter physics.
Amongst various magnetic topological materials, axion
insulator is currently attracting a particular attention
because of magnetoelectric effect associated with ahypothetic quasiparticle called axion. Despite thenumerous theoretical predictions, a realization of axion
insulator has still been one of the hottest experimentalchallenges. Recently, Zhang et al [1] predicted thatEuIn2As2, which undergoes the antiferromagnetic (AFM)
transition at TN = 16 K, exhibits the coexistence of axion
insulator and higher-order topological insulator in a single
material. Whereas EuIn2As2 inherently has a bulk bandinversion at a single point in the Brillouin zone (BZ), atransition to the AFM state opens an energy gap in gaplesssurface state leading to the axion-insulator phase. To reveal
the electronic structure related to the exotic properties of
EuIn2As2, we have performed a systematic ARPES studyby using energy-tunable synchrotron light [2].
2 ExperimentHigh-quality single crystals of EuIn2As2 weresynthesizedbyflux
method.
Angle-resolved
photoemission spectroscopy (ARPES) measurements wereperformed with a DA30 electron analyzer at BL28A. We
used circularly polarized light of 50-100 eV. The energy
and angular resolutions were set to be 5-20 meV and 0.3˚,respectively. Crystals were cleaved in situ in an ultrahighvacuum better than 1×10-10 Torr along the (0001) crystal
plane. As seen from the core-level photoemission spectrumin Fig. 1(b), clean nature of the cleaved sample surface canbe confirmed.
3 Results and Discussion
Figure 1(a) shows ARPES intensity mapping as afunction of in-plane wave vector (kx and ky) of EuIn2As2.
One can immediately recognize a circular intensity spot
centered at the G point. This intensity is attributed to thehole pocket, as seen from the energy dispersion in Fig. 1(c)along a k cut crossing the G point which signifies a highly
dispersive holelike band crossing EF. From the invarianceto photon energy and the comparison with the bandcalculation, we attribute the holelike band to the surface
band (S1). On the other hand, we found that the week
intensity inside the S1 band is of bulk origin. Upon

entering the AFM phase, the B1 band exhibits a marked
reconstruction characterized by the emergence of a “M”shaped band near EF [schematically shown in Fig. 1(d)].
The qualitative agreement with the first-principles bandstructure calculations suggests the occurrence of bulk-bandinversion at the G point in the AFM phase. EuIn2As2 thus
provides an excellent opportunity to study the exotic
quantum phases associated with axion insulator andhigher-order topology.

Fig. 1:(a) Fermi-surface mapping, (b) core-level spectrum,and (c) band dispersion around the Γ point of EuIn2As2. (d)Schematic band diagram in the AFM phase. “SS”, “S1”denotes the surface band while “B1” to the bulk band.
Acknowledgement
We thank M. Kitamura, K. Horiba, and H. Kumigashirafor their assistance in the ARPES measurements. We also
thank T. Kim, C. Cacho (Diamond Light Source), and A.
Arian, V. N. Strocov (Swiss Light Source) for theircollaboration on this work.
References[1] D. Zhang et al., Phys. Rev. Lett. 122, 206401 (2019).
[2] T. Sato et al., submitted.
* t-sato@arpes.phys.tohoku.ac.jp