Photon Factory Activity Report 2019 #37 (2020)

BL-2A/2018G653

Inverted band structure of topological Dirac-semimetal candidate CaAuAs
Kosuke Nakayama,1,2 Zhiwei Wang,3 Daichi Takane,1 Seigo Souma,4,5* Yuya kobota,1 Miho Kitamura,6 Koji
Horiba, 6 Hiroshi Kumigashira, 6,7 Takashi Takahashi,1,4,5 Yoichi Ando,3 and Takafumi Sato1,4,5
1Department of Physics, Tohoku University, Sendai 980-8578, Japan
2
PRESTO-JST, Tokyo 102-0076, Japan
3
Institute of Physics II, University of Cologne, Köln 50937, Germany
4
Center for Spintronics Research Network, Tohoku University, Sendai 980-8577, Japan
5
WPI-AIMR, Tohoku University, Sendai 980-8577, Japan
6
Photon Factory, Institute of Materials Structure Science, High Energy Research Organization,
1-1 Oho, Tsukuba, 305-0801, Japan
7
IMRAM, Tohoku University, Sendai 980-8577, Japan
1 Introduction
Topological Dirac semimetals (TDSs), a threedimensional analogue of graphene, are attracting particularattentions since they are a useful platform to study an
intriguing quantum physics triggered by three-dimensional
Dirac fermions. Recent first-principles band-structure
calculations have predicted that CaAuAs possesses a pairof Dirac cones close to the Fermi level (EF) along the ΓAline of the bulk Brillouin zone (BZ) due to the bulk-bandinversion and the protection by the C3 symmetry [1]. The
predicted simple band structure with no EF crossings ofother topologically trivial bands makes CaAuAs an ideal
system to search for exotic properties associated with bulk
Dirac fermions. To examine possible 3D-TDS nature of
CaAuAs, we have performed soft x-ray ARPES to reveal
the electronic structure relevant to 3D Dirac fermions andclarify its possible link to the exotic physical properties [2].
2 ExperimentHigh-quality single crystals of CaAuAs were grown by
the self-flux method. Angle-resolved photoemission
spectroscopy (ARPES) measurements were performed
with a SES2002 electron analyzer at BL2. We used linearlypolarized light of 300-600 eV. The energy and angularresolutions were set to be 150 meV and 0.2˚, respectively.
Crystals were cleaved in situ in an ultrahigh vacuum better
than 1×10-10 Torr along the (0001) crystal plane. Sample
temperature was kept at T = 40 K during the ARPES
measurements.
3 Results and Discussion
By utilizing bulk-sensitive soft-x-ray photons fromsynchrotron radiation, we experimentally determined thebulk valence-band structure in the 3D bulk BZ of CaAuAs,
and found a semimetallic band overlap between holelike
As 4p and electron-like Au 6p bands. As schematicallyshown in Fig. 1(a), our ARPES measurements revealed
that the Fermi surface (FS) of CaAuAs consists of 3Delectronlike (e1) and holelike (h1) pockets centered at theΓ and A points, respectively, as well as a quasi-2D holelike(h2) cylinder along the ΓA line. The formation of a quasi2D cylindrical FS is probably associated with the holeFig. 1: (a) Schematic 3D view of the FS for CaAuAs
deduced from the ARPES experiment. (b) Experimental
band dispersion around the Γ point in kz=0 plane, comparedwith the calculated band dispersion with spin-orbit
coupling along the ΓM line (solid curves) [1].
doping to the crystal. After taking the hole doping into
account, we found a good agreement between theexperimental and calculated bulk bands. In particular, theband dispersion at kz = 0 coincides nearly perfectly withthe calculation in which the chemical potential is shifted
downward by 300 meV [1]. The present result
demonstrates that the bulk-band structure of CaAuAs iswell captured by the band-structure calculations. Becauseof the high tunability in the chemical composition of thiscompound, CaAuAs provides a precious opportunity for
exploring novel quantum phenomena associated with the
Dirac fermions, and for investigating the novel propertiesof 3D Dirac quantum phase transition from TDS to otherexotic topological phases.
Acknowledgement
We thank T. Kim, C. Cacho (Diamond Light Source),
and S. A. Ekahana, M. Shi (Swiss Light Source) for theircollaboration on this work.
References[1] B. Singh et al., Phys. Rev. B 98, 085122 (2018).
[2] K. Nakayama et al., Phys. Rev. B (R), in press.
* s.souma@arpes.phys.tohoku.ac.jp