Photon Factory Activity Report 2018 #36 (2019)

BL-28A, BL-2A/2018S2-001, 2016G555

Nodal-line fermions protected by nonsymmorphic symmetry in Ta3SiTe6
Takafumi Sato,1,2,3,* Zhiwei Wang,4 Kosuke Nakayama,2 Seigo Souma,1,3 Daichi Takane,2 YukiNakata,2 Takashi Takahashi,1,2,3 Koji Horiba,5 Hiroshi Kumigashira,5,6 and Yoichi Ando4
1 WPI-AIMR, Tohoku University, Sendai 980-8577, Japan
2 Department of Physics, Tohoku University, Sendai 980-8578, Japan
3 Center for Spintronics Research Network, Tohoku University, Sendai 980-8577, Japan
4 Institute of Physics II, University of Cologne, Köln 50937, Germany
5Photon Factory, Institute of Materials Structure Science, High Energy Research Organization,
1-1 Oho, Tsukuba, 305-0801, Japan
6 IMRAM, Tohoku University, Sendai 980-8577, Japan
1 Introduction
The search for new types of topological materials
hosting nodal fermions is currently one of emergent topics
in condensed-matter physics. Recently, it was theoretically
proposed that layered ternary telluride Ta 3SiTe6 hosts thenodal fermions protected by the nonsymmorphic glide
mirror symmetry [1]. This material crystalizes in theorthorhombic structure [Fig. 1(a)] with the space group No.
62 (Pnma). First-principles band-structure calculations
show that, when the spin-orbit coupling (SOC) is neglected,
Ta3SiTe6 displays a four-fold-degenerate (eight-folddegenerate if counting spin) nodal line on the SR line in
the bulk Brillouin zone (BZ) shown in Fig. 1(b) due to theband crossing protected by the glide mirror symmetry. It isalso suggested that when the SOC is included, the four-folddegeneracy on the SR line is slightly lifted and as a result
the hourglass-like dispersions appear in the close vicinityof EF. To examine such intriguing predictions, it is highly
desirable to experimentally establish the electronic band
structure of Ta3SiTe6.
2 ExperimentHigh-quality single crystals of Ta3SiTe6 were grown by
the chemical vapor transport method by using I2 astransport
agent.
Angle-resolved
photoemission
spectroscopy (ARPES) measurements were performed
with a DA30 electron analyzer at BL28. To excite
photoelectrons, we used linearly polarized VUV light of40-100 eV. The energy and angular resolutions were set tobe 6-20 meV and 0.2˚, respectively. We also measured thecore-level spectrum with 600-eV photons at BL2. Crystals
were cleaved in-situ along the (001) plane in an ultrahighvacuum of 1×10-10 Torr and showed a shiny mirror-likesurface indicative of high quality of the crystal. Sample
temperature during measurements was kept at T = 30 K.
3 Results and Discussion
We show in Fig. 1(c) the ARPES-intensity mapping at
EF as a function of in-plane wave vectors (kx and ky) [2].
We observe two types of Fermi surfaces, one is an open__
Fermi
surface with a holelike character, surrounding the Γ__and Y points with strong wiggling along__ the kx direction.
__
Another Fermi surface encloses the M and X points,

forming an open Fermi surface with an electronlike
character. By utilizing the energy-tunable photons fromsynchrotron radiation, as shown in the normal-emission
spectra in Fig. 1(d), we have determined the band structurein the 3D BZ, and found that the energy bands in thevalence-band show Dirac-like dispersions which present aband degeneracy at the R point of the bulk orthorhombic
Brillouin zone (not shown). This band degeneracy extends
one-dimensionally along the whole SR high-symmetryline, forming the nodal lines protected by the glide mirror
symmetry of the crystal. Our result thus suggests that theglide symmetry is highly useful to search for new platformof topological semimetals hosting nodal lines.

Fig. 1: (a) Crystal structure and (b) BZ of Ta3SiTe6. (c)ARPES intensity at EF plotted as a function of kx and ky.
(d) Plot of normal-emission ARPES intensity as afunction of kz [2].
References[1] S. Li et al., Phys. Rev. B 97, 045131 (2018).
[2] T. Sato et al., Phys. Rev. B 98, 121111(R) (2018).
* t-sato@arpes.phys.tohoku.ac.jp