Photon Factory Activity Report 2016 #34 (2017)

BL-28A/2015S2-003

Dirac-nodal-arc surface state in the nodal-line semimetal HfSiS
D. Takane 1, Z. Wang,2 S. Souma,3,4,* K. Nakayama,1 C. X. Trang,1 T. Sato,1,4
T. Takahashi,1,3,4 and Yoichi Ando2
1Department of Physics, Tohoku University, Sendai 980-8578, Japan
2
Institute of Physics II, University of Cologne, Köln 50937, Germany,
3
WPI-AIMR, Tohoku University, Sendai 980-8577, Japan
4
Center for Spintronics Research Network, Tohoku University, Sendai 980-8577, Japan
1 Introduction
The discovery of topological insulators triggered thesearch for new types of topological materials based onother symmetries, as represented by topologicalcrystalline insulators where gapless SSs are protected by
the space-group symmetry (specifically mirror symmetry)of crystal. Topological semimetals are recently becoming
a leading material platform for realizing such novel
topological states [1]. In contrast to conventional
semimetals with a finite band-overlap between valenceband (VB) and conduction band (CB), topologicalsemimetals are categorized by the band-contacting naturebetween VB and CB in the Brillouin zone (BZ); pointcontact (Dirac/Weyl semimetals) or line-contact (linenode semimetals; LNSMs). The existence of threedimensional (3D) Dirac semimetals was first confirmedby angle-resolved photoemission spectroscopy (ARPES)
where the VB and CB disperse linearly in all k axes andcontact each other at the point (Dirac point) protected byrotational symmetry of the crystal. Recent ARPES studies
on noncentrosymmetric transition-metal monopnictides
have clarified pairs of bulk Dirac-cone bands and Fermiarc surface states, supporting their Weyl-semimetallic
nature [2]. While the existence of Weyl semimetals withpoint nodes has been confirmed experimentally, theexperimental studies of LNSMs with line node are
relatively scarce despite many theoretical predictions.
2 ExperimentHigh-quality single crystals of HfSiS were synthesizedwith a chemical vapor transport method. ARPES
measurements were performed with an Omicron-Scienta
SES2002 electron analyzer with energy-tunable
synchrotron light at BL28A in Photon Factory. We usedcircularly polarized light of 36–200 eV. The energy andangular resolutions were set at 10–30 meV and 0.2º,respectively. Samples were cleaved in situ along the (001)crystal plane in an ultrahigh vacuum of 1×10-10 Torr, andkept at 30 K during the measurements.
3 Results and Discussion
We plot in Fig. 1(a) the ARPES intensity as a functionof two-dimensional (2D) wave vector at the Fermi level(EF) measured at hn = 48 eV. One finds a “banana”_ _shaped Fermi surface (FS) elongated _along the XX
direction, together with small pockets at X. The bananalike feature gradually expands upon increasing EB, andevolves into two _diamonds at EB ≥ 0.5 eV, whereas thesmall pockets at X gradually shrink and finally disappear(not shown). The outer and inner diamonds arise from theholelike Si 3p and the electronlike Hf 5d bands,respectively.
Inside the banana-like FS, we found the additionalband (X1) which is attributed to bulk band states. By
switching the polarization of circularly polarized light, wealso observed the X2 band (not shown) which shows the__dispersion
symmetric to the X1 band with respect to the G__M line. This suggests that the actual band has an X-shapetilted in E-k space, resulting from a merger of X1 and X2
shown in Fig. 1(b). These results led us to conclude that__ __the X-shaped dispersion extends along a line on the GM[black line], which can be viewed as an arc of Dirac node
extending one-dimensionally in k space. This Dirac-node
arc is apparently different from the Dirac cone of TIs andgraphene where the upper and lower cones intersect at apoint in k space. The X1 and X2 bands is insensitive tophoton energy supports their surface-state (SS) origin.
Obviously, these unexpected SSs pose a significant
challenge in our understanding of topological LNSMs.
The unexpected SSs containing a novel Dirac-node arc
could be a consequence of some topology which is yet tobe discovered in LNSMs. Whatever its origin, the Diracnode arc found hear has a peculiar characteristic, that acharge neutrality point is always present in the surface
state as long as the arc crosses EF.

Fig. 1: (a) Fermi surface plot of HfSiS. (b) Schematicband dispersion in 3D E-k space for X1 and X2 band.
References[1] A. A. Burkov, Nature Mat. 15, 1145 (2016).
[2] S. Souma et al., Phys. Rev. B 93, 161112R (2016).
[3] D. Takane et al., Phys. Rev. B 94, 121108R (2016).
* s.souma@arpes.phys.tohoku.ac. jp