Photon Factory Activity Report 2014 #32 (2015) B

BL-28A/2012S2-001, 2014G177

Anisotropic electronic structure of BaFe2As2 in the electronic nematic phase
Keisuke Koshiishi1, Liang Liu1, Kozo Okazaki1, Hakuto Suzuki1, Jian Xu1, Masafumi Horio1,Hiroshi Kumigashira2, Kanta Ono2, Masamichi Nakajima3, Shigeyuki Ishida3, Kunihiro Kihou3,
Chul-Ho Lee3, Akira Iyo3, Hiroshi Eisaki3, Shin-ichi Uchida1 and Atsushi Fujimori1
12

Department of Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, JapanPhoton Factory, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki, 3050801, Japan
3National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba,Ibaraki, 305-8568, Japan

1 Introduction
A parent compound of 122-type iron-based
superconductors BaFe2As2 (Ba122) is paramagnetic withthe tetragonal structure at room temperature. At
structural-transition temperature of TS = 142 K, Ba122
undergoes a tetragonal-to-orthorhombic structuraltransition
that is coincident with spin-density-wave(SDW) phase transition. In this material, electronic
nematicity, in which the rotational C4 symmetry ofelectronic structure is broken, has been observed in theAFM state close to the superconducting dome in thephase diagram through resistivity [1], optical conductivity[2], and ARPES [3] measurements. Thus the origin of theelectronic nematicity is considered as one of a key issues
to understand the superconductivity. Recently, it has beenreported that the electronic nematicity persists up to 170
K well above the transition temperature [4], indicating theappearance of orbital order [5]. Here, we report theelectronic structure of Ba122 in the electronic nematic
phase observed by ARPES.
2 ExperimentHigh-quality single crystals of Ba122 were grown by
the self-flux method and post-annealed. To observe theintrinsic electronic structure of Ba122, samples weredetwinned by applying uniaxial compressive pressure in
the a-b plane with a sample holder which is similar to thatused in Ref. [6]. ARPES measurements were performedat beamline 28A of Photon Factory using circularly
polarized light with the photon energy of 63 eVcorresponding to kz ~ 2π/c. A SIENTA SES-2002
electron analyzer was used with the total energy
resolution of ~ 20 meV. The crystals were cleaved in situ
at T =20 K, and the measurements were carried out in an
ultrahigh vacuum of ~ 4×10-10 Torr.
3 Results and Discussion
To characterize the ordered state in the electronic
nematic phase proposed in Ref. [4], we studied the band
structure of Ba122 in the temperature ranges from 20 to150 K across the transition temperature Ts. Figure 1(a)shows Fermi-surface mapping in the kx-ky plane. A hole
pocket, two electron pockets contiguous to the hole
pocket, and small but high-intensity small electron pockets

Fig. 1: ARPES spectra of BaFe2As2. (a) Fermi Surface
mapping in kx-ky plane at 20 K. (b) Evolution of theenergy-momentum plots with temperature. The cuts weretaken across the Dirac cone near the X point along a red
broken line shown by in panel (a). Raw spectra andsecond-derivative plots of MDCs are shown in left andright panels respectively.
corresponding to crossing points of linear bands forming
Dirac cones were observed. The shapes of the Fermi
surfaces of Ba122 thus determined are depicted in theinset of Fig. 1(a). Energy-momentum plots at sometemperatures across TS are shown in Fig. 1(b). The Diraccone is confirmed at 80 K. Although the band dispersions
is broadened as the temperature increases, a feature of
the Dirac cone persists above TS. The difference of theintensity between the two linear bands seems to be due
to matrix element effects. This results suggest that somekind of antiferro-orbital order or its strong fluctuation thathas the same translational symmetry as the SDW statepersists above the transition temperature.
References[1] J.-H. Chu et al, Science 329, 824 (2010).
[2] M. Nakajima et al., Proc. Natl. Acad. Sci. USA 108, 12238(2011).
[3] M. Yi et al., Proc. Natl. Acad. Sci. USA 108, 6878 (2011).
[4] S. Kasahara et al., Nature 486, 382 (2012).

[5] H. Kontani et al., Phys. Rev. B 84, 024518 (2011).
[6] Y. Kim et al., Phys. Rev. B 83, 064509 (2011).
* keisuke@wyvern.phys.s.u-tokyo.ac.jp