Photon Factory Activity Report 2016 #34 (2017)

BL-28A/2016G096, 2015S2-003

Superconducting gap of PrFeAsO1-y observed by angle-resolved photoemission
spectroscopy
Kenta Hagiwara1,*, Masafumi Horio1, Keisuke Koshiishi1, Suguru Nakata1,Motoyuki Ishikado2, Shigeyuki Ishida3, Hiroshi Eisaki3, Shinichi Shamoto4,Kanta Ono5, Hiroshi Kumigashira5, Teppei Yoshida6 and Atsushi Fujimori1
1

Department of Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku Tokyo 113-0033, Japan
2
Comprehensive Research Organization for Science and Society, 162-1 Shirakata, Tokai, Naka
Ibaraki, 319-1106, Japan
3
National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, 305-8568,
Japan
4Japan Atomic Energy Agency, 765-1 Funaishikawa, Tokai, Naka, Ibaraki, 319-1184, Japan
5
Photon Factory, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki, 3050801, Japan
6
Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-nihonmatsu,
Sakyo-ku, Kyoto 606-8501, Japan

1 Introduction
In the 1111-type iron-based superconductors,
superconductivity is realized by electron doping throughsubstituting F for O or Co for Fe, or introducing oxygen
deficiencies [1, 2, 3]. According to previous angleresolved photoemission spectroscopy (ARPES) studies [4,5, 6], however, the 1111 compounds exhibit heavily holedoped electronic structures for cleaved surfaces, which
has been attributed to charge transfer because of the polar
surfaces. Superconducting gaps for these surfaces havebeen reported recently Charnukha et al. [6]. Theyobserved an edge shift around the superconducting
critical temperature (Tc). However, the relationshipbetween the surface electronic structure and thesuperconductivity has not been clarified yet. Therefore,
more detailed knowledge about the superconducting gap
at the surface is required.
In this work, we have measured the temperaturedependence of the superconducting gap of the 1111
compounds using ARPES in order to understand thesuperconductivity at the surface.
2 ExperimentHigh-quality single crystals of the electron-doped
superconductor PrFeAsO1-y (Tc = 24 K) were synthesized
by the high pressure technique as described in Ref. [7].
ARPES measurements were performed at BL-28A of
Photon Factory using circularly polarized light with thephoton energy of 42.5 eV. A SCIENTA SES-2002
electron analyzer was used. The total energy resolution of~ 20 meV. The crystals were cleaved in situ at T =8 K.
The measurements were carried out in an ultrahighvacuum of ~ 10-10 Torr.
3 Results and Discussion
Figure 1(a) shows a Fermi surface (FS) mapping for thePrFeAsO1-y at T = 8 K. One can clearly observe a large

circular hole pocket and a middle-size circular hole
pocket, referred to as “Outer” and “Inner”, respectively.
This result is consistent with previous reports [4, 5, 6].
Figure 1(c) shows symmetrized energy distribution curves(EDCs) at the Fermi wave vector (kF) of the “Outer” FS
indicated in Fig. 1(b). One can see that an energy gap
opens at low temperature. In order to quantify the energy
gap, we have estimated gap depth and the gap area fromthe symmetrized EDCs and have plotted them in Fig. 1(c).
One can clearly see an energy gap opening even above Tc.
There are two possibilities for the gap opening above Tc.
One is that superconductivity at the surface occurs at ahigher temperature than the bulk Tc. Another possibility isthat superconductivity at the surface is realized by theproximity effect of the superconducting bulk and apseudo-gap persists up to ~80 K. To make conclusion
about this issue, further investigation is required.
(b)

(a)

OuterCut #1

(c)

(d)

Fig. 1 ARPES results of PrFeAsO1-y (Tc = 24 K)

Photon Factory Activity Report 2016 #34 (2017)(a) Fermi surface mapping at T = 8 K.
(b) Energy-momentum plot along Cut #1 in Fig. 1 (a).
(c) Symmetrized EDCs at kF of the “Outer” FS in Fig. 1(b).
(d) Temperature dependence of the gap magnitude.
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* hagiwara@wyvern.phys.s.u-tokyo.ac.jp