Photon Factory Activity Report 2017 #35 (2018)

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

Investigation of extended superconducting dome in protect-annealed
Pr1.3-xLa0.7CexCuO4Chun LIN1,*, Masafumi HORIO1, Keisuke KOSHIISHI1, Suguru NAKATA1, Kenta HAGIWARA1,Yuxuan WAN1, Masahiro SUZUKI1, Kanta ONO2, Koji HORIBA2, Hiroshi KUMIGASHIRA2,
Shin-ichiro IDETA3, Kiyohisa TANAKA3, Taro OHGI4, Takayuki KAWAMATA4, Hidetaka SATO4,Yoji KOIKE4, Tadashi ADACHI5, and Atsushi FUJIMORI1
1Department of Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan2
Photon Factory, KEK, 1-1 Oho, Tsukuba, Ibaraki, 305-0801, Japan
3
UVSOR Facility, Institute for Molecular Science, Okazaki, 444-8585, Japan
4
Department of Applied Physics, Tohoku University, Aramaki, Aoba-ku, Sendai, 980-8579, Japan
5
Department of Engineering and Applied Sciences, Sophia University, Tokyo, 102-8554, Japan
1 Introduction
Electron-doped cuprate superconductors are usuallycharacterized by the T’ structure (Fig.1 (b) [1]), a muchnarrower superconducting (SC) dome, and the morerobust antiferromagnetic (AF) phase in the phase diagram
than their hole-doped counterparts (Fig. 1 (d) [2]). Unlike
the T structure of the hole-doped La2-xSrxCuO4 (LSCO)(Fig.1 (a) [1]), in the T’ structure, copper atoms are
surrounded by four oxygen atoms in the square-planar
coordination. Nevertheless, in real systems, some
impurity oxygen atoms remain at the apical sites forminga structure shown in Fig. 1 (c). The apical oxygen atoms
induce high disorder and are harmful to superconductivity.
This leads to insulating as-grown samples, and superconductivity emerges only after annealing [3], which isbelieved to remove the apical oxygen atoms [2].
The SC state in single crystals of Pr2-y-xLayCexCuO4
with relatively high Tc can be realized for low Ce
concentration [4]. Recently, Adachi et al. successfully
synthesized bulk SC single crystals of Pr1.3-xLa0.7CexCuO4(PLCCO) with x = 0.05, 0.10, and 0.15 (Tc ~ 27 K, Fig. 2(d), pink circles) by utilizing an improved novel “protectannealing” method [5,6]. From angle-resolved photoemission spectroscopy (ARPES) studies of PLCCO after
protect annealing, we found that the AF correlation of x =0.10 samples can be strongly suppressed and the electron
content estimated from Fermi surface area cansignificantly deviate from the Ce doping level (Fig. 1 (e)[6]). This has also been confirmed by the recent study

targeting on conventionally annealed Pr1-xLaCexCuO4-δ
[7]. Although the oxygen reduction during annealing may
induce additional electrons into the system, the generally
accepted phase diagram of electron-doped cuprates
regards the nominal Ce doping level as the electronconcentration. Therefore, the SC dome of electron-doped
cuprates may not be that narrow as we see in Fig. 1 (d). Inorder to systematically investigate the possibly extended
SC dome of PLCCO after protect annealing, we have
performed ARPES measurements on single crystals of
PLCCO with different Ce doping levels and annealing
conditions to estimate the electron concentration.
2 Experiment
ARPES measurements were performed at PF BL-28A
and UVSOR BL5U with the total energy resolution of~30 meV. The incident light was circular polarized with55 eV photon energy and linearly polarized (s) with 60
eV photon energy respectively. The samples were cleaved
and measured in situ at temperature ≤ 10 K and pressure≤ 1.5 × 10-10 Torr.
Single crystals of Pr1.3-xLa0.7CexCuO4 with Tc varying
from 28 K to 16 K were synthesized by the travelingsolvent floating-zone method. The basic annealing steps
consist of protect annealing, low-temperature annealing,and dynamic annealing. The dynamic annealing separated
the annealing processes into a few stages providing timefor oxygen atoms to travel from the bulk to the surface,(e)Fig. 1. (a-b) T and T’-type structures [1]. (c) T’-type structure with the apical oxygen atom. (d) Generally accepted phase
diagram of hole-doped (left) and electron-doped (right) cuprates, where R represents the rare earth elements [2]. (e)Fermi surface (inset) and new phase diagram of PLCCO after protect annealing plotted as Tc vs nFS [6].

Photon Factory Activity Report 2017 #35 (2018)

(a)

(b)

(c)

(d)

Fig. 2. (a-c) Fermi surface intensity plot integrated within ±10 meV around EF and tight-binding fitting results for xCe =0.10 and 0.15 with different reduction conditions. Sample Tc and tight-binding fitting parameters were also indicated. (d)Extended SC dome in the new phase diagram of PLCCO (magenta shaded area). Solid markers were plotted as Tc vs nFS(electron number estimated from Fermi surfaces) which compose the new phase diagram. Empty markers [6,7,9] wereplotted as traditional way: Tc vs xCe. Green curve comes from the data of hole-doped LSCO [12,13] and was plottedagainst xSr. In order to compare, LSCO’s optimal Tc of 37 K was scaled to PLCCO’s optimal Tc of 28 K in our study.
improving the oxygen homogeneity of the samples. To
systematically estimate the real electron concentration aseries of samples with Ce doping content x = 0.10 and
0.15 as well as different annealing conditions have beenmeasured. We classify the crystals with the same Ce
doping level by the reduction status, i.e., under-reduced(UR), optimally reduced (OP), and over-reduced (OR),
which can be estimated by the annealing time and the Tc.
According to the Luttinger theorem, i.e., the number ofelectrons is given by the Fermi surface volume [8], one
can use ARPES to accurately measure the electron
concentration.
3 Results and Discussion
The Fermi surfaces determined by ARPES in Fig. 2 (ac) indicate that there is no AF pseudogap on the entire
Fermi surfaces for both x = 0.10 and 0.15, i.e., AF
correlation was strongly suppressed, unlike those earlystudies based on the conventionally annealed samples [2]and consistent with the previous study on the strength ofthe same protect annealing method [6]. In order tocalculate the Fermi surface area [10,11], we used thetight-binding model as the fitting function. Tc was plottedagainst electron content estimated from the Fermi surface
area (nFS) in Fig. 2 (d). In the same figure, Tc from someprevious studies was also plotted against nominal Ce
doping level [6,9]. The current systematic study indicatesthat nFS can be significantly larger than the Ce doping
level and thus reveals an extended SC dome in the newphase diagram of PLCCO after protect annealing (Fig. 2(d), magenta shaded area). Thanks to the superiorities

resulting from the novel protect annealing method [5,6],
the SC dome in our study is even more extended than thatobtained from the conventionally annealed samples (Fig.
2 (d), blue shaded area [7]). In order to compare present
results with those of the hole-doped counterpart, we also
reproduced the scaled data from LSCO [12,13] as a green
curve in the same figure. This new phase diagram ofPLCCO after protect annealing exhibits similar or evenmore extended SC dome than that of hole-doped LSCO,
thus providing a clue for understanding the asymmetry of
the phase diagram of cuprate superconductors. Furtherstudies are in process so as to confirm the boundaries of
the new phase diagram.
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* clin@wyvern.phys.s.u-tokyo.ac.jp