Ⅲ-2. Atomic & Solid State Physics (Muon)

RIKEN Accel. Prog. Rep. 50 (2017)

μSR study of impurity eﬀects on the Cu-spin correlation andsuperconductivity in the undoped superconductor T’-La1.8 Eu0.2 CuO4
T. Kawamata,∗1,∗2 K. Ohashi,∗2 T. Adachi,∗1,∗3 T. Takamatsu,∗2 M. Kato,∗2 I. Watanabe,∗1 and Y. Koike∗1,∗2

∗1
∗2
∗3

RIKEN Nishina Center
Department of Applied Physics, Tohoku University
Department of Engineering and Applied Sciences, Sophia
University- 231 -Raw asymmetry (%)

Raw asymmetry (%)

T’-La1.8Eu0.2Cu1-yMyO4

Raw asymmetry (%)

It has long been believed that high-Tc superconductivity appears through hole and electron doping into
antiferromagnetic mother compounds Ln2 CuO4 (Ln:lanthanide elements) with the K2 NiF4 -type (T-type)
and Nd2 CuO4 -type (T’-type) structures, respectively.
Recently, however, in adequately reduced thin ﬁlms ofT’-Nd2−x Cex CuO4 , superconductivity was observed ina wide range of x and even in the undoped mothercompound of x = 0.1) In undoped T’-Ln2 CuO4 , it
was suggested that the electronic state is explained
based on the strong electronic correlation:2) carriers
are induced at the Fermi level by the collapse of thecharge-transfer gap between the upper Hubbard bandof Cu3dx2 −y2 and the O2p band.2) The electronic stateand superconductivity in undoped T’-Ln2 CuO4 have,therefore, attracted great interest.
For the undoped superconductor T’-La1.8 Eu0.2 CuO4(T’-LECO), our recent study of impurity eﬀects on thesuperconducting critical temperature, Tc , has revealed
that T’-LECO shows d-wave superconductitiy mediated by the spin ﬂuctuation and that the electronic
state is similar to that in optimally hole-doped andoverdoped T-LSCO.3) A previous μSR study using aT’-LECO sample (Tc = 15 K) showed that a shortrange magnetic order appears in the almost whole region of the sample.4) Accordingly, in order to investigate the electronic state and superconductivity, theeﬀect of impurities on the development of the Cu-spincorrelation in undoped T’-LECO was studied. We
performed the zero-ﬁeld (ZF) and longitudinal-ﬁeld
μSR measurements for another sample of T’-LECO(Tc = 20 K) and impurity-substituted samples of T’La1.8 Eu0.2 Cu1−y M y O4 (M = Ni, Zn) at RIKEN-RAL.
Figure 1 shows the ZF spectra of T’-La1.8 Eu0.2 Cu1−y M y O4 (M = Ni, Zn; y = 0, 0.01). In all samples, we
ﬁnd that (a) the spectra at 150 K show a slow depolarization of muon spins, (b) the depolarization becomes
faster gradually with decreasing temperature, and (c)the spectra show an exponential-like depolarization at1.5 K, indicating the development of the magnetic order. The volume fraction of the magnetic order at1.5 K is estimated to be almost the same in all samples.
This shows that the magnetic order does not appear
around Ni and Zn in the superconducting regions of
the impurity-substituted samples. This behavior has
been observed in overdoped T-LSCO5) and is consistent with the results of impurity eﬀects on Tc .3)

y=020

15150K50K

30K
20K

1.5K

M = Ni, y = 0.01
2015150K50K

30K
20K

1.5K

M = Zn, y = 0.01

2015150K50K100130K
20K

1.5K

234Time (μsec)

56Fig. 1. Zero-ﬁeld μSR time spectra of T’-La1.8 Eu0.2 Cu1−y M y O4 (M = Ni, Zn; y = 0, 0.01).

From the temperature dependence of the depolarization change, moreover, it is found that the magnetic order develops in the Ni- (Zn-) substituted sample below the temperature higher (lower) than thatin the non-substituted sample. These impurity eﬀects
indicate that the magnetic order is due to the nearest exchange interaction in a localized spin system.
The development of the magnetic order is enhancedby Ni with a larger magnetic moment than that ofCu, while the order is suppressed owing to the spindilution eﬀect by the substitution of nonmagnetic Zn.
These results indicate a strong electronic correlationand suggest the occurrence of phase separation into
superconducting and magnetic-ordered regions in T’LECO. The strong electronic correlation model2) inundoped T’-Ln2 CuO4 is strongly supported.
References
1) O. Matsumoto et al.: Physica C 469, 924 (2009).
2) T. Adachi et al.: J. Phys. Soc. Jpn. 82, 063713 (2013).
3) K. Ohashi et al.: J. Phys. Soc. Jpn. 85, 093703 (2016).
4) T. Adachi et al.: J. Phys. Soc. Jpn. 85, 114716 (2016).
5) T. Adachi et al.: Phys. Rev. B 78, 134515 (2008).