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

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Magnetism and superconductivity in underdoped region of T*-typeLa1−x/2 Eu1−x/2 Srx CuO4−y Fy
M. Takahama,∗1,∗2 T. Taniguchi,∗1 D. P. Sari,∗3,∗4 I. Watanabe,∗3 and M. Fujita∗1
In the study of high-transition-temperature cuprates,
the doping evolution of magnetism and its relationshipwith superconductivity are important issues. It had
been considered that the superconductivity is inducedby carrier doping into Mott insulators.1) However, theappearance of superconductivity in RE2 CuO4 (RE =rare earth) without eﬀective carrier doping (undopedsuperconductivity) was clariﬁed by improving material synthesis and annealing techniques.2,3) After the
discovery of undoped superconductivity, the broad superconducting (SC) phase was reported.4)
Recently, we succeeded in synthesizing T*-type
structured La1−x/2 Eu1−x/2 Srx CuO4 (LESCO) in
which the oxygen coordination around a copper ion
is ﬁve and reported the magnetic and SC properties as functions of Sr concentration.5,6) Our muon
spin rotation/relaxation (µSR) measurement of LESCO revealed the presence of a spin-glass state andthe absence of static magnetism in the as-sintered
non-SC and high-pressure oxidated SC samples, respectively. Therefore, the short-range magnetic order disappears with the emergence of superconductivity, suggesting competition between the two states.
However, as it is challenging to synthesize a singlephase sample of LESCO with a smaller x, the electronic state of the underdoped (UD) region of T*type cuprates has not been investigated. Thus, wenewly synthesized La1−x/2 Eu1−x/2 Srx CuO4−y Fy (LESCOF), in which the hole concentration can be reduced by increasing y while keeping the single-phase
T*-type structure. The evidence of low carrier density in both as-sintered and oxidation-annealed LESCO was reported by x-ray absorption spectroscopy
measurement,6) and LESCO with x ≲ 0.23 corresponds
to UD La2−x Srx CuO4 . Therefore, LESCOF with x =0.18 is expected to be located in a more UD region.
The magnetism in UD LESCOF was studied by µSR
measurements at RIKEN-RAL.
Figure 1(a) shows the temperature dependence ofthe zero-ﬁeld µSR time spectra of as-sintered LESCOF
with x = 0.18 and y = 0.14. Exponential-type spectra are observed below ∼80 K, and muon spin precession was observed at the lowest temperature of 5.1 K.
The appearance of precession suggests the emergenceof long-range magnetic order. Furthermore, the temperature at which the spectra change from Gaussiantype to exponential is higher than that in the pristine∗1∗2∗3∗4Institute for Materials Research, Tohoku University
Department of Physics, Tohoku University
RIKEN Nishina CenterCollege of Engineering, Shibaura Institute of Technology

Fig. 1. Zero-field µSR time spectra of (a) as-sintered and(b) oxidation-annealed La1−x/2 Eu1−x/2 Srx CuO4−y Fywith x = 0.18 and y = 0.14.

LESCOF with x = 0.18,4) indicating the developmentof static magnetism upon underdoping. On the other
hand, the oxidation-annealed SC sample shows no evidence of magnetic order down to 2 K (Fig. 1(b)).
Two characteristic features of magnetism were clariﬁed in the present study. Firstly, a long-range magnetic ordered state exists in the lightly doped sampleof as-sintered T*-type LESCOF. Combined with previously reported results,4) the short-range spin-glass
state is replaced by the long-range state upon underdoping, as is the case with other cuprates. Secondly,such an ordered state disappears with annealing inconnection with the emergence of superconductivity.
Therefore, our results suggest that strong Cu-spin correlations cause the superconductivity in T*-type LESCO, and that the spin ﬂuctuations play an essential role in the mechanism of superconductivity in thelightly hole-doped region.
References

1) N. P. Armitage et al., Rev. Mod. Phys. 82, 2421 (2010).
2) A. Tsukada et al., Solid State Commun. 133, 427(2005).
3) T. Takamatsu et al., Appl. Phys. Express 5, 073101(2012).
4) O. Matsumoto et al., Physica C 469, 924 (2009).
5) S. Asano et al., J. Phys. Soc. Jpn. 88, 084709 (2019).
6) S. Asano et al., J. Phys. Soc. Jpn. 89, 075002 (2020).

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