Tadashi Adachi

To investigate the relationship between the so-called stripe correlations and the high-<italic>T</italic>_csuperconductivity, we have carried out magnetic susceptibility and zero-field muon spin relaxation measurements in partially Fe-substituted La₂₋<italic>_x</italic>Sr<italic>_x</italic>Cu₁₋<italic>_y</italic>Fe<italic>_y</italic>O₄(LSCFO). It has been found that the Fe substitution induces successive magnetic transitions in the overdoped regime: a spin-glass transition of Fe³⁺spins due to the Ruderman–Kittel–Kasuya–Yosida interaction based on itinerant spins at high temperatures and a stripe-order transition of localized Cu²⁺spins at low temperatures. The stripe-order transition disappears at the hole concentration per Cu of ∼0.28 which coincides with the endpoint of the superconductivity in pristine La₂₋<italic>_x</italic>Sr<italic>_x</italic>CuO₄, suggesting that the stripe correlations are closely related to the high-<italic>T</italic>_csuperconductivity in the overdoped regime as well as in the underdoped regime. For the spin-glass state, it has been found that the contribution of polarized itinerant spins to the muon-spin depolarization is not negligible. Taking into account neutron-scattering results obtained by He et al. [<ext-link ext-link-type="uri" xlink:href="http://doi.org/10.1103/PhysRevLett.107.127002" xlink:type="simple">Phys. Rev. Lett. <bold>107</bold>, 127002 (2011)</ext-link>] that a spin density wave (SDW) order has been observed in overdoped LSCFO, the present results demonstrate the presence of a novel coexistent state of an SDW order of itinerant spins and a spin-glass state of Fe³⁺spins.

To investigate the electronic state of Ce-free and Ce-underdoped high-<italic>T</italic>_ccuprates with the so-called T′ structure, we have performed muon spin relaxation (μSR) and specific heat measurements of Ce-free T′-La_1.8Eu_0.2CuO_4+δ(T′-LECO) polycrystals and Ce-underdoped T′-Pr_1.3−<italic>_x</italic>La_0.7Ce<italic>_x</italic>CuO_4+δ(T′-PLCCO) single crystals with <italic>x</italic>= 0.10. The μSR spectra of the reduced superconducting samples of both T′-LECO with <italic>T</italic>_c= 15 K and T′-PLCCO with <italic>x</italic>= 0.10 and <italic>T</italic>_c= 27 K have revealed that a short-range magnetic order coexists with the superconductivity in the ground state. The formation of a short-range magnetic order due to a very small amount of excess oxygen in the reduced superconducting samples strongly suggests that the Ce-free and Ce-underdoped T′-cuprates are strongly correlated electron systems.

For the understanding of Ce-doped high-<italic>T</italic>_ccuprates with the so-called T′-structure, we have performed ^63,65Cu and ¹³⁹La NMR measurements in T′-Pr_1.3−<italic>_x</italic>La_0.7Ce<italic>_x</italic>CuO_4+δsingle crystals with <italic>x</italic>= 0.10 and 0.15. The Néel temperature (<italic>T</italic>_N) was evaluated from the temperature (<italic>T</italic>) dependence of the ¹³⁹La NMR line width. For <italic>x</italic>= 0.10, <italic>T</italic>_Ndecreased from 190 K in the as-grown crystal to 60 K in the oxygen-reduced crystal annealed at 800 °C. For <italic>x</italic>= 0.15, <italic>T</italic>_Nof 170 K in the as-grown crystal decreased to zero through the same heat treatment. The oxygen reduction strongly suppressed the antiferromagnetic (AF) order. ^63,65Cu NMR spectra had a narrow center line and broad satellites with a small electric field gradient in the paramagnetic state. In the AF state, in contrast, ^63,65Cu NMR spectra had a large electric field gradient. The <italic>T</italic>dependence of the ⁶³Cu nuclear spin–lattice relaxation rate (1/<italic>T</italic>₁) showed the existence of large spin fluctuations in the paramagnetic state. The spin fluctuations seem to play a key role in the appearance of superconductivity in the T′-cuprates.