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Abstract
In order to investigate the electronic state of Ce-free and Ce-underdoped high-Tc cuprates with the so-called T' structure, we have performed muon-spin-relaxation (muSR) and specific-heat measurements of Ce-free T'-La_1.8_Eu_0.2_CuO_4+d_ (T'-LECO) polycrystals and Ce-underdoped T'-Pr_1.3-x_La_0.7_Ce_x_CuO_4+d_ (T'-PLCCO) single crystals with x=0.10. The muSR spectra of the reduced superconducting samples of both T'-LECO with Tc=15K and T'-PLCCO with x=0.10 and Tc=27K 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 tiny amount of excess oxygen in the reduced superconducting samples strongly suggest that the Ce-free and Ce-underdoped T'-cuprates are regarded as strongly correlated electron systems.
Highlights
The μSR spectra of the reduced superconducting samples of both T’-LECO with Tc = 15 K and T’-PLCCO with x = 0.10 and Tc = 27 K have revealed that a short-range magnetic order coexists with the superconductivity in the ground state
A surprising result has come out in thin films of T’-cuprates in which superconductivity appears without doping of electrons. [6] This has been followed by a result using T’-NCCO thin films that the superconducting (SC) transition temperature, Tc, of ∼ 28 K at Ce-free x = 0 decreases monotonically with increasing x and disappears at x ∼ 0.20. [7] The superconductivity in the parent compounds of the T’-cuprates has been confirmed using polycrystalline samples. [8, 9] These findings have been brought about through the removal of excess oxygen from as-grown samples more effectively than that ever reported
In the case of weak electron correlation, a simple band-metal state has been proposed, [10] while in strong electron correlation, calculations based on the local-density approximation (LDA) combined with the dynamical mean-field theory (DMFT) have revealed the possibility of the closing of the so-called charge-transfer (CT) gap between the upper Hubbard band (UHB) of the Cu 3dx2−y2 orbital and the O 2pσ band observed in the T-cuprates. [11, 12] In the case of moderate electron correlation, the appearance of superconductivity due to doublons and holons has been proposed
Summary
The μSR spectra of the reduced superconducting samples of both T’-LECO with Tc = 15 K and T’-PLCCO with x = 0.10 and Tc = 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 tiny amount of excess oxygen in the reduced superconducting samples strongly suggest that the Ce-free and Ce-underdoped T’-cuprates are regarded as strongly correlated electron systems.
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