Materials Science Laboratory, *Tokyo Univ. of Agricul. & Technol.
Commonly, high-Tc cuprates are considered as doped-Mott insulators (charge transfer insulators).
We have shown that the parent compounds of electron-doped superconductors,
T’-RE2-xCexCuO4 (RE: rare-earth ion) at x = 0 are not insulators but superconductors [1, 2]. A complex oxygen diffusion process lies beneath the contradiction of earlier reported and our present results. Superconducting parent compounds are a novelty in solid state physics and the physical origin of the superconducting state requires clarification. We have prepared high quality Pr2CuO4 thin films by MBE.
Superconductivity is induced after a specially designed two-step post-annealing
process is applied. Systematic optimization of the annealing conditions
resulted in Pr2CuO4 thin films with a Tc of 26 K (ΔTc < 0.5 K) and metallic conduction [ρRT = 400 µΩcm, RRR = 10] (Fig. 1). The resistivity value is lowest
[ρ (30 K) ~ 40 µΩcm] for Tred = 475 - 500ºC whereas Tc is highest (Tc ~ 26 K) for Tred = 500ºC. A strong diamagnetic signal was observed (Fig. 2) in parallel
configuration with an onset at 23 K, confirming a robust superconducting
response of Pr2CuO4. A nearly ideal oxygen sub-lattice and therefore intrinsic properties of the parent compounds are unveiled after applying an optimal post-annealing process. Our observations are supported by recent theoretical considerations where the parent compounds of T’-structure cuprates are not exclusively Mott insulators (charge-transfer insulators) [3, 4].
[1] O. Matsumoto et al., Phys. Rev. B 79 (2009) 100508R.
[2] H. Yamamoto et al., Solid State Commun. 151 (2011) 771.
[3] C. Weber et al., Nature Phys. 6 (2010) 574; Phys. Rev. B 82 (2010) 125107.
[4] H. Das and T. Saha-Dasgupta, Phys. Rev. B 79 (2009) 134522.
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