Superconducting Parent Compound Pr_2CuO_4 Achieved by Special Post-Reduction

摘要

It is commonly believed that the parent compounds of high-T_c cupratres are, universally, charge transfer insulators and triggered by Mott physics. In our experiments using metal-organic decomposition (MOD), however, accumulating evidences show that the parent compounds of "electron-doped" superconductors, RE_(2-x)Ce_xCuO_4 [RE = rare earth ion] with x = 0, are not Mott insulators but superconductors [1-5]. They have a T_c of 30 K and crystallize in the Nd_2CuO_4 (T) structure. Most likely, the sharp contradiction between our results and commonly achieved data originates from the complicated oxygen chemistry in these materials. The as-synthesized specimens contain a fair amount of impurity interstitial oxygen. Throughout the reduction process it is required to remove exclusively impurity oxygen while preserving regular oxygen site occupied in order to obtain superconductivity. With decreasing x the constraints of the reduction process are getting more tight. In this study, we systematically investigated the post-annealing process using MBE-grown T-P_2CuO_4 films. The MBE films were reduced ex-situ in a tubular furnace following a specially designed 2-step process, as in the case of MOD films. The films were annealed at T_a = 700 - 850℃ in a reducing atmosphere (P_(O2) = 2 × 10~(-5)- 2 × 10~(-3) atm) and finally reduced at a lower temperature T_(red) = 450 - 700℃ under vacuum (< 10~(-4) Torr). The film properties systematically changed with T_a, P_(O2), and T_(red). The optimized T_(red) varies from 475℃ to 650℃ mainly depending on T_a, since the microstructure and grain size of the films are determined by T_a. Optimal superconducting properties are T_c of 26 K, while ρ(300 K.) = 250 μΩcm, and RRR ～ 10. We believe the combination of thin-film synthesis and specially designed post-reduction process enabled us to obtain nearly intact CuO_2 planes. Samples prepared by above-mentioned method unveiled the intrinsic properties of the parent compounds, which are not triggered by Mott physics. This result also agrees with the recent calculation result indicating the parent compounds with T structure are not charge transfer insulators [6-8].