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Accueil du site > Doping-induced antiferromagnet-superconductor transformation in RBa2Cu3O6+x crystals : What can we learn from the c-axis conductivity ?

In strongly anisotropic high-Tc cuprates, the c-axis charge transport appears to be remarkably sensitive to the spin ordering in CuO2 planes. Both the establishment of the long-range AF order and relatively weak modifications of the spin structure such as spin-flop or metamagnetic transitions bring about large changes, reaching an order of magnitude, in the c-axis resistivity of lightly hole-doped and electron-doped crystals. This sensitivity of the interplane charge transport to the spin order can be employed for tracing the evolution of the spin state with doping, temperature, or magnetic field. A systematic study of the c-axis transport may therefore help in clarifying the mechanism of transformation, upon charge doping, of an antiferromagnetic Mott insulator into a superconducting metal, which has challenged researchers since the discovery of high-Tc superconductivity in cuprates.


Following this strategy we have studied in detail the c-axis resistivity and magnetoresistance in RBa2Cu3O6+x (R = Lu, Y) single crystals with doping levels spanning the range of the AF-SC transformation. Sharp AF transitions observed in crystals even with low TN  18-20 K have allowed us to map precisely the phase diagram and to single out the impact of spin ordering on the c-axis charge transport. We have found that, in the doping region of the AF-SC transformation, the RBa2Cu3O6+x crystals exhibit a 3D metallic conduction until the SC or AF ordering intervenes at low temperatures. The Néel ordering competes with SC by strongly inhibiting the charge motion, yet nevertheless the two orders seem coexist in a certain range of hole doping. Indeed, the magnetoresistance measurements of RBa2Cu3O6+x crystals with Tc<15-20 K give a clear picture of the magnetic-field induced superconductivity suppression and recovery of the long-range AF state. Interestingly, we find that the magnetic field not only suppresses the superconductivity, but also stabilizes the static AF order, increasing the Néel temperature. What still remains to be understood is whether the AF order actually persists in the SC state or just revives when the superconductivity is suppressed, and, in the former case, whether the antiferromagnetism and superconductivity reside in nanoscopically separated phases or coexist on an atomic scale.


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