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The Underdoped-overdoped Transition in Yba 2 Cu 3 O X

The overdoped regime of YBa 2Cu3Ox is accessible with heavy oxygen dopings close to x = 7. The crossover from the underdoped to the overdoped regimes occurs around xopt = 6.92, nearly coinciding with a displacive structural phase transition [1]. It is suggesting to associate the structural instability close to optimum doping with a kind of barrier limiting Tc. The 123 bi-layer cuprates exhibit comparatively low “optimum” Tc‘s around 90 K, but chemical trends and pressure experiments imply that Tc is not yet at its optimum value. In this communication we discuss a mechanism being possibly responsible for the barrier limiting Tc of YBa2Cu3Ox, and stabilizing the overdoped regime for x ≥ 6.95.