Cu-63 and Y-89 NMR study of an optimally doped YBa2Cu3O6.94 single crystal

We report Cu-63 spin-spin and spin-lattice relaxation rates (T-63(2)-1 and T-63(1)-1) and Y-89 magnetic hyperfine shift (K-89) in the normal state of an optimally doped single-crystalline sample YBa2Cu3O6.94 with T-c=93 K (which corresponds to the maximal T-c for the YBa2Cu3O6+x family). The comparison between ((T1T)-T-63)(-1) and (T-63(2g))(-1) evidences that the spin pseudogap does not open above T-c at optimal doping, although the static spin susceptibility starts to decrease well above T-c, at T-0= 150 K. Comparing the NMR data of this optimally doped sample with those of two other ''90 K'' samples previously studied by our group, an underdoped YBa2Cu3O6.92 and an overdoped YBa2C3O7, strongly suggests that the crossover to the spin pseudogap regime occurs precisely at the optimal doping level. A phenomenological phase diagram based an the NMR results in the normal states is proposed. We discuss to which extent our data agree with the predictions of the magnetic scaling theory.