NMR investigation of low energy excitations in highTcsuperconductors

We review the NMR results in the normal state ( T > T c ) of superconducting YBa 2 Cu 3 O 6+ x single crystals as a function of oxygen hole doping. We show that the characteristic energy Γ = πωχ'/χ'' for low-energy excitations can be deduced both from NMR (for antiferromagnetic Q AF and zero wave vector), and from inelastic neutron scattering for Q AF only. In underdoped (d T c /d x > 0) samples Γ( 0 ) is found to be constant down to T c , while Γ( Q AF ) starts to increase below ∼ 150 K, i.e., well above T c , reflecting the opening of the pseudogap for magnetic excitations at Q AF . In overdoped samples (d T c /d x < 0) there is only the superconducting gap opening at T c , supporting the phase diagram based on the spin-charge separation.