Abstract
We report observation and resonance-Raman studies of spin-pair excitations in
Y
Ba
2
Cu
3
O
6+
x
(0.37 <
x < 0.7) and
YBa
2
Cu
4
O
8 superconductors. These citations at energy ∼ 3
J (2700±150
cm
−1), similar to two-magnon excitations in insulators, are evidence that antiferromagnetic fluctuations with spatial extent of at least three lattice constants are not overdamped in the underdoped superconductors. With optimal hole doping, where
T
c
>80
K, and in the overdoped
TlBa
2
CuO
6 superccconductors the two-magnon peak can no longer be seen above the flat Raman continuum.