  In the copper-oxide parent compounds of the high-transition-temperature
superconductors, the valence electrons are localized, one per copper site, due
to strong intraatomic Coulomb repulsion. A symptom of the localization is
antiferromagnetism, where the spins of localized electrons alternate between up
and down. The superconductivity appears when mobile 'holes' are doped into this
insulating state, and it coexists with antiferromagnetic fluctuations. In one
approach to the coexistence, the holes are believed to self-organize into
'stripes' that alternate with antiferromagnetic (insulating) regions within
copper-oxide planes. Such an unusual electronic state would necessitate an
unconventional mechanism of superconductivity. There is an apparent problem
with this picture, however: measurements of magnetic excitations in
superconducting YBa(2)Cu(3)O(6+x) near optimum doping are incompatible with the
naive expectations for a material with stripes. Here we report neutron
scattering measurements on stripe-ordered La(1.875)Ba(0.125)CuO(4). We show
that the measured excitations are, surprisingly, quite similar to those in
YBa(2)Cu(3)O(6+x) (i.e., the predicted spectrum of magnetic excitations is
wrong). We find instead that the observed spectrum can be understood within a
stripe model by taking account of quantum excitations. Our results support the
concept that stripe correlations are essential to high-transition-temperature
superconductivity.
