  The pseudogap is one of the most pervasive phenomena of high temperature
superconductors. It is attributed either to incoherent Cooper pairing setting
in above the superconducting transition temperature Tc, or to a hidden order
parameter competing with superconductivity. Here we use inelastic neutron
scattering from underdoped YBa(2)Cu(3)O(6.6) to show that the dispersion
relations of spin excitations in the superconducting and pseudogap states are
qualitatively different. Specifically, the extensively studied "hour glass"
shape of the magnetic dispersions in the superconducting state is no longer
discernible in the pseudogap state and we observe an unusual "vertical"
dispersion with pronounced in-plane anisotropy. The differences between
superconducting and pseudogap states are thus more profound than generally
believed, suggesting a competition between these two states. Whereas the
high-energy excitations are common to both states and obey the symmetry of the
copper oxide square lattice, the low-energy excitations in the pseudogap state
may be indicative of collective fluctuations towards a state with broken
orientational symmetry predicted in theoretical work.
