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Abstract

The pseudogap is one of the most pervasive phenomena of high-temperature superconductors(1). It is attributed either to incoherent Cooper pairing setting in above the superconducting transition temperature, T c, or to a hidden order parameter competing with superconductivity. Here, we use inelastic neutron scattering from underdoped YBa2Cu3O6.6 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(2-4) 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(5-8).