Magnetic Dynamics in Underdoped YBa₂Cu₃O₇₋ₓ: Direct Observation of a Superconducting Gap

Polarized and unpolarized triple-axis neutron measurements were performed on an underdoped crystal of YBa2Cu3O7-x (x = 0.4 ± 0.02, Tc = 62.7 K). Our results indicate, contrary to earlier evidence, that the spin excitations in the superconducting state are essentially the same as those in the fully doped material except that the unusual 41 meV resonance is observed at 34.8 meV. The normal state spin excitations are characterized by a weakly energy-dependent continuum whose temperature dependence shows the clear signature of a superconducting gap at Tc. The enhancement at the resonance is accompanied by a suppression of magnetic fluctuations at both higher and lower energies