Magnetic Dynamics in UnderdopedYBa2Cu3O7−x: Direct Observation of a Superconducting Gap

Polarized and unpolarized triple-axis neutron measurements were performed on an underdoped crystal of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ ( $x=0.4\mathrm{}\pm 0.02$, $T_c=62.7\mathrm{}\mathrm{K}$). 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 $T_c$. The enhancement at the resonance is accompanied by a suppression of magnetic fluctuations at both higher and lower energies.