Shifting of the magnetic-resonance peak to lower energy in the superconducting state of underdoped YBa2Cu3O(6.8)

Inelastic neutron scattering has been used to determine the dynamic spin fluctuations in an underdoped high-temperature superconductor $\rm YBCO _{6.8}$ single crystal. The magnetic resonance, that occurs around 40 meV in overdoped samples, is shifted to a lower energy, $E_{\rm r} = 34\,\rm meV$. A constant ratio, $E_{\rm r}/ k_{\rm B} T_{\rm C}= 4.9 \pm 0.2$, almost independent of the doping level, is found. According to numerous theoretical approaches, this finding supports the idea that the resonance energy is proportional (approximately twice) to the superconducting gap