Shifting of the magnetic-resonance peak to lower energy in the superconducting state of underdoped YBa2Cu3O6.8

Inelastic neutron scattering has been used to determine the dynamic spin fluctuations in an underdoped high-temperature superconductor YBCO6.8 single crystal. The magnetic resonance, that occurs around 40 meV in overdoped samples, is shifted to a lower energy, E-r = 34 meV. A constant ratio, E-r/k(B)T(C) = 4.9 +/- 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.