Magnetic Quantum Oscillations in YBa2Cu3O6.61 and YBa2Cu3O6.69 in Fields of Up to 85 T: Patching the Hole in the Roof of the Superconducting Dome

We measure magnetic quantum oscillations in the underdoped cuprates YBa2Cu3O6+x with x = 0.61, 0.69, using fields of up to 85 T. The quantum-oscillation frequencies and effective masses obtained suggest that the Fermi energy in the cuprates has a maximum at hole doping p approximate to 0.11-0.12. On either side, the effective mass may diverge, possibly due to phase transitions associated with the T = 0 limit of the metal-insulator crossover (low-p side), and the postulated topological transition from small to large Fermi surface close to optimal doping (high p side).