Quasiparticle mass enhancement approaching optimal doping in YBa2Cu3O6$+$x

Unconventional superconductivity is almost always found in proximity to other broken-symmetry states. The high-Tc cuprates exhibit a rich phase diagram that includes both spin and charge order. Previous quantum oscillation studies on YBa2Cu3O6$+$x have addressed how broken symmetry reconstructs the Fermi surface, but the evolution of the Fermi surface toward optimal doping was missing due to the high upper-critical fields. We use magnetic fields approaching 100 tesla to measure quantum oscillations in YBa2Cu3O6$+$x at three new doping levels with Tcs of 75, 81, and 91 K. We find that the quasiparticle effective mass is strongly enhanced approaching a hole doping of p$=$0.18---the same doping where experimental signatures of broken symmetry terminate and superconducting properties are enhanced. This is suggestive of a quantum critical point underlying the superconducting dome near optimal doping.