1. Unconventional superconductivity, where Cooper pairing is driven by something other than electron-phonon coupling, often appears in proximity to magnetic order. The heavy-fermion and organic superconductors provide two families of materials that serve as examples of this phenomenon. By varying pressure or chemical doping, the transition temperature to the magnetic phase can be suppressed to T = 0, resulting in a quantum phase transition, and a quantum-critical point (QCP) when critical behavior is observed. This QCP often coincides with the maximum superconducting T c, suggesting that unconventional superconductivity and quantum criticality are linked. High-T c cuprates lack this magnetic transition near optimal T c, but may instead have a different type of QCP driving superconductivity. Magnetic fields exceeding 90 teslas were utilized to search for a QCP near optimal doping in the high-T c cuprate superconductor YBa 2 Cu 3 O 6+x. We found that the quasiparticle effective mass is greatly enhanced as the material is doped toward optimal T c (Fig.1)—a common signature of increased electron-electron interactions in the vicinity of a QCP. An extrapolation of the data to determine the doping at which the maximum mass enhancement occurs finds the same doping at which superconductivity survives to the highest magnetic fields (Fig.2), suggesting that quantum- criticality drives or enhances superconductivity in this system. Underlying any QCP is a broken-symmetry phase of matter, such as magnetism. It remains to be discovered what the broken- symmetry phase is in the high-T c cuprates: one possible contender is charge-density wave order, which has recently been shown to extend to near-optimal T c in YBa 2 Cu 3 O 6+x. Facilities: 65 T and 100 T magnets at the Pulsed Field Facility Acknowledgements : Canadian Institute for Advanced Research; DOE BES “Science at 100 T” References: B.J. Ramshaw, S.E. Sebastian, R.D. McDonald, James Day, B.S. Tan, Z. Zhu, J.B. Betts, Ruixing Liang, D.A. Bonn, W.N. Hardy, N. Harrison, “Quasiparticle mass enhancement approaching optimal doping in a high-Tc superconductor.” Science 348:6232 (317-320). 2015 Quasiparticle mass enhancement approaching optimal doping in a high-T c superconductor B. J. Ramshaw 1, S. Sebastian 2, R. D. McDonald 1, J. Day 3, B. Tan 2, Z. Zhu 1, J. B. Betts 1, R. Liang 3, W. N. Hardy 3, D. A. Bonn 3, N. Harrison 1 1. Los Alamos National Labs – Pulsed Field Facility; 2. Cambridge University; 3. University of British Columbia Funding Grants: DOE BES “Science at 100 Tesla”; National Science Foundation (NSF DMR-1157490) Figure 1: Oscillations of the magnetoresistance. (A) The magnetoresistance of YBa 2 Cu 3 O 6+x for x = 0.75, 0.80, and 0.86, which gives T c = 75, 86, and 91 K. Left panels show the raw resistance up to 90 T; right panels show the oscillatory component after a smooth, monotonic background has been removed. (B) Effective mass as determined from the temperature dependence of the oscillation amplitude using standard Lifshitz-Kosevich analysis. Figure 2: Superconducting phase diagram in a magnetic field. Blue contours denote T c (p) at 0, 15, 30, 50, 70, and 82T (right axis). The inverse of the effective mass (left axis) is overlaid, with the orange star data points coming from the data in Fig.1, and the white points coming from an earlier work. Note that the extrapolation of the inverse effective mass is p=0.18—the same doping around which superconductivity is most resistant to a magnetic field.