02385nas a2200253   4500008004100000245009000041210006900131260001500200300000800215490000800223520164100231100002001872700002201892700002101914700001501935700001601950700001201966700001801978700001901996700001702015700001802032700001702050856006402067       2015                        eng d00aQuasiparticle mass enhancement approaching optimal doping in a high-Tc superconductor0 aQuasiparticle mass enhancement approaching optimal doping in a h  c2015/04/16  a3170 v3483 a<p>Thirty years on, and the mechanism of superconductivity in copper-oxide superconductors remains a mystery. Knowledge of their normal nonsuperconducting state is also incomplete; however, we do know that the more robust the superconductivity, the higher the magnetic fields required to suppress it. Ramshaw et al. studied samples of three different compositions of the copper-oxide YBa2Cu3O6+Î´ in magnetic fields exceeding 90 T. They found that as the oxygen content increased toward the point of the maximum transition temperature, the conducting electrons became heavier and heavier. This mass enhancement reflected an increase in electronic correlations, which in turn may be a signature of a quantum critical point.Science, this issue p. 317 In the quest for superconductors with higher transition temperatures (Tc), one emerging motif is that electronic interactions favorable for superconductivity can be enhanced by fluctuations of a broken-symmetry phase. Recent experiments have suggested the existence of the requisite broken-symmetry phase in the high-Tc cuprates, but the impact of such a phase on the ground-state electronic interactions has remained unclear. We used magnetic fields exceeding 90 tesla to access the underlying metallic state of the cuprate YBa2Cu3O6+Î´ over a wide range of doping, and observed magnetic quantum oscillations that reveal a strong enhancement of the quasiparticle effective mass toward optimal doping. This mass enhancement results from increasing electronic interactions approaching optimal doping, and suggests a quantum critical point at a hole doping of pcrit â‰ˆ 0.18.</p>
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