Inverse correlation between quasiparticle mass and Tc in a cuprate high-Tc superconductor

Close to a zero-temperature transition between ordered and disordered electronic phases, quantum fluctuations can lead to a strong enhancement of electron mass and to the emergence of competing phases such as superconductivity. A correlation between the existence of such a quantum phase transition and superconductivity is quite well established in some heavy fermion and iron-based superconductors, and there have been suggestions that high-temperature superconductivity in copper-oxide materials (cuprates) may also be driven by the same mechanism. Close to optimal doping, where the superconducting transition temperature T_c is maximal in cuprates, two different phases are known to compete with superconductivity: a poorly understood pseudogap phase and a charge-ordered phase. Recent experiments have shown a strong increase in quasiparticle mass m* in the cuprate YBa₂Cu₃O_7-δ as optimal doping is approached, suggesting that quantum fluctuations of the charge-ordered phase may be responsible for the high-T_c superconductivity. We have tested the robustness of this correlation between m* and T_c by performing quantum oscillation studies on the stoichiometric compound YBa₂Cu₄O₈ under hydrostatic pressure. In contrast to the results for YBa₂Cu₃O_7-δ, we find that in YBa₂Cu₄O₈, the mass decreases as T_c increases under pressure. This inverse correlation between m* and T_c suggests that quantum fluctuations of the charge order enhance m* but do not enhance T_c.