ALBERT

Description: 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 YBa 2 Cu 3 O 7- 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 YBa 2 Cu 4 O 8 under hydrostatic pressure. In contrast to the results for YBa 2 Cu 3 O 7- , we find that in YBa 2 Cu 4 O 8 , 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 .