Multi-band mass enhancement towards critical doping in a pnictide superconductor

Near critical doping, high-temperature superconductors exhibit multiple anomalies associated with enhanced electronic correlations and quantum criticality. Quasiparticle mass enhancement approaching optimal doping has been reported in quantum oscillation measurements in both cuprate and pnictide superconductors. Although the data are suggestive of enhanced interactions, the microscopic theory of quantum oscillation measurements near a quantum critical point is not yet firmly established. It is therefore desirable to have a direct thermodynamic measurement of quasiparticle mass. Here we report high-magnetic field measurements of heat capacity in the doped pnictide superconductor BaFe2(As1−xPx)2. We observe saturation of the specific heat at high magnetic field in a broad doping range above optimal doping which enables a direct determination of the electronic density of states recovered when superconductivity is suppressed. Our measurements find a strong total mass enhancement in the Fermi pockets that superconduct. This mass enhancement extrapolates to a mass divergence at a critical doping of x = 0.28. In an Fe-based superconductor, the variation of quasiparticle masses across a broad doping range is found to culminate in a mass divergence at critical doping — the doping corresponding to the highest superconducting critical temperature. Such a quasiparticle mass divergence hints to a quantum critical origin of the phase diagram of high-temperature superconductors. A microscopic theory explaining this behavior still has to be established, and a direct measurement of quasiparticle masses can give input to theory. Camilla Moir at Florida State University and colleagues used high magnetic fields to suppress superconductivity in BaFe2(As1-xPx)2 and measure the density of states in the normal state, which allows to determine the quasiparticle masses from all Fermi pockets, finding a mass divergence near critical doping. These results will guide theoretical investigations of quantum criticality in high-temperature superconductors.