Abstract
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.
Highlights
A mass divergence at critical doping has been deduced from quantum oscillation measurements at high magnetic fields up to 90 T in the cuprate superconductor YBa2Cu3O6+δ,1,2 and in the pnictide superconductor, BaFe2(As1−xPx)2.3–5 These measurements, together with measurements of upper critical magnetic field,[6] elastoresistivity,[7] and magneto-transport[8] in BaFe2(As1 −xPx)[2], as well as elastic moduli[9] and specific heat studies[10,11] in other doped BaFe2As2 compounds (Ba122), provide mounting evidence for a quantum critical origin of the phase diagram in high-temperature superconductors
The H behavior of correlated superconductors such as Ba122 pnictides. (C/T) is characteristic of a line-node in the superconducting gap of BaFe2(As1−xPx)[2], which is corrpobffiffiffiorated by other measurements.[17–25]
These observations demonstrate that some Fermi pockets must have an even stronger mass enhancement than that reported for the βpocket alone[3] and some pockets couple more strongly to quantum fluctuations than does the β-pocket
Summary
A mass divergence at critical doping has been deduced from quantum oscillation measurements at high magnetic fields up to 90 T in the cuprate superconductor YBa2Cu3O6+δ,1,2 and in the pnictide superconductor, BaFe2(As1−xPx)2.3–5 These measurements, together with measurements of upper critical magnetic field,[6] elastoresistivity,[7] and magneto-transport[8] in BaFe2(As1 −xPx)[2], as well as elastic moduli[9] and specific heat studies[10,11] in other doped BaFe2As2 compounds (Ba122), provide mounting evidence for a quantum critical origin of the phase diagram in high-temperature superconductors. The electronic specific heat measures the total quasiparticle density of states, which is proportional to the sum of quasiparticle masses on all Fermi pockets in quasi-twodimensional (2D) systems such as Ba122. What has been missing is a direct measurement of the normal state density of states in high-temperature superconductors, from which the sum of quasiparticle masses from all Fermi pockets can be determined. We utilize high magnetic fields to fully suppress superconductivity and reveal the doping evolution of the electronic density of states in the normal state of.
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