Evidence on the nature of superconductivity in La1.85Ba0.15CuO4from specific heat measurements

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Abstract
Information on the nature of the superconducting state in La1.85Ba0.15CuO4 has been obtained from high-precision differential specific heat measurements on superconducting La1.85Ba0.15CuO4, and non-superconducting La2CuO4 and La1.85Ba0.15Cu0.9Ni0.1O4 samples between 1.5 and 300 K. From the very small jump of 0.07 mJ mol-1 K-2 in C/T at Tc approximately 29 K the authors obtain (dHc/dTc)=33 Oe K-1 and Hc(0)=475 Oe, where Hc is the thermodynamic critical field. A condensation of real-space Bose pairs (e.g. bipolarons) is excluded by the author's results, which indicate that a fraction of around 0.06 of the electrons within kTc of the Fermi surface are in Cooper pairs. The estimated electron mass enhancement (1+ lambda ) with lambda approximately 0.82 is lower than expected for conventional phonon-mediated superconductivity with Tc approximately 29 K and indicates that an alternative, possibly excitonic, pair interaction may be involved. Phonon spectra are determined that are in agreement with theoretical predictions, and evidence is obtained for strong high-temperature anharmonicity. A striking anomaly at 58 K in the specific heat of the superconducting compound may indicate a soft phonon mode.