Ni-impurity effects on the superconducting gap of La_[2-x]SrxCuO4 studied from the magnetic field and temperature dependence of the electronic specific heat

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The magnetic field and temperature dependence of the electronic specific heat C_[el] have been systematically investigated in La_[2-x]SrxCu_[1-y]NiyO4 (LSCNO) in order to study Ni-impurity effects on the superconducting (SC) gap. In LSCNO with x = 0.15 and y = 0.015, the value of γ (≡C_[el]/T) at T = 0 K, γ0, is enhanced under the magnetic field H applied along the c axis. The increment of γ0, Δγ0, follows the Volovik relation Δγ0 = A√H, characteristic of the SC gap with line nodes, with prefactor A similar to that of a pure sample. The C_[el]/T versus T curve under H = 0 shows a d-wave-like SC anomaly with an abrupt increase at Tc and T-linear dependence at T << Tc, although the γ0-value in the C_[el]/T versus T curve increases with increasing Ni concentrations. Interestingly, as the SC part of C_[el]/T, C_[el]/T - γ0 ≡ γs, decreases in LSCNO, Tc is reduced in proportion to the decrease of γs. These findings can be explained phenomenologically by a simple model in which Ni impurities bring about strong pair breaking at the edges of the coherent nodal part of the Fermi surface but in the vicinity of the nodes of the SC gap. The reduction of the SC condensation energy U0 in LSCNO, evaluated from C_[el] at T ≲ Tc, is also understood by the same model.
2012-04-01