Variation of temperature-linear specific heat with doping in (La_1-xSr_x)₂CuO₄

Low-temperature specific heat of (La_1-xSr_x)₂CuO₄ has been studied between 2 and 20 K by systematically varying the Sr concentration x, in order to clarify the origin of non-zero γ, the temperature-linear specific-heat coefficient, frequently observed in high-T_c superconducting oxides. Identical procedure is used to prepare samples for the specific-heat measurement and for the determination of the structural, magnetic and superconductive phase-transition temperatures as a function of x. The value of γ is nearly zero in the antiferromagnetic ordering region, but increase abruptly with x to about 4 mJ/K² mol in the insulating, but not antiferromagnetic region. By increasing x further into the superconductive region, γ tends to be smaller: 1.6 mJ/K²mol for x = 0.075. We argue that the non-zero γ is not due to inhomogeneity of the samples, but due to electronic origin intrinsic to this system.