Low temperature specific heat studies on the pairing states of high-T_c superconductors: A brief review

Low temperature specific heat (LTSH) data on a variety of high-T_c superconductors, such as YBa₂Cu₃O₇, La_1-xSr_xCuO₄, Bi₂Sr₂CaCu₂O₈, and Ba_0.6K_0.4BiO₃ are reviewed. The agreement between the experimental data and theoretical predictions, such as T²-dependence of specific heat at zero magnetic field and H^1/2-dependence of electronic specific heat is widely discussed within the scenario of d-wave superconductivity. Impurity scattering effects and scaling model on d-wave superconductivity are verified using Zn- and Ni-doped La_1-xSr_xCuO₄. The low energy quasiparticle density of states N(E)=N(0)+E^1/2 are deduced from dirty d-wave superconductors. Absence of paramagnetic contribution to LTSH is found both in superconducting and non-superconducting underdoped samples suggesting that a mechanism be yond Kondo screening model maybe required to explain its magnetic property.