Inhomogeneous superconductivity in both underdoped and overdoped regimes of high-T_c cuprates

Our recent experimental works on the inhomogeneity of superconductivity in both underdoped and overdoped regimes of La_2-xSr_xCuO ₄ are reviewed. In order to estimate the superconducting (SC) volume fraction in a sample, bulk-sensitive magnetic-susceptibility and specific-heat measurements have been carried out, using La_2-xSr_xCuO ₄ single crystals of good quality with x 0.05-0.3. As a result, it has been found that the SC volume fraction is not 100% in both underdoped and overdoped regimes, suggesting the occurrence of a phase separation into SC and non-SC regions. Moreover, strong vortex-pinning effects have been observed in moderate magnetic fields in the overdoped regime, suggesting that the phase separation in the overdoped regime is not macroscopic but microscopic.