Thermal-conductivity study on the electronic state in the overdoped regime of La_2-xSr_xCuO₄: Phase separation and anomaly at x ∼ 0.21

Measurements of the in-plane thermal conductivity, κ_{a b}, in magnetic fields parallel to the c-axis have been carried out in the overdoped regime of La_2-xSr_xCuO₄ with x = 0.18 - 0.30. At low temperatures, κ_{a b} is enhanced by the application of magnetic field for x < 0.22 due to the Volovik effect, while κ _{a b} is suppressed for x > 0.22. No Volovik effect for x > 0.22 is explained as being due to the phase separation into superconducting and normal-state regions. Moreover, the enhancement of κ_{a b} has been found to be singularly weak at x ∼ 0.21 where T_c is a little suppressed in zero field. A so-called stripe order might be stabilized by the application of magnetic field at x ∼ 0.21, as in the case of x ∼ 0.115.