Thermal conductivity of La_2-x Sr_x CuO₄ (0.05 ≤ × ≤0.22)

A systematic study of the thermal conductivity of single-crystal samples of the La_2-xSr_xCuO₄ superconductive system and comparison with that of La_1.60-xNd_0.40Sr _xCuO₄ (x = 0.12, 0.15 and 0.20) have demonstrated that this measurement is a useful indirect probe of mechanisms that suppress phonons. The data distinguish octahedral-site rotational or charge-order fluctuations above a structural order-disorder transition and two-phase fluctuations associated with locally cooperative atomic displacements within two-dimensional sheets that segregate hole-rich and hole-poor regions in a mixed-valent system. The former suppress phonons in both the basal plane and along the apical axis, whereas the latter only suppress phonons in the basal plane. The data support a spinodal phase segregation below room temperature into the parent and the superconductive phases in the underdoped compositional range as well as the superconductive and metallic phases in the overdoped compositions; they also support the existence of locally cooperative bond-length fluctuations in the normal state of the superconductive phase that prevent the formation of a percolative matrix capable of supporting phonons. Restoration of the phonons below T_c signals a long-range, dynamic ordering of the bond-length fluctuations which implies stabilization of a travelling charge-density wave with possible hybridization of electrons and phonons below to give heavy vibrons that pair in the T_c superconductive phase.