Local lattice fluctuations as a function of chemical pressure

Here we report local lattice fluctuations as a function of chemical pressure in the high T-c superconductors at the optimum doping (delta =0.16). The chemical pressure is determined by the mismatch between the rock-salt M-O (M= Ba, Sr, La) layers and the CuO2 layers. High k-resolution Cu K-edge EXAFS has been used to measure the strain of the Cu-O bond lengths and the amplitude of local lattice distortions probed by the Debye-Waller factor of the Cu-O bonds in different materials under increasing chemical pressure. We find that in the normal metallic state, showing non-Fermi liquid transport behavior, the Debye-Waller factor reaches a maximum at a critical value of the strain eta (c) of the CuO2 plane. The results provide a direct evidence that at the maximum critical temperature T-c the system has diverging local lattice fluctuations.