High critical temperature by resonant quantum confinement: Evidence for polarons ordering at T ^*≈1.5T _c in Bi-2212 and La-214 by EXAFS

Summary

The temperature dependence of local Cu site conformations in single-domain crystals of Bi₂Sr₂CaCu₂O_8+y (Bi-2212) and La_1.85Sr_0.15CuO₄ (La-214) has been determined by EXAFS (Extended X-ray Absorption Fine Structure), a fast (10⁻¹⁵ s) and local probe. Two sets of Cu site conformations characterized by the Cu−O(apical) bond distances and tilting of copper square plane have been found at T<T ^*∼1.5T _c. The difference between the long and short Cu−O(apical) distance appeared to be 0.17 Å and 0.12 Å at T<T ^* in Bi-2212 and La-214 systems, respectively. The polarons are associated with the short Cu−O(apical) bond distance. The ratio between the number of short and the total number of Cu−O(apical) bonds (N _short/N _tot∼0.42 in both systems) is a measure of the relative area of distorted and undistorted domains. By using this result we have been able to show the polaronic nature of the one-dimensional structural anharmonic modulation of the CuO₂ plane in Bi-2212 where the polarons have dimension W=(11±1) Å at T<T ^*≈1.4T _c and are condensed into a one-dimensional generalized Wigner charge density wave (CDW). It appears that superconductivity is stabilized at thigh temperature by resonant quantum confinement in a superlattice of stripe width L where k _Fx∼k_Fy∼π/L.