Disordering effects in the atomic structure of fine-crystalline HTSC YBa2Cu3O y

Abstract

The atomic structure of YBa2Cu3O y fine-crystalline HTSC samples with various average crystallite sizes 〈D〉 ranging from 0.4 to 2 μm and an oxygen concentration y close to the optimal value for superconductivity (y ≈ 6.93) is investigated by the neutron diffraction technique. We have found some effects associated with the redistribution of cations and oxygen atoms and with variations in the positions of atomic layers in the unit cell, which are not observed in macrocrystalline samples. In all probability, these effects appear due to nonequilibrium conditions of synthesis required for obtaining this compound in the fine-crystalline state. The results have made it possible to explain the peculiar physical properties of fine-crystalline YBa2Cu3O y samples (in particular, the coexistence of high superconducting transition temperatures T c and noticeably lower values of magnetization in strong magnetic fields for T < T c ). It is shown that a nanoscale structural inhomogeneity exists in fine-crystalline YBa2Cu3O y samples with the optimal oxygen content and changes the fundamental superconducting parameters, viz., the magnetic field penetration depth and the coherence length.