NOVEL STRUCTUR AL PHENOMENA AT THE MAXIMUM T_c IN123 AND HgBa_2CuO_(4+δ) SUPERCONDUCTORS: EVIDENCE FOR A STRUCTURAL RESPONSE THAT COMPETES WITH SUPERCONDUCTIVITY

摘要

Structural distortions that compete with superconductivity have been investigated in two systems where oxygen content can be used to vary the doping continuously from the under doped state, through the maximum Tc into the over doped state. In the 123 system, (La_(1-x)Ca_x)(Ba_(1.75-x)La_(0.25+x))Cu_3O_(7+δ), the buckling of the CuO_2 planes goes through a maximum at the maximum T_c. In HgBa_2CuO_(4+δ), where buckling of the CuO_2 planes is not available as a structural degree of freedom, there is a plateau at the maximum T_c where the unit cell volume expands as oxygen is added while the charge transfer and T_c remain constant. These unusual structural phenomena upon crossing through the maximum T_c are hypothesized to be a response of the crystal structure to the electronic structure, with the structural distortions competing with superconductivity, or lowering the T_c from what it would otherwise be.