Are the high Tc superconducting materials bulk superconductors or grain boundary percolating network superconductors? (abstract)

Abstract

We have studied the physical properties of high T_c Y‐Ba‐Cu‐O_x superconducting materials with levitation, ac susceptibility, macroscopic resistivity, resistivity as measured by scanning tunneling microscopy (STM) and Hall effect. Levitation experiments show that the powder of the as‐prepared material does not levitate at liquid nitrogen while pellets and powders that have been heated above 450 °C do levitate (are superconducting). These experiments seem to indicate that clustering and intimate contact of fine grains are necessary for levitating. The ac susceptibility experiments show that diamagnetism is extremely sensitive to pellet density. The higher the density and the smaller the field amplitude the less diamagnetic is the system. This is interpreted as evidence for a surface, not a bulk effect. Resistivity at 17 °C measured with macroscopic contacts is of the order of 10 Ω cm. When measured with microscopic STM contacts, a clear semiconducting behavior is observed. This observation does not preclude grain boundary regions with metallic conductivity. We conjecture that the bulk of the grain is semiconducting with a conducting percolative network of grain boundaries that is the source of the superconductivity. We propose a model of a superconductor‐metal‐superconductor percolative network based on the excitonic model of Allender, Bray, and Bardeen to explain the high values of T_c. Hall measurements show large effective masses in agreement with an excitonic model.