LOW-TEMPERATURE INTERNAL-FRICTION SPECTRUM OF CALCIUM-DOPED POLYCRYSTALLINE YBA2CU3O6+X

Internal friction measurements (IF) in the kilohertz frequency range were performed on Ca doped polycrystalline YBa2Cu3O6+x. The samples were also characterized by DC electrical resistivity and X-ray measurements. The IF spectra obtained in the present investigation show an increase in the low temperature mechanical loss maxima at 95 K and 115 K with increasing Ca content. This could be interpreted as being due to an increase in the electrical charge density, the Y3+-ions being replaced by the lower valent Ca2+-ions, thus creating additional holes. Furthermore the fact that doping increases the electrical charge density is shown by a decrease in the electrical resistivity with increasing Ca content. Thus the low temperature IF peaks could be attributed to the hopping of electronic defects in the CuO2 planes and in the CuO chains, whose density increase with Ca content.