A low-temperature internal friction study of Y124 superconductors

A ceramic sample of YBa2Cu4O8 (Y124) was synthesized at atmospheric pressure with the addition of NaNO3 as an enhancer. The internal friction was measured from liquid nitrogen temperature to room temperature at kilohertz frequency using the vibrating reed method. Instead of a broad internal friction peak around 220 K observed in YBa2Cu3O7-delta (Y123) a small step in the internal friction spectrum was observed around 200 K with increase of temperature in Y124. This internal friction behaviour is similar to that of Sr-substituted YBa0.8Sr1.2Cu3O7-delta. We expect that the internal friction step in Y124 originates from charge carrier crossover. The small size of the change in internal friction around 200 K in Y124 is due to suppression of the lattice response compared with that in undoped Y123.