Specific heat study on CuxBa2Ca3Cu4Oy

Specific heat anomaly was studied on Cu x Ba 2 Ca 3 Cu 4 O y . ΔC/ T c , a height of peak of the specific heat at the superconducting transition ( T c ) divided by T c , is less than 60% of that of optimally doped YBa 2 Cu 3 O 7-δ . This suggests that a large density of states (DOS) remains to prohibit the creation of Bardeen-Cooper-Schrieffer (BCS) pairs at T c . It is much larger than the residual DOS at 0 K. We interpret that the “ missing DOS ” start to become BCS pairs at a temperature lower than T c , as proposed by an NMR study [Phys. Rev. B 61 (2000) 9707], though we have not yet seen clear evidence of this.