Correlation of Superconductivity with the Infrared Anomaly in (La_1-xM_x)₂(Cu_1-yN_y)O₄ (M=Ca, Sr and Ba; N=Ni and Zn)

For La 2 CuO 4 based high- T c superconducting oxide systems, three infrared absorption peaks by phonons, which were labeled as P 1 , P 2 and P 3 from the higher energy side, were observed in the energy region between 250 and 800 cm -1 . The intensity of the peak P 1 changed sensitively depending on the concentration of substituted ions. We call the fact the infrared anomaly. Superconductivity appears for samples for which the intensity ratio I (P 1 )/ I (P 2 ) is nearly zero. This correlation between the infrared anomaly and supeconductivity has been confirmed to be common not only for (La 1- x M x ) 2 CuO 4 (M=Ca, Sr and Ba) but also for (La 1- x Sr x ) 2 (Cu 1- y N y )O 4 (N=Ni and Zn). From mode assignments, the anomalous peak P 1 is inferred to correspond to stretching motions of Cu-O bonds along the bond direction perpendicular to the c -axis.