INFRARED ANOMALY, LOCAL ITINERANT CARRIERS AND SUPERCONDUCTIVITY IN YBA2(CU1-XCOX)3O7-DELTA

Infrared spectra have been measured for YBa2(Cu1-xCox)3O7-d oxides as a function of the sintering temperature and concentration x. In the spectrum, the infrared absorption peak at about 560cm has been observed to disappear for particular samples although it is allowed by the selection rules; i.e. the infrared anomaly. The infrared anomaly has been confirmed to correlate with appearance of superconductivity controlled both by the quenching temperature and by the substitution for Cu. The anomalous mode has been assigned as the Cu-0 stretching motion in the conduction plane. It has been concluded that the infrared anomaly is mainly caused by shielding of the dipole moments due to the particular phonon mode by locally itinerant carriers. From the continuous variation of the infrared anomaly, the itinerant length has been estimated to be about twice the lattice constant a. A possible relevance of the infrared anomaly with the t-J model has been discussed. © 1990, Elsevier Science Publishers B.V. (North-Holland). All rights reserved.