OPTICAL STUDY OF ANTIFERROMAGNETIC SINGLE-CRYSTALS Y1-XPRXBA2CU3O6 INHIGH MAGNETIC-FIELDS

Infrared measurements and Linear-spin-wave calculations for single-cry stal Y1-xPrxBa2Cu3O6 (x = 0.0, 0.4, 1.0) in magnetic fields up to 30 T show that absorption features which have been ascribed to magnon exci tations are very insensitive to the applied field. The substitution of Pr3+ for Y3+ leads to an additional absorption feature, which does ha ve a strong field dependence. This excitation is assigned to an interm ultiplet transition in the Pr3+ ion. The zero-field temperature depend ence of this absorption shows clear evidence of an interaction between Cu and Pr spins.