Magnetic excitations in PrBa2Cu4O8 explored by Raman scattering

Polarized Raman scattering spectra of PrBa2Cu4O8 were studied in the temperature range 10 to 300 K. An anomalous temperature dependence of the two-magnon peak was found, which softens upon cooling from about 2590 cm(-1) at 300 K to 2480 cm(-1) at 10 K. The observed behavior is opposite to what is known for RBa2Cu3O6, R2CuO4 (R = rare earth) and KMnF3-type antiferromagnets, where the two-magnon peak shifts to higher frequencies upon decreasing the in-plane Cu-O bond length due to either lowering temperature or applying pressure. The experimental data are discussed in view of the semiconductor-metal transition which, according to recent optical and transport studies, occurs in PrBa2Cu4O8 at T* approximate to 150 K.