Spin dynamics and structural phase transitions in quasi-2D antiferromagnets R$_2$CuO$_4$ (R${}={}$Pr, Sm, and Eu)

Spin and lattice dynamics of R$_2$CuO$_4$ (R${}={}$Pr, Sm, and Eu) crystals were studied over the frequency and temperature ranges $20$–$250$ GHz and $5$–$350$ K, respectively. The absorption coefficients of the R$_2$CuO$_4$ crystals (R${}={}$Pr, Sm, and Eu) were found to change dramatically at temperatures of, respectively, $20$, $80$, and $150$ K over a broad frequency range above $120$ GHz. The absorption jumps were caused by the structural phase transitions. Broad spin-wave bands were observed in the high-temperature phases of all crystals studied. Absorption lines due to lattice dynamics were observed near the temperatures of structural phase transition over a broad frequency range, including the frequencies corresponding to the spin-wave bands.