Spin-phonon coupling in antiferromagnetic Bi2 Sr2 CoO6+δ: An infrared reflectance study

The ab -plane infrared and optical reflectance of single-crystal Bi2 Sr2 CoO6+δ (0.4<δ<0.5) have been measured over a wide frequency range (50-55000 cm-1) and at temperatures between 20 and 330 K. The room-temperature infrared spectrum displays an insulating characteristic. The optical gap determined from the infrared conductivity (∼0.53 eV) is consistent with thermal activation energy from dc transport measurements. Upon passing through the 265 K antiferromagnetic ordering transition, a softening of the phonon mode near 205 cm-1 correlates well with the temperature-dependent normalized square of the sublattice magnetization. Furthermore, the magnetic-order-induced splitting of the phonon modes at about 238 and 376 cm-1 is observed. Additionally, the phonon mode at about 588 cm-1 exhibits a Fano-type line shape. Since no appreciable structural change was detected at low temperatures in x-ray diffraction studies, all of these observables suggest a complex nature of spin-phonon coupling in this material.