Far Infrared Spectroscopy of CsNiCl₃ Type Crystals
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
A far infrared interferometer has been interfaced to a microcomputer and programs developed to enable fourier transform spectroscopy to be carried out in real time. A Heᶾ cooled bolometer-superconducting magnet system has been constructed for far infrared Zeeman spectroscopy. The bolometer element is magnetically shielded from the field using superconducting lead. The theory of the transmission of polarized radiation through slabs of non-isotropic dielectric crystals mounted on a dielectric substrate is developed. The resulting equations allow general features of far infrared transmission spectra to be examined. For example they show that for "thicker" crystals, the transverse optical mode absorption lines are asymmetric while the longitudinal optical mode absorption lines are symmetric. For large angles of incidence, an extra absorption line is predicted to occur above the longitudinal optic mode frequency for crystals of low dielectric constant. The infrared-active phonon spectra of thirteen crystals of the CsNiCl₃ type structure are reported and assigned to specific lattice modes. Using polarized, Zeeman spectroscopy, two magnon lines are identified in the quasi 1 dimensional antiferromagnet RbCoBr₃ at 100.0±0.5 cm⁻¹ and 120.0±0.5 cm⁻¹ , each having a g value of 3.8±0.4. These lines are fitted using a perturbed Ising model to give a longitudinal interchain exchange strength of 51 cm⁻¹ and a relative longitudinal interchain exchange value of 0.06 for RbCoBr₃ at 1.7 K. A possible antiresonance line is reported at 114.6 cm⁻¹ for RbCoBr₃ and preliminary work is reported in the search for magnons in other CsNiCl₃ type crystals.